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Transverse Load and Temperature Sensing Using Multiplexed Long-Period Fiber Gratings

Transverse Load and Temperature Sensing Using Multiplexed Long-Period Fiber Gratings hv photonics Article Transverse Load and Temperature Sensing Using Multiplexed Long-Period Fiber Gratings 1 , 1 1 2 Ismael Torres-Gómez * , Alejandro Martínez-Rios , Gilberto Anzueto-Sánchez , Daniel. E. Ceballos-Herrera and Guillermo Salceda-Delgado Centro de Investigaciones en Óptica AC (CIO), Loma del Bosque 115, Lomas del Campestre, Leon, Guanajuato 37150, Mexico; amr6@cio.mx (A.M.-R.); gilberto.anzueto@cio.mx (G.A.-S.) Instituto de Ingeniería, Universidad Nacional Autónoma de México (UNAM), Ciudad Universitaria, Alcaldía Coyoacán, Mexico City 04510, Mexico; DCeballosH@iingen.unam.mx Facultad de Ciencias Físico Matemáticas, Universidad Autónoma de Nuevo León (UANL), Avenida Universidad S/N, San Nicolás de los Garza, Nuevo León 66455, Mexico; guillermo.salcedadl@uanl.edu.mx * Correspondence: itorres@cio.mx Abstract: The simultaneous measurement of transverse load and temperature using two long- period fiber gratings multiplexed in the wavelength domain is presented experimentally. For this, a mechanically induced long-period fiber grating (MI-LPFG) and a long-period fiber grating inscribed by a continuous-wave CO laser (CO LPFG) are connected in cascade. First, the transverse load 2 2 and the temperature measurements were individually performed by the multiplexed long-period fiber gratings configuration. The MI-LPFG is subject to a transverse load variation from 0–2000 g with steps of 500 g, whereas the CO LPFG is unloaded and they are kept at room temperature. Similarly, the CO LPFG is subject to a temperature variation from 30 to 110 C by increments of 20 C, while the MI-LPFG with a constant transverse load of 2000 g is kept at room temperature. Subsequently, the simultaneous measurement of the transverse load and the temperature is performed by the multiplexed long-period fiber grating following the steps outlined above. According to the experimental results, the transverse load and temperature measurement present high repeatability Citation: Torres-Gómez, I.; Martínez- Rios, A.; Anzueto-Sánchez, G.; Ceballos- for the individual and simultaneous process. Moreover, the multiplexed LPFGs exhibit low cladding- Herrera, D.E.; Salceda-Delgado, G. Trans- mode crosstalk of transverse load and temperature. The coarse wavelength-division multiplexing verse Load and Temperature Sensing (CWDM) of long-period fiber gratings is an attractive alternative technique in optical fiber distributed Using Multiplexed Long-Period Fiber sensing applications. Gratings. Photonics 2021, 8, 1. https://dx.doi.org/10.3390/photonics8 Keywords: transverse load sensor; temperature sensor; long-period fiber gratings; primary rejection band; coarse wavelength-division multiplexing; optical fiber distributed sensing Received: 30 October 2020 Accepted: 16 December 2020 Published: 22 December 2020 1. Introduction Long-period fiber gratings (LPFGs) are versatile components widely studied with Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims relevant applications in telecommunications, fiber-optic lasers, and sensing systems [1–3]. in published maps and institutional Concerning sensing applications, LPFGs offer high sensitivity to external perturbations affiliations. of the surrounding medium, immunity to electromagnetic fields, passive measurements, fast response, low insertion loss, small backscattering, compactness, and remote moni- toring. These properties make LPFGs very attractive in developing physical, chemical, and biological fiber optic sensors [4–6]. Currently, several methods have been reported to Copyright: © 2020 by the authors. Li- produce LPFGs, such as exposure to ultra-violet (UV), electric arc discharge, CO laser ra- censee MDPI, Basel, Switzerland. This diation, femtosecond laser radiation, mechanical pressure, hydrogen-oxygen flame heating, article is an open access article distributed and ion implementation, among others [7–13]. Despite the progress made in the fabrication under the terms and conditions of the methods of LPFGs, there are still some challenges to harnessing the potential of LPFGs in Creative Commons Attribution (CC BY) sensing applications, such as the development of low-cost interrogation systems [14] and license (https://creativecommons.org/ licenses/by/4.0/). Photonics 2021, 8, 1. https://dx.doi.org/10.3390/photonics8010001 https://www.mdpi.com/journal/photonics Photonics 2021, 8, 1 2 of 9 the optical fiber distributed sensing applications by the wavelength-division multiplexing of LPFGs in cascade. Two different approaches have been reported for the optical fiber distributed sensing applications with wavelength-division multiplexing of LPFGs. The first method uses two similar concatenated LPFGs to conform a Mach–Zehnder interferometer [15]. In this way, one can have two or more Mach–Zehnder interferometers with distinct cavity lengths in series to measure different parameters simultaneously [16,17]. However, interferometric optical fiber sensors produce differential outputs. Therefore, they require complex demod- ulation techniques such as filtering the carrier frequencies in the frequency domain and the unwrapped phase processes to extract the external perturbations [18]. The second method entails implementing two or more different LPFGs in series in such form that their refer- ence rejection bands do not overlap and operate independently [19–22]. Although these schemes require simple demodulation techniques, the wavelength-division multiplexing of LPFGs is delimited because LPFGs usually generate multiple rejection bands. However, mechanically induced long-period fiber gratings (MI-LPFGs) with a primary rejection band have recently been reported using laminated plates [23]. Such MI-LPFGs with a principal rejection band facilitate the use of the CWDM technique for LPFG sensors in cascade. In this report, we demonstrate experimentally the simultaneous measurement of transverse load and temperature using two multiplexed long-period fiber gratings. For this, an MI-LPFG is connected with a CO LPFG in cascade to measure transverse load and temperature, respectively. These LPFGs are notable for having a prominent attenuation band over a wide wavelength range. As far as we know, this is the first time that the technique of wavelength-division multiplexing using LPFGs with a prominent attenuation band is presented. The work’s structure comprises the following sections: Section 1 ex- plains an antecedent on the two approaches of the multiplexing LPFGs reported previously, highlighting their scope and limitations. The relevance of the wavelength-division multi- plexing technique using LPFGs with a prominent attenuation band and their application in distributed sensing is also presented. The general principle of the phase-matching in LPFGs is described in Section 2. Section 3 describes the experimental arrangement and its principal features. Also, it describes in detail the implementation of the MI-LPFG and the inscription of the CO LPFG. In Section 4, the individual characterization of the MI-LPFG and the CO LPFG is presented when the LPFGs are under transverse load and temperature, respectively. Also, we show the simultaneous measurement of transverse load and temperature. Finally, Section 5 presents the most relevant findings of the work. 2. LPFGs Principle Long-period fiber gratings result from a periodic refractive index modulation pro- duced in the core of a single-mode optical fiber. The long-period fiber gratings operate as modal couplers allowing the light transfer from the fundamental mode in the core (LP ) to different co-propagating high-order cladding modes (LP , m = 2, 3, 4 . . .). This coupling 0m results in a discrete set of rejection bands in the grating transmission spectrum due to the scattering of the high-order cladding modes at the interface between the cladding and the external medium. Where the resonant central wavelength ( ) of the individual rejection bands must fulfill the phase-matching condition [7], = (n n )L (1) m 01 0m where n and n represent the effective refractive indices of the LP mode in the core 01 0m 01 and the LP mode in the cladding, respectively, and L is the period of the refractive index 0m modulation in the long-period fiber grating. The number of rejection bands in an LPFG depends upon the structure and material composition of the host single-mode optical fiber and the corresponding refractive index modulation. In general, LPFGs in single-modal optical fiber with a cosinusoidal refractive index modulation and a typical period from 100–600 m, usually present spectrum trans- missions with three to five rejection bands in the spectral range from 1200–1700 nm [7], Photonics 2021, 8, 1 3 of 9 Photonics 2020, 7, x FOR PEER REVIEW 3 of 9 where the attenuation depth of the rejection bands typically is more profound as the cou- pling mode’s order of the cladding increases. LPFGs with this number of the rejection typically is more profound as the coupling mode’s order of the cladding increases. LPFGs with this bands and their spectral side lobes limit the wavelength-division multiplexing of LPFGs number of the rejection bands and their spectral side lobes limit the wavelength-division in cascade [16]. However, in the last years, it has been shown that LPFGs with a primary multiplexing of LPFGs in cascade [16]. However, in the last years, it has been shown that LPFGs with rejection band over a wide wavelength range using different inscription techniques is a primary rejection band over a wide wavelength range using different inscription techniques is feasible [23–27]. Long-period fiber gratings with a primary rejection band facilitate the feasible [23–27]. Long-period fiber gratings with a primary rejection band facilitate the deployment deployment of fiber optic distributed sensing systems based on the wavelength-division of fiber optic distributed sensing systems based on the wavelength-division multiplexing of LPFGs. multiplexing of LPFGs. 3. Setup and Components 3. Setup and Components Figure 1a illustrates the experimental configuration schematic for measuring the Figure 1a illustrates the experimental configuration schematic for measuring the transverse load transverse load (TL) and the temperature (T) by the multiplexed LPFGs. The experimental (TL) and the temperature (T) by the multiplexed LPFGs. The experimental setup consisted of an MI- setup consisted of an MI-LPFG connected to a CO LPFG in cascade. The CO LPFG 2 2 LPFG connected to a CO2 LPFG in cascade. The CO2 LPFG was located over an electric hot plate was located over an electric hot plate where the temperature can be manually controlled. where the temperature can be manually controlled. The input end of the MI-LPFG was connected to The input end of the MI-LPFG was connected to a white light source (WLS; AQ-4303B), a white light source (WLS; AQ-4303B), and the output end of the CO2 LPFG was connected to an and the output end of the CO LPFG was connected to an optical spectrum analyzer (OSA; optical spectrum analyzer (OSA; AQ-6315A). For each proof, the transmission spectrum of the AQ-6315A). For each proof, the transmission spectrum of the cascaded LPFGs was recorded cascaded LPFGs was recorded by the optical spectrum analyzer, while the spectral resolution was set by the optical spectrum analyzer, while the spectral resolution was set to 1 nm. The fiber to 1 nm. The fiber used in the double grating configuration is a standard single-mode fiber (SMF-28) used in the double grating configuration is a standard single-mode fiber (SMF-28) for for telecommunications. The LPFGs were separated 10 cm in the single-mode optical fiber. Figure 1b telecommunications. The LPFGs were separated 10 cm in the single-mode optical fiber. Figure 1b illustrates the photography of the experimental configuration. illustrates the photography of the experimental configuration. Figure 1. Experimental configuration: (a) schematic and (b) photography. Figure 1. Experimental configuration: (a) schematic and (b) photography. The MI-LPFG can be achieved when the optical fiber is compressed between a flat The MI-LPFG can be achieved when the optical fiber is compressed between a flat aluminum aluminum plate and a laminated plate, see Figure 1b. The laminated plate consisted of plate and a laminated plate, see Figure 1b. The laminated plate consisted of a parallel assembling of a parallel assembling of single-edged utility blades [23]. The laminated plate had a length single-edged utility blades [23]. The laminated plate had a length of approximately 30 mm, an of approximately 30 mm, an average period of 490  10 m, and an average duty cycle average period of 490 ± 10 µ m, and an average duty cycle of the refractive-index modulation of 0.2. of the refractive-index modulation of 0.2. Figure 2a shows the transmission spectrum Figure 2a shows the transmission spectrum of the MI-LPFG when a constant transverse load of 2000 of the MI-LPFG when a constant transverse load of 2000 g is applied between the plates. g is applied between the plates. As can be seen, its spectrum transmission shows a primary rejection As can be seen, its spectrum transmission shows a primary rejection band at 1279.3 nm with band at 1279.3 nm with a sidelobe at 1338.4 nm and shallow rejection bands at 1386.0 and 1484.2 nm. a sidelobe at 1338.4 nm and shallow rejection bands at 1386.0 and 1484.2 nm. The primary The primary rejection band’s attenuation depth was 14.4 dB, whereas the attenuation depth for the rejection band’s attenuation depth was 14.4 dB, whereas the attenuation depth for the lateral shallow rejection bands was lower than 1.5 dB. According to the reference spectrum lateral shallow rejection bands was lower than 1.5 dB. According to the reference spectrum transmission of the MI-LPFG, its average insertion loss is lower than 0.25 dB for the above conditions. transmission of the MI-LPFG, its average insertion loss is lower than 0.25 dB for the above It should be noted that the MI-LPFG has no attenuation bands in the spectral range from 1540–1640 nm, although a small portion of light can propagate through the cladding, as can be inferred from the background loss induced by the MI-LPFG in that spectral range. Photonics 2020, 7, x FOR PEER REVIEW 4 of 9 On the other hand, the CO2 LPFG was inscribed using a continuous-wave CO2 laser glass processing system (Laser Master LZM-100). It has a length of 37.5 mm and a period of 0.75 mm. In the inscription process, a fiber section is heated during 120 ms with a power discharge of 20 W; then, with the same power, the fiber is pulled for 60 ms. In order to have a principal rejection band in the spectral range of 1540–1580 nm, a period of 0.75 nm was determined empirically based on an extensive experimental study. Figure 2b shows its transmission spectrum. As can be seen, the transmission spectrum displays a primary rejection band at 1553.8 nm with 9.8 dB and shallow rejection bands at 1220.2, 1289.7, 1370.8, and 1493.7 nm with attenuation depths lower than 2.2 dB. According to the reference spectrum transmission, the average insertion loss is lower than 0.85 dB. The insertion loss is due to the scattering produced by the heated points irradiated by the continuous- wave CO2 laser. The CO2 LPFG was fixed on an aluminum holder (12 × 2 × 1 cm) by commercial epoxy Photonics 2021, 8, 1 4 of 9 putty. The CO2 LPFG was sat on the aluminum holder by pasting its two ends with a physical separation of 10 cm. Then the aluminum holder with the CO2 LPFG was placed over the electric hot plate, see Figure 1b. The maximum operating temperature of the epoxy putty was 110 °C. For its part, Figure 2c illustrates the transmission spectrum of the MI-LPFG and the CO2 LPFG in cascade when a conditions. It should be noted that the MI-LPFG has no attenuation bands in the spectral constant transverse load of 2000 g is applied in the MI-LPFG. We can observe the primary rejection range from 1540–1640 nm, although a small portion of light can propagate through the bands of the LPFGs and the overlapping of their shallow rejection bands. The insertion loss of the cladding, as can be inferred from the background loss induced by the MI-LPFG in that cascaded LPFGs is less than 1.1 dB regarding the reference transmission spectrum. spectral range. Figure 2. The transmission spectrum of (a) mechanically induced long-period fiber grating (MI- Figure 2. The transmission spectrum of (a) mechanically induced long-period fiber grating (MI- LPFG), LPFG (b), ) CO (b) CO LPFG, 2 LPFG( , c (c )) multiplexed multiplexed MMI-LPFG I-LPFG and and CO2CO LPFGLPFG. . 2 2 4. ExpOn erim the ent and R other esu hand, lts An the alys CO is LPFG was inscribed using a continuous-wave CO laser 2 2 glass processing system (Laser Master LZM-100). It has a length of 37.5 mm and a period Once the experimental setup was installed, the transverse load on the MI-LPFG was increased of 0.75 mm. In the inscription process, a fiber section is heated during 120 ms with a power from 0 to 2000 g with increments of 500 g, while the CO2 LPFG remains unloaded. Both LPFGs in the discharge of 20 W; then, with the same power, the fiber is pulled for 60 ms. In order to proofs stayed at room temperature (27 ± 3 °C). Figure 3a shows the transmission spectrum evolution have of the multip a principal lexed LPFG rejection s when band the load in the on th spectral e MI-LPFG range incre of ase 1540–1580 s. As a result, nm, the a atten period uatioof n dept 0.75 h nm of the leading rejection band of the MI-LPFG got more profound as the transverse load increased. In was determined empirically based on an extensive experimental study. Figure 2b shows contrast, the attenuation depth of the principal rejection band of the CO2 LPFG remains practically its transmission spectrum. As can be seen, the transmission spectrum displays a primary unchanged. Figure 3b,c shows the spectrum transmission evolution of the primary rejection bands of rejection band at 1553.8 nm with 9.8 dB and shallow rejection bands at 1220.2, 1289.7, 1370.8, the multiplexed LPFGs when the transverse load increases. No wavelength shift is observed in the and 1493.7 nm with attenuation depths lower than 2.2 dB. According to the reference MI-LPFG primary rejection band. In contrast, the principal rejection band of the CO2 LPFG presents spectrum transmission, the average insertion loss is lower than 0.85 dB. The insertion loss is a tiny wavelength shift to larger wavelengths that can be practically considered negligible. Figure 4 due to the scattering produced by the heated points irradiated by the continuous-wave CO illustrates the attenuation depth evolution of the primary rejection bands of the multiplexed LPFGs laser. The CO LPFG was fixed on an aluminum holder (12  2  1 cm) by commercial versus the transverse load on the MI-LPFG. The leading rejection band of the MI-LPFG presents a epoxy putty. The CO LPFG was sat on the aluminum holder by pasting its two ends nonlinear increase, while the attenuation depth of the principal rejection band of CO2 LPFG shows a with a physical separation of 10 cm. Then the aluminum holder with the CO LPFG was small variation. It is important to note that the shallow rejection bands of the MI-LPFG also got 2 deeper placed over the electric hot plate, see Figure 1b. The maximum operating temperature of when the transverse load increased, but they do not interfere with the primary rejection band of the CO2 LPFG. the epoxy putty was 110 C. For its part, Figure 2c illustrates the transmission spectrum of the MI-LPFG and the CO LPFG in cascade when a constant transverse load of 2000 g is applied in the MI-LPFG. We can observe the primary rejection bands of the LPFGs and the overlapping of their shallow rejection bands. The insertion loss of the cascaded LPFGs is less than 1.1 dB regarding the reference transmission spectrum. 4. Experiment and Results Analysis Once the experimental setup was installed, the transverse load on the MI-LPFG was increased from 0 to 2000 g with increments of 500 g, while the CO LPFG remains unloaded. Both LPFGs in the proofs stayed at room temperature (27  3 C). Figure 3a shows the transmission spectrum evolution of the multiplexed LPFGs when the load on the MI-LPFG increases. As a result, the attenuation depth of the leading rejection band of the MI-LPFG got more profound as the transverse load increased. In contrast, the attenuation depth of the principal rejection band of the CO LPFG remains practically unchanged. Figure 3b,c shows the spectrum transmission evolution of the primary rejection bands of the multiplexed LPFGs when the transverse load increases. No wavelength shift is observed in the MI- LPFG primary rejection band. In contrast, the principal rejection band of the CO LPFG presents a tiny wavelength shift to larger wavelengths that can be practically considered negligible. Figure 4 illustrates the attenuation depth evolution of the primary rejection bands of the multiplexed LPFGs versus the transverse load on the MI-LPFG. The leading rejection band of the MI-LPFG presents a nonlinear increase, while the attenuation depth of the principal rejection band of CO LPFG shows a small variation. It is important to note 2 Photonics 2021, 8, 1 5 of 9 Photonics 2020, 7, x FOR PEER REVIEW 5 of 9 that the shallow rejection bands of the MI-LPFG also got deeper when the transverse load Photonics 2020, 7, x FOR PEER REVIEW 5 of 9 increased, but they do not interfere with the primary rejection band of the CO LPFG. Photonics 2020, 7, x FOR PEER REVIEW 5 of 9 Figure 3. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO LPFG versus Figure 3. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO2 LPFG versus 2 Figure 3. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO2 LPFG versus applied weight on the MI-LPFG. applied weight on the MI-LPFG. applied weight on the MI-LPFG. Figure 3. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO2 LPFG versus applied weight on the MI-LPFG. Figure 4. Attenuation depth of the main rejecti on bands of the MI-LPFG and the CO2 LPFG versus Figure applie 4.d Attenuation weight on the M depth I-LPFG. of the main rejection bands of the MI-LPFG and the CO LPFG versus Figure 4. Attenuation depth of the main rejection bands of the MI-LPFG and the CO2 LPFG versus applied weight on the MI-LPFG. Figure 4. Attenuation depth of the main rejection bands of the MI-LPFG and the CO2 LPFG versus applied weight on the MI-LPFG. Next, the temperature in the CO2 LPFG was increased from 30 to 110 °C by steps of 20 °C using applied weight on the MI-LPFG. an electric Next, hot the plate temperatur , whereas the e in MIthe -LPF CO G with LPFG a con was stant incr transver eased se fr lo om ad of 30 20 to 00 110 g waC s kept by steps at room of Next, the temperature in the CO2 LPFG was increased from 30 to 110 °C by steps of 20 °C using temperature. Figure 5a shows the transmission spectrum evolution of the multiplexed LPFGs when 20 Next, C using the tem an perature electri c inhot the plate, CO2 LPFG was whereas incre thease MI-LPFG d from 30 with to 110 a constant °C by steps transverse of 20 °C usload ing an electric hot plate, whereas the MI-LPFG with a constant transverse load of 2000 g was kept at room the temperature in the CO2 LPFG was increased. Figure 5b,c shows the transmission spectrum an electric hot plate, whereas the MI-LPFG with a constant transverse load of 2000 g was kept at room of 2000 g was kept at room temperature. Figure 5a shows the transmission spectrum temperature. Figure 5a shows the transmission spectrum evolution of the multiplexed LPFGs when tem evo perat lution ure. of Fi the gure prima 5a ry sho re ws ject th ion e trans band miss s of ion the spect multipl rum exed evo LP lutio FGs. n of As th can e mu be lti see plexed n, the L reject PFGio s when n band evolution of the multiplexed LPFGs when the temperature in the CO LPFG was increased. the temperature in the CO2 LPFG of the CO was 2 LP incre FG shi ased. fted Fi tog wards ure 5b,c longer show wavelen s thgths e tra with nsmiss a slight ion decrea spectrum se in the attenuation depth. the temperature in the CO2 LPFG was increased. Figure 5b,c shows the transmission spectrum Figure 5b,c shows the transmission spectrum evolution of the primary rejection bands Meanwhile, the leading rejection band of the MI-LPFG presents small variations in the attenuation evolution of the primary reject evo ion lu band tion o s f of the th prima e mury ltipl reject exed ion LP band FGs. s of As the can mube ltipl see exed n, th LP e FGs. reject As iocn an band be see n, the rejection band of the multiplexed LPFGs. As can be seen, the rejection band of the CO LPFG shifted depth and wavelength shift. Figure 6 shows the wavelength shift of the primary rejection bands of of the CO2 LPFG shifted towards longer wavelengths with a slight decrease in the attenuation depth. of the CO2 LPFG shifted towardstowar longer dswavelen longer wavelengths gths with a slight withdecrea a slight se decr in th ease e atten in the uation attenuation depth. depth. Meanwhile, multiplexed LPFGs concerning the spectrum transmission of the multiplexed LPFG at 30 °C. The CO2 Meanwhile, the leading rejection band of the MI-LPFG presents small variations in the attenuation the leading rejection band of the MI-LPFG presents small variations in the attenuation Meanwhile, the leading rejection band of the MI-LPFG presents small variations in the attenuation LPFG principal rejection band shows a linear wavelength shift when the temperature is increased. depth and wavelength shift. Figure 6 shows the wavelength shift of the primary rejection bands of depth and wavelength shift. Figure 6 shows the wavelength shift of the primary rejection depth and wavelength shift. Figure 6 shows the wavelength shift of the primary rejection bands of This rejection band shows a temperature sensitivity of ~50 pm/°C, and its R-squared factor of the multiplexed LPFGs concerning the spectrum transmission of the multiplexed LPFG at 30 °C. The CO2 bands of multiplexed LPFGs concerning the spectrum transmission of the multiplexed multiplexed LPFGs concerning the spectrum transmission of the multiplexed LPFG at 30 °C. The CO2 LPFG linear pr fitting incipal is reject 0.9989. ion On band the shows other a hli an ne d, ar th wave e MIle -L ngth PFG shi leading ft wh en reject the ion tem bpe anra d ture pres ent is incr ed ea a sma sed. ll LPFG at 30 C. The CO LPFG principal rejection band shows a linear wavelength shift LPFG principal rejection band shows a linear wavelength shift when the temperature is increased. oscillating wavelength shift due to the overlapping with the shallow rejection bands at 1289.7 nm of This rejection band shows a temperature sensitivity of ~50 pm/°C, and its R-squared factor of the when the temperature is increased. This rejection band shows a temperature sensitivity This rejection band shows a temperature sensitivity of ~50 pm/°C, and its R-squared factor of the the CO2 LPFG. Similarly, the attenuation depth of the leading rejection band of the MI-LPFG is linear fitting is 0.9  989. On the other hand, the MI-LPFG leading rejection band presented a small of 50 pm/ C, and its R-squared factor of the linear fitting is 0.9989. On the other hand, linear fitting is 0.9989. On the other hand, the MI-LPFG leading rejection band presented a small slightly altered. oscillating wavelength shift due to the overlapping with the shallow rejection bands at 1289.7 nm of the MI-LPFG leading rejection band presented a small oscillating wavelength shift due to oscillating wavelength shift due to the overlapping with the shallow rejection bands at 1289.7 nm of the CO2 LPFG. Similarly, the attenuation depth of the leading rejection band of the MI-LPFG is the overlapping with the shallow rejection bands at 1289.7 nm of the CO LPFG. Similarly, slightly altered. the CO2 LPFG. Similarly, the attenuation depth of the leading rejection band of the MI-LPFG is the attenuation depth of the leading rejection band of the MI-LPFG is slightly altered. slightly altered. Figure 5. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO2 LPFG versus applied temperature on the CO2 LPFG. Figure 5. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO LPFG versus Figure 5. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO2 LPFG 2 versus applied temperature on the CO LPFG. applied temperature on the CO2 LPFG. Figure 5. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO2 LPFG versus applied temperature on the CO2 LPFG. Photonics 2020, 7, x FOR PEER REVIEW 6 of 9 Photonics 2021, 8, 1 6 of 9 Photonics 2020, 7, x FOR PEER REVIEW 6 of 9 Photonics 2020, 7, x FOR PEER REVIEW 6 of 9 Figure 6. The wavelength shift of the main rejection bands of MI-LPFG and CO2 LPFG versus applied temperature on CO2 LPFG. Figure 6. The wavelength shift of the main rejection bands of MI-LPFG and CO LPFG versus applied Figure 6. The wavelength shift of the main rejection bands of MI-LPFG and CO2 LPFG versus applied Then, the transverse load on the MI-LPFG and the temperature in the CO2 LPFG were temperature on CO LPFG. temperature on CO2 LPFG. Figure 6. The wavelength shift of the main rejection bands of MI-LPFG and CO2 LPFG versus applied simultaneously measured. Thus, the transverse load was increased from 0 to 2000 g by increments of temper Then, aturthe e on CO transverse 2 LPFG. load on the MI-LPFG and the temperature in the CO LPFG were 500 g in the MI-LPFG, while the temperature was also increased from 30 to 110 °C by steps of 20 °C in Then, the transverse load on the MI-LPFG and the temperature in the CO2 LPFG were simultaneously measured. Thus, the transverse load was increased from 0 to 2000 g by the CO2 LPFG. Figure 7a shows the transmission spectrum evolution of the multiplexed LPFGs for these simultaneously measured. Thus, the T h trea n n , sv th eer st e r a lo ns av d e rw sea sl o in ad c re oa ns etd h ef rM om I- L 0P t Fo G 2 0 a0 n0 d gt h bey tie nm cr p eem rat eu nr te s o in f the CO2 LPFG were increments of 500 g in the MI-LPFG, while the temperature was also increased from 30 conditions. Figure 7b,c shows the primary rejection bands evolution of the multiplexed LPFGs when simultaneously measured. Thus, the transverse load was increased from 0 to 2000 g by increments of 500 g in the MI-LPFG, while the temperature was also increased from 30 to 110 °C by steps of 20 °C in to 110 C by steps of 20 C in the CO LPFG. Figure 7a shows the transmission spectrum the transverse load and the temperature were increased, respectively. Figure 8a shows the leading 500 g in the MI-LPFG, while the temperature was also increased from 30 to 110 °C by steps of 20 °C in the CO2 LPFG. Figure 7a shows the transmission spectrum evolution of the multiplexed LPFGs for these evolution of the multiplexed LPFGs for these conditions. Figure 7b,c shows the primary rejection band th ’s e a CtO te 2n Lu Pa FtG io . n Fi g d u erp e t7 h a s b h eo h w av s t ih oe r tirn a ntsh m ei s M sio I- n L sP pF eG ctr u w m h e ev no t lu hte io tn r a on f t sh vee m rsu el tlio pa le d x eid n c LrP eF as Ge ss f ofro tr h ese conditions. Figure 7b,c shows ther p ejection rimary bands rejectioevolution n bands ev of olu the tion multiplexed of the multiLPFGs plexed when LPFGsthe whtransverse en load and the conditions. Figure 7b,c shows the primary rejection bands evolution of the multiplexed LPFGs when different temperatures at the CO2 LPFG. The attenuation depth shows a nonlinear increase, similar to temperature were increased, respectively. Figure 8a shows the leading rejection band’s the transverse load and the temperature were increased, respectively. Figure 8a shows the leading the transverse load and the temperature were increased, respectively. Figure 8a shows the leading the results obtained previously in Figure 4. On the other hand, Figure 8b shows the principal rejection attenuation depth behavior in the MI-LPFG when the transverse load increases for different rejection band’s attenuation depth behavior in the MI-LPFG when the transverse load increases for rejection band’s attenuation depth behavior in the MI-LPFG when the transverse load increases for band’s wavelength shift in the CO2 LPFG with respect to spectrum transmission at 30 °C for the above temperatures at the CO LPFG. The attenuation depth shows a nonlinear increase, similar to different temperatures at the CO2 LPFG. The attenuation depth shows a nonlinear increase, similar to different temperatures at the CO2 LPFG. The attenuation depth shows a nonlinear increase, similar to conditions. Thethe prirn esults cipal obtained rejection b pr aeviously nd’s centin er F w igur avee le4 n.gtOn h in the the other CO2 hand, LPFGFigur show e s8 b a shows linear the the results obtained previously in Figure 4. On the other hand, Figure 8b shows the principal rejection the results obtained previously in Figure 4. On the other hand, Figure 8b shows the principal rejection principal rejection band’s wavelength shift in the CO LPFG with respect to spectrum wavelength shift towards longer wavelengths. The rejection band shows a temperature sensitivity of band’s wavelength shift in the CO2 LPFG with respect to spectrum transmission at 30 °C for the above band’s wavelength shift in the CO2 LPFG with respect to spectrum transmission at 30 °C for the above transmission at 30 C for the above conditions. The principal rejection band’s center ~50 pm/°C, and its R-squared factor of the linear fitting was 0.9986. conditions. The principal rejec ct oin od ni tib oa nn s.d T ’sh ec ep nrtie nr c ip w ala v re ejle ecn tig otn h bia n n d t’h se ce C nO te2 r L wP aF ve G le n sg h to hw in s a th el in Ce O a2r LPFG shows a linear wavelength in the CO LPFG shows a linear wavelength shift towards longer wavelengths. wavelength shift towards longer wavelengths. The rejection band shows a temperature sensitivity of wavelength shift towards longer wavelengths. The rejection band shows a temperature sensitivity of The rejection band shows a temperature sensitivity of 50 pm/ C, and its R-squared factor ~50 pm/°C, and its R-squared factor of the linear fitting was 0.9986. ~50 pm/°C, and its R-squared factor of the linear fitting was 0.9986. of the linear fitting was 0.9986. Figure 7. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO2 LPFG versus Figure 7. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO LPFG versus applied transverse load on MI-LPFG and temperature on the CO2 LPFG, simultaneously. 2 Figure 7. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO2 LPFG versus applied transverse load on MI-LPFG and temperature on the CO LPFG, simultaneously. applied transverse load on MI-LPFG and temperature on the CO2 LPFG, simultaneously. Figure 7. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO2 LPFG versus applied transverse load on MI-LPFG and temperature on the CO2 LPFG, simultaneously. Figure 8. (a) Attenuation depth of MI-LPFG, and (b) wavelength shift of the CO2 LPFG for the Figure 8. (a) Attenuation depth of MI-LPFG, and (b) wavelength shift of the CO LPFG for the Figure 8. (a) Attenuation depth of MI-LPFG, and (b) wavelength shift of the CO2 LPFG for the simultaneous measurement of transverse load and temperature, respectively. simultaneous measurement of transverse load and temperature, respectively. simultaneous measurement of transverse load and temperature, respectively. The MI-LPFG displays a leading rejection band at 1280  1 nm, while the CO LPFG Figure 8. (a) Attenuation depth of MI-LPFG, and (b) wavelength shift of the CO2 LPFG for the 2 presents a principal rejection band at 1553.80 nm at room temperature (27 C). The wave- simultaneous measurement of transverse load and temperature, respectively. Photonics 2021, 8, 1 7 of 9 Photonics 2020, 7, x FOR PEER REVIEW 7 of 9 The MI-LPFG displays a leading rejection band at 1280 ± 1 nm, while the CO2 LPFG presents a length separation between the primary rejection bands allows the CWDM of LPFGs with principal rejection band at 1553.80 nm at room temperature (27 °C). The wavelength separation low cladding-mode crosstalk. When the transverse load in the MI-LPFG was increased from between the primary rejection bands allows the CWDM of LPFGs with low cladding-mode crosstalk. 0–2000 g, the principal rejection band of the CO LPFG underwent attenuation depth varia- When the transverse load in the MI-LPFG was increased from 0–2000 g, the principal rejection band tions lower than 0.2 dB and a wavelength shift lower than 0.2 nm with respect to the of the CO2 LPFG underwent attenuation depth variations lower than ±0.2 dB and a wavelength shift initial spectrum transmission. We assume that these random variations are due to the white lower than ±0.2 nm with respect to the initial spectrum transmission. We assume that these random light source output power stability combined with the insertion loss induced by the MI- variations are due to the white light source output power stability combined with the insertion loss LPFG. Note that the MI-LPFG transverse load sensitivity was obtained at a constant room induced by the MI-LPFG. Note that the MI-LPFG transverse load sensitivity was obtained at a temperature. However, it is well known that attenuation depth and the central wavelength location of the rejection bands are influenced by the temperature on the MI-LPFGs [28]. constant room temperature. However, it is well known that attenuation depth and the central It had been observed that with a temperature increase on the MI-LPFGs, the rejection bands wavelength location of the rejection bands are influenced by the temperature on the MI-LPFGs [28]. shift to longer wavelengths, and their attenuation depth partially decreases. In this sense, It had been observed that with a temperature increase on the MI-LPFGs, the rejection bands shift to to include the effect of the temperature on the MI-LPFG response, the transverse load longer wavelengths, and their attenuation depth partially decreases. In this sense, to include the effect sensitivity, and the wavelength shifting sensitivity of the principal rejection band can be of the temperature on the MI-LPFG response, the transverse load sensitivity, and the wavelength calibrated at different temperatures. On the other hand, when the temperature in the CO shifting sensitivity of the principal rejection band can be calibrated at different temperatures. On the LPFG was increased from 30 to 110 C, the leading rejection band in the MI-LPFG experi- other hand, when the temperature in the CO2 LPFG was increased from 30 to 110 °C, the leading enced an attenuation depth variation lower than 0.4 dB and a wavelength shift lower than rejection band in the MI-LPFG experienced an attenuation depth variation lower than ±0.4 dB and a 0.3 nm with respect to the initial spectrum transmission. We assume that these variations wavelength shift lower than ±0.3 nm with respect to the initial spectrum transmission. We assume are also due to the white light source output power stability and the overlapping between that these variations are also due to the white light source output power stability and the overlapping the leading rejection band in the MI-LPFG and the CO LPFG shallow rejection band at between the leading rejection band in the MI-LPFG and the CO2 LPFG shallow rejection band at 1289.7 nm. In the last case, increasing the separation between the LPFGs can significantly 1289.7 nm. In the last case, increasing the separation between the LPFGs can significantly reduce the reduce the overlapping effect since cladding light will be attenuated by the high index overlapping effect since cladding light will be attenuated by the high index polymer coating of the polymer coating of the optical fiber section between LPFG. On the other side, the random optical fiber section variations bet of wee the n power LPFG.transmission On the other spectr sid um e, intr the oduced random by va the riations white light of th sour e po cewer can be eliminated using a broadband light source by combining two superluminescent diodes at transmission spectrum introduced by the white light source can be eliminated using a broadband 1280 and 1550 nm. light source by combining two superluminescent diodes at 1280 and 1550 nm. The above results were replicated when we simultaneously measured the transverse The above results were replicated when we simultaneously measured the transverse load and load and the temperature in the multiplexed LPFGs, respectively. Figure 9a displays the temperature in the multiplexed LPFGs, respectively. Figure 9a displays a comparison between a comparison between the individual and simultaneous measurement of the attenuation the individual and simultaneous measurement of the attenuation depth in the MI-LPFG. As can be depth in the MI-LPFG. As can be seen, except for the attenuation depth corresponding to seen, except for the attenuation depth corresponding to 1500 g, the remainder attenuation depth 1500 g, the remainder attenuation depth points preserve a close correlation. This difference points preserve a close correlation. This difference at 1500 g can be due to the repeatability of the MI- at 1500 g can be due to the repeatability of the MI-LPFG. Meanwhile, Figure 9b illustrates LPFG. Meanwhile, Figure 9b illustrates a comparison between the wavelength shift in the CO2 LPFG a comparison between the wavelength shift in the CO LPFG for the individual and for the individual and simultaneous temperature measurement, where one can observe a close simultaneous temperature measurement, where one can observe a close correlation between correlation between individual and simultaneous measurements of the temperature. According to individual and simultaneous measurements of the temperature. According to these results, these results, the multiplexed LPFGs operate with low cladding-mode crosstalk. The experimental the multiplexed LPFGs operate with low cladding-mode crosstalk. The experimental results demonstrate the simultaneous measurement of transverse load and temperature by the results demonstrate the simultaneous measurement of transverse load and temperature multiplexed LPFGs. by the Th multiplexed e current mu LPFGs. ltiplexed The LPFGs current arrangement multiplexed uses LPFGs an MI arrangement -LPFG; howeve uses r, it an is MI- possible to pro LPFG; duce L however PFGs with , it is a possible primary to reject produce ion band LPFGs by with other a pri inscr mary iptio rn ejection methods band such by other as inscription methods such as electric arc discharge and femtosecond laser irradiation [25–27]. electric arc discharge and femtosecond laser irradiation [25–27]. These LPFGs may allow more These LPFGs may allow more flexible schemes of distributed sensing applications based flexible schemes of distributed sensing applications based on multiplexed LPFGs in cascade. on multiplexed LPFGs in cascade. Figure 9. Individual and simultaneous measurement of (a) attenuation depth in the MI-LPFG and Figure 9. Individual and simultaneous measurement of (a) attenuation depth in the MI-LPFG and (b) (b) wavelength shift in the CO LPFG. wavelength shift in the CO2 LPFG. Photonics 2021, 8, 1 8 of 9 5. Conclusions The simultaneous measurement of transverse load and temperature using two mul- tiplexed long-period fiber gratings has been demonstrated experimentally in this work. We have used an MI-LPFG with a CO LPFG connected in cascade to measure the transverse load and the temperature, respectively. The experimental results show that the transverse load and temperature measurements show low cladding-mode crosstalk between the mul- tiplexed LPFGs. In fact, the cladding-mode crosstalk can be canceled by increasing the length of the fiber between the gratings. The low or absence of cladding-modes crosstalk, simplify the interrogation method since only changes in transmission need to be measured. It is well known that distributed sensing applications are still a trend in the development of optical fiber sensors based on multiplexed LPFGs. In this regard, we propose the CWDM of long-period fiber gratings with a leading rejection band to determine multiple parameters simultaneously. To our best knowledge, this is the first time that coarse multiplexing of LPFGs has been reported for multiple parameter sensing using LPFGs with a leading rejection band. The new concept of multiplexed LPFGs using MI-LPFG with only one leading rejection band can facilitate the CWDM of several LPFGs in cascade. This technique can improve the implementation of fiber optic distributed sensing systems based on the wavelength-division multiplexing of LPFGs. 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Transverse Load and Temperature Sensing Using Multiplexed Long-Period Fiber Gratings

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hv photonics Article Transverse Load and Temperature Sensing Using Multiplexed Long-Period Fiber Gratings 1 , 1 1 2 Ismael Torres-Gómez * , Alejandro Martínez-Rios , Gilberto Anzueto-Sánchez , Daniel. E. Ceballos-Herrera and Guillermo Salceda-Delgado Centro de Investigaciones en Óptica AC (CIO), Loma del Bosque 115, Lomas del Campestre, Leon, Guanajuato 37150, Mexico; amr6@cio.mx (A.M.-R.); gilberto.anzueto@cio.mx (G.A.-S.) Instituto de Ingeniería, Universidad Nacional Autónoma de México (UNAM), Ciudad Universitaria, Alcaldía Coyoacán, Mexico City 04510, Mexico; DCeballosH@iingen.unam.mx Facultad de Ciencias Físico Matemáticas, Universidad Autónoma de Nuevo León (UANL), Avenida Universidad S/N, San Nicolás de los Garza, Nuevo León 66455, Mexico; guillermo.salcedadl@uanl.edu.mx * Correspondence: itorres@cio.mx Abstract: The simultaneous measurement of transverse load and temperature using two long- period fiber gratings multiplexed in the wavelength domain is presented experimentally. For this, a mechanically induced long-period fiber grating (MI-LPFG) and a long-period fiber grating inscribed by a continuous-wave CO laser (CO LPFG) are connected in cascade. First, the transverse load 2 2 and the temperature measurements were individually performed by the multiplexed long-period fiber gratings configuration. The MI-LPFG is subject to a transverse load variation from 0–2000 g with steps of 500 g, whereas the CO LPFG is unloaded and they are kept at room temperature. Similarly, the CO LPFG is subject to a temperature variation from 30 to 110 C by increments of 20 C, while the MI-LPFG with a constant transverse load of 2000 g is kept at room temperature. Subsequently, the simultaneous measurement of the transverse load and the temperature is performed by the multiplexed long-period fiber grating following the steps outlined above. According to the experimental results, the transverse load and temperature measurement present high repeatability Citation: Torres-Gómez, I.; Martínez- Rios, A.; Anzueto-Sánchez, G.; Ceballos- for the individual and simultaneous process. Moreover, the multiplexed LPFGs exhibit low cladding- Herrera, D.E.; Salceda-Delgado, G. Trans- mode crosstalk of transverse load and temperature. The coarse wavelength-division multiplexing verse Load and Temperature Sensing (CWDM) of long-period fiber gratings is an attractive alternative technique in optical fiber distributed Using Multiplexed Long-Period Fiber sensing applications. Gratings. Photonics 2021, 8, 1. https://dx.doi.org/10.3390/photonics8 Keywords: transverse load sensor; temperature sensor; long-period fiber gratings; primary rejection band; coarse wavelength-division multiplexing; optical fiber distributed sensing Received: 30 October 2020 Accepted: 16 December 2020 Published: 22 December 2020 1. Introduction Long-period fiber gratings (LPFGs) are versatile components widely studied with Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims relevant applications in telecommunications, fiber-optic lasers, and sensing systems [1–3]. in published maps and institutional Concerning sensing applications, LPFGs offer high sensitivity to external perturbations affiliations. of the surrounding medium, immunity to electromagnetic fields, passive measurements, fast response, low insertion loss, small backscattering, compactness, and remote moni- toring. These properties make LPFGs very attractive in developing physical, chemical, and biological fiber optic sensors [4–6]. Currently, several methods have been reported to Copyright: © 2020 by the authors. Li- produce LPFGs, such as exposure to ultra-violet (UV), electric arc discharge, CO laser ra- censee MDPI, Basel, Switzerland. This diation, femtosecond laser radiation, mechanical pressure, hydrogen-oxygen flame heating, article is an open access article distributed and ion implementation, among others [7–13]. Despite the progress made in the fabrication under the terms and conditions of the methods of LPFGs, there are still some challenges to harnessing the potential of LPFGs in Creative Commons Attribution (CC BY) sensing applications, such as the development of low-cost interrogation systems [14] and license (https://creativecommons.org/ licenses/by/4.0/). Photonics 2021, 8, 1. https://dx.doi.org/10.3390/photonics8010001 https://www.mdpi.com/journal/photonics Photonics 2021, 8, 1 2 of 9 the optical fiber distributed sensing applications by the wavelength-division multiplexing of LPFGs in cascade. Two different approaches have been reported for the optical fiber distributed sensing applications with wavelength-division multiplexing of LPFGs. The first method uses two similar concatenated LPFGs to conform a Mach–Zehnder interferometer [15]. In this way, one can have two or more Mach–Zehnder interferometers with distinct cavity lengths in series to measure different parameters simultaneously [16,17]. However, interferometric optical fiber sensors produce differential outputs. Therefore, they require complex demod- ulation techniques such as filtering the carrier frequencies in the frequency domain and the unwrapped phase processes to extract the external perturbations [18]. The second method entails implementing two or more different LPFGs in series in such form that their refer- ence rejection bands do not overlap and operate independently [19–22]. Although these schemes require simple demodulation techniques, the wavelength-division multiplexing of LPFGs is delimited because LPFGs usually generate multiple rejection bands. However, mechanically induced long-period fiber gratings (MI-LPFGs) with a primary rejection band have recently been reported using laminated plates [23]. Such MI-LPFGs with a principal rejection band facilitate the use of the CWDM technique for LPFG sensors in cascade. In this report, we demonstrate experimentally the simultaneous measurement of transverse load and temperature using two multiplexed long-period fiber gratings. For this, an MI-LPFG is connected with a CO LPFG in cascade to measure transverse load and temperature, respectively. These LPFGs are notable for having a prominent attenuation band over a wide wavelength range. As far as we know, this is the first time that the technique of wavelength-division multiplexing using LPFGs with a prominent attenuation band is presented. The work’s structure comprises the following sections: Section 1 ex- plains an antecedent on the two approaches of the multiplexing LPFGs reported previously, highlighting their scope and limitations. The relevance of the wavelength-division multi- plexing technique using LPFGs with a prominent attenuation band and their application in distributed sensing is also presented. The general principle of the phase-matching in LPFGs is described in Section 2. Section 3 describes the experimental arrangement and its principal features. Also, it describes in detail the implementation of the MI-LPFG and the inscription of the CO LPFG. In Section 4, the individual characterization of the MI-LPFG and the CO LPFG is presented when the LPFGs are under transverse load and temperature, respectively. Also, we show the simultaneous measurement of transverse load and temperature. Finally, Section 5 presents the most relevant findings of the work. 2. LPFGs Principle Long-period fiber gratings result from a periodic refractive index modulation pro- duced in the core of a single-mode optical fiber. The long-period fiber gratings operate as modal couplers allowing the light transfer from the fundamental mode in the core (LP ) to different co-propagating high-order cladding modes (LP , m = 2, 3, 4 . . .). This coupling 0m results in a discrete set of rejection bands in the grating transmission spectrum due to the scattering of the high-order cladding modes at the interface between the cladding and the external medium. Where the resonant central wavelength ( ) of the individual rejection bands must fulfill the phase-matching condition [7], = (n n )L (1) m 01 0m where n and n represent the effective refractive indices of the LP mode in the core 01 0m 01 and the LP mode in the cladding, respectively, and L is the period of the refractive index 0m modulation in the long-period fiber grating. The number of rejection bands in an LPFG depends upon the structure and material composition of the host single-mode optical fiber and the corresponding refractive index modulation. In general, LPFGs in single-modal optical fiber with a cosinusoidal refractive index modulation and a typical period from 100–600 m, usually present spectrum trans- missions with three to five rejection bands in the spectral range from 1200–1700 nm [7], Photonics 2021, 8, 1 3 of 9 Photonics 2020, 7, x FOR PEER REVIEW 3 of 9 where the attenuation depth of the rejection bands typically is more profound as the cou- pling mode’s order of the cladding increases. LPFGs with this number of the rejection typically is more profound as the coupling mode’s order of the cladding increases. LPFGs with this bands and their spectral side lobes limit the wavelength-division multiplexing of LPFGs number of the rejection bands and their spectral side lobes limit the wavelength-division in cascade [16]. However, in the last years, it has been shown that LPFGs with a primary multiplexing of LPFGs in cascade [16]. However, in the last years, it has been shown that LPFGs with rejection band over a wide wavelength range using different inscription techniques is a primary rejection band over a wide wavelength range using different inscription techniques is feasible [23–27]. Long-period fiber gratings with a primary rejection band facilitate the feasible [23–27]. Long-period fiber gratings with a primary rejection band facilitate the deployment deployment of fiber optic distributed sensing systems based on the wavelength-division of fiber optic distributed sensing systems based on the wavelength-division multiplexing of LPFGs. multiplexing of LPFGs. 3. Setup and Components 3. Setup and Components Figure 1a illustrates the experimental configuration schematic for measuring the Figure 1a illustrates the experimental configuration schematic for measuring the transverse load transverse load (TL) and the temperature (T) by the multiplexed LPFGs. The experimental (TL) and the temperature (T) by the multiplexed LPFGs. The experimental setup consisted of an MI- setup consisted of an MI-LPFG connected to a CO LPFG in cascade. The CO LPFG 2 2 LPFG connected to a CO2 LPFG in cascade. The CO2 LPFG was located over an electric hot plate was located over an electric hot plate where the temperature can be manually controlled. where the temperature can be manually controlled. The input end of the MI-LPFG was connected to The input end of the MI-LPFG was connected to a white light source (WLS; AQ-4303B), a white light source (WLS; AQ-4303B), and the output end of the CO2 LPFG was connected to an and the output end of the CO LPFG was connected to an optical spectrum analyzer (OSA; optical spectrum analyzer (OSA; AQ-6315A). For each proof, the transmission spectrum of the AQ-6315A). For each proof, the transmission spectrum of the cascaded LPFGs was recorded cascaded LPFGs was recorded by the optical spectrum analyzer, while the spectral resolution was set by the optical spectrum analyzer, while the spectral resolution was set to 1 nm. The fiber to 1 nm. The fiber used in the double grating configuration is a standard single-mode fiber (SMF-28) used in the double grating configuration is a standard single-mode fiber (SMF-28) for for telecommunications. The LPFGs were separated 10 cm in the single-mode optical fiber. Figure 1b telecommunications. The LPFGs were separated 10 cm in the single-mode optical fiber. Figure 1b illustrates the photography of the experimental configuration. illustrates the photography of the experimental configuration. Figure 1. Experimental configuration: (a) schematic and (b) photography. Figure 1. Experimental configuration: (a) schematic and (b) photography. The MI-LPFG can be achieved when the optical fiber is compressed between a flat The MI-LPFG can be achieved when the optical fiber is compressed between a flat aluminum aluminum plate and a laminated plate, see Figure 1b. The laminated plate consisted of plate and a laminated plate, see Figure 1b. The laminated plate consisted of a parallel assembling of a parallel assembling of single-edged utility blades [23]. The laminated plate had a length single-edged utility blades [23]. The laminated plate had a length of approximately 30 mm, an of approximately 30 mm, an average period of 490  10 m, and an average duty cycle average period of 490 ± 10 µ m, and an average duty cycle of the refractive-index modulation of 0.2. of the refractive-index modulation of 0.2. Figure 2a shows the transmission spectrum Figure 2a shows the transmission spectrum of the MI-LPFG when a constant transverse load of 2000 of the MI-LPFG when a constant transverse load of 2000 g is applied between the plates. g is applied between the plates. As can be seen, its spectrum transmission shows a primary rejection As can be seen, its spectrum transmission shows a primary rejection band at 1279.3 nm with band at 1279.3 nm with a sidelobe at 1338.4 nm and shallow rejection bands at 1386.0 and 1484.2 nm. a sidelobe at 1338.4 nm and shallow rejection bands at 1386.0 and 1484.2 nm. The primary The primary rejection band’s attenuation depth was 14.4 dB, whereas the attenuation depth for the rejection band’s attenuation depth was 14.4 dB, whereas the attenuation depth for the lateral shallow rejection bands was lower than 1.5 dB. According to the reference spectrum lateral shallow rejection bands was lower than 1.5 dB. According to the reference spectrum transmission of the MI-LPFG, its average insertion loss is lower than 0.25 dB for the above conditions. transmission of the MI-LPFG, its average insertion loss is lower than 0.25 dB for the above It should be noted that the MI-LPFG has no attenuation bands in the spectral range from 1540–1640 nm, although a small portion of light can propagate through the cladding, as can be inferred from the background loss induced by the MI-LPFG in that spectral range. Photonics 2020, 7, x FOR PEER REVIEW 4 of 9 On the other hand, the CO2 LPFG was inscribed using a continuous-wave CO2 laser glass processing system (Laser Master LZM-100). It has a length of 37.5 mm and a period of 0.75 mm. In the inscription process, a fiber section is heated during 120 ms with a power discharge of 20 W; then, with the same power, the fiber is pulled for 60 ms. In order to have a principal rejection band in the spectral range of 1540–1580 nm, a period of 0.75 nm was determined empirically based on an extensive experimental study. Figure 2b shows its transmission spectrum. As can be seen, the transmission spectrum displays a primary rejection band at 1553.8 nm with 9.8 dB and shallow rejection bands at 1220.2, 1289.7, 1370.8, and 1493.7 nm with attenuation depths lower than 2.2 dB. According to the reference spectrum transmission, the average insertion loss is lower than 0.85 dB. The insertion loss is due to the scattering produced by the heated points irradiated by the continuous- wave CO2 laser. The CO2 LPFG was fixed on an aluminum holder (12 × 2 × 1 cm) by commercial epoxy Photonics 2021, 8, 1 4 of 9 putty. The CO2 LPFG was sat on the aluminum holder by pasting its two ends with a physical separation of 10 cm. Then the aluminum holder with the CO2 LPFG was placed over the electric hot plate, see Figure 1b. The maximum operating temperature of the epoxy putty was 110 °C. For its part, Figure 2c illustrates the transmission spectrum of the MI-LPFG and the CO2 LPFG in cascade when a conditions. It should be noted that the MI-LPFG has no attenuation bands in the spectral constant transverse load of 2000 g is applied in the MI-LPFG. We can observe the primary rejection range from 1540–1640 nm, although a small portion of light can propagate through the bands of the LPFGs and the overlapping of their shallow rejection bands. The insertion loss of the cladding, as can be inferred from the background loss induced by the MI-LPFG in that cascaded LPFGs is less than 1.1 dB regarding the reference transmission spectrum. spectral range. Figure 2. The transmission spectrum of (a) mechanically induced long-period fiber grating (MI- Figure 2. The transmission spectrum of (a) mechanically induced long-period fiber grating (MI- LPFG), LPFG (b), ) CO (b) CO LPFG, 2 LPFG( , c (c )) multiplexed multiplexed MMI-LPFG I-LPFG and and CO2CO LPFGLPFG. . 2 2 4. ExpOn erim the ent and R other esu hand, lts An the alys CO is LPFG was inscribed using a continuous-wave CO laser 2 2 glass processing system (Laser Master LZM-100). It has a length of 37.5 mm and a period Once the experimental setup was installed, the transverse load on the MI-LPFG was increased of 0.75 mm. In the inscription process, a fiber section is heated during 120 ms with a power from 0 to 2000 g with increments of 500 g, while the CO2 LPFG remains unloaded. Both LPFGs in the discharge of 20 W; then, with the same power, the fiber is pulled for 60 ms. In order to proofs stayed at room temperature (27 ± 3 °C). Figure 3a shows the transmission spectrum evolution have of the multip a principal lexed LPFG rejection s when band the load in the on th spectral e MI-LPFG range incre of ase 1540–1580 s. As a result, nm, the a atten period uatioof n dept 0.75 h nm of the leading rejection band of the MI-LPFG got more profound as the transverse load increased. In was determined empirically based on an extensive experimental study. Figure 2b shows contrast, the attenuation depth of the principal rejection band of the CO2 LPFG remains practically its transmission spectrum. As can be seen, the transmission spectrum displays a primary unchanged. Figure 3b,c shows the spectrum transmission evolution of the primary rejection bands of rejection band at 1553.8 nm with 9.8 dB and shallow rejection bands at 1220.2, 1289.7, 1370.8, the multiplexed LPFGs when the transverse load increases. No wavelength shift is observed in the and 1493.7 nm with attenuation depths lower than 2.2 dB. According to the reference MI-LPFG primary rejection band. In contrast, the principal rejection band of the CO2 LPFG presents spectrum transmission, the average insertion loss is lower than 0.85 dB. The insertion loss is a tiny wavelength shift to larger wavelengths that can be practically considered negligible. Figure 4 due to the scattering produced by the heated points irradiated by the continuous-wave CO illustrates the attenuation depth evolution of the primary rejection bands of the multiplexed LPFGs laser. The CO LPFG was fixed on an aluminum holder (12  2  1 cm) by commercial versus the transverse load on the MI-LPFG. The leading rejection band of the MI-LPFG presents a epoxy putty. The CO LPFG was sat on the aluminum holder by pasting its two ends nonlinear increase, while the attenuation depth of the principal rejection band of CO2 LPFG shows a with a physical separation of 10 cm. Then the aluminum holder with the CO LPFG was small variation. It is important to note that the shallow rejection bands of the MI-LPFG also got 2 deeper placed over the electric hot plate, see Figure 1b. The maximum operating temperature of when the transverse load increased, but they do not interfere with the primary rejection band of the CO2 LPFG. the epoxy putty was 110 C. For its part, Figure 2c illustrates the transmission spectrum of the MI-LPFG and the CO LPFG in cascade when a constant transverse load of 2000 g is applied in the MI-LPFG. We can observe the primary rejection bands of the LPFGs and the overlapping of their shallow rejection bands. The insertion loss of the cascaded LPFGs is less than 1.1 dB regarding the reference transmission spectrum. 4. Experiment and Results Analysis Once the experimental setup was installed, the transverse load on the MI-LPFG was increased from 0 to 2000 g with increments of 500 g, while the CO LPFG remains unloaded. Both LPFGs in the proofs stayed at room temperature (27  3 C). Figure 3a shows the transmission spectrum evolution of the multiplexed LPFGs when the load on the MI-LPFG increases. As a result, the attenuation depth of the leading rejection band of the MI-LPFG got more profound as the transverse load increased. In contrast, the attenuation depth of the principal rejection band of the CO LPFG remains practically unchanged. Figure 3b,c shows the spectrum transmission evolution of the primary rejection bands of the multiplexed LPFGs when the transverse load increases. No wavelength shift is observed in the MI- LPFG primary rejection band. In contrast, the principal rejection band of the CO LPFG presents a tiny wavelength shift to larger wavelengths that can be practically considered negligible. Figure 4 illustrates the attenuation depth evolution of the primary rejection bands of the multiplexed LPFGs versus the transverse load on the MI-LPFG. The leading rejection band of the MI-LPFG presents a nonlinear increase, while the attenuation depth of the principal rejection band of CO LPFG shows a small variation. It is important to note 2 Photonics 2021, 8, 1 5 of 9 Photonics 2020, 7, x FOR PEER REVIEW 5 of 9 that the shallow rejection bands of the MI-LPFG also got deeper when the transverse load Photonics 2020, 7, x FOR PEER REVIEW 5 of 9 increased, but they do not interfere with the primary rejection band of the CO LPFG. Photonics 2020, 7, x FOR PEER REVIEW 5 of 9 Figure 3. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO LPFG versus Figure 3. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO2 LPFG versus 2 Figure 3. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO2 LPFG versus applied weight on the MI-LPFG. applied weight on the MI-LPFG. applied weight on the MI-LPFG. Figure 3. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO2 LPFG versus applied weight on the MI-LPFG. Figure 4. Attenuation depth of the main rejecti on bands of the MI-LPFG and the CO2 LPFG versus Figure applie 4.d Attenuation weight on the M depth I-LPFG. of the main rejection bands of the MI-LPFG and the CO LPFG versus Figure 4. Attenuation depth of the main rejection bands of the MI-LPFG and the CO2 LPFG versus applied weight on the MI-LPFG. Figure 4. Attenuation depth of the main rejection bands of the MI-LPFG and the CO2 LPFG versus applied weight on the MI-LPFG. Next, the temperature in the CO2 LPFG was increased from 30 to 110 °C by steps of 20 °C using applied weight on the MI-LPFG. an electric Next, hot the plate temperatur , whereas the e in MIthe -LPF CO G with LPFG a con was stant incr transver eased se fr lo om ad of 30 20 to 00 110 g waC s kept by steps at room of Next, the temperature in the CO2 LPFG was increased from 30 to 110 °C by steps of 20 °C using temperature. Figure 5a shows the transmission spectrum evolution of the multiplexed LPFGs when 20 Next, C using the tem an perature electri c inhot the plate, CO2 LPFG was whereas incre thease MI-LPFG d from 30 with to 110 a constant °C by steps transverse of 20 °C usload ing an electric hot plate, whereas the MI-LPFG with a constant transverse load of 2000 g was kept at room the temperature in the CO2 LPFG was increased. Figure 5b,c shows the transmission spectrum an electric hot plate, whereas the MI-LPFG with a constant transverse load of 2000 g was kept at room of 2000 g was kept at room temperature. Figure 5a shows the transmission spectrum temperature. Figure 5a shows the transmission spectrum evolution of the multiplexed LPFGs when tem evo perat lution ure. of Fi the gure prima 5a ry sho re ws ject th ion e trans band miss s of ion the spect multipl rum exed evo LP lutio FGs. n of As th can e mu be lti see plexed n, the L reject PFGio s when n band evolution of the multiplexed LPFGs when the temperature in the CO LPFG was increased. the temperature in the CO2 LPFG of the CO was 2 LP incre FG shi ased. fted Fi tog wards ure 5b,c longer show wavelen s thgths e tra with nsmiss a slight ion decrea spectrum se in the attenuation depth. the temperature in the CO2 LPFG was increased. Figure 5b,c shows the transmission spectrum Figure 5b,c shows the transmission spectrum evolution of the primary rejection bands Meanwhile, the leading rejection band of the MI-LPFG presents small variations in the attenuation evolution of the primary reject evo ion lu band tion o s f of the th prima e mury ltipl reject exed ion LP band FGs. s of As the can mube ltipl see exed n, th LP e FGs. reject As iocn an band be see n, the rejection band of the multiplexed LPFGs. As can be seen, the rejection band of the CO LPFG shifted depth and wavelength shift. Figure 6 shows the wavelength shift of the primary rejection bands of of the CO2 LPFG shifted towards longer wavelengths with a slight decrease in the attenuation depth. of the CO2 LPFG shifted towardstowar longer dswavelen longer wavelengths gths with a slight withdecrea a slight se decr in th ease e atten in the uation attenuation depth. depth. Meanwhile, multiplexed LPFGs concerning the spectrum transmission of the multiplexed LPFG at 30 °C. The CO2 Meanwhile, the leading rejection band of the MI-LPFG presents small variations in the attenuation the leading rejection band of the MI-LPFG presents small variations in the attenuation Meanwhile, the leading rejection band of the MI-LPFG presents small variations in the attenuation LPFG principal rejection band shows a linear wavelength shift when the temperature is increased. depth and wavelength shift. Figure 6 shows the wavelength shift of the primary rejection bands of depth and wavelength shift. Figure 6 shows the wavelength shift of the primary rejection depth and wavelength shift. Figure 6 shows the wavelength shift of the primary rejection bands of This rejection band shows a temperature sensitivity of ~50 pm/°C, and its R-squared factor of the multiplexed LPFGs concerning the spectrum transmission of the multiplexed LPFG at 30 °C. The CO2 bands of multiplexed LPFGs concerning the spectrum transmission of the multiplexed multiplexed LPFGs concerning the spectrum transmission of the multiplexed LPFG at 30 °C. The CO2 LPFG linear pr fitting incipal is reject 0.9989. ion On band the shows other a hli an ne d, ar th wave e MIle -L ngth PFG shi leading ft wh en reject the ion tem bpe anra d ture pres ent is incr ed ea a sma sed. ll LPFG at 30 C. The CO LPFG principal rejection band shows a linear wavelength shift LPFG principal rejection band shows a linear wavelength shift when the temperature is increased. oscillating wavelength shift due to the overlapping with the shallow rejection bands at 1289.7 nm of This rejection band shows a temperature sensitivity of ~50 pm/°C, and its R-squared factor of the when the temperature is increased. This rejection band shows a temperature sensitivity This rejection band shows a temperature sensitivity of ~50 pm/°C, and its R-squared factor of the the CO2 LPFG. Similarly, the attenuation depth of the leading rejection band of the MI-LPFG is linear fitting is 0.9  989. On the other hand, the MI-LPFG leading rejection band presented a small of 50 pm/ C, and its R-squared factor of the linear fitting is 0.9989. On the other hand, linear fitting is 0.9989. On the other hand, the MI-LPFG leading rejection band presented a small slightly altered. oscillating wavelength shift due to the overlapping with the shallow rejection bands at 1289.7 nm of the MI-LPFG leading rejection band presented a small oscillating wavelength shift due to oscillating wavelength shift due to the overlapping with the shallow rejection bands at 1289.7 nm of the CO2 LPFG. Similarly, the attenuation depth of the leading rejection band of the MI-LPFG is the overlapping with the shallow rejection bands at 1289.7 nm of the CO LPFG. Similarly, slightly altered. the CO2 LPFG. Similarly, the attenuation depth of the leading rejection band of the MI-LPFG is the attenuation depth of the leading rejection band of the MI-LPFG is slightly altered. slightly altered. Figure 5. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO2 LPFG versus applied temperature on the CO2 LPFG. Figure 5. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO LPFG versus Figure 5. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO2 LPFG 2 versus applied temperature on the CO LPFG. applied temperature on the CO2 LPFG. Figure 5. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO2 LPFG versus applied temperature on the CO2 LPFG. Photonics 2020, 7, x FOR PEER REVIEW 6 of 9 Photonics 2021, 8, 1 6 of 9 Photonics 2020, 7, x FOR PEER REVIEW 6 of 9 Photonics 2020, 7, x FOR PEER REVIEW 6 of 9 Figure 6. The wavelength shift of the main rejection bands of MI-LPFG and CO2 LPFG versus applied temperature on CO2 LPFG. Figure 6. The wavelength shift of the main rejection bands of MI-LPFG and CO LPFG versus applied Figure 6. The wavelength shift of the main rejection bands of MI-LPFG and CO2 LPFG versus applied Then, the transverse load on the MI-LPFG and the temperature in the CO2 LPFG were temperature on CO LPFG. temperature on CO2 LPFG. Figure 6. The wavelength shift of the main rejection bands of MI-LPFG and CO2 LPFG versus applied simultaneously measured. Thus, the transverse load was increased from 0 to 2000 g by increments of temper Then, aturthe e on CO transverse 2 LPFG. load on the MI-LPFG and the temperature in the CO LPFG were 500 g in the MI-LPFG, while the temperature was also increased from 30 to 110 °C by steps of 20 °C in Then, the transverse load on the MI-LPFG and the temperature in the CO2 LPFG were simultaneously measured. Thus, the transverse load was increased from 0 to 2000 g by the CO2 LPFG. Figure 7a shows the transmission spectrum evolution of the multiplexed LPFGs for these simultaneously measured. Thus, the T h trea n n , sv th eer st e r a lo ns av d e rw sea sl o in ad c re oa ns etd h ef rM om I- L 0P t Fo G 2 0 a0 n0 d gt h bey tie nm cr p eem rat eu nr te s o in f the CO2 LPFG were increments of 500 g in the MI-LPFG, while the temperature was also increased from 30 conditions. Figure 7b,c shows the primary rejection bands evolution of the multiplexed LPFGs when simultaneously measured. Thus, the transverse load was increased from 0 to 2000 g by increments of 500 g in the MI-LPFG, while the temperature was also increased from 30 to 110 °C by steps of 20 °C in to 110 C by steps of 20 C in the CO LPFG. Figure 7a shows the transmission spectrum the transverse load and the temperature were increased, respectively. Figure 8a shows the leading 500 g in the MI-LPFG, while the temperature was also increased from 30 to 110 °C by steps of 20 °C in the CO2 LPFG. Figure 7a shows the transmission spectrum evolution of the multiplexed LPFGs for these evolution of the multiplexed LPFGs for these conditions. Figure 7b,c shows the primary rejection band th ’s e a CtO te 2n Lu Pa FtG io . n Fi g d u erp e t7 h a s b h eo h w av s t ih oe r tirn a ntsh m ei s M sio I- n L sP pF eG ctr u w m h e ev no t lu hte io tn r a on f t sh vee m rsu el tlio pa le d x eid n c LrP eF as Ge ss f ofro tr h ese conditions. Figure 7b,c shows ther p ejection rimary bands rejectioevolution n bands ev of olu the tion multiplexed of the multiLPFGs plexed when LPFGsthe whtransverse en load and the conditions. Figure 7b,c shows the primary rejection bands evolution of the multiplexed LPFGs when different temperatures at the CO2 LPFG. The attenuation depth shows a nonlinear increase, similar to temperature were increased, respectively. Figure 8a shows the leading rejection band’s the transverse load and the temperature were increased, respectively. Figure 8a shows the leading the transverse load and the temperature were increased, respectively. Figure 8a shows the leading the results obtained previously in Figure 4. On the other hand, Figure 8b shows the principal rejection attenuation depth behavior in the MI-LPFG when the transverse load increases for different rejection band’s attenuation depth behavior in the MI-LPFG when the transverse load increases for rejection band’s attenuation depth behavior in the MI-LPFG when the transverse load increases for band’s wavelength shift in the CO2 LPFG with respect to spectrum transmission at 30 °C for the above temperatures at the CO LPFG. The attenuation depth shows a nonlinear increase, similar to different temperatures at the CO2 LPFG. The attenuation depth shows a nonlinear increase, similar to different temperatures at the CO2 LPFG. The attenuation depth shows a nonlinear increase, similar to conditions. Thethe prirn esults cipal obtained rejection b pr aeviously nd’s centin er F w igur avee le4 n.gtOn h in the the other CO2 hand, LPFGFigur show e s8 b a shows linear the the results obtained previously in Figure 4. On the other hand, Figure 8b shows the principal rejection the results obtained previously in Figure 4. On the other hand, Figure 8b shows the principal rejection principal rejection band’s wavelength shift in the CO LPFG with respect to spectrum wavelength shift towards longer wavelengths. The rejection band shows a temperature sensitivity of band’s wavelength shift in the CO2 LPFG with respect to spectrum transmission at 30 °C for the above band’s wavelength shift in the CO2 LPFG with respect to spectrum transmission at 30 °C for the above transmission at 30 C for the above conditions. The principal rejection band’s center ~50 pm/°C, and its R-squared factor of the linear fitting was 0.9986. conditions. The principal rejec ct oin od ni tib oa nn s.d T ’sh ec ep nrtie nr c ip w ala v re ejle ecn tig otn h bia n n d t’h se ce C nO te2 r L wP aF ve G le n sg h to hw in s a th el in Ce O a2r LPFG shows a linear wavelength in the CO LPFG shows a linear wavelength shift towards longer wavelengths. wavelength shift towards longer wavelengths. The rejection band shows a temperature sensitivity of wavelength shift towards longer wavelengths. The rejection band shows a temperature sensitivity of The rejection band shows a temperature sensitivity of 50 pm/ C, and its R-squared factor ~50 pm/°C, and its R-squared factor of the linear fitting was 0.9986. ~50 pm/°C, and its R-squared factor of the linear fitting was 0.9986. of the linear fitting was 0.9986. Figure 7. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO2 LPFG versus Figure 7. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO LPFG versus applied transverse load on MI-LPFG and temperature on the CO2 LPFG, simultaneously. 2 Figure 7. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO2 LPFG versus applied transverse load on MI-LPFG and temperature on the CO LPFG, simultaneously. applied transverse load on MI-LPFG and temperature on the CO2 LPFG, simultaneously. Figure 7. The transmission spectrum of (a) multiplexed LPFGs, (b) MI-LPFG, (c) CO2 LPFG versus applied transverse load on MI-LPFG and temperature on the CO2 LPFG, simultaneously. Figure 8. (a) Attenuation depth of MI-LPFG, and (b) wavelength shift of the CO2 LPFG for the Figure 8. (a) Attenuation depth of MI-LPFG, and (b) wavelength shift of the CO LPFG for the Figure 8. (a) Attenuation depth of MI-LPFG, and (b) wavelength shift of the CO2 LPFG for the simultaneous measurement of transverse load and temperature, respectively. simultaneous measurement of transverse load and temperature, respectively. simultaneous measurement of transverse load and temperature, respectively. The MI-LPFG displays a leading rejection band at 1280  1 nm, while the CO LPFG Figure 8. (a) Attenuation depth of MI-LPFG, and (b) wavelength shift of the CO2 LPFG for the 2 presents a principal rejection band at 1553.80 nm at room temperature (27 C). The wave- simultaneous measurement of transverse load and temperature, respectively. Photonics 2021, 8, 1 7 of 9 Photonics 2020, 7, x FOR PEER REVIEW 7 of 9 The MI-LPFG displays a leading rejection band at 1280 ± 1 nm, while the CO2 LPFG presents a length separation between the primary rejection bands allows the CWDM of LPFGs with principal rejection band at 1553.80 nm at room temperature (27 °C). The wavelength separation low cladding-mode crosstalk. When the transverse load in the MI-LPFG was increased from between the primary rejection bands allows the CWDM of LPFGs with low cladding-mode crosstalk. 0–2000 g, the principal rejection band of the CO LPFG underwent attenuation depth varia- When the transverse load in the MI-LPFG was increased from 0–2000 g, the principal rejection band tions lower than 0.2 dB and a wavelength shift lower than 0.2 nm with respect to the of the CO2 LPFG underwent attenuation depth variations lower than ±0.2 dB and a wavelength shift initial spectrum transmission. We assume that these random variations are due to the white lower than ±0.2 nm with respect to the initial spectrum transmission. We assume that these random light source output power stability combined with the insertion loss induced by the MI- variations are due to the white light source output power stability combined with the insertion loss LPFG. Note that the MI-LPFG transverse load sensitivity was obtained at a constant room induced by the MI-LPFG. Note that the MI-LPFG transverse load sensitivity was obtained at a temperature. However, it is well known that attenuation depth and the central wavelength location of the rejection bands are influenced by the temperature on the MI-LPFGs [28]. constant room temperature. However, it is well known that attenuation depth and the central It had been observed that with a temperature increase on the MI-LPFGs, the rejection bands wavelength location of the rejection bands are influenced by the temperature on the MI-LPFGs [28]. shift to longer wavelengths, and their attenuation depth partially decreases. In this sense, It had been observed that with a temperature increase on the MI-LPFGs, the rejection bands shift to to include the effect of the temperature on the MI-LPFG response, the transverse load longer wavelengths, and their attenuation depth partially decreases. In this sense, to include the effect sensitivity, and the wavelength shifting sensitivity of the principal rejection band can be of the temperature on the MI-LPFG response, the transverse load sensitivity, and the wavelength calibrated at different temperatures. On the other hand, when the temperature in the CO shifting sensitivity of the principal rejection band can be calibrated at different temperatures. On the LPFG was increased from 30 to 110 C, the leading rejection band in the MI-LPFG experi- other hand, when the temperature in the CO2 LPFG was increased from 30 to 110 °C, the leading enced an attenuation depth variation lower than 0.4 dB and a wavelength shift lower than rejection band in the MI-LPFG experienced an attenuation depth variation lower than ±0.4 dB and a 0.3 nm with respect to the initial spectrum transmission. We assume that these variations wavelength shift lower than ±0.3 nm with respect to the initial spectrum transmission. We assume are also due to the white light source output power stability and the overlapping between that these variations are also due to the white light source output power stability and the overlapping the leading rejection band in the MI-LPFG and the CO LPFG shallow rejection band at between the leading rejection band in the MI-LPFG and the CO2 LPFG shallow rejection band at 1289.7 nm. In the last case, increasing the separation between the LPFGs can significantly 1289.7 nm. In the last case, increasing the separation between the LPFGs can significantly reduce the reduce the overlapping effect since cladding light will be attenuated by the high index overlapping effect since cladding light will be attenuated by the high index polymer coating of the polymer coating of the optical fiber section between LPFG. On the other side, the random optical fiber section variations bet of wee the n power LPFG.transmission On the other spectr sid um e, intr the oduced random by va the riations white light of th sour e po cewer can be eliminated using a broadband light source by combining two superluminescent diodes at transmission spectrum introduced by the white light source can be eliminated using a broadband 1280 and 1550 nm. light source by combining two superluminescent diodes at 1280 and 1550 nm. The above results were replicated when we simultaneously measured the transverse The above results were replicated when we simultaneously measured the transverse load and load and the temperature in the multiplexed LPFGs, respectively. Figure 9a displays the temperature in the multiplexed LPFGs, respectively. Figure 9a displays a comparison between a comparison between the individual and simultaneous measurement of the attenuation the individual and simultaneous measurement of the attenuation depth in the MI-LPFG. As can be depth in the MI-LPFG. As can be seen, except for the attenuation depth corresponding to seen, except for the attenuation depth corresponding to 1500 g, the remainder attenuation depth 1500 g, the remainder attenuation depth points preserve a close correlation. This difference points preserve a close correlation. This difference at 1500 g can be due to the repeatability of the MI- at 1500 g can be due to the repeatability of the MI-LPFG. Meanwhile, Figure 9b illustrates LPFG. Meanwhile, Figure 9b illustrates a comparison between the wavelength shift in the CO2 LPFG a comparison between the wavelength shift in the CO LPFG for the individual and for the individual and simultaneous temperature measurement, where one can observe a close simultaneous temperature measurement, where one can observe a close correlation between correlation between individual and simultaneous measurements of the temperature. According to individual and simultaneous measurements of the temperature. According to these results, these results, the multiplexed LPFGs operate with low cladding-mode crosstalk. The experimental the multiplexed LPFGs operate with low cladding-mode crosstalk. The experimental results demonstrate the simultaneous measurement of transverse load and temperature by the results demonstrate the simultaneous measurement of transverse load and temperature multiplexed LPFGs. by the Th multiplexed e current mu LPFGs. ltiplexed The LPFGs current arrangement multiplexed uses LPFGs an MI arrangement -LPFG; howeve uses r, it an is MI- possible to pro LPFG; duce L however PFGs with , it is a possible primary to reject produce ion band LPFGs by with other a pri inscr mary iptio rn ejection methods band such by other as inscription methods such as electric arc discharge and femtosecond laser irradiation [25–27]. electric arc discharge and femtosecond laser irradiation [25–27]. These LPFGs may allow more These LPFGs may allow more flexible schemes of distributed sensing applications based flexible schemes of distributed sensing applications based on multiplexed LPFGs in cascade. on multiplexed LPFGs in cascade. Figure 9. Individual and simultaneous measurement of (a) attenuation depth in the MI-LPFG and Figure 9. Individual and simultaneous measurement of (a) attenuation depth in the MI-LPFG and (b) (b) wavelength shift in the CO LPFG. wavelength shift in the CO2 LPFG. Photonics 2021, 8, 1 8 of 9 5. Conclusions The simultaneous measurement of transverse load and temperature using two mul- tiplexed long-period fiber gratings has been demonstrated experimentally in this work. We have used an MI-LPFG with a CO LPFG connected in cascade to measure the transverse load and the temperature, respectively. The experimental results show that the transverse load and temperature measurements show low cladding-mode crosstalk between the mul- tiplexed LPFGs. In fact, the cladding-mode crosstalk can be canceled by increasing the length of the fiber between the gratings. The low or absence of cladding-modes crosstalk, simplify the interrogation method since only changes in transmission need to be measured. It is well known that distributed sensing applications are still a trend in the development of optical fiber sensors based on multiplexed LPFGs. In this regard, we propose the CWDM of long-period fiber gratings with a leading rejection band to determine multiple parameters simultaneously. To our best knowledge, this is the first time that coarse multiplexing of LPFGs has been reported for multiple parameter sensing using LPFGs with a leading rejection band. The new concept of multiplexed LPFGs using MI-LPFG with only one leading rejection band can facilitate the CWDM of several LPFGs in cascade. This technique can improve the implementation of fiber optic distributed sensing systems based on the wavelength-division multiplexing of LPFGs. 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Published: Dec 22, 2020

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