Spectral power stabilization against temperature variations in multimode fiber Bragg gratings

Fiber Bragg gratings (FBGs) have been extensively used for single-point and multi-point measurements, mostly inscribed in single-mode fibers. However, it is feasible to inscribe FBGs in multimode fibers, which resist bending and can perform discriminative sensing of multiple physical parameters. When using a simple experimental setup to measure the temperature dependence of the dip in the transmission spectrum, significant fluctuations in its spectral power can be observed. Therefore, this study shows that the temperature-dependent spectral power fluctuations in multimode FBGs can be mitigated using a reflectometric configuration with suppressed modal interference, leading to higher-reliability temperature sensing.

Fiber Bragg gratings (FBGs) have been extensively used for single-point and multi-point measurements, mostly inscribed in single-mode fibers.However, it is feasible to inscribe FBGs in multimode fibers, which resist bending and can perform discriminative sensing of multiple physical parameters.When using a simple experimental setup to measure the temperature dependence of the dip in the transmission spectrum, significant fluctuations in its spectral power can be observed.Therefore, this study shows that the temperature-dependent spectral power fluctuations in multimode FBGs can be mitigated using a reflectometric configuration with suppressed modal interference, leading to higher-reliability temperature sensing.© 2024 The Author(s).][3][4][5][6][7][8] One of the most popular and commercially successful optical fiber sensors is the one that utilizes fiber Bragg gratings (FBGs).These specialized reflectors are inscribed within a short segment of an optical fiber and reflect specific wavelengths of light. 9)FBGs are widely recognized as one of the most fundamental optical devices and have a wide range of practical applications in communication and sensing.][16][17] When fiber gratings are inscribed in single-mode fibers (SMFs), they reflect only a single wavelength of light, known as the Bragg wavelength λ = 2 • n eff • Λ, where n eff represents the effective refractive index of the fiber core and Λ denotes the grating pitch.In contrast, when FBGs are inscribed in multimode fibers (MMFs), they exhibit multiple reflection peaks around the fundamental Bragg peak due to the coupling between different linearly polarized (LP) modes.MM-FBGs utilize the properties of both MMFs and FBGs, which offer the advantage of multiple peaks that can be utilized for discriminative sensing of various physical parameters.For instance, they have been demonstrated to enable simultaneous temperature and strain sensing, 18,19) as well as strain and bending measurements. 20)However, a drawback of using MM-FBGs in a simple experimental setup is that the power of the transmitted spectral dip tends to exhibit significant variations with temperature.The cause of this phenomenon as well as methods for its suppression has not been reported previously.
In this study, we reveal that the temperature-dependent power variation of the transmitted spectrum of MM-FBGs is caused by modal interference within the MMF.In addition, we experimentally demonstrate that power fluctuations can be suppressed by utilizing the reflected spectral peak instead of the transmitted spectral dip.This paper builds on preliminary findings from a previous conference paper, 21) enriching it with detailed analysis on spectral changes in a folded SMS structure employing a multimode circulator.
We utilized an MM-FBG inscribed along a 2-mm-long Sect. of a silica graded-index (GI-) MMF with a core diameter of 50 μm, designed to have a primary Bragg wavelength around 1550 nm using a phase mask method.Figure 1 shows that, when broadband light is injected into the MM-FBGs, peaks emerge at each Bragg wavelength corresponding to the diffraction order m.][24] The Bragg wavelength for the fundamental mode, i.e. m = 1, can be expressed as λ B = 2 • n • Λ, where λ B refers to the Bragg wavelength for the fundamental mode, and n represent the refractive index at the center of the MMF core.However, for higher-order dominant-the increase in grating pitch Λ or the modes, the optical path is curved, resulting in a larger grating pitch Λ but a smaller refractive index n.Therefore, the Bragg peak of higher-order mode (l ¢ B ) shifts to longer wavelength when the grating pitch increasing (l ¢ B < λ B ), and shifts to shorter wavelengths when the refractive index n decreases (l ¢ B > λ B ). Thus, the position of the Bragg peak of higher-order modes is determined by which factor is dominant.
The experimental setup utilized in this study is depicted in Figs.2(a) and 2(b).In the configuration (a), a 2.2-m-long FBG-inscribed MMF is sandwiched between two 1-m-long SMFs.The output of an amplified spontaneous emission (ASE) light source (ASE-FL7002, Thorlabs) is injected into this probe, and its transmitted spectrum is observed using an optical spectrum analyzer (OSA, AQ6370, Yokogawa Electric).In the configuration (b), an optical circulator composed of SMFs was utilized to direct the output of the ASE light source into the FBG-inscribed MMF.The sensor heads were attached to the heating plate with minimized strain and avoided external forces in both configurations, and no geometric deformations occurred in the experiments.The reflected spectrum was then observed using the OSA.An exact measurement was also performed using a setup where large bending losses were applied near the open end of the MMF, suppressing Fresnel reflection at the end face by >60 dB.During the FBG heating process, a 10-cm-long Sect.centered on the FBG was fixed on a heating plate, and the corresponding spectra were obtained in the range of 35 °C-70 °C with steps of 3.5 °C.
First, the transmitted and reflected spectra obtained from each experimental setup are presented in Fig. 3 (a: transmission, b: reflection with no loss, and c: reflection with loss).Multiple peaks and dips unique to the MM-FBG were observed in all experimental setups.Additionally, regardless of the experimental setup, the Bragg wavelength of the main mode shifted towards longer wavelengths with increasing temperature, which exhibits a dependence coefficient of approximately 12.7 pm °C−1 , as shown in Fig. 4(a) [derived from Fig. 3(c)].On the other hand, although the temperaturedependent variation of the dip power was prominent in Fig. 3(a), the temperature-dependent variation of the peak power was suppressed in Figs.3(b) and 3(c).][27][28][29] Modal interference in the MMF of the SMS structure results in a spectral pattern observed over a much wider bandwidth than the FBG reflection bandwidth, and the modal interference pattern shifts with temperature It is a reasonable assumption that the significant temperature-dependent power fluctuations of the dips are caused by the overlapping of this effect.In contrast, in reflectometric configuration, the folded SMS structure is formed by the reflection of light at the distal facet of the MMF. 30)owever, due to the low reflection coefficient at the end face (approximately 4%), the effect of modal interference can be reduced (even when a large loss is applied at the end of the MMF to suppress the reflectivity to nearly 0%, no significant change was observed in the power fluctuations).
Our findings indicate that the temperature dependence of the dip power in the transmitted spectrum of an MM-FBG is a result of modal interference in the MMF.However, utilizing a reflectometric configuration can effectively reduce the influence of modal interference and mitigate the temperature dependence of the peak power.Thus, at present, modal interference poses a challenge to sensing reliability when using MMF-FBGs.However, by using this modal interference, it may be feasible to achieve simultaneous measurements of multiple physical quantities with higher accuracy than the conventional method of relying solely on multiple peaks.Further research is necessary to explore this possibility in greater depth.052001-3 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd

Fig. 1 .Fig. 2 .
Fig. 1.Schematic of peaks appearing at each Bragg wavelength corresponding to the diffraction order m.

Fig. 4 .
Fig. 4. (a) Temperature dependence of the Bragg wavelength (measured in the reflectometric configuration with a loss at the MMF end).The blue line is a linear fit.(b) Temperature dependencies of the relative powers measured in the transmissive configuration (red), the reflectometric configuration with no loss (blue), and that with a loss at the MMF end (green).