Multimode Fiber Bragg Grating Research Articles

Spectrum design of multimode fiber Bragg gratings based on suppression of mode coupling

The direct femtosecond laser inscription method is very competitive in the fabrication of fiber Bragg gratings, but its refractive index modification region is very limited and leads to low reflectivity and wide spectra of multimode fiber gratings. In this work, the relationship between the refractive index modification region, the coupling coefficient, and the spectrum of the multimode fiber grating is investigated based on the intrinsic mechanism of mode coupling; thus the annular and eccentric elliptic refractive index modification regions are accordingly proposed to suppress mode couplings in the multimode fiber Bragg grating, so as to achieve a highly discrete spectrum and narrow spectrum of the multimode fiber Bragg grating, respectively. This work can provide a solid theoretical foundation for the spectrum design of the multimode fiber Bragg grating, which can be utilized to customize the spectrum according to application scenarios such as optical communication, fiber lasers, filtering, and sensing.

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.

Controllable Light Scattering on Fiber Bragg Gratings in Multimode Fibers: Tailoring Angular Emission for Advanced Fiber-Based Light Sources

Controllable Light Scattering on Fiber Bragg Gratings in Multimode Fibers: Tailoring Angular Emission for Advanced Fiber-Based Light Sources

Wavelength stabilization of high-power laser diodes using Bragg gratings inscribed in their highly multimode fiber pigtails

Efficient wavelength stabilization of an off-the-shelf high-power laser diode operating at 976 nm is demonstrated by using a highly multimode fiber Bragg grating (FBG). This first-order grating is inscribed with 400 nm femtosecond pulses inside the 200 µm/0.22 NA pure silica core of the diode's fiber pigtail. The FBG reduces the wavelength thermal drift of the 70 W diode by a factor of 12 while also reducing its emission linewidth by a factor of 2.8. At maximum output power, a power penalty of only 6% is measured. This promising approach offers a robust and compact scheme to stabilize the spectrum of high-power laser diodes that are particularly useful for fiber-laser pumping.

Highly Sensitive Dual Parameter Sensor Based on a Hybrid Structure with Multimode Interferometer and Fiber Bragg Grating Fabricated by Femtosecond Laser

A hybrid sensing configuration for simultaneous measurement of strain and temperature based on fiber Bragg grating (FBG) written in an offset multimode fiber (MMF) interferometer using femtosecond laser pulse is proposed and demonstrated. A Mach–Zehnder interferometer is formed by splicing a section of MMF between two single-mode fibers (SMFs) and a high interference fringe of up to 15 dB is achieved. The sensing experimental results show a strain sensitivity of −1.17 pm/με and 0.6498 pm/με for the dip of MZI and Bragg peak, while a temperature sensitivity of 42.84 pm/°C and 19.96 pm/°C is measured. Furthermore, the matrix analysis has found that the strain and temperature resolution of the sensor are as high as ±12.36 με and ±0.35 °C, respectively. In addition, the sensor has merits of simple fabrication, good spectral quality, and high resolution, which shows attractive potential applications in dual-parameter sensing.

Simultaneous vector bending and temperature sensing based on eccentric multi-mode fiber Bragg gratings

Vector bending sensing is a key parameter in posture detection and structural health monitoring. Eccentric fiber Bragg gratings (FBGs), which can measure vector bending and temperature simultaneously, were fabricated by femtosecond laser direct writing in multi-mode fiber. The intensity of the FBG is only sensitive to curvature when the temperature is below 260 ℃. Rectangular structure was adopted to resolve bending direction. The intensity difference of the symmetrically inscribed FBG realizes the multiplication of the bending sensitivity, and the bending sensitivity is 4.42 dB/m−1. In addition, the difference between the two symmetrical peaks eliminates the power fluctuation noise caused by the fluctuation of the light source and the contamination of the fiber end. The wavelength of the FBG is only sensitive to temperature when the curvature is in the range of −2.353 m−1–2.353 m−1, and the temperature sensitivity is more than 13 pm/℃.

Twelve-Wavelength-Switchable Thulium-Doped Fiber Laser With a Multimode Fiber Bragg Grating

In this article, a precisely controlled, switchable thulium-doped fiber laser (TDFL) with high optical signal-to-noise ratio (OSNR) is proposed and demonstrated. A home-made multimode fiber Bragg grating (MMFBG) with multireflection peaks was used as an optical filter to select the desired lasing channel. Using a manually controlled fusion splicer, the axial misalignment was applied between the single mode fiber and the MMFBG to switch the output wavelength. In the experiment, the number of switchable lasing lines was up to 12, and the fluctuation of the center wavelength and output power was less than 0.03 nm and 0.67 dB, respectively. A maximum slope efficiency of 1.3% was obtained when the axial misalignment was 0 μm, and the OSNR was higher than 65 dB. In addition, the proposed fiber laser was used as an optical fiber laser sensor to quantify the strain applied on the MMFBG, and a sensitivity of 1.16 pm/μ∈ was obtained.

Fabrication and modification of temperature FBG sensor: role of optical fiber type and Cu sputtered thickness

This paper presents the effects of optical fiber type and copper coating thickness on fiber Bragg grating (FBG) temperature sensitivity. Three types of FBGs including single-mode, step-index, and graded-index multimode fibers were fabricated via the phase mask writing technique. In the temperature range of 30 °C to 80 °C, the step-index multimode FBG exhibits the highest temperature sensitivity of 11.8 pm °C−1 compared to single-mode and graded-index multimode FBG with the sensitivities of 10.5 pm °C−1 and 11.4 pm °C−1, respectively. The temperature sensitivity of the step-index multimode FBG sensor was further improved through the deposition of copper nanolayer which has a greater thermal expansion coefficient than glass. The FBG probe sputtered with a 759.2 nm-thick copper layer achieves the highest temperature sensitivity of 14.2 pm °C−1 compared to those with 306.5 nm and 562.3 nm-thick copper layers which show the sensitivities of 13.2 pm °C−1 and 13.4 pm °C−1, respectively.

Comparison of Fiber Bragg Grating based on SMF and MMF over Temperature Sensitivity

We have successfully fabricated and demonstrated a simple, cost-effective and easy to use of fiber Bragg grating (FBG) based on single mode, and multimode fiber which have been employed for temperature monitoring. The study purposely compared the performance of two types of FBGs; single mode FBG (SM-FBG) and multimode FBG (MM-FBG). The FBGs sensor is fabricated by phase mask technique which being exposed to ArF excimer laser with 20 mm uniform grating length and 99% reflectivity. The proposed FBG is studied for temperature monitoring starting at room temperature until 120 °C, and the configurations with SM-FBG and MM-FBG achieved a sensitivity of 10.9 pm/°C and 13.23 pm/°C, respectively whereas linear response correlation coefficient of 0.98229 and 0.99929. These show the MM-FBG has better sensitivity to be used in sensor applications.

Liquid level and temperature sensing by using dual-wavelength fiber laser based on multimode interferometer and FBG in parallel

Abstract The detection of liquid level and temperature based on a fiber ring cavity laser sensing configuration is presented and demonstrated experimentally. The sensing head contains a fiber Bragg grating (FBG) and a single-mode–cladding-less–single-mode multimode interferometer, which also functions as wavelength-selective components of the fiber laser. When the liquid level or temperature is applied on the sensing head, the pass-band peaks of both multimode interference (MMI) filter and FBG filter vary and the two output wavelengths of the laser shift correspondingly. In the experiment, the corresponding sensitivities of the liquid level with four different refractive indices (RI) in the deep range from 0 mm to 40 mm are obtained and the sensitivity enhances with the RI of the liquid being measured. The maximum sensitivity of interferometer is 106.3 pm/mm with the RI of 1.391. For the temperature measurement, a sensitivity of 10.3 pm/°C and 13.8 pm/°C are achieved with the temperature ranging from 0 °C to 90 °C corresponding to the two lasing wavelengths selective by the MMI filter and FBG, respectively. In addition, the average RI sensitivity of 155.77 pm/mm/RIU is also obtained in the RI range of 1.333–1.391.

Dual-point reflective refractometer based on parallel no-core fiber/FBG structure

Abstract A novel dual-point reflective fiber-optic refractometer based on multimode interference (MMI) effect and fiber Bragg grating (FBG) reflection is proposed and experimentally demonstrated, which adopts parallel structure. Each point of the refractometer consists of a single mode-no core-single mode fiber (SNS) structure cascaded with a FBG. Assisted by the reflection of FBG, refractive index (RI) measurement can be achieved by monitoring the peak power variation of the reflected FBG spectrum. By selecting different length of the no core fiber and center wavelength of the FBG, independent dual-point refractometer is easily realized. Experiment results show that the refractometer has a nonlinear relationship between the surrounding refractive index (SRI) and the peak power of the reflected FBG spectrum in the RI range of 1.3330–1.4086. Linear relationship can be approximately obtained by dividing the measuring range into 1.3330–1.3611 and 1.3764–1.4086. In the RI range of 1.3764–1.4086, the two sensing points have higher RI sensitivities of 319.34 dB/RIU and 211.84 dB/RIU, respectively.

High-sensitivity thermometer based on singlemode-multimode FBG-singlemode fiber

A fiber Bragg grating (FBG) sensing configuration for temperature measurement based on singlemode–multimode FBG–singlemode fiber was proposed and experimentally demonstrated. The configuration was formed by fabricating an FBG in a multimode fiber, which was sandwiched between two singlemode fibers. Both theoretical and experimental analyses were performed. An average sensitivity of 266.25pm/°C and a resolvable index change of 3.75×10−2°C were obtained in the temperature range from −40°C to +40°C. The proposed thermometer could find useful applications in biology, medicine, and material science because of its superior properties such as high sensitivity, compact size, easy fabrication, linear response, ease of interconnection with other in-fiber optical components, and low cost.

Multipoint refractive index and temperature fiber optic sensor based on cascaded no core fiber-fiber Bragg grating structures

A multipoint fiber optic sensor based on two cascaded multimode interferometer (MMI) and fiber Bragg grating (FBG) structures is proposed and demonstrated for simultaneous measurement of refractive index (RI) and temperature. The MMI is fabricated by splicing a section of no-core fiber (NCF) with two single-mode fibers. The suitable NCF lengths of 19.1 and 38.8 mm are selected by simulations to achieve wavelength division multiplexing. The two MMIs are sensitive to RI and temperature with the maximal RI sensitivities of 429.42228 and 399.20718 nm/RIU in the range of 1.333 to 1.419 and the temperature sensitivities of 10.05 and 10.22 pm/°C in the range of 26.4°C to 100°C, respectively. However, the FBGs are only sensitive to the latter with the sensitivities of 10.4 and 10.73 pm/°C. Therefore, dual-parameter measurement is obtained and cross-sensitivity issue can be solved. The distance between the two sensing heads is up to 12 km, which demonstrates the feasibility of long-distance measurement. During measurement, there is no mutual interference to each sensing head. The experimental results show that the average errors of RI are 7.61×10−4 RIU and 6.81×10−4 RIU and the average errors of temperature are 0.017°C and 0.012°C, respectively. This sensor exhibits the advantages of high RI sensitivity, dual-parameter and long-distance measurement, low cost, and easy and repeatable fabrication.

A simple equation to describe cross-sensitivity between temperature and refractive index in fiber Bragg gratings refractometers

In this paper, we propose a simple nonlinear equation to describe the cross-sensitivity between temperature and refractive index in fiber Bragg grating refractometers. A numerical analysis using Comsol Multiphysics software was performed to understand how the effective index changes with the temperature and refractive index. The sensor response to refractive index (from 1.33 to 1.45) and temperature (from 15 °C to 45 °C) was fitted using a surface equation. Even though the refractive index range considered being significantly large, and the sensor response being nonlinear, proper concordance between computer simulations and experiments point out that this multivariable equation is suitable to describe the Bragg wavelengths as a function of temperature and refractive index. When applied to multimode fiber Bragg gratings, the proposed equation can be used to construct a system of equations to calculate temperature and refractive index from the measured Bragg wavelengths.

Simultaneous measurement of strain, temperature and refractive index based on multimode interference, fiber tapering and fiber Bragg gratings

We report the development of an optical fiber sensor capable of simultaneously measuring strain, temperature and refractive index. The sensor is based on the combination of two fiber Bragg gratings written in a standard single-mode fiber, one in an untapered region and another in a tapered region, spliced to a no-core fiber. The possibility of simultaneously measuring three parameters relies on the different sensitivity responses of each part of the sensor. The results have shown the possibility of measuring three parameters simultaneously with a resolution of 3.77 με, 1.36 °C and 5 × 10−4, respectively for strain, temperature and refractive index. On top of the multiparameter ability, the simple production and combination of all the parts involved on this optical-fiber-based sensor is an attractive feature for several sensing applications.

High-Temperature Strain Sensing Using Sapphire Fibers With Inscribed First-Order Bragg Gratings

Strain sensor designs and strain measurements based on single-crystal sapphire fibers with inscribed first-order fiber Bragg gratings for applications up to 600 °C are presented. We report on all the details of two different sensor designs; for instance, we show that the resolution of multimode sapphire fiber Bragg grating (SFBG) strain sensors is about $\Delta{l/l}=10^{-5}$ (10 $\mu\text{strain}$ ), which is comparable with state-of-the-art high-temperature sensors. We apply our sensors for the determination of the thermal expansion coefficients of high-temperature steel alloys, showing a good match to known values. Hence, we believe that SFBG sensors may represent a promising alternative to currently used non-optic-based strain-detecting devices.

Результаты экспериментальных исследований маломодовых режимов волоконных брэгговских решеток на многомодовых световодах

Nowadays fiber Bragg gratings (FBGs) are widely used in various devices and applications of fiber optics. They become a basic component for fiber optic sensor networks. The most of those both commercial and prototype fiber optic sensor systems are based on well-researched either singlemode or multimode regime of signal transmission over optical fibers. However some works are concerned with researches of FBGs written on silica multimode optical fibers (MMF) for various photonics and fiber optics applications. Thus, FBGs written on graded-index MMFs are used in optical filters of fiber optic data transmission systems 10G designed for local area networks and radio-over-fiber systems covering micro- and pico-cells. Some works are focused on FBGs written on step-index MMFs. Slanted or tilted multimode FBGs are also widely used in vibration, temperature, deformation etc. fiber optic sensors. Another group of publications is devoted to conventional FBGs on silica telecommunication graded-index MMFs 50/125 as well as 62.5/125 and experimental approbation of various fiber optic sensors based on them. However, the most part of mentioned papers are focused on just analysis of FBG spectral response during laser-excited optical signal propagation that corresponds to a few-mode regime. Few-mode regime adds a new another one dimension to parameter space that is associated with guided modes of particular order which limited number (from two to a few dozens) transfer the most part of optical signal power over tested optical fiber. Optical fiber operating in a few-mode regime of data transmission is interferometer that in combination with FBG can be considered as an alternative basis for design of elements with new unique properties. This work is concerned with experimental researches of few-mode regimes of FBG operating during transferring of laser based optical signals. We tested FBGs written on silica graded-index multimode fibers 50/125 Cat. OM2+/OM3 with Bragg wavelength 1550 nm by using them in a set of developed experimental schemes based on excitation of multimode fibers by corresponding laser sources. The researches were also focused on analysis of both spectral and pulse responses under changing of selected mode mixing and power diffusion processes due to tension and/or stress local and distributed action to FBG or sensor fiber. Results of spectral and pulse response measurements at the output of schemes with installed described FBGs are represented.

Dynamic gate algorithm for multimode fiber Bragg grating sensor systems.

We propose a novel dynamic gate algorithm (DGA) for precise and accurate peak detection. The algorithm uses a threshold-determined detection window and center of gravity algorithm with bias compensation. We analyze the wavelength fit resolution of the DGA for different values of the signal-to-noise ratio and different peak shapes. Our simulations and experiments demonstrate that the DGA method is fast and robust with better stability and accuracy than conventional algorithms. This makes it very attractive for future implementation in sensing systems, especially based on multimode fiber Bragg gratings.

Multimode Interference in Single Mode–Multimode FBG for Simultaneous Measurement of Strain and Bending

In this paper, a discriminating measurement of strain and bending based on single mode–multimode fiber Bragg grating (FBG) structure is investigated experimentally. Experimental results show that the reflected modes of single mode–multimode FBG exhibit different responses toward the bending of transmitting multimode fiber because of the effect of multimode interference, which can be used for ratiometrical detection of bending, meanwhile the wavelength of the reflected modes is immune from bending but sensitive to strain. Using the different responses of the reflected modes of single mode–multimode FBG to bending, strain and bending can be discriminated by measuring simultaneously the wavelength shift of FBG resonance and the reflected intensity ratio between different modes.

Measuring Bragg gratings in multimode optical fibers.

Fiber Bragg gratings (FBG) in multimode optical fibers provide a means for cost-effictive devices resulting in simplified and robust optic sensor systems. Parasitic mode effects in optical components of the entire measurement system strongly influence the measured multi-resonance reflection spectrum. Using a mode transfer matrix formalism we can describe these complex mode coupling effects in multimode optical systems in more detail. We demonstrate the accordance of the theory by two experiments. With this formalism it is possible to understand and optimize mode effects in multimode fiber optic systems.