Fiber Bragg Grating Parameters Research Articles

Optimization of Dual-Wavelength linear cavity Erbium-Doped fiber laser based on pairs of fiber Bragg gratings

There are few reports in the field of linear cavity dual-wavelength fiber lasers about the laser output characteristics when 3-dB fiber loop mirror (FLM) and Fiber Bragg grating (FBG) are used as full reflective mirrors, respectively. Additionally, there is scarce research on further tuning of the FBG parameters (reflectivity and center wavelength interval) to study dual-wavelength fiber lasers. In this paper, we compare the performances of dual-wavelength lasing outputs between the 3-dB FLM-FBG cavity and the FBG-only cavity in a linear-cavity erbium-doped fiber laser (EDFL) based on the polarization hole burning effect. The results show that the optical signal-to-noise ratios (OSNRs) of the dual-wavelength lasing outputs with the FBG-only cavity are approximately 4 ∼ 5 dB higher than those with the 3-dB FLM-FBG cavity. Furthermore, the lasing output performances based on the FBG-only cavity are compared when the reflectivities of the output-mirror FBGs are the same or different. It is observed that the dual-wavelength laser with the same output reflectivity leads to higher OSNR, slope efficiency, and stability. Moreover, by varying the center wavelength interval of pairs of FBGs in the FBG-only cavity to values of 4 nm, 8 nm, and 12 nm respectively. It is shown that wider intervals result in higher OSNRs but lower laser slope efficiency. Finally, we demonstrate a dual- wavelength linear cavity EDFL operating at room temperature with two lasing wavelengths of 1550 nm and 1562 nm. The laser results in OSNRs around 50.62 dB and 50.93 dB; center wavelength variations of less than 0.030 nm and 0.035 nm; output power fluctuations of less than 0.061 mW and 0.059 mW, respectively.

Research and development of requirements for the spectral parameters of fiber bragg gratings for the system of diagnostics of composite structures

Research and development of requirements for the spectral parameters of fiber bragg gratings for the system of diagnostics of composite structures

All-Silicon Packaging Technology for Fiber Bragg Gratings and Its Application in Tilt Sensor

A fiber Bragg grating (FBG) sensor includes three main parts, an FBG, a sensor substrate, and a packaging material. The most commonly used packaging material is epoxy resin adhesive, which is prone to aging and failure, which further affects the performance and life of a sensor. This research provides an innovative approach that enables all-silicon sensor fabrication, where the substrate and packaging material are prepared by silicon glass, which is consistent with silicon optical fiber. In the packaging process, there are three main bottleneck problems, which are considered in this study: 1) highly elastic processing of glass used in the sensor elastic element; 2) thermal fixation between the silicon packaging material and substrate; and 3) selecting appropriate FBG parameters and fixing methods. In the proposed approach, an elastic glass cantilever beam and an FBG are packaged with molten glass powder. The proposed approach is verified by tests, and the test results show that the packaged FBG has good package quality and sensing performance. The theoretical analysis of the wavelength drift phenomenon during the packaging is presented. Finally, the introduced packaging method is applied to the fabrication of an FBG tilt sensor. The sensor shell, cantilever beam, temperature compensation beam, mass block, and other components are designed and made of silicon glass. The sensing principle and performance tests of the tilt sensor have been described in detail. The tilt sensor shows good linearity and repeatability. The presented packaging technology shows good applicability and has promising applications in the fabrication of high-performance FBG sensors.

Direct femtosecond laser inscription of an IR fluorotellurite fiber Bragg grating.

This study proposes a novel, to the best of our knowledge, development of fluorotellurite glass fiber Bragg gratings (FBGs). Shell-like morphology was achieved using a single femtosecond laser pulse illuminated through the fiber's polymer coating. Different FBG fabrication methods and parameters were systematically studied to optimize performance. The fluorotellurite FBG exhibited a high sensitivity to writing laser power and reflectivity saturation effect in repetitive writing. A low-insertion-loss fluorotellurite FBG with a reflectivity of over 99% and bandwidth of less than 1 nm was successfully inscribed. The flexible inscription methods can write an FBG at any wavelength in the fluorotellurite glass transparent window, and are applicable to infrared fiber lasers or sensors.

FBG performance enhancement for sensing and EDFA gain flattening applications

The importance of fiber Bragg gratings (FBGs) is rapidly increasing due to their wide variety of applications. We present herein a full analytical study related to a simple optimization of the performance of FBGs for sensing applications and erbium-doped fiber amplifier (EDFA) gain flattening. In sensing applications, such as strain and temperature change detection, the most important issues are the sensitivity of the sensing process and the maximum value that can be sensed before saturation. Here, we optimized the FBG parameters to obtain the best sensing performance. EDFA gain flattening is performed using one long-period fiber grating and one uniform tilted fiber Bragg grating to compensate the narrow and wide peaks in the EDFA gain variation with input signal wavelength for an Al/P-silica EDFA, respectively. Two different input signal powers are used, applying our flattening technique in each case. The best peak-to-peak value reached is 0.87 dB, corresponding to a flattening efficiency of 81.17%. The validity of this flattening technique is also evaluated through an intensive comparison with other types of EDFAs, where the flattening efficiency is illustrated and compared in each case. We thus optimize FBG performance in a new and simple way for application in sensing and gain flattening applications.

Fiber Bragg Grating Sensor Based on Refractive Index Segment Code of Mobile Modulation

Fiber Bragg Grating (FBG) is achieved by refractive index modulation. Temperature and strain can be measured by FBG. The echo spectral distribution is determined by the refractive index of the fiber and the grating pitch. Therefore, there are few types of conventional refractive index modulation methods, and there are few types of echo spectrum. In this paper, an index code modulation based on IoT method was proposed. The FBG is divided into a plurality of interval segments in the method. The spectral shape of the echo is controlled by encoding and modulating each interval. When the segment size satisfies some requirements, the positive and negative modes of the sub-FBG are calculated by the coupled-mode theory, and then the mode functions of each segment are solved by the matrix transmission algorithm. The equivalent function of the reflectance distribution field is obtained. According to the relevant parameters of FBG, the mathematical model of the FBG modulation spectral distribution is established by the refractive index modulation method. Through the simulation analysis of the fractional refractive index modulation, the results show that the echo spectral peak, half-width, etc. have a larger tunable range. In the experiment, a femtosecond laser was used to accurately encode and modulate the refractive index, and two different types of segmented FBGs were fabricated according to design requirements. The experimental results show that the design of different segmented structures has a significant impact on the spectral distribution of the echo. Spectral shape control can be achieved by refractive index modulation method.

Design of a flattening filter using Fiber Bragg Gratings for EDFA gain equalization: an artificial neural network application


 
 
 This paper presents a proposal for the non-uniform gain compensation of an Erbium-doped fiber optic amplifier (EDFA) in a Wavelength Division Multiplexed (WDM) system using Fiber Bragg Gratings (FBG). In this proposal, the multilayer perceptron feed-forward artificial neural network with backpropagation was trained under the secant method (one-step secant) and was selected according to mean square error measurement. The proposal optimizes FBG parameters such as center frequency, rejection level and length in order to determine a filtering response based on a reduced number of FBGS that will be used to flatten the non-linear response of the amplifier gain and avoid the per-carrier treatment of a standard flattening filter. While an artificial neural network with a 7-10-6 structure demonstrated the feasibility of equalizing the gain of an EDFA using as few as three FBGS, a 25-18-12 structure improved the results when the configuration consisted of an FBG array of six resonances that provided similar results to that featured by the standard gain-flattening filter. The proposal was evaluated in an amplified WDM system of eight optical carriers located between 195-196.4 THz.
 
 

Dynamic Interaction Process Analysis Between Failure-Detecting Light Pulse and 2D Optical Encoder in Link Health Detection System for PON

We explore the dynamic interaction process between the failure-detecting light pulse and fiber Bragg grating (FBG) based two-dimensional (2-D) optical encoder in a link health detection system (LHDS) for a passive optical network (PON). The dynamic interaction model for PON-LHDS with a 2-D optical encoder supporting 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m-1</sup> users is derived by discussing the overlap between the failure-detecting light pulse spectrum and the FBG reflection spectrum, where m is the number of the failure-detecting light sources. A series of received light pulses for PON-LHDS with six wavelength channels involving 32 users are obtained based on the model, considering the variation of the FBG parameters. The simulation results show that the central wavelength difference between the failure-detecting light pulse and the coding FBG is the key factor affecting a system performance that 0.1 nm central wavelength difference will reduce the equivalent reflection coefficient to 0.89 and result in a 6.56-dB peak power difference of the coded signal. The PON-LHDS with a 32-user 2-D optical encoder is a setup, and the experiment results indicate that the peak power of the coded signal between two channels has a 2.8-dB difference due to the central wavelength difference, consistent with the calculation results of the dynamic interaction model.

Development of an FBG Sensor for Measuring Large Range and Multi-Directional Settlement

In this paper, one advanced settlement sensor with the advantages of wide measurement range and multi-directional displacement measurement has been designed and fabricated based on multidisciplinary technologies, including fiber Bragg grating (FBG) sensing technology, 3D printing technology, and others. First, one novel structure of settlement sensor was designed by introducing the concept of a hinge joint, which mainly consists of four components, including internal rigid beam, external rigid beam, elastic FBG sensing beam, and hinge joint. The FBG sensing element inside the sensor can rotate unconstrained without disturbing stress field underground, overcoming the strengthening problem commonly existed in conventional settlement sensors. It is also worth noting that all sensor components were designed and fabricated with 3D printing technology, while the two most popular and effective 3D printing technologies, including fused deposition modeling (FDM) and stereolithography (SLA) technology were separately used for fabricating elastic sensing beam and rigid beam segments, respectively. Furthermore, the FBG sensors were successfully embedded into the printed polylactic acid (PLA) without sacrificing its sensing performance, and the superiority of FBG sensing technology, such as high sensitivity, high accuracy, strong resistance, light weight, and serial connection have been achieved. After completing the fabrication of developed sensor, the measurement performance parameters of elastic FBG sensing beam (includes two paralleled FBG sensors) including measuring linearity, repeatability, and measurement range have been calibrated and verified. Finally, working performance of the assembled settlement sensor from calibration was examined, characterized by a maximum measurement range -160 ~ +160, and a measurement sensitivity of 0.0066 pm/° (in terms of inclination degree) or 0.012 mm/pm (in terms of settlement).

A Sensitivity Enhanced Microdisplacement Sensing Method Improved Using Slow Light in Fiber Bragg Grating

Fiber Bragg grating (FBG) sensors have been widely used in all industrial fields due to their advantages of small size, corrosion resistance, strong antijamming capability, and easy to reuse. Traditional FBG sensors can measure microdisplacement, but low sensitivity and resolution limit its applications in the field of precision measurement. There are sidelobes near the FBG bandgap whose transmittivity changes with wavelength, and the transmitted light has a huge group delay time due to the light here transmitting back and forth numerous times. This paper proposes and demonstrates a sensitivity enhanced microdisplacement sensor using slow light in FBGs through numerical simulations and experiments. A theoretical model of gratings is established by the coupled mode theory and transfer matrix method, and the mechanism of slow light generated in FBG is studied theoretically and also by numerical simulation analysis. This paper determined the set structure parameters of FBG by studying the slow light characteristics of different types of gratings with different structure parameters and influences of structure parameters on the FBG slow light characteristics. A microdisplacement sensing system is designed and built, and finally a sensitivity of 15.0786 mW/mm and a resolution of 66 nm are reached in the range of 0–60 $\mu \text{m}$ , which is 13 times more than the traditional grating sensing with light intensity demodulation.

A novel method of optimal apodization selection for chirped fiber Bragg gratings

Apodization is a crucial technology for improving the dispersion performance of fiber Bragg gratings (FBGs). In this paper, we focus on how to select an optimal apodization for chirped fiber Bragg gratings and demonstrate a novel apodization selection method capable of choosing the optimal apodization based on the relationship between the bandwidth change and variation in average group delay ripple (GDR). Compared with current approaches, the novel method can select an optimal apodization profile and parameter for FBGs easily and accurately. Two numerical experiments are used to demonstrate the advantages of this method, one exhibits the different performances of five different apodization profiles, the other evaluates the influence of FBG parameters such as grating length, period, chirp, and index change on the apodization performance.

Real time transverse-force sensor based on polarization properties of fiber Bragg grating and cross-sensitivity compensation

We present a new method for real-time transverse force sensor based on the measurement of the polarization properties of a uniform fiber Bragg grating (FBG) written into standard single mode fiber. Unlike the usual spectral analysis of FBG sensors, we demonstrate here that the amplitude of the first Stokes parameters of a uniform FBG in transmission can be used to obtain transverse force value. The influences of incident angle of linear polarized light launched into FBG on sensor performance are analyzed. Experimental results measured by means of a tunable laser source and a polarimeter are presented. We also propose a kind of grating structure with triangular-shaped transmission spectrum to reduce the influences of cross-sensitivity. The compensation effect can satisfy the requirements of the practical application with optimized grating parameters.

Analysis of optical switching in a Yb3+-doped fiber Bragg grating by using self-phase modulation and cross-phase modulation

The optical performances of the all-optical switching based on Yb3+-doped fiber Bragg grating (FBG) are investigated under the case of self-phase modulation (SPM) and cross-phase modulation (XPM). For the SPM case, the optical bistability of FBG under different parameters is investigated. It shows that the width of the hysteresis loop and threshold switching power are strongly dependent on the fiber grating length, fiber grating detuning, and coupling constant. For the XPM case, the expressions of the threshold switching power in the different detuning range are given. The influence of parameters about different detuning and coupling constant to the threshold switching power and extinction ratio are also studied. In comparison with the SPM case, the switching power of FBG under XPM case can be reduced to less than 20 mW by optimizing the parameters of FBG.

Sensitivity Enhancement of Strain Sensing Utilizing a Differential Pair of Fiber Bragg Gratings

In strain measurement applications, the matched fiber Bragg gratings (FBG) method is generally used to reduce temperature dependence effects. The FBG parameters have to be designed to meet the requirements by the particular application. The bandwidth and slope of the FBG has to be balanced well, according to the measurement range, accuracy and sensitivity. A sensitivity enhanced strain demodulation method without sacrificing the measurement range for FBG sensing systems is proposed and demonstrated utilizing a pair of reference FBGs. One of the reference FBGs and the sensing FBG have almost the same Bragg wavelength, while the other reference FBGs has a Bragg wavelength offset relative to the sensing FBG. Reflected optical signals from the sensing FBG pass through two reference FBGs, and subtract from each other after the detection. Doubled strain measurement sensitivity is obtained by static rail load experiments compared to the general matched grating approach, and further verified in dynamic load experiments. Experimental results indicate that such a method could be used for real-time rail strain monitoring applications.

Relative Intensity Noise Reduction by Optimizing Fiber Grating Fabry–Perot Laser Parameters

A set of nonlinear rate equations that can describe an external cavity laser with any arbitrary external optical feedback (OFB) level are derived. A comprehensive study on the relative intensity noise (RIN) characteristics of a fiber grating Fabry-Perot is performed numerically. In this paper, fiber Bragg grating (FBG) is used as a wavelength lasing selective element to control the external OFB level, thereby control the RIN. In addition to the external OFB level, the effect of other external cavity parameters such as temperature, injection current, cavity volume, gain compression factor, and FBG parameters on RIN characteristics is investigated. The temperature dependence (TD) of RIN is calculated according to TD of laser parameters instead of well-known Parkove relationship. Results show that by optimization, the peak value of the RIN can be reduced down to around -150 dB/Hz. The optimum and the shortest external cavity length that provides the minimum RIN is found to be around 3.1 cm. In addition, by optimization, the relaxation oscillation frequency of RIN spectra is shifted toward around 5.6 GHz.

Sensing With Slow Light in Fiber Bragg Gratings

On the edge of the bandgap in a fiber Bragg grating (FBG) narrow peaks of high transmission exist at frequencies where light interferes constructively in the forward direction. In the vicinity of these transmission peaks, light reflects back and forth numerous times across the periodic structure and experiences a large group delay. Since the sensitivity of a phase sensor to most external perturbations is proportional to the reciprocal of group velocity, in these slow-light regions the sensitivity of an FBG is expected to be significantly enhanced over traditional FBG sensors operated around the Bragg wavelength. In this paper, we describe means of producing and operating FBGs that support structural slow light with a group index that can be in principle as high as several thousand. We present simulations elucidating how to select the FBG parameters, in particular index modulation, length, and apodization, to generate such low group velocities, and quantify the very large improvement in strain and temperature sensitivities resulting from these new slow-light configurations. As a proof of concept, we report an FBG with a group index of 127, or a group velocity of ~2,360 km/s. This is by far the lowest group velocity reported to date in an FBG. Used as a strain sensor, this slow-light FBG is shown to be able to detect a strain as small as 880 fe/ √Hz , the lowest value reported for a passive FBG sensor.

Specifics of Bragg gratings inscription and characterization in polarization-maintaining Yb-doped fiber for DFB lasers

Fiber Bragg gratings (FBGs) with a length of about 4 cm and a phase shift of about π are written in the polarization-maintaining fiber doped with ytterbium ions. A tunable polarized single-frequency laser is employed for the measurement of the FBG reflection spectrum. A relatively high spectral resolution allows the estimation of the FBG parameters. Polarized single-frequency ytterbium-doped distributed-feedback (DFB) fiber lasers radiated at about 1.1 μm are constructed using the written FBGs. The passive line width of the laser radiation (about 50 kHz) is estimated from the beat spectrum of the two DFB lasers.

Effect of Grating Parameters on Mode-Locked External Cavity Lasers

This paper deals with numerical solutions of actively mode-locked fiber grating semiconductor lasers using a time-domain solution of coupled wave equations and rate equations. Simulation of linearly chirped tanh apodized fiber Bragg grating (FBG) utilized in hybrid soliton pulse source (HSPS) shows an extreme increase in the mode-locking frequency range of HSPS. Our model predicts transform-limited pulses over a frequency range of 1.6 GHz (1.8-3.4 GHz) for this grating around a system operating frequency of 2.5 GHz, with a pulsewidth of 46 ps required for a practical soliton transmission system, whereas it ranges to about 1.3 GHz (2-3.3 GHz) for the linearly chirped raised-cosine flat top and 850 MHz (2.1-2.95 GHz) for linearly chirped Gaussian apodized. Furthermore, in this study, the effects of FBG parameters, such as peak reflectivity, grating length, grating chirp, and modulation index, on output of mode-locked HSPS are also described for the first time. The numerical results indicated that although pulsewidths decreased with the increase in grating chirp, shorter grating lengths gave shorter pulses, and the modulation index and peak reflectivity of the grating did not significantly affect the pulsewidths.

The modelling of Fiber Bragg Grating

Fiber Bragg Gratings (FBGs) attract great attention due to their present and prospective applications in fiber-optical communication systems and modern opto-electronics. FBGs are components having a wide range of applications in the fields of communication, lasers and sensors. This study is a theoretical study in the field of FBG. Our aim is, by using coupled-mode theory and changing-of-index variation function, to model the FBG and to elucidate the appropriate parameters of FBG. Diffraction spectra of a FBG and its first and second degrees were obtained. Maximum reflection of FBGs versus saturation coefficient was also examined.

On a fiber grating sensor system with the capacity of cross-sensitivity discrimination

A novel fiber Bragg grating (FBG) sensor system based on an interrogating technique by two parallel matched gratings was designed and theoretically discussed. With an interrogation grating playing the role of temperature compensation grating simultaneously, the wavelength drifts induced by temperature and strain were discriminated. Additionally, the expressions of temperature and strain were deduced for our solution, and dual-value problem and cross sensitivity were solved synchronously through data processing. The influence of the FBG's parameters on the dynamic range and precision was discussed. Besides, the change of environment temperature cannot influence the dynamic range of the sensor system through temperature tuning. The system proposed in this paper will be of great significance to accelerate the real engineering applications of FBG sensing techniques.