Multichannel Fiber Bragg Grating Research Articles

Full C-band covered and DWDM channelized high channel-count all-fiber orbital-angular-momentum mode generator based on the fiber gratings.

To generate the orbital-angular-momentum (OAM) modes at multiple wavelengths, which exactly fit with the dense-wavelength-division-multiplex (DWDM) channel grids, is important to the DWDM-based OAM mode-division-multiplex (MDM) fiber communication system. In this study, a full C-band covered and DWDM channelized OAM mode generator is firstly proposed and experimentally demonstrated, which is realized especially by using a broadband helical long-period fiber grating (HLPG) combined with a phase-only sampled multichannel fiber Bragg grating (MFBG). As a proof-of-concept example, the DWDM channelized two complementary 51-channel OAM mode generators have been successfully demonstrated, each of which has a channel spacing of 100 GHz (∼0.8 nm), an effective bandwidth of ∼40 nm, a high azimuthal-mode conversion efficiency of 90%, and high uniformities in both inter- and intra-channel spectra as well. To the best of our knowledge, this is the first time for proposal and experimental demonstration of such a high channel-count and DWDM channelized first-order OAM mode (l = 1) generator, which can also be used for multichannel higher-order OAM mode generation as long as the utilized HLPG is capable of generating a broadband higher-order OAM mode. The proposed device has potential applications to DWDM-based OAM fiber communications, OAM comb lasers, OAM holography, and OAM sensors as well.

A flexible multiplexing method of FBG sensors based on OFDR

A flexible multiplexing method of fiber Bragg grating (FBG) sensors based on optical frequency domain reflectometer (OFDR) is proposed in this paper to reduce the influence of spectral shadow effect on measurement and improve the reliability and measurement flexibility of FBG sensing network. In this method, the interrogation principle of FBG position and wavelength shift based on OFDR is analyzed, and as a guide, the flexible multiplexing FBG array is designed, in which multi-channel identical weak reflection FBGs are interrogated using a photoelectric detector. Meanwhile, the crosstalk beat noise generated by the flexible multiplexing FBG array is avoided so as to improve the performance of the measurement. The experiment results show that the method presented in this paper can achieve the flexible multiplexing of multi-channel FBGs with a single acquisition channel, while the measurement accuracy can be guaranteed.

Temperature Field Measurement of Photovoltaic Module Based on Fiber Bragg Grating Sensor Array.

Studying the temperature field of photovoltaic modules is important for improving their power generation efficiency. To solve the problem of traditional sensors being unsuitable for measuring the spatial temperature field, we designed a real-time detection scheme of the photovoltaic module temperature field based on a fiber Bragg grating (FBG) sensor array. In this scheme, wavelength division multiplexing and space division multiplexing technologies were applied. The multi-channel FBG sensor strings were arranged on the surface and in the near field of the photovoltaic module. Different FBG strings were selected through optical switches, and the wavelength of the FBG string was addressed and demodulated using the tunable laser method and a peak-seeking algorithm. A measurement experiment of the photovoltaic module temperature field was carried out in an outdoor environment. The experimental results showed that the fluctuation law of the photovoltaic module surface and near-field temperature is basically consistent with that of solar radiation power. The temperature of the photovoltaic module decayed from the surface to space. Within 6 mm of the photovoltaic module surface, the temperature sharply dropped, and then the downward trend became flat. The lower the solar radiation power and the higher the wind speed, the faster the temperature decay. This method provides technical support for measuring the temperature field of a photovoltaic module and other heat source equipment.

Multi-Wavelength, Passively Q-Switched, Single-Frequency Fiber Laser

A passively Q-switched single-frequency fiber laser with stable multi-wavelength operation is presented in this report. The laser cavity is constructed by using a 13-mm Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> /Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> co-doped phosphate fiber with a multi-channel fiber Bragg grating and a semiconductor saturable absorber mirror attached to each fiber end, respectively. Quad-wavelength pulsed lasers are obtained and all of them maintain single-frequency operation. By controlling the pump power and the temperature of laser cavity, the Q-switched pulse operation is realized with repetition rates range of 126.6–350.9 kHz, a narrowest pulse duration of 104 ns, and a maximum pulse energy of 55.4 nJ. To the best of our knowledge, this is the first study to realize a Q-switched fiber laser operating at multiple wavelengths with single longitudinal mode. Furthermore, the evolutions of the pulse energy and repetition rates for different wavelength numbers are further analysis theoretically. This type of single-frequency pulsed fiber laser has extensive application prospect in fiber sensing and lidar systems.

Multichannel fiber bragg grating based on DC-sampling method

We have proposed and numerically demonstrated a DC-sampling method for designing a multichannel filter from a single-channel fiber Bragg grating (FBG). Unlike the previous sampling methods reported to date where the sampling function (either an amplitude-only or a phase-only one) is utilized to modulate the AC part of the induced index-modulation in the seed single-channel FBG, here an additionally periodic index-modulation with a slowly varied envelope, named the DC-sampling function in the study, is especially added to the DC part of the induced index-modulation in the seed FBG. We theoretically demonstrate that the proposed DC-sampling function acts equivalently like the phase-only sampling, which can be applied to produce a multichannel FBG and make the maximum utilization of the fiber as well. By using the proposed method, a 9-channel FBG with almost an ideal spectrum has been successfully demonstrated. Fabrication tolerances for this kind of multichannel have also been analyzed. The proposed method enables us to produce a multichannel filter from a single channel FBG just by using the direct UV exposure without the need of the expensive and the complex phase mask. Moreover, the proposed method would provide us much more flexibilities to fabricate any other new kinds of fiber gratings than ever before.

Multichannel fiber Bragg grating for temperature field monitoring.

We demonstrate a multichannel fiber Bragg grating (MC-FBG) based distributed temperature field sensor with millimeter-order spatial resolution. The MC-FBG was designed by using the layer peeling (LP) algorithm with a tailored group delay characteristic and fabricated using seamless UV-inscription. We have achieved a 21-channel MC-FBG with 0.2 nm bandwidth of each channel and 0.5 nm channel gap. The sensor was tested by using a temperature field distribution. Experimental results show that the sensor had a spatial resolution of 3 mm and could measure a maximum temperature gradient of 7.85 °C/mm.

Multichannel High-Speed Fiber Bragg Grating Interrogation System Utilizing a Field Programmable Gate Array

We demonstrate a multichannel high-speed fiber Bragg grating (FBG) interrogation system utilizing a field programmable gate array (FPGA). A temperature-controlled Fourier domain mode locking laser was operated with a sweep rate of 50.7kHz, sweep bands of 35nm, and an output power of 0.91mW. This laser enabled an FBG measurement speed of up to 9.9 s. Real-time processing could be achieved by incorporating a high-speed data acquisition system (DAQ) mounted on an analog to digital converter and an FPGA. The DAQ performed in-line processing with four 14-bit analog channels and a sampling frequency of 250MHz. The internals of the FPGA were implemented parallel processing for high-speed FBG detection to allow for the simultaneous measurement of the multiplexed FBG in three channels. We demonstrated that this system is capable of high-speed vibration measurements above several tens ofkHz with a multichannel.

A reconfigurable microwave photonic filter with flexible tunability using a multi-wavelength laser and a multi-channel phase-shifted fiber Bragg grating

We propose a photonic scheme to realize a reconfigurable microwave photonic filter (MPF) with flexible tunability using a multi-wavelength laser (MWL) and a multi-channel phase-shifted fiber Bragg grating (PS-FBG). The proposed MPF is capable of performing reconfigurability including single bandpass filter, two independently bandpass filter and a flat-top bandpass filter. The performance such as the central frequency and the bandwidth of passband is tuned by controlling the wavelengths of the MWL. In the MPF, The light waves from a MWL are sent to a phase modulator (PM) to generate the phase-modulated optical signals. By applying a multi-channel PS-FBG, which has a series of narrow notches in the reflection spectrum with the free spectral range (FSR) of 0.8 nm, the +1st sidebands are removed in the notches and the phased-modulated signals are converted to the intensity-modulated signals without beating signals generation between each two optical carriers. The proposed MPF is also experimentally verified. The 3-dB bandwidth of the MPF is broadened from 35 MHz to 135 MHz and the magnitude deviation of the top from the MPF is less than 0.2 dB within the frequency tunable range from 1 GHz to 5 GHz.

Comparison of RF Photonics-Based Beamformers for Super-Wide Bandwidth Phased Array Antennas

We demonstrate the NI AWRDE E-CAD tool-based simulation experiments to compare the three arrangements of photonic beam forming networks. The results confirm clearly the benefits of the proposed arrangement based on combination of multichannel fiber Bragg grating and switchable optical delay lines providing super-wide operating bandwidth and the better economical characteristics of microwave-band phased array antennas.

Laboratory core flooding experiments in reservoir sandstone under different sequestration pressures using multichannel fiber Bragg grating sensor arrays

This paper describes an innovative method for a fiber-optic sensing technology based on multichannel fiber Bragg grating (FBG) sensor arrays, which was applied to deionized water and free-CO2 core flooding experiments on saturated Shaximiao sandstone core specimens under various sequestration pressures. Three FBG sensor arrays, bonded to the sandstone surface along the axial and radial directions, dynamically monitored the strain responses during the wetting phase (water) or non-wetting phase (CO2) displacement processes. It was found that with the increase in the confining pressure or pore pressure, the relative strain increased throughout the experiment. For the water flooding experiment, the proportions due to rise the sequestration pressures are different in each scenario and FBG arrays, and they are generally positive increase, which can also be ascribed to the effective pressure. Meanwhile, the initial arrival time of the precisely characterized strain histories revealed the fronts of the injected water as well as the detailed implications of the CO2 plume migration during the core-scale flooding process. These FBG data from water/CO2 flooding experiments are of great significance for geophysicists and reservoir engineers to better establish the accumulations of raw data and theoretical mechanism reserves for further experimental analysis, numerical simulation and field applications in the CCUS projects and can enhance the minimal experimental data available in the literature to provide a new data set in the geomechanical field for water-CO2 two-phase fluid flow.

Investigation of the dynamic strain responses of sandstone using multichannel fiber-optic sensor arrays

In the process of rock mechanical experiments, strain-response measurement is a most fundamental and most essential procedure for geomechanical researchers. The main objective of this paper is to point out the feasibility and the superiority of the application of a novel multichannel fiber Bragg grating (FBG) sensor arrays for dynamic strain-response measurements of cylindrical specimen subjected to uniaxial compression. The principle, design, and embedment of multichannel FBG sensors used in the experiment are briefly described. To fully monitor the strain history of the sandstone cylinder in uniaxial compression, six circumferential FBG sensors, four lateral FBG sensors, linear variable differential transformers (LVDT) built-in machine have been utilized for spatially monitoring small radial and axial strains along the height of the specimen, respectively. The experimental results indicate that the proposed FBG sensors can successfully provide a full-field view of the surface strains, as well as detect the potential crack locations within the specimen, and strains measured by multichannel FBG sensors are in good agreement with the results of LVDT, especially in the axial strains. Hence, it could be inferred that multichannel FBG sensor arrays are capable of measuring dynamic strain responses of sandstone specimen in multistage compression, which would greatly strengthen experimental basis for further application and theoretic research of in-situ field monitoring.

Microwave photonic integrator based on a multichannel fiber Bragg grating.

We propose and experimentally demonstrate a microwave photonic integrator based on a multichannel fiber Bragg grating (FBG) working in conjunction with a dispersion compensating fiber (DCF) to provide a step group delay response with no in-channel dispersion-related distortion. The multichannel FBG is designed based on the spectral Talbot effect, which provides a large group delay dispersion (GDD) within each channel. A step group delay response can then be achieved by cascading the multichannel FBG with a DCF having a GDD opposite the in-channel GDD. An optical comb, with each comb line located at the center of each channel of the FBG, is modulated by a microwave signal to be integrated. At the output of the DCF, multiple time-delayed replicas of the optical signal, with equal time delay spacing are obtained and are detected and summed at a photodetector (PD). The entire operation is equivalent to the integration of the input microwave signal. For a multichannel FBG with an in-channel GDD of 730 ps/nm and a DCF with an opposite GDD, an integrator with a bandwidth of 2.9 GHz and an integration time of 7 ns is demonstrated.

基于粒子群算法的多通道光纤布拉格光栅滤波器优化设计

基于粒子群算法的多通道光纤布拉格光栅滤波器优化设计

Analysis and Applications of Phase Shifts for Multichannel Fiber Bragg Grating

Analysis and Applications of Phase Shifts for Multichannel Fiber Bragg Grating

Effective algorithm for high-channel-count multichannel fiber Bragg grating designs

We present an efficient approach to design a high-channel-count multichannel fiber Bragg grating by assigning optimal sets of delay coefficients and constant phases to the corresponding channel responses. Based on approximate Fourier transform, the delay coefficients are chosen to separate all the single-channel gratings into several groups spatially in the grating structure, and the constant phases in each group are optimized to minimize the maximum index modulation to be approximately the square root of the maximum of the number of the channels in all groups times larger than that of the one-channel grating. Design examples demonstrate that the proposed method has advantages of low index modulation, low algorithmic complexity, and suitability for multichannel fiber Bragg grating designs with either identical or nonidentical spectral responses.

Effects of group delay ripple on optical beam-formers for pulsed antenna arrays

A semi-analytical formulation is presented to quantify the effects of group delay ripple on the performance of an optical beam-former. For delay elements, implemented with commercially available multi-channel fiber Bragg gratings, an impulse response is introduced and calculated for the group delay ripple. To do this, the transfer function of the fiber Bragg grating is parted into two dispersive and non-dispersive sections. This will also ease the demonstration of the fiber Bragg grating as a true time delay element. The formulation is applied to an ultra-wideband pulsed array of antennas. As an example the array factor and electric field patterns of a four element antenna array is derived, using certain available measured data. The distortion in the temporal-angular radiation patterns due to the non-ideal true time delay behavior may be predicted using the presented work. It also facilitates to distinguish between the effects of long and short ripples. This new mathematical investigation is applicable to any linearly chirped fiber Bragg grating other than multi-channel ones. It also includes corrections to previously published formulations.

Multichannel Arbitrary-Order Photonic Temporal Differentiator for Wavelength-Division-Multiplexed Signal Processing Using a Single Fiber Bragg Grating

A multichannel photonic temporal differentiator implemented based on a single multichannel fiber Bragg grating (FBG) for wavelength-division-multiplexed (WDM) signal processing is proposed for the first time to our knowledge. The multichannel FBG is designed using the discrete layer peeling (DLP) algorithm together with the spatial sampling technique. Specifically, the DLP algorithm is used to design the spectral response of an individual channel, while the spatial sampling is employed to generate a multichannel response. The key feature of the proposed temporal differentiator is that WDM signals at multiple optical wavelengths can be simultaneously processed. Two sampling techniques, the phase-only and the amplitude-only, are employed. The use of the phase-only sampling technique to design a 45-channel first-order and second-order temporal differentiator is performed, and the use of the amplitude sampling technique to design a 3-channel first-order and second-order temporal differentiator is also performed. A proof-of-concept experiment is then carried out. A 3-channel first-order differentiator with a bandwidth of 33.75 GHz and a channel spacing of 100 GHz is fabricated. The use of the fabricated 3-channel FBG to perform first-order temporal differentiation of a 13.2-GHz Gaussian-like optical pulse with different optical carrier wavelength is demonstrated.

Simple index modulation profile with fast-converging design optimization for multichannel fiber Bragg grating filters

In this paper, a novel hybrid algorithm featuring a simple index modulation profile with fast-converging optimization is proposed towards the design of dense wavelength-division-multiplexing systems (DWDM) multichannel fiber Bragg grating (FBG) filters. The approach is based on utilizing one of other FBG design approaches that may suffer from spectral distortion as the first step, then performing Lagrange multiplier optimization (LMO) for optimized correction of the spectral distortion. In our design examples, the superposition method is employed as the first design step for its merits of easy fabrication, and the discrete layer-peeling (DLP) algorithm is used to rapidly obtain the initial index modulation profiles for the superposition method. On account of the initially near-optimum index modulation profiles from the first step, the LMO optimization algorithm shows fast convergence to the target reflection spectra in the second step and the design outcome still retains the advantage of easy fabrication.

Analysis of designing multichannel fiber Bragg gratings with different inverse design algorithms

We have analyzed different inverse design algorithms for the synthesis problems of fiber gratings. An example of dense wavelength-division-multiplexing (DWDM) multichannel fiber Bragg grating (MCFBG) for optical fiber communication was synthesized and simulation results were analyzed and compared with use of the discrete layer-peeling (DLP) inverse scattering algorithm and Lagrange-multiplier constrained optimization (LMCO) algorithm. Simulation results show the LMCO method was more flexible than the DLP method.

Design of High-Channel-Count Multichannel Fiber Bragg Gratings Based on a Largely Chirped Structure

We propose a novel technique to design a high-channel-count, multichannel fiber Bragg grating (FBG) based on a largely chirped structure. The minimization of refractive-index modulation has been widely discussed in the previously demonstrated multichannel grating designs. The complexity of the grating structure, however, is also important from the point of view of practical fabrication. In this paper, the degree of grating complexity (DGC) is defined. We show that the DGC of a multichannel grating can be significantly reduced by designing a grating with a largely chirped structure. A detailed grating design process based on this technique is discussed, by which four multichannel gratings are designed and numerically studied, for applications such as periodic and nonperiodic spectral filtering, chromatic dispersion compensation and dispersion slope compensation. The proposed theory and examples show that different gratings with high-channel-count and multichannel responses can be designed and fabricated using a single commercially available phase mask, and all the gratings can be realized by a conventional FBG fabrication facility since the gratings have a low DGC with low index modulation.