Design Of Fiber Bragg Gratings Research Articles

Design of High Performance Triangular Fiber Bragg Grating Temperature Sensor using OptiSystem and OptiGrating Software

This paper presents on the design of high performance triangular Fiber Bragg Grating (FBG) as temperature sensor. This triangular FBG is designed and simulated using OptiGrating Software. The Triangular FBG design is exported to OptiSystem Software. In the OptiSystem software, the overall triangular FBG temperature sensor system is designed and simulated. It found that the triangular FBG has higher temperature sensitivity which is 37.5pm/°C than the standard FBG which is around 13pm/°C. It also has high linearity of 99.2% which specifies the consistency of wavelength shift as the temperature is increased. It also has higher resolution due to its sharp peak and smaller bandwidth.

Design and development of type-1 FBG based high temperature sensor

Thermal metrology is critical in many high-energy industrial processes to optimize the process efficiency, energy consumption and quality of end product. Here, we present the design and development of a double thimble type standard type-1 fiber Bragg grating (FBG) high temperature sensing probes. Two different probes named probe-1 and probe-2 are developed by encapsulating type-1 FBGs in two stainless steel thimbles with ceramic filling. The probes are experimentally tested in a high temperature box furnace and their responses are interrogated by an optical spectrum analyzer. FBGs encapsulated in both probe-1 and probe-2 exhibited a linear red-shift in their Bragg wavelength with increase in temperature from 30 °C to 700 °C with sensitivities of 13.86 ± 0.1 pm/°C and 14.10 ± 0.2 pm/°C which are 4.8% and 3.16% lower than temperature sensitivity of bare FBG, respectively. A deformation in the Bragg spectrum of probe-1 is observed at temperatures above 500 °C however, probe-2 is found to be thermally stable up to 700 °C. An estimated life-time of about 6 years is offered by the optimized probe-2 with a negligible hysteresis and repeatability of 1°C–5°C.

Design of a Weak Fiber Bragg Grating Acoustic Sensing System for Pipeline Leakage Monitoring in a Nuclear Environment

We propose a design method of a weak fiber Bragg grating (FBG) array, in which the reflectivity of the FBG increases along the fiber, and this increase is limited by the dynamic range of the distributed optical fiber sensing (DOFS) system. For a DOFS system, this design method can not only well compensate for the impact of fiber loss, within a certain range and caused by radiation, on the deterioration of the system performance, but also can reduce the requirements for the dynamic range of the system in nuclear environments. Based on this design method, we propose a short-distance acoustic detection system based on broadband ultraweak FBGs assisted phase-sensitive optical time domain reflectometry ( $\Phi $ -OTDR), and assess the system’s ability to detect pipeline leakages. This work not only provides guidance for the design of FBG sensing systems but also provides a new promising scheme for the application of DOFS system in nuclear environments.

Design of fiber bragg grating (FBG) temperature sensor based on optical frequency domain reflectometer (OFDR)

In this paper, the simulation of Fiber Bragg Grating (FBG) as a temperature sensor is conducted. The FBG temperature sensor is designed based on Optical Frequency Domain Reflectometer (OFDR) concept. A continuous wave (CW) laser is used as the optical source and it is transmitted to two FBGs. The two FBGs reflection spectra will produce a beat frequency that can be detected using a Radio Frequency (RF) Spectrum Analyzer. Any temperature change will shift Bragg wavelength, thus produce a shift for the beat frequency. In this work, an FBG with temperature sensitivity 10 pm/˚C is employed. It is found that by using this technique, a high-resolution temperature sensor can be designed with temperature resolution of 0.1˚C.

Transmission line method for the simulation of fiber Bragg gratings.

A new method for the analysis and design of fiber Bragg gratings (FBGs) based on the theory of transmission lines has been developed and verified both theoretically and experimentally. The method is an extension of the coupled-mode theory and utilizes the equivalent transmission lines in order to simulate any type of grating, with an easy and direct implementation. The method provides the ability to analyze the optical devices without using full wave approaches, while also facilitating the incorporation of core materials with a complex or non-linear refractive index, non-uniform distributions of the grating's refractive index, and tilted and phase-shifted gratings. The approach also allows the design of the grating for a given reflection spectra. Numerical results of the method's application on a randomly varied inscription of the refractive index of a FBG have also been simulated and discussed. Using this method, the characteristics of an erbium-doped (ED)-FBG have been simulated and the predictions verified experimentally.

Strain transfer analysis of fiber Bragg grating sensor assembled composite structures subjected to thermal loading

Optical fiber sensing technology has been widely used for thermal strain detection of civil structures subjected to the environmental temperature variations. Due to the fragile material nature, bare optical fibers are seldom used in practice, as they cannot resist the harsh service environment. A packaging measure is usually required to enhance the durability of optical fiber-based sensors for reliable long-term detection. However, the existence of packaging and bonding materials brings about strain transfer loss, which makes the optical fiber readings not accurately represent the actual strains of the host material. This paper presents a combined experimental and theoretical study on a four-layer optical fiber sensing composites (i.e., the host material, an adhesive bonding layer, the sensing fiber, and its protection layer) subjected to thermal loading to correct the above-mentioned strain transfer error. Aluminum and polypropylene (PP) plates with different thermal expansion coefficients were surface-bonded with fiber Bragg grating (FBG) sensors and then exposed to various temperatures (30–70 °C). Theoretical study was developed to investigate how the strain measurement error was influenced by the thermal deformation incompatibility of multiple contacted layers (i.e., the optical fiber, the protective layer, the adhesive layer and the hosting material) with different thermal expansion coefficients. The accuracy and effectiveness of the proposed strain transfer function was then validated through comparisons with the test results. Based on the theoretical analysis, the sensitivity of the relevant material and geometrical parameters on the strain transfer efficiency was discussed to instruct the design of FBG based sensors. The proposed error-modification formula can be used to effectively improve the strain measurement accuracy and instruct the optimum design of FBG based sensors applied to civil structures under thermal loading.

Effects of fiber Bragg grating design on dual-grating demodulation performance

Dual-grating demodulation has been both effective and simple in most fiber Bragg grating (FBG) sensors because it involves self-demodulation, and in theory, temperature effects are eliminated. Here, by applying the coupled-mode theory, influences of FBG design parameters such as grating length, refractive index modulation depth, and apodization type on the dual-grating demodulation performance are investigated. It is found that particular performance concerns and priorities can be appropriately addressed by proper selection of FBG design parameters. As an example, experiments using FBGs with properly selected parameters based on our analysis are carried out and compared with the previous experiments using available FBGs. The results show that proper FBG parameter selection can be effective for particular performance requirement.

Top-hat random fiber Bragg grating.

We examined the possibility of using noise or pseudo-random variations of the refractive index in the design of fiber Bragg gratings (FBGs). We demonstrated theoretically and experimentally that top-hat FBGs may be designed and fabricated using this approach. The reflectivity of the fabricated top-hat FBG matches quite well with that of the designed one.

Picosecond optical signal processing based on transmissive fiber Bragg gratings

We report the experimental realization of an ultrafast (terahertz-bandwidth) linear optical signal processor, particularly a picosecond flat-top optical pulse shaper, based on a fiber Bragg grating (FBG) working in transmission. The used FBG design technique, based on a specially apodized linearly chirped FBG, enables the synthesis of readily feasible devices with processing bandwidths well in the terahertz range. The specific device reported here is successfully demonstrated for reshaping ultrashort (400fs FWHM) optical Gaussian-like pulses into 2ps flat-top pulses.

A Practical Approach to Synthesize Multi-channel Fiber Bragg Grating with Right-Angled Triangular Spectrum

A novel approach is proposed towards the design of fiber Bragg gratings with multi-channel right-angled triangular spectrum. Firstly, a single-channel grating is synthesized utilizing an adaptive quantum particle swarm optimization with the piecewise constant mutated factor. Meanwhile, the reflectivity spectrum with good linear edge for a short grating is obtained. Then, for its merits of easy fabrication, the superposition method is adopted to design multi-channel gratings with initial spectral distortion. Finally, this distortion is optimized by the method in the first step. It is shown that the design outcomes still retain the features of easy fabrication and short length. Such gratings would be useful as wavelength-interrogation devices with multiple physical parameters in optical sensor systems.

Cross-Entropy Optimization for the Design of Fiber Bragg Gratings

This paper considers the synthesis problem of a low-dispersion fiber Bragg grating (FBG) filter. This problem can be formulated as a nonlinear objective problem in practice. However, solving this problem with traditional methods is difficult because of its multimodal and ill-conditioned character. Inspired by the efficient ability of cross-entropy optimization (CEO) method to find near-optimal solutions in large search spaces, we propose the application of CEO method to search for design variables to satisfy the target design criteria. Computer simulation results show that the proposed CEO method can successfully achieve the target values of the design specifications. It also provides a reliable convergence to the near-optimum solution. These findings indicate that the CEO algorithm is more appropriate for the FBG filter design compared with conventional particle swarm optimization method.

Analysis of strain transfer of six-layer surface-bonded fiber Bragg gratings

A theoretical analysis of strain transfer of six-layer surface-bonded fiber Bragg gratings (FBGs) subjected to uniform axial stress is presented. The proposed six-layer structure consists of optical fiber, protective coating, adhesive layer, substrate layer, outer adhesive layer, and host material, which is different from the four-layer case of common acknowledgement. A theoretical formula of strain transfer rate from host material to optical fiber is established to provide an accurate theoretical prediction. On the basis of the theoretical analysis, influence parameters of the middle layers that affect the average strain transfer rate of the six-layer surface-bonded FBG are discussed. After the parametric study, a selection scheme of sensor parameters for numerical validation, which makes the average strain transfer rate approach unity, is determined. Good agreement is observed between numerical results and theoretical predictions. In the end, the six-layer model is extended to the general situation of multiple substrate layers, which lays a theoretical groundwork for the research and design of surface-bonded FBGs with substrate layers in the future.

Design of FBG En/decoders in Coherent 2-D Time-polarization OCDMA Systems

A novel fiber Bragg grating (FBG)-based en/decoder for the two-dimensional (2-D) time-spreading and polarization multiplexer optical coding is proposed. Compared with other 2-D en/decoders, the proposed en/decoding for an optical code-division multiple-access (OCDMA) system uses a single phase-encoded FBG and coherent en/decoding. Furthermore, combined with reconstruction-equivalent-chirp technology, such en/decoders can be realized with a conventional simple fabrication setup. Experimental results of such en/decoders and the corresponding system test at a data rate of 5 Gbit/s demonstrate that this kind of 2-D FBG-based en/decoders could improve the performances of OCDMA systems.

Point-by-point inscription of apodized fiber Bragg gratings

We demonstrate apodized fiber Bragg gratings (FBGs) inscribed with a point-by-point (PbP) technique. We tailor the grating phase and coupling amplitude through precise control over the longitudinal and transverse positions of each laser-inscribed modification. This method of apodization is facilitated by the highly localized, high-contrast modifications generated by focused IR femtosecond laser inscription. Our technique provides a simple method for the design and implementation of PbP FBGs with complex apodization profiles.

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.

Design of fiber Bragg gratings with arbitrary reflective spectrum

The reflective spectrum of fiber Bragg gratings (FBGs) might be regarded as the spatial Fourier transform (FT) of the refractive index modulation when the reflection is low. Therefore, the normalized amplitude and the phase which are obtained from the inverse FT of the expected reflection shape are the significant factors for designing the intensity and phase modulation of the index change, respectively. By carefully incorporating and designing the intensity and the phase modulations, we obtain FBGs with arbitrary reflective spectra including cosine/sine, triangle, sawtooth, trapezia and upside-down triangle shapes. Simulation results based on the piecewise uniform method further confirm the proposed method. Such FBGs can easily find their applications in signal generation, parameter demodulation, and so on.

High-order passive photonic temporal integrators

We experimentally demonstrate, for the first time to our knowledge, an ultrafast photonic high-order (second-order) complex-field temporal integrator. The demonstrated device uses a single apodized uniform-period fiber Bragg grating (FBG), and it is based on a general FBG design approach for implementing optimized arbitrary-order photonic passive temporal integrators. Using this same design approach, we also fabricate and test a first-order passive temporal integrator offering an energetic-efficiency improvement of more than 1 order of magnitude as compared with previously reported passive first-order temporal integrators. Accurate and efficient first- and second-order temporal integrations of ultrafast complex-field optical signals (with temporal features as fast as approximately 2.5ps) are successfully demonstrated using the fabricated FBG devices.

Inverse scattering for the one-dimensional Helmholtz equation: fast numerical method

The inverse scattering problem for the one-dimensional Helmholtz wave equation is studied. The equation is reduced to a Fresnel set that describes multiple bulk reflection and is similar to the coupled-wave equations. The inverse scattering problem is equivalent to coupled Gel'fand-Levitan-Marchenko integral equations. In the discrete representation its matrix has Töplitz symmetry, and the fast inner bordering method can be applied for its inversion. Previously the method was developed for the design of fiber Bragg gratings. The testing example of a short Bragg reflector with deep modulation demonstrates the high efficiency of refractive-index reconstruction.

Design of FBG En/Decoders in Coherent 2-D Time-Wavelength OCDMA Systems

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A novel fiber Bragg grating (FBG)-based en/decoder for the two-dimensional (2-D) time-spreading wavelength-hopping optical coding is proposed. Compared with other 2-D en/decoders, the proposed en/decoding for an optical code-division multiple-access (OCDMA) system uses a single phase-encoded FBG and coherent en/decoding. Furthermore, combined with reconstruction-equivalent-chirp technology, such en/decoders can be realized with a conventional simple fabrication setup. Experimental results of such en/decoders and the corresponding system test at a data rate of 5 Gb/s demonstrate that this kind of 2-D FBG-based en/decoders could improve the performances of OCDMA systems. </para>

Optimization algorithm for ultrabroadband multichannel aperiodic fiber Bragg grating filters

We consider an approach to designing complex multichannel filters for ultrabroadband applications. In contrast to earlier approaches that are restricted to specific problems, our generalized method can be applied to any conceivable fiber Bragg grating (FBG) multichannel design. Our method accommodates hundreds of filter channels that are spaced unevenly in wavelength, with variable widths, depths, and shapes. We demonstrate the power of the method for an FBG design with 150 narrow channels over the region 1400-1800 nm.