Length Of Fiber Bragg Grating Research Articles

Analysis of ultraviolet fringes contrast on first and second order Fiber Bragg gratings written by prism interferometers

A numerical analysis on the refractive index modulation in first and second order of type I fiber Bragg gratings (FBGs) written by prism interferometer fringes is presented. The analysis of FBG written by biprism interferometers has been carried out to optimize the writing position and by Lloyd prism interferometer to optimize FBG length with respect to ultraviolet (UV) beam parameter. It is analytically shown that in the biprism fringe depth, the fiber positions of maximum reflectivity in first and second orders are different and both are less than the distance of maximum beam overlap. The refractive index modulation of FBGs written by Lloyd prism varies along the FBG length. The evolution and saturation of the FBGs written by biprism and Llyod prism are different due to difference in magnitude and/or profile of the UV fringes contrast in the FBG writing plane.

Time-frequency analysis of long fiber Bragg gratings with low reflectivity

A new technique to investigate the spatial distribution of the reflection spectrum along fabricated long weak fiber Bragg gratings (FBG) is experimentally demonstrated, together with its potential applications for distributed fiber sensing and broadband signal processing. A short pulsed coherent light signal is launched into a FBG and the signal frequency is scanned through the FBG reflection spectrum. When the pulse duration is set much shorter than the transit time through the grating a time-resolved reflected signal can be obtained for each signal frequency. It informs about the distribution of the refractive index periodic perturbation along the entire FBG length, hence the uniformity or frequency chirp information of the fabricated FBG. This technique has been implemented to demonstrate a distributed temperature sensing system with high spatial resolution and to also realize a robust all-fiber tunable delay line for broadband signals.

대신호 시영역 모델을 이용한 광섬유 격자 외부 공진 레이저 다이오드의 해석

A large-signal time-domain model is implemented to analyze an FG-LD (Fiber Grating Laser Diode) in which a reflective laser diode is hybrid-integrated with a fiber Bragg grating (FBG). When the length of the externally integrated resonator is 8 mm, in which the effective FBG length of 2.1 mm is included, a static frequency chirp of 0.44 GHz and a dynamic frequency chirp of 6.4 GHz are observed. In addition, it is also observed that the eye of the 10Gbps NRZ signal is well open. The FG-LD is expected to be a cost-effective solution for a 10Gbps-class single wavelength laser covering a span of 50 km range.

Composite failure prediction of π-joint structures under bending

In this article, the composite -joint is investigated under bending loads. The “L” preform is the critical component regarding composite -joint failure. The study is presented in the failure detection of a carbon fiber composite -joint structure under bending loads using fiber Bragg grating (FBG) sensor. Firstly, based on the general finite element method (FEM) software, the 3-D finite element (FE) model of composite -joint is established, and the failure process and every lamina failure load of composite -joint are investigated by maximum stress criteria. Then, strain distributions along the length of FBG are extracted, and the reflection spectra of FBG are calculated according to the strain distribution. Finally, to verify the numerical results, a test scheme is performed and the experimental spectra of FBG are recorded. The experimental results indicate that the failure sequence and the corresponding critical loads of failure are consistent with the numerical predictions, and the computational error of failure load is less than 6.4%. Furthermore, it also verifies the feasibility of the damage detection system.

Fiber Bragg grating sensor for two-phase flow in microchannels

A new non-intrusive measurement technique for two-phase flow in microchannels is presented. The development of an evanescent field-based optical fiber Bragg grating (FBG) sensor is described, and experiments coupled with flow visualization demonstrating the performance of this sensor are presented. Two adjacent 1-mm FBGs in etched D-shaped fiber are embedded into the surface of a PDMS microchannel. Experiments are conducted in both droplet and slug flow regimes and high-speed digital video is captured synchronously with the sensor data. The FBGs exhibit an on–off type response to the passage droplets which is shown to correlate precisely with the passage of the liquid phase. This correlation enables the measurement of droplet average velocity and size using only the sensor data. In addition to the use of both FBG signals for the purpose of measuring droplet speed and size, it is shown that for droplets larger than the FBG length, a single FBG can be used to estimate the convection velocity and size of fast moving droplets. This sensing method is potentially useful for monitoring two-phase flow in fuel cells and microfluidic applications such as micro-heat exchangers and lab-on-a-chip systems.

Sinusoidally chirped fiber Bragg grating for mode-locked application

In this study, a mode-locked hybrid soliton pulse source (HSPS) utilizing sinusoidal chirped fiber Bragg grating (FBG) is reported for the first time, using the time-domain solution of coupled wave equations and rate equations. The sinusoidal chirped FBG provides a wider bandwidth by adjusting the reversion coefficient or chirp rate even if the FBG length is short. Numerical results also indicate that an HSPS-utilized sinusoidal chirped FBG produces shorter pulses in the 25−72 ps range, whereas the pulses range from 31 to 97 ps for a linearly chirped tanh apodized grating, and from 30 to 80 ps for a linearly chirped Gaussian apodized grating, along with an increase in the mode-locking frequency range.

Study on the spectral response of fiber Bragg grating sensor under non-uniform strain distribution in structural health monitoring

Many theoretical studies have been developed to study the spectral response of a fiber Bragg grating (FBG) under nonuniform strain distribution along the length of FBG in recent years. However, almost no experiments were designed to obtain the evolution of the spectrum when a FBG is subjected to non-uniform strain. In this paper, the spectral responses of a FBG under non-uniform strain distributions are given and a numerical simulation based on the Runge-Kutta method is introduced to investigate the responses of the FBG under some typical non-uniform transverse strain fields, including both linear strain gradient and quadratic strain field. Experiment is carried out by using loads applied at different locations near the FBG. Good agreements between experimental results and numerical simulations are obtained.

Flexible tunable spectrum sliced microwave photonic filter using parallel Fabry-Pérot filters and Fiber Bragg Grating

A simple and low-cost spectrum sliced microwave photonic filter with high tunability has been implemented. This is based on use of two Fabry-Pérot filters with same free spectral range connected in parallel but one filter with an external fiber delay. An extension of this configuration with Fiber Bragg Grating (FBG) as delay element is also demonstrated. This architecture overcomes the tuning difficulties of a single Fabry-Pérot filter employed as a slicing element. Flexible tunability, large overall free spectral range and high quality factor have been obtained with the proposed configuration owing to the use of fiber delay/FBG. Wide and step tuning has been achieved in the frequency range of 41.1GHz to 50.23GHz in steps of 2.28GHz. The additional components are another Fabry-Pérot filter and fiber delay/FBG which makes the proposed architecture highly compact and cost-effective. Most of the practical difficulties faced in use of optical fiber are eliminated using FBGs. The characteristics of proposed configuration with different applied delays using fiber and FBG were tabulated. An analysis of flexible tunable property of the filter was made in terms of length of fiber delay and grating length of FBG and the results were plotted. Also quality factor calculation of the proposed filter was discussed.

DBR光纤激光器有效腔长测量新方法

A novel method to measure the effective cavity length of distributed Bragg reflector (DBR) fiber laser with fiber Bragg grating (FBG) Fabry-Perot cavity as the resonator is proposed. The effective cavity length of DBR fiber laser is accurately measured by measuring the frequency spacing of two adjacent longitudinal modes in the cavity without the precise physical length of FBGs. The measuring accuracy as high as 10-5 mm can be obtained by setting the resolution bandwidth of the spectrum analyzer.

System impairment of birefringence in fiber Bragg gratings in optical networks

Considering the presence of birefringence in fiber Bragg gratings (FBGs), the spectral polarization mode dispersion and polarization-dependent-loss-induced power penalty of eye opening is simulated for a network consisting of cascaded FBGs with various index apodization profiles. It is shown that with the same FBG length, 3dB bandwidth, and the birefringence value, FBGs with Gaussian index apodization profiles introduce the highest power penalty compared to FBGs with other apodization profiles, such as raised cosine, Blackman, etc. When there is wavelength misalignment up to 0.1nm between the central FBG's resonance and the wavelength of the laser source, the birefringence-induced power penalty can be up to a few times higher than the case with no wavelength misalignment.

Development of Distributed Long-gage Fiber Optic Sensing System for Structural Health Monitoring

The essence of structural health monitoring (SHM) requires an innovative sensing system similar to a human nervous system throughout the whole body to catch comprehensive information without losing structural integrity. In a recent research by the authors, an integrated structural assessment strategy based on distributed strain sensing technologies is proposed [1], which has been dedicated to utilize the strain distribution throughout the full or some partial areas of structures to detect the arbitrary and unforeseen damage. To implement such strategy, a typical distributed fiber optic sensing system is developed by extending the gage length of fiber Bragg grating (FBG) and then arranging the long-gage FBG sensors in series. This article summarizes the packaging design and manufacture method of such long-gage FBG sensors and verifies the performance of the developed sensors and the distributed sensing system by using a series connection of long-gage FBG sensors. Combined with the practical application in steel and reinforced concrete (RC) structures, the efficiency and ascendancy of the proposed distributed fiber optic sensing system in SHM are elaborated.

Effects of the phase shift split on phase-shifted fiber Bragg gratings

In proximity side-writing fiber Bragg gratings (FBGs) using a phase shifted phase mask, the phase shift in the phase mask is split into two half-amplitude phase shifts. The effects of the phase-shift split on FBGs is investigated. A FBG with a split π-phase shift is modeled as two cascaded and tightly coupled grating-based Fabry-Perot filters. A compact expression of its reflectivity spectrum is obtained showing spectral asymmetry errors and a shift of the transmission peak. Our numerical results are in good agreement with previous experimental results. These effects decrease with increasing FBG length and become negligible for gratings more than a few millimeters long.

Dependence of Fiber Bragg Grating Characteristics on its Length

A fiber Bragg grating (FBG) is normally made with a length of 10 to 30 mm. Depending on the application, however, a short length may be desired, especially in sensor applications for local detection. Under the most common fabrication conditions using the phase mask method, we have fabricated FBGs with lengths ranging from 0.35 to 15 mm using different excimer laser irradiation times. Although 0.35-mm-long FBGs can be made with laser exposure times exceeding 1 h, the practical minimum FBG length appears to be around 3 mm with exposure times in the range of 3 to 5 min.