High speed demodulation method of identical weak fiber Bragg gratings based on wavelength-sweep optical time-domain reflectometry

Abstract

The identical weak reflection Fiber Bragg gratings (FBGs) with large capacity has become one of the central issues of optical fiber sensing field in the engineering application.Currently,wavelength division multiplexing (WDM) and time division multiplexing (TDM) are two major multiplexing techniques.For a WDM system,the maximum number of FBGs is limited by the spectral bandwidth of laser.So the identical weak FBGs are proposed to break through the limitation of the multiplexing capacity.For large-capacity multiplexing of identical weak FBGs,TDM technique is commonly used.In a TDM system,the spectral information of all FBGs can be obtained by some pulsed light with different wavelengths.However,with increasing the number of identical weak FBGs in TDM system,some problems such as complex demodulation process and slow response time are highlighted in the current various demodulation methods. Thus in this paper we propose a new high-speed demodulation method combined with wavelength-sweep optical timedomain reflectometry (WSOTDR) which is different from the pulsed light in optical time domain reflectometry (OTDR), namely a continuous wavelength-sweep light source is used in WSOTDR.In this method,the reflected signals of identical weak FBG at each position will be distinguished from others in time domain through optical delay effect,hence the location information of each FBG could be acquired,and meanwhile the wavelength information of all the identical weak FBGs could be obtained through high-frequency periodical wavelength-swept spectrum in just one wavelength scanning period.In order to calibrate the error of FBG demodulation which is caused by optical delay at high-speed wavelength sweep,we propose a self-calibration method in which two different wavelength-sweep rates are used to obtain the inherent delay parameters of each FBG.In practical application,we use this self-calibration method in the initial stage of demodulation because the inherent delay parameters are usually stable after the layout of an identical weak FBGs network.So the demodulating speed at the working stage of this system is not affected by this self-calibration method. In this paper,by setting up a Fourier domain mode locking laser as an output of continuous wavelength-sweep and highspeed (3.27×106 and 2.72×106 nm/s) light,an identical weak FBG sensing network which consists of 18 FBGs is tested in three experiments.In the initial calibration experiment,we use the self-calibration method to calibrate the inherent delay parameters of each FBG and to verify the accuracy of the system by comparing with the measurement result of spectrum analyzer.In the temperature experiment,the wavelength of each FBG is demodulated from 30 to 100 ℃ in order to test the demodulation linearity of the system.Then in the vibration experiment,a dynamic measurement of 3.6 kHz vibration of FBG is demonstrated with a demodulating speed as fast as 120 kHz,and a 0-60 kHz frequency spectrum is analyzed to prove the speed.The experimental results show that the demodulation error is less than 15 pm, the resolution is 1pm,the linearity is above 0.998,and the demodulating speed reaches 120 kHz.

Design, Optimization, and Experimental Evaluation of Slow Light Generated by π-Phase-Shifted Fiber Bragg Grating for Use in Sensing Applications

“…The use of fiber Bragg gratings (FBGs) in sensor applications have been extensively studied over past decades. Sensing with optical sensors is developing very rapidly, many fiber optics network elements used today can be found in light generation [ 1 ], demodulation [ 2 ], or used in optical delay lines [ 3 ]. Apart from a smaller size, the immunity to electromagnetic interference and the ability to measure multi-parameter phenomena, FBG sensors offer many advantages.…”

Research on a high-precision calibration method for tunable lasers

“…Tunable lasers have attracted significant interest for optical fiber sensing owing to their ability to generate scanning laser continuously and uniformly over a specified wavelength band [1,2]. Therefore, the performance of tunable lasers has a major impact on optical sensing systems [3]. However, due to the structure characteristics of the tunable laser, mechanical aging and environmental impact, the output wavelength and scanning rate are always fluctuating and deviating from the set value, resulting in measurement errors [4].…”

“…However, these methods are complex and costly because of the introduction of several femtosecond lasers. By adopting multi-wavelength etalons, a piecewise linear approach can be used to calibrate tunable lasers [3,13]. This method is very simple, but it has to assume that the output wavelength of the laser in the piecewise region changes linearly, so the correction accuracy is inadequate.…”