Field-Programmable Gate Array-Based Multichannel Measurement System for Interrogating Fiber Bragg Grating Sensors

Abstract

We develop a field-programmable gate array (FPGA)-based multichannel measurement system for sensing multiplexed fiber Bragg gratings (FBGs), using a high-speed wavelength-swept laser. The wavelength-swept laser, operated with Fourier-domain mode locking (FDML), exhibits a sweep frequency of 50.7 kHz and sweep band of ∼60 nm. A breakthrough in multichannel and real-time measurement is achieved by implementing an FPGA with unique parallel-processing circuits. This FPGA enables the implementation of a high-speed centroid-peakdetection circuit for FBG detection, which can be operated with a sampling frequency of 250 MHz. In multichannel measurement systems, the light propagating through each sensor path produces a delay, which reduces the measurement accuracy. This can be solved by implementing a delay-correction technique, which utilizes the time difference between two spectra obtained by a bidirectional scan of the FDML laser. The system is demonstrated to exhibit multichannel sensing and high measurement time resolution of 9.9 µs, without being affected by the delay.

Interrogation System With Automatic Recognition and Delay Correction Functions of Fiber Bragg Gratings by Pulse Modulation With Wavelength-Swept Laser

“…The laser optical system consists of an FDML laser, an optical amplifier (OAMP), and a fiber optical switch (FSW). The FDML laser is a ring resonator type laser consisting of a semiconductor optical amplifier (SOA1117, Thorlabs), 4 kmlong optical fiber, and fiber Fabry-Perot tunable filter (FFP-TF, Micron) [30], [32]. The FFP-TF is driven by an oscillator (OSC, 33612A, Agilent) with a sinusoidal wave signal V C of a sweep frequency f m of 50.7 kHz.…”

“…The fiber length L K (K = 1 to 3) from the reference position P S of this interrogation system to the center position of each FBG without optical offset fiber is listed in Table I. The reference position P S corresponds to the optical fiber length obtained by measuring the wavelength sweeping characteristic of the FDML laser [30], [32]. Here, an optical offset fiber O F K (K = 1 to 3) with the fiber length of L O F_K (K = 1 to 3) were placed in the front of each FBG to consider the long distance installation of the FBG.…”

“…Our previous research proposed a method of correcting delay by using the bidirectional sweeping of the high-speed wavelengthswept laser. The developed systems were equipped with a field programmable gate array (FPGA) and performed real-time measurement without being affected by the delay [30]- [32].…”

“…Therefore, even when multiplexing the FBG, reflection signals from FBGs can be recognized individually. The interrogation system can perform delay correction by using the bidirectional sweeping of the wavelength-swept laser [32]. Bidirectional sweeping offers twice the measurement rate of unidirectional sweeping.…”

Development of real‐time vibration measurement system by fiber Bragg gratings with pulse light

“…A number of techniques were proposed for FBG measurement using spectroscopy, 10,11 laser demodulation, 12 or wavelength scanning 13–21 . Particularly, FBG Bragg wavelength shift can be measured via phase change using interferometry 22 ; this technique offers higher sensitivity than other methods.…”