Abstract. We constructed a temperature-controlled Fourier domain mode locking (TC-FDML) laser capable of high-speed wavelength sweeping and developed a real-time fiber Bragg grating (FBG) measurement system. The TC-FDML laser can perform high-speed wavelength sweeping at a sweep frequency of 50.7 kHz with a scan range of ∼60 nm in the 1.55-μm band. This system uses a data acquisition system mounting an analog/digital converter and field programmable gate array that enables real-time FBG measurement at a sampling frequency of 250 MHz. Using bidirectional wavelength sweeping by the TC-FDML laser, the system has a measurement time resolution of 9.9 μs. We show that the developed system can measure high-speed vibrations of several kHz and perform simultaneous and continuous measurements of multiple FBGs for a period of one hour. © The Authors.Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

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.

We develop a high-speed interrogation system with a wavelength-swept laser for sensing multiplexed fiber Bragg gratings (FBGs). High-speed and multipoint sensing is achieved by means of a buffered Fourier domain mode-locked (FDML) laser, a recently proposed high-speed wavelength-swept laser. This laser increases the measurement rate several times by tailoring the laser output with a buffer stage. The developed buffered FDML laser successfully achieves a measurement rate of 202.8 kHz using a general FDML laser driven at a sweep rate of 50.7 kHz. In order to detect reflection signals of FBGs at the high measurement rate, the measurement system introduces digital signal processing using a field programmable gate array (FPGA). However, FBG measurements with wavelength-swept lasers are affected by the propagation time (delay) in the optical fiber connecting from the laser to the FBG sensor, which reduces measurement accuracy. To overcome this limitation, the developed system uses a delay correction method with bidirectional sweep of the buffered FDML laser. The interrogation system with the buffered FDML laser achieves a time resolution of 4.9 µs without being affected by the delay.

Recently, optical image coding using a circular Dammann grating (CDG) has been proposed and investigated. However, the proposed technique is intensity based and could not be used for three-dimensional (3D) image coding. In this paper, we investigate an optical image coding technique that is complexamplitude based. The system can be used for 3D image coding. The complex-amplitude coding is provided by a circular Dammann grating through the use of a digital holographic recording technique called optical scanning holography. To decode the image, along the depth we record a series of pinhole holograms coded by the CDG. The decoded reconstruction of each depth location is extracted by the measured pinhole hologram matched to the desired depth. Computer simulations as well as experimental results are provided.

We propose an interrogation-system with automatic recognition and delay correction functions of fiber Bragg gratings (FBGs) by pulse modulation with a wavelength-swept laser. By pulse-modulating the light of a wavelength-swept laser, light with an arbitrary wavelength bandwidth can be extracted as the pulsed light. In the automatic recognition function of FBGs, the pulsed light is sequentially controlled to match the wavelength bandwidth of each FBG. This recognition method enables the selection and detection of a reflection signal from a single FBG. Therefore, reflection signals from multiple FBGs can be recognized individually. When multiple FBGs are installed at long distances, the reflection signal of each FBG is affected by the propagation time (delay). In the interrogation system with the wavelength-swept laser, the delay lowers the measurement accuracy. Therefore, a delay correction function using bidirectional sweeping of the wavelength-swept laser is used. The wavelength-swept laser using Fourier domain mode locking (FDML) is driven at a sweep frequency of 50.7 kHz with a sweep bandwidth of ∼60 nm. This study demonstrates that pulse-modulated FDML laser can automatically recognize reflection signals from multiple FBGs installed at arbitrary long distances. When the recognition process is complete, the interrogation system can perform real-time measurement with a time resolution of 9.9 µs, without being affected by delay that occurs when installing at a long distance.