Koji Kajiwara

Proposal and experimental verification of Bragg wavelength distribution measurement within a long-length FBG by synthesis of optical coherence function

In this paper, a sensor system for measuring continuous Bragg wavelength distribution in a long-length fiber Bragg grating is newly proposed, using synthesis of optical coherence function (SOCF), which is one of the spatial resolving techniques used for reflectometry. Experimental results are also reported. In the process of synthesizing optical coherence function, it is found that an apodization scheme is necessary to obtain the reflection spectrum of local section in a long-length FBG around the coherence peak. As a verification of this method, the detection of local Bragg wavelength shift due to temperature change within a short section in a long-length FBG is demonstrated experimentally.

Multiplexed long-length fiber Bragg grating distributed sensors based on synthesis of optical coherence function

Recently, we have reported a scheme for measuring Bragg-wavelength distribution inside a long-length fiber Bragg grating using synthesis of optical coherence function, which is one of the reflectometry techniques based on interferometer and heterodyne detection with an optical frequency-shifter and an electronic band-pass filter. However, the system did not have enough measurement range since it measured the distribution with only one grating. For extending the range, long-length FBG sensors are multiplexed and distribution in each grating is measured. In the process of multiplexing FBGs, a scheme for compensating the beat frequency shift due to the high speed sweep of the optical center frequency of the light source is introduced in the system.

Measurement of Bragg-Wavelength Distribution in a Long-Length Fiber Bragg Grating with High Speed Sampling

We have performed a measurement of Bragg wavelength distribution in a long-length fiber Bragg grating (FBG) with kHz order sampling rate. We have also realized the total distribution measurement with a repetition rate faster than 1 Hz. The proposed sensing system is based on a reflectometry technique “synthesis of optical coherence function”. The advantage of this system is the capability of random access distributed measurement and fast speed interrogation. At two points of a long-length FBG, we have performed the reflection spectrum variation measurement when an 80 Hz vibration is applied to part of the FBG.

Measurement of Bragg wavelength distribution in a long-length fiber Bragg grating by synthesis of optical coherence function

A sensor system for measuring continuous Bragg wavelength distribution in a long-length fiber Bragg grating is newly proposed and implemented, based on the technique of synthesis of optical coherence function. Experimental results and simulations are comparatively examined.

Distributed sensing in a long-length FBG based on synthesis of optical coherence function with 1-kHz sampling rate

Measurement of Bragg wavelength distribution inside a long-length fiber Bragg grating is demonstrated with a higher sampling speed based on synthesis of optical coherence function to realize a real time sensing. The local reflection spectrum is acquired with 1-kHz sampling rate and the total distribution is measured with 1-Hz measurement speed.

Synthesis of optical coherence function to sweep the 0th peak by phase modulation

We propose a method to shift whole the shape of synthesized optical coherence function along an optical path by phase modulation. We can synthesize periodical delta function type optical coherence peaks by frequency modulation of a light source to realize distributed measurement of back-scattering and/or reflections along an optical fiber by sweeping the coherence peak as measurement window. In this study, we propose and demonstrate a method to synthesize and sweep the 0th coherence peak by phase modulation with the same waveform as that for the laser frequency modulation. Basic experiments of optical reflectometry are successfully demonstrated with the proposed method.

Single coherence peak extraction among synthesized periodical peaks by different beat frequencies for elongation of measurement range in multiplexed long-length distributed FBG sensors

A scheme to extract only one coherence peak among multiple peaks synthesized in synthesis of optical coherence function (SOCF) technique is proposed, and elongation of measurement range of multiplexed distributed long-length FBG sensors is demonstrated. In the SOCF systems, distributed measurement is carried out by sweeping the position of a coherence peak synthesized with optical frequency modulation of a laser source. Since, only one coherence peak is allowed to exist in the measurement range for the distributed measurement, the measurement range was limited within the interval of the peaks. In the proposed scheme, only one peak can be extracted among the multiple coherence peaks by using different heterodyne beat frequency caused in each peak when the laser center frequency is swept in high speed for shaping the FBG reflection spectrum. Extracted single peak is successfully swept over the conventional measurement range in our multiplexed distributed long-length FBG sensor system.

Spatial resolution enhancement by external phase modulation in long-length FBG sensing system based on synthesis of optical coherence function

External phase modulation is newly combined with the synthesis of optical coherence function for improvement of spatial resolution in distributed sensing system using a long-length fiber Bragg grating. The spatial resolution is improved from previously-reported 9.8 mm to 4.0 mm.