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.
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.
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.
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.
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