Fiber cavity ring-down(CRD) technology has attracted widespread attentions for its high sensitivity, and has extensive applications in many fields requiring high sensitivity measurement. In this paper, we propose and implement a refractive index sensor based on fiber CRD spectroscopy. A pulse of light passes through a loop resonator formed by two high-splitting ratio couplers and comes out repeatedly. A segment of ~4cm fiber with thin cladding was designed as a refractive index sensor and inserted into the fiber cavity. The sensor appeared different transmission losses with different surrounding refractive indices, which can be measured by detecting the ring-down curves. The experimental results show that the system has a linear response to the surrounding refractive index with a sensitivity of 182.17 μs /RIU in the range of 1.3575~1.4430 at the center wavelength of 1553nm.
Signal processing is studied for Sagnac fiber distributed sensors. Wavelet de-noising and wavelet decomposition
are comparatively used to analyze signals in frequency domain to give smoothed frequency spectra, based on which null
frequencies of the spectra are found more accurately. The analysis is helpful to more accurately locate the disturbance.
Temporal imaging is one of the important research issues using time-lens. The theory of
temporal imaging using time lens is discussed briefly. The experiment to perform optical pulses
compression is demonstrated and the problem is further discussed by numerical simulation in this article.
The growth of long period fiber grating written in H2-loaded fiber within one hour immediately after fabrication was
measured and analyzed. Fast deepen on difference of refraction index was obviously observed in all the experiments,
but the peak wavelength to time and cross-coupling coefficient to time curves are fitted better in exponential decay
function than power function, and suggest a same variation pattern in difference of refraction index.
Two fiber Bragg gratings with ~35mm spatial interval are written in the self-made photosensitive Erbium-doped fiber
(PEDF) to form a fiber Bragg grating Fabry-Perot (FBG F-P) cavity. A distributed Bragg reflector (DBR) fiber laser is
built up based on the FBG F-P cavity. 1.8mW laser output is achieved with 63mW pump power. By calculating the
mode-spacing using the effective length of the FBG F-P cavity, the output of the DBR fiber laser is proved to be singlelongitudinal-
mode (SLM).
The time delay of the light reflected from and transmitted through the uniform fiber Bragg grating (FBG) are computed
and shown to be equal. The physical meaning of the time delay is discussed based on tunneling effect and energy storage
function of the FBG. The energy density of the optical field distributed along the FBG and the total energy stored in the
FBG are calculated based on the coupled mode theory (CMT). The time needed for light to form the stable energy
density distribution is computed and shown to be equal to the time delay of the FBG.
Temporal-Spectral Imaging of optical pulses constitutes a technique for the measurement of
fast optical spectral. The experiment to perform real-time spectral analysis of optical pulses in fibre by
time lens is demonstrated in this article. The use of time lens in the demodulation of the grating
wavelengths is discussed in this article.
A novel fibre Bragg grating (FBG) hydrophone system is introduced in the paper. The influence of the sound pressure on
the FBG is transformed to light intensity measurement with tuned laser. Elastic material and matched FBGs are
employed to enhance the sensitivity of the hydrophone system. The hydrophone system can operate in a wide acoustic
frequency range from 100Hz to 3kHz and good linear relationship is observed between the output light intensity and the
sound pressure.
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