To develop a practical fiber laser hydrophone, demodulation method to get the wavelength shift of the fiber laser
hydrophone is a key technology. A novel scheme for demodulating the phase shift is proposed to eliminate the affect of
the three unsymmetrical outputs from 3×3 coupler. A special polarization-insensitive unbalanced Michelson fiber
interferometer based on the 3×3 coupler is employed to transpose the wavelength shift into phase shift of the
interferometer. The interferometer just has two outputs. A certain length of the fiber in an arm of the interferometer is
wrapped onto a PZT tube to get a high frequency phase modulation signal whose amplitude is bigger than π rad, so the
direct and alternate item of the two outputs from the interferometer can be gotten. The two outputs from the
interferometer are detected by photoelectrical detectors, and the digitized acquisition data from the detectors are
processed by Labview program to demodulate the phase shift. The gains of the two outputs are adjusted real-time to get
two symmetric signals after their direct items were subtracted, and the two symmetric signals are processed through
differentiator and cross-multiplier to demodulate the acoustic pressure signal. The sensitivity experiment shows that the
new demodulation scheme can get a steady output and the phase output is linear to the pressure applied on the fiber laser
sensor when the outputs from the interferometer are unsymmetrical. The amplitude fluctuate of the phase output is less
than 5% when the fiber laser hydrophone is under stable acoustic pressure at a frequency point.
To develop a practical fiber laser hydrophone, demodulation method to get the wavelength shift of the fiber laser
hydrophone is a key technology. A homodyne coherent demodulation scheme utilizing a 3×3 coupler has the advantage
of passive detection without phase or frequency modulation in the reference arm and wide dynamic range. But the
amplitude of phase output from the passive homodyne coherent demodulation system based on a 3×3 coupler for fiber
laser hydrophone is instable during a lake test. To solve this problem, the principle of the passive homodyne phase
demodulation scheme based on a 3x3 coupler for the fiber laser hydrophone is investigated and a digital demodulation
system is built. A unbalanced Mach-Zehnder fiber interferometer based on the 3×3 coupler is employed to transpose the
wavelength shift into phase shift of the interferometer. Through theoretical analysis and simulation calculation, the
relations between amplitude discrepancies and the phase differences with the phase output are finally educed. The
theoretical analysis and simulation calculation shows that the amplitude discrepancies and the phase differences of the
three outputs from the interferometer result in the amplitude instability of phase output. To solve these problems, we can
get three symmetrical outputs by adjusting the gains of the three outputs. Both the simulation calculate and the
underwater acoustic experiment show that the phase output of homodyne coherent demodulation system based on a 3x3
coupler can be steady by adjusting the gains of the three outputs, when the 3×3 coupler is asymmetric.
Planar gratings have wide applications and, till date, many methods for the fabrication of gratings have been reported. Ultrashort pulse laser has been used for the machining of gratings primarily due to its ability of direct ablation and its capability to fabricate sub-wavelength structures. In this paper, we present a novel direct ablation technique for the fabrication of planar gratings by interfering ultrashort pulses in a novel optical configuration. This technique not only simplifies the optical setup, but also immunizes the system to extraneous and inherent vibrations, thus enabling planar gratings of good edge acuity. In addition, this technique ensures that gratings are formed only on the focal point. The grating line width can also be adjusted without much change to the optical configuration. With this technique, we have successfully fabricated planar grating of different line-widths on a silicon substrate. Effect of pulse number, and the laser threshold on the grating quality has been qualitatively studied using SEM analysis. This method offers a novel technique for the fabrication of surface relief profile on the metal surface by direct ablation. The optical setup is immune to vibration, at the same time cost effective and fast. Gratings have wide applications and this fabrication technique can be realized commercially.
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