A novel interferometric fiber optic geophone is introduced in this paper. This geophone is mainly used for geo-acoustic signal detection. The geophone use one of the three orthogonal components of mandrel type push-pull structure in mechanically and single-mode fiber optic Michelson interferometer structure with Faraday Rotation Mirror (FRM) elements in optically. The resonance frequency of the geophone is larger than 1000Hz. The acceleration sensitivity is as high as 56.6 dB (0dB re 1rad/g) with a slight sensitivity fluctuation of ±0. 2dB within the frequency band from 20Hz to 200Hz. The geo-acoustic signals generated by underwater blasting are detected successfully. All the channels show good uniformity in the detected wave shape and the amplitudes exhibit very slight differences. The geo-acoustic signal excitated by the engine of surface vehicles was also detected successfully.
Current research on photonic crystal fiber (PCF) for acoustic sensing was focused on the PCF’s pressure sensitivity
enhancement. However, whether the enhancement of the PCF’s pressure sensitivity can be actually realized is still
controversial. Practical hydrophone, utilizing PCFs, to manifest its superior sensitivity to normal single mode fibers
(SMFs) for acoustic sensing, should be made. Account to this point of view, actual hydrophone was fabricated. Index
guiding PCF was used, the fiber core is solid silicon dioxide (SiO2), and the cladding is SiO2 filled with lots of periodical transverse circular air hollows. The PCF, mounted on an air-backed mandrel for structural sensitivity enhancement, was
used as a sensing arm of the fiber Michelson interferometer. The other arm, so called reference arm, was made of SMF.
Faraday rotator mirrors (FRM) were spliced in the end of each interferometric arm account for polarization induced
phase fading, which is a common scheme in fiber interferometric sensing systems. A similar hydrophone, with all the
same structure except that the PCF was exchanged into SMF, was also fabrication to make the contrast. The narrowlinewidth
and frequency-tunable optical fiber laser was used to achieve high accuracy optical interferometric
measurement. Meanwhile, the phase generated carrier (PGC) modulation-demodulation scheme was adopted to
interrogate the measurand signal. Experiment was done by using acoustic standing-wave test apparatus. Linearity
characteristics of the two hydrophones were measured at frequency 100Hz, 500Hz, and 1000Hz, experimental results
showed that the maximum error of the linearity was 10%, a little larger than the theoretical results. Pressure sensitivities
of the PCF hydrophone and the SMF hydrophone were measured using a reference standard PZT hydrophone in the
frequency range from 20 Hz to 1600 Hz, the measurement data showed that the sensitivity of the PCF hydrophone was
about -162.8 dB re. rad/μPa, with a fluctuation ±1.3 dB over the whole measured frequency range, compared with the
sensitivity of -163.6 dB re. rad/μPa and a almost the same fluctuation of the SMF hydrophone. The contrast result shown
that the sensitivity of the PCF hydrophone is only a little larger than that of the SMF hydrophone, the reason was that the
structure sensitivity enhancement played a weightier role in pressure sensitivity.