A sonobuoy system based on a fiber optic vector hydrophone (FOVH) is demonstrated. Phase Generated Carrier– Arctangent (PGC-ATAN) demodulation algorithm was used to acquire real-time underwater acoustic signals. After the optimal design of the laser configuration, the background noise of the FOVH is -104.3dB re rad √ Hz at 1 kHz, with an acceleration sensitivity of 41.5dB re rad/g which allows the system detecting signals at DSS0. The theoretical derivation of FOVH directivity is proposed and the design criterion is discussed. The ratio of the minimum to the maximum amplitude of the FOVH directivity is -35dB by symmetrical structure design of the FOVH. A lake trial shows that the maximum detection range of the sonobuoy system is more than 15km for an acoustic signal of 210dB re μPa, and the bearing of a moving target can be estimated.
LPGs with ultra-low strain and temperature sensitivities are reported in germanosilicate fibre with a W-shaped index profile. Through low temperature annealing, the temperature and strain coefficients are dramatically reduced to dλ/dT ~ - (8.75×10-2) pm/K and dλ/d epsilon ~ (2.21×10-2) pm/μepsilon respectively. As a control reference, LPGs in standard SMF-28 fibre were inscribed but were not found to exhibit similar reductions in sensitivity; this illustrates that the design of the W-shaped, with its inner cladding, plays a key role in altering the LPG performance.
The excited state lifetime of Er3+ and gain coefficient of an Er3+/Yb3+ co-doped phosphosilicate optical fibre with and without hydrogen loading can be recovered and even improved by high temperature annealing. Consequently processes optimizing room temperature component fabrication such as hypersensitisation are found to offer no added value for high temperature operation. The implications for harsh environmental laser performance are discussed.
Regeneration of long period gratings (LPGs) in hydrogen-loaded, boron-codoped germanosilicate fibre is studied. The regenerated LPGs survive temperatures exceeding T > 950 °C. High temperature stability and stable spectra of these gratings were studied under different annealing temperatures. Regeneration was faster at higher temperatures, and occurred within 75 minutes in the region of 850 °C to 950 °C. These regenerated LPGs also exhibit ultra-low strain sensitivity.
The acoustic pressure sensitivities of hollow-core photonic bandgap fibers (HC-PBFs) with different thicknesses of silica outer-cladding and polymer jacket were experimentally investigated. Experiment with a HC-PBF with 7 μm-thick silica outer cladding and 100 μm-thick Parylene C jacket demonstrated a pressure sensitivity 10 dB higher than the commercial HC-1550-02 fiber and 25 dB higher than a standard single mode fiber. The significant enhancement in sensitivity would simplify the design of fiber hydrophones and increase the number of sensors that could be multiplexed in a single fiber.
On the basis of the complex cavity model, the feedback-induced instabilities which may cause coherence collapse in
single-mode distributed feedback (DFB) fiber laser sensors are analyzed. We measure the longitudinal operating mode
and the frequency shift of DFB fiber laser due to Rayleigh backscattering, and obtain quantitative agreement between
simulation and experiments in long-time investigations. All these study can provide references to design a novel form of
DFB fiber laser sensor system utilizing external optical feedback, and the feasibility of the sensor has been proved.