2 μm soliton lasers in a bidirectional nonlinear polarization evolution Tm3+-doped fiber oscillator

Funding Information: This work is financially supported by National Natural Science Foundation of China ( 61905150 ; 61805281 ); Fundamental Research Funds for the Central Universities, China (Grant No. 3072022CFJ2501 ; 3072022CF2506 ); Natural Science Foundation of Guangdong Province, China ( 2019A1515010732 ).Peer reviewedPublisher PD

Ultrathin interferometric hydrophone towed line array based on uwFBG

In order to improve the suitability of the fiber hydrophone towing line array, we propose an interferometric hydrophone towed line array with a diameter of 1.7 mm. The line array is an integration of sensitization and package, embedding grating sensors in the composite materials of Kevlar and polyurethane in an additional pultrusion process. The theory and experiment indicate that polymer cladding of pultrusion is an effective way to improve phase sensitivity, increasing mechanical strength and decreasing cable diameter simultaneously. Then, a hydroacoustic detection system, with a quasi-distributed ultra-weak fiber Bragg grating towed line array, based on unbalanced interference is established and its phase sensitivities are tested experimentally. The result is measured by the vibration liquid column method and compared with a standard piezoelectric underwater acoustic detector. Over the frequency range of 200–2000 Hz, the system has a good amplitude–frequency response. We obtain an average phase sound pressure sensitivity of about −147.96 dB (re 1 rad/μPa) and a relative flatness response of 3 dB. The experimental results show that this system provides the possibility of an integrated hydrophone towed linear array with high adaptability and reliability

Pretreatment of Ultra-Weak Fiber Bragg Grating Hydrophone Array Based on Cubic Spline Interpolation Using Intensity Compensation

The demodulation algorithm based on 3 × 3 coupler in a fiber-optic hydrophone array has gained extensive attention in recent years. The traditional method uses a circulator to construct the normal path-match interferometry; however, the problem of increasing the asymmetry of the three-way signal to be demodulated is easily overlooked. To provide a solution, we report a pretreatment method for hydrophone array based on 3 × 3 coupler demodulation. We use cubic spline interpolation to perform nonlinear fitting to the reflected pulse train and calculate the peak-to-peak values of the single pulse to determine the light intensity compensation coefficient of the interference signal, so as to demodulate the corrected three-way interference signal. For experimental verification, ultra-weak fiber Bragg gratings (uwFBGs) with reflectivity of −50 dB are applied to construct a hydrophone array with 800 sensors, and a vibratory liquid column method is set up to generate a low-frequency hydroacoustic signal. Compared to the traditional demodulation algorithm based on a 3 × 3 coupler, the pretreatment method can improve the consistency of interference signals. The Lissajous figures show that cubic spline interpolation can improve the accuracy of monopulse peak seeking results by about 1 dB, and intensity compensation can further lead to a much lower noise density level for the interference pulse amplitude—specifically, more than 7 dB at 5~50 Hz—and the signal-to-noise ratio is improved by approximately 10 dB at 10 Hz. The distinct advantages of the proposed pretreatment method make it an excellent candidate for a hydrophone array system based on path-match interferometry