The stretched pulse has narrower pulse duration and high pulse energy than conventional soliton, so that the stretched pulse can improve the potential application values of 2 μm mode-locked fiber laser in remote sensing, mid-infrared source, material processing and other applications. In this paper, the dispersion management method is adopted to accurately manage the dispersion in the cavity using a commercial ultra-high numerical aperture fiber and a common single-mode fiber. Based on the nonlinear polarization rotation technology to achieve mode-locking, when the pump power is set to 645 mW, the polarization controller is adjusted to achieve stretched pulse output with a de-chirped pulse duration of 581 fs. The center wavelength is 1939.26 nm and the 3-dB bandwidth is 21.8 nm. The repetition rate is 28.9 MHz and the signal to noise ratio is 54.21 dB. At the maximum pump power, the direct output power of the resonant cavity is 9.98 mW, and the corresponding single pulse energy is 0.34 nJ.
A distributed vibration sensing technique using double-optical-pulse based on phase-sensitive optical time-domain reflectometry (ϕ-OTDR) and an ultraweak fiber Bragg grating (UWFBG) array is proposed for the first time. The single-mode sensing fiber is integrated with the UWFBG array that has uniform spatial interval and ultraweak reflectivity. The relatively high reflectivity of the UWFBG, compared with the Rayleigh scattering, gains a high signal-to-noise ratio for the signal, which can make the system achieve the maximum detectable frequency limited by the round-trip time of the probe pulse in fiber. A corresponding experimental ϕ-OTDR system with a 4.5 km sensing fiber integrated with the UWFBG array was setup for the evaluation of the system performance. Distributed vibration sensing is successfully realized with spatial resolution of 50 m. The sensing range of the vibration frequency can cover from 3 Hz to 9 kHz.
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