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Using high-bandwidth feedback, we have synchronized the pulse train from a mode-locked semiconductor laser to an optical clock and achieved the lowest timing jitter (20 fs, 10 Hz -10 MHz) for such devices.OCIS codes: (320.7160) Ultrafast technology, (120.3930) Metrological instrumentation A mode-locked laser diode (MLLD) provides a compact and reliable source of an optical frequency comb in the 1.5 p m wavelength region. Such a source is attractive for establishing frequency reference grids for telecommunications. In light of recent developments of optical atomic clocks, [l] MLLD-based comb systems can play a central role in clock signal distributiodtransfer to remote sites. A low-jitter MLLD also has important applications for ultrafast analog-to-digital conversion. Our experimental approach employs an ultrawide bandwidth optical frequency comb generated by a mode-locked femtosecond Tisapphire laser that is phase locked to a highly stable optical frequency standard. The optical second harmonic output of a MLLD will be compared against the Tksapphire comb in order to establish optical phase coherence between the two systems.

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Brillouin optical time-domain analysis (BOTDA) using distributed Brillouin amplification (DBA) only requires a milliwatt-level pump to achieve a sensing range beyond 100 km, which provides a powerful tool for temperature/strain sensing. However, similar to the majority of other long-range BOTDAs, the state-of-the-art reports require > 1000 times average, severely restricting the sensing speed. The blind area over tens of kilometers caused by the nonuniform Brillouin response and parasitic amplitude modulation (AM) are crucial factors affecting the signal-to-noise ratio (SNR). Here, a comprehensive performance optimization and substantial enhancement for BOTDA sensors was presented by the direct demodulation of an injection-locked dual-bandwidth probe wave. Injection locking (IL) can completely eliminate the impact of AM noise; dual-bandwidth probe enables self-adaptive pulse loss compensation, thereby intensifying the SNR flatness along the ultralong fiber, and direct probe demodulation can overcome nonlocal effects and allows ∼19.7 dB enhancement of probe input power. Therefore, using only 100 times average, ∼148.3 km sensing, and ∼5 m spatial resolution were achieved with < ∼0.8 MHz standard deviation of Brillouin frequency shift (BFS) over a broad range (∼131.7 km). The reduction in averages was more than 10 times that of the reported majority of long-range BOTDAs. Such performances were achieved without using time-consuming or post-processing techniques, such as optical pulse coding and image denoising. Because this approach is compatible with optical chirp chain technique without frequency sweeping, fast acquisition (0.3 s) was also realized, which has the potential for fast sensing at 3.3 Hz along a ∼150 km fiber.

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