Highly coherent linearly-swept fiber laser based on composite optical phase-locked loop

We present a dual-loop composite optical phase-locked loop (OPLL) composed of an acousto-optic frequency shifter based external modulation loop and a piezo based direct modulation loop for the generation of highly coherent swept-frequency fiber laser. It allows offering a larger loop bandwidth and gain, permitting an efficient linearization and coherence enhancement. We have verified experimentally a highly coherent swept-frequency fiber laser with a high fidelity and linearized frequency sweep with sweep range of ~8.2 GHz at ~164 GHz/s sweep rate, accompanied with a peak-to-peak frequency error as low as ~130 kHz. The in-band noise of the coherent beat note has been effective suppressed by almost 60 dB within a loop bandwidth up to ~80 kHz . The proposed linearly swept-frequency fiber laser provides a straightforward optimization of the real-time sweep control and far distance ranging in various scenarios, which is supposed to be a beneficial tool in both industrial and commercial applications.