Excess Intensity Noise in a Nonlinear Amplifying Loop-Mirror-Based Mode-Locked Laser from a Non-Reciprocal Phase Bias

We demonstrate a low-intensity-noise, nonlinear amplifying loop-mirror-based mode-locked fiber laser by optimizing the polarization of the non-reciprocal phase bias and the pump current. If the angle of the waveplate in the non-reciprocal phase bias to the polarization axis of a polarization-maintaining fiber is not carefully aligned, parasitic polarization is induced. The parasitic polarization affects the minimum pump power and dynamic range of pump power for mode-locking, the intensity noise, and the comb power. To reduce intensity noise, the angle of the waveplate for the non-reciprocal phase bias is adjusted, and then the pump power is adjusted. The waveplate angle minimizing the intensity noise maximizes the dynamic range of the pump power for mode-locking and output power. As a result, the relative intensity noise has been suppressed by more than 32 dB at 15 kHz Fourier frequency. The polarization extinction ratio at the non-reciprocal phase bias is critical since it can determine a cavity loss and quality factor of a laser oscillator. Therefore, the additional polarizers cannot improve the intensity noise once the angle is mismatched and the polarization extinction ratio is degraded.