Optical frequency combs serve as a core technology for optical clocks and frequency transfer, and their linewidth directly impacts the precision of frequency measurements. Consequently, linewidth compression has been a major research focus in the field of frequency combs. The noise of laser has a significant impact on the performance of the carrier-envelope offset frequency (f_ceo), and different mode-locking mechanisms of the laser result in distinct noise properties. Additionally, intensity fluctuations in the pump source also affect the phase noise of the laser.
In this work, a polarization-maintaining figure-9 mode-locked laser (F9L) is established by incorporating dispersion management technology, with the repetition rate continuously tunable in the range of 197.8-200.65 MHz. Compared with a previously developed nonlinear polarization rotation (NPR) mode-locked laser of the same repetition rate, the F9L exhibited superior phase noise performance. Within the 1 Hz-1 MHz integration range, the phase noise of NPR and F9L is 222.4 ps and 18.5 ps, respectively. Based on the F9L, an all-polarization-maintaining optical frequency comb system was built. The spatial light from the laser is coupled into the fiber through a collimator, and the average output power after a bidirectional pumping amplifier reaches 395 mW. Amplifier-output pigtail fiber length was controlled to manipulate the evolution of higher-order solitons. When the output fiber length is trimmed to 41 cm, the pulse width is measured to be 78 fs after Gaussian fitting. The pulsed light was launched into a section of highly nonlinear fiber, generating a supercontinuum spectrum that fully covers the 1000-2000 nm. A carrier-envelope offset frequency (f_ceo) signal with a signal-to-noise ratio of 47 dB was successfully obtained with a common path f-2f interferometer.
Under driving by the same LDC8020 pump source, the free-running f_ceo linewidths of the NPR and F9L mode-locked lasers were measured as 221.5 kHz and 11.4 kHz, respectively. Additionally, the effects of pump current noise and the angle of the 1/8 waveplate inside the F9L cavity on the f_ceo linewidth were systematically studied. For the pump current noise analysis, two types of current sources with different noise levels, namely Thorlabs LDC8020 (20 μA RMS) and Thorlabs CLD1015 (10 μA RMS), were employed. When the F9L was driven by the lower-noise CLD1015 current source, the free-running f_ceo linewidth was further narrowed to 6.6 kHz, and the multi-peak structure in the spectrum was eliminated, demonstrating the positive role of optimizing pump current noise in linewidth compression. Regarding the waveplate angle, experiments were conducted at angles of 45°, 55°, and 65°. It was found that an appropriate waveplate angle (55° in this case) balanced the modulation depth and intracavity loss, effectively suppressing amplified spontaneous emission (ASE) quantum noise and minimizing phase noise, thereby achieving the optimal f_ceo linewidth. Finally, the standard frequency deviations of the repetition rate and f_ceo were 0.376 mHz and 0.263 mHz, respectively, under two consecutive days of locking.