Optical frequency combs serve as a core technology for optical clocks and frequency transfer, with its noise characteristics determining the precision of frequency measurement. The linewidth of the carrier-envelope offset frequency (f_ceo) is a key characterization of the noise properties of the optical frequency comb. Therefore, achieving linewidth compression and optimization of f_ceo through noise suppression represents a critical research objective in this field. The noise of laser can significantly affects the performance of the f_ceo, and different mode-locking mechanisms of the laser result in distinct noise properties. Additionally, intensity fluctuations in the pump source can also affect the phase noise of the laser.

In this work, a polarization-maintaining figure-9 mode-locked laser (F9L) is established based on dispersion management technology, with a continuously tunable repetition rate in a 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 exhibits superior phase noise performance. Within the 1 Hz–1 MHz integration range, the phase noise of NPR and F9Lare 222.4 ps and 18.5 ps, respectively.

Using the F9L, a fully polarization-maintaining optical frequency comb system is 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. The length of the amplifier-output pigtail fiber is controlled to manipulate the evolution of higher-order solitons. When the output fiber length is trimmed to 41 cm, the pulse width measured after Gaussian fitting is 78 fs. The pulsed light is launched into a section of highly nonlinear fiber, generating a supercontinuum spectrum that fully covers a range of 1000–2000 nm. A f_ceo signal with a signal-to-noise ratio of 47 dB is successfully obtained with a common path f-2f interferometer.

Under the same LDC8020 pump source, the free-running f_ceo linewidths of the NPR and F9L mode-locked lasers are measured to be 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 are 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), are employed. The experimental results show that the CLD1015 produces lower relative intensity noise from the same laser diode and lower phase noise from the same F9L than the LDC8020. When the F9L is driven by the lower-noise CLD1015 current source, the free-running f_ceo linewidth is further narrowed to 6.6 kHz, and the multi-peak structure in the spectrum is eliminated, demonstrating a positive role in optimizing pump current noise in linewidth compression. Regarding the waveplate angle, the experiments are conducted at angles of 45°, 55°, and 65°. It is found that an appropriate waveplate angle (55° in this case) can balances 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, under two consecutive days of locking, the standard frequency deviations of the repetition rate and f_ceo are 0.374 mHz and 0.263 mHz, respectively.