Dual-comb spectroscopy (DCS) based on optical frequency comb (OFC) has attracted much attention in recent years because of its high sensitivity, high resolution, and high sampling rate. At present, there are three technical routes to achieve DCS: coherent DCS, adaptive DCS, and single-cavity DCS. The single-cavity DCS system is constructed by two pulse sequences with a small repetition frequency difference from a single laser, and the two mode-locked pulse sequences share part or the whole laser cavity, which suppresses common-mode noise and maintains good coherence, making the system possess simple structure, high coherence, and easy integration. Given the advantages above, it has become one of the research hotspots in recent years.

A linear all-polarization-maintaining dual-wavelength mode-locked erbium-doped fiber laser is used in the experiment. The semiconductor saturable absorber (SESAM) is used to assist mode-locking, and the wavelength division multiplexer (WDM) with polarization-dependent loss is fused with the polarization-maintaining erbium-doped fiber to generate a periodic Lyot filter. By adjusting the pump power and the polarization state in the cavity, the laser can switch between single-wavelength and dual-wavelength mode-locked states.

When the pump power is increased to 60 mW, the laser begins to output continuous light near the central wavelength of 1560 nm. When the pump power is increased to 106 mW, and the wave plate is rotated to adjust the polarization state in the cavity, the laser begins to exhibit a mode-locked state. At this time, the mean output power of the fiber laser is 1.03 mW. In the single-wavelength mode-locked state, the central wavelength of the two pulse sequences is 1564.2 nm, the bandwidth of 3 dB is 2.8 nm, and there are obvious Kelly sidebands on both sides of the spectrum [Fig. 2(a)]. The repetition frequency is 22.81 MHz, the signal-to-noise ratio (SNR) is 52 dB, and the spectrum diagram in the 550 MHz range indicates that the mode-locked laser is stable at this time [Fig. 2(b)]. The two pulse sequences are spaced 43.8 ns apart, corresponding to the repetition rate [Fig. 2(c)]. The autocorrelation curve under Gaussian fitting is 1.4 ps [Fig. 2(d)]. Due to the presence of the Lyot filtering effect, when the pump power is increased to 176 mW, the single-wavelength mode-locking is switched to asynchronous dual-wavelength mode-locking by changing the polarization state in the cavity. The central wavelengths of the two asynchronous pulse sequences are 1563.0 and 1568.4 nm, and the 3 dB bandwidths are 1.6 and 1.3 nm, respectively [Fig. 3(a)]. The repetition frequencies of the two pulse sequences are 22.813252 and 22.813571 MHz, with a repetition frequency difference of 319 Hz, and the SNR is 60.2 and 57.1 dB, respectively [Fig. 3(b)]. The maximum spectral offset of the two center wavelengths in one hour is less than 0.3 nm, and the root-mean-square (RMS) value of power fluctuation is 0.21% [Figs. 3(c) and 3(d)]. Figures 4(a)-4(e) show the fragment diagrams of each time period of the oscilloscope under the asynchronous dual-wavelength mode-locked state. Asynchronous dual-wavelength mode-locking can be converted to synchronous one when the pump power is increased to 460 mW. The central wavelengths of the two pulse sequences are 1562.1 and 1567.3 nm, and the corresponding 3 dB bandwidths are 0.5 and 0.6 nm respectively [Fig. 5(a)]. The repetition frequency is 22.81353 MHz and the SNR is 56 dB [Fig. 5(b)]. A synchronous dual-wavelength oscilloscope trace diagram is shown in Figs. 5(c) and 5(d). To prove whether it is a synchronous dual-wavelength pulse, commercial filters are used to filter out components of different wavelengths. Oscilloscope trajectories of different wavelength components after filtering are shown in Figs. 6(a)-6(c). The maximum deviation of the central wavelength of both pulse sequences in one hour is less than 0.4 nm, and the RMS value of the power fluctuation is 0.17%, indicating that the mode-locked state is stable [Figs. 7(a) and 7(b)].

In summary, the experiment verifies the all-polarization-maintaining dual-wavelength mode-locked erbium-doped fiber laser based on SESAM. A periodic Lyot filtering effect is formed by combining a WDM with the polarization-maintaining erbium-doped fiber. By controlling the pump power and adjusting the polarization state in the cavity, asynchronous and synchronous dual-wavelength mode-locked states can be obtained. Because of the dispersion in the laser cavity, the mode-locked pulses with different center wavelengths have different repetition frequencies, and a repetition frequency difference of ~319 Hz can be formed between the two asynchronous dual-wavelength mode-locked pulses. The research results provide a new method for the realization of all-polarization-maintaining, repeatable, and stable dual-wavelength mode-locked lasers, which is of great significance in the application of DCS.