Objective Compared with the traditional laser, fiber laser has the advantages of simple structure, high stability, and small size, becoming the research focus of practical optical comb. The structure of fiber-based optical comb with nonlinear polarization rotation (NPR) mode-locking is simple, but the refractive index of non polarization-maintaining fiber is easily affected by vibration and temperature, and the long-term stability of mode-locked state and phase-locked state of the oscillator is poor. The optical frequency comb based on semiconductor saturable absorption mirror (SESAM) can improve the influence of environmental noise due to the fully polarization maintaining fiber structure and appropriate damping measures. However, SESAM has the risk of light-induced damage during long-term use. It has a relaxation time of several hundred femtoseconds to picoseconds, bringing additional phase noise and instability to the optical comb. Most of the reports about the long-term stable operation of the comb were obtained in the constant temperature laboratory. In this study, we report a compact optical frequency comb with all polarization maintaining fiber structure. The system had kept stable operation for a long time in the external environment of 10 ℃ temperature fluctuation, and the mixed gas detection experiment was completed. This paper provides a feasible method for realizing practical optical frequency comb.

Methods In the study, a mode-locked fiber laser with nonlinear amplifying loop mirror (NALM) was used as the seed source of the optical comb, which was placed in a sealed cavity to achieve isolation from the external environment. Then, the optical power amplification, pulse width compression, and spectrum broadening were studied by using cascaded erbium-doped fiber amplifier, single-mode polarization-maintaining fiber (PMF), and highly nonlinear fiber (HNLF). The broadened spectrum was injected into the f-2f self-referenced detector to detect the carrier envelope offset frequency (f₀) signal. Then, by precision controlling the local temperature of the mode-locked fiber oscillator, the length of a piezoelectric transducer, and the current of the pump diode, long-term repetition rate (f_r) and f₀ locking of the optical frequency comb was achieved. Finally, a high non-linear dispersion shifted fiber (HN-DSF) was connected to the comb application end to generate flat spectrum at 1350--1550 nm. Besides, a gas sample cell filled with ¹²CO and ¹²C₂H₂was used to exam the mixed gas detection experiment.

Results and Discussions The repetition rate of the mode-locked laser was 75.27 MHz, and the average power was 4.1 mW. The pulsed light was amplified to 196 mW by a cascaded erbium-doped fiber amplifier, and then the pulse width was compressed to less than 100 fs by a section of PMF fiber. The supercontinuum of the compressed pulse was realized by a segment of HNLF fiber from which the spectrum covered 1000--2200 nm [Fig. 2(a)]. The f-2f self-referenced detector detected f₀ signal with signal to noise ratio of 40 dB and line width of 5 kHz [Fig.2(b)--2(c)]. In long-term operation, the drifts of f_r and f₀ (24 h) in the open-loop were reduced from 6.2 kHz and 310 MHz to 0.51 kHz and 26.9 MHz, respectively [Fig. 4(a) and (b)]. The drifts were further reduced to 10 Hz and 700 kHz in the partially feed-back loop, which satisfied the servo capability of the phase-locked circuit of the system [Fig. 5(a) and (b)]. Finally, the standard deviations of f_r and f₀ after locking were 358 μHz and 248 mHz in 100 hours operation, respectively [Fig. 6(a)]. In the experiment of gas mixture detection based on the optical frequency comb, a flat spectrum from 1250--1650 nm was generated from a segment of HN-DSF fiber, covering the absorption spectra of various gas molecules including ¹²C₂H₂, ¹²CO, and H₂O [Fig. 2(d)]. The obtained ¹²C₂H₂ gas detection data was basically consistent with the simulation results of HITRAN database, and the standard deviation of their deviations was only 2.4% [Fig. 7(c)].

Conclusions An all polarization-maintaining fiber optical frequency comb for outdoor application is proposed. The size of the optical frequency comb is 330 mm×340 mm×65 mm. The frequency stability of the optical frequency comb system is about 4×10 ^-12 in 1 s, and the system can keep f_r and f₀ stable for a long time under the temperature fluctuation of (22±5) ℃. In addition, in the experiment of wide spectrum detection of mixed gas based on the optical comb, the gas detection data obtained is consistent with the simulation results of HITRAN database, indicating that the optical comb system can be rudimentarily used for outdoor applications such as spectral analysis.