Dual-comb operation in an all-polarization-maintaining ultrafast holmium-doped fiber laser

We present a dual-comb fiber laser in the wavelength range of 2.1 μm from an all-polarization-maintaining holmium-doped fiber laser based on a reflective nonlinear amplifying loop mirror (Figure-9) scheme. The dynamics of the laser, where net cavity dispersion is controlled by varying the length of the intra-cavity dispersion-compensating fiber (DCF), has been fully investigated. The laser is found capable of mode-locking with a net cavity dispersion value ranging from net anomalous (-0.19 ps2) to net normal (0.11 ps2). More interestingly, without any intentional design, dual-comb operation with a relatively small offset frequency, down to a few Hertz, is observed in the system. The dual-comb operation is believed to arise from direction multiplexing in the loop cavity and polarization multiplexing in the linear arm, and is likely to be a generic solution for Figure-9 lasers. Such a dual-comb system with a characteristic small frequency difference can provide sample interval with higher precision in asynchronous optical sampling (ASOP) applications.