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 mu m 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 dispersioncompensating 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 ps 2 ) to net normal (0.11 ps 2 ). 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.