Novel multiwavelength Er³⁺-doped fiber laser with double-pass Mach-Zehnder comb filter

Compact and stable multiwavelength lasers (MWL) are attractive for wavelength division multiplexing (WDM) system. Recently, multiwavelength Er³⁺ -doped fiber lasers have been reported by several groups. In this paper, we propose and demonstrate a novel fiber MWL by inserting a double-pass Mach-Zehnder (M-Z) interferometer as a comb filter in the fiber laser cavity. The two output ports of the fiber M-Z device are spliced with a piece of Er³⁺ -doped fiber in between to provide the optical gain. One of the input ports is connected with a WDM fiber loop which totally reflects light at 1550 nm and transmits light at 980 nm, and the other input port is connected with a partially reflected fiber loop as the output reflector to the laser cavity. 10 lines lasing simultaneously are obtained when the Er³⁺ -doped fiber is immersed into liquid nitrogen. By controlling the optical path difference between the two arms in the M-Z device, the lasing peak wavelengths and their spacings can be adjusted to meet the ITU recommendation for WDM applications.