Mode structure evolution of a modeless multiwavelength Raman fiber laser

Multiwavelength Raman fiber lasers (MRFLs) are used in a number of applications. A longitudinal mode structure of light in MRFL is directly related to the temporal properties of laser radiation. In this Letter, we demonstrate a modeless multiwavelength Raman fiber laser based on a low-reflection Sagnac loop mirror (SLM). The MRFL exhibits a discrete multi-longitudinal mode structure at a low pump power level. The difference in longitudinal mode spacing for various polarization states of the MRFL leads to the appearance of sub-peak discreteness and recombination at different frequencies in the RF spectrum. As the pump power increases, the discrete longitudinal modes of each polarization mode gradually broaden until adjacent modes overlap and the MRFL undergoes a decoherence process. The low-reflection SLM effectively facilitates broadening longitudinal modes to a modeless state. Modeless MRFL exhibits highly stable temporal intensity distribution.