Investigation of the tuning mechanism in a pump-induced tunable Lyot filter and its applications

An optically controlled tunable Lyot filter is presented, and the mechanism underlying its pump-induced wavelength tuning is illustrated. The filter employs a structure composed of spliced polarization-maintaining ytterbium-doped fiber (PM-YDF) and polarization-maintaining erbium-doped fiber (PM-EDF) with a 90^∘ slow-axis offset. Wavelength tuning is achieved by adjusting the injected pump power in the PM-YDF. The PM-EDF serves a dual function of expanding the filter's free spectral range (FSR) and acting as a saturable absorber (SA). A detailed analysis attributes the mechanism of pump-driven wavelength tuning in the filter to birefringence changes in the PM-YDF, caused by both thermal effects and electronic transitions. Experimental results demonstrate that the thermal effect serves as the primary cause of the birefringence variation, while the change induced by electronic transition is one order of magnitude smaller. Integration of the filter into the laser ring cavity enabled a single-frequency (SF) tunable laser with a tuning range from 1554.7 nm to 1561 nm and a linewidth of 315 Hz. The results show that the filter is an effective choice for wavelength tuning in laser systems.