Four-wavelength-switchable SLM fiber laser with sub-kHz linewidth using superimposed high-birefringence FBG and dual-coupler ring based compound-cavity filter

We propose and demonstrate a four-wavelength-switchable erbium-doped fiber laser (4WS-EDFL) with a four-channel superimposed high-birefringence fiber Bragg grating (SI-HBFBG) and a dual-coupler ring based compound-cavity (DCR-CC) filter. Both for the first time, a SI-HBFBG as a four-channel reflective filter is used in a multi-wavelength switchable fiber laser to define wavelength channels and a DCR-CC filter is used to select a single mode from dense longitudinal-modes in a fiber laser. We present in detail how to design, fabricate, and characterize the DCR-CC filter with both theoretical analysis and experimental results, which we believe is the first systematic approach for making a compound-cavity based filter used for selecting single-longitudinal mode (SLM) in a fiber laser. The enhanced polarization hole burning effect in a 2.9 m long erbium-doped fiber, coiled inside a three-loop polarization controller, and the polarization-mismatch-induced losses are introduced into the laser cavity to achieve wavelength-switching operations. We show that the 4WS-EDFL can be switched among fifteen lasing states, including four single-wavelength operations, six dual-wavelength operations, four three-wavelength operations and one four-wavelength operation, all with high stability. For demonstration, in switchable single-wavelength operations, the four SLM lasing outputs measured are all with an optical signal to noise ratio of >80 dB, a linewidth of <700 Hz, a relative intensity noise of ≤-156.7 dB/Hz at frequencies over 3 MHz, an output power fluctuation of ≤0.555 dB and excellent polarization characteristics.