Tunable Active Dual-Coupler Ring Based Compound-Cavity Filter for Single-Longitudinal-Mode Fiber Lasers

This paper introduces a novel active dual-coupler ring based compound-cavity (ADCR-CC) filter, specifically designed for use in single-longitudinal-mode (SLM) fiber lasers, which features adjustable full-width at half-maximum (FWHM) and transmittance. A comprehensive methodology for the design, fabrication, and characterization of the ADCR-CC filter is proposed as an innovative approach to fabricating actively controlled compound-cavity filters for SLM operations in fiber lasers. Both theoretical analysis and experimental results demonstrate that adjusting the pump power of the ADCR-CC filter enables control over the filter's FWHM and transmittance. Optimizing the pump power allows the ADCR-CC filter to simultaneously achieve a FWHM below 7 MHz and transmittance exceeding 0.99. To illustrate the longitudinal-mode selectivity afforded by the ADCR-CC filter, we constructed a dual-wavelength-switchable erbium-doped fiber laser (DWS-EDFL) leveraging the ADCR-CC filter and a high-birefringence fiber Bragg grating (HB-FBG). The DWS-EDFL can be switched among three lasing states, including two single-wavelength operations and one dual-wavelength operation, all with high stability. In single-wavelength mode, each output exhibits an optical signal-to-noise ratio of >78 dB, a linewidth of <720 Hz, a relative intensity noise of ≤−155.1 dB/Hz@>3 MHz, an output power fluctuation of ≤0.32 dB, and superior polarization characteristics.