Experimental study of MMI structures in a switchable continuous-wave thulium-doped all-fiber laser

Switchable multi-wavelength laser emission from a thulium-doped all-fiber laser is reported by implementing a tapered and a non-tapered multi-modal interference (MMI) filters. The MMI structure relies on a coreless optical fiber spliced in between two single-mode optical fibers. For the non-tapered case, a minimum insertion loss of 12.60 dB is achieved around the 2-μm region, from which stable generation of commutable dual-wavelength emission at 1986.34 nm and 2017.38 nm is obtained. On the other hand, the tapered MMI structure performs a minimum insertion loss of 8.74 dB at the 2-μm region, allowing a stable triple-wavelength emission at 1995.4 nm, 2013.3 nm, and 2038.3 nm. In addition, commutable dual-wavelength emission was also obtained at 1997.9 nm and 2032.1 nm. The generated laser lines perform bandwidths of around 50 pm, low peak spectral power fluctuations and signal-to-noise ratio of 50 dB.