Switchable, Widely Tunable and Interval-Adjustable Multi-Wavelength Erbium-Doped Fiber Laser Based on Cascaded Filters

A novel individually switchable, widely tunable, and interval-adjustable multi-wavelength erbium-doped fiber laser (EDFL) is experimentally demonstrated in this article. It is based on the cascaded structure of SMF-TMF-SMF (STS) filter and Lyot filter with the assistance of nonlinear polarization rotation (NPR). The homemade STS filter is a comb filter that formed by splicing a section of two-mode fiber (TMF) and two segments of single-mode fiber (SMF) with core-offset structures. Compared with other multimode interferences, the spectrum of STS filter is more uniform. When inserted into the laser cavity, this comb filter functions as the wavelength selector with wavelength interval of ~0.8 nm. The insertion loss of the filter is only ~0.4 dB. By properly adjusting the polarization controllers (PCs) in the laser cavity, individually switchable wavelengths with different flexible lasing states can be obtained, such as tunable adjacent single-, dual-, triple-, quadruple-, and quintuple-wavelength outputs. The tuning range can up to 30 nm, from 1530 to 1560 nm. When the number of output wavelengths is 1~3, the tuning process can keep the step size of 0.8 or 1.6 nm. The non-adjacent dual- and triple-wavelength outputs, as well as adjacent sextuple- and septuple-wavelength outputs can also be achieved. The output characteristics, such as flexible switchability and tunability, are difficult to implement by other methods.