Nonlinearity enhances operation of fiber ring laser

Parametric four-wave mixing in nonlinear fiber allows continuous wave or mode-locked fiber ring lasers to achieve ultrastable, superior performance for high-power multiwavelength output. Fiber lasers, especially multiwavelength erbium-doped fiber lasers (MW-EDFLs), have attracted considerable interest for po-tential applications in optical test and measurement, and optical wavelength-division-multiplexing communication and sensing systems.1 Compared with compact semiconductor-based lasers, EDFLs are competitive because of their all-fiber structure aswell as their capacity to provide high power and ultrashort pulse width.2 They can be used in applications that require multiple wavelength sources, with small, equal wavelength spacing, a large number of peaks within a broad band, and high output uniformity across the channels. These requirements pose a very challenging task for building a cost-effective multiwavelength EDFL for continuous wave (CW) or pulsed operation. Previously, due to the homogeneously broadened gain prop-erty,manyMW-EDFLsweredevelopedwithwavelength spacing larger than homogeneous linewidth (∼3.5nm) to overcome gain competition.AlthoughaMW-EDFLwith16wavelengthshasbeen accomplishedby cooling an erbium-dopedfiber (EDF) to 77Kus-ing liquidnitrogen, the increasedcomplexity andcostmake it im-practical for real applications. Many different approaches have also been explored for developing MW-EDFLs, including using polarization or spatial hole burning, using independent gainme-dia, frequency shifting, and phasemodulation.3–7 Here, we demonstrate a novel room-temperature-operated MW-EDFL with wavelength spacing less than homogeneous broadening linewidth, based on interchannel four-wave mixing (FWM).8 The gain-clamping effect in EDF is compensated by the parametric FWMbetween multiwavelength channels in a highly nonlinear fiber section that is inserted into the fiber ring cavity. First, we implemented a stable CW MW-EDFL based on highly nonlinear photonic crystal fiber (HNL-PCF) and sampled-fiber Bragg grating (SFBG), as described in Figure 1.W