Wavelength-Switchable Fiber Laser Based on Long-Period Fiber Grating Written on Polarization-Maintaining Photonic Crystal Fiber

摘要

Here we propose a wavelength-switchable erbium-doped fiber ring laser using a temperature-insensitive spectral polarization-dependent loss (PDL) element and two fiber Bragg gratings (FBGs). The fiber PDL element was fabricated by inscribing a long-period grating (LPG) on polarization-maintaining photonic crystal fiber (PMPCF) with a 10.6 mu m CO2 laser. The LPG fabricated on PMPCF, referred to as PMPCF-LPG, has the characteristics of a fiber polarizer at two specific wavelengths due to the birefringence of PMPCF and the co-directional mode coupling of the LPG. The two wavelengths at which the fabricated PMPCF-LPG acts as a polarizer are two resonance wavelengths (similar to 1528.58 and similar to 1555.90 nm) of the PMPCF-LPG, obtained for orthogonal input polarization states. By considering these two resonance wavelengths of the PMPCF-LPG, the Bragg wavelengths of two FBGs, which determine lasing wavelengths in our wavelength-switchable laser, were selected as similar to 1527.71 and similar to 1554.74 nm. As the temperature sensitivity of the PMPCF birefringence is 30 times lower than that of the birefringence of conventional polarization-maintaining fiber (PMF), the fabricated PMPCF-LPG could facilitate more stable switching operation between the two lasing wavelengths in comparison with a previous fiber laser employing an LPG inscribed on conventional PMF as a wavelength-switching filter. The lasing wavelengths of our laser could be switched by controlling input polarization of the PMPCF-LPG with a polarization controller, and temperature-insensitive wavelength switching operation was experimentally demonstrated over a temperature range of 25-100 degrees C.