All-fiber second-order optical vortex laser based on a single strong modulated long-period grating

We here reported a single wavelength second order vortex mode lasing in optical fiber system. For this, we implemented a high efficient mode conversion between the fundamental guiding mode(LP01 mode) and the second azimuthal order guiding mode(e.g.LP21 mode), and an appropriate mode modulation that converted the mode from LP21 mode to the second order optical vortex(OV) mode. The high efficient mode conversion was realized by utilizing a single intra-cavity strong modulated long-period fiber grating (SM-LPFG) that enables a higher than 20 dB(>99%) direct mode conversion efficiency over a bandwidth range of 3 nm. And the mode modulation was fulfilled with high-order mode polarization controller that control the polarization state of the resonating mode to transform the LP21 mode to the 2nd order OV mode. Experiment results showed that a single wavelength of 1542.88 nm of the 2nd order OV mode laser was obtained with a 3 dB bandwidth 0.22 nm and over 27 dB signal to noise ratio. As far as we know, this was the first report on the implementation of the 2nd order OV mode lasing in optical fiber system by only one intra-cavity SM-LPFG in the laser cavity.