High fidelity amplification of radially-polarized laser in few-mode polarization-maintaining fiber amplifier

A high-power 20/400-mu m few-mode polarization-maintaining (PM) fiber amplifier with a radially polarized output beam is firstly demonstrated and investigated. The reconstructed beam profiles and mode contents of the output laser were obtained using a numerical method. Experimentally, a home-made fiberized mode convertor was employed to convert the LP01 mode to be TM01 mode (radially polarized beam) with an average power of 2 W. In addition, the mode purity and polarization purity were 98.76%, and 98.5%, respectively. Based on a few-mode PM fiber amplifier, instead of the commonly used non-PM fiber amplifier, the radially polarized beam was scaled up to 192.1 W, while preserving the donut-shaped intensity distribution. The corresponding mode purity and polarization purity were measured to be approximately 97% and 98.2%, respectively, which indicates the polarization characteristic of the TM01 mode was preserved well. The beam quality factor (M-2) was measured to be similar to 2.2. Additionally, long-term stability in terms of both output power and polarization state were also verified experimentally. To the best our knowledge, this is the highest output power from a few-mode PM fiber amplifier laser system, while maintaining a high mode purity and a high polarization purity for radially polarized beam.