Pico-second vortex output from a large-mode area fiber amplifier

Abstract

High intense optical vortex pulses, exhibiting an orbital angular momentum, have potentially to be applied in various fields, such as optical tweezers, laser micro-fabrication, and laser acceleration. Several researchers have successfully demonstrated ways to produce vortex outputs through mode-conversion in an optical fiber, including a stressed optical fiber as well as a hollow fiber. However, they used mostly a passive fiber as well as a continuous-wave laser. In this paper, we present the production of intense pico-second vortex pulses by selectively coupling a pico-second master laser into LP 11 modes in a stressed large mode-area fiber amplifier with an off-axis fiber injection method, for the first time.The experimental setup is shown in Fig. 1. A continuous-wave mode-locked Nd:Gd 0.6 Y 0.4 VO 4 laser with a pulse-width of 4.5 ps and a pulse repetition frequency of 150 MHz was used for the master laser, and its output power was 100 mW. The master laser output was off-axially injected to a large-mode-area Yb doped double-clad fiber (core diameter, φ30 µm, NA 0.06; clad diameter, 400 µm, NA 0.46) amplifier, thereby yielding highly efficient coupling into two orthogonal LP 11 modes. Stress was appropriately applied along a specific direction, so that the two orthogonal LP 11 modes were 90° out of phase at the end of the fiber. The output from the fiber was then converted to a circularly annular beam. The fiber amplifier was also pumped by a 30 W fiber-coupled 975 nm laser diode. The output power reached up to 4.4 W at the maximum pump power, corresponding to a peak power of ~6 kW (Fig. 2). To confirm that the annular output exhibited a phase singularity, we also analyzed interferometric fringes formed by the laser output beam and its laterally sheared copy. Then, a branch like a fork, due to a phase singularity, was observed (Fig. 2).In conclusion, we have produced intense pico-second vortex pulses by selectively coupling a pico-second master laser into the LP 11 mode in a stressed large mode-area fiber amplifier with an off-axis fiber coupling method. In particular, this method is capable of directly producing amplified intense pico-second vortex pulses from a simple fiber amplifier without any phase elements.