High energy narrow-linewidth ytterbium-doped pulsed fiber amplifier

Abstract

Narrow-linewidth high-power fiber pulsed sources have many applications, such as laser remote sensing and spectroscopy. In this article, an all fiber 1064nm single-frequency pulsed laser in Master Oscillator Power Amplifier (MOPA) configuration and the corresponding experimental results are presented. The laser consists of four Polarization-Maintaining (PM) fiber amplification stages. In the final stage, 1.8m long large mode area (LMA) double-clad gain fiber with core diameter of 25μm and numerical aperture (NA) of 0.06 is utilized. In the system, considering the low power of the seed laser, a continuous-wave (cw) seed laser is externally modulated by an acousto-optic modulator (AOM). The modulated pulse repetition frequency (PRF) of the first and the second amplification stage are chosen to be 10KHz to minimize the Amplified-Spontaneous-Emission (ASE). Then another AOM is inserted to change the PRF from 10KHz to 100Hz, which also eliminates build-up of optical power emitted within the same bandwidth of the ASE filter. Besides, in order to prevent the pulse steepening in the amplification stage, the modulated pulse in the first AOM is pre-shaped and last pulsed shape with almost flat top are demonstrated. The pulse energy is increased by more than 80% through that special pulse shaped method. In the end, pulse energy of 100μJ with 500ns pulse duration and 100Hz repetition rate is obtained. Near diffraction limited beam quality (M² ~ 1.1) is also proved by coiling the fiber at a diameter of 10-cm.