Abstract

Fiber based master oscillator power amplifier (MOPA) systems generating nanosecond to microsecond pulses with variable pulse shape, duration, and repetition frequency are interesting for a wide range of applications due to their unmatched adaptability. These systems, based on a pulsed diode laser serving as the seed source, have so far demonstrated up to 27mJ pulse energy and up to 2.7MW peak power [1]. Additionally, it was recently shown that there exists an analytical solution for pre-compensating the pulse distortions (caused by gain saturation) accumulated in the amplifier chain for arbitrary pulse shapes [2]. All this transforms fiber integrated MOPA systems in extremely versatile devices for science and industry. Most MOPA systems are seeded by Fabry-Perot diode laser seed sources driven by an electronic arbitrary pulse shape generator. Unfortunately, these seed sources have a tendency to emit a spectrum having randomly occurring features of very high spectral brightness due to the low number of longitudinal modes emitted from the short laser cavity. A typical output spectrum of such a pulsed fabry perot diode laser investigated in our lab is shown in fig. 1. Features like the marked peak in the spectrum lead to a low threshold for stimulated Brillouin scattering (SBS), which is one of the main challenges for further power scaling of fiber MOPA systems employing pulsed Fabry-Perot diode lasers as the seed source [3,4]. In this contribution we demonstrate, for the first time to the best of our knowledge, a novel approach for a diode seeded fiber MOPA system requiring neither external modulators nor external resonators to solve the problem of the few longitudinal modes of the seed source. This approach employs a pulsed superluminescence diode as the seed source. A superluminescence diode (SLD) is a diode laser with the emitter facets antireflection coated and / or tilted. This ensures that over a large span of the pump current the SLD will not reach the laser threshold, leading to a smooth output spectrum free of any longitudinal modes. The bandwidth of the SLD depends on the gain material, the length of the gain region and gain narrowing. The very low finesse of the cavity of the SLD results in a photon cavity lifetime approaching the single pass time. A semiconductor and package design optimized for low parasitic capacity should allow for rise times in the picosecond regime. The output spectrum in pulsed operation is independent of the pulse shape, duration and repetition frequency (Fig. 2). The SLD output (>150mW peak power) was amplified with a double stage fiber amplifier to an average power of 1W. Arbitrary pulse shapes down to 10ns duration could be achieved without the observation of SBS. Pre-compensation of the pulse distortion caused by gain saturation in the amplifier chain was demonstrated, and several output pulse shapes are shown in fig. 3.

© 2011 Optical Society of America