IEEE PHOTONICS TECHNOLOGY LETrERS, VOL. 6. NO. 9, SEPTEMBER 1994 I067 Mode-Locked Operation of a Master Oscillator Power Amplifier zy Alan Mar, Roger Helkey, John Bowers, David Mehuys, and David Welch zyxw Abstract- We report the first mode-locked operation of a tapered amplifier MOPA to generate short optical pulses with record high average powers and pulse energies. The MOPA is constructed using a discrete semiconductor oscillator and amplifier operating at 940 nm wavelength. Pulses of 4.2 ps duration are generated with average powers of 2% mW and peak powers of 28.1 W. The energy of these pulses was 118 pJ, corresponding to energies more than an order magnitude greater than the gain saturation energy in the mode-locked laser itself, and also corresponds to energies internal in the amplifier in excess of the gain saturation energy. Although the amplifier saturation energy limits the pulse energy obtainable in a mode-locked laser, the MOPA configuration demonstrates that generation of pulses with energies in excess of the amplifier saturation energy is feasible in a post-amplification stage. I. INTRODUCTION ODE-LOCKED semiconductor lasers have drawn con- M siderable attention as compact, reliable, and relatively inexpensive sources of short optical pulses. Advances in the design of such lasers have resulted in vast improvements in pulsewidth and noise performance, at a very wide range of repetition rates. An attractive application for these lasers would be to serve as alternatives for large benchtop laser systems such as dye lasers and solid-state lasers. However, mode-locked semiconductor lasers have not yet approached the performance of such systems in terms of output power. Conventional mode-locked semiconductor lasers operate with average output powers of typically a couple of milliwatts. This output power has been improved through the use of arrayed lasers zyxwvutsrqpo [ 1 1, and of post-amplification and compression to achieve high peak powers zyxwvutsrqp [2]. In this letter we describe the use of a tapered amplifier MOPA using a mode-locked master oscillator source. Tapered amplifier MOPAs have been demonstrated to be capable of multi-Watt CW powers by using Ti:Sapphire injection 131, by external diode injection 141, and also by using a monolithically integrated master oscillator zyxwvut [5]. The gain in such amplifiers is maximized by minimizing the parasitic gain depletion by amplified spontaneous emission (ASE) noise, especially at the input end of the amplifier. The power output and pulse energy from a mode-locked laser with an intra-waveguide saturable absorber is limited by Manuscript received February 9, 1994; revised May 9, 1994. zyxwvutsrq A. Mar, R. Helkey, and J. Bowers are with the Department of Computer and Elecmcal Engineering, University of California, Santa Barbara, CA USA. D. Mehus and D. Welch are. with Spectra Diode Laboratories, San Jose, CA USA. IEEE Log Number 94043 1 1. the saturation energy, where A is the active region cross-section, zyx hv the photon energy, r the confinement factor, and zyxw dgldn the differential gain. At large pulse energies compared to ESat, pulse broad- ening due to gain saturation in the amplifier and reduced pulse shaping in the absorber causes ineffective net pulse shaping each round trip in the laser, preventing mode-locking at such energies 161. The use of a post-amplification stage provides the advantage of allowing the mode-locked master oscillator to be independently optimized (at lower pluse energies) from the power amplification stage. In addition, tapered single- pass amplifiers have the advantage of increased pulse energies bzcause the saturation energy can be made relatively large ai the flared output end of the amplifier. By expanding the g.iin cross-section area along the length of the amplifier, as the amplified power grows, a more uniform power density and degree of gain saturation is maintained throughout the amplifier. The spectral and temporal distortion effects of gain saturation are also less deleterious in a single-pass post- amplification stage, as opposed to the effect in the mode- lccked oscillator, where pulse evolution occurs over many round trips. 11. EXPERIMENTAL SET-UP -4 schematic diagram of the mode-locked MOPA is shown in Fig. 1. An external cavity two-section laser emitting at 940 nin is used as the master oscillator. The active region consisted 01 three 8 nm 1110 zGa0 8As quantum wells separated by 10 nrn GaAs barriers, with A10 2Gao 8As separate confinement regions on a GaAs substrate. Passive mode-locking is initiated b! biasing the short section of this laser below transparency, forming a center of saturable absorption. The overall device length was 650 pm, with an absorber section length of 70 pin and the balance of the device used for gain. Details on the characteristics of such devices have been described previously [6]. The use of a 6 cm length external cavity results in a mode-locked pulse repetition rate of 2.5 GHz. Tlre amplifier employs a strained InGaAs quantum well active region, and has a 4 pm wide single-mode input waveguide which expands within the device to 130 pm width at the output facet. The amplifier is mounted p-side down on a copper heatsink for CW operation, and both the input and output facets of the amplifier are AR coated. Two AR- coated lenses are used to image the output of the master 1041-1135/94$04.00 0 1994 IEEE