Ultrashort Pulse Generation with Semiconductor Modelocked Lasers Using Saturable Absorbers Based on Intersubband Transit
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Ultrashort Pulse Generation with Semiconductor Modelocked Lasers Using Saturable Absorbers Based on Intersubband Transitions in GaN/AlGaN Quantum Wells Faisal R. Ahmad, Paul George, Jahan Dawlaty, Fahan Rana and William J. Shaff Department of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853, U.S.A. ABSTRACT We present a novel scheme for generating high energy ultrashort pulses in modelocked semiconductor lasers using a fast saturable absorber based upon intersubband transitions (ISBT) in GaN/AlGaN superlattice. The fast electron relaxation time (~100-300 fs) in this saturable absorber enables the generation of stable sub-100 fs pulses at the communication wavelength (1.55µm) with high pulse energies (5-10 pJ). INTRODUCTION Semiconductor modelocked lasers are compact, electrically pumped sources of optical pulses that can be used in various photonic and telecommunication applications. To date, semiconductor modelocked lasers have been used to generate picosecond pulses with energies usually not exceeding more than 0.5 pJ. Conventional monolithic semiconductor modelocked lasers that generate pulses at the communication wavelength use interband transitions in InGaAsP/InP multiple quantum well for both the gain medium as well as the saturable absorber. The main limitation of these lasers is the long relaxation time of the absorber, which is typically 1ns. The use of a fast saturable absorber with a fast relaxation time (~100-300 fs) will enable ultrashort, high energy pulse generation by providing stability at high pulse energies as well as enhanced pulse shaping. Such an absorber can be realized by utilizing the ISBT in GaN/AlGaN quantum wells. At the communication wavelength, this has a significantly faster relaxation time compared to interband based semiconductor saturable absorbers [1, 2]. In this paper the term modelocking by fast saturable absorption (FSA) is extended to describe the regime of modelocking in which the relaxation time of the absorber is on the order of the pulse duration. LASER DESIGN The key for producing ultrashort pulses with large pulse energies is to have gain medium with a large bandwidth and a saturable absorber with a fast relaxation time. The homogeneously broadened gain bandwidth of a typical quantum well active region is limited to around 20 nm. By engineering the quantum well structure, up to 300 nm of inhomogeneously broadened bandwidth can be achieved [3]. Fast saturable absorber based modelocking offers better stability at higher pulse energies compared to slow saturable absorbers [4, 5, 6]. A physical realization of a saturable absorber which has a relaxation time 100-300 fs is based on the ISBT in GaN/AlGaN multiple quantum well. The proposed device will comprise of an electrically pumped gain section coupled to the fast saturable absorber section based on the ISBT. In this aspect, the proposed device is different from earlier monolithic devices in which the active region and the saturable absorber component are different regions along the same wave
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