Wavelength Tunable Mode-Locked Quantum-Dot Laser Jimyung Kim a , Myoung-Taek Choi, Wangkuen Lee, and Peter. J. Delfyett b College of Optics and Photonics/CREOL & FPCE, University of Central Florida, 4000 Central Florida Blvd., Orlando, Florida 32816, USA ABSTRACT We study the characteristics of wavelength tunable quantum-dot mode-locked lasers using a curved two-section device, external grating, and optical bandpass filter. Wide wavelength tunability is demonstrated due to the fact that the center wavelength of mode-locking is extended to excited state transitions as well as ground state transitions of the quantum- dot gain media. Keywords: Mode-locked laser, Quantum-dot, Ground-state, Excited-state. 1. INTRODUCTION Quantum Dot (QD) lasers have been intensively studied for their attractive characteristics, i.e. low threshold current, temperature insensitivity, feedback insensitivity, low linewidth enhancement factor (LEF), etc [1-3]. The broad gain spectrum due to the inherent dot size fluctuation during the growth process of self-assembled QDs [4] is very attractive in several applications, such as optical amplifiers with wide gain bandwdith, lasers with a wide tuning range for wavelength-division-mulitplexing (WDM) and spectroscopy, and mode-locked lasers for short pulse generation. Monolithic mode-locked lasers have been demonstrated by several research groups [5-7]. External cavity mode-locked lasers (ECML) have many advantages over monolithic mode-locked lasers, such as wavelength and repetition rate tunability, and flexibility that allows the modification of the cavity design with other optical components. In this study we investigate the characteristics of tunable mode-locked lasers by using a curved two-section mode-locked laser, external gratings, and optical bandpass filters. The overall tuning wavelength range is greatly extended, over 100 nm, because of the contribution from excited state (ES) transitions, as well as normal ground state (GS) transitions. The output pulse train characteristics of GS and ES mode-locking were experimentally measured, including the optical spectrum, RF spectrum and intensity autocorrelation. 2. DEVICE QD two section devices and a QD Semiconductor Optical Amplifier (SOA) are fabricated from a QD wafer using a standard lithography and wet etching methods. The wafer has the active region which has 10 layers of self-assembled InAs/GaAs quantum dots, covered with 5 nm In 0.15 Ga 0.85 As, grown by molecular beam epitaxy. The QD two-section device consists of both gain and saturable absorber (SA) sections. The device length and waveguide width are 2 mm and 5 um, respectively. The length of the SA section is 250 µm. The gain section is curved and terminated at an angle of 7° to the cleaved facet to minimize the back reflection from the facet. The Figure 1 shows the waveguide structure and contact pad. The QD SOA is used to amplify the energy of the pulses generated from the laser oscillator. The waveguide of the QD SOA is 7° tilted. Various SOA lengths have been tested, since an appropriate length of SOA is required for sufficient gain for both GS and ES. It was found that a 1.8 mm SOA worked well for a broad range of input wavelengths. a jmkim@creol.ucf.edu b delfyett@creol.ucf.edu Enabling Photonics Technologies for Defense, Security, and Aerospace Apps. II, edited by Michael J. Hayduk, Andrew R. Pirich, Eric J. Donkor, Peter J. Delfyett, Jr., Proc. of SPIE Vol. 6243, 62430M, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.673753 Proc. of SPIE Vol. 6243 62430M-1