IEEE JOURNAL OF QUANTUM ELECTRONICS; VOL. 39, NO. 11, NOVEMBER 2003 1409 High-Gain Quantum-Dot Semiconductor Optical Amplifier for 1300 nm Zoltan Bakonyi, Hui Su, Member, IEEE, George Onishchukov, Luke F. Lester, Senior Member, IEEE, Allen L. Gray, Timothy C. Newell, and Andreas Tünnermann, Member, IEEE Abstract—Using an AlGaAs–GaAs waveguide structure with a six-stack InAs–InGaAs “dots-in-a-well” (DWELL) gain region having an aggregate dot density of approximately cm , an optical gain of 18 dB at 1300 nm has been obtained in a 2.4-mm-long amplifier at 100-mA pump current. The optical bandwidth is 50 nm, and the output saturation power is 9 dBm. The dependence of the amplifier parameters on the pump current and the gain recovery dynamics has also been studied. Index Terms—Quantum dots (QDs), quantum-dot optical am- plifiers, semiconductor lasers, semiconductor optical amplifiers (SOAs). I. INTRODUCTION R ECENT progress in quantum dot (QD) technology has led to the development of unique QD laser diodes with low threshold current, high output power, high temperature sta- bility, high modulation bandwidth, and low chirp [1]–[6]. Also, QD semiconductor optical amplifiers (QD-SOAs) have very promising features that could provide breakthrough improve- ment of SOA performance in optical networks. There is more than an order of magnitude decrease of the gain recovery time [7]–[13] and of the linewidth enhancement factor [14]–[16], resulting in reduction of signal distortions caused by cross-gain and cross-phase modulation. Moreover, similar to QD lasers, QD-SOAs could have a lower power consumption and a higher temperature stability than common multiple-quantum-well (MQW) and bulk SOAs. Theoretical simulations of QD-SOAs [17] have shown that the QD-SOA could achieve very good performance: saturation power of dBm, gain above 45 dB, and noise figure of about 4 dB at the same time. Last but not least, the QD technology allows the mixing of different sizes of dots in one structure to realize extremely broad-band amplifiers ( 300 nm) with inhomogeneous broadening [14], [18]–[20].This feature would be especially promising for use in broad-band wavelength division multiplexing (WDM) systems. Manuscript received March 12, 2003; revised June 24, 2003. This work was supported in part by the German Research Council under Grant DFG LE 755/7, the U.S. Department of Commerce under Grant BS123456, and the National Science Foundation under Grant ECS-0084498. Z. Bakonyi, G. Onishchukov, and A. Tünnermann are with the Institute of Applied Physics, Friedrich-Schiller University of Jena, D-07743 Germany (e-mail: zbakonyi@iap.uni.de; george.onishchukov@uni-jena.de). H. Su and L. F. Lester are with the Center for High Technology Mate- rials, University of New Mexico, Albuquerque, NM 87106 USA (e-mail: luke@chtm.unm.edu). A. L. Gray and T. C. Newell are with Zia Laser, Inc., Albuquerque, NM 87106 USA (e-mail: info@zialaser.com). Digital Object Identifier 10.1109/JQE.2003.818306 Fig. 1. Atomic force micrograph image of the InAs–InGaAs DWELL active region showing an average dot density of cm . In this paper, we report on such parameters of a 1300-nm QD-SOA as gain, saturation power, linewidth enhancement factor, and noise figure. The dependence of some amplifier parameters on pump current and the gain recovery dynamics has been also studied. II. RESULTS A six-stack InAs–InGaAs “dots-in-a-well” (DWELL) ampli- fier structure was grown by solid source molecular beam epitaxy (MBE) on a (001) GaAs substrate using conditions and design criteria similar to those published previously [21], [22], the only exception being the number of QD layers. As shown in Fig. 1, the average dot density in a layer is about cm . This is among the highest values that have been achieved for GaAs-based QD optoelectronic devices emitting at greater than 1300 nm [23]–[26]. The amplifier was fabricated using tilted ridge waveguide geometry. The 4- m ridge was formed with inductively coupled plasma etching using BCl to remove part of AlGaAs cladding. Self-alignment of the ridge was achieved by protecting it with SiNx during a wet oxidation of the re- maining AlGaAs cladding layer. Ti/Pt/Au was evaporated for the metal contact to the p GaAs cap layer. The substrate was lapped and polished to a thickness of 100 m and AuGe/Ni/Au was deposited for the n-type contact. The waveguide length is 2.4 mm. The cleaved facets with a tilt angle of 6.8 were left un- coated and provided a sufficiently low level of back reflection 0018-9197/03$17.00 © 2003 IEEE Authorized licensed use limited to: UNIVERSITY OF NEW MEXICO. Downloaded on June 29, 2009 at 11:29 from IEEE Xplore. Restrictions apply.