IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 8, AUGUST 2003 1023 Amplification of Femtosecond Pulses Over by 18 dB in a Quantum-Dot Semiconductor Optical Amplifier E. U. Rafailov, P. Loza-Alvarez, W. Sibbett, G. S. Sokolovskii, D. A. Livshits, A. E. Zhukov, and V. M. Ustinov Abstract—We demonstrate amplification ( 18 dB) of 200-fs pulses in a quantum-dot (QD) semiconductor amplifier. Our measurements have shown that such QD devices can provide amplification of femtosecond pulses over a spectral range that exceeds 100 nm. Index Terms—Femtosecond pulses, quantum dots (QDs), semi- conductor amplifier. I. INTRODUCTION S EMICONDUCTOR optical amplifiers (SOAs), both bulk and quantum-well based, are currently of significant practical interest in data communication applications because of their small size, high gain, and compatible optoelectronic characteristics. Recently, efficient room-temperature emission from quantum-dot (QD) lasers operating at 1.3- m wavelength has been achieved [1] and significant progress in the fabrication of QD lasers using metal–organic chemical vapor deposition and molecular beam epitaxy (MBE) techniques has allowed a number of groups to investigate devices such as SOAs based on QD structures [2], [3]. These QD SOAs are becoming of primary interest in the amplification of femtosecond pulses because one of the key parameters that influences the emission spectra and the optical gain in QD devices is the spectral extent broadening associated with the distribution of dot sizes. Thus, the study of the transmission characteristics of femtosecond pulses in QD SOA structures and devices is of key importance. In this letter, we present experimental data that demonstrate optical amplification exceeding 18 dB where measurements were made for incident pulse energies in the range from 0.01 to 3 pJ. II. EXPERIMENTAL RESULTS The QD structure studied was grown on an n+ GaAs (100) substrate by MBE. The laser active region consists of three planes of self-organized QDs separated by 33-nm-thick GaAs spacer layers. Each QD plane was formed by deposition of 2.5 monolayers of InAs followed by 5.5-nm-thick InGaAs quantum well ( 15%). Both were grown at 485 C. The density of QDs per layer was 5 10 cm and the sizes were estimated to be 5 12 12 nm. The n- and p-type Manuscript received January 27, 2003; revised April 22, 2003. E. U. Rafailov, P. Loza-Alvarez, and W. Sibbett are with the School of Physics and Astronomy, University of St. Andrews, St. Andrews Fife KY16 9SS, U.K. (er8@st-andrews.ac.uk). G. S. Sokolovskii, D. A. Livshits, A. E. Zhukov, and V. M. Ustinov are with the Ioffe Physico-Technical Institute, St. Petersburg 194021, Russia. Digital Object Identifier 10.1109/LPT.2003.815362 Fig. 1. Simplified measurement scheme. Al Ga As cladding layers were doped with Si and Be, respectively, with a carrier concentration of 5 10 cm . The active region was placed in the centre of a short-period Al Ga As nm GaAs nm superlattice which acts as a waveguiding layer. The total thickness of the laser waveguide was about 0.35 m. The structure was terminated by a 0.6- m thickness GaAs contact layer that was heavily doped with Be. The 3-mm-long ridge (7 m) waveguide laser which oper- ates at 1.25 m was mounted p-side down on a Cu-heatsink and was tested at a constant room temperature of 20 C. The threshold current was 70 mA, and typical optical powers of 40 mW centered at 1257 nm were generated from the front facet at a pump current 450 mA. Following these initial tests, both facets were antireflection (AR) coated to reduce the reflec- tivity to less than 0.5%. The continuous-wave (CW) ampli- fied spontaneous emission (ASE) from this amplifier device was centerd at 1250 nm. A simplified optical scheme of our experimental setup is illustrated in Fig. 1. The optical parametric oscillator (OPO) that we used was synchronously pumped by a Kerr-lens mode-locked Ti : sapphire laser operating at a repetition rate of 84 MHz and delivering 1.5 W of average output power. These pump laser pulses, which were centered at 820 nm and had typ- ical durations around 110 fs, were focused with a 50-mm focal length lens into a periodically-poled lithium niobate crystal that had a grating period of 21.5 m. The OPO cavity consisted of two curved broad-bandwidth high-reflectivity mirrors and one flat output coupler for selected wavelengths around 1.3 m. The OPO signal wavelengths were readily tunable from 1.16 to 1.4 m [4]. In the spectral range of 1.18–1.28 m that concerns this assessment work, the pulse durations were 200 fs. The excitation pulses from the OPO were modulated using a chopper and coupled into the QD waveguide device, using a high numerical aperture ( 60) aspheric lens. After propagation 1041-1135/03$17.00 © 2003 IEEE