629 IEEE PHOTONICS TECHNOLOGY LEITERS, VOL zyxwvutsrqp 7. NO 6, JUNE 1995 zyxwvutsrqp 20 Gbit/s High-Performance Integrated MQW TANDEM Modulators and Amplifier for Soliton Generation and Coding N. Souli, F. Devaux, A. Ramdane, Ph. Krauz, A. Ougazzaden, F. Huet, M. zyxw Cad, Y. Sorel, J. F. Kerdiles, M. Henry, zyxwvutsrqp G. Aubin, E. Jeanney, T. Montallant, J. Moulu, B. Nortier, and J. B. Thomine Abstract- We report on the integration of two InGaAsP- InGaAsP MQW electroabsorption ridge modulators and an am- plifier on the same active layer. The two modulatorsare separated by a 500-pm ridge waveguide optical amplifier, in order to prevent electrical crosstalk, and to compensate for optical losses. The first modulator is used as a soliton generator, and the second codes the generated pulses. The amplifier showed 8.2-dB gain for 120-mA injected current. This allowed a low fiber to fiber insertion loss of 9 dB for the tandem. We present here for the first time to our knowledge, a 20 Gbit/s operation for a tandem- amplifier device, together with transmission experiments. I. INTRODUCTION S a consequence of the growing interest for 1.55-pm A optical soliton transmission, many laboratories are inves- tigating how to generate and code these optical pulses in a sta- ble and reliable manner. Electro-absorption (EA) modulators have been demonstrated as zero-jitter, stable and frequency flexible high speed soliton sources [l], by simply driving them with a sinusoidal signal [2]. The integration of a second modulator should allow to code the pulses. However, the fiber to fiber insertion loss resulting from this double component integration, even though better than in the hybrid case, is still of about 15 dB [3]. Furthermore, electrical crosstalk might appear as a limiting factor when operating the device at high bit rates [4], although it does not seem to be the case in a more recent work [SI. In this letter we report on a novel TANDEM device, based on a simple integration scheme using one single active layer, which includes an optical amplifier [6] between the two modulators to reduce the electrical crosstalk (<40 dB), and to compensate for optical losses (fiber to fiber loss <9 dB). This component is operated at 20 Gbit/s and the BER is measured. Soliton propagation is also demonstrated. Manuscript received October 12, 1994; revised February 7, 1995. This work was supported in part by the EEC through RACE WELCOME project. N. Souli, F. Devaux, A. Ramdane, Ph. Krauz, A. Ougazzaden, F. Huet, and M. CmC are with France Telecom, CNET. BP 107, 92225 Bagneux Cedex, France. Y. Sorel, J. F. Kerdiles, and M. Henry are with France Telecom, CNET, route de Tregastel, BP 40, 22301, Lannion, France. G. Aubin, E. Jeanney, T. Montallant, J. Moulu, B. Nortier, and J. B. Thomine are with France Telecom, CNET, 38-40, rue du GBn6ra.l Leclerc, 92 131 Issy les Moulineaux, France. IEEE Log Number 9410663. NRZ coding signal zyx amplifier current h region Fig. 1. soliton generator and MOD2 is the coder. Schematic structure of the component and connections, MODl is the 11. STRUCTURE AND FABRICATION The layers of the structure were grown by atmospheric pres- sure MOCVD on a n +InP substrate. The undoped common active core consists of 13 periods of 12 nm thick InGaAsP wells zyxwvu (Aph = 1.57 pm), and 6 nm thick InGaAsP barriers (Aph = 1.15 pm), surrounded by two 30-nm thick confining layers. This is topped by a 1.5-pm thick p+InP cap layer and 0.12-pm thick p+InGaAs contact layer. Ridge waveguides of 3 pm width were formed by a combination of wet and zy dry etching. Low capacitance contact pads are then deposited on planarizing polyimide. The lengths of the first modulator (MODl), which is the soliton source, the amplifier section, and the second modulator (MOD2), which is the coder, are 136, zyxwvut 500, and 100 pm (Fig. l), respectively. The amplifier section length was optimized for less than 40-dB electrical crosstalk between the bonding wires. An isolation resistance of more than 10 MR between the device sections, is achieved by means of H+ ion beam implantation of the p+InP layer in a 10-pm wide channel. The final device was indium soldered to a specific FW submount, which includes 2 microstripe lines and a contact pad for amplifier polarization. The optical facets were antireflection coated. Light is emitted from a tunable laser at 1.55 pm, TE polarized and coupled in and out through single mode fibers using selfoc lenses. This operating wavelength corresponds to a 50-nm detuning relatively to the active layer photolumines- cence peak which is around 1500 nm. The amplifier was driven with a 120 mA current. The absorption characteristics versus reverse bias were measured for each modulator (see Fig. 2). MODl showed 25-dB extinction ratio and MOD2 17 dB for 1041-1135/95$04,00 zyxwvuts 0 1995 IEEE