IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 1, JANUARY 2002 27 40-Gb/s Tandem Electroabsorption Modulator Beck Mason, Abdallah Ougazzaden, Charles W. Lentz, Kenneth G. Glogovsky, C. Lewis Reynolds, George J. Przybylek, Ronald E. Leibenguth, Terry L. Kercher, John W. Boardman, Michael T. Rader, J. Michael Geary, Frank S. Walters, Larry J. Peticolas, Joseph M. Freund, S. N. George Chu, Andrei Sirenko, Ronald J. Jurchenko, Mark S. Hybertsen, Leonard J. P. Ketelsen, and Greg Raybon Abstract—In this letter, we have developed a tandem electroab- sorption modulator with an integrated semiconductor optical am- plifier that is capable of both nonreturn-to-zero and return-to-zero (RZ) data transmission at 40 Gb/s. The tandem modulator consists of a broad-band data encoder and a narrow-band pulse carver. The pulse carver is able to produce 5-ps pulses with more than 20 dB of extinction. The on-chip semiconductor optical amplifier provides up to 8.5 dB of fiber-to-fiber gain and enables the modulator to be operated with zero insertion loss. Devices have been realized with greater than 40-GHz bandwidth, and 13-dB dynamic extinction for a 2.5-V swing. For optimized designs bandwidths of nearly 60 GHz have been realized. Using these devices penalty free RZ data trans- mission over a 100-km dispersion compensated fiber link has been demonstrated with a received power sensitivity of 29 dBm. Index Terms—Electroabsorption modulators, semiconductor optical amplifiers. I. INTRODUCTION E LECTROABSORPTION modulated sources are key components for current and next generation 40-Gb/s fiber transport systems. Electroabsorption modulators (EAMs) offer advantages in compactness, low cost, compatibility with monolithic integration, and low drive voltages. In this letter, we describe the design, fabrication, and transmission performance of 40-Gb/s tandem EA modulators capable of both nonre- turn-to-zero (NRZ) and return-to-zero (RZ) operation. The device structure consists of two short multiple-quantum-well (MQW) modulators, a monolithically integrated semiconductor optical amplifier (SOA), and spot-size converters (SSCs) on both the input and output. For RZ transmission, one modu- lator functions as a pulse carver, and the other one operates as a data encoder. For NRZ operation, the pulse carver is not used. The tandem modulator design was realized using semi-insulating InP regrown deep ridge buried heterostructure technology. Modulation bandwidths of up to 57 GHz have been demonstrated for the smallest device geometries. The carver/encoder configuration with the SOA operates with up to 8.5-dB fiber-to-fiber gain at a 0-V modulator bias. This device is intended for long-haul RZ data transmission. There are significant advantages to using short pulse RZ transmission for long distance 40-Gb/s communications Manuscript received June 25, 2001; revised August 9, 2001. B. Mason, A. Ougazzaden, C. W. Lentz, K. G. Glogovsky, C. L. Reynolds, G. J. Przybylek, R. E. Leibenguth, T. L. Kercher, J. W. Boardman, M. T. Rader, J. M. Geary, F. S. Walters, L. J. Peticolas, J. M. Freund, S. N. G. Chu, A. Sirenko, R. J. Jurchenko, M. S. Hybertsen, and L. J. P. Ketelson are with Agere Systems, Breinigsville, PA 18031 USA. G. Raybon is with Lucent Technologies, Holmdel, NJ 07733 USA. Publisher Item Identifier S 1041-1135(02)00017-4. Fig. 1. Tandem modulator cross section. systems [1]. For these systems, a transmitter is required that can generate short optical pulses and encode them using data modulation. A semiconductor-based tandem electroabsorption modulator (TEAM) offers a single compact device incorpo- rating both stable short pulse generation and data modulation functions. The first demonstration of a TEAM was at 5 Gb/s using two discrete InGaAsP bulk modulators [2]. Fiber-to-fiber insertion loss resulting from the two components was 15 dB. To reduce the optical losses, tandem modulators integrated with optical amplifiers have been reported. As a result, tandem device fiber-to-fiber insertion loss was reduced to 9 dB, and operation at 20 Gb/s was demonstrated [3]. In this letter, we report on a TEAM with better than 0-dB insertion loss and greater than 40-GHz bandwidth. II. DEVICE DESIGN The tandem modulator consists of a semiconductor optical amplifier sandwiched between two EAMs (see Fig. 1). Spot size converters are used on the input and output waveguide to im- prove the optical coupling efficiency. The modulators and the SOA have a MQW active layer with a thin separate confine- ment heterostructure. The device is fabricated using a deep ridge buried heterostructure process. Modulators with varying lengths of 80, 100, and 120 m were evaluated with SOA lengths of 400 and 600 m. In this letter, we show results for two different modulator active layer designs. The first structure (Type I) has a 0.2- m-thick depletion region. In the second structure (Type II), we increased the depletion re- gion thickness to 0.3 m. These two structures were designed to investigate the tradeoff between the bandwidth and the extinc- tion ratio for EAMs of varying lengths. Type I devices have a narrow depletion region which maximizes the internal field, and thus, the change in absorption for a given drive voltage. Type II devices have a wider depletion region giving them a smaller junction capacitance, which will result in a higher bandwidth for a given device length, assuming that the radiatively coupled (RC) bandwidth limit due to the junction capacitance dominates over the parasitic effects. 1041–1135/02$17.00 © 2002 IEEE