Generating Spectral-Efficient Duobinary Data Format from Silicon Ring Resonator Modulators Lin Zhang 1 , Muping Song 1,2 , Jeng-Yuan Yang 1 , Wei-Ren Peng 1 , Scott Nuccio 1 , Raymond G. Beausoleil 3 and Alan E. Willner 1 1. Dept. of Electrical Engineering, Univ. of Southern California, Los Angeles, CA 90089, USA, Email: linzhang@usc.edu 2. Dept. of Info. and Electro. Engineering, Zhejiang University, Hangzhou, Zhejiang, 310027, China 3. HP Laboratories, Palo Alto, CA 94304, USA Abstract: Two types of ultra-small silicon microring-based duobinary modulators are proposed and compared. Required drive voltage is as low as 0.77 volt. at 10 Gb/s. Extinction ratio is > 15 dB. Introduction There has been much excitement in the optical communications community regarding the use and performance of various advanced data formats. In general, spectrally efficient formats transmit more information in a narrower bandwidth, resulting in: (a) lower modulator bandwidth requirements, (b) higher tolerance to chromatic dispersion, and (c) smaller channel spacing. One such data modulation format is duobinary (DB) [1]. Techniques to generate DB have typically used Mach-Zehnder modulators (MZM) in which a ¼ bit rate electrical filter will generate a 3- level drive signal for a dual-drive MZM [2]. Ring resonators have generated much interest over the past few years since they can selectively operate on the amplitude, frequency and phase on an optical signal passing through it [3-10]. One of the exciting trends in our community is silicon photonics. Novel ring-resonator based optical filters have been demonstrated [3-5] while high-speed modulators exhibited >Gbit/s operation [6]. In general, such resonators have the potential to be efficient in terms of space and power consumption. Recently, it has been shown that ring resonators can be used to generate advanced modulation formats such as differential-phase-shift-keying (DPSK) [7] and differential-quadrature-phase-shift-keying (DQPSK) [8]. A laudable goal could be to use ring resonators instead of a MZM to generate DB. We discuss two ways to use ring resonators as a generator of duobinary signals. One consists of a low- speed microring modulator that plays a role as an electrical filter and modulator simultaneously to convert electrical DPSK signal to optical duobinary signal. This is called Type I, as shown in Fig. 1. The other is based on a dual-arm interferometric structure, each arm with a microring modulator to obtain DPSK signals that have the same logic pattern but a time offset between each other. This is called Type II. We show that 10-Gbit/s DB signals can be successfully generated in both ways, with a drive voltage as low as 0.77 volt. This is expected to have applications in access networks [10] and chip-scale scenarios. The DB generation is characterized in terms of extinction ratio, signal bandwidth and eye-opening penalty. Error free detection (BER<10 -9 ) can be obtained. Principle and results Silicon microring modulators have been used for DPSK modulation [5], in which an over-coupled ring resonator is driven by voltage signal. Input continuous wave experiences a phase shift but keeps constant power in pulse durations, as shown in Fig.1. We note that the modulator has a bandwidth limited by carrier transit time in silicon and photon lifetime in the resonator. One can intentionally keep the modulator bandwidth lower than the data rate of the driving DPSK signal. In this way, the modulator could act as a filter and convert the electrical DPSK to optical DB signal. For 10 Gb/s DB generation, a low-speed, say ~5 GHz, microring modulator can be used, with transit time as long as 50 ps and cavity Q-factor of 22000. This way only 0.77-volt drive signal is needed. Fig. 1. Principle of two types of silicon microring-based duobinary modulators. Type II duobinary generator operates as a delay interferometer, in which an electrical delay t is introduced between the two identical driving DSPK signals. For a 10 Gb/s DB generation, two 10 GHz ring modulators are needed. As shown in Fig. 1, the modulated DPSK and its delayed version (by t) constructively interfere with each other to obtain DB. The required higher speed modulators are composed of low-Q resonators, which means an increment of drive voltage. 1.88 volt is needed when Q=16000. Shorter transit time like 23 ps is also desired. In both types, the ring radius is 5 μm. Figure 2 shows a comparison of DB signal spectra and detected eye-diagrams for the two types of the Tu.3.C.4 ECOC 2008, 21-25 September 2008, Brussels, Belgium Vol. 2 - 95 1 978-1-4244-2228-9/08/$25.00 (c) 2008 IEEE