466 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 4, FEBRUARY 15, 2012 InP-Based Comb Generator for Optical OFDM Nicolas Dupuis, Christopher R. Doerr, Fellow, IEEE, Liming Zhang, Member, IEEE, Long Chen, Nick J. Sauer, Po Dong, Lawrence L. Buhl, and Dongwahn Ahn, Member, IEEE Abstract—We describe in more detail a novel InP-based comb generator for optical orthogonal frequency division multiplexed (OFDM) transmission. The device integrates a Mach–Zehnder in- terferometer inside an amplified ring. In order to maximize the number of lines in the spectrum, the phase modulators were de- signed with a 3-step asymmetric quantum well structure to max- imize the index change with voltage change. We demonstrate a 6-line frequency comb which is tunable over 80 nm. The linewidth of the comb is limited by only the input laser, which makes this de- vice especially suitable for coherent optical OFDM applications. Index Terms—Comb generator, indium phosphide, integrated optics, phase modulator. I. INTRODUCTION O PTICAL orthogonal frequency division multiplexing (OFDM) is an attractive transmission format for high-bit-rate-per-channel systems (100 Gb/s and beyond) due to its high spectral efficiency and robustness against chromatic dispersion and polarization mode dispersion [1]. In optical OFDM, the channel spacing between the subcarriers is equal to the symbol rate. An optical OFDM transmitter usually com- prises a frequency comb generator, a filter, a demultiplexer, a modulator array, and a power combiner. Some key requirements for a comb generator in an optical OFDM implementation are a good spectral flatness, an accurate channel spacing and a narrow linewidth if using coherent detection. Furthermore, in order to make the transmitter more flexible, the ability to arbitrarily tune the center wavelength and line spacing of the optical comb should be considered. Different approaches have been proposed for comb generation. A first approach is to use a mode-locked laser (MLL) [2]. In this case, the center wavelength of the comb is fixed by the laser design, and the channel spacing must be a multiple of the cavity-mode spacing. Another approach is to modulate a Continuous-wave (CW) laser output with a phase modulator at the desired channel spacing frequency [3]. Some advantages are that it is wavelength tunable, and that the frequency channel spacing is limited by only the bandwidth of the phase modulator, which makes the transmitter flexible. However, this method requires driving the modulator by 10s of radians peak to peak, which generally requires very high drive voltages. Also, obtaining a flat spectrum is challenging. A possible enhancement is to include the phase modulator inside Manuscript received August 04, 2011; revised October 11, 2011; accepted October 19, 2011. Date of publication October 25, 2011; date of current version February 01, 2012. The authors are with Bell Laboratories, Alcatel-Lucent, Holmdel, NJ 07733 USA (e-mail: nicolas.dupuis@alcatel-lucent.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2011.2173463 a reseeded amplified loop [4]. This method allows a broadening of the spectrum by successive phase modulation of the input light. In another paper, the authors use a slightly different method inserting a single-sideband modulator (SSBM) inside a reseeded amplified loop [5]. The comb generation is based on the concept of recirculating frequency shifting using the SSBM [5]. Some other approaches have also been recently reported in Si photonics. For instance [6] reported an on-chip optical parametric oscillator utilizing non-linear four-wave mixing in a silicon nitride micro-ring to generate a frequency comb. This solution is very compact. However, the linewidths may increase as the mixing products grow and the frequency spacing is not adjustable. In [7], we demonstrated a wavelength tunable InP-based photonic integrated circuit (PIC) that gen- erates a comb from an input CW laser. The device consists of a Mach–Zehnder interferometer (MZI) sinusoidally driven in quadrature and inserted inside an amplified feed-back loop. It requires only a 6.7-V p-p sinusoidal driver, and the center wavelength and comb spacing are completely adjustable. In this paper, we explain the design in more detail and show additional data on the phase modulators and amplified ring. II. DESIGN AND FABRICATION A. Comb Generator We propose to generate a comb using a 2 2 MZI which in- cludes an AC phase modulator (PM) and a DC phase shifter (PS) in each arm. The two PMs are sinusoidally driven in quadrature to generate the sidebands. In order to enhance the phase modu- lation, one output of the MZI is amplified and connected back to one input. The multiple phase modulation of the light through the MZI allows the generation of the sidebands. The amplifier is used to compensate for the losses of the loop. In Fig. 1, we show a schematic of the comb generator (CG). The loop also integrates a separate PS for fine tuning of the phase inside the ring. The phase shifters are mainly used for fine adjustment of the flatness of the comb. The ring adds some inherent loss because the PMs are driven in quadrature, which prevents complete destructive or constructive interference at the output of the 2 2 coupler. These losses depend on the phase relations between the MZI and the loop. Assuming that the 2 2 couplers are 3-dB couplers, one can compute the output of the comb generator, , by solving the following system: (1) 0733-8724/$26.00 © 2011 IEEE