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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 24, DECEMBER 15, 2006
Demonstration of Spectral Phase O-CDMA Encoding and Decoding in Monolithically Integrated Arrayed-Waveguide-Grating-Based Encoder Jing Cao, R. G. Broeke, N. K. Fontaine, C. Ji, Y. Du, N. Chubun, K. Aihara, Anh-Vu Pham, F. Olsson, S. Lourdudoss, and S. J. Ben Yoo
Abstract—We report on successful spectral phase encoding and decoding operation in a pair of monolithically integrated InP encoder chips, each consisting of an arrayed waveguide grating (AWG) pair and an eight-channel electrooptic phase shifter array. The monolithic fabrication process includes anisotropic reactive ion etching and planarizing hydride-vapor-phase-epitaxy lateral regrowth to realize buried hetero-waveguide structures in AWGs and phase shifters. Electrooptical modulation in the phase shifter arrays in the encoder chip achieved Walsh-code-based optical code-division multiple access (O-CDMA) encoding and decoding. The matched-code encoding–decoding operation resulted in error-free performance in the presence of an interferer, indicating good potential for O-CDMA network applications. Index Terms—Arrayed waveguide grating (AWG), monolithic integration, optical code-division multiple access (O-CDMA), spectral encoder and decoder.
I. INTRODUCTION HE optical code-division multiple-access (O-CDMA) technology provides promising solutions for future all-optical access networks by offering truly flexible assignments of the vast networking capacity [1], [2]. Instead of wavelength channels or time slots used in wavelength-division multiplexing (WDM) or time-division multiplexing (TDM) networks, O-CDMA utilizes optical codes to configure network access. O-CDMA networks can also exploit new secure networking utilizing code translation and code modulation [3], [4]. While the feasibility of spectral-phase-encoded time-spreading (SPECTS) O-CDMA has been demonstrated using free space bulk optics [5], practical O-CDMA systems require compact and robust monolithic integration [6]. Previous integrated O-CDMA encoders are either based on silica arrayed-waveguide gratings (AWG) with nonprogrammable external phase shifters [7] or based on ring micro-resonators with relatively slow heater-based phase
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Manuscript received July 13, 2006; revised October 12, 2006. This work was supported in part by the Defense Advanced Research Projects Agency (DARPA) and the Space and Naval Warfare (SPAWAR) Systems Center under Agreement N66001-02-1-8937. J. Cao, R. G. Broeke, N. K. Fontaine, C. Ji, Y. Du, N. Chubun, K. Aihara, A.-V. Pham, and S. J. B. Yoo are with the Department of Electrical and Computer Engineering, University of California, Davis, CA 95616 USA (e-mail:
[email protected];
[email protected];
[email protected];
[email protected];
[email protected];
[email protected];
[email protected];
[email protected];
[email protected]). F. Olsson and S. Lourdudoss are with the Department of Microelectronics and Information Technology, Royal Institute of Technology, KTH-Electrum 229, S-16440 Kista, Sweden (e-mail:
[email protected];
[email protected]). Color versions of Figs. 2–5 are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2006.887189
shifters [8]. On the other hand, SPECTS O-CDMA encoders and decoders realized on the InP/InGaAsP material system can offer rapid code reconfigurations by employing electrooptical phase shifters. They also offer the possibility of realizing monolithically integrated O-CDMA transmitters and receivers by incorporating lasers and detectors on the same chip [6]. This paper presents error-free 10-Gb/s encoding and decoding operation of InP-based encoder and decoder devices in tandem, fabricated by employing hydride vapor-phase epitaxy (HVPE) [9] facilitates monolithic integration with exceptionally planar morphology compared to previously demonstrated InP AWGs with metal–organic chemical vapor deposition (MOCVD) regrown buried hetero (BH) waveguides [10]. II. OPERATION PRINCIPLE AND FABRICATION OF THE ENCODER CHIP A SPECTS O-CDMA system uses spectral encoding of ultrashort optical pulses for data transmission [11]. Only if the encoder code and decoder code are conjugates of each other, the decoder restores the phase conditions of the initial pulse reproducing an ultrashort optical pulse with high peak power. The SPECTS O-CDMA receiver discriminates the properly decoded pulse from incorrectly decoded pulses with nonlinear threshold detection. The receiver detects a “1” bit only if a pulse has sufficient peak power (correctly decoded). Otherwise, the receiver detects a “0” bit for a time-spread pulse. Fig. 1(a) shows the O-CDMA encoder mask layout. The AWG pair achieves spectral demultiplexing and multiplexing. The phase modulators between the AWGs apply a phase shift corresponding to the O-CDMA code to each demultiplexed spectral channel. The input and output waveguides are chosen for optimal wavelength match of the two AWGs. Fig. 1(b) shows the transmission spectrum of an O-CDMA encoder with 32-dB insertion loss, which can be greatly reduced by improving the fiber coupling and the fabrication process. The fabricated encoder chip was wire-bonded and packaged in a butterfly package for programmable electrical access to the phase shifter arrays. Fig. 1(c) and (d) shows the packaged chip and a scanning electron micrograph (SEM) picture of the chip. III. EXPERIMENTAL DESCRIPTION AND RESULTS Fig. 2 shows the O-CDMA testbed schematic including the wire-bonded InP O-CDMA encoder and decoder. Each encoder and decoder consists of an identical pair of eight-channel AWGs
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CAO et al.: DEMONSTRATION OF SPECTRAL PHASE O-CDMA ENCODING AND DECODING
Fig. 1. (a) O-CDMA encoder chip layout, (b) encoder transmission spectrum, (c) packaged O-CDMA encoder chip, and (d) SEM picture of the device.
Fig. 2. Experimental setup for spectral encoding and decoding implementation of InP encoder and decoder.
with 180-GHz channel spacing. The total wavelength spanned by the eight channels is approximately 1.4 THz, which is sufficient for encoding subpicosecond pulses. Lensed fibers couple light in and out of the chips. The LiNbO3 modulator modulates PRBS the intensity of the pulse train with a 10-Gb/s data signal. The modulated signal is split into two branches. One branch passes through the InP encoder, which performs as the intended user. The other branch acts as the interferer. It is time delayed and subsequently encoded in the spatial light modulator (SLM). At the receiver side, a nonlinear optical loop mirror (NOLM) provides time gating, and a highly nonlinear fiber (HNLF) provides peak-power thresholding. The combination of the NOLM and thresholder suppresses incorrectly decoded pulses. More details can be found in [5], where the same testbed is used with free-space spatial-light modulators (SLM) instead of InP chips. Fig. 3 shows the time-domain intensity traces following the encoding-decoding operations on a pseudorandom bit sequence (PRBS)-modulated signal with a Walsh W5 code encoding [11110000]. Here, 1 denotes phase shift and 0 means 0 phase shift for the respective phase shifters. On the decoder side different codes are applied to evaluate the encoder/decoder performance. Fig. 3(a) shows the decoder output without any code applied but with phase error compensation. Cross-correlation
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Fig. 3. Cross-correlation traces of (a) the cascaded encoder-decoder output with only phase error compensation applied to the phase modulators, (b) the encoder output under W5 encoding, (c) the decoder output for correctly decoded signal, and (d) the decoder output for incorrectly decoded signal. (Solid lines are experimental results and dotted lines are simulated results. W5 code is [11110000]).
frequency-resolved optical gating (XFROG) was utilized to measure and calibrate the channel phase errors [12]. Through accurate measurements of the channel phase errors and proper phase compensation, a clear short pulse is reconstructed. Ringing peaks occur at 5.5-ps intervals due to the AWG’s comb-shaped spectral responses. Fig. 3(b) shows the encoder output with a W5 code. Fig. 3(c) shows the correctly decoded signal when the W5 conjugate code is applied to the decoder. Fig. 3(d) shows the incorrectly decoded pulse when using the W7 conjugate code instead of the W5 conjugate code. The decoder output shown in ps, Fig. 3(d) shows reduction in peak power and a null at as expected, and corresponds to W3 by “code-translation” (i.e., ) [4]. The encoding and decoding simulation results are based on an internally developed Matlab program, taking into account the actual AWG spectral response and the built-in phase errors within each spectral channel. All four cases indicate reasonably good matching between the experimental (solid lines) and simulated results (dotted lines). Introduction of ps can provide seleca time-gate with a 3 ps window at tive detection of correctly decoded pulses. Additional nonlinear optical thresholding subsequent to the time-gating provides enhanced contrast in the selective detection. Fig. 4 shows the BER measurement results of the InP chipbased O-CDMA encoding–decoding experiment. The BER measurement was conducted downstream to the O-CDMA receiver (Fig. 2). The back-to-back measurements denoted as circles ( symbols) were taken by bypassing the O-CDMA encoder and decoder pair. The single user case, which is also the correctly decoded signal ( symbols), and the two-user case with one interferer ( symbols) were taken with the InP encoder/SLM encoder and InP decoder in place. The received power in all BER curves is defined as the total average power input to the “O-CDMA receiver.” Both the single user and two-user case demonstrated error-free operation. The power
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 24, DECEMBER 15, 2006
case which is also the correctly decoded signal ( symbols) and two-user case with one interferer ( symbols) both demonstrated error-free operation. The power penalty for the single . user and two-user are 2 and 7 dB, respectively, at The incorrectly decoded signal reached an error floor of which is comparable to that of the InP chips. There is only a 3 dB power penalty for using the InP chips compared to the SLM-based pulse shapers for the two-user case. IV. CONCLUSION
Fig. 4. BER measurement results and eye-diagrams for back-to-back ( symbols), single-user case which is also the correctly decoded signal ( symbols), and two-user case ( symbols) for InP encoder and decoder chips.
We have successfully demonstrated error-free SPECTS O-CDMA spectral encoding and decoding operation in the presence of an additional interferer using monolithic eight-channel optical-CDMA encoder and decoder chips realized on InP. The comb-shaped AWG spectrum of the chips only caused a small reduction in receiver sensitivity with respect to an ideal rectangular SLM based encoder/decoder spectrum. The experimental results imply a good potential for realizing integrated InP-based O-CDMA transmitter and receiver microsystems. REFERENCES
Fig. 5. BER measurement results and eye-diagrams for back-to-back ( symbols), single-user case which is also the correctly decoded signal ( symbols), and two-user case ( symbols) for SLM encoder and decoder.
penalty for the single user and two-user case are 7.4 and . This is mainly due to 10.2 dB, respectively, at power lost in the ringing peaks (outside the time-gate) of the corrected decoded signal, and partially from signal-to-noise ratio degradations attributed to the encoder insertion loss and the subsequent EDFA amplification process. The incorrectly decoded signal (the interferer) did not result in BER of 0.5 while the correctly decoded signal was blocked. Instead, it primarily because the reached a BER error floor at 10 “O-CDMA receiver” is optimized to reject a large number of interferers in the presence of a correctly decoded user by using a highly saturated high-power EDFA [13]. In addition, the short (8-chip) code does not spread the pulse energy sufficiently outside the time-gate, and the residual signal from incorrect decoding (without an additional user with correct decoding) into the EDFA resulted in a finite amount of signal detection. Either a larger code set (e.g. 128-chip) or a narrower time gate could bring the BER up to 0.5. The effect of the comb-shaped spectral response of the InP encoder chips on the BER performance was compared to the BER measurements using two SLM-based pulse shapers as the encoder and decoder. These pulse shapers have an ideal flat spectral response. Fig. 5 shows the BER results. The single-user
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