Spectral Encoding and Decoding of Monolithic InP OCDMA Encoder

We report the optical-coding operation of monolithic, ultra-compact optical-CDMA encoder and decoder pair, consisting of InP based integrated AWGs and phase modulators. The encoder and decoder successfully demonstrate eight-bit Walsh code based encoding and decoding. Introduction The Optical Code Division Multiple Access (O-CDMA) technology is a promising approach for future alloptical access networks, thanks to its potentials for providing very flexible and high-capacity access to the vast networking capacity available [1]. While the feasibility of O-CDMA has been demonstrated in free space optical systems [2], monolithic chip-scale integration is required for reliable, low-cost, and largescale commercial deployment of the O-CDMA technology [3]. The arrayed waveguide grating (AWG) is an attractive dispersive element as part of an OCDMA spectral phase encoder or decoder, due to its integration compatibility in the InP based material system and its high spectral resolution. Previous work reported an AWG-based encoder using Si technology and a non-programmable external phase shifters [4]. This paper reports on the spectral encoding and decoding of 10 Gbit/s femtosecond pulses using fully-monolithic AWG-based O-CDMA encoder realized in an InP material system with planarized BH waveguides using Hydride Vapor Phase Epitaxy (HVPE) regrowth. The encoder incorporates an electro-optic phase shifter array for rapid code reconfigurations with negligible power consumption. This approach discussed previously [5] is suitable for future large-scale chip level integration for O-CDMA transmitters and receivers. Principle In a Spectral Phase Encoded Time Spreading (SPECTS)-OCDMA system O-CDMA encoder encodes the pulses by dividing the pulse spectrum into N spatially separated spectral components. Subsequently, the phase shifters apply 0 or π phase shift to each component before the encoder recombines these spectral components. The set of phase shifts for the N spectral components forms the O-CDMA code. The encoded pulse spreads out in the time domain with a corresponding reduction in the peak power. On the receiver side, a decoder (same configuration as the encoder but operating in reverse) decodes the encoded pulse. The decoder restores the phase conditions of the initial unencoded pulse and the original pulse shape only if the conjugate code is used. Otherwise, the decoded pulse will also spread out in time. Fig. 1 shows the packaged OCDMA encoder chip (top) and the chip layout (bottom). An ultra-short pulse enters AWG-1 and spatially separates into eight slices. An electro-optical phase shifter array then applies the O-CDMA code to the spectral slices, before AWG-2 routes the encoded pulse into a single output waveguide. The delay lines serve to equalize the optical path lengths of the spectral components passing through the AWG pair. The fabricated encoder chip was wire-bonded packaged in a butterfly package to allow programmable electrical access to the phase shifter arrays. In Out 7 6 5 4 3 2 1