Code-division multiple-access (CDMA) communication system allows multiple users to access the network simultaneously using unique codes. Optical CDMA has the advantage of using optical processing to perform certain network applications, like addressing and routing without resorting to complicated multiplexers or demultiplexers. The asynchronous data transmission can simplify network management and control. Therefore, OCDMA is an attractive candidate for LAN application. Particularly, OCDMA can provide a secure network connection providing dynamic encoding. Our DARPA OCDMA project proposed a chip-scale OCDMA system. The investigation involves research in all aspects of optical CDMA technologies ranging from innovative Indium Phosphide (InP) device fabrication, to orthogonal optical coding, and to OCDMA network architecture design and simulations.
Our approach to optical CDMA utilizes spectral encoding and decoding of optical ultra-short pulse for a bulk optics tabletop demonstration. A coherent ultra-short optical pulse representing one bit of information is spatially spread in spectral domain by diffraction grating, a Spatial Light Modulator (SLM) is applied to introduce a relative different phase shift (address code) among the different spectral components. The reflected light from the SLM travels through the grating one more time and reassembles into a single optical beam. The receiving system is similar to the transmitting system, except a conjugate phase shift must be applied to the according spectral component to recover the encoded pulse. When the phase shift of the transmitting system and the receiving system do not match, the spectral phase shifts are rearranged but not removed and the pulse remains spread in time with low intensity. With proper threshold detection, the desired user can successfully receive the
transmitted information.
The advantage of spectral phase encoding as opposed to temporal amplitude coding is that "time spread" signals maintain their high-speed nature throughout the system; i.e., no signal bandwidth is sacrificed for the coding, a problem which worsens with increasing code complexity.
In order to realize this OCDMA system on a chip scale device, a novel InP device including Photonic Band-gap (as the ultra-short pulse source), Arrayed Wave-guide Grating (as the spread spectrum device), phase modulator (as the phase coding device), SOA based Mach-Zehder Interferometer (as the threshold detector) will be integrated on a single chip.