Resolution of security vulnerabilities in SPECTS O-CDMA and field trial performance

The global demand for secure high-speed and high-capacity Internet communications has stimulated the need to develop optical fiber local access networks. Optical code division multiple access (O-CDMA), which offers flexible bandwidth allocation and simplified network management as well as potential physical layer security, is a promising candidate for future local access networks. Spectral phase encode time spreading (SPECTS) is a coherent approach to the implementation of O-CDMA, and it potentially supports a large number of users while maintaining a high speed. This technique applies a unique code assigned to a user as a sequence of phase shifts across the spectrum of the user's ultrashort optical pulses. The receiver relies on the correlation of the code families to distinguish the desired user's signal from multiuser signals. This dissertation focuses on resolutions to certain challenges in SPECTS O-CDMA, including reported security issues and the influence of impairments in optical fiber links.;One security vulnerability is that SPECTS O-CDMA offers weak confidentiality protection because the feasible O-CDMA codes are limited to several code families. An eavesdropper needs only to try a finite number of codes to intercept data, provided that he knows that spectral coding is being used. This dissertation experimentally demonstrates a phase masking concept which applies a mask of random phase variations to all of the users in a network to effectively hide the phase relations of individual code. The phase mask serves as a group security key to enhance confidentiality.;The central contribution of this dissertation is the demonstration of a security enhancement method called four-state encoded data modulation. This technique resolves another security vulnerability in SPECTS O-CDMA networks, which is the ability of an eavesdropper to intercept single user data without O-CDMA decoding. The four-state encoded data modulation technique encodes the user data in the electrical domain and distributes the encoded sequence across four distinct waveforms. It effectively defeats eavesdropping by energy detection or differential phase-shift keying detection and enhances security on the upstream links in SPECTS O-CDMA networks. This dissertation presents an FPGA based 2.5 Gb/s four-state encoder/decoder employing parallel programming and reports on the corresponding security enhanced SPECTS O-CDMA testbed demonstration.;Demonstration of SPECTS O-CDMA concepts in a laboratory testbed is the first step in investigating multiuser performance. This dissertation presents the optimization of the pulse shaper based SPECTS encoder for the high spectral resolution required for long code lengths. This was the first implementation of a high resolution, multi-channel pulse shaper in cylindrical optics with excellent performance, despite the lack of low aberration cylindrical lenses.;After laboratory based testbed demonstrations, field trials are required for evaluation of the impact of physical layer impairments that accumulate over long distances of field fiber. This dissertation presents a field trial of SPECTS O-CDMA over 150km of field fiber link. The SPECTS O-CDMA signals are extremely susceptible to fiber dispersion. This field trial employed dispersion compensating fiber and a tunable dispersion slope compensator (TDSC) for dispersion management. The TDSC is a bulk optics pulse shaper with a mirror bent in an "S" shape to provide a cubic-function spectral phase. It handles a significant amount of residual dispersion slope from the field fiber link. The success of this field trial shows the viability of SPECTS O-CDMA in a realistic telecommunication environment and its potential over wide or metropolitan access networks.