Optical Communications Project

Summary

Communication systems often make use of frequency or wavelength diversity to increase channel capacity. This approach is particularly appropriate for optical communication systems because of the very large bandwidths available. Optical communications can be divided into two general areas. Point-to-point optical fiber links are the focus of intense commercial development and application, while work on optical multi-user networks is at a earlier stage of research. Our group conducts interdisciplinary research in both areas, using a combination of theory, simulations, and experiments.

Wavelength division multiplexing (WDM), the use of several distinct, separated wavelengths on a single fiber, is revolutionizing point-to-point optical links in the telecommunications industry. The separate wavelengths, however, are not truly independent but are coupled together by nonlinear interactions in the fiber material. Nonlinear four wave mixing (FWM), combined with low dispersion fibers and Er-doped fiber amplifiers, results in severe performance degradation that limits capacity and span lengths. We have developed a WDM wavelength shift keying technique with balanced detection that completely cancels FWM interference to first order, and in addition eliminates other broad bandwidth noise. Our modeling shows that this technique substantially improves performance relative to standard On-Off WDM encoding. Experimental testing of the technique is underway.

For a multiuser network the parameters of interest are the total number of users that can subscribe to the network, and the number of users that can be active simultaneously on the network for a given quality of service. The number of active users on a network is generally a small fraction of the total number of subscribers due to the bursty nature of computer traffic, but even so, design parameters of interest for a network are as many as 1000 subscribers and 100 active users. This means that network channels must have capacities approaching a terabit per second. Thus, the user interface, which extracts the desired data from the combined data stream on the channel, must be a passive optical system to avoid electronic speed limitations. We are investigating using wavelength encoding to support many more users than simple WDM can. Each user is assigned a unique combination of wavelengths as their code, a technique we call spectral code division multiplexing (SCDM). Obviously, many more combinations exist than single wavelengths. The key to performance is picking codes that have low cross correlations so that a correlation receiver can isolate the desired data. Such codes have been developed for radio frequency systems, where they are generally used in the time domain and called spread spectrum systems. These codes are bipolar, with (+1, -1) elements, and unfortunately cannot be used directly in the optical domain because optical phase cannot be determined directly.

We have invented (and patented) a new technique that permits these codes to be used in optical fiber systems while maintaining their desirable properties. This work was done in collaboration with Prof. Aazhang and his group. Our approach has a major advantage in that it avoids electronic speed limitations: the spreading or processing gain, and number of users, is independent of the transmission rate, and the combined, very high speed data on the network channel is processed by a passive all optical decoder. We have built a prototype two-user system, made bit error rate measurements, studied the influence of interference, and examined the performance limitations due to encoder/decoder optical properties. Currently we are working on integrating the encoder/decoder onto an integrated photonics chip.

This work has been supported by the Texas Advanced Technology Program, the U.S. Air Force Office of Scientific Research, and Rice University. For further information, please consult the publications below, or contact me at: young@ece.rice.edu

Selected Publications

Planar lightwave circuit design for programmable complementary spectral keying encoder and decoder
C.-H. Lee, S. Zhong, X. Lin, J. F. Young, and Y. J. Chen, Electronics Letters Vol. 35, 1813-1815 (1999). Abstract

Measurements of BER performance for bipolar encoding of an SFS
T. Dennis and J. F. Young, Journal of Lightwave Technology Vol. 17, 1542-1546 (1999). Abstract

Optical implementation of bipolar codes
T. Dennis and J. F. Young, IEEE Journal of Quantum Electronics Vol. 35, 287-291 (1999). Abstract

All-Optical Encoders and Decoders for Spectral CDMA using Bipolar Codes
T. Dennis and J. F. Young, in All-Optical Networking: Architecture, Control, and Management Issues, J. M. Senior and C. Qiao, Eds., SPIE Proceedings Vol. 3531 (1998). Abstract

Experimental demonstration of bipolar codes for optical spectral amplitude CDMA communication
L. Nguyen, T. Dennis, B. Aazhang, and J. F. Young, Journal of Lightwave Technology Vol. 15, 1647-1653 (1997). Abstract

System and Method for Performing Optical Code Division Multiple Access Communication using Bipolar Codes
J. F. Young, B. Aazhang, and L. Nguyen, United States Patent 5,760,941 (June 2, 1998). Abstract

Reduction of four-wave mixing crosstalk in multi-wavelength channels using wavelength coding
Y. Guo, B. Aazhang, and J. F. Young (unpublished). Abstract

Photoelectric current distribution and bit error rate in optical communication systems using a superfluorescent fiber source
L. Nguyen, J. F. Young, and B. Aazhang, J. Lightwave Technology Vol. 14, 1455-1466 (1996). Abstract

All-Optical CDMA with Bipolar Codes
L. Nguyen, B. Aazhang, and J. F. Young, Electronics Letters Vol. 31 No. 6, 469-470 (16 March 1995). Abstract

Recent Presentations

Wavelength Shift Keying Technique to Reduce Four-Wave Mixing Crosstalk in WDM
C. Xiang and J. F. Young, IEEE Lasers and Electro-Optics Society 1999 Annual Meeting, San Francisco, CA, November 1999. Summary

Performance Modeling of a Planar Waveguide Based Spectral Encoding System
C. D. Babich and J. F. Young, IEEE Lasers and Electro-Optics Society 1999 Annual Meeting, San Francisco, CA, November 1999. Summary

All-Optical Encoders and Decoders for Spectral CDMA using Bipolar Codes
T. Dennis and J. F. Young, All-Optical Networking: Architecture, Control, and Management Issues, Boston, MA, November 1998. Abstract

Wavelength Encoding for Optical Networks
T. Dennis and J. F. Young, 1998 Int. Conf. on Applications of Photonic Technology, Ottawa, Canada, July 1998 (Invited).