Cedric F. Lam

Experimental demonstration of bipolar optical CDMA system using a balanced transmitter and complementary spectral encoding

We demonstrate a novel balanced differential optical transmitter for spectrally encoded optical code-division multiple-access (CDMA) systems. The proposed structure is suitable for making optical signaling bipolar using complementary spectral encoding. An optical CDMA link with a pair of programmable transmitter and receiver is tested at the OC-3 transmission speed (155 Mb/s) for single-channel transmission. Unmatched code rejection is also demonstrated.

Maximizing spectral utilization in WDM systems by microwave domain filtering of tandem single sidebands

We present an optical tandem single-sideband receiver that enables the detection of signals having different information in the two sidebands of the same optical carrier. The technique relies on the use of a dual-electrode Mach-Zehnder modulator and achieves heterodyne detection without the use of an optical local oscillator. Sharp filtering requirements are met in the electrical domain, eliminating the need for wasteful guardbands.

To spread or not to spread: the myths of optical CDMA

In a spread spectrum system, the signal is transmitted using a bandwidth much larger than that required for the transmitted data. By spreading the signal energy over a very broad bandwidth, one can obtain a multitude of benefits. Firstly, one can make the signal difficult to detect by lowering its spectral density and hiding it under noise. Secondly, spreading also makes jamming a much more difficult task, as a high jamming power is required over a broad spectrum.

A tunable wavelength demultiplexer using logarithmic filter chains

This paper proposes a novel wavelength division multiplexer (WDM) demultiplexer design for dense WDM networks. The proposed demultiplexer consists of rectangular shaped periodic frequency filters connected in series. These rectangular filters can be formed with an apodized one-dimensional (l-D) photonic crystal structure on a ridged semiconductor waveguide. A design example is given for such filters. In such designs, we can use a moderate refractive index contrast, in discrete groups of many layers, to achieve the same stop-band width that we would get with a large contrast. Apart from being compact in size, to demultiplex an arbitrary wavelength from N wavelengths, only log/sub 2/ N switches and filter stages are required. This type of filter has a large potential to be used in integrated photonic implementation and packet switched dense WDM applications.

Response characteristics of thin-film-heated tunable fiber Bragg gratings

We investigate the thermal response of a tunable fiber Bragg grating (FBG) with a metal coating on bare fiber surface as a heater both theoretically and experimentally. By solving a differential equation of temperature as a function of time, together with some reasonable approximation, we obtained an explicit description of the thermal response characteristics with the structures heat capacity and a time constant as its parameters. Using a squared wave modulation current and a tunable laser, we measured the temporal response of the tunable FBG. The obtained response curves match our solution. A time constant in the range of subseconds was deduced. Discussions on the tuned FBG spectra are also given at the end to explain the linewidth broadening effect at higher temperature.

Crosstalk in a metro-scale ring with passive optical add/drop

We find that crosstalk can severely degrade transmission in a metro-scale ring with passive optical add/drop nodes. The worst degradation is in the downstream signals, which are affected by the ASE and pattern-dependent chirp of the direct-modulated lasers attached to the ADD ports. Same channel crosstalk due to leakage through the FBG filter elements also plays a role. With appropriate add/drop design, crosstalk can be effectively suppressed, allowing penalty-free operation of an 80-km ring comprising a hub and four add/drop nodes.

Multiwavelength optical code division multiplexing

In this paper, we review the previous work in optical code division multiple access (CDMA) systems. Owing to the explosive growth of bandwidth demand in recent years, the current trend in optical communication system designs is to achieve one bit per hertz utilization of the available bandwidth in the optical fiber. Full orthogonality is important in order for optical CDMA systems to achieve high throughput. We describe two spectrally encoded optical CDMA systems which both give us full orthogonality. The throughput of the non-coherent spectral amplitude encoded system is limited by both speckle noise interference and shot noise interference. The multi-wavelength spectral phase encoded system is limited by shot noise only. Performance analysis is also given in this paper.

Experimental demonstration of composite packet switching on a WDM photonic slot routing network

We propose photonic slot routing ring networks that use a novel packet stacking technique to add/drop packets, which are simultaneously time and wavelength division multiplexed. Packet stacking, routing, and unstacking with four wavelengths was successfully demonstrated.

Experimental demonstration of spectrally encoded optical CDMA systems using Mach-Zehnder encoder chains

Summary form only given. Recently, there has been tremendous interest in applying spread spectrum and code division multiple access (CDMA) techniques in optical fiber communication systems. Contrary to radio CDMA systems, which employs electric-field encoding and correlation to achieve orthogonality, most of the optical communication systems use intensity modulation and direct detection. To achieve true orthogonality in optical CDMA systems, the balanced detection scheme and spectral intensity coding need to be used. We shows the block diagram of the bipolar spectrally encoded CDMA system. The transmitter consists of a pair of broadband optical sources connected in a balanced fashion. The input data differentially modulate the intensity of the two balanced sources.

A novel dynamic crosstalk characterization technique for 3-D photonic crossconnects

We introduce a novel sensitive technique to characterize dynamic crosstalk in three-dimensional MEMS photonic crossconnects (PXCs). This nonintrusive technique enables end users to evaluate PXC dynamic performances without special access to the internal photonic-switching fabric. Our results confirm that while dynamic crosstalk can be significantly higher than static crosstalk, it has little system performance impact under normal operating conditions. We discuss methods to mitigate the effects of dynamic crosstalk.