J.E. Midwinter

Modeling of four-wave mixing and gain peaking in amplified WDM optical communication systems and networks

A theoretical model is presented for analyzing the propagation of densely spaced WDM optical signals through a cascade of erbium-doped fiber amplifiers and single-mode optical fibers with nonuniform chromatic dispersion. By combining a numerical solution for the EDFA and an analytical expression for FWM components generated through the cascade, the model allows a realistic system analysis which includes gain peaking effect, amplified spontaneous emission accumulation and the effect of dispersion management on the four-wave mixing efficiency. The FWM power distribution at the end of the multi-amplifier transmission link is computed taking into account the phase relation between FWM light amplitudes generated within different sections of the link. The transmission of many WDM channels, evenly spaced around 1547.5 nm, has been analyzed for various dispersion management techniques and propagation distances. Numerical results point out the importance of such a model for a realistic design of WDM optical communication systems and networks. A proper choice of chromatic dispersion, amplifier characteristics, span length, input signal powers and wavelengths, combined with the use of gain equalizing filters, allows to maximize the transmission distance ensuring acceptable signal-to-noise ratio (SNR) and limited SNR variation among channels.

Spatioangular multiplexed storage of 750 holograms in an Fe:LiNbO3 crystal.

We propose a volume holographic storage scheme, spatioangular multiplexing, which is a hybrid of both angular and spatial multiplexing. We describe the recording procedure, for a preliminary experiment in which 750 holograms were recorded (at room temperature) in a single photorefractive crystal of iron-doped lithium niobate and report the successful replay of all images with negligible cross talk and with an average diffraction efficiency of 0.5%.

Planar optical implementation of crossover interconnects.

We propose a planar optical configuration for implementation of crossover interconnects. The planar overlay allows two-dimensional pixel arrays located on a wafer-scale integrated circuit to be interconnected in a compact and vibrationally robust configuration. By using an acute-angle microprism array with apex angles of 70.52 deg fabricated on a silicon substrate, we show the experimental result of the crossover interconnections. Some practical limits of the planar optical crossover interconnections are also discussed.

Photonic switching technology: component characteristics versus network requirements

Components for switching in the optical domain offer substantially different characteristics than electronic switches. However, except in special cases these characteristics do not immediately map well onto the network requirements as currently perceived so that there remains great challenge in establishing a viable technology. A variety of photonic switching technologies are discussed in this context. They are: passive pathway switches with electrical control active-path optical switches with electrical control; optically controlled electronic logic; optically controlled optical logic; and wavelength routing technologies. Optimum component and technology choices are discussed. >

An OXC Architecture Suitable for High Density WDM Wavelength Routed Networks

A novel optical crossconnect architecture, capable for tens of terabit throughput and allowing the integration of wavelength and space switching, is presented for both WP and VWP networks. The architecture offers significant advantages in terms of crosstalk performance and node complexity. In the architecture, there are no crossovers and crosspoints. Therefore, the signal-to-crosstalk ratio per wavelength channel can be very high since each channel is processed independently from the others. This will also allow to introduce other all-optical techniques like adding/dropping of OTDM channels as well as optical regeneration. The proposed architecture is wavelength and link modular and particularly suitable for crossconnects with a very large number of wavelength channels. Performance aspects of a single as well as of a cascade of crossconnects have been simulated using a simulation tool.

Optically interconnected switching networks

Work on hybrid multistage switching networks is presented. A simple mapping rule is used to form a network using two-dimensional perfect shuffle interconnections, and other 2-D patterns are developed from other mappings. The general conditions for a 1-D net to be mappable into two dimensions are established, and it is pointed out that in practice all multistage interconnection networks (MINs) are mappable-a suitable mapping transformation is what is required. It is shown how to use the mapping process to produce hybrid MINs with a variable electronic island size. It is concluded that the design flexibility exists to take multistage processors and transform them into formats which are highly suitable for the employment of optical interconnects. >

Design of a Holographic Concave Grating Used as a Multiplexer/Demultiplexer in Dense Wavelength-routed Optical Networks with Subnanometer Channel Spacing

Abstract The performance parameters of a free-space concave grating multiplexer/demultiplexer have been evaluated for applications in wavelength routed optical networks. The conditions for the stationarity of astigmatism with wavelength were rigorously derived and a novel design method was adopted to minimize the optical aberrations. The performance of this system is close to diffraction-limited over the wavelength ranges 1480–1520 nm or 1530–1570 nm. The optical isolation is better than − 30 dB for a channel spacing of 0·5 nm and the losses due to aberrations are estimated to be less than 4 dB. The results can be used to design a spectrograph suitable for wavelength cross-connection in all-optical networks.

Low-chirp, 2.5 Gbit/s directly modulated fiber grating laser for WDM networks

Densely spaced wavelength-routed optical transport and access networks will require low-cost, stable, low-chirp narrow linewidth lasers operating on precisely specified wavelengths. Directly modulated semiconductor lasers with short external fiber gratings (FGL) can optimally satisfy these requirements. Although 10-Gbit/s externally modulated soliton transmission using a mode-locked FGL over 27,000 km was reported, wavelength chirp and transmission performance of directly modulated FGLs have not been investigated at bit rates higher than 155 Mbit/s for NRZ transmission. In this paper, we report on the very low chirp performance of a directly modulated FGL at 2.5 Gbit/s. Transmission over distances of >300 km in standard fiber and >10,000 km in dispersion-shifted fiber was demonstrated.

Design and performance of concave holographic gratings for applications as multiplexers/demultiplexers for wavelength routed optical networks

The design method and the performance of two types of con- cave gratings for operation as multiplexers/demultiplexers, with 10 nm/mm reciprocal linear dispersion, required to multiplex/demultiplex channels separated by 0.5 nm in wavelength, are presented. The first type of grating is designed in such a way that the principal ray of the central wavelength in the spectrum is diffracted at normal angles to the image field. The second type of grating is a Rowland circle mount. In each case, two recording configurations are obtained. The performances of the mounts are almost identical. In the first configuration, the coupling loss is less than 22.7 dB and the optical isolation is better than 237 dB. In the second configuration, the losses are less than21.8 dB and the optical isolation is better than 243 dB. Finally, a Rowland circle grating with 4 nm/mm reciprocal linear dispersion, which can multiplex/ demultiplex at least 175 channels with loss less than 22.6 dB and crosstalk better than 228 dB, is presented. These spectrographs can be used for wavelength routing applications in wavelength division multi- plexing (WDM) networks.

Thermo-optic bistable devices: Theory of operation in freestanding films

This paper presents a theory for the operation of thin-film thermo-optic bistable devices. Special attention is paid to free-standing films. After a review of nonlinear Fabry—Perot resonators and how they work we solve the heat equation in the thin film subject to a heating effect. Switching speed and optical power consumption are functions of the rate of change of temperature in the presence of a heat source. The interplay of heat loss from the surface of the film with heat diffusion within the film complicate the dynamics considerably. Quantitative results are derived for the dependence of speed and power on the dimensions of the active region of the optical element. It is found that diffusion transients can effect more rapid switching at the expense of power but only for infrequent operation.