The routes to higher transport capacity are clear. i. Increase the TDM rate up to the electronic limit (10-40 Gbit/s) ii. Move to the optical domain to achieve higher per fibre transport a. using multiple wavelength channels (DWDM) b. using optical TDM. Both routes have their attractions. If we now wish to carry out all-optical routing, we have a variety of options. i. Access the underlying digital data structure and operate in the manner of an electronic router or digital switch. This seems unrealistic for optics. ii. Route complete wavelength channels in the DWDM system after demultiplexing them, possibly with wavelength shifting at nodes, and probably using some form of space switch. iii. Time demultiplex the OTDM data stream, space switch the discrete channels and then recombine them using some form of time-slot interchange processor.

The author joined the U.K. optical communications program in 1971, when it had already been running for at least six years and he then observed it during its most formative phases. This is a personal account of some of its triumphs and its disasters that may carry lessons for others in the future.

We describe the performance of a 100 channel free-space concave grating demultiplexer with 1dB transmission passband of ±7GHz (FWHM 14–22GHz). The influence of optical aberrations and defocusing on the transmission spectrum is demonstrated. Crosstalk between neighbouring channels separated by >0.3nm is lower than −25dB and no penalty due to crosstalk is observed using 2.6Gbit/s directly modulated DFB lasers. Penalty-free transmission through a cascade of 30 demultiplexers is demonstrated in an optical fibre recirculating loop experiment, and the permitted tolerances on the laser frequency misalignments for a larger number of cascaded demultiplexers are investigated.

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.

A communication network resource control algorithm that allocates bandwidth in a provably proportionally fair manner is described. This algorithm is believed to be more rapidly convergent and computationally straightforward than those based on utility optimisation methods for elastic broadband traffic described previously.

It is shown that both economic and control theory based analogies are useful in designing policy based next generation 'unified network' control architectures. These unified networks will be capable of delivering all of the services of today's circuit switched and packet switched networks with a combined control, management and billing system. They will have bandwidth on demand capability and would deliver guaranteed quality of service for future real time Internet applications. The paper reports progress on a joint industrial-academic research project into the dynamic resource control (DRC) of such networks. A first generation DRC unified network architecture is proposed and an overview presented of five more detailed papers describing the way in which global and local 'resource broker' policies must be married together to design the most competitive, responsive and stable overall control architecture. The detailed papers show how the policies are translated first into rules, then algorithms and show the results of traffic flow modelling, numerical simulation and mathematical analysis of overall network response and stability. The most recent of these papers is the first proposal to the authors' knowledge of st matrix method in the complex frequency domain for determining the general stability of an arbitrary topology network using dynamic congestion priced feedback flow control. This analysis technique gives much greater confidence that such networks can be designed for high response speed and unconditional stability.

Low-penalty WDM transmission of 3 densely-spaced (0.12-0.6 nm) directly-modulated fibre grating lasers at 2.6 Gbit/s over 117 km of standard fibre is reported, for the first time. WDM transmission experiments show that fibre grating lasers operate error-free over a wider temperature range compared to a directly modulated DFB laser.

Very low wavelength chirp (static <0.07 GHz/mA and dynamic <0.09 GHz/mA) was measured for a directly-modulated fibre grating laser with an angled facet semiconductor chip and a short fibre grating. Low-penalty transmission at 2.5 Gbit/s over 312 km of standard singlemode fibre has been demonstrated using this low-chirp device.

The concave grating stationary anastigmatic spectrograph has been modified into a novel retroreflective scheme by introducing a convex mirror concentric to the intermediate image. This provides double dispersion, diffraction-limited imaging of a long entrance slit, and substantial compensation of the stationary chromatic astigmatism. One can realize 140 independent multi/demultiplexers with 160 channels each simultaneously in the described device for optical network applications. Ray-tracing spot diagrams are presented.

A design procedure for two aberration-corrected spherical concave gratings operating in the spectral range 91-103 nm (extreme-ultraviolet) is presented. The first configuration is a Rowland circle mount whereas the second is based on the conditions of stationarity with respect to wavelength for the second-order aberration coefficients, as well as the condition for stationary meridional coma. The first design has better resolution whilst the latter design has a better overall degree of aberration correction. Holographic gratings recorded with two diverging stigmatic sources are considered for both designs. The theoretical performance of these gratings is very good across the spectral range of interest. The combination of simplicity and comparatively low cost together with very good aberration correction performance makes these grating designs attractive for the Lyman Far-Ultraviolet Spectroscopic Explorer.

WDM transmission and demultiplexing of 4 directly-modulated DFB lasers at 2.5 Gbit/s, with very narrow channel spacing (50 GHz) over 3000 km of dispersion-shifted fibre has been performed using a single-span recirculating loop. A high-performance free-space concave grating demultiplexer is used, for the first time, in a WDM transmission experiment.

Two subfamilies of concave grating superstationary anastigmatic mounts that provide minimum chromatic aberrations are described. The obtained approximate formulas can be used to design flat-field spectrographs and multi/demultiplexers for optical communication networks. Two specific mounts and their performance are presented.

The general equations for parameters of concave grating mounts that provide stationary and superstationary astigmatism at the wavelength of correction are derived for the first time, to the best of our knowledge. These can be used to design grating multi/demultiplexers for wavelength-division multiplexed optical communication systems and high-resolution, narrow-band spectrographs. Important special cases of stationary anastigmatic mounts and their performance are presented.

Summary form only given. Wavelength-routed optical networks (WRONs) without wavelength conversion offer a huge potential in simplifying routing and bandwidth management for future core transport networks. Their feasibility depends on the number of wavelengths necessary to satisfy a given network logical connectivity and traffic demand. We investigate the wavelength requirements in networks with variable physical connectivity, because such a quantitative relationship is vital in optimizing the WRON design with reference to existing or planned optical networks that are neither full nor regular.

A free-space concave ruled grating multi/demultiplexer with singlemode fibre input/outputs for subnanometre wavelength routing applications is reported. In the wavelength range of 16 nm around 1.5 mu m, and channel spacing of 0.4-0.8 nm (53-106 GHz), operation of the device as a wavelength routing switch with fibre-to-fibre loss of 12-14 dB is demonstrated.

We describe the transmission and crosstalk performance of a low-loss (/spl ap/6 dB) concave free-space grating demultiplexer at 1.5 /spl mu/m for dense WDM networks, and report, for the first time, on the penalty-free operation at 2.5 Gbit/s, with channel spacing 0.2-0.6 nm.

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. © 1996 Society of Photo-Optical Instrumentation Engi- neers.