Abstract: In an optical communication system, a train of optical pulses is transmitted such that each pulse carries a set of multiplexed channels. In a receiver, the error rate for each channel is monitored and a subset of channels having the most favorable rate is selected to carry a first data stream. The remaining channels carry a second data stream. Error rate performance is determined on the basis of error detecting codes carried by the channels in the first and second data streams.

Abstract: Optical signals in an optical communication system are transmitted as a train of optical pulses which are multiplexed to provide a large number of channels. Error correction coding is applied to data carried by the channels using both interchannel coding and serial coding of individual channels. When multiplexed by wavelength division multiplexing, a waveguide array is used as a dispersive device having a characteristic of frequency selectivity which is locked by a monitoring and control system to a set of spectral lines from the single pulsed laser which generates the pulses. In the receiver, clock signals are generated by extracting clock signals from the earliest and latest received channel signals and performing interpolation to obtain clock signals for the remaining channels. When multiplexed using a spectral modulation technique, individual channels are represented by distinct sinusoidal modulations in frequency space which are then detected in the receiver by Mach-Zehnder filters.

Abstract: Optical timing detection, for phase comparison of optical signals, or clock recovery is achieved by arranging an interferometer to be responsible to data transitions. A pulse train is fed into both arms of the interferometer. An optical amplifier (20) enables the interference condition to be changed when the data is fed into one arm. The output changed if the data transitions lose synchronization with the pulse train.

Abstract: In an optical transmission system, an optical signal is transmitted to an optical element which is sensitive to changes in the optical power of the signal. Changes in optical power are anticipated and damped by controlling the transmitter output power, or an external damping element. For WDM systems, wavelengths can be added or removed without causing rapid changes in total power which would otherwise disturb the output of downstream optical amplifiers, and cause bit errors.

Abstract: An optical transmission system includes means for measuring optical dispersion in the optical path, and a controllable element such as a dispersion compensator, operable in dependence on the measured value of dispersion. A low frequency dither on the optical signal causes timing jitter which varies according to the dispersion in the optical path. The timing jitter is extracted from a clock signal recovered from the optical signal. This jitter is correlated with the original dither to remove jitter effects caused by other mechanisms. Thus a value for dispersion is derived which can be used for monitoring or control purposes.

Abstract: An interferometer, such as a Mach-Zehnder type is fed with a pulse train. One arm is fed with a data stream, so as to modulate the pulse train according to the data stream. This enables the data stream to be regenerated, or sampled without converting from optical form into electrical form. Thus all optical regeneration, multiplexing, demultiplexing or retiming to remove jitter, can be achieved. An optical amplifier (20) in one arm of the interferometer enables the interference condition of the interferometer to be varied, to cause the pulse train to be modulated by the data.

Abstract: A method of monitoring transients in an optical transmission system having two or more optical elements is disclosed. The method comprises detecting the existence of a transient in the system and determining the cause of the transient or the location of the cause of the transient by monitoring variations in system characteristics.

Abstract: A waveguide device for optical signals in an optical communication system relies upon photonic bandgap material to guide the optical signal via a plurality of pathways. A two-dimensional photonic bandgap material is formed in a planar slab of dielectric medium by a two-dimensional lattice in which discontinuities in the lattice region define waveguides. A waveguide array formed in the device allows multiple Mach-Zehnder interferometer devices to be constructed including small radius turns without significant losses. An optical signal equalizer formed of such a device relies upon the transfer function defined by a multiple arm Mach-Zehnder having arms of different length. By applying controlled modulation to propagation through the arms, an adaptive equalizer is formed. The adaptive equalizer has application in correcting gain tilt in line amplifiers of optical communications systems.

Abstract: In an optical transmission system employing optical amplifiers a method of regulating the gain of such an amplifier uses a non-linear control system whose non-linearity of operation is provided at least in part by the enabling/disabling of a portion of the control system by the operation of a transient magnitude threshold sensor.

Abstract: The variables and parameters previously understood to affect the gain spectrum of an optical amplifier 13 were: (1) the wavelengths to be amplified; (2) the input power levels at those wavelengths; (3) the characteristics of the amplifying medium 20; (4) the insertion loss spectra of the amplifier's components, including any filter(s) used for gain flattening; (5) the pump band chosen to pump the amplifying medium 20; and (6) the total amount of pump power supplied in the chosen pump band. An additional fundamental variable has been identified which can be used to control the gain spectrum of an optical amplifier 13, namely, the center wavelength of the spectrum of the pump's output power within the chosen pump band. Methods and apparatus for using this variable for this purpose are disclosed.For example a, transmission system is disclosed having a transmitter 11 and a receiver 10 connected by an optical fiber 12.

Abstract: A field replaceable element of an optical transmission system includes an operating unit (1), a self test means (2), and non volatile memory (3) for storing self test results on-board. Parameters of the internal operation of the unit may be stored, to facilitate long term analysis of component ageing, and to facilitate diagnosis of faults.

Abstract: An optical transmission system comprises a number of optical terminals connected by optical waveguides such as optical fibers. Nonlinear processes occurring within an optical transmission medium of the waveguide or within the optical terminals is detected by a monitor which outputs monitored data representative of the products of nonlinearity. The data is used to control the power of the optical signal such that power is regulated to avoid the onset of the nonlinear process. The data may also be used to indicate an alarm condition to an operator. A monitor for four wave mixing detection utilizes dither signals applied to respective frequency components of a wavelength division multiplexed signal, products of the four wave mixing process being detected in the monitor by correlation between a sample of a received optical signal and reference data representative of dither induced modulation in the four wave mixing product.

Abstract: The invention provides a method and system for transmitting very high bit rates over an optical link, using unidirectional and bidirectional WDM technology. The 1550 nm window for optical transmission is spatially separated into two bands, "Red" and "Blue", and the channels in each band are selected so that respective wavelengths present a substantially equal gain tilt. A fiber amplifier designed according to spatial separation and wavelength selection of the invention is disclosed, along with multiple span WDM network topologies.

Abstract: An add/drop multiplexer/demultiplexer (ADM) for switching, modulating and attenuating optical signals in a fiber optic network employing wavelength division multiplexing (WDM) is disclosed. The ADM is equipped an optical multiplexer for splitting an input WDM signal into individual optical signals, leading to respective 2.times.2 switches. Each switch has another input originating from a plurality of "add lines", and selects one of its inputs to be dropped and the other to continue along a main signal path. The retained signals may be modulated and attenuated prior to being tapped and finally multiplexed together by a WDM multiplexer. The tapped signals are optoelectronically converted and fed back to a controller, preferably a digital signal processor running a software algorithm, which controls the switching, modulation and attenuation.

Abstract: An optical transmission system comprises a transmitter and a receiver linked by an optical path having amplifier stations and incorporating a reflectometer for determining the location of an optical fault in the path. Transmitted signals are tapped from the path at an amplifier station and are processed by a correlation technique to detect the relative timing of reflections from a fault and thereby to determine the location of that fault.

Abstract: An optical communications system has a control system for controlling the transmission of signals between system elements via waveguides. The occurrence of counterpropagation occurring due to scattering or other bulk properties of the waveguides is monitored under the control of the control system at selected system elements such as bidirectional optical amplifiers. A reverse signal found to be consistent with a scattering process is analysed in terms of a power distribution as a function of delay relative to transmission of an outgoing signal and any effects due to discrete scattering events are identified and removed from the power distribution data. The revised data is used to quantify the effects of the scattering process.

Abstract: Data carrying optical signals are subjected to equalization or pulse shaping of the optical signal waveform. A plurality of optical tap signals derived from the optical signal are used to control a modulator operating on an input signal to provide an output signal having the desired waveform. A Mach-Zehnder interferometer having semiconductor optical amplifiers in each arm provides modulation by the effect of cross modulation induced by propagating the respective tap signals through a selected one of the semiconductor optical amplifiers. Various forms of transversal filter are provided by selecting the number of optical taps, assigning positive or negative weights and appropriate delays. The effects of dispersion in optical signals can be mitigated by utilizing optical taps with negative weights to subtract tail portions from the leading and trailing edges of a signal pulse.

Abstract: The invention provides a method and system for transmitting very high bit rates over an optical link, using unidirectional and bidirectional WDM technology. The 1550 nm window for optical transmission is spatially separated into two bands, "Red" and "Blue", and the channels in each band are selected so that respective wavelengths present a substantially equal gain tilt. A fiber amplifier designed according to spatial separation and wavelength selection of the invention is disclosed, along with multiple span WDM network topologies.

Abstract: The invention is directed to a failure detection system and method for detecting malfunction of an optical amplifier module with one or multiple transmission channels. The failure detection system comprises a unit for measuring a performance parameter of the module; a unit for providing an expected performance parameter; and a comparator unit for receiving the performance parameter and the expected performance parameter and producing an error signal when the performance parameter substantially departs from the expected performance parameter. The system also includes a display/alarm unit for receiving the error signal and accordingly signaling failure of the module. The performance parameter is an output value of the module; a correspondence between an output value and an input value for a transmission channel; a figure of merit (FOM.sup.t=t); a set of gains (g) for all transmission channels; and the dynamic range of the amplifier module.

Abstract: In an optical transmission system, an optical signal is transmitted to an optical element which is sensitive to changes in the optical power of the signal. Changes in optical power are anticipated and damped by controlling the transmitter output power, or an external damping element. For WDM systems, wavelengths can be added or removed without causing rapid changes in total power which would otherwise disturb the output of downstream optical amplifiers, and cause bit errors.