Abstract: A variable gain optical amplifier, in which homogeneous gain broadening is dominant, has first and second fixed gain rare-earth doped optical waveguide amplifiers (21, 23) optically in series together with an intervening variable attenuation optical attenuator (22). This arrangement circumvents the problem of gain tilt encountered when operating such amplifiers under variable gain conditions. An alternative form of the module has variable gain waveguide amplifiers, but these are co-regulated so that the aggregate of their gain at a wavelength within the gain spectrum is maintained constant. A further alternative form of module is employed in a concatenation of such modules. In such a concatenation, the gain of individual modules is allowed to vary, but the aggregate of the gain, at a wavelength within the gain spectrum, of all the waveguide amplifiers of all the modules of the concatenation is maintained constant.

Abstract: A polarization diversity receiver has an optical section for converting the received optical signal into four or five polarization diverse component optical signals that substantially represent amplitude and polarization state information of the received optical signal, by respective polarization transformations to respective points on a Poincaré sphere, the points being equally spaced apart to maximise polarization diversity, even in the worst case input polarization state. Detectors produce component electrical signals from each of the component optical signals, for electronic processing to compensate for PMD. By reducing the number of component optical signals significant cost and size reductions are enabled. The need for precise polarization tracking in the receiver can be reduced or eliminated completely. Balanced detectors can be used to reduce the number of electrical signals. The electrical processing can use sequence detection.

Abstract: A method of compensating the polarisation mode dispersion (PMD) of an optical transmission path carrying clocked digital data traffic involves modulating the state of polarisation (SOP) of the light launched into the transmission path, and employing consequential frequency components in an electrical signal detected at the far end of the transmission path to regulate the birefringence of a polarisation state controller portion of a birefringent element inserted between the transmission path and the receiver's photodetector. The SOP modulation, when represented on a Poincaré sphere, has an oscillatory rotational component at a frequency f1 about a first axis of the sphere and an oscillatory rotational component at a frequency f2 about a second axis of the sphere that is orthogonal to said first axis, and where f1≠f2, f1≠2f2, and f2≠2f1.

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: An apparatus and a method for compensating for polarisation mode dispersion in an optical signal. The apparatus includes a chirped Bragg reflector extending longitudinally along an optical waveguide, the waveguide being susceptible to stress birefringence; and at least one tuning element located at a position along the length of the waveguide. The tuning element is operable to, in use, apply a stress to the waveguide so as to alter the magnitude and the orientation of the birefringence of the waveguide for compensation of the polarisation mode dispersion of the optical signal. Higher order PMD compensation can be achieved by providing two such tuning elements along the waveguide.

Abstract: A variable gain optical amplifier, in which homogeneous gain broadening is dominant, has first and second fixed gain rare-earth doped optical waveguide amplifiers (21, 23) optically in series together with an intervening variable attenuation optical attenuator (22). This arrangement circumvents the problem of gain tilt encountered when operating such amplifiers under variable gain conditions. An alternative form of the module has variable gain waveguide amplifiers, but these are co-regulated so that the aggregate of their gain at a wavelength within the gain spectrum is maintained constant. A further alternative form of module is employed in a concatenation of such modules. In such a concatenation, the gain of individual modules is allowed to vary, but the aggregate of the gain, at a wavelength within the gain spectrum, of all the waveguide amplifiers of all the modules of the concatenation is maintained constant.

Abstract: A variable gain optical amplifier, in which homogeneous gain broadening is dominant, has first and second fixed gain rare-earth doped optical waveguide amplifiers (21, 23) optically in series together with an intervening variable attenuation optical attenuator (22). This arrangement circumvents the problem of gain tilt encountered when operating such amplifiers under variable gain conditions. An alternative form of the module has variable gain waveguide amplifiers, but these are co-regulated so that the aggregate of their gain at a wavelength within the gain spectrum is maintained constant. A further alternative form of module is employed in a concatenation of such modules. In such a concatenation, the gain of individual modules is allowed to vary, but the aggregate of the gain, at wavelength within the gain spectrum, of all the waveguide amplifiers of all the modules of the concatenation is maintained constant.

Abstract: A method of measuring the polarization mode dispersion (PMD) of an optical transmission path carrying clocked digital data traffic involves modulating the state of polarization (SOP) of the light launched into the transmission path, and detecting consequential frequency components in an electrical signal detected at the far end of the transmission path. The SOP modulation, when represented on a Poincaré sphere, has an oscillatory rotational component at a frequency f1 about a first axis of the sphere and an oscillatory rotational component at a frequency f2 about a second axis of the sphere that is orthogonal to said first axis, and where f1≠f2, f1≠2f2, and f2≠2f1.

Abstract: The problem of gain tilt in an optically amplified WDM system is avoided by using SBS (Stimulated Brillouin Scattering) to limit the power levels of the individual WDM channels at the outputs of at least selected amplifiers in the transmission path. The SBS limits the power by the creation of a dynamic amplitude Bragg reflective grating. Alternative mode of creating such gratings are also described, one relying upon the Kerr effect, and another relying on creating a thermal grating in a medium, such as a liquid crystal that exhibits a refractive index that is strongly temperature sensitive.

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: Polarization mode dispersion in an optical signal transmitted through a waveguide of a communications system is compensated by separating the dispersed signal into components corresponding to principal polarization states. The components are delayed by respective delays differing by a delay increment which is controlled to correspond to the dispersion delay and the delayed components are recombined to provide a dispersion compensated optical output signal. Each of the delays is provided by an chirped Bragg reflector forming part of a delay line, the Bragg reflectors comprising optical fibres with chirped intracore index gratings. Transducers or temperature controllers acting on one of the fibres allows dimensional control of the grating periodicity such that the position of Bragg reflection is variable. Wavelength division multiplexed optical signals are compensated using sampled gratings which allow a common Bragg reflection position for each wavelength.

Abstract: An optical multiplexer/demultiplexer with an improved spectral characteristic is provided by two diffraction gratings (61, 60) arranged optically in tandem, the gratings being arranged to provide free spectral ranges differing by a factor of at least two, and having a coupling between them that carries over into the second grating information concerning the dispersion afforded by the first grating.

Abstract: One facet of each of a set of N semiconductor laser chips is provided with an antireflection coating through which the chip emission is coupled, via a wavelength multiplexer, with a partially reflecting Bragg reflector providing an N-way wavelength multiplexed laser output for pumping an optical amplifier. The wavelength multiplexer may be constituted by a cascade arrangement of Mach-Zehnders.

Abstract: An Optical Transmission System includes a monitor for outputting monitored data representative of the extent to which an optical signal is degraded by polarization mode dispersion. The monitor measures the degree of depolarization present in a sample of the optical signal derived from an optical tap, the depolarization being associated with polarization variation at the bit rate frequency of a signal data stream carried by the optical signal. The monitored data is used to control operation of the system to reroute the optical path taken by the optical signal in the event of a threshold level of polarization mode dispersion being exceeded. A transmitter of the optical signal may also be controlled to minimize polarization mode dispersion by varying the polarization state of the optical signal at launch so as to reduce the measured monitor data. The monitor functions during normal transmission of optical signals without the need for shutting down links of the system for measurement purposes.

Abstract: A control system for an etalon arrangement 4, in an optical transmission system involves applying a dither to a laser 1 upstream of the arrangement, monitoring the output of the arrangement, and using a control means 3 to lock the laser frequency to the etalon arrangement response, or vice versa. The control means operates on the basis of a ratio of derivatives of the monitored output with respect to wavelength, to enable locking to a point on the response curve, independent of power variations.

Abstract: An optical communications source has a laser optically coupled with an external optical modulator (25). The laser has a long optical cavity and a narrow spectral bandwidth Bragg reflector (24) so that the longitudinal mode separation of the laser emission is small compared with the spectral bandwidth of the reflector. This makes the laser operate multi-mode, but without any significant dispersion penalty.

Abstract: An interferometric optical multiplexer 10 for use in optical communication systems and especially for use in optical communication systems employing fibre optical amplifiers is disclosed. The interferometric optical multiplexer has two input ports 12, 14, an output port 16 and an output monitoring port 18. The interaction of the input signals is monitored by a photodetector situated in the monitoring part 18. In conjunction with a feedback circuit 24 wavelength separation of signals input at pods 12, 14 is ensured.

Abstract: The emission waveband of a data modulated laser signal is locked to the reflection waveband of a chirped Bragg grating dispersion compensator by means of a phase locked loop regulating the temperature of the laser or the magnitude of the drive current applied to it.

Abstract: A laser stud mount wherein one terminal connection is provided by the stud and the other by a strip line. One embodiment includes a light guide extending through the stem of the stud mount or along the surface of the stud base for monitoring the light output from the back end of the laser. The monitored light from the back end of the laser can be used to regulate the laser drive.