TRANSMITTER AND TRANSCEIVER FOR A COHERENT OPTICAL SYSTEM

Abstract: An optical transmitter having a single optical source (10) coupled to a modulator (14) for providing an information modulated signal and an optical amplifier (13) for providing an amplified phase reference signal which signals are combined by a coupler (15) for onward transmission. Duplex operation of a travelling wave optical amplifier 13 facilitates the implementation of a coherent optical transceiver 22 with a single optical amplifier (13). A separate reference signal oscillator, therefore, is not required at a remote receiver. The bi-­ directional amplifier 13 also amplifies the incoming signal, removing the need for a separate dedicated amplifier before the receive (11). The transceiver may be adapted for homodyne or heterodyne operation.

Description: The present invention relates to coherent optical systems and in particular a transmitter and transceiver for use in coherent optical systems. Any optical system involving the transmission and reception of optical signals must cope with the problems introduced by signal degradation over the transmission medium. Some methods of improving the quality of signal reception are well known. It is recognised, for example, that coherent optical systems, with optical receivers using heterodyne or homodyne techniques in particular, offer a considerable improvement over standard direct intensity systems. Conventionally, such coherent systems require a strong reference signal to be mixed with an incoming modulated signal in a receiver. Generally, this reference signal is provided by a secondary laser, or local oscillator, in the receiver itself. Additionally, for homodyne reception, the reference signal must be precisely phase locked with the incoming modulated signal. With one laser to originate the transmission and another to act as local oscillator, the difficulties involved in achieving this phase locking are severe. Published European Patent Application EP-A-0168192 discloses a coherent transmission system in which a single laser transmitter is used to provide both modulated and reference signals. Both signals are sent together in orthogonal polarisations. At a receiver, the reference signal is divided off and separately amplified to drive a coherent detector. This system suffers from problems caused by birefringence in the transmission medium (eg. an optical fibre) which may introduce polarisation dependent phase shifts in the two signals. It is an object of the present invention to provide a novel method of operating an optical amplifier to counteract the problems of signal degradation. It is a further object of the invention to provide novel and improved optical transceiver topologies which avoid or at least mitigate the difficulties mentioned above. According to a first aspect of the present invention an optical transmitter for use in a coherent optical system comprises a single optical source coupled to a modulating means for generating a modulated information signal characterised in that: a) the optical source is also coupled to an optical amplifier to provide an amplified, unmodulated reference signal; and that b) there is a coupling means for combining the modulated information signal and the amplified unmodulated reference signal in the same polarisation state. A transmitter according to the invention in its first aspect avoids the need for two separate optical sources, one as a transmitter, one as a local oscillator, in coherent systems. Through selective amplification of an unmodulated proportion of the source signal a transceiver according to the invention is able to provide an output signal which itself contains the strong reference signal needed for demodulation of the modulated signal at a remote receiver. Furthermore, since the reference signal and the modulated signal are both derived from the same source the problem of phase locking in homodyne receivers is overcome. According to a second aspect of the present invention an optical transceiver for use in a coherent optical system comprises an optical transmitter according to the first aspect of the invention, the optical amplifier being adapted to provide bi-directional optical amplification in one direction for the transmitted reference signal and in the other direction both for a received information signal and a received reference signal; and a receiver including a coherent detection receiver coupled to the amplifier for receiving an input signal comprising a modulated information signal and an unmodulated reference signal from a remote transmitter amplified by the amplifier. The duplex use of the optical amplifer avoids the necessity for a pre-amplifier dedicated to the receiver alone, thereby offering a further reduction in component count and complexity relative to earlier transceivers. Preferably the amplifier is a travelling wave or other substantially non-resonant amplifier. Preferably also the light travels through the amplifying medium of the amplifier once in each direction. The wavelengths transmitted by the transmitter and received by the receiver in which both incoming and outgoing signals in an optical system can be amplified without requiring separate amplifiers, for example wavelengths may be, but need not necessarily be, different. So long as the amplifier is driven to amplify the light linearly according to the input amplitude then the light from one path will not substantially interfere with light from another. Since the light from each path is amplified independently, the light input from both different paths may be amplified simultaneously. The method is therefore particularly appropriate for duplex optical systems. In a preferred embodiment of the present invention in its second aspect the optical transceiver comprises an optical source for generating a source signal; an optical receiver for receiving an input signal; a bi-directional optical amplifier for amplifying the input signal and a first proportion of the source signal; modulating means for applying a signal modulation to a separate second proportion of the source signal; first optical coupling means for coupling the amplified input signal to the receiver and for coupling the first proportion of the source signal from the source into the amplifier and the second proportion of the source signal from the source into the modulating means; and second optical coupling means for combining the modulated second proportion and the amplified first proportion of the source signal into an output signal and for coupling the input signal into the amplifier. The transceiver may be adapted for heterodyne operation by including wavelength shifting means for shifting the wavelength of the second proportion of the source signal. The combined output signal then comprises the amplified first proportion of the source signal as reference signal together with a wavelength-shifted, modulated second proportion. Conveniently, the first coupling means comprises a directional coupler and an optical isolator to prevent any signals coupling back into the source. Similarly, the second coupling means also conveniently comprises a directional coupler and an optical isolator to prevent the input signal coupling into the modulating means. Preferably, the optical source comprises a laser and provides a narrow linewidth, single mode coherent source signal. Preferably also, the optical source is tunable to allow channel selectivity in transmission. Likewise conveniently the receiver includes filter means to permit local channel selection when the input signal comprises a plurality of channels, for example. Where a transceiver is one of many forming nodes in an optical network, for instance, the tunability of source and receiver will enable any one transceiver to communicate with any other via an otherwise passive optical network. The invention in its various aspects will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 illustrates the operation of a bi-directional optical amplifier as used in the second aspect of the invention; Figure 2 is a schematic of one embodiment of a transmitter according to the invention in its first aspect; Figure 3 is a schematic of an embodiment of a transceiver according to the invention in its second aspect; and Figure 4 is a schematic of an alternative embodiment of a transceiver according to the invention in its second aspect. In Figure 1, a travelling wave optical amplifier 1, capable of optically bi-directional operation, is shown connected between two optical transmission paths 2, 3. Two optical signals 4, 6 of wavelengths λ₁, λ₂ respectively are injected into the amplifier 1 in optically opposed directions along the paths 2, 3. These signals 4, 6, are then subject to linear amplification in the amplifier and emerge as amplified signals 5, 7 along the paths 3, 2 optically opposed to those along which they were originally injected. (The signals are shown deflected in Figure 1 for convenience of illustration only). The signals are amplified independently, even when simultaneously present, as the amplification is controlled to be substantially linear. It is also not necessary for the wavelengths λ₁, λ₂ to be different. A travelling wave amplifier 1 as shown, is suitably adapted for bi-directional operation, since the signals make only one pass through the amplifier and the optically opposed exit and entry points are well defined for each signal. However, it will be appreciated that other amplifiers may equally be suitable for use according to the method so long as they fulfill the requirements that the amplification is substantially linear and that the signals transfer between paths after amplification. The operation of an optical amplifier by the method described facilitates improvements in optical transceiver topology as will be described with reference to Figures 3 and 4. Figure 2 illustrates schematically the topology of a homodyne coherent optical transmitter according to the present invention. The transmitter 22 of Figure 2 includes a tunable, transmitting laser source 10. A directional coupler 12 connects the laser 10 to a bi-directional optical amplifier 13 and to a modulator 14. The amplifier 13 and modulator 14 are themselves further coupled via a second directional coupler 15 to the input/output ports A, B of the transmitter. In operation, the laser 10 is tuned to provide light output at wavelength λ₁. This light is split by the directional coupler 12, a proportion passing to the modulator 14 whilst the remainder passes amplifier 13. The modulator 14, in this example, is a polarisation sensitive planar waveguide device of a type known in the art, it is therefore necessary to ensure that light entering the modulator has the appropriate polarisation. For this purpose, a polarisation controller 18 is provided at the output of the laser 10. The position of the polarisation controller 18 is not crucial. It could equally well be placed immediately before the modulator 14, for example, in which case a polarisation controller will be required after the modulator to bring it into the correct polarisation orientation with respect to the amplified reference signal. The modulator 14 imposes the desired modulation on the proportion of light passed to it and passes this modulated light to the second directional coupler 15. The remainder of the light from the laser 10 is unmodulated. However, this light is amplified in the amplifier 13 before it too is passed to the second directional coupler 15. This coupler 15 combines the modulated and unmodulated but amplified, light signals. The combined signals are then output into an appropriate optical communication network via either or both of the I/O ports A, B. The output signal therefore comprises an information carrying, modulated portion and an unmodulated, automatically phase locked, amplified portion. The output signal thus contains its own strong homodyne reference for use in demodulating modulated portion at a remote receiver. Consequently, there is no need for a dedicated local oscillator reference laser at that remote receiver. Referring now to Figure 3 there is shown a transceiver according to the second aspect of the present invention in which has been added to the transmitter of Figure 2 a receiver 11 containing an optical detector coupled to the previously free arm of the coupler 12. Considering the receive function of the transceiver of Figure 3, an input signal of wavelength λ₂, not necessarily the same as λ₁, arriving via either or both I/O ports A, B is coupled by the second directional coupler 15 into the bi-directional amplifier 13. An optical isolator 16 prevents coupling of any of the input signal into the modulator 14. The amplifier 13 passes the amplified input signal to the receiver 11 via the first directional coupler 12. A second optical isolator 17 prevents any of this signal coupling into the laser 10. The receiver 11 includes a narrow-band optical filter 19 to allow fine tuning of the wavelength selection. It may be desirable in some circumstances to have a heterodyne transceiver rather than a homodyne transceiver. Heterodyne systems may be more tolerant to imperfections in laser line width and disturbances such as phase-noise, for example. An embodiment of the invention as a heterodyne transceiver 23 is illustrated in Figure 4. The components generally correspond with those in Figure 3 and are numbered accordingly, with one addition. In this embodiment a wavelength shifter 20 is inserted before the modulator 14. The wavelength shifter 20 introduces the necessary intermediate frequency difference for a heterodyne system in the form of a wavelength shift Δλ₁. Consequently, and as shown, the output signal comprises an amplified, unmodulated reference signal at wavelength λ₁ together with a modulated signal at shifted wavelength λ₁ + Δλ₁. Similarly, the input signal from a remote transceiver comprises a reference signal at wavelength λ₂ and a modulated signal at wavelength λ₁ + Δλ₁. It will be appreciated that, in this embodiment, the receiver 11 is suitably adapted to demodulate the intermediate frequency signal derived from the combined input signal, as in a conventional heterodyne system. In addition to the benefits from reduced component court and complexity already mentioned, optical transceivers as described above offer considerable advantages in budgeted power gain. A representative budget is: Bi-directional amplifier gain - transmission +15dB Bi-directional amplifier gain - reception +15dB and, using only output A or B: Additional coupler losses - 6dB Excess receiver tuner loss (eg Fabry-Perot) - 1dB Excess modulator loss - 1dB Additional isolator losses - 2dB This represents a gain of approximately 20dB by virtue of te bi-directional operation of the amplifier when compared with conventional coherent systems incorporating a single amplifier. The nett additional power budget compared with a conventional homodyne system is therefore in the region of 20 dB. The homodyne technique itself is theoretically capable of about 20 dB improvement over standard direct intensity detection systems, depending on the technique adopted, but in a less than optimum system about 10 dB may be practicable. In this instance, however, it is the combination of duplex working and transmission of the signal and reference oscillator taken together with the local oscillator tuning ability that provide the performance in shorter distance networks. Heterodyne variants of the same arrangement are also possible.

Claims: An optical transmitter for use in a coherent optical system comprising a single optical source (10) coupled to a modulating means (14) for generating a modulated information signal characterised in that: a) the optical source (10) is also coupled to an optical amplifier (13) to provide an amplified, unmodulated reference signal; and that b) there is a coupling means (15) for combining the modulated information signal and the amplified unmodulated reference signal in the same polarisation state. An optical transmitter according to claim 1 including wavelength shifting means (20) for shifting the wavelength of the signal from the optical source (10) before modulation thereof by the modulating means (14). An optical transmitter according to any one of claims 1 and 2 wherein the optical source (10) comprises a laser and means for tuning the source signal. An optical transceiver for use in a coherent optical system comprising an optical transmitter as claimed in any preceding claim, the optical amplifier (13) being adapted to provide bi-directional optical amplification in one direction for the transmitted reference signal and in the other direction both for a received information signal and a received reference signal; and a receiver (11) including a coherent detection receiver coupled to the amplifier (13) for receiving an input signal comprising a modulated information signal and an unmodulated reference signal from a remote transmitter (A) amplified by the amplifier (13). An optical transceiver according to claim 4 including an optical isolator (16) arranged to prevent any signal coupling back into the source (10). An optical transceiver according to any one of claims 4 and 5 including an optical isolator (17) arranged to prevent the input signal coupling into the modulating means (14). An optical transceiver according to any one of claims 4 to 6 wherein the receiver (11) includes a filter means (19) for selecting one input signal from a plurality of input signals.

Inventors: STANLEY, IAN WILLIAM
Applicants: BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1988
Main classification: H04B10
IPC codes: H04B10