Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/60—Receivers
  • H04B10/61—Coherent receivers
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/60—Receivers
  • H04B10/61—Coherent receivers
  • H04B10/615—Arrangements affecting the optical part of the receiver
  • H04B10/6151—Arrangements affecting the optical part of the receiver comprising a polarization controller at the receiver's input stage
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/60—Receivers
  • H04B10/61—Coherent receivers
  • H04B10/64—Heterodyne, i.e. coherent receivers where, after the opto-electronic conversion, an electrical signal at an intermediate frequency [IF] is obtained

Definitions

• This invention relates to optical receivers and in particular coherent optical receivers.
• the input signal is divided into two or more paths, and each path is combined with a separate local oscillator signal.
• the local oscillator signals are arranged to have a known fixed phase relationship to each other. However, the phase difference between the local oscillator signals and the input signal is not fixed. In practice the local oscillator laser will generally be offset frequency locked to the input signal. This maintains an average IF frequency which can be a fraction of the modulation bandwidth.
• the ideal multiport detection scheme would employ two local oscillators with a quadrature phase relationship between them. This arrangement should give the same receiver sensitivity as an equivalent single detector heterodyne receiver, i.e. 3dB worse than perfect homodyne detection.
• Operating a multiport detection receiver with more than two detectors produces further reduction in performance and considerably complicates the electronic circuits needed to process the electrical signals from the photodetectors.
• generation of the correct phase relationship required for implementation with only two detectors, between the two local oscillator signals of 90° for in-phase and quadrature detection has been found to present significant problems. To avoid these problems it has been proposed by A.W. Davies (op/cit) to employ a three-phase local oscillator and use the special properties of a six-port fused optical fibre coupler to implement the system.
• the present invention aims to provide an improved multiport receiver capable of operating stably with two detectors.
• optical multiport coherent detection is achieved by employing relative polarisation rotation between the optical local oscillator signal and the input signal to provide the required phase relationship for in-phase and quadrature detection.
• the present invention solves the problem of providing local oscillator signals in phase and in quadrature with corresponding input signal components by using, and where necessary controlling, the polar isationn properties of the input and/ or the local oscillator signal such that their respective signal powers can be evenly divided by polarisation splitting. It is important to note that, unlike hitherto, in the solution provided by the present invention the local oscillator signal, or the input signal, does not always necessarily divide individually into two signal components phase shifted by 90° of angle with respect to each other; instead, it is only the necessary instantaneous phase relations between input and local oscillator signal components which are continuously maintained.
• the invention may, for example, employ a linearly polarised input signal whose power is divided evenly into two orthogonally linearly polarised components, and a local oscillator signal which is circularly polarised and similarly divided.
• the input signal components are in phase, and the local oscillator signal components 90° out of phase.
• both the input signal and local oscillator signal are elliptically polarised, provided their relative polarisations are such that the instantaneous phase relation required for in-phase and quadrature detection can be achieved.
• the components of neither the input signal nore the local oscillator signal have individually a phase difference of 90° of angle, but the instantaneous in-phase and quadrature relationship required for multiport detection can nevertheless be achieved.
• an optical coherent receiver which comprises an optical input for an information modulated signal, an optical local oscillator, and power dividing means for evenly dividing the input signal power and the local oscillator signal power between an in-phase and a quadrature detection path
• the power dividing means comprise a polarisation splitter
• polarisation control means are provided to control the relative polarisation of the input signal and the local oscillator signal for in-phase and quadrature detection to occur .
• the polarisation control means need to be capable of ensuring a desired polarisation of the input signal.
• a method of performing optical coherent detection comprises evenly dividing optical input signal power and optical local oscillator signal power between an in-phase and a quadrature detection path by polarisation splitting, and controlling the relative polarisation of the input signal and the local oscillator signal for in-phase and quadrature detection to occur.
• a substantially linearly polarised input signal and a substantially circularly polarised local oscilaltor signal are employed.
• elliptically polarised input and local oscillator signals may be employed.
• Figure 1 is a schematic diagram of an optical coherent receiver in accordance with the present invention
• Figures 2A, 2B and 2C are schematic diagrams of detection circuits for use with the receiver of Figure 1 , and suitable for ASK, DPSK, and PSK modulation schemes , respectively
• Figure 3 is a schematic diagram of an alternative circuit for polarisation control .
• a digitally modulated optical input s ignal passes along fibre path 1 and is coupled into a pol arisation control device 2 which is itself controlled by control circuitry 28.
• the input pol arisation to the control device 2 may be of any arbitrary state.
• the control ler 2 is used to adjust the input polarisation to a known state such that the received signal is divi ded half/half between two detectors 5 and 6 by a pol arisation splitter 26. Most simply, the input signal wi l l be adjusted to be in a l inear state of pol arisation with appropriate orientation.
• the output of the pol arisation control ler 2 is then combined with the output of a local oscillator l aser 25 in a beam combiner device 3.
• the combiner 3 coul d be a partially reflecting mirror or a fibre combiner.
• the pol arisation of the local oscil lator signal is also chosen to have a sui table polarisation state as expl ained below.
• the state woul d be circular polarisation and this can be obtained by the use of a suitable retardation pl ate 4 interposed between the local oscillator laser 25 and the combiner 3.
• the combined input and local oscillator signal is coupled, by optical fibre, to a pol arising beam spl itter 26.
• the outputs from the polarising beam splitter are coupled to two separate photo detectors 5 and 6.
• circuits 7 and 8 are then processed by circuits 7 and 8 before addition in circuit 9.
• the detail of circuits 7 and 8 will depend on the type of modulation used, and Figures 2a to 2c illustrate some suitable examples.
• each of the circuits 7 and 8 may comprise, as shown in Figure 2a, an amplifier, filter 11 and non-linear device 12.
• the latter could be a square law detector or rectifier.
• the circuits 7 and 8 may consist of the following: a signal amplifier 13, a low pass filter 14, a signal splitter 15, two electrical paths 16 with a difference in length of propagation time equivalent to the bit period and a multiplier device 17.
• the circuits 7 and 8 each contain an amplifier 18, an IF carrier recovery circuit 19 and a multiplier 20.
• the combined electrical output from the two channels 9 is then further filtered before regeneration 21 in the usual manner.
• an automatic frequency control loop (AFC) 22 is included to enable the local oscillator laser to track the average frequency of the input signal with a fixed offset. This offset is usually called the IF as in heterodyne detection.
• Control signals to the circuit 28 controlling the polarisation control device 2 may be obtained by detecting an electrical signal at twice the IF at point 9. This is achieved by using a band pass filter 23. Further control signals may be obtained by comparing the average signal level from the two receivers in comparitor/ integrator combination 24.
• the polarisation control system may be expected to use techniques such as dither modulation to aid its operation.
• Figure 3 shows the relevant portions of an alternative, and for its greater simplicity preferable, embodiment of a polarisation control circuit which may be substituted in the receiver of Figure 1.
• the control signal for polarisation control is now derived directly from the outputs of the in-phase and quadrature detection circuits 7, 8.
• the outputs of the detector circuits 7, 8 are applied to a double balanced mixer circuit 50.
• the output of the double balanced mixer is a sin(theta)cos(theta) product, which is zero when theta is 90° (or a multiple of
• the present invention provides a relatively simple technique for deriving stable local oscillator signals with quadrature phase relationships which promises better system performance than schemes using, for example, three or more local oscillator phases.
• the key feature of the technique according to the invention is that use is made of the properties of relative polarsation between the input and the local oscillator signals to obtain the relative phase differences required for detection.
• the received signal is linearly polarised
• the local oscillator signal is circularly polarised to generate the required 90° phase difference between the two local oscillator signals.
• the input signal will often be somewhat elliptically polarised from the attempt to split the input signal power exactly half/half between the two detectors, and the state of polarisation of the local oscillator signal will then have to be such as to maintain the appropriate phase relation between its components and those of the received signal.
• Other important aspects of the invention are the capability it provides to demodulate signals of various formats and the control systems necessary for stable operation. The principles should find particular appl ication to optical fibre systems working the wavelength regions 0.8 to 1.7 microns but of course could also be used throughout the optical spectrum with suitable components .

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Optical Communication System (AREA)
• Digital Transmission Methods That Use Modulated Carrier Waves (AREA)
• Measuring Instrument Details And Bridges, And Automatic Balancing Devices (AREA)

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• 1985
  • 1985-06-06 GB GB8514264A patent/GB8514264D0/en active Pending
• 1986
  • 1986-06-06 JP JP61503509A patent/JPS63500067A/ja active Pending
  • 1986-06-06 AU AU59941/86A patent/AU5994186A/en not_active Abandoned
  • 1986-06-06 EP EP19860903514 patent/EP0227729A1/de not_active Withdrawn
  • 1986-06-06 WO PCT/GB1986/000327 patent/WO1986007513A1/en not_active Ceased

Non-Patent Citations (1)

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