Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1988-12-15
1992-01-28
Kuntz, Curtis
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359154, H04B 1000
Patent
active
050847791
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to coherent optical systems and in particular a transmitter and transceiver for use in coherent optical systems.
2. Description of Related Art
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 recognized, 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 168192 A2 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 (e.g. an optical fibre) which may introduce polarisation dependent phase shifts in the two signals.
SUMMARY OF THE INVENTION
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 and to an optical amplifier to provide an amplified, unmodulated reference signal; and a coupling means for combining the modulated information signal and the amplified, unmodulated reference signal.
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 amplifier 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 wav
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British Telecommunications public limited company
Kuntz Curtis
Negash Kinfe-Michael
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