Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1994-11-08
1996-08-20
Pascal, Leslie
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359179, 359188, 359341, 359349, 372 18, 372 26, 372 28, 372 32, H04B 1012, H04B 1017
Patent
active
055484336
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to the recovery of a clock signal from a data stream, and in particular to an all-optical system for carrying out such a function. The invention also encompasses a novel mode-locked laser.
In this specification the term "optical" is intended to refer to that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultraviolet regions at each end of the visible region which are capable for example of being transmitted by dielectric optical waveguides such as optical fibres.
BACKGROUND ART
Clock or timing recovery circuits have the purpose of extracting a timing wave at the symbol rate from a conditioned signal pulse stream. Once recovered, such a timing wave might then be used, for example, to clock subsequent signal processing stages for the pulse stream. The present invention is particularly concerned with the case where the original data stream is in the form of a modulated optical signal, as might be found, for example, in an optical telecommunications system. Hitherto, in order to recover a clock signal from such an optical data stream the optical signal has been converted from the optical to the electronic domain using e.g. a photodetector, and the electronic signal subsequently filtered to derive the timing wave. Since the bandwidth of the electronic circuitry is inevitably far smaller than that of the optical circuits, this clock recovery stage acts as a bottle-neck, limiting the performance of the system of which it forms a part.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a system for recovery of a clock from an optically encoded signal comprising:
a mode-locked laser;
a modulator in the optical path of the optical cavity of the mode-locked laser;
means for applying an optically encoded input signal to the modulator; and
means for outputting an optical pulse stream from the mode-locked laser;
the modulator in response to the optically encoded signal modulating the phase and/or amplitude of light in the optical path of the laser cavity thereby locking the phase and frequency of the output pulse stream to the timing wave of the optically encoded signal.
The present invention provides an all-optical clock recovery system, suitable for use with an RZ pulse stream. A laser is mode-locked via a modulator driven by the optical stream of data. This results in the generation in the laser of a pulse stream corresponding to the timing wave of the optical signal applied to the modulator. This all-optical system can operate at data rates as high as 100 GHz, far above the rates attainable with conventional electronic clock recovery circuits. Moreover the resulting pulse stream can be employed directly in further optical processing applications, without requiring an additional electro-optical conversion stage. It is suitable for use therefore with optical multiplexing, regeneration and memory access circuits.
Preferably the output pulse stream is used to clock a subsequent optical processing stage for the optically encoded signal.
Preferably the modulator is a non-linear optical modulator (NOM) connected in common with an optical transmission path for the optically encoded signal and with the laser cavity.
Preferably, the laser cavity includes a passive signal shaping element.
The inclusion of a passive shaping element in the cavity makes the output pulse stream more robust, and less susceptible to variations in the modulating data.
Preferably the modulator is arranged as a cross-phase modulator (XPM).
Preferably the mode-locked laser is a fibre laser. The use of a fibre laser is particularly advantageous in facilitating integration of the system with optical fibre-based transmission systems. Preferably the active medium of the mode-locked laser serves as part of the optical transmission path for the optically encoded signal, thereby amplifying the signal.
The configuration adopted for this preferred aspect of the invention, usi
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British Telecommunications public limited company
Pascal Leslie
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