Optical clock recovery

Details Optical clock recovery Optical clock recovery

1993-08-02

1995-08-22

Boudreau, Leo H.

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

359158, H04J 1408, H04B 1000

Patent

active

054445604

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to clock recovery, and in particular to the recovery of a clock signal from a digital optical data signal in which there is no significant component at the clock frequency.


BACKGROUND OF THE INVENTION

In return-to-zero (RZ) coding the frequency spectrum of a coded signal will include a strong peak at the clock frequency. The presence of such a strong peak means that clock recovery can be achieved simply by filtering at the clock frequency.
In non-return-to-zero (NRZ) and similar coding systems there is no such peak in the frequency spectrum of the coded signals. Indeed, the frequency spectrum of such signals will show a dip to near zero power at the clock frequency. Consequently, with NRZ and similar coding systems, it is not possible simply to filter out a component at the clock frequency in order to perform clock recovery.
The most common form of digital coding in which the coded data contains no significant component at the clock frequency is NRZ coding. For ease and clarity of description, the present invention is described herein with reference to NRZ data and NRZ coding. It is to be understood, however, that the invention is also applicable to coding systems which are not, strictly-speaking, NRZ, but which nevertheless produce data streams which do not contain a significant component at the clock frequency, and for which clock recovery is required. Hence, throughout the description, unless the context clearly requires otherwise, references to NRZ systems should be taken to include systems which are not NRZ but to which the present invention is nevertheless applicable.
For processing NRZ coded data signals, it is necessary to generate a clock signal synchronous with the data signals. In processing NKZ optical signals, it is conventional simply to convert the NRZ optical signals into NRZ electrical signals, the NRZ electrical signals then being processed electronically to generate a clock signal. If the signal processing is being carried out preparatory to onward transmission of optical NRZ signals, further conversion is required to generate NRZ optical signals from the processed NRZ electrical signals. Such conversion, which is currently employed in regenerators and at exchanges, is disadvantageous for several reasons. Thus, in regenerators, the complexity and power consumption of the requisite electronic circuitry are limiting factors. Moreover, both in exchanges and regenerators, the use of electronic processing imposes, in the long-term, an upper limit on the optical bit-rate used.
The need for this electronic processing also stands in the way of the realisation of one of the next major advances sought in optical telecommunications, namely all-optical routing.


SUMMARY OF THE INVENTION

The present invention provides a technique whereby a synchronous clock signal can more readily be generated. The technique can also be used in applications in which a received optical signal is passed for onward transmission without regeneration, a clock signal being generated from an input optical signal without destruction of that input signal. The invention is thus applicable to systems for all optical-routing.
The present invention provides a method of generating a clock signal from a digital optical data signal not having a significant clock component, the method comprising the steps of: device; the digital optical data signal in the device, and generating a further signal from the device by means of said detection; and the digital optical data signal.
Advantageously, said non-linear variation includes a non-linear photon density variation, and said further signal includes an optical signal which is amplified by the opto-electronic device, the amplified further signal leaving the opto-electronic device via an optical output.
Preferably, said non-linear variation includes a non-linear variation in electrical carrier concentration, and said further signal includes an electrical signal which leaves the opto-electronic device via an electrical output. In this case,

REFERENCES:
patent: 5339185 (1994-08-01), Kaede
"Clock Extractor Using Saturable Association in a Semiconductor Nonlinear Optical Amplifier" Barnsley et al., IEEE Photonics Technology Letters vol. 3, No. 9, Sep. 1991, pp. 832-834.
"Absorptive nonlinear semiconductor amplifiers for fast optical switching", Barnsley et al., SPIE, vol. 1378 Optically Activated Switching (1990), pp. 116-126.
"Optical Digital High-Speed Transmission; General Considerations and Experimental Results", Albrecht et al., IEEE Photonics Transactions on Microwave Theory and Techniques, vol. 30, No. 10, Oct. 1982, pp. 1535-1547.
"Clock Extraction Using Saturable Absorption in a Semiconductor Nonlinear Optical Amplifier", Barnsley et al., IEEE Photonics Technology Letters, vol. 3, No. 9, Sep. 1991, pp. 832-834.
"Coherent Optical Tapping Using Semiconductor Optical Amplifier", Emura et al., IEEE Photonics Technology Letters, vol. 2, No. 8, Aug. 1990, pp. 565-567.
"All-Optical Timing Extraction Using a 1-5.mu.m Self Pulsating Multielectrode DBF LDF", Jinno et al., Electronics Letters, vol. 24, No. 23, 10 Nov. 1988, pp. 1426-1427.
"Optical Retiming Regenerator Using 1-5.mu.m Wavelength Multielectrode DFB LDs", Jinno et al., Electronics Letters, vol. 25, No. 20, 28 Sep. 1989, pp. 1332-1333.
"Broadband Operation of InGaAsP-InGaAs Grin-SC-MQW BH Amplifiers with 115 mW Output Power", Bagley et al., Electronics Letters, vol. 26, No. 8, 12 Apr. 1990, pp. 512-513.

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