Coherent light generators – Particular active media – Semiconductor
Patent
1993-11-08
1997-02-11
Bovernick, Rodney B.
Coherent light generators
Particular active media
Semiconductor
372 46, 372 50, H01S 319
Patent
active
056028628
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to clock extraction, and in particular to clock extraction from digital optical signals.
BACKGROUND AND SUMMARY 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. Clock extraction can then be achieved by filtering at the clock frequency and rectifying the result. However, this involves signal conversion to electronic form. It is much preferable to be able to extract the clock frequency by optical means.
Demand for broadband services (such as high quality data transfer, high definition television and video conferencing) may require telecommunications networks to operate with TBit/s capacities by the first decade of the next century. In order to meet this capacity demand, all-optical or "transparent" networks have been proposed, which networks employ either high speed optical time division multiplexing (OTDM) or wavelength division multiplexing (WDM) to attain the high data-rate. The transparent optical networks rely on optical switching and routing to maintain a transparent path between the source and destination nodes.
A transparent optical network may lie above the top electronic switched transport layer of a "synchronous digital hierarchy" (SDH). A synchronisation between the traffic on the transparent optical network and the switched transport layer is required for the transparent optical network to be compatible with the SDH. This can be achieved, for example by the use of optical switches at the intermediate network nodes, these switches requiring a clock synchronisation signal from incoming traffic, the synchronisation signal also being used for the demultiplexing of channels in an OTDM system.
The networks may also need to support services with a very broad range of bit-rates from 100 MBit/s (eg. video) to many Gbit/s (eg. multiplexed data). In order to maintain this network transparency, the clock extraction technique used needs to be flexible to bit-rate.
In transmission systems, electronic clock recovery circuits are generally used, conventional techniques using electronic filtering in the post detection circuitry. For instance, a high Q electrical filter may be used to extract the clock component in a received data modulation spectrum. The lack of tunable narrow bandpass electronic filters will, however, introduce an electronic "bottleneck" into the otherwise transparent network. If the modulation spectrum does not contain a clock component, such as in non-return to zero (NRZ) format data, then an additional electronic nonlinearity is needed to generate one. Within transparent optical network architectures, electronic clock recovery techniques are disadvantageous as they are bit-rate sensitive, require the tapping of the optical signal which results in power loss, and can also require wide band electronics.
Methods of all-optical clock extraction, particularly where the clock frequency can be tuned over a wide frequency range would be extremely useful.
The present invention is based on the use of a self-pulsating semiconductor laser. Such a device is known, the self-pulsation being caused by self Q-switching within the device caused by instabilities induced by regions of saturable absorption coupling with regions of high gain. The repetition rate of emitted pulses can be controlled by varying the current to either region of a two-region device, and has been found to vary approximately as 1/I.sup.1/2. In a paper entitled "Conditions for Self-Sustained Pulsation and Bistability in Semiconductor Lasers", J. Applied Physics, Vol 58, number 4, pp 1689-1692 (1985), M Ueno and R Lang have shown, by theoretical analysis, that self-pulsation only occurs at certain ratios of the carrier lifetimes, .tau..sub.g .tau..sub.a, and differential gain, (.delta.g.sub.g .delta.n) (.delta.g.sub.a .delta.n), where the subscripts "g" and "a" refer to the gain and absorbing regions respectively, "g" is the material gain (or loss ) and "n" is the carrier density. In general, p
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Barnsley Peter E.
Robertson Michael J.
Wickes Howard J.
Bovernick Rodney B.
British Telecommunications public limited company
Wise Robert E.
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