Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1997-09-22
2002-04-30
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06381056
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical signals and to optical signal generation for use in optical communications, and finds particular application in optical time division multiplexing (OTDM).
2. Related Art
The use-of OTDM signals currently offers both access to aggregate data capacities beyond the reach of commercial broadband electronics, and the additional flexibility of optical routing without recourse to high capacity electronic switches.
Typically, short pulses may be encoded and interleaved to produce a traditional OTDM data sequence, or modulators may be used to shape pulses and form an optically multiplexed signal by combining several such sequences. Both of these optical techniques require multiple optical paths and accurate synchronisation of the optical path lengths. It is also important in an OTDM interleaver in these known arrangements to exhibit a high enough extinction ratio to avoid interference effects at its output between data channels. Furthermore, the maximum line rate (eg 100 GHz) of an OTDM system is determined to a large extent by the width of the base rate leg 10 GHz) pulses, as highlighted in “Transmission of a true single polarisation 40 Gbitis soliton data signal.”, Electronics Letters, vol. 29, no. 11, pp990-992.
One alternative method of producing an OTDM signal is described in “All-optical time division multiplexing using four-wave mixing”, Electronics Letters, vol. 30, no. 20, pp 1697-1698. In this paper OTDM is achieved by modifying a 100 GHz 1547 nm optical signal by time-delayed 6.3 Gbit/s signals to generate sub-channels in a 1557 nm 100 Gbit/s signal via four-wave mixing. This method requires a series of wavelength division multiplexers each adding a sub-channel, or data channel, to an OTDM stream. Whilst this method relaxes the constraints on the extinction ratio of the 100 GHz signal, precise control of optical delays is still required.
This and other methods known to the applicants deal only with what may be termed “bright pulse” OTDM transmission.
SUMMARY OF THE INVENTION
The present inventors have realised that it is both possible and advantageous to implement optical communications systems in which “dark pulses” replace “bright pulses” as the information-bearing component.
According to one aspect, the present invention provides a method of generating an optical data signal, wherein dark pulses representative of one or more data sequences are imposed by at least two dark pulse generators onto an optical input signal received by said generators for subsequent transmission along an optical fibre, the dark pulse generators being in optically coupled alignment with the optical input.
The optical input may comprise a substantially continuous burst of optical radiation, such as might be provided by the output of a continuous wave optical signal generator. Alternatively, the optical input may comprise a pulse train such as might be provided by an optical clock. The effect on an incoming pulse train such as that provided by an optical clock might then be that bright pulses are missing from the pulse train.
In this specification, a “dark pulse” is a temporal gap, or region of reduced intensity radiation, in incoming optical radiation, or light beam. (Although generally in optical communications, the emphasis lies on speed and therefore short pulse lengths are advantageous, the term “dark pulse” should not be taken to indicate of itself a limitation on the length of the temporal gap, or region of reduced intensity radiation,)
An advantage of using dark pulses in place of bright pulses is that optical signal generation may be simplified, as will be discussed in the subsequent description.
Each of the dark pulse generators may provide a respective data signal and these may be interleaved so as together to provide an OTDM signal. A particularly convenient way of providing the interleaving is to fabricate the dark pulse generators on a common substrate, optically aligned so that the output of one, carrying its dark pulse data train, is fed straight to the next which can then add its own in a different time slot of the OTDM signal. As is further discussed below, this avoids the use of optical delay lines although it is still necessary to provide electrical synchronisation between the dark pulse generators.
(It should be noted that embodiments of the invention are not limited to OTDM however as there may clearly be other applications which benefit from the present invention. It would be possible for instance to produce a single output signal by means of consecutive dark pulses being generated by consecutive dark pulse generators. Such an arrangement may still benefit from a speed advantage.) Use of dark pulses in OTDM is particularly advantageous. While pulse alignment remains important for dark pulse OTDM, the extinction ratio is less of a problem than in bright pulse OTDM. This is because, in bright pulse OTDM, there have to be provided multiple parallel optical paths so that each pulse generator can add its pulses without “blotting out” pulses imposed by another pulse generator. These multiple optical paths have to be recombined and that causes interference problems because of phase variations in the background. The random interference causes errors unless there is an extinction ratio of about for instance 40 dB in a four channel system. In dark pulse OTDM however, there is only the one optical path, therefore there does not have to be recombination of the paths and the interference effects described simply do not happen. Although there is still a constraint on the extinction ratio in dark pulse OTDM, it is a question of the power budget at the receiver. A reasonable extinction ratio in a four channel system for dark pulse OTDM is more likely to be of the order of 15 dB.
In a preferred embodiment, each one of the dark pulse generators generates dark pulses for just one data channel of the OTDM signal. Electronics would thus only limit the data rate of a single data channel. The overall optical signal data rate could then be well beyond the aggregate data rate of commercial broadband electronics.
Preferably, a dark pulse generator comprises an optical modulator having both high and low optical transmission states and a high optical extinction ratio (although, as discussed above, not as high as that required for bright pulse OTDM). The operation in one or other state may for example be determined by an electrical bias signal. The optical extinction ratio, as discussed above, could for instance be signicantly less than 40 dB in a four data channel system, for instance lying in the range 10 to 30 dB, and a reasonable value being of the order of 15 dB.
The applicants have shown that a suitable optical modulator is an electro-absorption modulator (EAM). A suitable electrical bias signal for an EAM comprises clock (for example a sinewave) and data components combined, for example, using a simple power splitting arrangement, where a dark pulse is formed when both clock and data components are negative. This arrangement obviates the need for signal processing in the electrical domain which would otherwise be necessary to provide a suitable data-encoded bias signal.
Preferably, an EAM is biased to provide high optical extinction for a short period of time to encode a dark pulse onto a light beam input. When no data is present, the electrical signal is arranged to bias the EAM to be in its high optical transmission state.
In a preferred embodiment of the present invention, a plurality of EAMs are optically cascaded and are arranged to generate dark pulses on a light beam from a single light source.
Preferably, each EAM is arranged to generate one OTDM channel. This arrangement has the advantage that each EAM leaves light substantially unperturbed between the dark pulses which it generates, and each subsequent modulator in the cascade can then modulate the unperturbed light. Thus, with suitable electrical timing it is possible to generate a high-speed OTDM data signal.
In theory, any number of EAMs ma
British Telecommunications public limited company
Nixon & Vanderhye P.C.
Pascal Leslie
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