Optical pulse seqeunce generator

Details Optical pulse seqeunce generator Optical pulse seqeunce generator

1996-03-25

1998-08-18

Scott, Jr., Leon

Coherent light generators

Particular beam control device

Control of pulse characteristics

372 6, 372 18, 372 26, H01S 310

Patent

active

057967652

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND TO THE INVENTION

The present invention relates to a method and apparatus for generating a desired optical pulse sequence in response to an input optical signal. The invention also encompasses an all-optical switch based on such pulse generation techniques. The present applicants, earlier International application PCT/GB93/00863 filed 26th Apr. 10 1993, and the paper by K. Smith and J. K. Lucek, Electronic Letters,28,1814 (1992) disclose a system in which a pulse sequence input to a modulator in the cavity of a mode-locked laser results in the laser outputting a clock signal corresponding to the timing waveform of the input data, i.e. having an output pulse corresponding to each bit position of the input pulse sequence, irrespective of whether there is a one or a zero (a pulse or a null) in a given bit position.
This clock may be used in producing a regenerated signal but this requires a further stage in which the recovered clock is modulated by the original signal.


SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a method of generating an optical pulse sequence comprising applying a repetitive optical signal to an optical modulator connected in the laser cavity of a mode-locked laser, and outputting a pulse sequence from the laser cavity, characterized in that the repeat period of the repetitive optical signal is equal to, or an integer multiple of, the cavity round-trip time, the laser thereby being driven to output pulses in bit positions corresponding to the non-zero pulses of the input repetitive optical signal.
The first aspect of the present invention provides an all-optical method for generating a desired pulse sequence. The output pulse sequence is found to have superior pulse characteristics with reduced noise and timing jitter, and so is suitable to be used as a regenerated signal to replace the input signal. As further described below, the output pulse sequence may also be used, for example, in the operation of an all-optical demultiplexer, or in other signal processing or switching devices.
The present inventors have found that if a repetitive signal, such as a repeated multi-bit word, or a sequence of OTDM frames all having zeros in certain predetermined bit positions, is used as the input data stream for a modulator in a laser cavity, and if the repetition period is matched to the round trip time of the cavity, or a multiple thereof, then instead of outputting a clock signal, the laser replicates the input pulse sequence. That is, the output from the laser, although as before being locked to the timing of the input pulse sequence, now has zeros where there are zeros in the input pulse pattern, or, in the case of an OTDM frame, zeros in those bit positions corresponding to those channels in the programming data stream which are set OFF.
Preferably the laser is arranged to continue to output the pulse sequence after the input pulse sequence has been removed or modified.
The method of this aspect of the invention may be used to provide an optical memory which maintains the pulse sequence at its output even after the original generating sequence has been removed. To this end, the cavity may include a pulse shaping element so that it functions as a hybrid active/passive mode-locked device. Then, after initial active mode-locking in response to the input pulse stream, the laser remains passively mode-locked to sustain the output pulse sequence. Appropriate pulse shaping devices are discussed in the above-cited paper and International application. Alternatively, the laser may be actively mode-locked by an additional modulator driven by an appropriate clocking signal.
According to a second aspect of the present invention, there is provided a method of controlling an optical switch,
characterized by a applying a repetitive optical signal to a modulator connected in the cavity of a laser thereby mode-locking the laser, the repeat period of the repetitive optical signal being equal to or an integer multiple of the cavity round-trip time,
out

REFERENCES:
patent: 5477555 (1995-12-01), Debeau et al.
patent: 5497386 (1996-03-01), Fontana
patent: 5513194 (1996-04-01), Tamura et al.
patent: 5577057 (1996-11-01), Frisken
patent: 5623362 (1997-04-01), Nitsuda et al.

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