GB2191644A

GB2191644A - Optical signal processing

Info

Publication number: GB2191644A
Application number: GB8613894A
Authority: GB
Inventors: Kevin Christopher Byron
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1986-06-07
Filing date: 1986-06-07
Publication date: 1987-12-16
Anticipated expiration: 2006-06-07
Also published as: GB2191644B; GB8613894D0

Abstract

An optical signal, which may be the output of a gas laser (1) which is not readily modulated, is modulated in a required manner with the aid of the Kerr effect. An optical beam output, for example from a semiconductor laser (4), which is modulated in the required manner is launched into a polarisation maintaining optical fibre (3) together with the optical signal output of the gas laser (1). The modulated optical beam is such as to induce the Kerr effect in the fibre in a manner corresponding to the modulation e.g. pulsed, and thus to induce corresponding polarisation variations in the beam from the other laser (4). The output of the fibre corresponding to that of the first laser is thus modulated in the required manner. To reduce the requirements on the semiconductor laser for high rate arrangements, a plurality of semiconductor lasers may be used, each suitably modulated and time division multiplexed together (Figure 2 not shown). <IMAGE>

Description

SPECIFICATION Optical signal processing This invention relates to optical signal processing and in particular, but not exclusively, to the modulation of the output of a laserfor high bit rate communications systems and optical pulse generation.

According to one aspect of the present invention there is provided a method of modulating an optical signal in a predetermined manner comprising the steps of providing an optical beam modulated in the predetermined manner, and launching the optical signal and said modulated optical beam into a polarisation maintaining optical fibre, the modulated optical beam serving to induce the optical Kerr effect in the fibre and corresponding polarisation variations in the optical signal, the output ofthefibre comprising the optical signal modulated in said predetermined manner.

According to another aspect ofthe present invention there is provided an arrangement for modulating an optical signal in a predetermined manner, comprising means for producing an optical beam modulated in the predetermined manner, a polarisation maintaining optical fibre, and means for launching the optical signal and said modulated optical beam into said optical fibre, wherein in use the modulated optical beam serves to inducethe optical Kerr effect in the fibre and corresponding polarisation variations in the optical signal, and the output ofthe fibre comprises the optical signal modulated in said predetermined manner.

Embodiments ofthe invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates schematically an optical pulse generator, and Figure2 shows a schematic arrangement for modulating a laser beam fora high bit rate communications system.

Some lasers, for example gas lasers, are not easily modulated without recourse to modelocking. It is thus proposed to modulate the output of another laserwhich is easily modulated and to usethisto modulate the output of the first mentioned laser.

This can be achieved by launching both the output of the first mentioned laser and the modulated output of the other laser into a polarisation maintaining optical fibre and employing the optical Kerr effect.

The other laser comprises a pump laser. In the presence of a continuous wave input signal from the first mentioned laser and a pulsed pump signal from the first mentioned laser and a pulsed pump signal of a sufficient power, the optical Kerr effect in the polarisation maintaining fibre serves to rotate the polarisation ofthe input signal for each pulse ofthe pump signal, andthusto modulatetheinputsignal.

The pump laser has to be modulated at the desired rate but can operate at a different wavelength and may be a laserthat is easierto modulate than a gas laser.

Figure 1 illustrates a basic optical pulse generator which comprises a gas laser 1 whose continuous wave output is directed towards a dichroic mirror 2 for reflection into one end of a polarisation maintaining optical fibre 3. A laser 4, which may be a semiconductor laser, has its output modulated, for example by adding a pulsed modulating signal to the dc bias current driving the laser, and this pulsed pump signal is transmitted to the fibre 3 via the mirror 2. Inthefibre3the pulsed pump signal induces refractive index variations due to the Kerr effect and these serve to correspondingly rotate the polarisation of the continuous wave output of laser 1 in a pulse-like manner. The output of fibre 3 is applied to a polariser 5 arranged to block output pulses of low intensity.The output ofthe polariser then corresponds to the output ofthe continuous wave gas laser 1 modulated at the same rate as the laser 4. The gas laser may have an output at say 1 .5m whereas the semiconductor laser has an output at 1 .3cm. Optical pulse generators are useful in various research applications, for exampleto study dispersion in optical fibres.

High bit rate communications systems require modulated laser beams and whilst an arrangement as described with respect to Figure 1 could be empioyed,the demands on the semiconductor laser 4are reduced if instead a plurality of semiconductor lasers were driven in a time division multiplexed manner. Such an arrangement is illustrated in Figure 2. A continuous wave gas laser 10 operating at say 1 .5m is modulated by a laser beam output from an arrangement consisting of three semiconductor lasers 11,12,13 and two couplers 14,15. Each laser is modulated as before at one third of the bit-rate.Both of the two laser beams are launched into the one end ofapolarisation maintaining optical fibre 16 by use of dichroic mirror 17. In thefibrethe polarisation of the output ofthe laser 10 is changed in a pulsed manner by the induced variations in refractive index due to the Kerr effect, as described above. Poly riser 18 serves to block low intensity output pulses. The output of the polariser corresponds to the output of the continuous wave gas laser 10 modulated in the corresponding manner to the semiconductor lasers 11,12,13, whose outputs are applied to thefibre in a time division multiplexed manner. Since the three semiconductor lasers are driven atone-third ofthe bit rate the reduced mark-space ratio allows them to be driven in a less demanding way for high bit-rate applications.

The polarisation maintaining optical fibre may, for example, be ofthe elliptical, bow-tie or panda type.

Claims

1. A method of modulating an optical signal in a predetermined manner comprising the steps of providing an optical beam modulated in the predetermined manner, and launching the optical signal and said modulated optical beam into a polarisation maintaining optical fibre, the modulated optical beam serving to induce the optical Kerr effect in the fibre and corresponding poiarisation variations in the optical signal,the output ofthefibre comprising the optical signal modulated in said predetermined manner.

2. A method as claimed in claim 1 ,wherein the optical beam modulated in the predetermined manner comprises the modulated output of a semiconductor laser.

3. A method as claimed in claim 1, wherein the optical signal the output of a gas laser.

4. A method as claimed in claim 1, wherein the optical beam modulated in the predetermined manner is produced by a plurality of semiconductor lasers whose outputs are suitably modulated and time division multiplexed together.

5. A method as claimed in claim 4 and comprising three modulated semiconductor lasers whose outputs are time division multiplexed together.

6. A method as claimed in any one of the preceding claims including the step of applying the fibreoutputto a polariser arranged to block low intensity output pulses.

7. A method of modulating an optical signal substantially as herein described with reference to and as illustrated in Figure 1 or Figure 2 ofthe accompanying drawings.

8. An arrangement for modulating an optical signal in a predetermined manner, comprising meansforproducing an optical beam modulated in the predetermined manner, a polarisation maintaining optical fibre, and means for launching the optical signal and said modulated optical beam into said optical fibre, wherein in use the modulated optical beam serves to induce the optical Kerr effect inthefibreand corresponding polarisation variations in the optical signal, and the output ofthe fibre comprises the optical signal modulated in said predetermined manner.

9. An arrangement as claimed in claim 8 wherein said means for producing the modulated optical beam comprises at least one semiconductor laser.

10. An arrangement as claimed in claim 8 or claim 9 wherein said launching means includes a dichroic mirror.

11. An arrangement as claimed in anyone of claims8to 10including a gas laserfromwhichthe optical signal is output.

12. An arrangement as claimed in anyone of claims 8to 11 and including a polariser, at the fibre output, arranged to block low intensity pulses.

13. An arrangement as claimed in claim 9 and comprising three modulated semiconductor lasers whose outputs aretime division multiplexed together.

14. An arrangement for modulating an optical signal substantially as herein described with reference to and as illustrated in Figure 1 or Figure 2 ofthe accompanying drawings.