WO2009068327A1

WO2009068327A1 - Optical pulse generation

Info

Publication number: WO2009068327A1
Application number: PCT/EP2008/053870
Authority: WO
Inventors: Antonella Bogoni; Luca Poti; Mirco Scaffardi; Paolo Ghelfi
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2007-11-28
Filing date: 2008-04-01
Publication date: 2009-06-04

Abstract

An optical pulse generator comprises a source of polarised optical pulses each having a first frequency spectrum, a length of non-polarisation maintaining highly nonlinear fibre (HNLF) connected to the source, and a polarisation rotator connected to the HNLF. An optical pulse output from the source is guided along the HNLF to the polarisation rotator, whereat the polarisation of the pulse is rotated, and wherein the rotated pulse is re-input to the HNLF, guided along the HNLF, and output from the HNLF as a modified optical pulse. The modified optical pulse has a second frequency spectrumthat is broader than the first frequency spectrum due to interaction between the optical pulse with the HNLF, and wherein the modified optical pulse is substantially polarised.

Description

OPTICAL PULSE GENERATION

TECHNICAL FIELD

The invention relates to a method of and apparatus for generating an optical pulse or pulses. The invention relates particularly, but not exclusively, to the generation of a "broad spectrum" pulse, that is, an optical pulse containing a broad range of frequencies, sometimes of the type known as a supercontinuum. In one aspect, the invention may find use in an optical communications network.

BACKGROUND

In photonic applications, such as in WDM/OTDM (wavelength division multiplexed/optical time division multiplexed) communications network, it can be useful to generate radiation at a variety of wavelengths. For example, optical sources able to generate short pulses of radiation at tunable wavelengths in the whole C-band have considerable potential in ultra-fast photonic applications. All-optical wavelength conversion of short pulse signals could be useful to allow wavelength routing in an ultra- fast hybrid WDM/OTDM network. In addition, replica generation at different wavelengths is useful for applications such as ultra-fast optical sampling.

It is known to provide tunable radiation sources such as tunable lasers. However, if it is required to transmit more than one wavelength of radiation simultaneously, more than one tunable laser must be provided, at significant expense.

It is an object of the invention to provide an alternative source of wavelength tunable radiation pulses. SUMMARY

According to a first aspect of the invention there is provided an optical pulse generator comprising: a source of polarised optical pulses each having a first frequency spectrum, a length of non-polarisation maintaining highly nonlinear fibre (HNLF) connected to the source, and a polarisation rotator connected to the HNLF, wherein an optical pulse output from the source is guided along the HNLF to the polarisation rotator, whereat the polarisation of the pulse is rotated, and wherein the rotated pulse is re-input to the HNLF, guided along the HNLF, and output from the HNLF as a modified optical pulse, wherein the modified optical pulse has a second frequency spectrum that is broader than the first frequency spectrum due to interaction between the optical pulse and the HNLF, and wherein the modified optical pulse is substantially polarised.

The modified pulse is substantially polarised because some of the alterations to the polarisation of the input pulse caused by the first passage through the HNLF are cancelled out by sending the rotated pulse along the same HNLF a second time. Because the pulse is rotated, the rotated pulse experiences different, and in some cases, opposite, alterations to those experienced by the input pulse, such that those second alterations tend to cancel out the first alterations.

The polarisation rotator may be operable to rotate the polarisation of the optical pulse output from the HNLF through between 60° and 120°, and preferably to rotate the polarisation of the optical pulse by substantially 90°. As described in detail below, 90° (or (90+nl 80)°, where n is an integer) is the degree of rotation that has been found to be most effective in cancelling out the polarisation fluctuations caused by passage through a HNLF.

The optical pulse may be guided along the HNLF to the polarisation rotator in a first direction, and guided along the same HNLF from the polarisation rotator in a second direction opposite to the first direction. The pulse may travel once in each of the first and second directions. Alternatively, the optical pulse may travel along the HNLF in the same direction twice. However, in both instances, the optical pulse passes along the same portion of HNFL both times, so that the optical pulse experiences substantially the same conditions in both passes. That is, the optical pulse travels along the same path twice, with its polarisation rotated when it travels in one direction as compared with its polarisation as it travels in the other direction.

The optical pulse generator may further comprise a polarisation beam splitter operable to direct at least some of the optical pulse from the source into the HNLF, and to direct at least some of the modified optical pulse from the HNLF to an output of the generator. The polarisation beam splitter may comprise a first output and a second output, wherein substantially all of the optical pulse emitted from the source is directed to the first output, because it has a first polarisation, whilst substantially all of the modified optical pulse is directed to the second output, because it has a second polarisation that is 90° to the first polarisation. This may be achieved, for example, using a polarising material which permits radiation of one polarisation to pass through it unimpeded, but diverts, for example, reflects, radiation of a perpendicular polarisation.

To feed the rotated pulse back into the HNLF in the second direction, the optical pulse may be reflected. The optical pulse may be rotated before, after, or before and after, it is rotated. In the latter case, the polarisation rotator may be a Faraday rotator mirror, which is able to both reflect optical radiation input to it, and rotate the polarisation of that radiation through 90°.

The modified optical pulse may be a broadband pulse containing a plurality of different wavelengths, such as a super continuum. The modified optical pulse may be a broadband pulse containing a plurality of wavelengths between 1525nm and 1575 nm (that is, wavelengths in the 'C-band' commonly used in the telecommunications industry). The HNLF may be specifically chosen because it is able to generate a supercontinuum in a particular frequency range, such as the C-band.

The source of optical pulses may be a mode locked laser. The mode locked laser may be operable to generate pulsed radiation at at least one frequency in the C-band, preferably a frequency of substantially 1550nm. The mode locked laser may employ regenerative feedback, or may rely on a clock signal, in order to mode lock the laser.

The duration of the first optical pulse input to the HNLF may be chosen to ensure that the second optical pulse output from the HNLF has an appropriate frequency spectrum. The first optical pulse may have a duration of between lps and 20ps. The first optical pulse may have a duration of between 5ps and 15ps.

The generator may further comprise an amplifier to boost the power of an optical pulse produced by the source before the optical pulse is input to the HNLF. The generator may be temperature controlled, perhaps to assist in maintaining a constant output.

According to a further aspect of the invention there is provided a node for a communications network comprising an optical pulse generator in accordance with the first aspect of the invention.

The node may further comprise a filter, such as a bandpass filter, operable to filter a selected wavelength from the modified optical pulse to produce a filtered optical pulse. The node may comprise one or more filters operable to select a plurality of wavelengths from the modified optical pulse to produce a plurality of filtered optical pulses. The filter or filters may be operable to produce the plurality of filtered optical pulses substantially simultaneously.

The node may further comprise a modulator operable to modulate one or more communication signals onto the filtered optical pulse or pulses.

According to another aspect of the invention there is provided a method of generating an optical pulse comprising: inputting a first pulse of polarised optical radiation having a first frequency spectrum to a non-polarisation maintaining highly nonlinear fibre (HNLF); rotating the polarisation of the optical pulse output from the HNLF; and re-inputting the rotated optical pulse into the HNLF to produce a modified pulse of optical radiation having a second frequency spectrum, wherein the second frequency spectrum is broader than the first frequency spectrum, and wherein the second pulse is substantially polarised.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows schematic view of an embodiment of an optical pulse generator in an optical communications network;

Figure 2 shows an optical pulse generator with a regenerative embodiment of a mode locked laser;

Figure 3 shows a harmonic embodiment of a mode locked laser;

Figure 4 shows (a) simulated pulses for input to a highly nonlinear fibre, (b) a simulated output spectrum for a lps width input pulse, and (c) a simulated output spectrum for a lOps width input pulse;

Figure 5 shows (a) an experimental input pulse at 500Mb/s (width

6ps), and (b) an experimental output spectrum for the pulse shown in (a); and Figure 6 is a flow diagram showing a method in accordance with the invention.

DETAILED DESCRIPTION

It is possible to generate "broad spectrum" light containing a range of wavelengths, such as a supercontinuum, by inputting light into a highly nonlinear fibre (HNLF). Nonlinear interactions within a HNLF result in the broadening of the frequency spectrum of light input to the HNLF and so produce output light with a broader frequency spectrum than the input light. The actual physical processes which result in that frequency broadening vary between fibres, but include phenomena such as four-wave mixing, self-phase modulation, and Raman scattering.

In order to generate a supercontinuum, a fibre usually has to present low chromatic dispersion (<1 ps/nm/km) in the spectral region of interest (in the example described below, that region is the C-band) and high nonlinear coefficient (>10 km^"1 W^"1). In particular, positive dispersion allows larger spectral broadening to be obtained, but with ripples in the spectrum profile, while negative dispersion reduces the efficiency of supercontinuum generation but guarantees a more flat spectrum. It is also important if it is desired to obtain a very flat supercontinuum that the dispersion profile is a function of longitudinal direction.

The length of HNLF required to generate a supercontinuum depends on the high nonlinear coefficient. In case of a coefficient of 10 krn W^"1 a typical length is 250-300meters.

The characteristics of the input radiation required to generate a supercontinuum also vary from fibre to fibre, and with fibre length. The duration and the peak power of the optical pulse required to be input to a given fibre can be determined using simulations.

The topic of supercontinuum generation is discussed in more detail in "Nonlinear Fibre Optics", G P Agrawal, Academic Press.

The polarisation of a signal input to an HNLF is also affected by nonlinear properties of the HNLF, such as birefringence. A supercontinuum produced in a HNLF is usually unpolarised, or has a polarisation that is arbitrary, making the supercontinuum unsuitable for use in optical communications.

We have realized that it is possible to generate an optical pulse, such as a supercontinuum, that is substantially unaffected by polarisation fluctuations caused by the HNLF using a commercial non-polarisation maintaining HNLF. A method and apparatus for the generation of such a pulse are described below.

Figure 1 schematically shows an optical pulse generator 1. The generator 1 is provided with a source 10 of polarised optical pulses. The radiation source may be may be external to the pulse generator, as shown in Figure 1 , or may be internal to the generator, as shown in Figure 2. The generator 1 comprises a highly nonlinear fibre (HNLF) Ia, which is operable to broaden the frequency spectrum of a pulse input to it, and which is not polarisation maintaining. The generator also comprises a polarisation rotator Ib, for rotating the polarisation of light input to it, and means for directing light output from the HNLF back into the HNLF, such as a mirror

Ic. An optical signal produced by the generator 1 is formed of one or more polarised pulses of radiation (a pulse train) that comprise wavelengths in the whole C-band (that is, wavelengths ranging between about 1525nm and 1575 nm). A pulse at a selected wavelength can thus be obtained by filtering the wide spectrum produced.

The generator may be housed within a node 30 in a communications network 40. An optical pulse output by the pulse generator 1 may be filtered using filter 50 to provide a filtered pulse of optical radiation at a selected wavelength, or a number of simultaneous pulses of radiation at different selected wavelengths. A communication signal 60 may be modulated onto that filtered pulse using modulator 70 for transmission in the network 40. Alternatively, the filtered (or unfiltered) radiation from the generator 1 may be used for another purpose within the node 20.

A more detailed structure of an optical pulse generator 1 is shown in Figure 2. The generator comprises two main blocks, a mode locked laser (MLL) 3 and a supercontinuum generator (SCG) 5. Both the SCG 5 and the MLL 3 can be managed by a user from a user interface 7. The generator 1 may be temperature controlled.

The purpose of the mode locked laser is to generate a train of pulses of a known centre frequency for input into the SCG. It will be appreciated that pulse generators other than the mode locked lasers described below would be suitable for that purpose.

The MLL 3 shown in Figure 2 comprises a resonant cavity with an erbium doped fibre amplifier 9 and an electro-optical modulator (EOM) 11 as nonlinear element.

The erbium doped fibre amplifier (EDFA) 9 comprises an erbium doped fibre (er-fiber) 13 as an active element and a pump laser 15 which is coupled into the erbium doped fibre 13 using a WDM coupler 17. The structure of the EDFA 9 is completed by an isolator 19. The EDFA 9 generates optical radiation at various frequencies in a known way so that a number of modes develop within the cavity 12. A variable optical delay line 25 can be used to vary the cavity length and thus to change the resonant frequency of the loop.

The EOM 11 is operable to force the relative phases of the different modes oscillating within the cavity to have the substantially the same value so that the modes interact in a constructive way in order to produce a train of pulses of restricted pulse width.

The generator 1 further comprises a variable optical filter 21. The purpose of the optical filter 21 is to set the central wavelength of the supercontinuum that will be generated. That central wavelength should preferably fit the spectrum portion of flat dispersion profile for a highly nonlinear fibre (HNLF) 23 in the SCG 5 (described later).

After the cavity isolator 19, a 50/50 coupler 27 splits out a copy of the signal circulating inside the loop. Substantially fifty percent of the signal remains circulating within the MLL, whilst the remaining fifty percent is directed into a 90/10 coupler 29. The 90/10 coupler 29 is used to send a low power signal to a regenerative feedback unit 31 , whilst ninety percent of the optical power is directed into a second 50/50 coupler 33, where it is further split in two identical replicas in order to create a monitor signal 35 and an input signal 37 to the SCG 5.

All the loop components described so far are polarisation maintaining.

In the regenerative version of the MLL shown in Figure 2; an electrical control signal (RF) for the EOM 11 is obtained by filtering and amplifying (with a pre-amplifier 39 and booster 41 cascade) the low power signal spilt from the cavity output by the 90/10 coupler. By correctly tuning the EOM bias 40 and the length of the feedback line 43, it is possible to mode-lock the ring laser. Two bandpass filters (BPF) in the regenerative feedback unit 31 are chosen so that they have a central frequency equal to a multiple of the cavity resonance frequency and their Q-factor allows the selection of only one mode. The presence of electrical elements in the cavity, especially high Q- factor bandpass filters results in a limitation of the pulse repetition rate. In fact, current electronics limits mean that it is not currently possible produce bandpass filters centered at frequencies above 40 GHz with a Q-factor suitable for this application. Therefore the pulse repetition rate of this particular embodiment is currently limited to a maximum of about 40 GHz.

Figure 3 shows an alternative harmonic MLL 3a which can be used instead of the regenerative MLL 3 shown in Figure 2. The harmonic MLL scheme differs from the regenerative one only in its electronic (rather than optical) section: the EOM control signal (RF) is here given by an external clock generator 45. In this case only bias tuning is needed to mode-lock the laser once the electric clock works at the right frequency. The frequency at which the clock generator operates is governed by the same characteristics in terms of central frequency and bandwidth as the regenerative feedback version of the MLL 3, and so the optical output has the same repetition rate restrictions.

In both cases a copy of the EOM control signal (RF clock out) is available at an output of the generator 1 to be used as clock signal if required. An isolator 47 on the RF clock output is used to avoid electrical reflections that could cause interferences.

As described above, part of the signal produced by the MLL is input to the SCG at 27. The supercontinuum generator comprises a high saturation power EDFA 49, and polarisation beam splitter (PBS) and a sigma circuit 51 , which comprises the highly nonlinear fibre (HLNF) 23 and a Faraday rotator mirror 53. The HNLF 23 is a non-polarisation maintaining fibre. Ideally the HNLF has a flat dispersion profile (i.e. a slope ~ 0) and a negative but close to zero dispersion coefficient. In order to generate a supercontinuum, nonlinear phenomena (such as self-phase modulation, Raman scattering and four-wave mixing) need to be established in the HNLF. The EDFA 49 is used to increase the power of the signal 37 output from the MLL cavity to generate a signal of sufficient power to induce those nonlinear effects. If the signal output from the MLL is already of high enough power, EDFA 49 will not be needed.

The amplifier output is connected to the polarization beam splitter (PBS). The PBS has two outputs, a polarisation maintaining output 55 which is set at 90 degrees to the polarisation of the signal 37 output from the MLL, and a non-polarisation maintaining output 57 through which radiation which is not of that particular polarisation passes. The non-polarisation maintaining output is connected to the sigma circuit 51 , whilst the polarisation maintaining output 55 is connected to the optical output of the device.

A signal entering the SCG 5 is first amplified by the EDFA 49 before being directed into the PBS. The signal passes through the non-polarisation maintaining output 57 of the PBS, because although the signal is polarised, it is does not have the polarisation required by the PBS. The signal passes into the HNLF 23 where nonlinear interactions with the glass of the HNLF generate a broadband output signal (a supercontinuum) in a known way.

The HNLF is not a polarisation maintaining fibre, and so exhibits various effects, such as birefringence, which can also affect the polarisation of a signal. Birefringence is used herein to mean that the refractive index of the fibre in one direction (a first birefringence axis) differs from the refractive index in a perpendicular direction (a second birefringence axis). Unless a wave happens to be parallel to one of those axes, this has the effect of splitting a polarised wave into two component waves, which travel at different speeds (as they both experience a different refractive index) resulting in changes to the state of polarisation of the wave during propagation due to the change in relative phase between the two components.

A Faraday mirror 53 is provided at the output of the HNLF remote from the PBS to compensate for this effect. The Faraday mirror rotates the state of polarisation of a signal input to it by 45° as it passes through a magnetic field, reflects the signal using a mirror, and rotates the signal by a further 45° as it passes through the magnetic field for a second time. The net effect is that the output from the HNLF reflected signal back into the HNLF having had its polarisation rotated by 90°. Other components which produce that effect could be provided rather than a Faraday mirror, such as a polarisation rotator and a mirror.

The signal thus travels through the HNLF twice, once in a first direction 59 and again in a second direction 61 that is opposite to the first direction. The state of polarisation of the wave components input to the HNLF in the second direction differs from the state of polarisation of the wave components output from the HNLF in the first direction by 90°.

Travelling in the first direction 59 the signal experiences random polarisation alterations within the HNLF. Considering the effect of birefringence, the wave can be considered as splitting into two components, each aligned with one of the birefringence axes, and each travelling at a different speed, so that the two components become out of phase. If the signal were simply reflected back into the HNLF in the same orientation, the components of the signal would enter the fibre at the same point, in particular, adjacent the same birefringence axis. This means that a component of the wave that travelled more slowly than the other would continue to travel more slowly. The wave components are therefore likely to remain out of phase when the signal is output from the HNLF, and the fluctuations in polarisation will continue.

However, if the signal enters the fibre at 90° to its original orientation, each component of the wave is adjacent a different birefringence axes. This means that the component that travelled more slowly in the first direction 59 will travel more quickly in the second direction 61 , and vice versa. Thus, by the time the components exit the fibre for the second time, the 'slow' component will have caught up the 'fast' components, and the components will be in phase once more (although polarised at 90 degrees to their original polarisation). In effect, the phase alterations experienced in the first direction are "cancelled out" by the perpendicular phase alterations experienced in the second direction. It will be appreciated that a rotation of (90+nl 80)°, where n is an integer, would result in the same effect, for example, 270°, or 450°. Any angular rotation (other than a multiple of 180) would result in some correction of the polarisation of the wave, but to a lesser effect.

When the supercontinuum, travelling in the second direction, reaches the PBS it has a polarisation that is 90° to the polarisation the signal had when it was first input to the PBS. The signal is thus directed into the polarisation maintaining output 55 of the PBS and toward the optical output of the generator. Any small portion of signal that happens to have a different polarisation is directed through the non-polarisation maintaining output towards the EDFA where it is stopped by an isolator inside the amplifier.

The configuration described is advantageous because it allows a non- polarisation maintaining HNLF to be used to generate a broad frequency spectrum signal that is substantially unaffected by the polarisation altering effects of the HNLF. The output signal is polarised in a way that is useful in an optical communications network. A method in accordance with the invention is summarised is Figure 6, which shows a flow diagram setting out the steps of the method. In step S l , first pulse of polarised optical radiation having a first frequency spectrum is input to a non-polarisation maintaining highly nonlinear fibre (HNLF). In step S2 the polarisation of the optical pulse output from the HNLF is rotated. Finally, in step S3, the rotated optical pulse is re-input to the HNLF to produce a second pulse of optical radiation having a second frequency spectrum. As a result of nonlinear interactions within the HNLF, the second frequency spectrum is broader than the first frequency spectrum. The second pulse is also substantially polarised, as a result of the rotation and re-input of the pulse.

In order to achieve a supercontinuum spectrum, the output of the MLL needs to satisfy certain values of pulse width and peak power for a given duty cycle. By tuning the EDFA output power, the required peak power can be set. The simulations shown in Figure 4 were obtained considering a highly nonlinear fibre of length L = 250m, with a flat dispersion profile (slope ~ 0.0064ps/nm²/Km) and dispersion ~ 0.35ps/nm/Km at 1550nm, nonlinear coefficient γ = 10W ¹Km^"1 and loss = 0.74dB/Km. Figure 4(a) shows two pulses of wavelength λ = 1550nm with differing widths of lps (broken line) and lOps (solid line). Each pulse has an input peak power that corresponds to the optimal case, that is to say to the maximum spectral broadening: Pin = 18W (lOps wide pulse) and Pin = 10 W (lps wide pulse).

The simulations in Figure 4 show that a narrow pulse width (in the order of lps) produces a narrower supercontinuum (also termed a comb) with well defined peaks and troughs (see Figure 4(b)), whilst a wider input pulse (in the order of lOps) produces a wider supercontinuum with a more complete range of frequencies (ie, without significant troughs, as shown in Figure 4(c)). The simulation in Figure 4(c) is preferred, as the frequency range of the output pulse is broader and more continuous, allowing a wide range of different frequencies of similar power to be selected from the pulse using appropriate filters. However, a pulse duration of between lps and 20ps, and in particular between 5ps and 15ps can be used to provide a useful range of output frequencies.

Figure 5 shows results obtained using a super continuum generator of the type described above. The results were obtained using a pulse repetition rate of 500 MHz. A train of 6ps pulses was fed into the SCG obtaining a supercontinuum spectrum greater than 50 nm in width with a low phase jitter (less than 40 fs in the range 1 kHz - 40 MHz) and low amplitude noise (less than 0.01 %).

It can be seen that the supercontinuum spectrum is not top-flat: a certain number of relative maxima are present. The quality of the converted signal improves if the central wavelength of a slicing filter used to select a train pulse from the supercontinuum comb is close to a wavelength of one of the relative maxima. If a wavelength that is far from the relative maxima is required, the noise variance on the converted signal increases.

The same results can be obtained using an optical source at 10 GHz. However, there are limitations on the repetition rate due to the problem of providing a high pulse repetition rate source (as described above electrical limits in the MLL mean that pulses cannot be produced at frequencies of much greater than 40GHz), and on the achievable peak power at the input of the nonlinear fibre.

Claims

1. An optical pulse generator comprising: a source of polarised optical pulses each having a first frequency spectrum, a length of non-polarisation maintaining highly nonlinear fibre (HNLF) connected to the source, and a polarisation rotator connected to the HNLF, wherein an optical pulse output from the source is guided along the HNLF to the polarisation rotator, whereat the polarisation of the pulse is rotated, and wherein the rotated pulse is re-input to the HNLF, guided along the HNLF, and output from the HNLF as a modified optical pulse, wherein the modified optical pulse has a second frequency spectrum that is broader than the first frequency spectrum due to interaction between the optical pulse with the HNLF, and wherein the modified optical pulse is substantially polarised.

2. The optical pulse generator of claim 1 wherein the polarisation rotator is operable to rotate the polarisation of the optical pulse output from the HNLF through between 60° and 120°.

3. The optical pulse generator of claim 1 or claim 2 wherein the polarisation rotator is operable to rotate the polarisation of the optical pulse output from the fibre by substantially 90°.

4. The optical pulse generator of any preceding claim wherein the optical pulse is guided along the HNLF to the polarisation rotator in a first direction and is guided along the same HNLF from the polarisation rotator in a second direction opposite to the first direction.

5. The optical pulse generator of claim 4 further comprising a polarisation beam splitter operable to direct at least some of the optical pulse from the source into the HNLF, and to direct at least some of the modified optical pulse from the HNLF to an output of the generator.

6. The optical pulse generator of claim 5 wherein the polarisation beam splitter comprises a first output and a second output, wherein substantially all of the optical pulse emitted from the source is directed to the first output, because it has a first polarisation, whilst substantially all of the modified optical pulse is directed to the second output, because it has a second polarisation that is 90° to the first polarisation.

7. The optical pulse generator of any one of claims 4 to 6 wherein the polarisation rotator is a Faraday rotator mirror.

8. The optical pulse generator of any preceding claim wherein the modified optical pulse is a broadband pulse containing a plurality of different wavelengths.

9. The optical pulse generator of claim 8 wherein the modified optical pulse is a supercontinuum.

10. The optical pulse generator of claim 8 or claim 9 wherein the broadband pulse contains wavelength substantially all between 1525nm and 1575 nm.

11. The optical pulse generator of any preceding claim wherein the source of optical pulses is a mode locked laser.

12. The optical pulse generator of any preceding claim wherein the first optical pulse has a duration of between lps and 30ps.

13. The optical pulse generator of claim 12 wherein the first optical pulse has a duration of between 5ps and 15ps.

14. A node for a communications network comprising the optical pulse generator of any one of claims 1 to 13.

15. The node of claim 14 further comprising a filter operable to filter a selected wavelength from the modified optical pulse to produce a filtered optical pulse.

16. The node of claim 15 further comprising one or more filters operable to select a plurality of wavelengths from the modified optical pulse to produce a plurality of filtered optical pulses.

17. The node of claim 16 wherein the filter or filters are operable to produce the plurality of filtered optical pulses substantially simultaneously.

18. The node of any one of claims 15 to 17 further comprising a modulator, operable to modulate one or more communication signals onto the filtered optical pulse or pulses.

19. A method of generating an optical pulse comprising: inputting a first pulse of polarised optical radiation having a first frequency spectrum to a non-polarisation maintaining highly nonlinear fibre (HNLF); rotating the polarisation of the optical pulse output from the HNLF; and re-inputting the rotated optical pulse into the HNLF to produce a modified pulse of optical radiation having a second frequency spectrum, wherein the second frequency spectrum is broader than the first frequency spectrum, and wherein the second pulse is substantially polarised.

20. The method of claim 19 wherein the polarisation of the optical pulse is rotated by substantially 90°.

21. The method of claim 19 or claim 20 wherein the optical pulse output from the HNLF is re-input to the HNLF by reflection from a surface.

22. The method of any one of claims 19 to 21 further comprising filtering the modifed optical pulse to produce one or more filtered pulses of radiation at one or more selected wavelengths.