GB2421336A

GB2421336A - Generating an arbitrary waveform wide band electrical signal

Info

Publication number: GB2421336A
Application number: GB0425526A
Authority: GB
Inventors: John Heaton
Original assignee: Filtronic PLC
Current assignee: Filtronic PLC
Priority date: 2004-11-19
Filing date: 2004-11-19
Publication date: 2006-06-21
Also published as: GB0425526D0; WO2006054090A1

Abstract

Generating an electrical signal with a wide band arbitrary waveform comprises a modulator block comprising a plurality of modulators 9-13, each modulator modulates an optical signal by a respective microwave signal; the modulator block further comprising an optical splitter 17 for splitting an optical signal received into a plurality of signal paths one to each modulator entrance; and an optical combiner 7 for combining the signals received at the modulator exits into a single signal at the output port; at last one of the optical splitter and optical combiner comprising an arrayed waveguide grating; an optical pulse generator 1 connected to the input of the modulator block via an input signal path; and a photodetector 18 connected to the output of the modulator block via an output signal path and at least one dispersive optical element arranged such that the pulses are dispersed by the at least one optical element either before or after modulation.

Description

2421336

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A METHOD OR DEVICE FOR GENERATING AN ELECTRICAL SIGNAL WITH A WIDE BAND ARBITRARY WAVEFORM

The present invention relates to a method or device for generating an electrical signal with a wide band arbitrary waveform. More particularly the present invention relates to such a method comprising the steps of providing an optical pulse, splitting the pulse, passing the split pulses through a plurality of modulators, recombining the pulse and passing the pulse to a photodetector, where either the splitting or recombining is performed an arrayed waveguide grating and the pulse is dispersed either before splitting or after recombining. The present invention also relates to a device for performing such a method.

Devices for generating arbitrary wideband waveforms are known. PCT/GB02/00019 discloses such a system. All such known systems however involve time division multiplexing (TDM) of the optical signal. Such systems are difficult and expensive to manufacture.

Accordingly, a first aspect of the present invention provides a device for generating an electrical signal with a wide band arbitrary waveform comprising a modulator block comprising an input port, an output port and a plurality of modulators, each modulator being adapted to modulate an optical signal passing from a modulator entrance to a modulator exit by a respective microwave signal;

the modulator block further comprising an optical splitter for splitting an optical signal received at the input port into a plurality of signal paths one to each modulator entrance; and an optical combiner for combining the signals received at the modulator exits into a single signal at the output port; at last one of the optical splitter and optical combiner comprising an arrayed waveguide grating;

the device further comprising

an optical pulse generator connected to the input port of the modulator block via an input signal path; and a photodetector connected to the output port of the modulator block via an output signal path,

the device further comprising at least one dispersive optical element arranged such that the pulses are dispersed by the at least one optical element either before or after modulation.

Such a device has the advantage that it can be manufactured from relatively simple passive optical components. It is relatively simple to manufacture and lightweight. This makes it particularly suitable for use in avionic systems including radar jamming and RF false target generation systems.

Preferably the optical splitter comprises an arrayed waveguide grating. Alternatively the optical splitter can be a wideband optical splitter.

The optical combiner can comprise an arrayed waveguide grating. Alternatively the optical combiner can comprise a wide band optical combiner.

Preferably, the input signal path comprises a dispersive optical element.

The output signal path can comprise a dispersive optical element.

The optical pulse generator can comprise a laser, preferably a mode locked laser.

Preferably, the laser provides a plurality of pulses, more preferably at regular intervals.

The dispersive optical element can be an optical fibre, preferably a fibre optic cable.

In an alternative aspect of the invention there is provided a method of generating an electrical signal with a wide band arbitrary waveform comprising the steps of

(a) providing an optical pulse;

(b) splitting the pulse by means of a splitter into a plurality of signal paths by means of an optical splitter;

(c) passing each of the split pulses through a separate microwave modulator wherein each split pulse is modulated by a respective microwave signal;

(d) combining the split pulses into a single output pulse by means of an optical combiner; and

(e) converting the output pulse into an electrical signal;

wherein at least one of the optical splitters or optical combiners comprises an arrayed waveguide grating;

the method further comprising the step of passing the optical pulse through a dispersive optical element either prior to or after modulation.

Preferably the optical splitter is an arrayed waveguide grating. Alternatively, the optical splitter can be a wide band optical splitter.

The optical combiner can be an arrayed waveguide grating. Alternatively, the optical combiner can be a wide band optical combiner.

The present invention will now be described by way of example only and not in any limitative sense with reference to the accompanying drawings in which figure 1 shows a first embodiment of a device according to the invention;

figure 2 shows a second embodiment of a device according to the invention;

figure 3 shows a third embodiment of a device according to the invention;

figure 4 shows a fourth embodiment of a device according to the invention;

figure 5 shows a fifth embodiment of a device according to the invention;

Shown in figure 1 is a first embodiment of a device according to the invention. The device comprises a mode locked laser (1) which provides a plurality of optical pulses at regular intervals. The pulses are passed through a dispersive optical element (2) comprising an optical fibre. The wavelength of light through the dispersive element depends upon the

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wavelength of the light and accordingly the different wavelengths of the pulse arrive at the end of the fibre at slightly different times.

Each pulse is then split into a plurality of signal paths (3-7) by a broadband optical splitter (8). In each signal path (3-7) is a microwave modulator (9-13) wherein the optical pulse 5 can be modulated by a microwave signal. Each microwave modulator (9-13) is an optical interferometer having first and second optical arms (14-15). A pulse entering the interferometer is split and passes down each of the optical arms (14-15) before being recombined at the interferometer exit (16). Each interferometer is adapted such that the microwave signal interacts with the signal in one of the arms (14) so affecting the 10 amplitude of the optical signal at the output of the interferometer (9-13).

On exiting the microwave modulators (9-13), the signals from each of the modulators are recombined into a single output signal by an arrayed waveguide grating (AWG) (17). The AWG (17) is effectively a complex wavelength filter combining a wavelength from the . * •. first modulator (9) with a second wavelength from the second modulator (10) and so on.

. •' * *1^ By providing the appropriate microwave signals to the modulators (9-13) one can produce • •••

a combined output signal of any arbitrary shape.

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The output signal is then passed to the photodetector (18) which converts the received

. * *, I optical signal to an electronic signal.

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Shown in figure 2 is a second embodiment of the invention. In the embodiment the optical 20 dispersive element (2) is placed between the output of the arrayed waveguide grating (17) and the photodetector (18). As with the embodiment of figure 1, pulses which leave the laser (1) are split by a broadband splitter (8) and sent to a plurality of modulators (9-13).

In the second embodiment however there is no dispersive element between the laser (1) and the modulators (9-13). All the wavelength elements of the pulses pass through the 25 modulators (9-13) simultaneously where they are each modulated by the microwave signal. After modulation the pulses are combined by the arrayed waveguide grating (17). The AWG (17) selects a first wavelength component from the first pulse, a second wavelength

component from the second pulse and so on. After exiting from the AWG (17) the new pulse is passed through the dispersive fibre optic cable (2) where the individual wavelength components are dispersed into a signal of arbitrary pre-determined shape. The signal is then passed to the photodetector (18) for conversion to an electronic signal.

Shown in figures 3 and 4 are third and fourth embodiments of the invention. In these embodiments the pulses are split into the plurality of signal paths by an AWG (17) such that only one wavelength component passes through each modulator (9-13) where they are individually modulated by microwave signals. After modulation the individual wavelengths are combined by a wide band optical combiner (19) before being passed to the photodetector (18) for conversion to the electronic signal. In the embodiment of figure 3 the pulses are dispersed by an optical fibre (2) prior to splitting such that the wavelength pass through the modulators (9-13) at slightly different times. In the embodiment of figure 4 the pulse is passed through the dispersive optical fibre (2) after recombination to disperse the pulse into an arbitraiy predetermined shape.

A fifth embodiment of the invention is shown in figure 5. In this embodiment the optical signal is split by an AWG (17) into a plurality of signal paths. The split signals are modulated by a plurality of modulators (9-13) before being recombined by a second AWG (17). After recombination the signal is dispersed into the predetermined arbitrary signal shape before being converted into an electronic signal. The embodiment of figure 5 is more efficient than the previous embodiments. In practice it is difficult to make broadband lxN splitters and Nxl recombiners.

In a further embodiment of the invention (not shown) the pulse is split and recombined by a pair of AWGs as in figure 5. The pulse is however dispersed by a fibre optic cable before splitting rather than after recombining.

Claims

1. A device for generating an electrical signal with a wide band arbitrary waveform comprising a modulator block comprising an input port, an output port and a plurality of modulators, each modulator being adapted to modulate an optical signal passing from a modulator entrance to a modulator exit by a respective microwave signal;

the device further comprising an optical pulse generator connected to the input port of the modulator block via an input signal path; and a photodetector connected to the output port of the modulator block via an output signal path the device further comprising at least one dispersive optical element arranged such that the pulses are dispersed by the at least one optical element either before or after modulation.

2. A device as claimed in claim 1, wherein the optical splitter comprises an arrayed waveguide grating.

3. A device as claimed in claim 1, wherein the optical splitter is a wideband optical splitter.

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4. A device as claimed in any one of claims 1 to 3, wherein the optical combiner comprises an arrayed waveguide grating.

5. A device is claimed in claim 2 wherein the optical combiner comprises a wide band optical combiner.

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6. A device as claimed in any one of claims 1 to 5, wherein the input signal path comprises a dispersive optical element.

7. A device as claimed in any one of claims 1 to 6 wherein the output signal path comprises a dispersive optical element.

8. A device as claimed in any one of claims 1 to 7 wherein the optical pulse 10 generator comprises a laser, preferably a mode locked laser.

9. A device as claimed in claim 8, wherein the laser provides a plurality of pulses, preferably at regular intervals.

10. A device as claimed in any one of claims 1 to 9, wherein the dispersive optical element is an optical fibre, preferably a fibre optic cable.

11. A method of generating an electrical signal with a wide band arbitrary waveform comprising the steps of

(a) providing an optical pulse;

20 (c) passing each of the split pulses through a separate microwave modulator wherein each split pulse is modulated by a respective microwave signal;

(e) converting the output pulse into an electrical signal;

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12. A method as claimed in claim 11, wherein the optical splitter is an arrayed waveguide grating.

13. A method as claimed in claim 11, wherein the optical splitter is a wide band optical splitter.

14. A method as claimed in any one of claims 11 to 13 wherein the optical combiner 10 is an arrayed waveguide grating.

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15. A method as claimed in 12, wherein the optical combiner is a wide band optical " *'. combiner.

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16. A device substantially as hereinbefore described.

A device substantially as hereinbefore described with reference to the drawings. A method substantially as hereinbefore described.

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19.

A method substantially as hereinbefore described with reference to the drawings.