GB2421335A

GB2421335A - Microwave signal digitiser comprising pulsed optical input signals, modulator, splitter and plurality of ADCs

Info

Publication number: GB2421335A
Application number: GB0425519A
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: GB0425519D0; WO2006054077A1

Abstract

A device for digitisation of microwave signals, comprises: ```a modulator 25 adapted to receive a microwave signal and to modulate an optical signal in the optical path in response to the microwave signal; ```an optical pulse generator 22; ```a dispersive optical path 23 extending from the output port of the pulse generator to the input port of the modulator; ```splitter means 29 for splitting the signal received from the output port of the modulator into a plurality of wavelength dependent signal paths; and ```a plurality of analogue to digital converters 33-35 , each connected to at least one wavelength dependent signal path for converting the received optical signal to a digital signal. The modulation may be amplitude modulation. The splitter may be an arrayed waveguide grating, AWG.

Description

2421335

-i-

A METHOD AND DEVICE FOR DIGITISING A MICROWAVE SIGNAL

The present invention relates to a method and device for digitising a microwave signal. More particularly but not exclusively, the present invention relates to a method of digitising a microwave signal by dispersing an optical pulse, modulating the dispersed pulse by the microwave signal, splitting the dispersed modulated pulse into wavelength dependent parts and digitising the signal in each of the parts. Additionally but not exclusively, the present invention also relates to a device for performing such a method.

Devices for digitising microwave signals are known. Such devices use either time division multiplexing (TDM) or wavelength division multiplexing (WDM). TDM systems provide a succession of optical pulses. These are modulated by the microwave signal. A high speed RF driven optical switching element sends successive pulses along different optical paths. An analogue to digital converter in each path digitises the received pulses. The high speed optical element is difficult and expensive to manufacture and is also relatively fragile.

WDM systems provide a series of pulses of different wavelengths. These are modulated by the microwave signal before being split into different optical paths by an arrayed waveguide grating before being digitised. It is difficult to generate a series of pulses with different wavelengths form one pulse to the next. Such WDM systems are complex and difficult to manufacture.

Accordingly, a first aspect of the present invention provides a device for digitisation of microwave signals, comprising:

a modulator having an input port and an output port and at least one optical path extending therebetween, the modulator being adapted to receive a microwave signal and to modulate the amplitude of an optical signal in the optical path in response to the microwave signal;

an optical pulse generator;

a dispersive optical path extending from the output port of the pulse generator to the input port of the modulator;

splitter means for splitting the signal received from the output port of the modulator into a plurality of wavelength dependent signal paths; and a plurality of analogue to digital converters, each connected to at least one wavelength dependent signal path for converting the received optical signal to a digital signal.

The device according to the invention is relatively simple to manufacture and can be achieved using relatively simple passive photonic components. It is lightweight and reliable and is particularly suitable for use in avionic applications, especially unmanned airborne vehicles.

Preferably, the optical pulse generator is adapted to generate a plurality of optical pulses at predetermined intervals, more preferably regular intervals.

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

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

Preferably, the pulses are sufficiently close together that a dispersed pulse overlaps with an adjacent dispersed pulse at the input port of the modulator.

Preferably, each of the plurality of analogue to digital convertors is connected to a different wavelength dependent signal path. Alternatively at least one of the plurality of analogue to digital converters can be connected to a plurality of wavelength dependent signal paths. The splitter means can be an arrayed waveguide grating.

Preferably, the modulator is. an optical interferometer having at least first and second optical arms and being adapted such that an optical signal in at least one of the arms is modulated by a received microwave signal.

In a further aspect of the invention there is provided a method of digitising a microwave signal comprising the steps of:

(a) providing an optical pulse;

(b) passing the optical pulse along a dispersive optical path;

(c)passing the dispersed pulse through a modulator wherein the dispersed pulse is modulated by a received microwave signal;

(d) splitting the dispersed modulated pulse into a plurality of wavelength dependent optical paths;

(e) digitising the signals received on the plurality of wavelength dependent paths to provide a digitised signal.

The method is relatively simple and can be enabled using simple passive optical components.

Preferably, the dispersive optical path is a optical fibre, more preferably a dispersive fibre optic cable;

The optical pulse can be provided by a laser, preferably a mode locked laser;

Preferably, the step of splitting the modulated dispersed pulse into a plurality of wavelength dependent optical paths is achieved by passing the dispersed modulated pulse through an arrayed waveguide grating;

The dispersed pulse can be modulated by passing the pulse through a plurality of arms of an optical interferometer wherein the pulse in at least one of the arms is modulated by the microwave signal.

The present invention will now be described by way of example only and not in any limitative sense with reference to the accompanying figures in which;

figure 1 shows a known time division multiplexing system for digitising a microwave signal;

figure 2 shown a known wavelength division multiplexing system for digitising a microwave signal;

figure 3 shows a device for digitising a microwave signal according to the invention; and figure 4 shows a further embodiment of a device according to the invention.

Devices for digitising microwave signals find application in high speed single shot oscilloscopes and spectrum analysers. In order to accurately digitise a microwave signal the signal must be sampled at a high sampling rate. Such a high sampling rate cannot easily be achieved using purely electronic systems. Photonic sources which can produce

pulses at a sufficiently high repetition rate to sample the microwave signal are to be preferred. Modern analogue to digital converters however cannot accept and digitise pulses at such a high repetition rate. It is therefore necessary to send successive pulses down different optical paths such that the analogue to digital converter in each of the paths receive pulses at a lower rate.

Shown in figure 1 is a known time division multiplexing system for digitising a microwave signal. The device comprises a pulsed laser (1) for providing pulses at a high repetition rate. The pulses are provided to a modulator (2) where they are successively modulated by a microwave signal. After modulation the pulses are sent to a high frequency switch (3) which divert successive pulses down different optical paths (4-7) where they are digitised by analogue to digital converters (8-11). The high frequency switch (3) is driven by a RF source (12). Such a high speed switch (2) is complex and also difficult and expensive to manufacture. The resulting device is also relatively heavy.

Shown in figure 2 is a known wavelength division multiplexing device for digitising a microwave signal. The device comprises a pulsed laser (13) for providing optical pulses at a high repeti tion rate. The laser cycles through a number of different frequencies with each frequency differing from the last. The pulses pass through a modulator (14) where they are successively modulated by a microwave signal. The pulses are then split into different optical paths (15-17) by an arrayed waveguide grating (18) where they are digitised by analogue to digital converters (19-21). An optical source providing successive pulses at different frequencies is difficult and expensive to manufacture.

Shown in figure 3 is a device for digitising a microwave signal according to the invention. The device comprises a mode locked pulsed laser (22) which provides pulses at a high repetition rate. These pulses are passed through a dispersive optical fibre (23). A dispersive fibre is one in which the speed of light through the fibre depends upon the wavelength of the light.

A pulse which enters into the fibre (23) comprises a plurality of frequencies, each of which travel through the fibre (23) at different speeds, arriving at the end of the fibre (23) at slightly different times, so broadening the pulse. In this embodiment, the pulses are sufficiently close together that at the end of the optical fibre (23) the dispersed pulses overlap with the front edge of one pulse overlapping the rear edge of the earlier pulse.

Each dispersed pulse enters into a microwave modulator (24). The modulator (24) comprises an optical interferometer (25) having first and second optical arms (16, 27).

Each pulse passes down both arms (26, 27). The dispersed pulse in one of the arms (26) is modulated by interaction with the microwave signal. The signals in the arms (26, 27) are then recombined at the output port (28) of the modulator (24) producing a pulse with an amplitude dependant on the microwave signal.

From the modulator (24) the dispersed modulated pulses pass to an arrayed waveguide grating (29). The arrayed waveguide grating (29) splits each pulse into its individual wavelength components and presents each of these components at a different output port (30-32). Connected to each output port is an analogue to digital converter (33-35). Each analogue to digital converter (33-35) digitises its respective received pulses to provide the digitised signal. As each pulse was dispersed before entering into the modulator (24) each of the wavelength components of the pulse passes through the modulator (24) at a slightly different time. Each wavelength of each pulse therefore samples the microwave signal at a slightly different time. The different wavelengths of each pulse issue from the arrayed waveguide grating (29) in the order in which they passed through the modulator (24) as shown in the figure. With such a device one can sample the microwave signal of several very close time intervals. Each analogue to digital converter (33-35) is however only required to make an analogue to digital conversion at the pulse rate of the optical source (22).

Shown in figure 4 is an alternative embodiment of a device according to the invention. In this embodiment, each of the analogue to digital converters (36, 37) is connected to a plurality of outputs from the arrayed waveguide grating (29). This increases

the rate at which pulses are received at each analogue to digital converter (36, 37) but reduces the number of analogue to digital converters (36, 37) required.

Claims

1. A device for digitisation of microwave signals, comprising:

an optical pulse generator;

2. A device as claimed in claim 1 wherein the optical pulse generator is adapted to generate a plurality of optical pulses at predetermined intervals.

3. A device as claimed in claim 2, wherein the optical pulse generator generates optical pulses at regular intervals.

4. A device as claimed in any one of claims 1 to 3, wherein the pulses are sufficiently close together that a dispersed pulse overlaps with an adjacent dispersed pulse at the input port of the modulator.

5. A device as claimed in any one of claims 1 to 4, wherein the optical pulse generator is a laser.

6. A device as claimed in claim 5 wherein the laser is a mode locked laser.

7. A device as claimed in any one of claims 1 to 6, wherein the dispersive optical path is an optical fibre, preferably a fibre optic cable.

8. A device as claimed in any one of claims 1 to 7, wherein each of the plurality of analogue to digital converters is connected to a different wavelength dependent signal path.

9. A device as claimed in any one of claims 1 to 7, wherein at least one of the plurality of analogue to digital converters is connected to a plurality of wavelength dependent signal paths.

10. A device as claimed in any one of claims 1 to 9 wherein the splitter means is an arrayed waveguide grating.

11. A device as claimed in any one of claims 1 to 10 wherein the modulator is an optical interferometer having at least first and second optical arms and being adapted such that a optical signal in at least one of the arms is modulated by a received microwave signal.

12. A method of digitising a microwave signal comprising the steps of:

(a) providing an optical pulse;

(b) passing the optical pulse along a dispersive optical path;

13. A method as defined in claim 12 wherein the dispersive optical path is an optical fibre, preferably a dispersive fibre optic cable.

14. A method as claimed in either of claims 12 and 13 wherein the optical pulse is provided by a laser, preferably a mode locked laser.

15. A method as claimed in any one of claims 12 to 14, wherein the step of splitting the modulated dispersed pulse into a plurality of wavelength dependent optical paths is achieved by passing the dispersed modulated pulse through an arrayed waveguide grating.

16. A method as claimed in any one of claims 12 to 15, wherein the dispersed pulse is modulated by passing the pulse through a plurality of arms of an optical interferometer wherein the pulse in at least one of the arms is modulated by the microwave signal.

17. A device as hereinbefore described.

18. A device as hereinbefore described with reference to the drawings.

19. A method as hereinbefore described

20.

A method as hereinbefore described with reference to the drawings.