WO2002054204A2

WO2002054204A2 - Apparatus for generating electrical signals with ultra-wide band arbitrary waveforms

Info

Publication number: WO2002054204A2
Application number: PCT/GB2002/000019
Authority: WO
Inventors: Philip Charles Jarrett Pring; John Michael Heaton; David Robert Wight
Original assignee: Esl Defence Limited; The Secretary Of State For Defence
Priority date: 2001-01-08
Filing date: 2002-01-04
Publication date: 2002-07-11
Also published as: GB0100425D0; GB0313257D0; AU2002219328A1; GB2385178B; GB2385178A; WO2002054204A3

Abstract

Apparatus (22) for generating electrical signals with ultra-wide band arbitrary waveforms, which apparatus (22) comprises an optical impulse generator (24) for generating pulsed optical signals, splitter means (26) for splitting the optical signals into a plurality of optical signals in parallel signal paths, and combiner and optical detector means (30) for combining and detecting the signals in the parallel signal paths to generate the electrical signals with ultra-wide band arbitrary waveforms.

Description

APPARATUS FOR GENERATING ELECTRICAL SIGNALS WITH ULTRA- WIDE BAND ARBITRARY WAVEFORMS

This invention relates to apparatus for generating electrical signals with ultra-wide band arbitrary waveforms.

In GB A 2341991 there is disclosed apparatus for generating electrical signals with ultra-wide band arbitrary waveforms. The apparatus comprises impulse generator means for generating signals, splitter means for splitting the signals into a plurality of signals in parallel signal paths, and combiner means for combining the signals in the parallel signal paths. Time delay means and amplified weighting means, which may be a scaling means, may be employed on the signals in the parallel signal paths in order to generate the desired output electrical signal waveform.

The present invention aims to modify the apparatus disclosed in GB A 2341991 in order to generate the electrical signals with the ultra-wide arbitrary waveforms by the use of optical components.

Accordingly, in one non-limiting embodiment of the present invention, there is provided apparatus for generating electrical signals with ultra-wide band arbitrary waveforms, which apparatus comprises an optical pulse generator for generating pulsed optical signals, splitter means for splitting the optical signals into a plurality of optical signals in parallel signal paths, and combiner and optical detector means for combining and detecting the signals in the parallel signal paths to generate the electrical signals with ultra-wide band arbitrary waveforms. The apparatus of the present invention may be used for generating electrical signals having an output waveform which can be extended from 1GHz (typical of electronic direct digital synthesis) to frequencies as great as 20GHz.

The apparatus may be one in which a plurality of optical impulse signal paths, combiner and optical detector means are used for combining and detecting the signals in the parallel signal paths to generate the electrical signals.

Preferably, the optical pulse generator is a pulsed laser generator. Other types of optical pulse generator may however by used.

The optical pulse generator may include amplitude stabilization control means.

The apparatus of the present invention may include time delay means for delaying the optical signals in the parallel signal paths in time relative to each other.

The apparatus of the present invention may include amplitude weighting means for weighting the optical signals in the parallel signal paths in amplitude relative to each other.

Preferably, the amplitude weighting means is a scaling means which enables a predetermined reduction of signal amplitude on the optical signals in each of the parallel signal paths.

The scaling means may comprise a plurality of scaling devices, one for each signal path. The apparatus may then include a plurality of direct digital synthesizers, there being one of the direct digital synthesizers for each one of the scaling devices, and each one of the direct digital synthesizers being such that it has an electrical output appropriate for driving the scaling devices.

The apparatus of the present invention may include a master clock for feeding the direct digital synthesizers and the optical impulse generator means.

The apparatus of the present invention may include means for combining the optical signals in the parallel signal paths, before detection of the combined optical signal to generate the required electrical output waveform.

The apparatus may include means for detecting the optical signals in each of the parallel signal paths, to produce parallel electrical signals before combination of the signals to generate the required electrical output waveform.

The apparatus of the present invention may be one in which the optical splitting and optical combining are effected using wavelength division demultiplexing and wavelength division multiplexing.

The present invention may enable the use of optical synchronization between waveform generation modules, each capable of generating electrical signals with ultra-wide band arbitrary waveforms, by using a single optical wave pulse generator and a master clock to provide a number of weighted, coherent electrical outputs.

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 shows apparatus as disclosed in GB A 2341991 for generating electrical signals with ultra-wide band arbitrary waveforms;

Figure 2 shows apparatus of the present invention which is like that shown in Figure 1 but which has been modified by the use of optical components;

Figure 3 shows apparatus for generating ultra-wide band arbitrary waveforms using optics and multiple optical pulse generators;

Figure 4 illustrates how a signal generated by the optical pulse generator has a wide optical bandwidth and may be divided by an optical splitter into a plurality of parallel optical signal paths; and

Figure 5 shows a waveform generator module for use in systems requiring multiple synchronized outputs.

Referring to Figure 1 , there is shown apparatus 22 which is in accordance with GB A 2341991 and which is for generating electrical signals of ultra-wide band arbitrary frequencies. The apparatus 22 comprises an impulse generator 24 for generating signals, splitter means 26 for splitting the signal into a plurality of signals in parallel signal paths 28, and combiner means 30 for combining the signals in the parallel signal paths 28. The impulse generator 24 is a single impulse generator. The splitter means 26 is an n-wave splitter, and the combiner means 30 is an n- wave combiner.

The apparatus 22 includes time delay means 32 in the form of a plurality of time delay elements 34 arranged one in each of the parallel signal paths 28. The time delay means 32 is for delaying the signals in the parallel signal paths 28 in time relative to each other. The apparatus 22 further includes amplitude weighting means 36 for weighting the signals in the parallel signal paths 28 in amplitude relative to each other. The amplitude weighting means 36 is a scaling means which enables a predetermined reduction of signal amplitude on the signals in each one of the parallel signal paths 28. The scaling means comprises a plurality of scaling devices 38, there being one of the scaling devices 38 arranged in each one of the parallel signal paths 28 as shown.

The apparatus 22 includes a plurality of direct digital synthesizers

DDSO, DDS1 , DDS2 DDSn as shown. There is one of the direct digital synthesizers for each one of the scaling devices 38.

The apparatus 22 includes a master clock 40 which feeds the direct digital synthesizers DDSO, DDS1 , DDS2...DDSn via line 42 and which feeds the impulse generator 24 via line 44. The apparatus 22 further comprises a low path filter 46 providing an output analog signal shown schematically as output analog signal 48.

The apparatus 22 operates such that a number "n" of direct digital synthesizers DDSO, DDS1 , DDS2...DDSn are operated from the common master clock 40. The master clock 40 also triggers the impulse generator 24 once per clock cycle.

The output of the impulse generator 24 is a single temporarily short burst of signal energy 50 with a temporal duration of at least "n" times less than the period of the master clock 40. The impulse generator output is passed to the splitter means 26 which splits the output of the impulse generator 24 into a plurality of parallel outputs along the parallel signal paths 28. The number "n" of the parallel signal paths 28 may vary from two parallel signal paths to in excess of 30 parallel signal paths. The number of the parallel signal paths may be an odd number or an even number.

The impulse outputs on the parallel signal paths 28 each pass to one of the scaling devices 38. The scaling devices 38 may each be a mixer, a variable attenuator, or any other suitable and appropriate scaling device. Each scaling device 38 scales its impulse in amplitude according to the direct digital synthesized signal feeding a port of the scaling device 38 via line 52. The port may be regarded as a second or controlled port.

The scaled impulses travel through the individual time delay elements 34 before they are recombined in the combiner means 30.

The apparatus 22 shown in Figure 1 and described in GB A 2341991 overcomes problems with previously known apparatus utilizing direct digital synthesis. The apparatus 22 utilizes an electrical impulse generator 24 and it does not use optical components. The use of optical components may be advantageous in certain circumstances in itself. In addition, the maximum bandwidth of the output waveform using the optical components can be extended from 1 GHz (typical of electronic direct digital synthesis) to frequencies as great as 20 GHz. The optical implementation of the apparatus shown in Figure 1 will now be described with reference to Figure 2. It will be seen that Figure 2 is similar to Figure 1 and similar parts have been given the same reference numerals for ease of comparison and understanding. In Figure 2, the impulse generator means 24 is an optical pulse generator. This optical pulse generator may include additional means for the control of pulse-to-pulse amplitude variations.

In Figure 2, the signal generated by the optical pulse generator 24 is split by an optical splitter 26 into a plurality of parallel optical signal paths 28. The parallel optical signals are each given an appropriate optical time delay 34, and amplitude scaling using an optical intensity modulator 38. The optical signals from the parallel signal paths 28 are combined and detected (30) to generate the desired output electrical waveform.

The optical signals from the parallel signal paths 28 may be detected before combination in the electrical domain, or they may be combined in the optical domain before detection, or the signal combination may be split between the optical and electrical domains.

One particular solution is shown in Figure 2 where the optical signals from the parallel signal paths 28 are combined in the optical domain 30, before detection by the detector 58 to generate an electrical output signal 56 which is filtered by a filter 46 in order to generate the desired output

electrical signal waveform.

A 50 channel system, using a 10 pS optical pulse width repeated at 1 nS intervals, may achieve 20 GHz bandwidth with low associated phase noise ( -120dBc) provided that the optical pulse amplitude can be controlled to better than approximately 3%.

The optical components utilized in the apparatus of the present invention can be realized in a number of technologies. For example, the plurality of optical paths 28 can be in free-space, in optical fibre, or optical waveguides defined on an optical chip. The other optical components will have appropriate interfaces to the transmission medium used in the optical paths.

Accordingly, the optical splitter means 26 can be implemented as a free-space component, or as an optical fibre component or as an integrated circuit component. The amplitude scaling means 38 can be an optical intensity modulator, realized using the electro-optic effect in a Mach- Zehnder interferometer or the electro-absorption effect. The optical intensity modulator can be implemented as an individual fibre pigtailed component, or as a bare chip with free-space coupling, or as an integrated component as part of a larger optical circuit.

The optical time delay means 34 may conveniently be implemented either as lengths of optical fibre, or as optical waveguides integrated on an optical circuit chip. The optical combiner means 30 can be realized using a lens (in a free space solution), or as an optical fibre component, or as an integrated optical component.

In one preferred embodiment as shown in Figure 2, the optical splitter means 26, the plurality of optical signal paths 28, the amplitude scaling means 38, the optical time delay means 34, and the optical combiner means 30 are all implemented as a single electro-optical integrated circuit 60. The single electro-optical integrated circuit 60 may be regarded as a wave packet assembler. This implementation provides miniaturization, with potential low-cost manufacture. The chip may be realized using known gallium arsenide electro-optical integrated circuit technology.

In this implementation of the present invention, the signal generated by the optical pulse generator 24 is split by an optical splitter 26 into a plurality of parallel optical signal paths 28 to produce a nominally identical pulsed optical signal in each of the signal paths (same amplitude, same optical wavelength). In an alternative implementation, the signal generated by the optical pulse generator 24 has a wide optical bandwidth, the optical splitter 26 divides the signal into a plurality of parallel optical signal paths 28,

each with a different optical wavelength by "wavelength demultiplexing" as illustrated in Figure 4. In this example, the optical combiner means 30 uses wavelength multiplexing to provide a stream of weighted pulses each of a different optical wavelength. This method of optical combination using wavelength multiplexing can be more efficient for time sequenced signal pulses, than the optical combiner in a single wavelength system.

In practice, an optical pulse generator 24 with wide optical bandwidth may be realized by a number of different methods. These include the use of a single laser source with very narrow pulse width (typically 100 fS) and broad spectral bandwidth, or the use of multiple wavelength-tuned optical sources to provide a comb of optical lines.

Commercial laser diode sources for dense wavelength division multiplexing are available to provide multiple optical channels for fibre-optic telecommunications. These are typically spaced at 0.8 nm intervals to provide of the order of 100 channels in the 1.5 to 1.6 micron optical wavelength band. Such a channel spacing would be suitable for this application, noting that the technologies used to implement the optical scaling means 38, optical time delay means 34, and optical detection means must all have a bandwidth wide enough to accommodate all of the wavelength channels used.

In a simple extension of the apparatus as shown in Figure 2, the outputs of dense wavelength division multiplexing laser sources can be brought together into a single fibre for pulse modulation. Thus the optical pulse generator 24 may be an array of laser sources, which are wavelength multiplexed together to provide a comb of wavelengths, which is then intensity modulated to provide an output signal with an appropriate time response.

Alternative designs are possible, where each of the plurality of parallel optical paths 28 has its own optical path generator, and an optical splitter may be used to distribute an optical synchronization signal. This design is equally applicable to the case where all paths operate at the same optical wavelength, and the case where the paths are sorted by optical wavelength and optical multiplexing is used for the signal combiner means 30.

Figure 3 illustrates an alternative design for the optical path generator 24, where there are multiple optical sources rather than a single optical source. Figure 3 shows one optical pulse generator which optical signal path 28 in the wave packet assembler. Although multiple optical pulse generators are used and all optical pulses need to be synchronized (which is an added complication), there may be benefits in adopting this approach, since a plurality of low power optical generators is required rather than a single high power optical pulse generator. Benefits may include lower cost, particularly if the optical pulse generators 24 and wave packet assemblers 60 can be integrated on a single semi-conductor chip, whereas a high power optical pulse generator may require an expensive EDFA (optical fibre amplifier). A number of similar designs are possible, where the outputs of a small number of low power optical pulse generators are each split to provide the total number of optical signal paths required.

In the implementations of the invention shown in Figures 2 and 3, there is a single output electrical signal waveform. A range of applications can be envisaged which require a number of coherent electrical outputs of variable phase/amplitude weighting and relative time delay. Examples include microwave transmit phased array antennas.

Referring to Figure 5, two or more waveform generator modules may be synchronized by sharing the same optical pulse generator triggered from a common master clock. Each waveform generator module may include optical amplification (at the input), and may include a wave packet assembler, the electronic direct digital synthesizers used to drive the wave packet assembler, the control electronics and a radio frequency photo- detector . The master clock may be shared between modules either directly, or through local pulse detection and clock recovery within each module. Thus the only required link between waveform generator modules may be a photonic fibre-optic connection to the optical pulse generator source. It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications may be effected.

Claims

1. Apparatus for generating electrical signals with ultra-wide band arbitrary waveforms, which apparatus comprises an optical pulse generator for generating pulsed optical signals, splitter means for splitting the optical signals into a plurality of optical signals in parallel signal paths, and combiner and optical detector means for combining and detecting the signals in the parallel signal paths to generate the electrical signals with ultra-wide band arbitrary waveforms.

2. Apparatus according to claim 1 in which a plurality of optical impulse signal paths, combiner and optical detector means are used for combining and detecting the signals in the parallel signal paths to generate the electrical signals.

3. Apparatus according to claim 1 or to claim 2 in which the optical pulse generator is a pulse laser generator.

4. Apparatus according to any one of the preceding claims in which the optical pulse generator includes amplitude stabilization control means.

5. Apparatus according to any one of claims 1 - 3 and including time delay means for delaying the optical signals in the parallel signal paths in time relative to each other.

6. Apparatus according to any one of the preceding claims and including amplitude weighting means for weighting the optical signals in the parallel signal paths in amplitude relative to each other.

7. Apparatus according to claim 6 in which the amplitude weighting means is a scaling means which enables a predetermined reduction of signal amplitude on the optical signals in each of the parallel signal paths.

8. Apparatus according to claim 7 in which the scaling means comprises a plurality of scaling devices, one for each signal path.

9. Apparatus according to claim 8 and including a plurality of direct digital synthesizers, there being one of the direct digital synthesizers for each one of the scaling devices, and each one of the direct digital synthesizers being such that it has an electrical output appropriate for driving the scaling devices.

10. Apparatus according to claim 9 and including a master clock for feeding the direct digital synthesizers and the optical impulse generator means.

11. Apparatus according to claim 10 and including means for combining the optical signals in the parallel signal paths, before detection of the combined optical signal to generate the required electrical output waveform.

12. Apparatus according to claim 10 and including means for detecting the optical signals in each of the parallel signal paths, to produce parallel electrical signals before combination of the signals to generate the required electrical output waveform.

13. Apparatus according to any one of the preceding claims in which the optical splitting and optical combining are effected using wavelength division demultiplexing and wavelength division multiplexing.