GB2144848A - Electrical signal spectrum analyser - Google Patents

Abstract

An electrical signal analyser comprises an acousto-optic cell 15 placed on the path of an input light beam 14 which produces a phase delay which varies across the width of the cell. A lens 21 then provides a Fourier Transform (FT) of the cell phase function in the FT plane 22 where a detector is placed to receive light transmitted through a slit 34 in an opaque screen. A periodic frequency modulated control signal C(t) modulated by an unknown electrical Signal S(t) is used to introduce acoustic waves in the cell 15 by driving a transducer 16 attached to the cell. The electrical signal S(t) can then be determined from the output of the detector 24. Phase information in the electrical signal can be determined by splitting the input light beam 14 to produce a second beam and directing the second beam to impinge on the FT plane 22 at an angle thereby producing a phase gradient across the FT plane. Two detectors placed behind respective adjacent slits can then provide phase information. <IMAGE>

Description

SPECIFICATION Electrical signal analysers This invention relates to electrical signal spectrum analysers.

Dispersive spectrum analysers such as prism of grating spectrqmeters are well known in the prior art for analysing light sources. It is also known to obtain radiation speectra by transform techniques. In particular, the Michelson interferometer has been employed to obtain Fourier transform spectra. These prior art techniques are restricted as regards speed of operation by the requirement to vary the position of an optical component in order to scan a spectrum. Dispersive spectrometers operate by rotating a prism or graing, and a Michelson interferometer by moving a mirror.

The principles of optical diffraction are also employed with a known light source to analyse the spatial characteristics of a diffracting object, as in holography for example.

It is also known to carry out spectral analysis of electrical signals by performing a Fourier analysis of the signal. One approach is to digitise the analogue signal and then to carry out a numerical Fourier analysis to obtain the signal spectrum. Alternatively, the signal may be analysed by a swept or tunable band-pass filter.

It is an object of the present invention to provide an electrical spectrum analyser capable of analysing electrical characteristics and employing an optical transform process.

The present invention provides an electrical signal analyser including an acousto-optic cell arranged in a light path from a light sourse, means to provide a Fourier transformed image of the cell, transducer means for introducing acoustic waves into the cell, a periodic, frequency modulated control signal source connected to an input of the transducer means, the electrical signal to be analised connected to the control signal source so as to frequency modulate the output signal from the control signal source, a reticle arranged for selective modulation of light from a frequency-modulated image of the Fourier transform of the cell, phase function and a detector arranged for selective detection of the modulated light.

The detector output depends (as will be described) on the spectral and spatical characteristics of the source of light incident on the acousto-optic cell, the spectrum of the electrical signal source and the spatial characteristics of the reticle. The periodic control signal causes the diffraction pattern of the Fourier Transform image to scan across the reticle and the resulting periodic variation in the light transmitted by the reticle is detected. Accordingly, an electrical signal analyser of the invention may be employed for analysis of the electrical signal source when the characteristics of the light source and reticle are known. Moreover, the analysis may be performed very rapidly, at a speed much faster than conventional spectrum analysers whilst using a single element, low noise high quality detector.

The acousto-optic cell preferably comprises a transparent having attached at one end thereof an input piezoelectric transducer; the input transducer being connected to the signal souce and having at the opposite end thereof an output transducer coupled to an acoustic load to provide a matched acoustic impedance to inhibit reflections within the cell.

Preferably the source of light is a quasi monochromatic laser. The laser output may be temporally modulated at a sufficiently high frequency that the spectral resolution of the electrical signal analyser is enhanced.

In one embodiment of the invention the reticle may comprise a single slit positioned such that a single diffraction order scans across the slit. Alternatively more than one slit may be provided, each having an associated detector such that different diffraction orders may be combined or different parts of the spectrum of the electrical signal in the Fourier Transform plane may be processes simultaneously. In the latter case the time required to process the signal spectrum will be reduced still further. The reticle slit positioning and spacing is therefore chosen to be appropriate to modulate the diffraction orders and each detector is connected to signal processing means for extracting the spectrum of the electrical signal source.

In a second embodiment for extracting information on the phase of the Fourier components of the spectrum of the electrical signal the electrical spectrum analyser further includes means to provide a second light path from the source whereby a beam of light is combined with the Fourier Transformed image of the acousto-optic cell such that the phase of the beam of light varies linearly across the reticle. Thus a spatial variation of phase in the Fourier Transform plane is superimposed on the Fourier Transform image. The resulting modulating effect on the detected signal will depend upon the phase of the Fourier components in the electrical signal applied to the acousto-optic cell.One way of extracting the phase information is to include a phase modulator in the second light path to produce a periodically varying phse delay in the light path, the Fourier transformed image being sampled at two positions in the Fourier Transform plane separated by less than the spacing of the interference fringes, the phase difference between the two samples is then measured and the resulting phase difference processed to give the phase. Advantageously the phase modulator is a Pockels Cell.

In an alternative arragement of the second embodiment the analyser may be used for signal recognition by inserting in the second light path an appropriate light modulator having a phase 1 amplitude modulation function complementary to an electrical signal to be detected.

In order that the invention might be more fully understood, an embodiment thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: Figures 1 to 3 are schematic drawings of an apparatus to illustrate the principles of operation of the electrical spectrum analyser Figure 4 is a schematic diagram of an electrical spectrum analyser; Figure 5 shows the detector output signal from the analyser of Fig. 4 when no electrical signal S(t) is present; and Figure 6 shows an arrangement of the analyser for measuring phase information in an electrical signal Referring to Fig. 1, there is shown an optical analyser in which parallel light 10 from a laser source (not shown) is incident on a first lens 11.The lens 11 focuses the light at the focus 1 2 of a second lens 1 3. Parallel light 14 passes through the transparent medium of an acousto-optic cell 1 5 having an input piezoelectric transducer 1 6 at one end thereof. The input transducer 1 6 is connected to a frequency-modulated electrical signal generator 18, and a transducer 1 7 at the opposite end of the cell is coupled to a circuit 1 9 so as to provide a matched acoustic load. The parallel light 14 is focussed at point 20 by a third lens 21 as a series of diffraction orders (not shown) appearing in the image of Fourier plane 22. Light from the diffraction orders or points 20 passes through a reticle 23 to a detector 24.

Referring now also to Figs. 2 and 3, the reticle 23 consists of a regularly spaced series of elongated transmissive zones 25 having a sinusoidal variation in transmissivity as shown at 26. In Fig. 'I, the zones 25 of the reticle 23 extend normally to the plane of the diagram, so the sinusoidal transmissivity variation is in the plane of Fig. 1 along the line of the focal points 20.

The principles of operation of the optical analyser of Fig. 1 to 3 are as follows. Consider a quasi monochromatic laser source of light producing the parallel light 14 passing through the acousto-optic cell 1 5. The transparent cell 1 5 produces a phase delay in the path of the light dependent on the refractive index of the cell material. Variations of phase delay along the length of the cell 1 5 can be described by a phase function. The lens 21 then produces a Fourier Transform of the cell phase function.

The signal generator 1 8 produces, via the input transducer 16, acoustic waves travelling along the cell 1 5 and acoustic reflections are suppressed by the matched load properties of the transducer 1 7 and circuit 1 9. If the output of the signal generator were to be of constant frequency fO, the Fourier transform image of the cell phase function would appear in the Fourier plane 22 as a series of diffraction orders centred at order zero. With constant frequency excitation, the cell 1 5 would behave similarly to a periodic transmission diffraction grating by virtue of the periodic acoustic stress variation along the cell producing a corresponding variation in refractive index.The intensity In of the nth diffraction order of the cell phase function would be proportional to the square of the nth order Bessel function, ie I,-k J, (A)2 (1) Where k is a constant and A is the argument of the nth order Bessel function Jn. The argument A is a function of source wavelength, cell physical properties and thickness and source amplitude. Accordingly, the intensity of the diffraction pattern is a function of wavelength, cell physical properties and thickness and source amplitude. Accordingly, the intensity of the diffraction pattern is a function of wavelength.The zero diffraction order would be located at the position of the geometric image of the light source, and the nth diffraction order would be displaced from the zero order by a distance Xn given by:

where n = order number f = electrical frequency of generator 18, d = distance from lens 21 to Fourier plane 22, As= wavelength of monochromatic source, and V = velocity of sound in the cell 1 5 Equation (2) demonstrates that the separation Xn of the nth and zero diffraction orders is proportional to order number, electrical frequency and source wavelength. Thus by frequency modulating the electrical signal there will be produced a change in Xn proportional to order number, frequency change and source wavelength, and the diffraction orders would return to their original positions after each frequency modulation (fm) cycle. By virtue of the sinusoidal transmissivity of the reticle 23, as the diffraction orders move towards and away from the zero order transversely of the reticle zones 25 during an fm cycle, the optical intensity passing to the detector 24 varies. If the frequency modulation is a saw-tooth or triangular function, the intensity variation will be sinusoidal. Accordingly the stationary reticle 23 selectively mo dulates a moving diffraction pattern produced by the cell 15.

The arrangement of Fig. 1 may be operated with other forms or reticle 23, for example for a source of sufficiently high optical intensity, a single slit reticle may be employed providing a Fourier transformation to provide a spectrum.

The above principles may be applied to determine the characteristics of an unknown electrical signal by using the signal to frequency modulate a carrier signal applied to the acousto-optic cell. The modulation of the detector output can then be used to determine the characteristics of the electrical signal. Fig.

4 illustrates an electrical spectrum analyser according to the invention. A monochromatic laser light source is employed and the electrical signal of interest s(t) is processed before being used to drive the acousto-optic cell 1 5.

The signal is processed by mixing it with a suitable frequency modulated control signal c(t). The electrical signal S(t) is multiplied by the frequency-modulated control signal C(t) by connecting the two signals to respective inputs 30, 31 of a multiplier 32. The control signal C(t) may for example be a periodic swept frequency signal with the frequency varying linearly with time between frequency limits w1 and w2. The output from the multiplier 32 is then used to drive the acoustooptic cell 1 5. The applied signal will then be S(w + wo) where wo is the instantaneous frequency of c(t). The light beam 14, modulated by the cell 15, will scan back and forth in the Fourier Transform plane 22 with an amplitude defined by the range of wo and at a rate determined by the FM period of wo.A reticle 33 with a single narrow slit 34 is placed in the Fourier Transform plane. The slit 34 is positioned off the optical axis of the analyser such that it is within the limit of the scan of the first diffraction order, say, of the modulated light beam. Fig. 5 illustrates the detector output 35 as a function of time with only the control signal c(t) applied to the acousto-optic cell 1 5. Variations in the detector output due to the electrical signal s(t) can then be extracted using appropriate processing electronics.

The frequency resolution of the analyser depends upon the width of the slit 34 and this can be made much narrower than the width of the detector 24. The resolution also depends upon, and is ultimately limited by, the width of the illuminated region of the acousto-optic cell 1 5.

In a conventional arrangement involving a signal scanned over an array of detectors, the band-width and resolution would be determined by the width of each detector element and the number of elements in the array. In the present invention using only a single detector behind a narrow slit the band-width can be increased by sweeping wo, the control frequency, over a wide range. Here the bandwidth is limited by the band-width of the acousto-optic cell.

The discussion of the invention has so far been concerned only with the detection of a sinle diffraction order. By using further appropriately located slits in the Fourier Transform plane use can be made of other diffraction orders with parallel processing. This method could be used to further extend the dynamic range of the analyser by electronically combining the individual outputs or, alternatively, different parts of the spectrum of S(t) could be sampled simultaneously. This latter approach would shorten the period required to sample the whole spectrum and could facilitate realtime comparison of individual frequency components in the S(t) spectrum.The dynamic range could also be improved by temporally modulating the laser monochromatic light source incident on the acousto-optic cell at a sufficiently high frequency (f) that variations in s(t) to be determined are well away from the 1 /f region of the detector.

An arrangement of the invention for measuring phase information in an electrical signal in addition to amplitude spectrum information is shown in Fig. 6. Features common to the Fig. 4 arrangement are indicated by like reference numerals. The parallel light 14 from a monochromatic laser light source is divided by a partly silvered plane mirror 40 such that one portion of the light passes through the acousto-optic cell 1 5 and a second portion 42 passes directly to the Fourier Transform plane 22 via a reflecting mirror 43 and a phase delay element 44.As in the Fig. 4 arrangement the electrical signal S(t) is mixed with a frequency modulated control signal c(t) and applied to the piezo-electric transducer 1 6 generating acoustic waves in the cell 1 5. A reticle 45, positioned in the Fourier Transform plane, is provided with two slits 46, 47 with a respective detector 48, 49 behind the slits.

The purpose of the redirected portion 42 of the input laser light beam 14 is to combine the Fourier spectrum in the Fourier Transform plane 22 with a linearly varying optical phase function obtained by virtue of the inclination of the redirected beam. The operation of the analyser can be understood by considering the simple case when the phase of the electrical signal S(t) is constant for all its Fourier components. The intensity distribution in the Fourier Transform plane will then appear as a cosine distribution, the amplitude bing spatially modulated by the amplitude components of the Fourier spectrum.The spatial frequency of the cosine variation is dependent on the wavelength of the light (A) and the angle (a) of the interfering beam 50 to the normal to the Fourier Transform plane 22 and is given by the expression: (27rsina)/A Consider now the more general case of a non-uniform phase for each Fourier component of the signal. This will produce an optical intensity function with the same envelope as previously, but now having localised shifts in the pattern spacing, previously a simple cosine function. The shift at a given point in the Fourier Transform plane 22 is precisely the phase of the associated Fourier component.

Thus, measurement of the shift function provides the phase information of the Fourier spectrum S(w). In a practical system the Fourier spectrum is continually changing since S(t) is a continuously changing function of time.

There are a number of ways in which the required phase information can be extracted.

For example, by including a phase modulator (eg a Pockels Cell) as the phase element 44 in the re-directed light path. The action of the phase modulator is made to cause a periodically varying phase delay with the result that the intensity of a given interference fringe will vary cosinusoidally with time. Consequently if the interference pattern is sampled at two positions in the Fourier Transform plane, with the positions separated by less than the fringe spacing, the phase of the sampled Fourier component can be obtained from the phase difference between the two samples. Thus the slits 46 and 47 are set a distance apart less than the fringe spacing. The phase difference between the signals at the output of the detectors 48 and 49 is then used together with the separation of the slits and the frequency of the phase modulator to determine the phase of the associated Fourier component. Many techniques will be apparent to those skilled in the art of signal processing for optically or electronically processing the signals from the detectors 48 and 49 to obtain the required phase information.

The Pockels cell phase modulator 44 may be replaced using alternative techniques of inserting phase and amplitude modulation function so that the analyser of Fig. 6 can be used for signal recognition. In this mode of operation, spatial light modulators or phase or transmission reticles replace the phase modulator 44. By choosing the appropriate phase and/or amplitude function for insertion in the redirected beam 42 input signals within S(t) can be automatically recognised from the Fourier plane when S(t) contains the complement to the function inserted in the redirected beam 42.

Claims (11)

1. An electrical signal analyser including an acousto-optic cell arranged in a light path from a light source, means to provide a Fourier transformed image of the cell, transducer means for introducing acoustic waves into the cell, a periodic frequency modulated control signal source connected to an input of the transducer means with the electrical signal to be analysed being connected to the control signal source so as to frequency modulate the output signal from the control signal souce, a reticle arranged for selective modulation of light from a frequency-modulated image of the Fourier transform of the cell phase function and a detector arranged for selective detection of the modulated light.

2. An electrical signal analyser as claimed in claim 1 wherein the acousto-optic cell comprises a transparent medium and has attached at one end thereof an input piezoelectric transducer; the input transducer being connected to the signal source at the opposite end thereof an output transducer coupled to an acoustic load to provide a matched acoustic impedance to inhibit reflections within the cell.

3. An electrical signal analyser as claimed in claim 1 or 2 wherein the source of light is a quasi monochromatic laser.

4. An electrical signal analyser as claimed in any one of claims 1 to 3 wherein the reticle comprises a single slit positioned such that a single diffraction order scans across the slit.

5. An electrical signal analyser as claimed in any one of claims 1 to 3 wherein the reticle comprises a plurality of slits, each having an associated detector such that different diffraction order can be combined or different parts of the spectrum of the elctrical signal in the Fourier Transform plane can be processed simultaneously.

6. An electrical signal analyser as claimed in any one of claims 1 to 3 for extracting information on the phase of the Fourier components of the spectrum of an electrical signal further including means to provide a second light path from the source whereby a beam of light is combined with the Fourier Transformed image of the acousto-optic cell such that the phase of the beam of light varies linearly across the reticle.

7. An electrical signal analyser as claimed in claim 6 wherein a phase modulator is included in the second light path to produce a periodically varying phase delay in the light path, the combined Fourier transformed image and light beam from the second light path being sampled at two positions in the Fourier Transform plane separated by less than the spacing of the interference fringes, the phase difference between the two samples being measured and the resulting phase difference processed to give the phase.

8. An electrical signal analyser as claimed in claim 7 wherein the phase modulator is a Pockels Cell.

9. An electrical signal analyser as claimed in any one of claims 1 to 3 including means to provide a second light path from the source whereby a beam of light from the second light path is combined with the Fourier Transformed image of the acousto-optic cell, and a light modulator inserted in the second light path having a phase-amplitude modulation function complementary to an electrical signal to be detected.

10. An electrical signal analyser substantially as described with reference to Figs. 1 to 5 of the accompanying Drawings.

11. An electrical signal analyser substantially as described with reference to Figs. 1 to 3 and 6 of the accompanying Drawings.