GB2246459A - Radio frequency spectrum analyser - Google Patents

Abstract

A radio frequency spectrum analyser comprises an electro-optic collinear filter (3) and detector means (8, 9, 10) for determining the wavelength of light output therefrom. The filter may include an electro-optic material exhibiting a variable birefringence under the influence of a variable electric field, and means for variably applying an electric field across the material, to permit the filter to be tuned. One particular frequency of the incident polarised light has its state of polarisation changed by the applied r.f. field and this changed state of polarisation enables this light to be deflected by a polarising beam splitter 7 to a diffraction grating 9. The angle at which the beam is diffracted onto a diode array 10 depends on the frequency of the light and hence of the applied r.f. field. <IMAGE>

Description

Radio Frequency SPectrum Analyser This invention relates to a radio frequency spectrum analyser which is of particular use in measuring frequencies in radar signals.

It is known to use a bulk wave Bragg cell for measuring the frequency and amplitude of radio frequency signals. The cell consist of a polished piece of acousto-optic material to which is bonded a piezo-electric transducer. The transducer converts an applied r.f. signal into an acoustic wave which propagates through the acousto-optic medium and acts optically as a moving diffraction grating. An incident monochromatic laser beam interacts with the acoustic grating producing a range of deflected beams, each corresponding to a separate frequency present in the acoustic beam. In such a device, the bandwidth is normally limited to one octave in frequency to avoid problems of signal aliasing caused by second harmonic diffractions.The diffracted optical beam is angularly deflected by an amount proportional to the acoustic frequency and experiences a frequency shift equal to the acoustic frequency. The frequency shift may be detected or the angular deflection may be measured to give the frequencies in the input r.f. signal.

A problem with this type of cell is that the attenuation of the acoustic wave in the material is proportional to the frequency raised to a power which is between about 1.4 and 2, depending on the material used.

This limits both the maximum operational frequency and hence the bandwidth which can be analysed and the frequency resolution of the device. A maximum centre frequency of 1OGHz is practical for this type of Bragg cell. Further, a maximum bandwidth of about 2 GHz is typical, so that full simultaneous coverage of any of the common radar bands is not possible with a single device.

Another type of device which may be used in r.f.

spectrum analysis is the acousto optic tunable filter, in which an acoustic wave derived from the r.f. signal and a polarised polychromatic optical beam interact in an acousto optic medium. The device is normally used for optical spectrum analysis, but may be used in an inverse mode for r.f. spectrum analysis. The acoustic wave corresponds to the input radio frequency and causes a change in polarisation and direction of one particular wavelength of the polarised light introduced into the device. The wavelength of the light whose polarisation is changed is detected to determine the r.f. frequency.

The r.f. frequency is inversely proportional to the optical wave length and is proportional to the birefringence of the material for the particular optical propagation directions. In practice, the radio frequency band analysable is again limited by acoustic attenuation to bandwidths between 1OMHz to 1GHz. The device may also operate in a collinear mode, in which the optical and acoustic beams travel in the same direction, and the acoustic wave causes a change in polarisation (but not in direction) of one particular wavelength of polarised light introduced into the device.

It has now been found that the use of an electro-optic material in a collinear tunable filter permits the detection of very much higher r.f.

frequencies.

Accordingly, the invention provides a radio frequency spectrum analyser, comprising an electro-optic collinear filter and detector means for determining the wavelength of the light output therefrom.

The detector means may comprise a linear array of photo diodes or CCD array and means for detecting the relative position and measuring the intensity of a light beam impinging thereon.

In a preferred embodiment of the invention, means are provided for selectively applying a voltage bias across the electro-optic material whereby the birefringence of the material may be varied. A variation of the birefringence permits variation of the centre frequency of the r.f. waveband which can be detected. The analyser can therefore be tuned to selected frequency ranges.

Reference is made to the drawings, in which: Figure 1 is a wave vector phase matching diagram for a collinear optical filter; Figure 2 is a diagrammatic plan view of a radio frequency spectrum analyser in accordance with one embodiment of the invention; and Figure 3 is a diagrammatic sectional view of a electro-optic wave guide forming part of the analyser shown in Figure 1.

Referring first to Figure 1, K1 represents the wave vector of the incident optical beam, KD represents the wave vector of the diffracted optical beam and K is the a wave vector of the diffraction grating, formed by the acoustic wave in the acousto optic case and by the electric field of the r.f. wave in the electro optic case. The optical wave vectors are scaled to the refractive index surfaces of the material and must terminate on those surfaces. Each of the surfaces represents a different optical polarisation. A particular incident optical wave vector is therefore converted to a diffracted optical wave vector of opposite polarisation by a particular r.f. or acoustic wave vector. This means that a particular incident optical wavelength is polarisation switched by a particular r.f. frequency.Operating the device between crossed polarisers gives a tunable optical filter the wavelength of the light passed being dependant on the applied r.f. signal.

In the analyser shown in Figure 2, a beam of polychromatic light from a wide bandwidth high intensity source such as a Xenon arc passes through a polariser 2 and into an r. f. waveguide 3 which may suitably comprise a rectangular channel formed in a substrate 4 (Figure 3) and containing an electro-optic material 5 provided with electrodes 6 therealong. The light beam passing along the channel interacts with the r.f.

signal in the manner of a collinear electro-optic filter, light of a particular wavelength having its polarisation changed by interaction with the grating formed by the electric field of the r.f. signal. The light beam 1 emerging from the waveguide 3 at its opposite end passes through a further polarising element 7 which is shown as a polarising beam splitter and separates the two polarisation states. The orientation of the beam splitter is chosen so that only light whose polarisation has been changed in passage through the waveguide is optically deflected onto a diffraction grating or other optically dispersive element 9 which diffracts light onto a linear photo-diode array 10, the position of the photo-diode or diodes in the array detecting light being determined by the wavelength of the light emerging from the waveguide 3 with polarisation switched. In the waveguide 3, the wavelengths at which change of polarisation of the light occurs depends upon the input microwave frequencies.

Thus, the light deflected by the beam splitter 7 contains only that wavelength or those wavelengths which correspond to the frequency or frequencies of the microwaves input at 11. The microwave frequency and the wavelength of the light whose polarisation is changed in the waveguide are linked by the formula:

wherein f is the r.f. frequency in Hz, c is the free space velocity of light,ssn is the birefringence of the waveguide for the optical polarisations, is the microwave dielectric constant and ss is the wavelength of the light.

Taking as an example a material having a value of = n = 0.001 and wherein lt, = 1.5 and the wavelength A = lem, it can be seen that the microwave frequency is 200 GHz. This is significantly higher than can be obtained with conventional devices. By changing this permits the analyser of the invention to be used for measuring radio frequencies directly in any of the common radar bands. Additionally, by applying a DC field across the electrodes 6, the value of an can be changed, thus permitting a different range of frequencies to be detected. By suitable selection of the waveguide material and its dimensions, an effective band width of 10% of the centre frequency can be detected. In the above example, this would give a bandwidth of 20 GHz.

The electro-optic material may be an electro-optic polymer or organic crystal, or an inorganic electro-optic crystal such as lithium tantalate, lithium niobate, or strontium barium niobate.

Claims (10)

1. A radio frequency spectrum analyser, comprising an electro-optic collinear filter and detector means for determining the wavelength of the light output therefrom.

2. An analyser according to claim 1, wherein the filter includes an inorganic electro-optic crystalline material.

3. An analyser according to claim 2, wherein the inorganic electro-optic crystalline material is lithium tantalate.

4. An analyser according to Claim 1, wherein the filter includes an organic electro-optic material.

5. An ana]yser according to claim 1, wherein the filter includes an electro-optic material exhibiting a variable birefringence under the influence of a variable electric field, and means for variably applying an electric field across said material, whereby the r.f. frequency of operation of the filter may be tuned.

6. An analyser according to any preceding claim, wherein the detector means comprises a linear array of light detectors.

7. An analyser according to claim 6, wherein the light detectors are photodiodes.

8. An analyser according to claim 6, wherein the detector means comprises a c.c.d. array.

9. An analyser according to claim 6, 7 or 8, wherein means are provided for detecting the position and measuring the intensity of a light beam impinging on the array.

10. A radio frequency spectrum analyser, substantially as described with reference to the drawings.