GB2144847A - Optical spectrum analyser - Google Patents

Abstract

An optical spectrum analyser comprises an acousto-optic modulator 15 places in the path of a light source to be analysed. Acoustic waves are generated in the modulator 15 by an attached transducer 16 driven by a cyclically variable alternating frequency source. A lens 21 produces a Fourier Transform (FT) of the modulator phase function at the FT plane 23 where a detector 24 is placed behind a slit 25 in an opaque screen (or a reticle, not shown) to view the optical image which is scanned across by the action of the modulator 15. The optical spectrum derived from the output of the detector 24 may represent the spectrum of a distant source or, with a suitable reticle, the spatial characteristics of the source. <IMAGE>

Description

SPECIFICATION Optical spectrum analysers This invention relates to optical spectrum analysers, where the term optical is to be interpreted to include ultra-violet and infra-red radiation as well as the visible spectrum of light.

Dispersive spectrum analysers such as prism or grating spectrometers are well known in the prior art. It is also known to obtain radiation spectra by transform techniques. In particular, the Michelson inter-ferometer 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 grating, 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 an object of the present invention to provide an optical spectrum analyser capable of analysing optical or spatial characteristics.

The present invention provides an optical spectrum analyser including an acousto-optic cell arranged in a path of a light source, transducer means for introducing acoustic waves into the cell to thereby produce a variable phase delay function in the path of the light, an alternating frequency signal source connected to an input of the transducer means, means connected to the signal source for varying the frequency of the signal source, means to produce a Fourier Transform of the cell phase function in an image plane, a reticle arranged for selective modulation of light in the image plane and a detector arranged for selective detection of the modulated light. The detector output includes (as will be described) information on the spectral and spatial characteristics of the source of light incident on the acousto-optic cell, and the spectral and spatial characteristics of the reticle.Accordingly, an optical spectrum ana lyser of the invention may be employed for spectral or spatial analysis of the light source or for spectral or spatial analysis of a reticle.

Moreover, the analysis may be performed very rapidly, for example within a single frequency modulation cycle of the acoustooptic cell. It can of course be made in a duration of a few modulation cycles, indeed in one-half of a cycle.

The acousto-optic cell is preferably a transparent medium having attached an input piezoelectric transducer and a matched load.

The input transducer is connected to the signal source and the a matched acoustic load inhibits reflections within the cell. In this mode the device operates over a continuous range of electrical driving signals, it can however be used in a standing wave mode.

In one embodiment the invention provides an optical spectrum analyser in which the frequency of the signal source is varied cyclically within a predetermined range; the reticle is preferably stationary and comprises one or more elongate transmissive zones arranged orthogonal to and in or near the plane of the diffraction orders of the frequency modulated image of the cell. The reticle spatial frequency is chosen to be appropriate to modulate the diffraction orders; the detector is connected to signal processing means for extracting from the detector output the spectrum of the light transmitted by the cell. For a reticle comprising a single transmissive zone or slit, the detector ouput is frequency modulated and is approximately equal to the optical spectrum whereas for a multiple slit reticle Fourier transformation of the detector output is needed.In practice the detector output may be electronically normalised and if necessary Fourier transformed to give the spectrum of interest.

This embodiment of the invention is capable of analysing a light spectrum in a single frequency-modulation cycles, of the order of 100 micro-seconds for an acousto-optic cell operative at ultrasonic frequencies. This spectral analysis if considerably faster than conventional dispersive spectrum analysers.

In a second embodiment, the invention provides an optical spectrum analyser for determining the spatial frequency characteristics of a reticle of unknown spatial characteristics arranged for modulation of the Fourier transformed image of the cell, the source of light and the signal source having given characteristics.

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: Figure 1 is a schematic drawing of an optical spectrum analyser of the invention; Figures 2 and 3 illustrate a sinusoidally transmissive reticle; and; Figure 4 is a schematic drawing of an optical spectrum analyser having a single slit reticle and showing the electrical circuitry in block diagram form.

Referring to Fig. 1, there is shown an optical spectrum analyser in which parallel light 10 from a distant 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 a transparent acousto-optic cell 1 5 containing a transparent medium having at one end input piezoelectric transducer. The input transducer 1 6 is connected to a frequency-modulated electrical signal generator 18, and at the other end of the cell a second transducer 1 7 is coupled to a circuit 1 9 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 elongate transmissive zones 25 having a sinusoidal variation in transmissivity as shown at 26. In Fig. 1, 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 spectrum analyser of Fig. 1 to 3 are as follows. Consider a remote monochromatic source of light producing the parallel light 14 passing through the acoustooptic 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 18 produces, via the input transducer 16, acoustic waves travelling along the cell 1 5. 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 be equivalent to a periodic transmission diffraction grating by virtue of the periodic acoustic stress variation along the cell producing a corresponding variation in a 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 In = k Jn (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 Vc = 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 a frequency modulated electrical signal would produce a change in X 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 is sinusoidal. Accordingly the stationary reticle 23 selectively modulates a moving diffraction pattern produced by the cell 1 5.

From the frequency modulation characteristics, the spatial frequency of the reticle 23 and the optical system parameters, the optical source wavelength may be extracted from the detected modulation frequency. Furthermore, the amplitude of the modulated optical signal is proportional to the intensity of the monochromatic source.

The apparatus of Figs. 1 to 3 may be employed to analyse a broad-band light source, which is equivalent to a number of super-imposed monochromatic sources. Accordingly, during frequency modulated excitation of the cell 15, the output of the detector 24 consists of a summation of many sinusoidal waveforms, each waveform being of a different frequency and amplitude corresponding to a respective monochromatic spectral component. The light from the cell 1 5 is in the form of a Fourier transform as previously described, and accordingly this transform must be inverted after detection to extract the broad-band spectrum. After Fourier transformation, the detector output yields the spectrum of the broad-band source. Since the argument of the nth order Bessel function (equation 1) depends on the optical wavelength the spectrum obtained after Fourier transformation of the detector output must be normalised to take this into account, as is described with reference to the Fig. 4 arrangement below.

A broad-band source spectrum may be obtained in a single frequency modulation cycles of the signal generator 1 8. The acoustic-optic cell 1 5 has an upper limit of frequency response up to Gigahertz and a spectrum may be obtained in 100 micro-seconds or less.

This is considerably faster than conventional disperse or interferometric instruments, which may take of the order of minutes or seconds respectively to produce a spectrum. The high speed of operation is highly advantageous in cases where transient or time-varying spectra are to be recorded, and for which there is insufficient time to employ conventional techniques.

The spectrum analyser of Fig. 1 may be operated with other forms of reticle 23, such as a reticle matched to a particular source geometry. Alternatively, for a source of sufficiently high optical intensity, a single slit reticle may be employed providing a Fourier transformation to determine the spectrum. Thus as shown in Fig. 4 the reticle 23 included only a single narrow slit 25 located at a distance h off the optical axis OA. In this arrangement, the detector output of the acoustic cell is frequency modulated and is (apart from effects due to the wavelength dependence of the Bessel function arguement) the actual optical spectrum (ie no Fourier transform of the detector output is needed).

The signal generator 1 8 supplying the fm signal to the acousto-optic cell 1 5 includes an oscillator 28 whose ouput frequency is controlled by a periodic signal produced by an external sweep generator 29. The output signal from the detector 25 is first processed by a filter 30 and then normalised by an amplitude normaliser 31 before connection to an oscilloscope display 32. The time base for the display 32 is controlled by the signal level from the external sweep generator 29. The sweep signal level is also used to control the gain of the amplitude normaliser 31.

In a further mode of operation, the invention may be employed as a spatial frequency spectrum analyser to obtain the geometry of a reticle. The reticle for spatial frequency spectrum analysis is placed in the position of the reticle 23 in Fig. 1, and the detector output modulation is then a function of the spatial characteristics of the reticle which then can be electronically extracted.

The invention may also be employed to detect optical sources of particular spatial characteristics, ie particular source geometries. In this application the reticle 23 is selected to have spatial frequency characteristics appropriate to the source geometry of interest, thus tuning the spectrum analyser selectively to a specific type of optical source. This makes it possible to record spectra of specific sources in the presence of interfering background radiation. Alternatively, light sources of specific spectral and spatial characteristics may be identified in the presence of background radiation by arranging the post-detection electronic circuits to respond only to the corresponding detector output modulation characteristics.

A spectrum analyser of the invention may be equipped with light colllecting optics, such as a telescope.

Claims (8)

1. An optical spectrum analyser including an acousto-optic cell arranged in a path of a light source, transducer means for introducing acoustic waves into the cell to thereby produce a variable phase delay function in the path of the light, an alternating frequency signal source connected to an input of the transducer means, means connected to the signal source for varying the frequency of the signal source, means to produce a Fourier Transform of the cell phase function in an image plane, a reticle arranged for selective modulation of light in the image plane and a detector arranged for selective detection of the modulated light.

2. An optical spectrum analyser as claimed in claim 1 wherein the acousto-optic cell is a transparent medium having attached an input piezoelectric transducer connected to the signal source and a matched acoustic load to inhibit reflections within the cell.

3. An optical spectrum analyser as claimed in claim 1 or 2 wherein the frequency of the signal source is varied cyclically within a predetermined range.

4. An optical spectrum analyser as claimed in any one of claims 1 to 3 wherein the rectile is stationary and comprises at least one elongate zone, with the or each zone arranged orthogonal to and in or near the plane of the diffraction orders of the frequency modulated image of the cell.

5. An optical spectrum analyser as claimed in claim 4 having a plurality of transmissive zones, the spatial frequency thereof being selected to modulate the diffraction orders.

6. An optical spectrum analyser as claimed in any one preceding claim wherein the detector output is connected to a signal processing means for producing an output corresponding to the spectrum of the light source.

7. An optical detector for determining the spatial frequency characteristics of a reticle comprising an optical spectrum analyser as claimed in any one of claims 1 to 5, the arrangement being such that the spectral characteristics of the light source and the signal source are known.

8. An optical spectrum analyser substantially as described with reference to Figs. 1 to 4 of the accompanying Drawings.