METHODS OF AND APPARATUS FOR MEASUREMENT USING ACOUSTO-OPTIC DEVICES

Abstract: Optical measuring apparatus includes a phase sensitive measuring device (8, 9, 14, 16) including an acousto-optic device adapted to modulate a parameter of a light beam (1), beam splitter means (5) to split said light beam into a reference beam (6) and a probe beam (7), detector means (8, 13) respectively to detect said reference beam and said probe beam after interrogation of an element under test, and measuring means (9, 14, 15) to measure the differential of an optical property of the element measured by said probe beam.

Description: Methods of and apparatus for measurement using Acousto-Optic Devices This invention relates to the use of acousto-optic measuring devices and, in particular, to differential optical measurements using such devices- Optical techniques are exploited in a very wide range of measurement technologies. An important factor in any measurement is the sensitivity to small changes in the physical property being measured. Therefore an invention which can enhance the sensitivity of several types of optical measurement has many potential applications. A feature of the present invention is the use of an acousto-optic in combination with phase sensitive detection techniques directly to make differential optical measurements. Acousto-optic devices rely on the interaction of optical and acoustic waves within certain transparent materials. They are extensively used in optical systems to control the intensity, wavelength and beam direction of light. The two types of acousto-optic device which are used in this invention are acousto-optical deflectors and acousto-optic tunable filters. As their names imply an acousto-optic deflector will electronically control the angle through which a light beam is deflected on traversing the device and an acousto-optic tunable filter will control the wavelength of light transmitted by the device. The important features of these devices are their very fast scan rate and continuous scan range. In both these respects acousto-optic devices are superior to their electromechanical equivalents, although more limited in angle and wavelength scan range. According to the present invention there is provided apparatus for enhancing the sensitivity of many types of optical measurement comprising an acousto-optical device and phase sensitive detection techniques directly to measure the differential of the optical properties of a sample or sensor element. The invention will now be particularly described with reference to the accompanying drawings in which: Figure 1 shows the reflectance of a layer of silver on a fused silica prism, Figure 2 is a schematic representation of apparatus used for measurement of differential reflectance, Figure 3 shows the reflectance of a layer of silver on a high index glass prism, Figure 4 illustrates schematically the measurement of differential transmission of a solution of potassium permanganate, Figure 5 is an indication of the optical transmission of a solution of potassium permanganate, Figure 6 is a schematic representation of apparatus used for measurement of small shifts in optical features, Figure 7 shows the shift in plasmon angle due to water vapour condensing on a silver layer, Figures 8 - 11 are explanatory diagrams, Figure 12 is a schematic representation of apparatus incorporating optical fibres for remote measurement; and Figure 13 is a diagrammatic representation of apparatus used for differential spectroscopy. Referring to the drawings, an optical measurement system uses an acousto-optic deflector or acousto-optic tunable filter to modulate the angle of incidence or wavelength of the light beam which is being used to probe a sample or sensor element. If the optical properties of the sample are angle (momentum) or wavelength (momentum or energy) dependent then a suitably placed photodetector will receive a modulated signal at the same frequency as the acousto-optic modulation. This signal can be fed to a lock-1n amplifier using the acousto-optic modulation frequency as the reference. In principle only that component of the photodetector signal which is due to the variation of the sample's optical properties will then be amplified. Because the size of the output signal from the amplifier is proportional to the modulation of the photodetector signal it is in fact a direct measurement of the differential of the sample's optical properties with respect to angle or wavelength. This is illustrated in Figure 1 where the dotted line shows the reflectance as a function of angle for a silver layer on a fused silica prism. The dip in reflectance is caused by a typical surface plasmon-polariton resonance at the silver air interface. The continuous line shows the differential of this reflectance, (dR/dθ), which was directly measured using an acousto-optic deflector to modulate the incident angle as illustrated in Figure 2. In this, a beam 1 of radiation from a HeNe laser 2 passes to an acousto-optical deflector 3 controlled by a frequency modulated drive signal from a voltage controlled frequency source 4. A partially reflecting mirror 5 splits the radiation into a reference beam 6 and a probe beam 7. The reference beam is detected by a photodiode 8 and passes to one input of an analogue divide circuit 9. The probe beam is reflected from a silvered layer on one surface 10 of a prism 11 mounted on a computer controlled rotating stage 12. It then passes to a photodiode 13 which is coupled to the other input of the analogue divide circuit 9. A signal representative of the ratio of the signal to reference voltages passes from the divider circuit to an input of a lockin amplifier 14 which is also fed from the signal generator 15 which which provides a frequency modulation signal to the VCFS. A differential signal passes from the lock-in amplifier to a microcomputer 16. The importance of measuring the true differential is that it allows the measurement of very weak optical features which would normally be dominated by large, perhaps noisy, background signal. Conventional beam amplitude modulation together with phase sensitive detection will also reduce noise but will not eliminate the effect of large background signals. The power of the differential measurement is shown in Figure 3 for an angle modulating implementation. When the reflectance of a thick film of silver deposited on a high index glass prism was measured using a standard amplitude modulated detection system it appeared to be constant over the incident angle range. However, modulating the incident angle with an acousto-optic deflector and with the lock-in amplifier set at very high gain (which accounts for the noise), the presence of a very weak surface plasmon resonance can be resolved from the differential reflectivity. The invention also finds application in wavelength scanning implementations such as differential liquid spectroscopy measurement. Normally in a differential liquid spectrometer the absorption of a cell containing the solution of interest is compared with that of a pure water reference cell. The system illustrated in Figure 4 has been used to measure the differential of this comparative absorption with respect to wavelength. A white light beam 20 is incident on an acousto-optic tunable filter 22. The filtered exit beam 24 passes to a sample cell 26 containing potassium permanganate and a reference cell 28 containing water to produce a probe beam 30 and a reference beam 32 which, after reflection by mirrors 34,36 are detected by respective signal and reference photodiodes 38,40. The output from these diodes is fed to an analogue divide circuit 42, lock-in amplifier 44 and microcomputer 46. A signal generator 48 provides a frequency modulation signal to the VCFS 50 and a reference signal for the lock-in amplifier. The microcomputer provides an additional wavelength scanning signal to the VCFS which drives the acousto-optic tunable filter. Figure 5 shows how the differential of the transmission of a cell containing a very dilute solution of potassium permanganate strongly enhances the weak absorption features in the comparative signal. A further embodiment of the invention allows the measurement of very small changes in the optical properties of the sample. This application requires the construction of the optical analogue of a phase locked loop. In practice this involves 5 feeding back the output from the lock-in amplifier to a voltage controlled frequency source which drives the acousto-optic device as illustrated in Figure 6. In this, a beam 60 of radiation from a HeNe laser 62 passes to an acousto-optical deflector 64 controlled by a frequency modulated drive signal 10 from a voltage controlled oscillator 66. A partially reflecting mirror 68 splits the radiation into a reference beam 70 and a probe beam 72. The reference beam is detected by a photodiode 74 and passes to one input of an analogue divide circuit 76. The probe beam is reflected from a silvered layer on one surface 1578 of a prism 80 mounted on a computer controlled rotating stage 82. It then passes to a photodiode 84 which is coupled to the other input of the analogue divide circuit 76. A signal representative of the ratio of the signal to reference voltages passes from the divider circuit to an input of a lock-in 0 amplifier 88 which is also fed from the signal generator 90 which modulates the voltage-controlled frequency source. A feedback signal also passes from the lock-in amplifier to the voltage-controlled frequency source. The output frequency of the voltage-controlled frequency source is continuously measured 5 by a frequency counter 92 and recorded by a microcomputer 94. If the phase of the feedback is correctly adjusted it is possible for the system to lock on to the angle or wavelength at which a particular optical feature occurs. For example if the laser beam after deflection by the 0 acousto-optic deflector is incident on the silver layer in Figure 1 at the angle for minimum reflectance, then the feedback signal from the lock-in amplifier (set to the acousto-optic deflector modulation frequency) is zero. However if the surface plasmon angle shifts in response to a physical change in the 5 sample then a large signal will be fed back from the amplifier to the voltage-controlled frequency source. This signal will pull the centre frequency of the voltage-controlled frequency source and so the deflection angle of the laser beam to restore a null signal. In this way the incident angle is locked to the 5 surface plasmon resonance and any shift in this angle will be measured as a shift in the centre frequency of the voltage-controlled frequency source. The fact that changes in the angle or wavelength of an optical feature are converted by the system to changes in frequency of the voltage-controlled frequency source results in a massive increase in sensitivity of the optical measurements. Typical drive frequencies for an acousto-optic deflector are 30-70MHz and those for an acousto-optic tunable filter are 80-160MHz and in principle these frequencies can be measured with a few Hertz accuracy giving a potential accuracy of 1 part in TO 7 . This is orders of magnitude more than can normally be achieved by direct measurement of angle or wavelength. The technique has been demonstrated by sensing small shifts in the plasmon angle caused by unsaturated vapours condensing on to a silver layer. A typical result is shown in Figure 7. In practice mechanical vibration, air currents and thermal drift has limited the sensitivity of the measurement to shifts of 10~ 4 degrees. However this is an improvement of two orders of magnitude over the the sensitivity previously obtained from this experiment. Also all of the noise generating factors can be reduced by careful device design. One possibility would be to integrate the acousto-optic device and sensor in a single crystal block. Figure 8 and 9 show embodiments used for detection of thin overlayers on metal surfaces. In these a beam of radiation 100 is reflected by a metal layer 102 on a surface of a prism 104 or grating 106. Alternatively, Figures 10 and 11 illustrate how an overlayer or waveguide of dielectric 108 can be added to the prism or grating of Figures 8 and 9. Small changes in the optical properties of the dielectric will then be measured with great accuracy. Figure 12 shows a modification to the embodiment of Figure 8 in which small changes δθ of the orientation of a mirror 110 in the path of the incident beam are measured. Referring to Figure 13, which shows apparatus for remote measurement using optical fibres, a beam of filtered radiation 120 from an acousto-optical tunable filter 122 is split into two component beams by mirrors 124,126. The component beams pass by way of optical fibres 128,130 to respective probe and reference gratings 132,134 and thence, by way of further fibres 136,138 to photodiodes 140,142 the output signals of which may be processed in the same manner as in the apparatus of Figure 4. Figure 14 shows an analogous apparatus used for differential spectroscopy using a sample cell 144 for a specimen under test and a reference cell 146 for a standard solution. The invention may be applied to many optical measurement techniques. The possibilities include applications which make use of the differential with respect to the angle of incidence (photon momentum) and those which use the differential with respect to the wavelength (photon energy). Specific differential applications include detection of thin overlayers (< 10~ 3 of a monolayer) on metal surfaces and detection of small changes of permittivity of an overlayer or waveguide and have applications in electrochemistry, gas sensing, biosensors, chromatography and pyroelectric infrared sensors. Wavelength differential applications include measurements which can be made remotely at the end of fibre optic waveguides, spectroscopy, detecting weak features on strong backgrounds and locking on to optical signals of varying wavelength, possibly for surveillance purposes.

Claims: CLAIMS: 1. Optical measuring apparatus including phase sensitive measuring means (8,9,14,16) characterised in that said apparatus includes acousto-optical means (3) to enhance the sensitivity of said phase-sensitive measuring means. 52. Optical measuring apparatus including phase sensitive measuring means (8,9,14,16) as claimed in claim 1 characterised in that said apparatus includes acousto-optic means adapted to modulate a parameter of a light beam (1), beam splitter means (5) to split said light beam into a reference beam (6) and a 10 probe beam (7), detector means (8,13) respectively to detect said reference beam and said probe beam after interrogation of an element under test, and measuring means (9,14,15) to measure the differential of an optical property of said element measured by said probe beam. 153. Optical measuring apparatus including phase sensitive measuring means (8,9,14,16) as claimed in claim 2 characterised in that said measuring means comprises divider means (9) fed from said detector means (8,13) adapted to provide a signal representative of the ratio of probe and reference signals to 20 amplifier means (14) fed from light beam modulator means (15). 4. Optical measuring apparatus including phase sensitive measuring means (8,9,14,16) as claimed in claim 3 characterised in that the output from said amplifier means (14) is fed back to a voltage controlled frequency source (66) which drives the 25 acousto-optic means (13). 5. Optical measuring apparatus including phase sensitive measuring means (8,9,14,16) as claimed in claim 1 characterised in that the acousto-optic means and detector means (8,13) are integrated in a single element. 306. Optical measuring apparatus including phase sensitive measuring means (8,9,14,16) as claimed in claim 1 characterised in that the acousto-optic means (3) is an acousto-optical deflector. 7. Optical measuring apparatus including phase sensitive 5 measuring means (8,9,14,16) as claimed in claim 6 characterised in that the probe beam is incident thereon at an angle selected for minimum reflectance. 8. Optical measuring apparatus including phase sensitive measuring means (8,9,14,16 ) as claimed in claim 6 characterised in that the acousto-optic deflector is driven at a frequency in the range 30-70MHz. 9. Optical measuring apparatus including phase sensitive measuring means (8,9,14,16) as claimed in claim 1 characterised in that the acousto-optic means is an acousto-optic tunable filter. 10. Optical measuring apparatus including phase sensitive measuring means (8,9,14,16) as claimed in claim 1 characterised in that the acousto-optic tunable filter is driven at a frequency in the range 80-160MHz.

Inventors: GASS PAUL ANTHONY, SAMBLES JOHN ROY, GASS, PAUL, ANTHONY, SAMBLES, JOHN, ROY
Applicants: BRITISH TECH GROUP, BRITISH TECHNOLOGY GROUP LIMITED
Application year: 1992
Main classification: G02F1
IPC codes: G01B11, G01J3, G01N21, G01N30