GB2309777A - Narrow angle scattering speckle interferometer - Google Patents

Abstract

An interferometer for measuring sample size and displacement includes an He-Ne laser coupled into a probe part by an optical fibre feed, a beam splitter 14 which splits the beam between a target and a narrow angle scatterer 16. The beam splitter 14 also combines the reflected signals from the target and the scatterer 16 and feeds them to signal conditioning magnifying optics comprising a converging lens 20 and magnifying lens 22. The resultant magnified signal is received by a quadrature detector 26 comprising four optical detectors which obtains four detector signals which are subsequently combined to eliminate noise and dc offsets. Both movement of apertures and detectors can be used to vary the phase of the received signal. The narrow angle scatterer 16 is mounted on a piezo-electric transducer 18 so as to allow its movement and alignment.

Description

INTERFER OMETER The present invention relates to an interferometer, in particular one which uses a speckle pattern which can be used to measure, for example, sample size or sample movement.

Laser based interferometers for measuring displacement and the like have been known for a long time. Early work in this field was published in Instrumentation Practice, December 1965, in an article entitled "The Laser Applied to Automatic Scale Measurement", by W R C Rowley and V W Stanley; and also in IEEE Transactions on Instrumentation and Measurement, Volume IM-15, Number 4, pages 146-149 December 1966, in an article entitled "Some Aspects of Fringe Counting in Laser Interferometers" by W R C Rowley.

These interferometers make use of a laser beam which is split between a reference mirror and a movable mirror on an object to be measured. The reflected beams from the two mirrors are then combined to produce an interference fringe which exhibits a sinusoidal variation with movement of the movable mirror.

Processing circuitry, which in the prior art systems is now well established, determines from the interference pattern and from its changes the position and movement of the movable mirror and thereby of the object being monitored.

Further details of the principles behind these prior art systems can be obtained from the cited articles.

There is a constant need to improve these interferometers so as to measure smaller distances. These systems have developed by providing more and more accurate signal processing and more and more accurate components within the apparatus. However, these systems still have limited resolution and have difficulty in retaining their accuracy.

The present invention seeks to provide an improved interferometer.

According to an aspect of the present invention, there is provided an interferometer including a laser source; a beam splitter; a narrow angle scatterer located to received a first part of the split beam, a second part of the split beam being directable to a target to be measured; a beam combiner for combining the beams from the target and from the narrow angle scatterer; detector means for detecting the combined beam; and processing means for processing the signal from the detector means.

Conventional interferometers of this type have always used wide angle scatterers. However, it has been found that replacing the wide angle scatterer with a narrow band scatterer can significantly improve efficiency. For example, in an experimental prototype, it was found that an interferometer with a wide angle scatterer had an efficiency of 50W, while the same interferometer with a narrow angle scatterer could have an efficiency of 95 and above.

The narrow angle scatterer may have a scatter surface produced from ground glass diffusers by etching. Any suitable other material could be used for the surface, for example a layer of granules adhered to a supporting substrate.

The scatter surface preferably have peak to valley variations of the order of 1/4 of a fringe, that is 1/4 of the sinusoidal variation produced during movement of the target.

There is preferably provided an optical stage between the beam combiner and the detector means and operative to amplify the combined beam before detection by the detector means. The optical stage preferably amplifies the beam by around ten times or more. Such amplification can minimise the effect of noise and of target differences.

The optical stage preferably includes a first converging lens optically in series with a magnifying lens.

The scatterer is preferably mounted on a piezo electric transducer which is movable by an electrical signal to align the scatter surface as required. The transducer is typically coupled to the processing means to provide closed loop control and calibration. In this system, as a result of the type of scatterer and the combination of components, it is typically only necessary to calibrate the apparatus on assembly. The apparatus keeps its accuracy thereafter. Moreover, the system is substantially immune to electrical interference, a feature which is of much importance in some applications to which this device is suited.

The system does not require highly accurate components to provide a resolution substantially greater than existing systems. In fact, experiments have shown that the preferred embodiment can measure distances of around 1/6000 of the thickness of a human hair with mirrors and lenses which are less accurately produced than those of known systems with significantly worse resolution.

The preferred system is also able to measure moving objects, not just fixed distances.

The detector means preferably includes a plurality of detectors each disposed to receive at least part of the combined beam.

Each detector is preferably arranged to detect the whole of the combined beam. In the preferred embodiment, there are provided four detectors, which may be arranged in a square array.

A plurality of detectors provides the ability to compare signals therefrom and to reduce or substantially eliminate noise, offsets and other signal disturbances.

Preferably, one or more of the detectors may be movable.

The detector means is preferably movable in a direction substantially orthogonal to the combined beam. This can help to keep the image position relative to the detecting means constant.

In an embodiment, there is provided aperture means optically in front of the detecting means. The aperture means preferably includes an aperture for each detector of the detecting means.

One or more of the apertures may be movable or otherwise alterable. The movable or changeable aperture can change the interference pattern, which in turn can keep the image position constant and can minimise the effect of different targets.

According to another aspect of the present invention, there is provided an interferometer including a laser source; a beam splitter; a scatterer located to received a first part of the split beam, a second part of the split beam being directable to a target to be measured; a beam combiner for combining the beams from the target and from the narrow angle scatterer; detector means including four detectors operative to provide phase quadrature signals of the combined beam; and processing means for processing the signals from the detectors.

An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a perspective view from above of an example of absolute speckle height probe in boxed form which includes the preferred embodiment of interferometer; Figure 2A is a schematic diagram of a prior art wide angle scatterer; Figures 2B and 2C are schematic diagrams of two embodiments of narrow angle scatterers; Figure 3 is a block diagram of the principal components of an embodiment of interferometer; Figure 4 is a block diagram of the principal components of another embodiment of interferometer; Figure 5 is a block diagram of the principal components of another embodiment of interferometer for use in detecting movement of a target;; Figure 6 is a block diagram of the principal components of an embodiment of interferometer with a movable detector; and Figure 7 is a block diagram of the principal components of another embodiment of interferometer.

Referring to Figure 1, the example of interferometer probe shown is useful for measuring very small distances of low mass targets and the like which can be close to the device itself. In particular, the target can be from 10 mm to 150 mm from the interferometer.

The example shown provides a measuring laser beam which is split from a principal laser beam produced by an He-Ne laser from a separate laser source as shown. The measuring laser beam has a diameter of 1 mm and can provide a dynamic range for the device of + 250 clam.

Within the probe, the measuring laser beam reflected back to the device off the target is combined with a reference beam, also split from the principal laser beam, and then detected by one or more detectors, which in turn provide a detector signal. This signal is passed through an output port of the device to signal conditioning electronics and then to digital signal processing stages.

The interferometer probe of Figure 1 includes a narrow angle scatterer which provides the reference beam which is combined with the beam reflected off the target. This gives the probe sufficiently greater resolution and accuracy than prior art interferometers, as discussed herein.

Referring to Figures 2A to 2C, the differences between wide and narrow angle scatterers are shown. In Figure 2A, a prior art wide angle scatterer has a rough speckle pattern provided by a dust ball, paint or the like. A laser beam reflected off the scatterer has a wide angle, in the example shown almost 900, which produces a random interference phenomenon.

It has been determined that this type of scatterer does not give adequate efficiency and that a more efficient scatterer, although not contemplated previously, can considerably improve efficiency and resolution.

Figures 2B and 2C show two examples of suitable narrow angle scatterers. In Figure 2B the scatterer shown is produced from ground glass diffusers by etching and may be supported on a substrate of soft aluminium or foil. The scatter surface can be adhered to the substrate. In Figure 2C the scatterer shown is formed from an array of micro particles adhered to a supporting substrate, which may be similar or the same as the substrate of the example of Figure 2B.

Figure 3 shows the principal components of an example of interferometer probe. The probe includes a beam input in the form of an optical fibre feed which in use is connected to, in this example, an He-Ne laser. A first mirror 12 reflects the incoming beam to a beam splitter 14 designed to reflect 50% of the incident rays thereon. The reflected beam part is directed in use to the target to be measured, while the transmitted beam part is directed to a narrow angle scatterer 16 of a type similar to those shown in Figures 2B and 2C. The size of the speckles or particles of the scatterer 16 are dependent on the diameter of the incident beam. This can be determined readily by the skilled person from the teachings herein.

The scatterer 16 is mounted on a piezo-electric transducer 18 which is coupled electrically to the processing and control circuitry of the interferometer (not shown) and which can thus be adjusted under a control voltage to align the scatterer 16 relative to the incident beam part.

The beam splitter 14 is also operative to combine the reflected beam parts from the scatterer 16 and from the target, to form a combined beam which is fed to signal conditioning optics. These optics include a first converging lens 20 which has in the preferred embodiment a speckled surface of small speckle size, and a magnifying lens 22 which is operative to magnify the incident beam to 10 times the size of the combined beam emerging from the beam splitter 14.

An optical conduit 24 includes a light receiving end 26 which is located close to the magnifying lens 22. The light receiving end 26 includes a support block 28 in which are mounted four optical fibres 30 (only two of which are visible in Figure 3) which are arranged in quadrature, that is at the four corners of a square. The optical fibres 30 are optically finished so as to provide optical ports for the receipt of the incoming beam. The optical fibres 30 preferably has diameters of S00m at their ends proximate the magnifying lens 22.

The support block is movable in directions orthogonal to the incident beam so as to keep the image position constant relative to the optical fibres 30, thereby to minimise the effect of different target positions.

At the other end of the optical conduit there are provided for optical detectors D1 to D4 (only D1 and D2 being visible in the Figure). These detectors D1 to D4 provide respective signals which are passed to signal processing circuitry, of conventional form, for generating an indication of distance or displacement of the object.

In use, the reflected signal from the scatterer 16 and from the target are combined to produce an interference fringe pattern.

The pattern varies sinusoidally with change in distance of the target relative to the device. The interference pattern is then amplified by the amplifying lens 22 and picked up by the four optical fibres 30. The light passing through the optical fibres 30 is then detected by the respective detectors D1 to D4, which in turn produce electrical signals representative of their respective incident light beam.

Each signal produced by the detectors D1 to D4 will include a dc offset and some noise. These two effects can be substantially eliminated by obtaining an aggregate of the combined signals.

For example, the signals could be combined by subtraction as follows: D1 - D2, D2 - D3, D3 - D4, D2 - D4, D1 - D3 and D4 - D1.

The combined signals can then be averaged to produce a resultant signal in which offsets and noise are substantially eliminated.

The resultant signal is then processed to determine from the detected fringe pattern the distance of the target from the device, in a manner analogous to the prior art methods. It is also readily possible to measure movement of the target, by detecting changes in the fringe pattern over a measuring period.

There is no need to adjust the device for measuring displacement, since it is only necessary to detect the changing interference pattern.

Calibration of the device is simple. A dummy target of predetermined type is placed at a predetermined distance from the device and the laser beam switched on to produce a combined beam pattern at the detectors D1 to D4. The piezo-electric transducer is then moved by a control voltage so as to move the scatterer 16 until the fringe pattern matches a predetermined base pattern.

In practice, the individual fringe pattern produced by the scatterer 16 is matched to the fringe pattern produced by the dummy target.

Once aligned, the scatterer remains aligned , in particular since it is immune to electrical interference.

It has been found that this probe, and the associated processing circuitry, is able to measure distances of less than 1/6000 of the thickness of a human hair, many times less than the minimum distances measurable with prior art devices.

An alternative embodiment is shown in Figure 4, in which the various components are the same apart from the signal detection part after the magnifying lens 22. In this embodiment, a quadrant photocell 40 is provided in place of the optical conduit 24 and photodetectors D1 to D4. However, as with the four detectors D1 to D4 of Figure 1, the quadrant photocell 40 provides four optical detectors which each give individual signals for subsequent processing.

In front of the quadrature photocell 40 there is provided a apertured disc 42 which has four apertures (only two of which are visible in the Figure). The disc 42 provides four individual beam images to the quadrant photocell 42. In the preferred embodiment, one or more of the apertures is movable or otherwise alterable, which can adjust the phase of the resultant processed interference pattern (by up to 90 ), for use in calibration. The adjustable aperture(s) can be adjusted by a screw adjustment or by a piezo-electric transducer similar to that which supports the scatterer 16.

In an alternative embodiment, the apertured disc 42 is omitted and one or more of the detectors or optical fibres 30 is made movable.

Another embodiment of probe is shown in Figure 5. In this embodiment, the scatterer 16 is located alongside the target. In this case, the beam splitter is in the form of a selective mirror which reflects the whole of the incoming laser beam towards a Wollaston prism 50, which in turn splits the beam into two prongs of a fork. A converging lens 54 focuses the split beam parts to the scatterer 16 and the target respectively.

The reflected beam parts return through the lens 54 and Wollaston prism 50, through the beam splitter 52 to the signal conditioning optics 20, 22. The beam reception is through an optical fibre conduit 24 as in the embodiment of Figure 1, although in this case one or more of the optical fibre ends is movable as described above.

Figure 6 shows another embodiment in which one of the detectors (in this case detector D1) can be moved. For this purpose, a second piezo-electric transducer PZT2 is coupled to a lever which is coupled at its free end to the detector D1. An arrangement of mirrors allows the various detectors D1 to D4 (only D1 and C-' being visible in this Figure) to be placed at any suitat - location and orientation in the device. This may be important for packaging purposes.

Figure 7 shows an embodiment of interferometer probe which is cf known form apart from the use of quadrature detectors 70, which are of the same form as the embodiment of Figure 4. Similarly, a quadrature optical fibre conduit of the type shown in Figures 3 or 5 may be used.

The above-described interferometers can be used to calibrate measuring apparatus for emerging technologies, such as scanning probe microscopes (STMs and AFMs) and conventional stylus instruments for surface texture measurement. Similarly, they can be used for checking the performance of nanomedhanism slideways; for the calibration of force, pressure and impact testing sensors; for direct measurement of strain, force and pressure; and in the assessment of vibration in machine tools and other systems. It is also believed that they could be used for medical purposes, such as measuring skin movement and other diagnostic tests.

Claims (17)

Claims:

1. An interferometer including a laser source; a beam splitter; a narrow angle scatterer located to received a first part of the split beam, a second part of the split beam being directable to a target to be measured; a beam combiner for combining the beams from the target and from the narrow angle scatterer; detector means for detecting the combined beam; and processing means for processing the signal from the detector means.

2. An interferometer according to claim 1, wherein the narrow angle scatterer has a scatter surface produced from ground glass diffusers by etching.

3. An interferometer according to claim 1 or 2, wherein the narrow angle scatterer has a scatter surface with peak to valley variations of the order of 1/4 of a fringe.

4. An interferometer according to claim 1, 2 or 3, including an optical stage between the beam combiner and the detector means and operative to amplify the combined beam before detection by the detector means.

5. An interferometer according to claim 4, wherein the optical stage amplifies the beam by around ten times or more.

6. An interferometer according to claim 4 or 5, wherein the optical stage includes a first converging lens optically in series with a magnifying lens.

7. An interferometer according to any preceding claim, wherein the scatterer is mounted on a piezo-electric transducer which is movable by an electrical signal to align the scatter surface.

8. An interferometer according to any preceding claim, wherein the detector means preferably includes a plurality of detectors each disposed to receive at least part of the combined beam.

9. An interferometer according to claim 8, wherein each detector is arranged to detect the whole of the combined beam.

10. An interferometer according to claim 8 or 9, wherein there are provided four detectors.

11. An interferometer according to claim 8, 9 or 10, wherein one or more of the detectors is movable.

12. An interferometer according to any preceding claim, including aperture means optically in front of the detecting means.

13. An interferometer according to claim 12, wherein one or more of the apertures is movable or otherwise alterable.

14. An interferometer including a laser source; a beam splitter; a scatterer located to received a first part of the split beam, a second part of the split beam being directable to a target to be measured; a beam combiner for combining the beams from the target and from the narrow angle scatterer; detector means including four detectors operative to provide phase quadrature signals of the combined beam; and processing means for processing the signals from the detectors.

15. An interferometer probe including input coupling means able to be coupled to a laser source; a beam splitter; a narrow angle scatterer located to received a first part of the split beam a second part of the split beam being directable to a target to be measured; a beam combiner for combining the beams from the target and from the narrow angle scatterer; detector means for detecting the combined beam; and output coupling means able to be coupled to processing means for processing the signal from the detector means.

16. An interferometer substantially as hereinbefore described with reference to Figure 3, 4, 5, 6 or 7.

17. An interferometer probe substantially as hereinbefore described with reference to and as illustrated in Figure 3, 4, 5, 6 or 7.