GB2355310A

GB2355310A - White light interferometer

Info

Publication number: GB2355310A
Application number: GB9922947A
Authority: GB
Inventors: Klaus Ehrmann
Original assignee: PREC LENS Manufacturing TECHNOLOGY Inc; OCULAR SCIENCES Ltd; OCULAR SCIENCES UK Ltd
Current assignee: PREC LENS Manufacturing TECHNOLOGY Inc; OCULAR SCIENCES Ltd; OCULAR SCIENCES UK Ltd
Priority date: 1999-09-28
Filing date: 1999-09-28
Publication date: 2001-04-18
Also published as: GB9922947D0

Abstract

An opto-electronic instrument to measure distances and/or the thickness of transparent objects (9) comprises a broad spectrum, low coherence light source (1), a photo-detector (13), optical components to guide the light to the measured object (9) and to guide the reflected light to the photo-detector (13), beam-splitters to split the light into measurement and reference beams and to recombine both beams for interferometric signal generation, and means (12, eg movable mirror, to vary the optical path length of the reference beam. One of the optical components, preferably the objective lens (8), generates large chromatic aberration. Use of chromatic aberration and white light illumination provides a large working range with a large capture range for surface tilt, while interferometric signal detection provides high sensitivity for surfaces or optical interfaces of low reflectivity. An interferometer is also claimed and may comprise a multimode optic fibre 2,4 and fibre coupler 3.

Description

2355310 Improvements in and relating to White-Light Interferometers The

invention relates to the field of surface measurement and surface quality quantification. In particular, it relates to measurements involving the detection of surfaces with low reflectivity and/or measurement of the thickness of thin transparent objects or layers, such as contact lenses.

Surface measurement devices have applications in many areas of industrial quality control and research metrology. Such devices can utilise optical senso. rs, which are typically based on laser interferometry or auto-focus sensors. More recently, instruments utilising white-light interferometry (also known as optical low coherence reflectometry) have been demonstrated. In that technique, broad-band light of low coherence is used in an interferometer arrangement. A beam of such light is split in two, a reference beam and a measurement beam, and one beam is reflected from a moving mirror whilst the other beam is reflected from the surface being measured. The two beams are then combined to generate an interferometric signal. Due to the low coherence, the interferometric signal only appears when the optical path length of the reference and the measurement beam are substantially equal.

The optical path length of the measurement beam is determined by the distance between the surface and the sensor head. The optical path length of the reference beam is determined by the position of the moveable mirror. By sweeping the mirror through the working range of the sensor head, the interferometric signal is generated when the mirror position makes the reference and measurement paths substantially equal. Recording the mirror position when the interference signal appears thus provides information about the distance between the sensor head and the surface being measured.

According to the invention, there is provided a measurement device for measuring the position of a surface, comprising: a source of polychromatic light; means to split a beam of light from the source into a probe beam and a reference beam; an optical system for delivering the probe beam to the surface and for collecting light reflected from the surface, the optical system exhibiting significant chromatic aberration; means for combining inter f erometri cal ly the collected light with the reference beam in order to provide a signal indicative of the position of the surface; wherein the chromatic aberration of the optical system is such that different spectral components of the probe beam are focused at different positions along the optical path of the probe beam and the interferometric signal can be obtained from the spectral component which is focused on the surface.

Preferably, the chromatic aberration would be sufficient to give the sensor a working range of more than Imm. For higher sensitivity and/or larger capture angles of tilted surfaces, a smaller working range may be more appropriate and the amount of chromatic aberration may be reduced. Preferably, the light would be broad-band light and of low coherence.

Typically, light reflected from the surface will only be effectively coupled back into the 6ptical system by which it is delivered if the light is focused at the surface and the tilt of the surface does not exceed a capture angle. Also, a small focal-spot size is needed for good lateral resolution in surface measurements. With conventional optics, a large depth of focus, a high numerical aperture (NA) and a small focal spot size cannot be achieved simultaneously. In conventional optical sensors, resolution of depth is typically achieved either by using collimated light or by scanning (by moving focusing optics) the focus of a light beam along a normal to the surface. The position of the surface can then be deduced from the interferometric signal or from the position of the focusing optics. Use of an optical system with a large chromatic aberration extends the depth of focus by focusing different wavelengths at different distances. Thus a high-NA objective lens, which will have a large capture angle, can be used whilst small spot sizes are maintained. Only the spectral component which is in focus on the measured surface will be reflected and captured as a measurement beam in the optical system and hence interfere with the broad band reference beam.

The optical system may include an optical fibre. The means to split the beam may be an end face of the optical fibre. Alternatively, the means to split the beam may be a surface of a lens, which may be used as an objective or collimating lens. Alternatively, the 'means to split the beam may be a polarising or non- polarising beam-splitter. Preferably, reflections from two or more surfaces can be used to measure the thickness of an object. Preferably, the surface(s) from which light is/are reflected is/are the surface(s) of a contact lens.

For thin transparent objects or transparent layers of different refractive indices, light will be reflected at each interface and corresponding interference signals will be detected, allowing the absolute position of the interfaces, and hence the thickness of each layer, to be determined. Known white-light interferometers have been used to detect surfaces having a reflectivity of less than 10-5.

The chromatic aberration may be attributable to one or more lenses. Preferably, the chromatically aberrative lens or lenses has/have a high numerical aperture.

Preferably the measurement device further comprises a sensor head containing at least part of the optical system. Preferably, the sensor head is immersible in liquids. Preferably, the sensor head is easily movable. Preferably, a signal can be generated for surfaces which are not normal to the probe beam. More preferably,' a signal can be generated for surfaces which are tilted by up to 30 degrees f rom the normal to the probe beam.

As explained above, an advantage of using a whitelight interferometric system having chromatic aberration is that it has a large depth of focus, enabling a long useable working range, and a large NA objective lens can be used to measure surfaces which are tilted significantly relative to the sensor axis. Being able to make such measurements with a white-light interferometric system opens up a wide range of applications. 20 The interferometric system may include a photodiode. Alternatively, the interferometric system may include a spectrometer. Usually the interferometric. system will include means for modifying the optical path length of the reference beam and means for recording changes in optical path length. In that case the optical path length is changed and the particular reflected spectral component (or a narrow band of such components) is used to produce an interferometric signal. The optical path length at which the interferometric signal occurs may then be deduced from the recorded changes. It is, however, also possible to use other arrangements; for example the optical path length of the signal beam may be obtained from the spectral position at which the interferometric signal is detected.

Also according to the invention-there is provided a method of measuring the position of a surface, comprising: providing a beam of polychromatic" light; splitting the beam into a probe beam and a reference beam; delivering the probe beam, through an optical system which exhibits significant chromatic aberration, to a surface to be measured; collecting in the optical system light reflected from the surface; combining interferometrically the collected light with the reference beam in order to provide a signal indicative of the position of the surface; wherein the chromatic aberration is such that different spectral components of the probe beam are focused at different positions along the optical path of the probe beam and the interferometric signal is obtained from the I spectral component which is focused on the surface. Such a method advantageously utilises a device as described above. Also according to the invention there is provided an interferometer comprising: 5 means for guiding light, the means including a surf ace from which the guided light is partially reflected to form a reference beam and partially transmitted to form a probe beam, the probe beam continuing to propagate to an object from which it is at least partially reflected to form a signal beam which is coupled back into the guiding means and returns past said surface; and means for combining the signal beam and the reference beam to give a signal indicative of the position of the object; wherein the reference beam travels along the optical path taken by the signal beam after it has returned past said surface. By that means, the effects on the accuracy of the interferometer resulting from changes in the length of said optical path are substantially eliminated because they affect the signal and the reference beam equally.

Use of one and the same fibre for the reference beam and the measurement beam eliminates any measurement errors due to changes in the length of the f ibre caused by, f or example, thermal expansions, mechanical bending, mechanical stress or creeping; thus, some of the stability problems usually associated with Michelson interferometers can be eliminated. Changes in fibre length affect both beams equally.

As will be clear from earlier passages, the interferometer may include a photodiode or a spectrometer.

Usually the interferometer will include means for modifying the optical path length of the reference beam and means for recording changes in optical path length. Also, the means for combining the signal beam and the reference beam to give a signal indicative of the position of the object may include means for recording the spectral position at which the interferometric signal is detected.

The means for guiding may be an optical fibre. Said surface may then be an end face of the fibre. Alternatively, the means for guiding may be a sequence of lenses. Said surface may then be a surface of one of the lenses.

The reflection from the end face could be enhanced, for example by applying a dielectric coating. Alternatives to utilising the reflection from the fibre end face to reflect the reference beam back into the system include using external polarising or non-polarising beam-splitters in the measuring head.

The measurement device defined above may incorporate the interferometer defined above.

An embodiment of the invention will now be described in greater detail, by way of example only, with reference to the accompanying drawings, of which:

Fig. 1 shows a low coherence, broad spectrum interferometer constructed according to the invention, Fig. 2 shows the sensor head of the interferometer of Fig. 1, and Fig. 3 is a schematic view of a soft contact lens profilometer incorporating the invention.

The measurement principle is based on white light interferometry (see Figures 1 and 2). A low coherent, broad spectrum light source 1 is coupled into a multimode optical fibre 2. Passing through a fibre coupler 3, the light is guided in a multimode fibre 4 to the sensor head 5. The sensor head consists of endpiece 6 of the optical fibre, a collimating lens 7 and an objective lens 8. A small percentage of the light is reflected back in the f ibre at the f ibre endf ace 6. This portion of the light acts as the reference beam for the interferometer. Light exiting the fibre 4 is collimated and focused onto the measured surface 9. The light reflected from the surface 9 is coupled back into the f ibre 4 and acts as the measurement beam. Both the reference beam and the measurement beam travel back to the coupler 3, where they are separated from the incoming light and fed into a second coupler 10, which splits both beams in half. One half is reflected from a fixed mirror 11, while the other half is reflected from a mirror 12 moving backwards and forwards, co-linearly with the beam. Both of the reflected beams travel back into the second coupler 10, where they recombine and exit at the port with the photo-detector 13 connected to it. An interference signal will be detected if the reference and the measurement beam have travelled exactly the same optical path length'. This occurs when the optical distance between the measured surface 9 and the endface 6 of the fibre inside the sensor head 5 equals the difference in distance between the second coupler 10 and the fixed mirror 11 and the second coupler 10 and the moving mirror 12. The position of the moving mirror 12 thus directly correlates with the distance of the measured surface 9 to the sensor head S. By monitoring the motion of the moving mirror 12 and recording its position when an interference signal is detected by the photo-detector 13, an absolute distance measurement reading is obtained.

For high precision applications, the typical working range (that is, the depth of focus) can be as large as 1 mm, whilst a spot size of less than 10 m is maintained.

It is possible to achieve resolutions of up to 10nm with a working range of 0. 1mm and a spot size of 31im. At the other extreme, working ranges of up to Smm can also be achieved with spot sizes increasing to 60m and a resolution of 2m. The depth resolution of the instrument is mainly determined by the device sensing the position of mirror 12.

The signal detected by photodiode 13 is processed in amplifier 20, filter 21 and peak-signal detector 22. The processed signal is combined in unit 23 with a signal giving information about the position of the moving mirror 12, to give the position of measured surface 9.

The effect of using an objective lens 8 having large chromatic aberration is shown in Fig. 2. Light leaving fibre 4 is spread into its spectral components, represented by 14, 15 and 16, by passage through the aberrative lens 8. The spectral components are focused at different planes 17, 18 and 19 respectively. The spectral component of light which is focused at the surface 9 is efficiently reflected back into fibre 4; the components which are not focused at the surface are not efficiently coupled.

An example of an application of the device shown in Figs. 1 and 2 is in a profilometer for soft contact lenses (Fig. 3). The thickness profiles of soft contact lenses can vary between 301im and 600pm. Sensor head 5 is mounted on rotary stage 24. It is used to measure the surface of soft contact lens 25, which is mounted on a lens holder and centring device 26. Device 26 is in turn mounted in a container 27, filled with saline, which is mounted on a second rotary stage 28.

The sensor head 5 moves over the front surface of the lens 25 in a semicircular motion whilst the position of the front and back surfaces of the lens are detected. Data are recorded in spherical co-ordinates at a predetermined lateral resolution. Use of the rotary stage 28 allows the scanning of profiles in different meridians. From all the data points, the complete three-dimensional shape of the lens can be reconstructed. The refractive index of the lens material needs to be known in order to convert the 10 optical thickness to physical thickness.

Claims

Claims

1. A measurement device for measuring the position of a surface, comprising: a source of polychromatic light; means to split a beam of light from the source into a probe beam and a reference beam; an optical system for deliverinq the probe beam to the surface and for collecting light reflected from the surface, the optical system exhibiting significant chromatic aberration; means for combining interferometrically the collected light with the reference beam in order to provide a signal indicative of the position of the surface; wherein the chromatic aberration of the optical system is such that different spectral components of the probe beam are focused at different positions along the optical path of the probe beam and the interferometric signal can be obtained from the spectral component which is focused on the surface. 2. A measurement device as claimed in claim 1, in which the optical system includes an optical fibre. 3. A measurement device as claimed in claim 2, in which the means to split the beam is an end face of the optical fibre.

4. A measurement device as claimed in claims 1 or 2, in which the means to split the beam is a surface of a lens. S. A measurement device as claimed in claim 4, in which the lens is used as an objective or collimating lens.

6. A measurement device as claimed in claim 1 or 2, in which the means to split the beam is a polarising or nonpolarising beam splitter. 7. A measurement device as claimed in any preceding claim, in which reflections from two- or more surfaces can be used to measure the thickness of an object. 8. A measurement device as claimed in any preceding claim, in which a signal can be generated for surfaces which are not normal to the probe beam. 9. A measurement device as claimed in claim 8, in which a signal can be generated which is indicative of the thickness of a transparent object with non-parallel surfaces. 10. A measurement device as claimed in claim 7, in which the surface (s) from which light is/are reflected is/are the surfaces of a contact lens. 11. A measurement device as claimed in any preceding claim, in which the chromatic aberration is attributable to one or more lenses. 12. A measurement device as claimed in claim 11, in which the chromatically aberrative lens or lenses has/have a high numerical aperture.

- 13. A measurement device as claimed in any preceding claim, which further comprises a sensor head containing at least part of the optical system. 14. A measurement device as claimed in claim 13, in which 5 the sensor head is immersible in liquids. 15. A measurement device as claimed in claim 13 or 14, in which the sensor head is easily movable. 16. A measurement device as claimed in any preceding claim, in which the interferometric system includes a photodiode. 17. A measurement device as claimed 'in any of claims 1 to 15, in which the interferometric system includes a spectrometer. 18. A measurement device as claimed in claim 17, in which the optical path length of the signal beam is obtained from the spectral position at which the interferometric signal is detected. 19. A measurement device as claimed in any preceding claim, in which the interferometric system includes means for modifying the optical path length of the reference beam. 20. A measurement device as claimed in any preceding claim, in which the interferometric system includes means for recording changes in optical path length.

21. A method of measuring the position of a surface, comprising:

16 - providing a beam of polychromatic light; splitting the beam into a probe beam and a reference beam; delivering the probe beam, through an optical system which exhibits significant chromatic aberration, to a surface to be measured; collecting in the optical system light reflected from the surface; combining interferometrically the collected light with the reference beam in order to provide a signal indicative of the position of the surface; wherein the chromatic aberration is such that different spectral components of the probe beam are focused at different positions along the optical path of the probe beam and the interferometric signal is obtained from the spectral component which is focused on the surface.

22. A method according to claim 21, utilising a device as claimed in any of claims 1 to 20.

23. An interferometer comprising:

means for guiding light, the means including a surface from which the guided light is partially reflected to form a reference beam and partially transmitted to form a probe beam, the probe beam continuing to propagate to an object from which it is at least partially reflected to form a signal beam which is coupled back into the guiding means and returns past said surface; and means for combining the signal beam and the reference beam to give a signal indicative of the position of the obj ect; wherein the reference beam remains confined to the optical path taken by the signal beam after it has returned past said surface. 24. An interferometer as claimed in claim 23, in which the means for combining the signal beam and the reference beam to give a signal indicative of the position of the object includes means for recording the spectral position at which the interferometric signal is detected. 25. An interferometer as claimed in claim 23 or 24, including means for modifying the optical path length of the reference beam.

26. An interferometer as claimed in any of claims 23 to claim 25, including means for recording changes in optical path length. 27. An interferometer as claimed in any of claims 23 to 26, in which the means for guiding is an optical fibre.

28. An interferometer as claimed in claim 27, in which said surface is an end face of the fibre. 29. An interferometer as claimed in any of claims 23 to 26, in which the means for guiding is a sequence of lenses.

30. An interferometer as claimed in claim 29, in which said surface is a surface of one of the lenses.

31. A measurement device as herein described with reference to the accompanying drawings. 32. An interferometer as herein described with reference to the accompanying drawings.