WO2009024344A1

WO2009024344A1 - Optical microprobe

Info

Publication number: WO2009024344A1
Application number: PCT/EP2008/006885
Authority: WO
Inventors: Peter Lehmann
Original assignee: Carl Mahr Holding Gmbh
Priority date: 2007-08-22
Filing date: 2008-08-21
Publication date: 2009-02-26
Also published as: DE102007039556B3

Abstract

An optical microprobe for common-path interferometers uses micro-optical components and utilizes a light exit surface (11) curved parallel to the optical spherical wave passing through it in order to generate a defined reference beam with an unambiguous phase angle of sufficient intensity and negligible dispersion difference relative to the measurement beam. As an alternative, the light exit surface can be situated so close to the object that as a result of lack of parallelism with respect to the wavefront passing through it, the disturbances brought about remain below a given limit value.

Description

Optical microprobe

The invention relates to an optical microprobe for focusing a light beam onto a measurement object.

Optical microprobes are components of optical sensor systems. They are brought to a test object and detect with high precision distance changes between the probe and the test object.

For the optical measurement of objects, DE 103 17 826 A1 discloses a method and a device for the interferometric measurement of distances, topographies or depth profiles. In this case, an interferometric arrangement is provided with an interferometer unit, which is connected via a fiber-optic device both to a light source and to an optical microprobe. Light is guided to a measuring object via the microprobe and received back by it. The light is then supplied to the interferometer to perform the desired measurement. For measurement is preferably kurzkohärentes Light used.

About the structure of the probe is not clear from this document.

The interferometric distance measurement is also known from DE 198 08 273 Al. An interferometer set up for this purpose is connected via a fiber optic device to optical probes which contain both a measurement light path and a reference light path. For measurement, short-coherent light is preferably used here as well. About the structure of the lens, i. an optical microprobe, this document gives little explanation.

In DE 100 57 539 Al an interferometric measuring device based on a fiber-based optical probe is described, in which the free, the measuring object facing end portion of the fiber polished provided with a diaphragm formed as a lens or prism treated against disturbing reflected light, bevelled, mirrored, anti-reflective or a combination of these measures.

No. 6,564,087 B1 describes optical probes in which lenses and prism elements for beam shaping and deflection are brought onto an optical fiber, so that the fiber leaves a focused light beam. The end surfaces from which the light emerges are either flat or convex. These probes are used in conjunction with an interferometric method, the so-called "Optical Coherence Tomography".

When using short-coherent light for metrological purposes, there are special requirements. If this light passes through a dispersion-related opti- through the medium, the different spectral components of the light move at different speeds. In an interferometer, the dispersion differences between the reference beam and the measuring beam must be extremely low, otherwise the interference ability of the light is lost.

Furthermore, it is desirable for the optical measurement if the measuring beam is focused with respect to the optical axis at as large an angle as possible, ie with the largest possible numerical aperture N _A. This should be as large as possible 0.1. As a result, high spatial resolutions and a high insensitivity to local inclinations of the surface of the measurement object can be achieved. At the same time, a large distance between the optical probe and the measurement object is often required. These requirements can only be met to a very limited extent with the optical microphones known today. Most of the prior art probes have a low numerical aperture (less than 0.1).

Therefore, a solution is sought in order to generate a defined reference beam with a clear phase position of sufficient intensity and negligible dispersion difference with respect to the measuring beam at larger numerical apertures within an optical microprobe.

In addition, such a microprobe should be manufactured and assembled with the least possible effort.

This object is achieved by the microprobe according to claim 1 as well as by the microprobe according to claim 2.

The microprobe according to the invention carry both the actual measuring beam, which is directed to the object to be measured and is reflected by this, as well as a reference beam, which is superimposed on the measuring beam and interferes with this. The system proves to be particularly robust because the measuring and the reference beam are guided largely together and the distances covered by either only the measuring beam or only the reference beam are kept short. As the measuring beam passes to and from the surface of the object, the reference beam passes to the light exit surface and is reflected back into the probe. As a result, the dispersion difference between the measuring beam and the reference beam is low and negligible or at least tolerable even with white light interferometry.

Furthermore, the microprobe according to the invention allows the achievement of large numerical apertures (e.g., ≥0.1). As a result, the optical microprobe is insensitive to local inclinations of the surface of the measurement object.

The optical microprobe according to the invention also allows easy adjustment of the intensity of the measuring beam in relation to or compared to the intensity of the reference beam. For this purpose, it suffices, for example, to design the reflection properties of the light exit surface appropriately.

The probe according to the invention can be manufactured and assembled in a simple manner with little effort, because of the exclusive use of simple optical elements which can be connected directly to one another. This results in a mechanically robust unit.

An essential idea of the invention is that the light exit surface of the optical probe is concave becomes. This is achieved, for example, by placing a glass rod with a concavely curved light exit surface between the GRIN lens (gradient index lens), GRIN fiber or any other condensing lens and the object surface. However, the light exit surface may alternatively also be arranged directly on the converging lens, the GRIN lens or the GRIN fiber and may also be concave. The optical fiber is used to illuminate the GRIN lens or converging lens. Between the optical fiber and the converging lens or GRIN lens, the light beam emerging from the optical fiber is fanned out. For this purpose, a distance between the optical fiber and the condenser lens or GRIN lens or alternatively a rod made of optically transparent and homgenous material, for example a glass rod, can be used. The optical axis of the optical fiber merges into the cylinder axis of the glass rod. The light emerging from the fiber core can initially spread divergently in the glass rod. The core cross section of the optical fiber thus assumes the function of an optical diaphragm through which light reflected at the measurement object can only pass when it is focused on the fiber core and the angle with respect to the optical axis within the acceptance angle caused by the numerical aperture of the fiber lies. Preferably, for example, the cylindrical GRIN lens is attached to the glass rod so that the cylinder axes of the glass rod and the GRIN lens coincide.

The GRIN lens first collimates the light beam and then focuses it to produce a convergent cone of light that leaves the GRIN lens. The light cone is guided via a concave light exit surface directly or alternatively via a cylindrical or conical glass rod in the direction of the measurement object. The concave light exit surface is then attached to the glass rod. At a defined distance to The light cone then exits the measuring object from the glass rod and converges as a convergent spherical wave to form a focal point which corresponds to the measuring point on the measuring object. For this purpose, it is preferred that the curvature of the concave light exit surface coincides with the curvature of the wavefront of the spherical wave at the location of the light exit. In other words, the individual light beams of the light cone converging towards the measurement object are perpendicular to the light exit surface and are therefore not broken. The wavefronts of the converging spherical wave thus extend parallel to the concave exit surface. This creates by (partial) reflection at this light exit surface, a defined reference beam whose light is coupled back into the fiber core. By contrast, the emerging portion of the light passes through the concave light exit surface without the individual light beams being deflected.

After the reflection on the measurement object, the light rays run back to the light exit surface and apart as a divergent spherical wave. They recede at least partially through the concave spherical segment surface formed by the light exit window, are guided through the optical system to the fiber core and coupled into the fiber.

The element arranged between the GRIN lens and the measurement object may be a glass rod, which may be, for example, cylindrical or frusto-conical. Also parts of the GRIN lens can be frusto-conical. This has the advantage that the probe geometry resembles the geometry of a conventional tactile roughness probe tip. Roughness probe tips also have a cone-shaped basic geometry so that they can glide over interfering edges on a measuring object during a stylus measurement. While it is advantageous if the light exit surface is a concave spherical segment surface whose center of curvature coincides with the focal point of the microprobe, variations of this arrangement are possible, which do not require a curved light exit surface. For this purpose, the optical element arranged between the GRIN lens and the measurement object, ie for example the glass cylinder or the glass cone, is dimensioned along the optical axis so that it extends directly to the measurement object whose surface is in the vicinity of the focal point , The light exit surface lies at a point at which the light cone is already focused so far that can be dispensed with the concave configuration of the light exit surface. This condition exists when the light path occupied by the element occupies at least 80%, better 90%, of the distance between the condenser lens and the focal point. The light dispersion between the light exit window and the object surface is so small that it hardly acts disturbing.

The arrangement of the optical element between the GRIN lens and the measurement object is not mandatory. Alternatively, the .Element can be dispensed with. The light then emerges directly from the GRIN lens, the end face of which is machined to create a concave spherical surface.

There are further modifications possible. For example, the GRIN lens can be replaced with a GRIN fiber of correspondingly smaller diameter. On the arranged between the glass fiber and GRIN fiber or GRIN lens element for Lichtauffächerung, which may have the form of a glass rod, for example, is omitted. The light exit surface can be provided directly on the GRIN fiber. In the between the converging lens or the GRIN lens and the measurement object to be arranged optical element, ie the glass rod or the glass cylinder and a deflection prism can be integrated, for example in the form of a light-reflecting (border) surface, so that the optical axis of the exiting light beam a defined non-zero angle with the optical axis of the optical fiber includes.

It should be noted that the optical elements, lenses and other components of the optical microprobe can be made of glass, transparent plastic or other suitable material. The optical microprobe according to the invention is particularly suitable for use in interferometric measuring devices or in confocal measuring devices. They are particularly suitable for operation with short-coherent light, e.g. white or colored light, in the visible or invisible wavelength range.

When used as a purely confocal probes, the light reflection which otherwise occurs at the light exit surface and which is used to generate a reference light beam during interferometric operation can be prevented by providing the light exit surface with a suitable coating, for example.

Further modifications, details and particularities will become apparent from the drawings, the description or claims. The description is limited to essential aspects of the invention and other conditions. The drawing discloses further details and can be used in addition. Show it:

1 shows a measuring device with optical microprobe in a schematic representation and Figures 2 to 8 modified embodiments of the optical microprobe of Figure 1 in each case in a schematic representation.

Figure 1 illustrates a measuring device 1, e.g. is designed as an interferometric measuring device. It has a measuring module 2 which contains one or more light sources, one or more interferometers and optionally an evaluation device. The light sources preferably produce broadband white or less broadband colored light. It is also possible to provide narrow-band light sources or light sources with a line spectrum or a single spectral line. To the measuring module 2, at least one optical fiber 3 is connected, which leads to an optical microprobe 4 and ultimately belongs to this. The optical microprobe 4 serves to focus a light beam 5 on a measurement object 6 or its surface.

Apart from at least the last end 7 of the optical fiber 3, the microprobe 4 includes an optical path 8, a converging lens 9 and an optical element 10, on which a concavely curved light exit surface 11 is formed. This is preferably a spherical surface or spherical segment surface whose center of curvature coincides with a focal point 12 of the microprobe 4. The end 7 of the optical fiber, the light path 8, the converging lens 9 and the element 10 have matching and adjoining optical axes 13, 14, 15, 16. The resulting common optical axis passes through the focal point 12.

The light path 8 is used for fanning the emerging from the optical fiber 3 light beam. The light path 8 may be formed by a suitable optical element such as a cylinder or rod 17 made of optically homogeneous transparent material such as glass or plastic, a truncated cone made of the same material, or the like. With a preferably flat surface 18 of the rod 17 connects to the optical fiber. 3 at. With an opposite also preferably flat surface 19 of the rod 17 connects to the converging lens 9 at.

The condenser lens 9 is preferably a GRIN lens 20 with e.g. Sectionally cylindrical and partially conical outer circumference. The GRIN lens 20 can abut directly against the surface 19 with a flat connection surface 21. The GRIN lens 20 serves as a converging lens for focusing the fanned out light beam 22 emitted by the rod 17.

The GRIN lens 20 has a preferably planar, the connecting surface 21 opposite another surface 23, to which the element 10 with a preferably turn flat surface, preferably immediately followed. For example, the element 10 may be a rod 24 made of optically transparent homogeneous material, which may be larger or, as shown, of lesser length, in which it becomes almost disc-shaped due to its shortness. On the rod 24, the light exit surface 11 is formed. The glass rod 24 allows the light bundle 25 focused by the GRIN lens 20 to pass uninterrupted to the focal point 12. However, the light exit surface 11 reflects a defined portion of the light in the described light path, i. the GRIN lens 20, the rod 17 and the optical fiber 3 back. For example, For example, the light exit surface 11 may be designed as a partially transmissive mirror. It can be used for this purpose, the natural reflection properties of the light exit surface. Alternatively, a light-reflecting coating, for example in the form of a metal vapor coating, can be provided which completely or partially covers the light exit surface 11.

The optical microprobe 4 described so far works tet as follows:

The microprobe 4 receives via the optical fiber 3 light, which out of the almost punctiform end face of the optical fiber 3 and into the rod 17 passes. It forms the cone-shaped light beam 22 with divergent marginal rays. The GRIN lens 20 refocuses the light beam with marginal rays converging toward the focal point 12. The light beam passes through the rod 24. At the light exit surface 22 it splits into measuring and reference beam. The serving as a measuring beam parts exits and runs convergent to the focal point 12. It is due to the (from the point of view of the focal point 12) concave shape little or not broken. The focal point 12 reflects parts of the light, which run away from the focal point 12 as a divergent spherical wave and strike the light exit surface 11, which is substantially parallel to the wavefront. This is thus the light entry surface. Here, the measuring light beam merges with the reference light beam reflected by the light exit surface 11 and together travel back through the GRIN lens and the rod 17 and the optical fiber 3 to the measuring module 2. There, the path differences between measuring light beam and reference light beam, if necessary, corrected and a resulting interference pattern can be evaluated.

The introduced measuring device 1 is suitable not only for interference optical measurement of the surface of the measuring object 6 but also for confocal microscopy or distance measurement. This can be done with almost non-reflective light exit surface 11. If the light exit surface 11 has certain reflection properties, for example in order to enable a mode switchover, this interferes little or not. In confocal microscopy, the measuring module 2 measures the intensity of the light reflected from the surface of the measuring object 6 and picked up by the microprobe 4. The strength is maximum when the surface of the measuring object 6 is located exactly in the focal point 12. A change in the distance between the micro-optical probe 4 and the measurement object 6 allows the determination of the brightness maximum and thus the height of the surface of the measurement object 6 in the intensity maximum.

At the so far presented microprobe 4 numerous modifications are possible. As FIG. 2 illustrates, the rod 24 or the element 10 can be dispensed with if the light exit surface 11 is attached directly to the GRIN lens 20. The spherically curved light exit surface 11 takes e.g. a part of the measuring object 6 facing surface 23 of the GRIN lens 20 a. Again, the concave curved light exit surface 11 is preferably curved with a constant radius with respect to the focal point 12. For the rest, the previous description applies on the basis of the already introduced reference numerals.

FIG. 3 illustrates a further modification. Instead of the bar 17, the light path 8 is realized by a free air gap, which is enclosed by a hollow cylinder 26, for example. At one end of the hollow cylinder 8, the end 7 of the optical fiber 3 can be held by means not further illustrated. At the other end of the hollow cylinder 26, the converging lens 9 is arranged. This can be designed as a GRIN lens or, as shown, as a glass body 27 with curved surfaces. Again, the light exit surface 11 may be formed by a surface of the converging lens 9. For the rest, the previous description applies on the basis of the same, already introduced reference numerals. Figure 4 illustrates a further modification of the invention. The peculiarity of the modification consists in the formation of the element 10. Its light exit surface 11 is flat here, ie formed without spherical curvature. For this, the rod 24 occupies at least 80%, preferably 90% of the total distance between the surface of the measuring object 6 and the surface 23 of the GRIN lens 20 facing the measuring object. Due to the small distance between the reference light beam generating light exit surface 11 of the focal point 12, the dispersion and phase differences between the reference light beam and the measuring light beam are largely negligible. In other words, the refraction of the light beam 25 occurring at the light exit surface 11 is hardly effective because of the small distance to the surface of the measurement object 6. In addition, the above description applies.

A further modification is shown in FIG. 5. This is based on the embodiment according to FIG. 4, with the difference that the rod 25 and the GRIN lens 20 have an at least partially conical lateral surface. Thus, the microprobe 4 is particularly slim at its the measuring object 6 end facing. In addition, it can easily slide over bumps, body edges and the like structures of the measurement object 6. With regard to the dimensioning of the length of the rod 24 applies, as already in connection with the embodiment of Figure 4 that its length L is greater than 90% of the focal length of the GRIN lens 20. the focal length is calculated as the distance of the GRIN lens from the surface of the DUT. The distance is the distance of the surface 23 to the focal point 12.

FIG. 6 illustrates another embodiment, again based on the embodiment of FIG. 1, However, the GRIN lens 20 and the rod 24 have a different from the cylindrical shape and the conical shape outer contour. They are rounded, for example. In addition, the light path 8 is again formed by the hollow cylinder 26. Otherwise, the previous description applies accordingly.

All of the above-described embodiments of the microprobe can be designed as sideways-looking probes by arranging a mirror 28 between the surface 23 of the converging lens 9 or GRIN lens 20 and the surface of the measuring object 6. This can deflect the optical axis of the exiting light beam 25 by a defined angle deviating from zero. This is shown in FIG. 7. FIG. 8 illustrates that the lower prism edge or prism end 29 of the rod or prism 24 can be dispensed with, so that the microprobe 4 can penetrate particularly deeply into blind holes and the like.

In addition to the above explanations, reference is made to the following alternative combinations of features:

1. An embodiment in which the optical fiber 3 is optically coupled to the first cylindrical rod 17 of optically homogeneous material, wherein the first cylindrical rod 17, the GRIN lens 20 is optically coupled. To the GRIN lens, a second rod or prism 24 is coupled from optically homogeneous material, wherein the axes of these optical elements approximately coincide with each other and facing away from the GRIN lens 20 end face 11 of the second cylindrical rod 24 is concave and as a light exit surface, if necessary serves as a light entry surface and as a mirror for generating a reference light beam. 2. An embodiment in which the microprobe 4 is an optical fiber 3, an optically to the optical fiber

3 coupled first cylindrical rod 17 of optically homogeneous material and a optically coupled to the first cylindrical rod GRIN lens 20 and a optically coupled to the GRIN lens 20 truncated cone of optically homogeneous material, wherein the axes of these optical elements approximately coincide with each other and the GRIN lens facing away from the end surface of the truncated cone is concave and serves as a light exit surface 11, optionally as a light entrance surface, and as a surface for generating a reference light beam.

3. An embodiment in which the optical microprobe

4 consists of an optical fiber 3, a GRIN lens 20 optically coupled to the optical fiber or a GRIN fiber and a truncated cone of optically homogeneous material optically coupled to the GRIN lens or GRIN fiber, the axes of these optical elements approximately coinciding with each other and the end face of the truncated cone facing away from the GRIN lens 20 or the GRIN fiber is concave and serves as light exit surface 11 or light entry surface and light-reflecting surface for generating a reference light beam.

4. An embodiment in which to the microprobe 4, an optical fiber 7, a optically coupled to the optical fiber cylindrical rod 17 of optically homogeneous material and a visually coupled to the cylindrical rod 17 GRIN lens 20, wherein the cylindrical rod 17 facing away Face surface of the GRIN lens 20 is concave at least near the optical axis 15 and as the light exit surface eleventh serves.

5. An embodiment in which belong to the microprobe 4, an optical fiber 3 and a GRIN lens 20 optically coupled to the optical fiber or a GRIN fiber, wherein the optical fiber 3 facing away from the end face of the GRIN lens or the GRIN fiber at least is concave near the optical axis 15 and serves as a light exit surface 11.

6. An embodiment in which the microprobe 4 has an optical fiber 3 and a light path 8 for fanning out the light beam emitted by the optical fiber 3, which hits a GRIN lens or GRIN fiber, wherein between the GRIN lens or GRIN lens Fiber is an optical element is provided, the measuring object 6 facing surface is formed as a plane surface and serves as a light exit surface 11, wherein the distance between the light exit surface 11 and the focal point 12 at most 20%, preferably at most 10% of the distance of the GRIN lens from the object surface is.

7. An embodiment in which the microprobe 4, an optical fiber 3, a optically homogeneous material coupled to the optical fiber 3 cylindrical rod 17, a visually coupled to the cylindrical rod 17 GRIN lens 20 and an optically coupled to the GRIN lens Element 10 comprises, wherein the element 10 is formed as a prism and deflects the optical axis of the system by a defined angle, wherein further the GRIN lens 20 facing away from the surface of the prism is concave and serves as a light exit surface 11. 8. In the microprobe according to any one of the numbers 2, 3 or 6, the truncated cone of optically homogeneous material may be integrated into a prism, which deflects the optical axis of the system by a defined angle, wherein the facing away from the GRIN lens face of the combined prisms Truncated cone is concave and serves as a light exit surface 11.

9. The diameter of the optical microprobe 4 may preferably be less than 5 mm.

10. All optical elements can be mechanically connected directly to each other.

The optical microprobe according to the invention uses micro-optical components and in particular uses a light exit surface 11 which is curved parallel to the optical spherical wave passing through it or which is so close to the measurement object 6, that disturbances caused by lack of parallelism between the light exit surface 11 and the wavefront passing through it stay below a given limit.

reference numeral

1 measuring device

2 measuring module

3 optical fiber

4 microprobe

5 light bundles

6 measuring object

7 fiber end

8 light path

9 condenser lens

10 element

11 light exit surface

12 focus

13-16 optical axes

17 staff

18, 19 area

20 GRIN lens

21 Connection surface 2 Light bundles 3 Surface 4 Bar / prism 5 Light bundles 6 Hollow cylinder 7 Glass body 8 Mirror 9 Prism edge

Claims

Claims:

1, optical microprobe (4) for focusing a light beam (5) on a measurement object (6), in particular for performing interferometric measurements, consisting at least of:

an optical fiber (3),

one to the optical fiber (3) coupled light path

(8) for fanning out a light beam emerging from the optical fiber (3),

a converging lens connected to the light path (8)

(9) for focusing the light beam (22) emerging from the optical fiber (3) onto a focal point (12),

with an optical element (10) connected to the condenser lens (9), which has a light exit surface (11) facing the focal point (12) and a) occupies at least 90% of the distance between the condenser lens (9) and the focal point (12) and / or b) in which the light exit surface (11) is concavely curved.

2. Optical microprobe (4) for focusing a light beam ^¬ (5) on a measurement object (6), in particular for performing interferometric measurements, consisting at least of:

an optical fiber (3),

one to the optical fiber (3) connected GRIN lens (20) or GRIN fiber for focusing the out the optical fiber (3) emerging light beam (22) on a focal point (12),

wherein the GRIN lens (20) has a concave curved light exit surface (11).

3. An optical microprobe according to claim 2, characterized in that between the optical fiber (3) and the GRIN lens (20) an optical path (8) for fanning out of the optical fiber (3) emerging light beam (22) is provided.

4. An optical microprobe according to claim 1 or 3, characterized in that the light path (8) by a rod (17) is formed of optically homogeneous material.

5. An optical microprobe according to claim 4, characterized in that the rod (17) is cylindrical.

6. An optical microprobe according to claim 4, characterized in that the rod (17) directly adjoins the optical fiber (3).

7. An optical microprobe according to claim 4, characterized in that the rod (17) directly adjoins the convergent lens (9).

8. An optical microprobe according to claim 1, characterized in that the convergent lens (9) is a GRIN lens (20).

9. An optical microprobe according to claim 1, characterized in that the optical element (10) is a rod (24) made of optically homogeneous material.

10. An optical microprobe according to claim 9, characterized in that the rod (24) is cylindrical.

11. An optical microprobe according to claim 9, characterized in that the rod (24) is frusto-conical.

12. An optical microprobe according to claim 9, characterized in that the light exit surface (11) is an end face of the rod (24).

13. An optical microprobe according to claim 9, characterized in that the light exit surface (11) is a side surface of the rod (24).

14. An optical microprobe according to claim 9, characterized in that the rod (24) has a light-reflecting surface (28), which redirects the light from the converging lens (9) incoming light to the light exit surface (11).

15. An optical microprobe according to claim 1 or 2, characterized in that the light exit surface (11) is partially mirrored.

16. An optical microprobe according to claim 1 or 2, characterized in that the light exit surface (11) is a spherical surface whose center of curvature coincides with the focal point (12).

17. Optical microprobe according to one of the preceding claims, characterized in that all optical Axes (13, 14, 15, 16) of all elements agree with each other.

18. An optical microprobe according to claim 1, characterized in that the optical fiber (3) is connected to an interferometer.

19. An optical microprobe according to claim 1, characterized in that the optical fiber (3) is connected to a confocal measuring device.