GB2332533A

GB2332533A - Optical system comprising beam splitter and Offner relay

Info

Publication number: GB2332533A
Application number: GB9726968A
Authority: GB
Inventors: David Steven Smith
Original assignee: IMAGE AUTOMATION Ltd
Current assignee: IMAGE AUTOMATION Ltd
Priority date: 1997-12-19
Filing date: 1997-12-19
Publication date: 1999-06-23
Also published as: GB2332533A9; GB9726968D0

Abstract

An optical system comprising a beam-splitting element 503 and an Offner Relay 505, the beam-splitting element for receiving an optical beam 501 input to the optical system and for providing a plurality of output beams to the Offner Relay such that a corresponding plurality of separate output images 506, 508 is formed. The optical system may include a plurality of beam-directing elements 504, 507 to direct the output beams from the beam-splitting element into the Offner Relay. The beam-splitting element may be a spatial filter and the system can include means 502 to focus the input beam to form an image at the position of the spatial filter. The system finds application in, inter alia, vibration measuring devices.

Description

2332533 Optical System

The present invention relates to optical systems and in particular to systems for separating component parts of an optical beam and to systems for mixing separate optical beams. In this specification the term "optical" is used for convenience and this and corresponding terms (such as "light") are intended to encompass wavelengths of radiation other than the visible, including, ultraviolet and infrared radiation.

There are some optical systems wherein there is a requirement to separate and re image component parts of an optical beam. The separation can be made by spectral, amplitude or spatial techniques. Where a valid separation can be made on a spatial basis, high efficiency can be achieved and where there is a need to preserve maximum signals in both paths, the spatial method is attractive. However, a disadvantage of using a spatial filter is that it must be located at an intermediate focal plane and there is consequently a need to relay both the component beams to their respective sources or targets.

For example, the requirement for an optical system to separate and reimage component parts of an optical beam arises in the field of vibration measurement. One forin of apparatus for measuring the vibration of a mechanical component, such as a car door or washing machine enclosure, detects the effect the vibration has on a light beam reflected from the component. In such an apparatus a light beam, normally a

2 laser beam, is focused to a spot on the component at the point at which the component's vibration is to be measured. A beam reflected from this point is shifted in frequency according to the frequency and amplitude of the vibration of the component at that point. If the reflected beam is caused to interfere with a suitable reference beam the vibration at the illuminated spot can be measured under a range of applied excitation conditions.

In such a Doppler laser vibration measurement system, it is convenient to be able to image the area surrounding the focused laser spot concurrently with vibration measurement for alignment or monitoring purposes. For this purpose an optical beam-splitter, separating a beam carrying an image of the component from the laser beam, can advantageously be used. Efficiency is high because an image of the laser on the testpiece will be very small, so that a spatial filter can be used to reflect the laser -beam out of the image of the test area with little loss of background data. This method contrasts with the other techniques in that amplitude beam- splitting necessarily diverts a part of both beams into the two directions, and in that for spectral beam splitters it is difficult to achieve high efficiency spectral splitting, particularly if the beam in question is convergent or divergent. Moreover, spectral beam- splitters tend to have the disadvantage of providing a visual or CC1)-(charge coupled device) video image which is coloured, or at least tinted.

A simple conventional arrangement for spatial separation of component parts of an optical image is shown in Figure 1. In this case, the object is a distant field 101

3 which is imaged onto beam-splitter 103 by an objective lens 102. The reflected portion is re-imaged by relay 104 onto receiver 105, while the transmitted portion is re-imaged onto receiver 107 by relay 106. In this case, where a laser beam is combined with an extended field, it is possible to use relatively simple optics for the laser point projection, but a higher level of correction is normally required for the extended field if reasonable quality image formation is required.

Refractive optics are the most commonly used solution, but it is possible to produce an optical relay using reflective components. For example, an all-mirror catoptric:

relay system is shown in Figure 2. This is fully described by Offner in US patent No. 3,748,015, July 24, 1973 to which reference can be made. It can be used to form a good quality, well-corrected image, free from chromatic aberration. The Offner relay belongs to the class of concentric imaging systems, since the curved mirror surfaces have centres of curvature at approximately or exactly the same point.

The operation of the relay is shown in Figure 2A. Light from object 201 travels to a large mirror 202, is reflected onto a small mirror 203, and back to the large mirror 202. A more detailed my diagram is shown in Figure 213 which shows a concave spherical mirror 202 and a convex spherical mirror 203. The two mirrors have centres of curvature nominally at the same point 223. The radius of curvature of the convex mirror 203 is approximately half of the radius of curvature of the concave mirror 202. The object and image planes for this system may be any plane 224 that includes the point 223. Rays 225 and 226 are shown diverging from an object point 4 201 in plane 224, being reflected from the concave mirror 202, then from the convex mirror 203, again from the concave mirror 202, and finally converging to focus on point 228 in plane 224. Figure 2B shows an alternative configuration of the relay, including a fold mirror 205.

The concentric optical relay design, illustrated by Figure 2C, includes no surfaces that are not nominally spherical or flat, and has a minimal number of optical surfaces. It can be extremely well corrected in the simple form shown; correction can be further improved by adding more near-concentric elements as indicated in Figure 21). This diagram shows the Offner relay improved by adding a refmcting, near- concentric meniscus element 230.

A real, spatially-inverted image is formed on the same plane as the object, at 204.

The Offner Relay is normally used at unity magnification but other magnifications are possible, albeit potentially at the cost of increased aberrations. Variants of this basic concentric design are possible. For example, the relay can be arranged with five reflections, three from the concave mirror 202, two from the convex mirror 203, when for unity magnification the radius of the convex mirror is two- thirds that of the concave mirror. Furthermore, the curvatures, tilts and separations of the mirrors may be allowed to change and the single concave mirror may be split into two mirrors (assigned to the two sequential reflections of concave surfaces) and these two mirrors may be allowed to change independently. Optimisation of a optical system may mean that in practice the mirrors may not be exactly concentric.

is Fundamentally an Offner Relay is a unity magnification catoptric: image- forming relay comprising a concave spherical mirror and a convex spherical mirror, the mirrors being supported with their centres of curvature substantially coincident, an object at an object location having an image which is a real image at a second location. The convex miffor is positioned about the centre of curvature to reflect to the concave mirror light from the object location initially reflected to the convex mirror from the concave miffor. Thereby light from the object location will be reflected at least twice at the concave mirror and at least once at the convex miffor before being focused at the second location. The power of the convex miffor multiplied by the number of reflections thereat is approximately equal to the power of the concave mirror multiplied by the number of reflections thereat but may be sufficiently less to produce at the second location a substantially stigmatic image of an object in the first location. The focused image at the second location can include third order tangential field curvature of one sense and higher order tangential field curvature of opposite sense.

Figure 3 shows a simple beam-splitting system using an Offner Relay, where a distant object, 301 is imaged onto the beam-splitter 303 by the objective lens 302. The transmitted (axial) component, is relayed onto the receiver 305 by the relay 304. The reflected component is relayed by the mirror system 306 and imaged onto the receiver 308 via the fold mirror 307.

Since the mirror relay is capable of diffraction-limited performance on axis, a second mirror system can also be used to re-image the laser beam.

6 A drawback with the systems of Figures 1 and 3 is the use of a substantial number of components, and a space-consuming layout.

A problem addressed by the present invention is that of providing a optical system which can separate two component parts of a light beam, for example a laser spot from an image, which is compact, reliable and efficient and which uses relatively few optical components.

According to a first aspect of the present invention there is therefore provided an optical system comprising a beam-splitting element and an Offner Relay, the beam splitting element receiving an optical beam input to the optical system and providing a plurality of output beams to the Offner Relay such that a corresponding plurality of separate output images is formed.

The beam-splitting element is preferably a spatial filter but could also be a spectral filter or a holographic optical element. A plurality of beam-directing elements can be included to direct the output beams from the beam-splitting element into the Offner Relay.

By using the Offner Relay to "process" all of the plurality of output beams the need for separate imaging systems is removed. An Offner Relay is particularly suitable for this application since it is designed for off-axis use where it introduces relatively little optical aberration since its component mirrors, being of opposite curvature, introduce 7 aberrations which tend to cancel (there are two reflections from the large mirror to one from the smaller mirror of twice the power). Furthermore, any necessary field curvature or other corrections can be made relatively easily.

According to a second aspect of the invention there is provided a method of separating component parts of an optical beam comprising the steps of. (a) providing an optical input beam to a beam-splitting element, and, (b) providing a plurality of output beams from the beam-splitting element to an Offner Relay. The beam-splitting element is preferably a partiallyreflecting spatial filter and the method may further comprise 10 focusing the input beam onto the spatial filter and using beam directing elements to direct the output beam from the spatial filter to the Offner Relay.

The beams in the system may propagate in the reverse directions to those described in the above described aspects of the invention. In this case, the system accepts a plurality of input beams and provides a single output beam and the "beam- splitter" functions as a beam-combiner.

In order to promote a fuller understanding of the above and other aspects of the invention, some embodiments will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a conventional arrangement for separation of component parts of a optical beam; 8 Figures 2A, 211, 2C and 21) show an Offner Relay, an Offner Relay with a fold mirror, a detailed ray diagram of an Offner Relay, and an improved Offner Relay; Figure 3 shows the system of Figure 1 with an Offner Relay incorporated; Figure 4 shows the disposition of light rays in an Offner Relay; and, Figure 5 shows an optical beam-splitting system embodying the present invention; and Figure 6 shows an interferometer for use with the optical system of Figure 5.

The Offner Relay provides for an input and corresponding output equally disposed about the axis of the system. As shown in figure 4, object 401 is re-imaged to point 404, and point 403 is re-imaged to point 402. It can be seen that the Offner Relay can be used for more than one relaying activity, for example, top to bottom and left to right.

The described embodiment uses the relay in this mode, combined with fold mirrors such that the intermediate image points are combined at a common third point (503), where the spatial combiner/filter plate is located. An objective lens, 502 forms an image of a distant scene and point (not shown), focusing beam 501 onto spatial filter, 503. (When the system is operating 1n reverse", as a beam-combiner, element 502 acts as an output beam collimator). The spatial filter can be either a reflective plate 9 with a hole to pass the test beam, or a transmitting plate with a reflective point to transmit the extended image and reflect the test beam. The former is the preferred option because it is desirable to ensure that the laser beam suffers minimum disruption and scatter and transmission through a hole will cause less loss and scatter than reflecting off a surface. Such a filter can be incorporated into a solid, opticallytransmissive cube to reduce aberrations although generally any cuboid, that is any rectangular parallelepiped shape will suffice. If tilted plate is used, the tilting will cause substantial aberrations (although keeping it thin reduces the effect); a block which is square to the beam introduces little aberration. The transmitted portion of the optical beam will be reflected into relay system 505 by fold mirror 504, and an image will be formed at point 506. Similarly, the reflected portion will be reflected into the relay 505 by fold mirror 507, and will be imaged to point 508. Thus, by making a dual use of the mirror relay system an advantageous means of mixing and/or separating the two beams is provided. It will be noted that the optical axes typically will intersect the fold mirrors in a common plane, and that this plane will be located between the object/image plane and the curved mirrors. To maintain optical path lengths, the mirror fold plane will be displaced towards the relay mirrors by the same optical path length as that between the fold mirrors and the beamsplitter. If all the reflections are at 901, the distance between the optical axis on the fold mirrors and the centre of the relay, will be half the distance between the input and output axes of the relay. (Note that if a solid block beanisplitter is used, the optical path will differ from the physical path). If the system is used with a transmissive plate, there will be slight focus shift in the transmitted component; this can be accommodated at the final focus point.

The system can be used in a vibration measuring system as already discussed, wherein a laser beam is focused onto a tested component and the reflected beam is used to determine the component's vibration concurrently with imaging the component. In this case, light 501 comes from the tested component and is separated into an image of the component at, say, 506, for presentation to a CCD camera, and a laser spot at, say, 508, carrying information about the vibration of the component. When so used, a second beam-splitter can be provided so that a laser beam passes through the relay out to the component under test and back through the relay after reflection from the component. Typically the CCD image, which is concurrently relayed, depends upon external illumination. Thus the reflected light from the component forms a first input beam to the relay system and two separate output beams are provided, a first output beam carrying an image of the tested component to the CCD camera, and a second output beam carrying the laser beam to the detector. The laser beam to be projected onto the component can advantageously be introduced (using a bearnsplitter or other means) into the second output beam as a counterpropagating second input beam to the relay, traversing the same path as the second output beam but in an opposite direction.

This laser beam forms a third output beam from the relay, issuing in a counterpropagating direction to the first input beam and illuminating a spot on the tested component. Variants are also possible. For example, if further splitting were required, additional bearnsplitting relays could be concatenated.

11 Figure 6, which shows a simplified interferometric detector which can be used with the optical system of Figure 5, clarifies the foregoing description. Figure 6 shows beamsplitter means 608, to separate a probe laser beam input 616 to the Offner Relay 600 from a reflected laser beam 618 output from the Offner Relay. Figure 6, for simplicity, does not show the optics of Figure 5 in detail, and in particular does not show optics associated with providing an image for a CCD camera.

Light 616 from laser 604 passes via fold mirror 606, beamsplitter 608 and objective lens 614 into Offner Relay 600. It illuminates a spot on the component under test 602 and the reflected light from the spot passes back through the Offner Relay to beamsplitter 608 again, and thence to detector 612. The detector detects interference between the reflected beam 618 and a reference beam 620 deriv ed from input beam 616.

The optical system of Figure 5 can be used for mixing or for separating optical beams, since it is symmetrical with respect to the direction of the light beams, and could even be used for simultaneous mixing and separation. Whilst for more conventional beam-splitting systems, simpler arrangements are possible, the mixing relay system could also be used with amplitude or spectral beam-splitters, particularly if there were a need to use a physically small beam-splitter component. Furthermore, in the above described vibration measurement application a suitable spatial filter could be used to provide a plurality of laser beams. For example, a 45' beam- splitter with a line of reflective points or transmissive apertures could be used to form a line 12 pattern to simultaneously measure vibration amplitude at a plurality of points on the component, whilst concurrently imaging it.

A benefit of this invention is that one set of components is used to reimage the two optical beam channels in a compact package, and that a means of combining the two channels is conveniently incorporated. Ile bearn-splitter can provide efficient bearn segregation over a wide range of wavelengths. Furthermore, the imaging capability of the relay system is such that diffraction-limited performance can be achieved for a single point on axis (for example, a laser beam can be focused to a 5 um spot) and adequate extended field performance can be realised - for example over the area of a %" or 1/3" CCD camera chip.

Many other effective alternatives will occur to those skilled in the art and it should be understood that the present invention is not limited to the illustrated embodiments.

is 13

Claims

1. An optical system comprising a beam-splitting element (503), and, an Offner Relay (505), the beam-splitting element for receiving an optical bewn (501) input to the optical system and for providing a plurality of output beams to the Offner Relay such that a corresponding plurality of separate output images (506, 508) is formed.

2. An optical system as claimed in claim 1 further comprising a plurality of beam-directing elements (504, 507) to direct the output beams from the beam splitting element into the Offner Relay.

3. An optical system as claimed in claim 1 or claim 2 wherein the beamsplitting element is a spatial filter and wherein the system includes means (502) to focus the input beam to form an image at the position of the spatial filter.

4. An optical system as claimed in claim 3 wherein the focusing means includes an objective lens and wherein the spatial filter includes a partially reflecting surface.

14

5. An optical system as claimed in claim 4 wherein the beam-directing elements are mirrors and wherein the spatial filter is cuboid.

6. An optical system as claimed in any one of claims 1 to 5 wherein the number of output beams is two.

7. An optical system as claimed in any preceding claim additionally comprising means to introduce into a selected output beam a further input beam substantially 10 counterpropagating to the selected output beam.

8. An optical system comprising a beam-combining element (503), and, is an Offner Relay (505), the beam-combining element for receiving a plurality of optical input beams to the system from the Offner Relay, each input beam corresponding to a separate object location (506, 508), and for providing an optical beam (501) output from the system.

9. An optical system as claimed in claim 8 further comprising a plurality of is beam-directing elements (504, 507) to direct the input beams from the Offner Relay into the beam - combining element.

10. An optical system as claimed in claim 8 or claim 9 wherein the beam- s combining element is a spatial filter and wherein the system includes means (502) to collimate the output beam from the spatial filter.

11. An optical system as claimed in claim 10 wherein the collimating means includes an objective lens and wherein the spatial filter includes a partially reflecting surface.

12. An optical system as claimed in claim 11 wherein the beam-directing elements are mirrors and wherein the spatial filter is cuboid.

13. An optical system as claimed in any one of claims 8 to 12 wherein the number of input beams is two.

14. An optical system as claimed in any one of claims 8 to 13 additionally comprising means to introduce into the output beam a further input beam, substantially counterpropagating to the output beam.

15. A vibration measuring device incorporating an optical system as claimed in any preceding claim.

16 16. A method of separating component parts of an optical beam comprising the steps of:

(a) providing an optical input beam (501) to a beam-splitting element (503), and, (b) providing a plurality of output beams from the beam-splitting element to an Offner Relay (505).

17. A method as claimed in claim 10 wherein the beam-splitting element is a 10 partially-reflecting spatial filter and finther comprising the steps of:

(c) focusing the input beam onto the spatial filter, and, is (d) using beam-directing elements (504, 507) to direct the output beams from the spatial filter to the Offner Relay.

18. A method of combining component parts of an optical beam comprising the steps of: (a) providing a plurality of optical beams (506, 508) input to the system 20 from an Offner Relay (505) to a beam-combining element (503), and (b) providing an optical output beam from the beam-combining element.

17

19. A method as claimed in claim 18 wherein the beam-combining element is a partially-reflecting spatial filter and further comprising the steps of:

(c) using bearn-directing elements (504, 507) to direct the input beams 5 from the Offner Relay to the spatial filter, and, (d) collimating the output beam from the spatial filter.

20. An optical system substantially as hereinbefore described with reference to Figure 5 of the drawings.