GB2360841A - Mounting block for optical devices - Google Patents

Abstract

A mounting block (<B>301</B>) is configured to mount a first optical device such as lens (<B>205</B>) and a second optical device such as a laser (<B>602</B>). The block includes mounting surfaces for supporting the optical devices and a beam splitter (<B>211</B>) supported within the block to effect a transfer of light at a substantially perpendicular angle between the devices. At least one of these surfaces is off-set from a ninety degree rectangular angle thereby ensuring that a light beam (<B>704</B>) being directed away from the block does not interfere with the light beam (<B>703</B>) entering the block.

Description

2360841 Mounting Block For Optical Devices The present invention relates

to a mounting block for mounting a first optical device and a second optical device, wherein said block includes mounting surfaces for supporting said optical devices and reflecting means supported within said block to effect a transfer of light at a substantially perpendicular angle between said devices.

Mounting blocks for optical devices producing light reflection of substantially ninety degrees are known. The blocks include mounting surfaces defining a block of substantially cuboid configuration; having mounting surfaces that are mutually normal to each other. The mounting surfaces have apertures thereby allowing light to be transmitted between the (first) optical device and co-operating optical devices, the latter being restrained by extension arms.

A problem with the known configurations is that undesirable interference effects may take place if light is reflected back along its originating path. This problem becomes particularly acute when generating coherent light by means of a laser, given that the returning light may be out of phase with light in the lasing cavity, resulting in unreliable lasing operations.

Problems of this type may be overcome in production equipment, such as the Type 2523 Torsional Vibration Meter produced by Briiei & Kjaer of Naerum, Denmark. In devices of this type, problems associated with optical alignment may be avoided by engineering components in a bespoke way so as to optimise the direction and orientation of the optical paths. It can also be achieved by including specific additional components to mitigate the problems, such as polarising filters. However, a problem with 2 this approach is that the addition of filters and similar devices adds to the overall cost of the instrument, while engineering components specific to a particular application results in the production of an instrument that does not have the same flexibility as similar instruments constructed from reusable components.

Reusable components for applications of this type are well known, such as the devices provided Linos Photonics GmbH of Mnigsallee 23, D 37081 G6ttingen, Germany. Cuboid blocks for assembling optical devices such as mixers are available from Linos Photonics but difficulties arise due to reflecting light. Consequently, sophisticated measures are required to achieve filtering, thereby increasing overall cost and assembly complexities.

According to a first aspect of the present invention, there is provided a mounting block for mounting a first optical device, in which said first optical device produces a light reflection of substantially ninety degrees, wherein said block includes mounting surfaces for connecting means arranged to connect extension means to said mounting surfaces, in which said extension means are configured to support co-operating optical devices to allow light to be transmitted between said first optical device and said co-operating devices via apertures in said mounting surfaces; and wherein at least one of said surfaces is off-set, from a normal cuboid angle.

The invention will now be described by way of example only, with reference to the accompanying drawings, of which:

Figure 1 shows a basic laser speckle angular displacement sensor of a V-form configuration; Figure 2 shows a device similar to that of Figure 1 configured as a mixer; Figure 3 shows and offset mounting block embodying the present 3 invention; Figure 4 shows the mounting block of Figure 3 with associated components; Figure 5 shows the assembly of Figure 4 with an additional beam splitter; Figure 6 shows a partially assembled instrument; Figure 7 shows a schematic representation of the instrument shown in Figure 6; Figure 8 shows an operational environment for the instrument shown in Figure 6; and Figure 9 shows an alternative embodiment of the block identified in Figure 3.

The preferred embodiment arose from development work directed at producing an improved laser-speckle angular-velocity sensor, of the type described by 13jarke Rose, Husain Iman, Steen G. Hanson, Harold T. Yura and ReM S. Hansen in Volume 37 of Applied Optics, published 10 April 1998. As is well known in the art, it is possible to produce coherent light by means of a laser and the cost of such devices has reduced significantly in recent years due to the availability of semi-conductor lasing elements. This coherent light may be reflected by smooth reflecting surfaces, such as mirrors used in optical devices for directing the light along particular orientations. However, when coherent light is reflected from a less than perfectly smooth surface, which will be termed a rough surface herein, a human observer viewing the reflected light experiences an effect known as speckle. Furthermore, head movement while this speckle effect is being observed causes the pattern to move and it is known that the direction of speckle movement will vary between short-sighted people and long-sighted 4 people. This effect occurs because the pattern is actually being formed by interference on the retina of the observer themselves; thus, the totality of the effect requires the presence of the observer who now forms part of the overall system.

A property of speckle is that the speckle pattern moves when movement occurs to the reflecting surface. Consequently, by analysing the movement of the reflected pattern it is possible to deduce certain movements of the moving object. This effect is related to the movement of the surface texture itself. The effect can distinguish angular motions from variations occurring due to movement towards and away from a detector, usually referred to as movement in the Z direction. Movement in the Z direction produces effects of different type, therefore a detector can be designed to make use of speckle while being resistant to Doppler effects.

A particular application of this effect involves observations concerning torsional oscillations of rotating shafts. Thus, pattern movements created by the rotation will also include information derived from fine movements occurring at the rotating surface. It is known for rotational oscillations to be considered using acoustic techniques but it is also known that these techniques are limited, particularly in situations where there is a significant degree of background noise. Laser techniques are not affected by noise of this type.

As the target moves, the speckle pattern becomes smeared, therefore it is necessary to produce a sequence of snapshots, whereafter it is possible for the information to be processed with respect to the time at which the snapshot was taken. With semi-conductor lasers, it is possible to apply current pulses thereby pulsing the laser but this in turn results in frequency variations known as chirp which in turn destroys the coherence of the emitted light; this being an essential property for the interference patterns to be produced.

A basic laser-speckle angular-velocity sensor is shown in Figure 1, directed at a rotating shaft 101, rotating in a direction indicated by arrow 102. The sensor includes a semi-conductor laser source 103, a detector 104 and a lens 105. A beam of coherent light 106 having a diameter of typically one millimetre is generated by laser 103 is directed at a target position 107 on the rotating shaft. Light scatters from the target position and a proportion of the scattered light is collected through lens 105. The lens is effectively focused at infinity, resulting in an out of focus spot being directed onto detector 104. With the spot being out of focus at the detector, it contains speckle that moves in a way dependent upon the angular displacement of the target (that is in the direction of arrow 102) and not in relation to translatory movement of the target in the X or Z directions, identified by arrows 108 and 110.

The path of light from the laser emitter 103 to the CCD detector 104 effectively defines a V-shape and consequently this arrangement is generally referred to as a V-form movement detector.

The scattering of light waves from the target spot 107 may be considered in terms of a Fourier transform by placing the lens 105 at a distance from the array camera 104 equal to its focal length f. The angle of the beam entering the lens 105 corresponds to the position off-set at its focus and with a spot diameter of one millimetre (effectively one thousand and five hundred wavelengths of red light wide) light travelling in one direction is collected. The collection of light in this way equates to an integration across the spot perpendicular to a line 109 along which the spot is being viewed. If this line is changed, another perpendicular summation is 6 produced; introducing the Cos (0) multiplying component of the Fourier transform. Consequently, integrations of individual directions of parallel light are effectively points on the Fourier integral and an image formed at the focal length f is the Fourier transform of the original target image. Other transforms could be considered, possibly including linear processing after defocusing.

If the target 101 moves by an angle 0, a scattered pattern moves through an angle 20. In addition, translation of the target surface may also occur resulting in incoherent changes to the total integral value. High processing speed is necessary in order to process the scattered light from a part of the target in successive samples covering a common area of the target, otherwise there is no correlation and therefore no way of inferring the rotation.

A target spot should not move by more than a fraction of the spot width per sample period and movements of a quarter of the target width, or even a tenth of the target width, produce better results. As a scattered field of light in front of the spot is swung around, an image is created by the Fourier transform on the detector 104 that slides across the pixels of the detector. If a slide occurs by a distance greater than one pixel, observability of the correlation is rapidly lost and frequencies are blurred out.

Consequently, the time sample rate (that is to say the flash period) must be short compared to time between samples. These constraints may be summarised as follows:

time between flashes - constrained by potential loss of correlation flash/time - constrained by the requirement to resist pixel blur.

With the increased availability of component devices required for 7 meeting these and other requirements, including the availability of relatively high powered computing systems, is has been realised that the overall cost of powerful instruments may be reduced substantially which could in turn lead to a widening of the field of application. However, in order to achieve this, it has been appreciated that cost effective solutions must also be identified in relation to the inclusion of associated components, or measures must be taken to reduce or eliminate the presence of these associated components.

A problem with the V-form arrangement shown in Figure 1 is that changes of target distance affect the image position at the detector. Also a relatively high-powered laser 103 is required in order for a signal received at detector array 104 to have an acceptable signal to noise ratio.

As an alternative to the V-form configuration shown in Figure 1, it is possible to construct an instrument with the illumination beam collinear with the observation light path. This can be achieved by using a partially reflecting prism to introduce the illuminating beam into the same but reversed path of the detector light. Prisms suitable for this purpose are often made in a cuboid form and these are liable to reflect a substantial amount of light back along the path from the laser source.

A laser speckle angular displacement sensor configured to exploit the properties of a mixer is illustrated in Figure 2. A rotating shaft 201 is substantially similar to shaft 101 of Figure 1 and is arranged to rotate in direction 202. The instrument has a laser 203 similar to laser 103 but requiring a lower power output for similar operating conditions. In this way, laser 203 is more easily implemented as a solid state device.

A front lens 205 has a focal length such that the lens focuses a target point 206 down the instrument to produce a nominal spot image at 8 207. At the nominal spot image there is a second lens 208 which conforms the image down onto the camera so that the camera array 209 may be considered as a second Fourier plane, the first Fourier plane following after lens 205 at position 210.

In addition, the instrument includes a beam splitter 211 and a reference arm 212. Light from the laser 203 is directed towards the beam splitter as indicated by arrow 213. At the beam splitter, part of the light generated by the laser is directed towards the target spot 206 as indicated by arrow 214. The remainder of the light generated by the laser 203 is directed towards the reference arm 212, as indicated by arrow 215. The reference arm is configured to create a beam, indicated by arrow 216 that appears to come through the nominal lens in such a way as to imitate the point source 206 at the target. Thus, the reference arm 212 is set up such as to focus the returned light in the centre of the nominal spot image 207.

The beam splitter 211 is a mixer for the returned light and the target returned light comes back through the front lens 205, as indicated by arrow 217, and is mixed with the light returned (216) from the reference arm 212.

If the target distance changes, the spot image moves axially along the light path which results in the reference beam and the target beam suffering wind-up of the relative phase. There are bounds as to how far this can go and an external computer is programmed to perform a significant degree of computation in order to compensate for this effect. Two wave fronts must be kept coherent with one another to within fine limits. These must be parallel or co-spherical. It is possible for them to be off-set but the degree of off-set must be accurately known.

The nature of the instrument is such that close tolerance problems exist with reference to the primary variables; thus, the focusing of the 9 lenses do not create particular problems but the axes of the instrument must be set up correctly. Thus, the physical configuration of the instrument is a very important factor when engineering an instrument to achieve optimum operational characteristics. Thus, when designing an instrument of this type, it is known to provide a solid metal block for securing the beam splitter 211 with arms rigidly attached thereto and including other rigid fixing members for securing the additional components.

Front lens 205 requires a slight skew on it so that the reflected beam of the main illumination going down the centre of the instrument does not shoot back towards the laser 203. However, a further problem exists in relation to light being directed back towards laser 203 within the configuration shown in Figure 2. In addition to its beam splitting surface 218, the beam splitter has physical external surfaces, such as surface 219.

These surfaces can be provided with anti-reflective coatings but even as little as one percent of the light generated by laser 203 and indicated by arrow 213 directed back towards the laser, indicated by arrow 220, reflected from surface 219, will produce severe problems in terms of the overall functionality of the instrument.

In scientific applications, it is known to add polarising filters, isolators and similar devices between the beam splitter 211 and the laser 203.

However, in terms of a commercial product, such devices add significantly to the overall cost and would therefore limit the realistic range of application.

The present invention overcomes this problem by introducing appropriate off-sets inherent to the construction of the instrument, thereby avoiding the need for the introduction of additional devices. When applied to the embodiment shown in Figure 2, laser 203 is physically connected in such a way that beam 213 strikes surface 219 at an off-set angle, thereby ensuring that any reflection is not reflected directly back to the lasing device.

An off-set angle is engineered by means of an off-set mounting block 301 shown in Figure 3 embodying the present invention. Mounting block 301 is configured to mount or secure a first optical device such as beam splitter 219; this device being of the type having faces that reflect light at substantially ninety degrees. The mounting block 301 includes mounting surfaces, such as surfaces 302 and 303. Extension means embodied as rods 304 connect to the mounting surfaces via additional support brackets 305. The extension means 304 allow light to be transmitted between the first optical device and co-operating optical devices by apertures 306 in the mounting surfaces. However, the mounting block itself is not configured as a normal cuboid in which all angles between mounting surfaces are ninety degrees. In this preferred embodiment, when viewed side-on, a cross section of the block presents a shape substantially rhomboid (with the acute corners chamfered off) such that at least one of the surfaces is off-set from a normal cuboid angle of ninety degrees. in this way, light may be directed at the block at an angle off-set from ninety degrees, such that reflected light is not directed back along the same orientation. This feature is emphasised more fully with reference to Figure 7, showing a fully assembled mixer.

The surfaces of the block are preferably offset by ten degrees but for alternative applications, the offset may range from five degrees to thirty degrees.

In the preferred embodiment, the off-set mounting block 301 is fabricated from metal and internal surfaces 307 have a matt black surface to minimise internal reflections. Interface plates 308 are attached to mounting surfaces 302, 303 of a size slightly larger than that of the mounting surfaces themselves. In this way, the interface plates introduce protrusions extending beyond the surfaces such that quick release clips 309 may be used to secure support brackets 305 to their respective mounting surfaces 303. The quick release clips 309 are attached to support brackets by means of screws 310, whereafter rods 304 are introduced into the support brackets 305 and secured firmly in place by screws 311.

The arrangement shown in Figure 3 is shown in its assembled form in Figure 4, in which interface plates 308 have been secured to mounting block 301 to provide securing protrusions. Quick release clips 309 have been attached to support brackets 305 thereby providing a quick release mechanism for facilitating the assembly and disassembly of the instrument.

Rods 304 are firmly attached to their respective support bracket 305 such that the whole arm, defined by rods 304, may be quickly attached and detached using respective release clips 309.

An assembled device similar to that shown in Figure 4 is also shown in Figure 5. In order to implement a mixer, of the type shown in Figure 2, arms extend from all four mounting surfaces in directions 501, 502, 503 and 504 so as to secure lens 205, reference arm 212, detector 214 and laser 203 respectively. During assembly, a beam splitter 505 is introduced into block 301 and mounted to a prism table adjuster 506, such as part 06 5064 produced by Linos Photonics. The prism table includes thumb screws 507 for adjusting tilt angles and setting axial position.

1 A partially assembled instrument is shown in Figure 6 in which a first arm 601 has lens 205 mounted thereon. A second arm 602 supports laser 203 and a third arm 603 supports the return devices 212. Detector 204 is then connected to block face 604. A schematic representation of the 12 instrument shown in Figure 6, having a detector 204 at location 701 is illustrated in Figure 7. Light from laser arm 602 is directed towards a target 702 and towards return arm 603. Laser speckle is returned via lens 205 and mixing occurs at splitter 211. Light generated by the laser is directed towards splitter 211 in the direction indicated by line 703. However, due to the off-set angle of block 301, any light reflected at the splitter 211 is returned back to laser device 602 in the direction of arrow 704. In this way, the reflected light is not directed back along the axis of the laser beam thereby ensuring that optimum performance is maintained. By use of off- set block 301, this is achieved without the introduction of complex filtering devices, while at the same time maintaining the physical stability and integrity of the instrument.

An operational environment for the instrument of Figure 7 is shown in Figure 8. The instrument is supported within a housing 801 and a laser beam is directed from the instrument towards operational machinery under test 802, including a rotating shaft 803. An output from CCD detector 701 is supplied to a higher powered personal computer 804, running application programs under a Windows NT platform for example. A personal computer 804 has interface devices, including a visual display unit 805, a keyboard 806 and a mouse 807.

The computer system may include many programs allowing the instrument to be used in many applications. The inclusion of the off-set mounting block 301 allows the nature of the instrument to be reconfigured with clips 309 facilitating the connection and reconnection of the operational arms.

An alternative mounting block 901 is shown in Figure 9, similar to the mounting block shown in Figure 3 but with enhancements made thereto.

13 The mounting block 901 includes mounting surfaces 902 and 903 and extension means 904 to facilitate applications where light is transmitted between co-operating optical devices. Like the mounting block shown in Figure 3, it is not configured as a normal cuboid in which all angles between mounting surfaces are at ninety degrees and, alternatively, provides a substantially rhomboid shape such that at least one of the surfaces is off set from a known normal cuboid angle of ninety degrees.

In addition, the arrangement shown in Figure 9 is provided with electrical connection sockets 911, 912. These sockets facilitate electrical connection between optical devices. In a preferred embodiment, these sockets make provision for electrical power to be provided to electrooptical devices assembled to the mounting block. In addition, electrical connections are also used for the transmission of data between optical devices and it is envisaged that instruments built using this system could include significant processing resources in the form of general purpose programmable integrated circuit possibly running embedded software. In particular, communication between devices could take place over a shared bus using a standard similar to the Small Computer Universal System Interface (SCUSI) or Universal System Bus (USB). In this way, a dedicated processor could address specific devices within the instrument thereby facilitating the transfer of data thereto or therefrom.

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Claims (11)

Claims

1 A mounting block for mounting a first optical device and a second optical device, wherein said block includes mounting surfaces for supporting said optical devices and reflecting means supported within said block to effect a transfer of light at a substantially perpendicular angle between said devices, characterised in that at least one of said surfaces is offset from a ninety degree rectangular angle.

2. A mounting block according to claim 1, including a mechanism for attaching optical devices to said mounting surfaces, wherein said mechanism is releasable by manual application without additional tools.

3. A mounting block according to claim 1 or claim 2, including electrical conducting means to facilitate electrical communication between said optical devices.

4. A mounting block according to any of claims 1 to 3, wherein said first optical device is a laser.

5. A mounting block according to any of claims 1 to 4, wherein said second optical device is a light sensitive device.

6. A mounting block according to any of claims 1 to 5, including means for mounting a third optical device.

7. A mounting block according to any of claims 1 to 6, wherein said reflecting means is partially transparent so as to perform a beam splitting operation.

8. A mounting block for mounting a first optical device and a second optical device substantially as herein described with reference to Figure 3, 4 and 5 or with reference to Figure 9.

IG Amendments to the claims have been filed as follows Claims 1. A mounting block, for mounting a first optical device and a second optical device, includes first and second mounting surfaces for supporting said first and second optical devices respectively and includes reflecting means supported within said block to effect a transfer of light at a substantially perpendicular angle between said devices, characterised in that said two mounting surfaces are offset from a ninety degree rectangular angle with respect to one another, 2. A mounting block as claimed in claim 1 in which the block is rhomboid in shape, the said two mounting surfaces forming adjacent sides of the rhomboid.

3. A mounting block as claimed in claim 1 or 2 in which the offset is in the range five to thirty degrees.

4. A mounting block as claimed in claim 3 in which the offset is ten degrees.

5. A mounting block according to any previous claim, including a mechanism for attaching optical devices to said mounting surfaces, wherein said mechanism is releasable by manual application without additional tools.

6. A mounting block according to any previous claim, including 1-7 electrical conducting means to facilitate electrical communication between said optical devices.

7. A mounting block according to any previous claim, wherein said first optical device is a laser.

8. A mounting block according to any previous claims, wherein said second optical device is a light sensitive device.

9. A mounting block according to any previous claim, including means for mounting a third optical device.

10. A mounting block according to any previous claim, wherein said reflecting means is partially transparent so as to perform a beam splitting operation.

11. A mounting block for mounting a first optical device and a second optical device substantially as herein described-with reference to Figure 3, 4 and 5 or with reference to Figure 9 of the accompanying drawings.