GB2183361A - Adjustable optical apparatus and spectrometers - Google Patents

Abstract

Optical apparatus includes an adjustable mounting for a lens, mirror or diffraction grating 11 in which a motor-driven leadscrew 37 displaces a rod 21 supporting the mirror 11, the rod 21 being itself supported by resilient strip hinges 22 and 23 which restrain part 11 from displacement lateral to the axis but allow limited axial movement. The rod slides without mechanical contact within cylindrical surfaces in the housing 20, the close fit and use of grease providing damping. An infra-red spectrometer utilizing the moving mirror as part of a Michelson cube interferometer configuration is described. <IMAGE>

Description

SPECIFICATION Adjustable optical apparatus and spectrometers This invention relates to sensitive optical apparatus in which an element such as a mirror has to be displaced in a precise manner so asto adjust the optical response in a controlled way, with minimal consequential misalignment and vibration.

The particularfield of application ofthe invention demanding these conditions is one in which the optical system involves setting up interference patterns between two light beams developed from the same source.

Although the invention will be described by referpence to optical systems, it is to be understood that optical' is to be interpreted broadly as including electromagnetic waves of any frequency to which it is practical to apply the techniques described. Specifically, this includes infra-red frequencies, butallopti- cal frequencies and higher and lower frequencies are included, as arethose frequencies applicableto X- rays and electron beams. In the latter connection, though reference will be made to lenses and mirrors as the adjustable elements, it is to be further understood that a diffraction grating or its equivalent could constitute an adjustable optical element two which the invention can be applied.

It iswell recog nized that the movement of mi rrors in an operating interferometer is to be avoided if this is at all possible. There are, however, applications in which it is desirable to displace a mirror in a cyclical manner so as to scan a range of frequencies systematically. Thus, in the infra-red spectrometer a test specimen is subjected to an infra-red beam which covers a wide frequency spectrum and the emerging beam is analysed spectrographically to determine the molecular composition of the specimen. This in volves the cycl ical tu ning of an interferometertodev- elop numerous spectral patterns, interferograms, which, with the aid of modern computing techniques, can be interpreted to establish the precise nature ofthe molecules present.

The optical adjustment means in spectrometers of prior construction are very difficultto manufacture and are costly owing to the precision ofthe assembly and control. The invention to be described is essentially simple and has been found to provide an adjustable system which can be manufactured atafraction ofthe cost of many prior art systems. This has proved somewhat surprising, especially as the performance in a spectrometer has proved a match for such prior expensive systems.

Though the invention to be described has been designed expresslyfor use in such a spectrometer, the same principles may find application in, for example, lasers, Fabry-Perot interferometers, optical sensors, specialized cameras, microscopes, telescopes and communication systems.

The Fabry-Perot interferometer, for example, warrants mention, because it comprises two semireflecting mirrors, the separation ofwhich can be varied by dispiacing one mirror. The latter is mounted on a nut threaded on an accurate screw. The nut is constrained to move exactly linearly so that, once the mirrors are adjusted to be parallel, they remain parallel as the separation is altered. However, even 30 years ago, this intrumentwas regarded as obsolescent owing to the difficulty of manufacturing itto the right tolerance level.

It has been replaced bythe Fabry-Perotetalon, which is much easierto manufacture because itcomprises two fixed mirrors and finds application where the optical medium between the mirrors can be adjusted to set up the interference conditions at a wavelength related to the mirror separation.

This, therefore, is an example of the long standing difficulties in manufacturing optical apparatus with movable mirrorsystemsfor precision application in interferometers.

Afurtherproblem inspectrometerdesign that of minimizing the vibration attendant upon mirrordisplacement. If the mirror makes its scan slowly, vibration is reduced, but then it takes longer to make an analysis ofthe data available. Modern computing facilities permit rapid analysis of interferograms, especially ifprogrammedto use the Fast FourierTransform, and this makes it possible, given enough sets of interferograms for a particulartest, to make very accurate analysis, limited only by the speed at which the mechanical system can operate.

It needs very little vibration at relatively lowfrequenciesto distort the reading obtained.Accordingly, the design of the mirror adjustment system must be particularly robust, lend itself to easy assembly and maintenance, and be smoothly operable with minimal likelihood of vibration being transmitted to the mirror.

This is possible by combining the original and novel features of the design now to be described.

According to the invention, optical apparatus includes an adjustable mounting which permits displacement of a lens or mirrorso astovarythe axial length of a ray path, and is characterized in thatthe mounting includes, in combination, at least one resilient support member, which restrains the lens or mirrorfrom displacement lateral to the axis but permits limited movement axially, and an axial rod rigidly connected to the lens or mirror, which provides both the means for adjusting the axial displacement and, via independent support means, the damping means, whereby to minimize vibration.

Accordingto afeatureofthe invention,the re- silient support member comprises a leaf spring arrangement in a plane perpendicularto the axis, with at Ieastthree radial arms rigidly connected to both the lens or mirror and a housing.

According to a furtherfeature of the invention,the arms ofthe resilient support member have a spiral form, being longer than the radial spacing between the lens or mirror and the housing. This permits axial displacementthrough a relatively long range,whilst still having a lateral restraint.

According to yet a further feature of the invention, the assembly formed bythelensormirrortogether with the axial rod is supported buy a pluralityofsaid resilient support members, whereby they assure axial alignment ofthe optical axis ofthe lens or mirrorwith said axial ray path.

According to still anotherfeature ofthe invention, the axial rod includes a piston section, which fits closelywithin a cylinderforming partofthe rigid housing ofthe apparatus, with no mechanical contact between the piston and cylinder. Damping can be enhanced by the provision of grease between the piston and cylinder surfaces.

According to a still furtherfeature of the invention, the meansforadjusting the axial displacement of the lens ormirrorcomprises also a very slender rod section connected between the axial rod and a leadscrew, the adjustment means comprising further a leadscrew nut which is rotatable.

The invention further provides apparatus in which the axial adjustment ofthe lens or mirror is coordina- ted with the operation of detection and recording means to facilitate analysis ofspectral data as a function of the optical ray path, there being additional calibration means involving a monochromatic light signal atconstantfrequencyformeasuringtheadju- sted position ofthe lens or mirror so as to provide calibration reference data throughoutthe adjustment.

The invention will now be described by reference to the accompanying drawings in which: Figure 1shows a schematic arrangement of a spectrometer.

Figure2 shows a cross-sectional view ofthe adjustable mirror mounting in the spectrometer shown in Figure 1.

Figure3showsthe planarform ofa resilient sup- port member used to restrain lateral motion of the mirror mounting of Figure 2.

This detailed description will be confined to such a spectrometer application which uses movable mirror elements. It will, however, be obvious that the same principles could be applied to adjustthe axial position of a lens. In this case the rod structure behind the mirrorwould need appreciable modification so as notto obscure too much ofthefield of view through the lens. Thus a slender rod attached to the centre of the lens would block the central field. However, the advantages of the focussing arrangement thus provided can still be exploited by restricting the optical field to the off-centre region.

The invention is, however, more likely to find application where mirrors are to be adjusted and one such application is in the end mirrorofa laser, where rapid adjustment of the mirror could be used to develop modulated frequency outputs, for example, or where slow adjustment by remote control for laser fine tuning is needed.

Referring now to Figure 1, the infra-red spectrome tercomprisesan infra-redsource 1 from which rays are directed via a mirror pair2 and a mirror3through an aperture4in the wall Sofa sample region 6. The radiation is directed through the sample 7 and passes for analysis into the interferometer section 8.

This section ofthe apparatus is of the basic Michel son 'cube' design. The radiation passes into a beam splitter 9 and divides between a ray path towards a fixed mirror 10 and an adjustable ray path towards an adjustable mirror 11. The reflected radiation recombines in the beam splitter and passesto another mirror pair 12, which focusses the composite radiation onto a detector 13.

The electrical signal emerging from the detector 13 can then be processed by means not shown, but including digitalizationtechniqueswhich permitthe data to be analyzed by a computer program operating by exploiting the Fast Fourier Transform to break the signal into its wave components and present the result on a visual display or a recorder chart.

In this particular apparatus the central field of view oftheapparatus is not used, becausethe mirrorpair 12 operates with the off-centre field. Thus, there is scope for measuring the actual position of an adjusted mirror 11 by providing a hole (not shown) in the beam splitter along the central axis. A laser producing monochromatic light at constantfrequency introduces a superimposed ray into the interferometer section along with the infra-red beam. By counting the transitions between constructive and destructive interference in the signal output at this laser frequ- ency it is possible to synchronize the detector sampling rate with the motion ofthe mirror 11.This laser is not illustrated in Figure 1, but it is evidentto those skilled in the artthatthe laser beam can be introduced in a variety of alternative ways, one being to set the laser in line with the main axis behind mirror 3, which then needs to be part-reflecting.

Referring to Figure 2, the interferometersection 8 in Figure 1 appears in a side-elevation view in relation to the plan view (ortop elevation) applicable in Figure 1.

It comprises a solid metal housing 20 which prov ides rigidity forthe Michelson cube system,the latter comprising the beam splitter 9, a fixed mirror 10 (not shown, but lying on the transverse axis through the centre ofthe beam splitter) and the movable mirror 11.Thismirrorcanbeadjusted bydisplacement along the main axis X-Xthroughthe main body of the housing 20.

The adjustable mounting forthis mirror 11 comprises an axial rod 21 having an enlarged section at one end in which the mirror 11 is rigidly mounted.

The rod 21 is supported by thin planar resilient support members 22 and 23 which act as circularstrip hinges or leaf springs and permit limited axial move ment ofthe rod 21 but restrain any movement lateral to the axis X-X. These members are made from hard beryllium-copper alloy and are shaped as shown in Figure 3.

As shown in Figure 3the centre of the member 22, denoted 24, is connected radiallyto the outer rim 25 by three arms 26,27 and 28, which progress in partspiral mannerthrough half a revolution beforees- tablishing the connection between centre and rim.

This particulardesign has beenfoundto bevery suitable for permitting substantial axial movement, whilst keeping the centre 24firmly in its axial position with no twist or distortion that could cause mis alignmentofthe rod and so affectthe orientation of mirror 11 during displacement.

Reverting to Figure 2,the rod 21, is seen to be positively located by these support members 22 and 23, the latter being fixed to the rod by a sleeve 29 and a threaded boss 30.

The rod 21 has two piston sections 31 and 32 which fit closely within cylinder sections (not numbered) that are part of the housing 20. There is no mechani cal contact across the piston and cylinder surfaces, buttheyare smeared with high vacuum grease in order to provide smooth damping of oscillations, without communicating vibration to the mirror assembly.

The axial adjustment of the rod 21 is achieved by a motor 33 which operates at extremely low speed through a pulley a belt system 34to drive a pulley wheel 35 which is coupled to a leadscrew nut 36. This engages with the leadscrew 37 which is connected via a veryslendertie rod 38to the rod 21. The con- nection between the tie rod 38 and rod 21 is made by a grub screw (not shown) at a central position between the two resilient support members 22 and 23, the tie rod extending through an axial hole in the rod 21. This putsthetie rod coupling at the centre of inertia ofthe moving system and minimizes any ten- dency for the forces exerted in the adjustment to tilt the system and so the mirror 11.

In operation, under appropriate control of motor 33, the leadscrew drive causes mirror 11 to move for ward and backwards along axis X-X in a cyclical manner. The spectrometer as a whole operates to feed a wide spectrum of infra-red radiation into the interferometer and, as the mirror cycles, it scans through repeated cycles of resonant frequencies, de- veloping an overall pattern of interference related to the particularfrequency components in the input radiation.

This input is modified by passage ofthe source radiation through a sample (eg. gas in a transparent container) and a referencefilterlocated at 14in Figure 1. Such techniques are familiartothose skilled in the art of infra-red spectrometry.

The output from the interferometer develops a complex sequence of waveforms in repetitive cycles and these are analyzed by conventional techniques to extract the ultimate information identifying the molecular composition ofthe test sample.

Claims (9)

1. Optical apparatus including an adjustable mounting which permits displacement of a lens or mirror so as to vary the axial length of a ray path, characterized in thatthe mounting includes, in combination, at least one resilient support member, which restrains the lens or mirrorfrom displacement lateral to the axis but permits limited movement axially, and an axial rod rigidly connected to the lens or mirror, which provides both the means for adjusting the axial displacement and, via independent support means, the damping means, wherebyto minimize vibration.

2. Optical apparatus according to claim 1,characterized in that the resilient support member comprises a leaf spring arrangement in a plane per pendicu larto the axis, with at least three radial arms rigidly connected to both the lens or mirror and a housing.

3. Optical apparatus according to claim 2, characterized in thatthe arms of the resilient support member have a spiral form, being longer than the radial spacing between the lens or mirrorandthe housing.

4. Optical apparatus according to any preceding claim, characterized in that the assembly formed by the lens or mirrortogetherwiththe axial rodissup- ported by a plurality of said resilient support members, whereby they assure axial alignment of the op- tical axis ofthe lens or mirror with said axial ray path.

5. Optical apparatus according to any preceding claim, characterized in that the axial rod includes a piston section, which fits closely within a cylinder forming part of the rigid housing of the apparatus, with no mechanical contact between the piston and cylinder.

6. Optical apparatus according to claim 5, characterized in that the piston and cylinder surfaces are separated by a layer of grease which enhances the damping of any vibrations in the axial direction.

7. Optical apparatus according to any preceding claim, characterized in that the means for adjusting the axial displacement of the lens or mirror comprises also a very slender rod section connected between the axial rod and a leadscrew,the adjustment means comprising further a leadscrew nut which is rotatable.

8. Optical apparatus according to claim 7, characterized in that the slender rod section extends through an axial hole in the axial rod and isconnec- ted to the rod at the centre of inertia of the rod and mirror assembly.

9. Optical apparatus according to any preceding claim, in which the axial displacement of the lens or mirror is coordinated with the operation of detection and recording means to facilitate analysis of spectral data as a function of the optical ray path, characterized by an additional calibration means involving a monochromatic light signal atconstantfrequency which measures the adjusted position of the lens or mirror so as to provide calibration reference data throughoutthe adjustment.