GB2162334A - Optical path length variation - Google Patents

Abstract

In interferometers using two beams (e.g. the Michelson type) means for varying the path length are difficult to manufacture. This difficulty is overcome by connecting a set of plane mirrors to a rotatable platform. As shown, two parallel mirrors 3, 4 are mounted on a platform 1 which is rotatable about an axis 2. A ray 6 is reflected by the mirrors and emerges in a direction parallel to the direction of entry. A plane mirror 5 perpendicular to the exit ray returns the ray along its own path. The device may be incorporated in an interferometer or in Fourier transform spectrometer. <IMAGE>

Description

SPECIFICATION Optical path length variation This invention relates to a device for varying the optical path length of a radiation beam comprising a plurality of plane mirrors rigidly connected together to form an assembly, means for directing an incident beam onto one mirror of the assembly, the mirrors being mounted so that the beam traverses the assembly by way of reflections from each of the mirrors and emerges from the assembly in a direction parallel to the direction of incidence on the one mirror, and means for rotating the assembly about an axis so as to change the angle of incidence of the beam on the mirrors and thereby vary the optical path length of the beam. A Fourier transform spectrometer, using a Michelson interferometer, is an example of an instrument in which such a device may be used.In such an instrument an input beam of radiation, which is to be analysed spectrally, is divided into two subbeams by a beam splitter, one subbeam being reflected back to the beam splitter along a path of fixed optical length and the other subbeam being reflected back to the beam splitter along a path of variable optical length for recombination with the first subbeam to form concentric circular interferences fringes on a radiation detector. The variation in detector output as a function of the varied path length can be analysed by suitable known Fourier transform methods to yield the spectrum of the input radiation beam.

To maintain the fringes centered on the detector during variation of the optical path length, the reflected beam must be returned along its outgoing path length, the reflected beam must be returned along its outgoing path to very close tolerances. For example, in a typical Fourier transform spectrometer working in the infra-red region at 4000cm~, in which it is desired to resolve 0.5cm-1, a stability of + 2 seconds of arc is required for reversing mirrors used with each subbeam.

It is known to provide the variable optical path length in one arm, or subbeam, of a Michelson interferometer by mounting a plane reflecting mirror on a carriage travelling on an air bearing slide. In such an arrangement the carriage and slide and necessarily relatively massive, stress relieved, castings leading to a relatively massive and expensive instrument. Also linear motion is more difficult to provide and control than rotational motion. Replacement of the movable plane reflecting mirror by a movable corner cube mirror has been proposed as a way of overcoming the problem of the angular stability of the plane mirror during movement.

In United States Patent Specification No. 4,383,762 a movable corner reflector produces optical path length variation while maintaining the retroreflected beam parallel to, but laterally offset from, the input beam. The retroreflected beam is reversed by a plane mirror so that the return beam retraces its path through the corner reflector and emerges coincident with the input beam as is required in an interferometer. The corner reflector is moved along the beam by rotating it about an axis laterally offset from the beam, the corner reflector moving along an arc of a circle. The lateral offset between the retroreflected beam and the input beam varies as the optical path length changes and the plane reversing mirror is of sufficient size to accommodate this change. Mainly, it is the translational movement of the corner reflector which produces the optical path length change.

If the corner reflector in the above cited United States Specification is a corner cube reflector, the arrangement is totally insensitive to rotation of the corner reflector about any axis, such as may occur due to bearing play. This is also true if the mirrors of the corner cube reflector have not been set accurately at right angles to one another. In this case, the plane reversing mirror is adjusted to be normal to the output beam from the corner reflector so that the return beam retraces its path as before. However such a corner cube reflector is relatively difficult to make and relatively massive to move.

If the corner reflector in the cited United States Specification is a pair of plane mirrors at right angles, these difficulties may be alleviated somewhat. But such a mirror pair has the serious inherent defect that the direction of the retroreflected beam is strongly affected by rotation of the mirror pair about a third axis at right angles to the input beam and at right angles to the intended rotation axis. The angle of incidence of the retroreflected beam on the plane reversing mirror may vary from normal, due to bearing play about the thrid axis and the angular error in the return beam is compounded as it retraces its path through the corner reflector.

It is an object of the invention to provide a device for varying the optical path length of a radiation beam, which uses only plane mirrors fixed in a moving component and in which the angular stability of the varied beam is relatively insensitive to rotation of the moving component about any axis, even in the presence of constructional errors in the device.The invention provides a device for varying the optical path length of a radiation beam comprising a plurality of plane mirrors rigidly connected together to form an assembly, means for directing an incident beam onto one mirror of the assembly, the mirrors being mounted so that the beam traverses the assembly by way of reflections from each of the mirrors and emerges from the assembly in a direction parallel to the direction of incidence on the one mirror, and means for rotating the assembly about an axis so as to change the angle of incidence of the beam on the mirrors and thereby vary the optical path length of the beam, characterised in that the mirrors are mounted in the assembly such that the emergent beam travels in the same direction as the incident beam.

In interferometers the subbeam of varied optical path length must be returned to the beam splitter. To this end, a device in accordance with the invention may be characterised in that a plane beam reversing mirror is provided in the path of and normal to the emergent beam, whereby the emergent beam retraces its path through the mirror assembly.

A device in accordance with the invention may be characterised in that the mirror assembly comprises a pair of parallel mirrors, the planes of the mirrors being separated from one another.

In this case the number of mirrors of the assembly is reduced to a minimum. These two mirrors may be extended so that the beam is reflected several times between the mirrors increasing the amount of optical path length change obtained for a given rotation of the assembly. However, the number of reflections is thereby increased with a consequent increase in reflection losses. It may be desirable to increase the separation of the planes of the two mirrors to obtain an increased change of optical path length.

However, in accordance with the invention, the number of mirrors of the assembly may be more than two, the mirrors of the assembly being at various angles to one another chosen to provide an emergent beam parallel to the incident beam.

If a pair of plane mirrors are used in the assembly and if they are exactly parallel, the emergent beam from the pair is always parallel to the incident beam regardless of the axis chosen for rotation. Optical path length variation is obtained provided the rotation produces a change in angle of incidence of the incident beam on the first mirror of the pair. As with a pair of plane mirrors at right angles, the lateral offset between incident and emergent beams varies and the plane reversing mirror must be made of sufficient lateral extent to accommodate this variation. In practice, the mirror pair cannot be made exactly parallel although they can be mounted on a rotary platform of sufficient rigidity to preserve any error in parallelism. In the presence of such an error, the emergent or output beam from the mirror pair will not be exactly parallel to the input or incident beam.But the plane reversing mirror can be adjusted to be normal to the output beam so that the beam retraces its path through the mirror pair. If the rotation axis is at an arbitrary angle to the mirror surfaces and the input beam, the rotation will produce only a second order change in angle of the output beam. However, if the device is characterised in that the rotation axis is substantially parallel to the mirror surfaces of the plane mirror pair and normal to the input radiation beam, this second order change is substantially reduced. Consequently this choice of rotation axis is currently preferred.

In a practical bearing for the rotation of the mirror pair, play is inevitable, giving rise to error rotations about axes other than the preferred axis. These rotations can be resolved into rotations about the input beam and about a third axis normal to the input beam and to the preferred axis.

Rotation about the input beam in a device in accordance with the invention and also in a device using a mirror pair at right angles merely has the effect of moving the output beam laterally across the plane reversing mirror while maintaining parallelism. Rotation about the third axis in a device in accordance with the invention has the effect of moving the normals to the two mirrors in opposite directions relative to the beam so that the beam error deviations produced by the two mirrors cancel. However, in a device using two mirrors at right angles, rotation about the third axis moves the normals together, compounding the error deviations. As mentioned above, in a practical parallel mirror pair, the mirrors will have an error in parallelism.In a device according to the invention rotation about the third axis in the presence of such an error produces only a second order change in beam angle.

A device in accordance with the invention may be used in, for example, a Michelson interferometer. In this case, the optical path length variation device does not have to be registered rigidly with respect to the beam splitter and the two subbeam reflectors. A massive supporting structure is then not necessary, enabling the use of lighter weight structures. A supporting structure of sheet metal may be used to register the beam splitter and the subbeam reflectors with respect to one another provided the metal sheets are attached to one another substantially at right angles and at points of kinematic contact between them in a stress free manner and provided the whole interferometer is enclosed in a thermally insulated case.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a device for varying the optical path length of a radiation beam, Figure 2 illustrates the calculation of the path length variation provided by the device of Fig.

1, Figure 3 illustrates the effect of an error rotation of the parallel mirror pair in Fig. 1, Figure 4 illustrates the effect of an error rotation of a mirror pair comprising a corner reflector, Figures 5a and 5b illustrate the effect of an error rotation of the parallel mirror pair of Fig. 1 in the presence of an error in parallelism of the pair, Figure 6 shows the schematic layout of a double interferometer, Figure 7 shows a Fourier transform spectrometer, and Figures 8, 9, 10 and 11 show details of the mechanical construction of a supporting structure for the spectrometer of Fig. 7.

Fig. 1 shows a plan view of a platform 1 mounted for rotation about an axis 2 at right angles to the platform. A pair of plane mirrors 3 and 4 are mounted rigidly on the platform with their facing mirror surfaces parallel to one another and to axis 2 so that they rotate as a unit about axis 2 when the platform is rotated relative to a supporting structure, not shown. A plane mirror 5 is mounted rigidly on the supporting structure normal to the intended direction of an input beam 6 of radiation. In use, the input beam 6 is incident once on each of mirrors 3 and 4 before energing from the mirror pair parallel to its original direction and laterally offset from it.

Beam 6 is then incident normally on mirror 5 and a ray of beam 6 retraces its original path through the mirror pair, emerging coincident with it. An input wavefront is therefore returned in coincidence with the original as is required in an interferometer, for example, when two subbeams split from an input beam are both reversed and recombined to form interference fringes.

Fig. 2 illustrates the change in optical path length produced by the mirror pair as they are rotated about axis 2. The path of a ray of beam 6 through the mirror pair is shown between two arbitrarily chosen planes 7 and 8 normal to beam 6 and separated by a distance D. A construction line 9 is normal to the beam 6 at its point of incidence on mirror 3 and construction line 10 is aligned with the outgoing beam. Distance y is the length of the beam between the points of incidence on the two mirrors separated by distance d and distance x is the longitudinal separation between the input and output parts of the beam. The optical path length, OPL, between planes 7 and 8 is given by: OPL = D + y - x From the diagram: d y = , and x = y sin (2a - 90") cosa from which it can be deduced that OPL = D + 2d cos a.

The lateral separation T between the input and output beams is given by T = 2d sin a, lying in the common plane of incidence of the beam on the two mirrors.

In the device of Fig. 1, the variable component of the total optical path length will be 4d cos a since the beam traverses the mirror pair twice. If the device is used in a typical Fourier transform spectrometer working in the infra-red region at 4000cm - 1 in which it is desired to resolve 0.5 cm-', a total OPL change of 20mm is required. In this case if d = 37.4mm, a rotation from 45 to 55 produces such a change. In this particular case the change in T is 8.4mm and mirror 5 must be of sufficient lateral size to accommodate this change.

In a practical device there will always be some residual error S in the parallelism of mirrors 3 and 4. It will normally be possible to fix these mirrors to platform 1 with sufficient rigidity so that this error does not change. If the error 6 is an angle in the plane of the drawing of Fig. 1, i.e. an angle about axis 2, the beam will have an angle of incidence 28 on mirror 5. This angle of incidence will be preserved, however, as the platform is rotated. Consequently mirror 5 can be adjusted in angle by 2 8 to return the beam along its incident path and provide exact compensation for the error.If the error is about axes other than axis 2, then provided the error is small, the compensation provided by adjusting mirror 5 is almost exact, there being a small, second order, angular movement of the beam incident on mirror 5 during platform rotation.

In a practical bearing provided for the rotation of the platform, there will be bearing play which can be resolved into small error rotations about the input beam and about a third axis normal to the input beam and axis 2. In the event that the two mirrors are exactly parallel an error rotation about the beam or about the third axis merely displaces the beam incident on mirror 5 out of the plane of Fig. 1, maintaining parallelism, and therefore giving no effect on the returned beam. In the presence of a residual error 8 in parallelism of mirrors 3 and 4, the situation for an error rotation about the beam is shown in Figs. 5a and 5b.In Fig. 5a, the mirrors and beam are shown schematically, the parallelism error 8, presumed here to be in the plane of the drawing, resulting in an emergent beam error 2 8. Rotation 11 about beam 6 rotates the common plane of incidence of the two mirrors out of the plane of the drawing, the emergent ray generating a cone of semi-vertical angle 2 8. Fig. 5b shows the effect on emergent beam angle for an error rotation H of the platform about beam 6. The change in emergent beam angle 0 is given by =280 That is, 0 is proportional to 8 but reduced in amount by a factor 2 8, where 8 is in radians.For example for an error oR of 1 minute of arc, or 0.00029 radian, the factor is 0.00058. Error rotation about the third axis in the presence of a parallelism error 8 is correspondingly reduced in its effect on emergent beam angle.

Fig. 3 and 4 illustrate an advantage of a device in accordance with the invention over a device in which a mirror pair comprising a corner reflector is moved along the beam to produce optical path length changes. An error rotation about the third axis produces no effect on the emergent beam direction, assuming the mirrors to be exactly parallel. In Fig. 3 such a rotation rotates one mirror normal 1 2 out of the plane of the drawing in one direction, whereas the other normal 1 3 is rotated in the opposite direction, producing exact compensation. In Fig. 4, the corner reflector pair, both normals rotate in the same direction, compounding the effect 11 of the error rotation on emergent beam direction.

A device for producing changes in optical path length in accordance with the invention may be contrasted with the device used in the so-called 'tilting' Michelson interferometer, illustrated and described on pages 131 and 132 of the textbook "The Design of Optical Spectrometers" by J.F. James and R.S. Sternberg, Chapman and Hall 1 969. In the 'tilting' Michelson, the rotating platform carries the beam splitter and two plane mirrors parallel to the beam splitter and one on either side of it. The beam incident on one of these plane mirrors from the beam splitter is reversed by a fixed plane mirror and retraces its own path. However, the beam incident on the other plane mirror is the recombined beam and traces this path only once on its way to the detector. Thus in the presence of an error rotation about either the input beam or about the third axis.The recombined beam is moved bodily sideways, undesirably displacing the centre of the fringe pattern on the detector. In the device in accordance with the invention, the beam splitter is stationary and the recombined beam is undisturbed.

Practically available beam splitters for use in a Fourier transform spectrometer working in the infra-red region have a limited wavelength range owing to problems of available materials versus the design of filter coatings to give 50% transmission and 50% reflection. To achieve a useful range, from 5000cm-1 to 200cm-1 for example, a pair of interchangeable beam splitters is required, each covering part of the range. In the 'tilting' Michelson, a beam splitter turret would have to be mounted on the rotatable platform, adding to the already large moving part. The stationary beam splitter in a device in accordance with the invention simplifies beam splitter changing.

In the event that the desired wavelength range can be covered by just two beam splitters, a device in accordance with the invention renders a simple 'double' spectrometer practicable. Fig.

6 is a schematic diagram of such a spectrometer. A wide band source 1 4 supplies diverging radiation of two off-axis concave collimating mirrors 1 5 and 1 6 which supply parallel beams of radiation to beam splitters 1 7 and 1 8 respectively. Beam splitter 1 7 functions effectively in the lower range of wavelengths and beam splitter 1 8 in the higher range, the two ranges overlapping to a small extent. A common plane beam reversing mirror 1 9 is provided for both beams in the arms of fixed optical path length.The plane parallel pair of mirrors 20 and 21, mounted on platform 1 for rotation as a unit about axis 2 as in Fig. 1, are of sufficient vertical extent to reflect both beams transmitted by the beam splitters. A common plane beam reversing mirror 22 is also of sufficient vertical extent for both beams. The lower recombined beam 24 is incident on a detector 25 when a hinged plane mirror 26 is lowered into position 27. The upper recombined beam 23 is reflected down onto mirror 2 by plane mirror 28. When hinged mirror 26 is lifted into a 45 degree position, lower beam 24 is removed from the detector and upper beam 23 is reflected onto the detector. In use, the platform 1 is rotated in one direction through an angle giving the desired optical path length change with mirror 26 in one position. Then, the mirror 26 is put into the other position and platform 1 rotated back to its initial position. The detector output is analysed during each rotation to extract the parts of the spectrum covered by each beam splitter.

It is another feature of a device in accordance with the invention that linear displacement of the mirror pair in any direction produces no change in emergent beam angle or in optical path length. The optical path length change produced is sensitive only to rotation about axis 2 of Fig.

1 for exact paralleism of the mirror pair and is sensitive to other rotations or displacements to only a very small extent in the presence of an error in parallelism of the two mirrors. These properties of a device in accordance with the invention make possible a fresh and more practical approach to the problem of constructing an interferometer in which one arm is required to be of variable optical path length. In particular a more practical Fourier transform spectrometer is possible when such an interferometer is used to produce the interferogram required in the spectrometer.

Fig. 7 shows a Fourier transform spectrometer in accordance with the invention. The optical components of the spectrometer are supported and registered in relative position by a sheet steel structure 29, which is box-like in general form and has internal walls. Structure 29 is constructed in a stress-free manner from sheet steel plates by a method which will be described later with reference to Fig. 8 to 11 inclusive.

Referring to Fig. 7, the input beam 31 is supplied by a broad-band infra-red radiation source 99 and collimated by concave mirror 100. A beam splitter 30 is attached to the floor of structure 29 at 45 to the beam 31. Subbeam 32 from the beam splitter is reflected by a stationary mirror 33 and returned to the beam splitter along a path of fixed optical path length.

Subbeam 6 is deflected through the rotatable parallel mirror pair 3 and 4 described with reference to Figs. 1, 2, 3 and 5. Rotation axis 2 of platform 1 is formed by a bearing set in the floor and/or ceiling of the structure. Subbeam 6 is reflected by a plane mirror 5 to retrace its path through the mirror pair in accordance with the invention along a path of optical path length which can be changed by rotating platform 1. Subbeam 6 and subbeam 32 form a combined output beam 35 which passes through the sample 95 whose absorption spectrum is to be measured. The centre of the interference pattern of beam 35 then falls on a detector 25.

The plane mirrors 5 and 33 are mounted upon angular tilt adjusters 34 which may be of the kind described in copending British Patent Application 8416263 (PHB 33094). One of the adjusters 34 provides tilt of the reflected beam in the plane of Fig. 7, the other adjuster providing tilt of its associated beam out of the plane of Fig. 7. When the instrument is set up during manufacture, the adjusters 34 are used to centre the fringe pattern of the recombined beam 35 on the detector 25. The adjusters are then locked in position relative to the structure in the stress-free manner described in the above copending patent Application.

The platform 1 is driven in rotation by a motor 102 mounted off the supporting structure. In this example, the motor is coupled by a vibration insensitive, constant-velocity coupling 103 to rotate a micrometer movement 104 bearing on a lever arm 105 attached to a shaft on platform 1 which rotates about axis 2. A pilot beam 106 of visible radiation from, for example, a heliumneon laser 90 is also fed through the optical paths of the interferometer, interference fringes being formed on a separate detector 94. The output of this detector is used in a waveform shaping circuit 96 to generate sampling instant signals which occur at intervals of one wavelength of the laser source or fractions thereof as the optical path length is changed.These sampling signals control the instants at which the output of detector 25 is sampled to provide inputs to a computer 97 which calculates the spectrum of beam 31 using, for example a program based on the known Cooley Tukey algorithm. In addition the platform drive motor 102 is controlled in speed by control circuit 101 in response to the sampling instant signals to keep the rate of sampling constant.

The off-structure mounting of the motor and its constant velocity coupling minimise the vibrational component of platform rotation, which component would degrade the output of both detectors.

The supporting structure 29 is suspended within a thermally insulating polystyrene foam box 36. The radiation souces, the detectors and any other heat dissipating components are external to box 36. Extended length clear apertures 37 and 38 of minimal diameter are provided for the input beam 31 and the output beam 35 respectively to minimse air movement in and out of box 36. With a wall thickness of 3.0cm, a thermal time constant of an hour or more can be realised for the structure 29.

To extend the wavenumber range of the spectrometer of Fig. 7, the double interferometer of Fig. 6 may replace the single interferometer shown in Fig. 7.

In use the optical path lengths of the two subbeams may be initially equalised with the platform 1 at one end of its working range of path length variation. The zero order fringe is then incident on the detector. Platform 1 is then driven in rotation to produce the change in optical path length necessary to realise the desired wavenumber resolution. A so-called single sided interferogram is then obtained. More desirably, the path lengths are equalised with the platform in the middle of its working range, the platform then being driven through the whole working range to produce a double sided interferogram less susceptible to phase errors.

The supporting structure 29 is constructed from sheets of mild steel by a method which relies only upon the stiffness of each sheet in its own plane, care being taken not to introduce any bending or twisting stress into each sheet during assembly of the structure. The sheets are attached to one another by a method based on kinematic principles and which reduces stress at fixing points to a minimum. Figs. 8, 9 and -10 show how two sheets are attached to one another at one kinematic point of contact and at right angles to one another.

Referring to Fig. 8, the edge 39 of one sheet 40 is provided with a foot 41 which extends clear of edge 39. A re-entrant cut-out 42 is provided in sheet 40, centered on foot 41. A cylindrical nut 43 is provided with diametrically opposed slots 44 extending part-way along the nut from one end and parallel to the cylinder axis. The nut length allows it to be inserted in cutout 42 and then slid down into-the inlet 46 to cut-out 42 with slots 44 engaging fitting shoulders 45 in sheet 40, the lower end of the nut not extending as far as the foot 41. Foot 41 is placed in contact with the other sheet 47 at right angles to sheet 40 with inlet 46 straddling a hole 48 in sheet 47. A screw 49 is inserted in hole 48 from below sheet 47 and screwed into the threaded portion of nut 43.Upon tightening screw 49, the sheets 40 and 47 are clamped together, clamping stress being confined to the locality of the sheets near shoulder 45 and hole 48. Figs. 9 and 10 show other view of the sheets, the nut and the screw. The hole 48 is enlarged to provide latitude for relative positional adjustment of the sheets.

Fig. 11 shows a typical box structure 50 of the kind suitable for use as the supporting structure 29 in Fig. 7. The attachment points between the sheets, described above with reference to Figs. 8, 9 and 10, are shown schematically as the foot in one sheet and the hole in the other sheet. Two rectangular sheets 51 and 52 are each bent into an L-shaped structure to provide four of the walls of a rectangular box. The latitude in adjustment of the attachment points allows all four of the feet 53, 54, 55 and 56 to be brought into contact with sheet 52 without elastically deforming either sheet when the screws are tightened. An end sheet 57 is provided with two feet 58 and 59 along the lower horizontal edge registering with holes 75 and 76 respectively in sheet 52 and a foot 77 on a vertical edge registering with a hole 78 in the vertical wall of sheet 52.Sheet 57 is also provided with holes 60 and 61 along its top horizontal edge registering with feet 62 and 63 on the horizontal wall of sheet 51 and with a hole 79 in the other vertical edge registering with a foot 80 on the vertical wall of sheet 51.

The feet 58, 59 and 77 can be rested on the respective walls of sheet 52 with the feet 62, 63 and 80 of sheet 51 in contact with the respective walls of sheet 57. Six screws are then inserted and tightened without stressing any of the three sheets.

An internal sheet wall 64 is shown attached, by way of illustration of the generality of this constructional method. Sheet 64 has two feet 65 and 66 on its lower edge, and a floot 81 on a vertical edge. Sheet 64 also has two tongues 67 and 68 on its upper edge and a tongue 82 on the other vertical edge which protrude through apertures 69, 70 and 83 in the walls of sheet 51. The apertures 69, 70 and 83 are shaped to provide feet registering with holes 71, 72 and 84 respectively in sheet 64. The feet 65, 66 and 81 can be rested on sheet 52 in registration holes 73, 74 and 85 respectively and with tongues 67, 68 and 82 in contact with the feet of apertures 69, 70 and 83. Six screws are then inserted and tightened without stressing any of the four sheets. It will be appreciated that further internal walls may be attached between two existing parallel walls using the same basic approach to provide a supporting structure for optical components which is light, rigid and unstressed and therefore not subject to hysteresis effects on temperature cycling. This light rigid structure is much easier to suspend within a thermally insulated enclosure. Any changes in structure temperature will now occur very slowly, all parts being at substantially the same temperature at any one time. All dimensions will therefore expand proportionately without generating bending or twisting moments in the whole structure.

Claims (16)

CLAIMS:

1. A device for varying the optical path length of a radiation beam comprising a plurality of plane mirrors rigidly connected together to form an assembly, means for directing an incident beam onto one mirror of the assembly, the mirrors being mounted so that the beam traverses the assembly by way of reflections from each of the mirrors and emerges from the assembly in a direction parallel to the direction of incidence on the one mirror, and means for rotating the assembly about an axis so as to change the angle of incidence of the beam on the mirrors and thereby vary the optical path length of the beam, characterised in that the mirrors are mounted in the assembly such that the emergent beam travels in the same direction as the incident beam.

2. A device as claimed in Claim 1, characterised in that a plane beam reversing mirror is provided in the path of and normal to the emergent beam, whereby the emergent beam retraces its path through the mirror assembly.

3. A device as claimed in Claim 1 or Claim 2, characterised in that the mirror assembly comprises a pair of parallel mirrors, the planes of the mirrors being separated from one another.

4. A device as claimed in Claim 3, characterised in that the rotation axis is substantially parallel to the mirror surfaces of the plane mirror pair and normal to the input radiation beam.

5. An interferometer comprising a beam splitter for dividing an input beam of radiation into two subbeams, first means for reflecting one subbeam back to the beam splitter along a path of fixed optical path length, and second means for reflecting the other subbeam back to the beam splitter along a path of variable optical path length to recombine with the one subbeam to form interference fringes, characterised in that the second means is provided by a device as claimed in any one of the preceding Claims.

6. An interferometer as claimed in Claim 5, characterised in that the mirror assembly additionally provides the optical path length variation device of a second interferometer, in that means are provided for switching the output beam of either interferometer onto an output path, and in that the beam splitters of the two interferometers function in different parts of the wavelength range of the input radiation.

7. An interferometer as claimed in Claim 6, characterised in that the input beams, subbeams and output beams of the two interferometers are parallel respectively to one another and in that the plane beam reversing mirrors are common to both interferometers.

8. A Fourier transform spectrometer comprising an interferometer for forming an interference fringe pattern in radiation to be spectrally analysed, a detector for receiving a selected portion of the fringe pattern, and means for processing the detector output signal as a function of variation in optical path difference of the subbeams to provide the spectrum of the radiation, characterised in that the interferometer is as claimed in Claim 5.

9. A Fourier transform spectrometer comprising an interferometer for forming an interference fringe pattern in radiation to be spectrally analysed, a detector for receiving a selected portion of the fringe pattern, and means for processing the detector output signal as a function of variation in optical path difference of the subbeams to provide the spectrum of the radiation, characterised in that the interferometer is as claimed in Claim 6.

10. A Fourier transform spectrometer as claimed in Claim 8 or Claim 9, characterised in that the beam splitter and the plane beam reversing mirrors are mounted in a supporting structure suspended within a thermally insulating enclosure.

11. A Fourier transform spectrometer as claimed in Claim 10, characterised in that the supporting structure comprises an assembly of metal sheets forming a rectangular box structure, each sheet being attached to other sheets of the assembly at points of kinematic contact with the other sheets and substantially at right angles to them at the points of contact.

1 2. A Fourier transform spectrometer as claimed in Claim 9, characterised in that the input beams, subbeams and output beams of the two spectrometers are parallel respectively to one another and in that the plane beam reversing mirrors are common to both spectrometers.

1 3. A device for varying the optical path length of a radiation beam substantially as described with reference to Figs. 1 to 5 of the accompanying drawings.

14. An interferometer substantially as described with reference to Fig. 6 of the accompanying drawings.

1 5. An interferometer substantially as described with reference to Fig. 7 of the accompanying drawings.

16. An interferometer substantially as described with reference to Figs. 6, 8, 9, 10 and 11 or to Figs. 7, 8, 9, 10 and 11 of the accompanying drawings.

1 7. A Fourier transform spectrometer substantially as described with reference to Fig. 7 or to Figs. 7 to 11 inclusive of the accompanying drawings.