Fabry-perot etalon and assembly therefor

Abstract

A Fabry-Perot etalon includes first and second mirrors (72) spaced from one another by a spacer element (70,80,100), which spacer element is provided with sides which provide support, respectively, for the first and second mirrors; an optical cavity (20) extending between the first and second mirrors (72) and having an optical axis; a plurality of cut-out surfaces (74-80) for supporting the spacer element (70,80,100) and being located symmetrically about the optical axis of the cavity (20); the cut-out surfaces (74-80) being concave cut-outs of the spacer element (70,80,100); wherein said cavity (20) is provided by a bore (18) within the spacer element; wherein a compressive force directed towards the centre of the cavity is applied at the support elements (74-80); and the support elements (74- 80) are positioned tetrahedrally about the spacer (80,90,100). The spacer may be in the shape of a cube, a rectangular cuboid or sphere or an oval spheroid. The concave cut-outs have a radius of curvature greater than a radius of curvature of the support elements (66) and are part spherical. The spacer structure is much more resistant to high inertial and vibratory forces. Another aspect provides an etalon with two pairs of partial mirrors (12) and evacuation elements through which the cavity may be evacuated even though mirrors overlay them. In another aspect spacer element is provided with a plurality of symmetry-balancing elements, preferably having a structure and dimensions the same as or very similar to the mirrors. There is also provided access through which one or more of the bores may be evacuated through mirrors or symmetry-balancing elements overlaying them.

Description

[0001] FABRY-PEROT ETALON AND ASSEMBLY THEREFOR

[0002] Field of the Invention

[0003] The present invention relates to a Fabry-Perot etalon and in particular to an etalon and assembly therefor which are insensitive to the effects of forces upon which the etalon may be subjected during use, in particular large inertial and differential forces.

[0004] In particular, preferred embodiments are directed to an etalon with improved insensitivities and corrections for the effects of vibration, acceleration and drift and to measure directionally-oriented parameters in three dimensions.

[0005] Background of the Invention

[0006] Optical cavities find wide application in atomic, optical and laser physics, telecommunications, astronomy, gravitational wave detection, and in tests of fundamental physics, where they are used as laser resonators, for power buildup, filtering, spectroscopy, and laser stabilization. In metrology, they are essential to the operation of optical atomic clocks, both enabling high- resolution spectroscopy and providing stability in the short-term during interrogation of the atomic transition. In this application, a laser is stabilized to a mode of a passive optical cavity consisting of two highly reflective mirrors contacted to an ultra-low-expansion ULE glass spacer. The frequency of the laser is defined by the optical length of the cavity, so for spectral purity, perturbations to this length must be minimized. To this end, the cavity is configured to be both highly isolated from its surroundings and designed to be insensitive to environmental fluctuations.

[0007] Several applications of optical clocks, including geodesy, tests of fundamental physics in space, and generation of ultra-stable microwaves for radar, demand that the supreme performance of an ultra-stable laser be available for use in a nonlaboratory environment with stabilities targeted at the 10-15level. For this purpose, the applicant has previously presented an etalon design based on a cubic geometry with a four-point tetrahedral support. The cavity is insensitive to the forces used to support it and has a maximum passive acceleration sensitivity of 2.5 x 10-11 / g.

[0008] For the cavity to be insensitive to inertial forces, there needs to be symmetry both in the geometry of the cavity and its mount, and in the forces acting at the supports. This ideal must be combined with the requirement for the cavity to be constrained in all degrees of freedom. The applicant’s preferred solution for a cubic spacer comprises four identical supports in a tetrahedral arrangement, which have been found to constitute a sufficient and symmetric constraint for a rigid body in three dimensions and their symmetrical arrangement with respect to an axis results in a cubic geometry. The optical axis passes through two opposing face centres of the cube and the supports are at four of the vertices of the cube, which also lie at the vertices of a tetrahedron. The vertices are truncated and the support surface of the supports is spherical. This aspect of the geometry ensures that the cavity, in addition to being constrained against displacement, is also constrained against rotation.

[0009] By symmetry, the cubic cavity is insensitive to inertial forces due to acceleration in all degrees of freedom (linear and rotational): axial displacements at the mirror centres are either zero or cancel out so that there is no net change of length on axis. A second-order sensitivity remains: that due to the inertial force arising from uniform rotation (centrifugal force). Using finite-element analysis, this is calculated, for a particular set of conditions, to be -5.3(+7.6) x 10-12s2for rotation about the optical axis (about axes perpendicular to the optical axis).

[0010] A compressive force, directed toward the centre of the cavity, is applied at each of the supports. Truncation of the cube vertices can nullify the effects of unsymmetrical distortions of the cavity caused by the compressive force or non- sym metrical spacer geometry.

[0011] Frictional forces at the supports mean that the preferred cavity is, in fact, constrained in 12 degrees of freedom, six of them corresponding to rigid-body motion arising from inertial forces and six corresponding to deformation arising from differential forces. For deformations where the supports are forced toward each other in any one of the directions defined by the edges of the cube, the length change on axis is nulled by virtue of the truncated geometry. For deformations where two supports are forced toward each other and two are forced away from each other, the axial displacements at the mirror centres are either zero or cancel out and there is no length change on axis. Thus, the cavity is insensitive to deformations arising from differential forces for all degrees of freedom.

[0012] In this embodiment, the cubic spacer is made from ultralow expansivity glass and preferably has an edge dimension of 50 mm. The vertices are preferably truncated to a depth of 6.7 mm toward the centre of the spacer. Three cylindrical bores of 5.1 mm diameter pass through the face centres of the cube and allow for evacuation of the cavity, although it will be appreciated that only one bore is required for evacuation in the spacer. The silica mirrors are 12.7 mm in diameter, 4.0 mm thick, and have a concave radius of curvature of 0.5 m. The skilled person will appreciate that these are indicative dimensions of the described embodiment and that in other embodiments one or more, or all, of these parameters may be altered.

[0013] The mirrors are optically contacted to the spacer and the cavity can have a finesse in excess of 300,000 dependent on the precise magnitude of the high reflectivity mirror coating. The cavity is housed within a support frame, which also has a cubic geometry, within a vacuum chamber. The direct cavity supports are made from compliant material such as nylon or PEEK and are hemispherical with a typical diameter of 12.7 mm. These are rigidly attached to posts mounted on the vertices of the cubical mounting frame within the chamber. The position of the cavity is placed symmetrically at the centre of the frame.

[0014] Laser light, for example from an Nd: YAG laser mounted on a separate platform, is transmitted to the cavity via an optical fibre and is frequency-locked to the cavity.

[0015] In summary, the applicant's earlier force-insensitive optical cavity has been designed for use in a nonlaboratory environment. A cubic geometry was chosen, to achieve the highest degree of symmetry between a four-point tetrahedral support and the optical axis. The cavity is constrained in all degrees of freedom and is insensitive to inertial forces acting in any direction. The vertices of the cube are truncated and this provides a means of nulling the sensitivity of the cavity to the compressive force by which it is held. In a robust experimental setup, the cavity operates in any orientation and a laser remains locked to it during inversion. The maximum acceleration sensitivity has been found to be 2.5 x 10-11 / g. This residual sensitivity is accounted for by error in the alignment of the cavity with respect to its supports or by error caused a small level of cavity geometry asymmetry.

[0016] The applicant’s earlier optical cavity has found considerable success due to its optical qualities and robustness of design. However, there are potential applications in which the cavity and its mounting arrangement could be subjected to very high forces and vibrations, with the risk that the cavity translates or rotates slightly across the supports as the level of vibration to which the unit is subjected is increased. This could cause malfunction of the etalon under such circumstances, with increased sensitivity and reduced performance, and potentially damage to the apparatus if the walk-off is large enough.

[0017] The applicant’s earlier design of Fabry-Perot etalon is disclosed in EP-2,638-418, the disclosure in which is incorporated herein by reference.

[0018] US-2002 / 005,988 and US-6,276,806 disclose other examples of prior art etalon and associated apparatus.

[0019] The article by David R Leibrandt et al entitled “Spherical reference cavities for frequency stabilization of lasers in non-laboratory environments” in Optics Express 3471, Vol. 19, No 4 (14 February 2011) discloses a spherical optical cavity.

[0020] Summary of the Present Invention

[0021] The present invention seeks to provide an improved Fabry-Perot etalon and apparatus therefor. The preferred embodiments seek to provide a structure of Fabry-Perot etalon which is insensitive or substantially insensitive to vibrational and acceleration forces to which the etalon may be subjected in practice. In the preferred embodiment, the Fabry-Perot etalon is designed such that its length, therefore frequency, is substantially insensitive to high inertial forces due to acceleration and / or vibration in all six degrees of freedom (three linear and three rotational) whilst at the same time being substantially insensitive to differential forces applied to the etalon. Additionally, the etalon of the present invention seeks to enable additional measurements and more importantly corrections due to vibration, acceleration and drift.

[0022] According to an aspect of the present invention, there is provided a Fabry- Perot etalon including: a spacer element; first and second mirrors spaced from one another by the spacer element, wherein the spacer element is provided with sides which provide support, respectively, for the first and second irrors; the spacer element comprising an optical cavity extending between the first and second mirrors and having an optical axis and a centre; and a plurality of cut-out surfaces located symmetrically about the optical axis of the cavity, the cut-out surfaces being concave cut-outs of the spacer element; wherein said optical cavity is provided by a bore within the spacer element; a plurality of support elements positioned about the spacer element and configured to apply a compressive force at respective cut-out surfaces of the spacer element directed towards the centre of the cavity.

[0023] This structure of optical spacer element, or cavity, enhances the insensitivity to differential forces applied to the etalon, particularly large forces that may be experienced in particular applications such as space applications and the like.

[0024] The arrangement of support elements can give the etalon insensitivity to inertial forces due to acceleration in all six degrees of freedom (three linear and three rotational). When combined with a spacer element as taught herein, the etalon is simultaneously insensitive to large inertial and differential forces.

[0025] Additionally, the concave vertices also ensure that, in the event of a large vibration, transient loss of contact of the support with the spacer vertex surface will be minimised, enabling the support point to hold its original position on the vertex surface. This will prevent possible walk-off of the support across the vertex surface as could be possible with flat vertices. These features are considered to provide an etalon with significantly better characteristics than known etalon assemblies.

[0026] Advantageously, the spacer is in the shape of a cube, but other shapes such as a rectangular cuboid or sphere or an oval spheroid may also be possible, provided the directions of the support forces are through the centre of the spacer. The support elements are preferably positioned tetrahedrally about the spacer and centred on the vertex concave surfaces along the diagonal lines through the centre of the cube.

[0027] There are preferably provided three bores in the spacer element, the bores extending orthogonally relative to one another through the centre-point of the spacer element and to the centre of respective faces of the spacer element.

[0028] The shape of each bore is most preferably cylindrical.

[0029] The etalon according may include eight supports each provided at a respective vertex cut-out of the spacer element.

[0030] The support elements preferably include spherical or part-spherical holding members.

[0031] The first and second mirrors have concave surfaces. The first and second mirror coating elements may be provided on mirror substrates made from silica or ultra-low expansion glass. There may be included an annulus on the rear flat surface of each mirror, the annulus preferably being made ultra-low expansion glass.

[0032] Preferably, the etalon can include up to three pairs of two mirrors contacted to the spacer to form three orthogonal cavities about the three bores.

[0033] In the preferred embodiments, the spacer vertices concave cut-outs have a radius of curvature greater than the radius of curvature of the support elements.

[0034] Advantageously, the cut-outs have an apex directed to a centre-point of the spacer. Preferably, all the cut-outs in the spacer are equivalently concave. Some embodiments may include one or more flat cut-outs in the spacer.

[0035] Preferably, the cut-outs have a concavity in the range of 50 -1000 mm ROC (radius of curvature), more preferably a concavity in the range of 200 mm ROC to 500 mm ROC.

[0036] According to another aspect of the present invention, there is provided a structure comprising a plurality of Fabry-Perot etalons including:

[0037] a spacer element;

[0038] three pairs of facing first and second mirrors spaced from one another by the spacer element, which spacer element is provided with sides which provide support, respectively, for the three pairs of first and second mirrors; optical cavities in the spacer element extending between the three pairs of first and second mirrors and having three orthogonal optical axes extending along the first, second and third bores within the spacer element, the bores crossing at a central point of the spacer, each pair of first and second mirrors being associated with a respective bore with the first and second mirrors disposed at opposing ends of the respective bore;

[0039] at least one evacuation element connecting the cavities to an external surface of the spacer;

[0040] a plurality of cut-out surfaces for supporting the spacer element and being located symmetrically about the optical axes of the cavity; and

[0041] support elements positioned about the spacer and configured to apply thereto a compressive force at the cut-out surfaces directed towards the centre of the cavities.

[0042] The bores are preferably orthogonal to one another.

[0043] The provision of a cavity with three orthogonal bores and associated pairs of reflective mirrors provides three orthogonally disposed optical cavities, which can allow further measurements and also enable the calculation of corrections due to vibration and acceleration, for example by determining differential measures from the three cavities.

[0044] Advantageously, the bores are circular in transverse cross-section and most preferably cylindrical. They bores may be identical in cross-section, and preferably, identical in length.

[0045] In the preferred embodiments, the mirrors are identical to one another. In other embodiments, for example in cases where the spacer is asymmetrical in at least one dimension, the mirrors may differ in size and / or mass.

[0046] The mirrors are preferably formed as structures disposed on and outside of the spacer. In the preferred embodiments, the mirrors are formed of a plurality of thin reflective coatings in a multi-layered arrangement, upon a mirror substrate element.

[0047] At least one evacuation element may be a bore access element disposed at one of more ends of a bore and which at an exterior surface of the spacer beyond a perimeter of an associated mirror. In the preferred embodiments, the access element is formed by an extended bore end at the exterior surface. The extended bore end is advantageously in the shape of a slot over which the mirror is disposed, and in which at least one end of the slot extends beyond a zone or perimeter of the mirror. Most preferably, the slot is centrally positioned under the mirror and both ends of the slot extend beyond the zone or perimeter of the mirror.

[0048] The access element or slot allows evacuation of the bore under the mirror outer annulus. In some embodiments, there are provided access elements or slots on opposite spacer faces, thereby maintaining symmetry in at least one axis.

[0049] Preferably, there are provided access elements at a plurality of bore ends and most preferably at every bore end. The access elements, when in the form of slots, may be disposed orthogonally to one another.

[0050] In another embodiment, at least one evacuation element may comprise one or more bore side branches extending from a respective bore to a surface of the spacer beyond a perimeter of the mirror. There are preferably provided a plurality of such bore side branches, advantageously disposed in a symmetrical arrangement in the spacer. More preferably, every bore is provided with one or more side branches.

[0051] In another embodiment, the at least one evacuation element may comprise one or more evacuation bores extending from the cavity crossing point to a surface of the spacer. In the preferred embodiments, the or each evacuation bore extends to a truncated surface of the spacer not used for support purposes.

[0052] Advantageously, the support elements are positioned tetrahedrally about the spacer and are configured to have applied thereto a compressive force directed towards the centre of the cavity.

[0053] Advantageously, the spacer is in the shape of a cube, a rectangular cuboid, sphere or an oval spheroid.

[0054] The first and second mirrors are advantageously curved. The first and second mirror elements may be provided on mirror substrates made from silica or ultra-low-expansion (ULE) glass. There may be included an annulus between each of mirror and their associated side of the spacer element, the annuli preferably being made of silica or ultra-low expansion glass. It is to be understood that if the spacer and mirror substrate are both made of ultra-low expansion glass, there is no need for a fused silica annulus. If the mirror substrate is fused silica, then a annulus in front, made of ultra-low expansion glass, is advantageous.

[0055] According to yet another aspect of the present invention, there is provided a Fabry-Perot etalon including:

[0056] a spacer element;

[0057] at least one pair of facing first and second mirrors spaced from one another by the spacer element, the spacer element being provided with sides which provide support, respectively, for the first and second mirrors;

[0058] an optical cavity in the spacer element extending between the first and second mirrors and having an optical axis; the optical cavity comprising a first bore within the spacer element;

[0059] the spacer element including a second bore, wherein the first and second bores extend along orthogonal axes;

[0060] at least one pair effacing symmetry-balancing elements having a mass substantially the same or the same as a mass of the first and second mirrors, the symmetry-balancing elements being disposed at and supported by sides of the spacer element adjacent ends of the second bore;

[0061] a plurality of cut-out surfaces in the spacer element for supporting the spacer element and being located symmetrically about the optical axis of the cavity; and

[0062] support elements positioned about the spacer and configured to apply thereto a compressive force at the cut-out surfaces directed towards the centre of the cavity.

[0063] The provision of the first and second mirrors alters the symmetry of the spacer and, it has been found, can lead to non-uniform ities particularly in high vibration and / or acceleration environments, as well as for applications requiring very low sensitivity. The provision of symmetry-balancing elements having masses substantially the same as the masses of the mirrors provides precise symmetry, in the preferred embodiments in all orthogonal directions of the spacer and associated cavity. In the preferred embodiment, the spacer element includes a third bore, with the first, second and third bores extending along orthogonal axes; and comprising a second pair of facing mirrors or a second pair of symmetry-balancing elements disposed at respective ends of the third bore.

[0064] The provision of a third bore in the spacer provides bores and associated mirrors or symmetry-balancing elements in all three orthogonal axes, thereby providing precise symmetry in every orthogonal axis.

[0065] The mirror coatings are preferably formed as layers disposed on the mirror substrates and outside of the spacer, and the symmetry-balancing elements have the same or substantially the same structures as the mirrors, excluding the coatings.

[0066] Most preferably, the mirrors are formed of a plurality of components in layered arrangement, at least one of the components being a mirror substrate, the symmetry-balancing elements being made of the same or similar components as the mirrors with the omission of the mirror coatings. In these preferred embodiments, the symmetry-balancing elements have the same structure and are formed of the same materials, save for the mirror coatings, and can therefore have characteristics equivalent to those of the mirrors. They could be said to be pseudo mirrors.

[0067] Advantageously, the symmetry-balancing elements are disposed over the ends of an orthogonal bore of the spacer not used for light transmission, thereby to have dispositions and characteristics as close as possible to the mirror pair(s) and associated spacer bore(s).

[0068] The symmetry-balancing elements in practical embodiments can be formed of uncoated mirror substrates, which coating normally provides the reflective mirror function.

[0069] In some embodiments, one or more of the symmetry-balancing elements is provided with an aperture therein, passing through to an associated bore which the symmetry-balancing element overlies, the aperture allowing for passage of gas to and / or from the bore.

[0070] Preferably, the aperture is disposed centrally to the associated bore.

[0071] Advantageously, the aperture is circular in cross-section. In another embodiment the spacer is provided with a bore access element at least at one end of the bore, which access element is exposed at an exterior surface of the spacer beyond a perimeter of at least one symmetry-balancing element or mirror. Preferably, the access element is formed by an enlarged bore end at the exterior surface. More preferably, the enlarged bore end is in the shape of a slot over which the symmetry-balancing element or mirror is disposed, and wherein at least one end of the slot extends beyond a zone or perimeter of the symmetry-balancing element or mirror. Most preferably, the slot is centrally positioned across the symmetry-balancing element or mirror and both ends of the slot extend beyond the zone or perimeter of the symmetry-balancing element or mirror.

[0072] The access element or slot allows evacuation of the bore under the symmetry-balancing element or mirror outer annulus. In some embodiments, there are provided access elements or slots on opposite spacer faces, thereby maintaining symmetry in at least one axis.

[0073] Advantageously, the support elements are positioned tetrahedrally about the spacer and are configured to have applied thereto a compressive force directed towards the centre of the cavity.

[0074] In preferred embodiments, the spacer has three orthogonal bores and three pairs of access elements or slots at the bore ends, wherein the pairs of access elements or slots are arranged in orthogonal axes.

[0075] In embodiments where one or more access elements or slots are provided, these may be disposed under a mirror or a symmetry-balancing element, or both. In the embodiments with an access element or slot, it is not necessary to have an aperture in one or more of the symmetry-balancing elements.

[0076] In this aspect, the invention provides a symmetry-balancing element pair to reduce asymmetry introduced by one or two axis cavity mirror sets in order to provide optimal vibration insensitivity.

[0077] Advantageously, the spacer is in the shape of a cube, a rectangular cuboid, sphere or an oval spheroid. There are preferably provided three bores in the spacer element, the bores extending orthogonally relative to one another through a centre-point of the spacer element and to the centre of respective faces of the spacer element.

[0078] The first and second mirrors are advantageously curved. The first and second mirror coatings may be provided on mirror substrates made from silica or ultra-low-expansion (ULE) glass. There may be included an annulus between each of mirror and their associated side of the spacer element, the annuli preferably being made of ultra-low-expansion ULE glass.

[0079] It will be appreciated that the aspect providing the symmetry balancing elements may also have a spacer with concave cut-out truncated edges as described in connection with the first disclosed aspect.

[0080] Other aspects and advantages of the invention taught herein will become apparent to the skilled person from the specific description that follows.

[0081] Brief Description of the Drawings

[0082] Embodiments of the present invention are described below, byway of example only, with reference to the accompanying drawings, in which:

[0083] Figures 1 to 3 show, respectively, a front elevational view, a plan view and a perspective view of an example etalon provided with a cuboidal spacer;

[0084] Figure 4 shows a transverse cross-sectional view of the etalon of Figures 1 to 3;

[0085] Figures 5 and 6 are respectively perspective and front elevational views of a finite-element model for a geometry representing a truncated cuboidal spacer of etalon, showing axial displacement under the action of a compressive force applied at four support elements provided tetrahedrally about the cube;

[0086] Figure 7 is a side elevational view of the original etalon of Figures 1 to 6 held by two of the tetrahedral supports (front) and two supports (back) fixed directly to a sample vacuum chamber (that is, without an intermediate cubic mounting frame);

[0087] Figure 8 is a side elevational diagram of a cubic spacer for an etalon according to the teachings herein; Figure 9 is a schematic diagram of a cubic spacer for an etalon according to the teachings herein;

[0088] Figure 10 is a schematic diagram of a rectangular cuboidal spacer for an etalon according to the teachings herein

[0089] Figure 11 is a schematic diagram of a spherical spacer for an etalon according to the teachings herein;

[0090] Figure 12 is a cross-sectional schematic view of an example support element for the etalon of any one of Figures 8 to 11;

[0091] Figures 13 to 15 show, respectively, a front elevational view, a plan view and a perspective view of an embodiment of etalon provided with a cuboidal spacer according to another aspect of the present invention;

[0092] Figure 16 shows a transverse cross-sectional view of the etalon of Figures 13 to 15;

[0093] Figures 17 and 18 are respectively perspective and front elevational views of a finite-element model for a geometry representing a truncated cuboidal spacer of etalon, showing axial displacement under the action of a compressive force applied at four support elements provided tetrahedrally about the cube;

[0094] Figure 19 is a side elevational view of the etalon of Figures 13 to 18 held by two of the tetrahedral supports (front) and two supports (back) fixed directly to a sample vacuum chamber (that is, without an intermediate cubic mounting frame);

[0095] Figure 20 is a side elevational view of a spacer of a Fabry-Perot etalon having a rectangular cuboidal shape;

[0096] Figure 21 is a schematic diagram of a spacer of a Fabry-Perot etalon having a spherical shape;

[0097] Figure 22 is a schematic plan view of a preferred embodiment of bore structure allowing evacuation of the bore in the presence of a mirror assembly over the bore;

[0098] Figures 23 to 26 are further schematic views of the embodiment spacer shown in Figure 22;

[0099] Figures 27 to 30 are schematic views similar to Figures 23 to 26, showing another embodiment of cavity evacuation elements; Figures 31 to 34 are schematic views similar to Figures 23 to 26, showing another embodiment of cavity evacuation elements;

[0100] Figures 35 to 37 show, respectively, a front elevational view, a plan view and a perspective view of an embodiment of etalon provided with a cuboidal spacer according to a third aspect of the present invention;

[0101] Figure 38 shows a transverse cross-sectional view of the etalon of Figures 35 to 37;

[0102] Figures 39 and 40 are respectively perspective and front elevational views of a finite-element model for a geometry representing a truncated cuboidal spacer of etalon, showing axial displacement under the action of a compressive force applied at four support elements provided tetrahedrally about the cube;

[0103] Figure 41 is a side elevational view of the etalon of Figures 35 to 40 held by two of the tetrahedral supports (front) and two supports (back) fixed directly to a sample vacuum chamber (that is, without an intermediate cubic mounting frame);

[0104] Figure 42 is a side elevational view of a spacer of a Fabry-Perot etalon having a rectangular cuboidal shape;

[0105] Figure 43 is a schematic diagram of a spacer of a Fabry-Perot etalon having a spherical shape;

[0106] Figure 44 is a schematic plan view of a preferred embodiment of bore structure allowing evacuation of the bore in the presence of a mirror assembly over the bore;

[0107] Figures 45 to 48 are further schematic views of the embodiment spacer shown in Figure 44;

[0108] Figures 49 to 52 are schematic views similar to Figures 45 to 48, showing another embodiment of cavity evacuation elements; and

[0109] Figures 53 to 56 are schematic views similar to Figures 45 to 48, showing another embodiment of cavity evacuation elements.

[0110] Description of the Preferred Embodiments

[0111] The general form of the earlier etalon spacer design can be appreciated from the example shown in Figures 1 to 6, which depict the structure disclosed by the applicant in its European patent number EP-2,638-418. The embodiments of spacers disclosed herein represent improvements over the applicant’s earlier design of spacer.

[0112] Referring to Figures 1 to 4, the example etalon 10 comprises a pair of reflective mirrors 12, 14 attached to a cuboidal spacer 16, which has truncated flat vertices 25. Figures 1 and 2 are, respectively, projections in the y-z and x-y planes respectively, while Figure 3 shows the geometry of the cubic etalon in perspective view showing 7 of the 8 truncated flat vertices, of which 4 vertices are used to support the etalon in tetrahedral configuration. Figure 4 is a cross-section through the plane z = 0. In Figures 1 to 3, the open (solid) circles indicate the positions of the vertex support points 22, 24, (26, 28) which are visible in (obscured by) the projection.

[0113] In practical embodiments, the mirrors 12, 14 may be partially reflective mirrors, as is known in the art. This applies to all embodiments of the invention.

[0114] The spacer 16 has in this example three cylindrical bores 18 which extend orthogonally relative to one another and pass through the centre-point of the cube to the centre-points of their respective faces. One of the bores is visible clearly in the cross-sectional view, Figure 4. The mirrors 12, 14 are bonded, on axis, to opposite ends of one of the bores 18. The bores 18 provide a void 20 in the spacer element which allow the propagation of light between the mirrors. For this purpose, the bores are preferably circular in transverse cross-section and cylindrical.

[0115] Mirrors 12, 14 may be bonded to opposite ends of one or more of the bores to form a plurality of optical, orthogonal, cavities 20.

[0116] The spacer 16 is advantageously made from ultra-low expansivity glass (ULE) and in this example has an edge dimension of 50 mm in this case. The two mirrors 12, 14 are fused-silica mirrors and are optically contacted at opposite ends of the bore 18 which lies along the x-axis. The mirror coating which lies on a typically concave surface of the mirror substrate is typically formed by alternate multiple layers of different refractive index material giving an overall resultant reflectivity. The vertices of the cube are truncated (as shown by reference numeral 25) in this example to a depth of 6.7 mm forming eight plane surfaces normal to the body diagonals of the cube. The mirrors 12, 14 have the following dimensions in this example: outer diameter = 12.7 mm; thickness = 4.0 mm; radius of curvature of concave surface = 500 mm; inner diameter of contacting annulus = 8.5 mm. It is to be appreciated that these dimensions are illustrative and not necessarily unique.

[0117] The mirrors 12 and 14 are typically curved, preferably spherically curved. An optical cavity is formed between the inner surfaces of the mirrors 12, 14 and the frequencies of the optical cavity modes are given by:

[0118]

[0119] where c is the speed of light, is the distance in vacuum between the points at the centres of the two mirror surfaces and N is an integer. Thus the frequency of a cavity mode is defined by the length of the cavity.

[0120] It will be appreciated that the distances between the centres of the mirror surfaces which are formed by the three pairs of mirrors 12 can be approximated as an integer number x the half-wavelength of the frequency of the light resonant within the cavity.

[0121] In this example, four supports 22, 24, 26, 28 arranged in a tetrahedral configuration constrain the cuboidal spacer 16 in all degrees of freedom and the optical axis is placed symmetrically with respect to these supports. In Figures 1 to 3, the open (solid) circles indicate the positions of the supports 22, 24, (26, 28) which are visible in (obscured by) the projection.

[0122] An inertial force due to acceleration acting on the etalon 10 will cause this to undergo elastic deformation. However, due to the high degree of symmetry, the length, and hence frequency, of the cavity mode is substantially insensitive to inertial forces due to acceleration in all degrees of freedom (linear and rotational).

[0123] A second-order sensitivity remains: that due to the inertial force arising from uniform rotation (centrifugal force). Using finite-element analysis, the fractional sensitivity is calculated to be -5.3(+7.6) x 10−12s2for rotation about the optical axis (about axes perpendicular to the optical axis).

[0124] More specifically, an inertial force acting on the etalon 10 causes it to undergo elastic deformation and the points at the centres of the mirrors 12, 14, which define the optical cavity length, are displaced. The modified cavity length is then:

[0125] / ' = I ~u2T / ) T(VS~ v,) ~ ) I,

[0126]

[0127] where u, v and w are the displacements in the x, y, and z direction respectively at the two mirror centres, denoted by the subscripts 1 and 2. The fractional change in length or frequency is given by:

[0128] AZ V r-z

[0129] Z ~ v ~ Z

[0130] For the symmetrical arrangement shown in Figures 1 to 4, Table 1 below gives the displacements under the six possible inertial forces due to acceleration that can act on the etalon 10 and the corresponding and fractional length changes. The quantities a~e denote displacements and are used to show the symmetries present. The column, Al / I, uses the leading term in the Taylor expansion of I'. In describing the direction of the inertial force acting upon the etalon 10, the following terms are used: axial - along x; transverse - along y, z: roll - rotation about x; pitch - rotation about y; yaw - rotation about z. Due to symmetry, the two transverse directions are identical, and pitch and yaw are identical. The terms In brackets (Tx, Ty, etc.) are the labels given to the degrees of freedom as they are shown in Figures 7 and 8. Table 1

[0131] j inertial force J" Hi fo I I Wi j~ L j

[0132] | axifo ( 1; j ( / a 0 1 0 1 0 i 0 i o i i transverse (.7 -J | 0 IL - b \ 1 - 5 11 - c | A- -. e | } - < - 11

[0133] o:;?;? ■ 9 1

[0134] 1 transverse ifo i 0 c:. e | b [

[0135] LL 0 ] 0 | 0~| 0 | 0 |

[0136] | pi id) ( 7fo) j 0 0 d • d! i e |! - r 1 j L: i:

[0137]

[0138] | yaw ( / A j | 0 0 c.. I i -e.. | d. / j I a 7! j y 2<ir- i j force / torque < axia? yq <> 0 0 1 0 i. 0 transverse foi 0 G | If fi L 4- PfoT

[0139] rifo (about a:) o 0 1 0 i) 0 J 0 0 pitch (about fo 0 0 I (7 d £: — f:< 1 + rd

[0140] yaw (about c) o 0 i e d j d L 1 + ( v )

[0141]

[0142] Axial force and roll result in no length change. Transverse force, pitch and roll result in equal and opposite transverse displacements at either end of the axis.

[0143] The fractional length change is proportional to the square of this transverse displacement. The sizes of c and e depend on the material properties and absolute dimensions of the etalon. For an etalon made from ULE (ultra-low expansivity glass; Young's modulus, 67.6 GPa; Poisson ratio, 0.17; density, 2.21 g / cm3) with an edge length of 50 mm, bore radius of 5 mm, mirror radius of 6.35 mm, mirror thickness of 4 mm and truncation at the cube vertices to a depth of 6.7 mm, a finite-element model for a spacer with flat vertices gives a fractional sensitivity to acceleration (transverse) of order 10“18 / ms“2and a fractional sensitivity to angular acceleration (pitch / yaw) of order 10'19 / s“2.

[0144] To constrain the etalon 10, a compressive force, directed towards the centre of the cavity, is applied at the supports 22, 24, 26, 28 and again this results in elastic deformation. In general, this causes a change in the length and frequency of the cavity mode, and one is then sensitive to changes in this applied force. In order to provide insensitivity against such deformation, the cuboidal spacer 16 is truncated at its vertices, the result being a structure in which the length of the cavity mode is made substantially insensitive to compression while retaining its insensitivity to inertial force. The principle is illustrated in Figures 5 and 6 described below.

[0145] Figures 1 to 4 show the geometry of the etalon 10 with its cuboidal spacer 16 supported at four points 22, 24, 26, 28 in a tetrahedral configuration. The supports 22, 24, 26, 28 are centred on the four vertex planes having normal vectors (1,1,1), ( 1, -1, -1 ), (-1, 1,-1 ) and (-1,-1,1) and the centres of contact form the vertices of a tetrahedron. This is the most symmetric arrangement for the four vertices of a tetrahedron with respect to the axis, which provides sufficient and symmetric constraint in three dimensions and with the forces acting on the four points 22, 24, 26, 28 being equal. In other embodiments, the spacer may be held at six or eight points. Other embodiments, of which an example is given below, may be supported at only two locations of the spacer.

[0146] For a cube with an edge dimension of 50 mm, the dimensions are preferably as follows: bore radius, 5 mm; mirror radius, 6.35 mm; mirror thickness, 4 mm; cube vertices truncated flat to a depth of 6.7 mm.

[0147] Referring now to Figures 5 and 6, these depict, respectively, perspective and front elevational views of a finite-element model of a geometry representative of a truncated cubic spacer element. They show axial displacement on application of a compressive force, directed towards the centre of the cavity, at the support points 22, 24, 26, 28. The zones 36 indicate zones of positive axial displacement; zones 38 indicate zones of negative axial displacement; while zones 40 are zones with zero axial displacement. The colour scale in zones 38 is truncated so that at the supports, where the displacement is largest, the plot is absent and one sees through to the other side of the cube. For the geometry shown, with the vertices cut to a depth of 6.7 mm, the axial displacements and length change on axis are zero. Thus, the etalon 10 is insensitive to inertial forces due to acceleration in all six degrees of freedom whilst simultaneously being substantially insensitive to a compressive force directed towards the centre of the cavity.

[0148] In a practical realisation, frictional forces act at the supports and this means that the cavity is, in fact, constrained in 12 degrees of freedom: six corresponding to rigid-body motion arising from inertial forces and six corresponding to deformation arising from differential forces. For deformations where the supports are forced towards each other in any one of the directions defined by the edges of the cube, the length change on axis is nulled by virtue of the truncated geometry. For deformations where two supports are forced towards each other and two are forced away from each other, the axial displacements at the mirror centres are either zero or cancel out and there is no net length change on axis. Thus, the cavity is insensitive to differential forces in all degrees of freedom.

[0149] Table 2 below gives the displacements for the six degrees of freedom corresponding to differential forces that can act on the etalon 10 and the corresponding fractional length changes. The quantities f-i denote displacements and are used to show the symmetries present. The column, Al / I, uses the leading term in the Taylor expansion of I'. The terms in brackets (Di, D2, etc.) are the labels given to the degrees of freedom as they are shown in Figures 7 and 8.

[0150] Table 2

[0151] Ar

[0152] i differential force (J* U2*1v2 w2

[0153] I axiai pinch f Q 0 0 0 2

[0154] 2<?

[0155] | transverse pinch (O2) g -9 0 0 0 0

[0156] I transverse pinch(O5) g -g 0 0 0 0.--rrrw

[0157] | transverse deformation (O4) h h 0 0 0 0 0

[0158] I axiai deformation (O5) 0 0 1 1 0 0 0

[0159] | axiai deformation (De) 0 0 0 0 i i 0

[0160]

[0161] n general, the deformations, Di-Ds, result in equal and opposite axial displacements at either end of the axis and the fractional length change is proportional to this displacement. However, for the geometry shown in Figures 5 and 6, for which the axial displacements are zero under the action of a compressive force acting towards the centre of the cavity, the displacements under deformations, Di-Ds, are also zero. The deformations, D1-D3, result in no length change.

[0162] Compression is, in fact, a special case of differential force and is equal to a sum of the deformations acting along the three orthogonal axes: =DI+D2+DS. It therefore follows that, as the axial displacement is zero for the deformations, Di-D3, for the particular geometry shown in Figures 5 and 6, the axial displacement under the action of a compressive force acting towards the centre of the cavity is also zero.

[0163] In an embodiment in which three orthogonal cavities are formed by bonding mirrors to opposite ends of all three bores, a depth of cut at the vertices can be found for which all three cavities are simultaneously insensitive to inertial forces, due to acceleration, and differential forces, in all degrees of freedom.

[0164] Referring now to Figure 7, this shows an example of a vacuum chamber 50 within which the etalon spacer 10 is disposed. The chamber 50, of known form, includes a housing or frame 60 provided with first set of port assemblies 64 located at the non-supported apices of the cubic spacer 10 and a second set of port assemblies 68 located at the apices of the cubic cavity which are supported. These assemblies 68 include four compliant hemispherical locators or support elements 66 for supporting an apex of the cubic cavity 10 in the manner described above and which are discussed further below.

[0165] As explained above, the etalon cavity 10 is held in balanced manner in the vacuum chamber 50 and so as to be substantially insensitive to external forces.

[0166] While the example of cubic spacer 10 described above is suitable for many applications, it can suffer from potential movement between the cavity 10 and its mounting frame 60 when subjected to mechanical vibration / shock during use in mobile situations in ground, marine aerospace and space environments, particularly where vibration and / or acceleration forces can be very high. More specifically, the existing spacer comprises flat surface cut-outs at four of the eight vertices in a tetrahedral arrangement against which mounting forces are symmetrically applied via four compliant hemispherical locators, acting on the central points of the cut-outs which define the cube diagonals, to hold the cavity cube 10 in place. There is a potential danger that the cube 10 might rotate or translate across the cut-outs (i.e. walk-off) as the vibration or acceleration level to which the unit is subjected is increased, particularly in extreme situations such as at launch into space. Walk-off due to high vibration levels can affect vibration insensitivity, loss of alignment and loss of mounting capability.

[0167] Referring now to Figures 8 and 9, these show an embodiment of cubic cavity etalon according to the present invention. The cubic cavity spacer 70 has a general form the same as that of the examples of Figures 1 to 6 described above, with opposing mirrors 72, of which only one is visible in the view of Figure 8. The vertices 74-80 of the spacer 70 are, instead of being flat as with the examples of Figures 1 to 6, concave. It will be appreciated that only the vertices 74-80 are visible in this Figures but in practice all the vertices preferably have the same shape.

[0168] Preferably, the vertex cut-outs 74-80 are concave with larger radii of curvature than the hemispherical locators 66, which it has been found can provide restoring surface contours able to bias the locators 66 so as to maintain their central location points under increased vibration and prevent walk-off.

[0169] The concave contours of the cut-outs 74-80 are in the preferred form of the embodiment shown in Figures 8 and 9 part spherical and having an apex directed precisely to the centre-point of the cubic spacer 70. In other words, the centrepoint of the sphere of which each concave cut-out forms a part (see circle 77 shown in dotted outline in Figure 8) is located along a line (see line 79 shown in dotted outline in Figure 8) passing from the centre of the cut-out to the centre of its diametrically opposed cut-out of the cubic spacer 70. Opposing concave cut-outs are diametrically opposed and facing, thus maintaining symmetry in the spacer 70. While not all eight vertices of the cubic spacer 70 may have associated locators 66, it is preferred that all eight vertices are equivalently concave. This ensures precise symmetry of the cubic spacer in all directions. It is not excluded, though, that in other embodiments only those vertices 74-80 which have associated locators 66 are formed with concave vertex surfaces, while the other vertices could have flat truncated edges, similar to the example of Figures 1 to 6.

[0170] The concavity of the concave cut-outs 74-80 may be in the range of 50 - 1000 mm ROC (radius of curvature), although a range of 200 mm ROC minimum to 500 mm ROC maximum is believed to be optimal. Length change is virtually constant at 200 mm ROC and above.

[0171] The inventors have discovered that above 500 mm ROC, the efficacy of the vertex concave curvature in preventing walk-off gradually reduces, because in practical terms the ROC begins to take the characteristics of a flat vertex.

[0172] It will be understood that these dimensions are suited to a cubic spacer having the dimensions given above. While for cubic spacers having of other sizes may benefit from concave cut-outs of different radii of curvature, it has been found that the above-mentioned radii of curvature are typically optimal for all practical sizes of etalon spacers.

[0173] The spacer 70 of Figures 8 and 9 has all the other features of the example of spacer 10 shown in Figures 1 to 7 and described above, so these will not be repeated here.

[0174] Referring now to Figure 10, this shows another embodiment of spacer falling within the scope of the invention. The spacer 90 of this embodiment, instead of being cubic as in the preceding examples and embodiment, is a rectangular cuboid shape, in other words is longer in one dimension relative to its size in the other orthogonal directions. The embodiment of Figure 10 is square in transverse cross-section but in other embodiments it may be rectangular in every dimensional cross-section (that is in the x, y and z planes).

[0175] Save for its rectangular cuboidal shape, the spacer 90 has a general form the same as that of the examples of Figures 1 to 6 described above, and the embodiment of Figures 8 and 9, with opposing mirrors 72, of which only one is visible in the view of Figure 10. The vertices 74’-80’ of the spacer 90 are, instead of being flat as with the examples of Figures 1 to 6, concave.

[0176] As with the previously described embodiment, the vertex cut-outs 74;-80’ are advantageously concave with larger radii of curvature than the hemispherical locators 66.

[0177] The concave contours of the cut-outs 74'-80’ are in the preferred form of the embodiment shown in Figure 10 part spherical and have an apex directed precisely to the centre-point of the rectangular cuboidal spacer 90. In other words, the centre-point of the circle of which each concave cut-out forms a part is located along a line passing from the circle centre-point to the centre of the rectangular cuboidal spacer 90. Opposing concave cut-outs are diametrically opposed and facing, thus maintaining symmetry in the spacer 90.

[0178] It will be appreciated that with concavities of this nature, because of the rectangular form of the spacer 90, the part-circular shape of the concavities will be uneven or non-symmetrical across the apex when viewed perpendicularly to the apex.

[0179] It is envisaged that in other embodiments, the symmetry of the concavity may be different, for instance to be centred to an imaginary cube rather than the centre of the rectangle (indicated by the dotted line 96 and imaginary centre point 94 in Figure 10).

[0180] As with the embodiment of Figures 8 and 9, while not all eight vertices of the rectangular cuboidal spacer 90 may have associated locators 66, it is preferred that all eight vertices are equivalently concave. This ensures precise symmetry of the spacer 90 in all directions. It is not excluded, though, that in other embodiments only those vertices 74’~80’ which have associated locators 66 are formed with concave vertex surfaces, while the other vertices could have flat truncated edges, similar to the example of Figures 1 to 6.

[0181] The concavity of the concave cut-outs 74’-80’ may be in the range of 50 - 1000 mm ROC (radius of curvature), although a range of 200 mm ROC minimum to 500 mm ROC maximum is believed to be optimal. Length change is virtually constant at 200 mm ROC and above.

[0182] The spacer 90 of Figure 10 has all the other features of the example of spacer 10 shown in Figures 1 to 6 and embodiment of Figures 8 and 9, and described above, so these will not be repeated here.

[0183] Referring now to Figure 11, this shows another embodiment of spacer falling within the scope of the invention. The spacer 100 of this embodiment, instead of being cubic or cuboidal as in the preceding examples and embodiments, is spherical in shape.

[0184] Save for its spherical shape, the cavity spacer 100 has a general form the same as that of the examples of Figures 1 to 6 described above, and the embodiments of Figures 8 to 10, with opposing mirrors 72, of which only one is visible in the view of Figure 11. The spacer 100 is, in this embodiment, provided with two diametrically opposing support elements 74”, 76” which, instead of being flat as with the examples of Figures 1 to 6, are also concave. Other embodiments may comprise more than a pair of support elements, for example with two or three pairs of facing support elements, with each pair being aligned orthogonally to the other pair(s). It is not excluded that there may be more than 6 support elements and concave spacer surfaces in some instances.

[0185] As with the previously described embodiments, the vertex cut-outs 74", 76” are advantageously concave with larger radii of curvature than the hemispherical locators 66.

[0186] The concave contours of the cut-outs 74”, 76” are in the preferred form of the embodiment shown in Figure 11 part spherical and have an apex directed precisely to the centre-point of the spherical spacer 100. In other words, the centre-point of the circle of which each concave cut-out forms a part is located along a line passing from the circle centre-point to the centre of the spherical spacer 100. Opposing concave cut-outs are diametrically opposed and facing, thus maintaining symmetry in the spacer 100.

[0187] As with the embodiments of Figures 8 to 10, while not all eight cut-outs of the spherical spacer 100 may have associated locators 66, it is preferred that all eight cut-outs 74”, 76” are equivalently concave. This ensures precise symmetry of the spherical spacer in all directions. In practice, the spacers may be supported in two opposing, four opposing or six opposing arrangements, that is in opposing pairs orthogonal to one another.

[0188] It is envisaged that in other embodiments only those vertices 74”, 76" which have associated locators 66 are formed with concave cut-out surfaces, while the other vertices could have flat cut-outs, similar to the example of Figures 1 to 6.

[0189] The concavity of the concave cut-outs 74”, 76” may be in the range of 50 - 1000 mm ROC (radius of curvature), although a range of 200 mm ROC minimum to 500 mm ROC maximum is believed to be optimal. Length change is virtually constant at 200 mm ROC and above. The spacer 100 of Figure 11 has all the other features of the example of spacer 10 shown in Figures 1 to 7 and embodiments of Figures 8 to 10, and described above, so these will not be repeated here.

[0190] Another embodiment, not shown, has a spacer in the form of an oval spheroid rather than a sphere. The elements of such a spacer are the same or equivalent as those of the previously described embodiments so are not repeated here.

[0191] Referring now to Figure 12, there is shown a preferred embodiment of support 66 for the etalon 80,90,100 taught herein. As is disclosed above, the etalon 80,90,100 is preferably provided with four supports of the type shown in Figure 12. Each cube vertex support area 74-80 includes a support 66 having a cylindrical shaft 110 and hemispherical tip 120 held in position by a conical housing (not shown) biased to apply pressure on the support 66 in a direction towards the centre-point of the etalon 80,90,100.

[0192] The supports 74-80 constrain motion in all six degrees of freedom (displacement and rotation). The etalon 80,90,100 is thus mounted on four supports 66. A compressive force directed towards the centre of the cavity 80, 90, 100 is applied at all the supports 74-80 constrains displacement.

[0193] Figures 13 to 34 are directed to another embodiment of the invention, in which three sets of orthogonal mirror pairs are disposed on the spacer element and provision is made for evacuation of the spacer cavity. The embodiments have a spacer with flat truncated support surfaces, this being one implementation of the embodiments according to this aspect of the invention. It is to be understood, however, that the embodiments relating to this aspect may instead be provided with concave truncated edge supports or cut-out surfaces (74-80) as per the embodiments of Figures 1 to 12. It is to be understood that the features of the various embodiments disclosed in Figures 1 to 35, and of the embodiments of the third aspect of the invention described below, may be combined with one another, that is in some to have concave cut-out surfaces (74-80) in the spacer, three orthogonal pairs of mirrors with cavities and an evacuation structure as disclosed and / or one or more pairs of pseudo mirrors or balancing elements. As there is overlap in the features of the embodiments described in relation to Figures 13 to 34, the description will not be unnecessarily repeated save for when there are differences relating to this second aspect of the disclosed invention.

[0194] Reference numerals have been kept the same whenever appropriate.

[0195] Referring to Figures 13 to 15, the etalon 10 comprises three pairs of reflective mirrors 12 attached to a cuboidal spacer 16, which has truncated flat vertices 25 in this example. Figures 13 and 14 are, respectively, projections in the y-z and x-y planes, while Figure 15 shows the geometry of the cubic etalon in perspective view showing seven of the eight truncated flat vertices, of which four vertices are used to support the etalon in tetrahedral configuration in the preferred embodiments. Figure 16 is a cross-section through the plane z = 0. In Figures 13 to 15, the open (solid) circles indicate the positions of the vertex support points 22, 24, (26, 28) which are visible in (obscured by) the projection.

[0196] In practical embodiments, the mirrors 12 may be partially reflective mirrors, as is known in the art. This applies to all embodiments of the invention.

[0197] The spacer 16 has three cylindrical bores 18 which extend orthogonally relative to one another and pass through the centre-point of the cube to the centrepoints of their respective faces. Two of the bores are visible clearly in the cross-sectional view of Figure 16. The mirrors 12 are bonded, on axis, to opposite ends of a respective bore 18. The bores 18 provide a void 20 in the spacer element which allow the propagation of light between the mirror pairs. For this purpose, the bores are preferably circular in transverse cross-section and cylindrical.

[0198] The mirrors 12 are bonded to opposite ends of all three bores 18 to form three orthogonal cavities 20.

[0199] The vertices of the cube are truncated (as shown by reference numeral 25) in this example to a depth of 6.7 mm forming eight plane surfaces normal to the body diagonals of the cube. The mirrors 12 have the following dimensions in this example: outer diameter = 12.7 mm; thickness = 4.0 mm; radius of curvature of concave mirror surface = 500 mm; inner diameter of contacting annulus = 8.5 mm. It is to be appreciated that these dimensions are illustrative and not necessarily unique. The vertices may be concave in other embodiments, as described above. The mirrors 12 have the characteristics described above. It will be appreciated that the distances between the centres of the mirror surfaces which are formed by the three pairs of mirrors 12 can be approximated as an integer number x the half-wavelength of the frequency of the light resonant within the cavity. In the preferred embodiments the mirrors 12 are all the same, as are the three bores 18. In other embodiments it is envisaged that the mirrors may differ from pair to pair, as may the bores. For example, the bores may be or different diameters, and the mirrors sized to match.

[0200] In the preferred embodiments, the mirrors 12 are formed as structures disposed on and outside of the spacer 10. Most preferably, the mirrors 12 are formed of a plurality of components in layered arrangement, at least one of the components being a reflective element.

[0201] For a cube with an edge dimension of 50 mm, the dimensions of the various components of the spacer are preferably as follows: bore radius, 5 mm; mirror radius, 6.35 mm; mirror thickness, 4 mm; cube vertices truncated to a depth of 6.7 mm.

[0202] Referring now to Figures 17 and 18, these depict, respectively, perspective and front elevational views of a finite-element model of a geometry representative of a truncated cubic spacer element. They show axial displacement on application of a compressive force, directed towards the centre of the cavity, at the support points 22, 24, 26, 28, as described above.

[0203] With all three orthogonal cavities covered by bonding mirrors to opposite ends of the bores, a depth of cut at the vertices can be found for which all three cavities are simultaneously insensitive to inertial forces, due to acceleration, and differential forces, in all degrees of freedom.

[0204] Referring now to Figure 19, this shows an example of a vacuum chamber 50 within which the etalon spacer 10 is disposed. The chamber 50 is as described above.

[0205] In this aspect of the invention, providing the three pairs of mirrors along the three orthogonal axes can provide comparative measurements usable in determining and compensating for spacer distortions dur to very high vibrations and / or acceleration. In particular, the three pairs of orthogonal mirrors 12 allow more measurements and more importantly corrections due to vibration, acceleration and drift, for example by determining differential measures from the three cavities 20.

[0206] Referring now to Figure 20, this shows another embodiment of spacer falling within the scope of the invention. The spacer 90 of this embodiment, instead of being cubic as in the preceding embodiment, is a rectangular cuboid shape, in other words is longer in one dimension relative to its size in the other orthogonal directions. The embodiment of Figure 20 is square in transverse crosssection but in other embodiments it may be rectangular in every dimensional cross-section (that is in the x, y and z planes).

[0207] Save for its rectangular cuboidal shape, the cavity spacer 90 has a general form the same as that of the examples of Figures 1 to 6 and 13 to 17 described above, and the embodiment of Figure 20, with three pairs of opposing mirrors 12 and three orthogonal bores (not visible in Figure 20).

[0208] In order to adjust for the non-cubic shape of the spacer 90, it is assumed that the vertices cut-outs should be angled such that the 4-point support forces should act through the centre of the spacer in order to maintain symmetry.

[0209] Additionally, the mirror pairs 12 may be of different mass, typically volume and dimensions, to balance the moment produced by the mirrors 12 and the extended dimension of the spacer.

[0210] The spacer 90 of Figure 20 has all the other features of the example of spacer 10 shown in Figures 1 to 7 and 13 to 19, and described above, so these will not be repeated here.

[0211] Referring now to Figure 21, this shows another embodiment of spacer falling within the scope of the invention. The spacer 100 of this embodiment, instead of being cubic or cuboidal as in the preceding examples and embodiments, is spherical in shape.

[0212] Save for its spherical shape, the cavity spacer 100 has a general form the same as that of the embodiments of Figures 1 to 8 and 13 to 20, with opposing mirrors 12. The mirrors 12 are disposed in pairs along orthogonal axes and align with bores also disposed with orthogonal axes. The spacer 100 of Figure 21 has all the other features of the previously described embodiments, so these will not be repeated here. For this example, instead of a tetrahedral support arrangement, the spacer may be provided with a 2-point or 3-point support arrangement.

[0213] Another embodiment, not shown, has a spacer in the form of an oval spheroid rather than a sphere. The elements of such a spacer are the same or equivalent as those of the previously described embodiments so are not repeated here.

[0214] The provision of mirrors 12 overlying all the bores 18 of the spacer 10 results in a loss of an evacuation path for gasses into and out of the spacer. The invention provides for evacuation paths which extend from the cavity 20 formed by the bores 18, to one or more external surfaces of the spacer. Below are described three embodiments of evacuation elements. It will be appreciated that the spacer could be provided with any number of evacuation elements, from a single one for the entire cavity, to one per bore 18, to a plurality per bore. The evacuation elements provide a passage for gasses into and out of the cavity, which may be the same, larger than or smaller than the diameter of the bores.

[0215] Referring now to Figure 22, there is shown a schematic plan view of a part of the surface of the spacer, showing a mirror 12 disposed over a bore 20 and which comprises a first embodiment of evacuation element.

[0216] The mirror 12 comprises a substrate 120, which in this example has an external diameter of about 12 mm or about 25 mm. Disposed on the substrate 120 is a mirror coating, which is preferably circular with an external diameter of, for example, about 6 mm, whereas the bore 20 has in this example a diameter of 5 mm.

[0217] The bore 20 is provided with an evacuation or access element 124, which in this embodiment is a slot 15 mm or 20 mm long cut into the polished spacer surface, and having a width typically of 5 mm and a depth of 5 mm. As will be apparent, both ends of the slot 124 extend beyond the perimeter of the mirror 12, thereby providing evacuation paths for the bore 20. The narrow configuration of the slot 124 provides mirror optical contact areas 126 on the polished surface of the spacer, thereby supporting the mirror 12. The slot 124 allows evacuation of the bore 20 under the mirror outer annulus 120.

[0218] In the preferred embodiments, there are slots 124 under all of the facing mirrors 12, which helps maintain symmetry along a single axis, although it will be appreciated that in other embodiments there may be just one slot, one or two pairs of slots. Pairs of slots 124 may be parallel or disposed along orthogonal axes.

[0219] In this aspect, the or each access element 124 is exposed at an exterior surface of the spacer beyond a perimeter of at least one mirror. The access element 124 is preferably formed by a cut-out in the exterior surface, in the example shown being in the shape of a slot over which the mirror is disposed, and wherein at least one end of the slot extends beyond a zone or perimeter of the mirror. The slot 124 is preferably centrally positioned across the mirror and both ends of the slot extend beyond the zone or perimeter of the mirror.

[0220] The access element or slot allows evacuation of the bore under the mirror outer annulus. In some embodiments, there are provided access elements or slots on opposite spacer faces, thereby maintaining symmetry in at least one axis.

[0221] Referring now to Figures 23 to 26, these show different views of a plurality of slot arrangements of Figure 22. The mirrors 12 at each face of the spacer cube can be seen and slots 124 disposed under each of the mirrors 12. Figure 25 in particular shows the slots 124 arranged along orthogonal axes to one another, although this is not essential as they could be parallel to one another. Figure 14 shows how the slots 124 provide access to the bores 18 for evacuating the cavity 20 in the spacer 10, thereby acting as evacuation elements.

[0222] The slots 124 are just one way of creating evacuation elements to evacuate the bores 20. Another example is the provision of one or more bore side branches 134, extending at an angle so as to open at the surface of the spacer beyond the perimeter of the mirror 12, and which at the other end connect to the bore 20, as can be seen in Figures 27 to 30, which show the same configuration of spacer 10 as Figures 22 to 26 but with different evacuation elements. Bore side branches 134 provide for evacuation of the cavity 20. As will be seen in the embodiment of these figures, there is preferably provided a pair of side branches 134 per bore, arranged in orthogonal orientations for the sake of balance, should this be required. The side branches 134 may have the same, smaller or larger diameters than the diameters of the bores 18. Furthermore, while a pair of side branches 134 is shown per bore end, in other embodiments there may be provided only one or more pairs of side branches 134, and in others singular side branches rather than pairs, again for an end of one or more bores, for both ends of one or more bores or for all the bores.

[0223] Figures 31 to 34 show yet another embodiment of evacuation elements. In this embodiment the evacuation elements are in the form of evacuation bores 144 extending from the centre of the cavity 20 to the apices of the spacer 10 not used for the support elements, in other words the free truncated apices. As with the earlier disclosed embodiments, the evacuation bores 144 could be of the same, smaller or larger diameter than the diameters of the optical bores 18. There may be provided a single evacuation bore 144, or multiple. Preferably, there are provided sufficient evacuation bores 144 to ensure symmetry of the spacer.

[0224] Figures 35 to 56 are directed to another embodiment of the invention, in which there are provided symmetry balancing elements substantially matching the mirror pair or pairs, providing three sets of orthogonal mirror or pseudo mirror pairs are disposed on the spacer element and provision is made for evacuation of the spacer cavity. The embodiments described have a spacer with flat truncated support surfaces, this being one implementation of the embodiments according to this aspect of the invention. It is to be understood, however, that the embodiments relating to this aspect may instead be provided with concave truncated edge supports or cut-out surfaces (74-80) as per the embodiments of Figures 1 to 12. It is to be understood that the features of the various embodiments disclosed herein may be combined with one another, that is in some to have concave cut-out surfaces (74-80) in the spacer, three orthogonal pairs of mirrors with cavities and an evacuation structure as disclosed and / or one or more pairs of pseudo mirrors or balancing elements.

[0225] As there is overlap in the features of the embodiments described in relation to Figures 35 to 56, the description will not be unnecessarily repeated save for when there are differences relating to this third aspect of the disclosed invention. Reference numerals have been kept the same whenever appropriate.

[0226] The general form of etalon spacer design can be appreciated from the embodiment shown in Figures 35 to 40.

[0227] Referring to Figures 35 to 38, the etalon 10 comprises a pair of reflective mirrors 12, 14 attached to a cuboidal spacer 16, which has truncated flat vertices 25. Figures 35 and 36 are, respectively, projections in the y-z and x-y planes respectively, while Figure 37 shows the geometry of the cubic etalon in perspective view showing 7 of the 8 truncated flat vertices, of which four vertices are used to support the etalon in tetrahedral configuration. Figure 38 is a cross-section through the plane z = 0. In Figures 35 to 37, the open (solid) circles indicate the positions of the vertex support points 22, 24, (26, 28) which are visible in (obscured by) the projection.

[0228] These Figures show small holes in the centre of each mirror or pseudomirror substrate on two axes. It will be appreciated that only one or two mirrors on one axis is necessary. In some embodiments it is optimal for holes to be provided in all six mirrors on the three axes for evacuation to maintain optimal symmetry.

[0229] In practical embodiments, the mirrors 12, 14 may be partially reflective mirrors, as is known in the art. This applies to all embodiments of the invention.

[0230] The spacer 16 has in this example three cylindrical bores 18 which extend orthogonally relative to one another and pass through the centre-point of the cube to the centre-points of their respective faces. Two of the bores are visible clearly in the cross-sectional view of Figure 38. The mirrors 12, 14 are bonded, on axis, to opposite ends of one of the bores 18. The bore 18 provides a void 20 in the spacer element which allows the propagation of light between the mirrors. For this purpose, the bore is preferably circular in transverse cross-section and cylindrical.

[0231] Mirrors are bonded to opposite ends of at least one of the bores across at least one orthogonal cavity

[0232] The spacer 16 has the characteristics described above in connection with the first and second aspects of the invention, so is not described again in connection with this aspect.

[0233] In the example of Figures 35 to 38, the etalon has a single optical bore 20. The other two, orthogonal, bores (of which one is shown in Figure 38) are provided for the sake of symmetry of the spacer 10 and also to allow for evacuation of the cavity. It will be appreciated that in other embodiments there could be two orthogonal bore cavities 20 with the third bore having a pseudo-mirror for evacuation and better symmetry.

[0234] For very sensitive applications, the mirror structures 12, 14-create an asymmetry, which while not material in many applications, can affect the performance of the etalon in more sensitive applications and in cases where the etalon may be subjected to particularly high vibrations and / or acceleration. In order to mitigate any such asymmetry, the spacer 10 is provided with a plurality of symmetry-balancing elements 27, in this embodiment disposed in pairs along orthogonal axes to the main optical bore 20 and in practice overlying the two other orthogonally disposed bores in the spacer 10. The symmetry-balancing elements 27 have the same or substantially the same mass as the mirrors 12, 14, thereby providing symmetry along all three dimensions.

[0235] In the preferred embodiments, the mirrors 12, 14 are formed as structures disposed on and outside of the spacer 10, and the symmetry-balancing elements 27 have the same or substantially the same dimensions as the mirrors 12, 14.

[0236] Most preferably, the mirrors 12, 14 are formed of a plurality of components in layered arrangement, at least one of the components being a reflective element, the symmetry-balancing elements 27 being made of the same or similar components as the mirrors 12, 14 with the omission of the reflective elements. In these preferred embodiments, the symmetry-balancing elements 27 have the same structure and are formed of the same materials, save for the reflective elements, and can therefore have characteristics equivalent to those of the mirrors 12, 14. They could be said to be pseudo mirrors. An example of a preferred mirror structure, and consequential symmetry-balancing element structure, is described below.

[0237] The symmetry-balancing elements 27 are preferably disposed over the ends of an orthogonal bore of the spacer 10 not used for light transmission, and thereby to have dispositions and characteristics as close as possible to the mirror pair(s) and associated spacer bore(s). The symmetry-balancing elements 27 in practical embodiments can be formed of uncoated mirror element materials or layers, which coating normally provides the reflective mirror function. The lack of a reflective coating in the symmetry-balancing elements means that a suitable optical cavity is not formed on that axis.

[0238] In some embodiments, one or more of the symmetry-balancing elements 27 is provided with an aperture 29 therein, passing to an associated bore which the symmetry-balancing element overlies, the aperture allowing for passage of gas to and / or from the bore.

[0239] The apertures 29 are preferably disposed centrally to their associated bore, and are advantageously circular in cross-section.

[0240] The symmetry-balancing elements 27 preferably have a structure and form as close as possible to those of the mirrors 12, 14 in order to be as close as possible in terms of mechanical symmetry with the mirrors 12, 14. It will be appreciated that the lack of a reflective surface may in some circumstances constitute a structural difference but this is likely to be immaterial in most cases. Should this make a material difference in some embodiments, this can be compensated for by a slight dimensional change in the symmetry-balancing elements to make up that difference. Similarly, the provision of apertures 29 in the symmetry-balancing elements 27 can affect the mass of the symmetry-balancing elements 27 and as a consequence cause them to differ slightly form the mirrors 12, 14. Again, this is likely to be immaterial in most cases and even if the difference is material, this can be compensated for by a slight dimensional change in the symmetry-balancing elements to make up that difference.

[0241] Referring now to Figures 39 and 40, these depict, respectively, perspective and front elevational views of a finite-element model of a geometry representative of a truncated cubic spacer element. What they show is consistent with the teachings above so these are not repeated here.

[0242] In an embodiment in which three orthogonal cavities are formed by bonding mirrors to opposite ends of all one or two bores, and a pair of symmetry-balancing elements to the ends of the other bore or bores, a depth of cut at the vertices can be found for which all three cavities are simultaneously insensitive to inertial forces, due to acceleration, and differential forces, in all degrees of freedom.

[0243] Referring now to Figure 41, this shows an example of a vacuum chamber 50 within which the etalon spacer 10 is disposed. The chamber 50 is as described above.

[0244] As explained above, the etalon cavity 10 is held in balanced manner in the vacuum chamber 50 and so as to be substantially insensitive to external forces.

[0245] While the example of cubic spacer 10 described above is suitable for many applications, it can suffer from potential movement between the cavity 10 and its mounting frame 52 when subjected to mechanical vibration / shock during use in mobile situations in ground, marine, aerospace and space environments, particularly where vibration and / or acceleration forces can be very high. More specifically, the existing spacer comprises flat surface cut-outs at four of the eight vertices in a tetrahedral arrangement against which mounting forces are symmetrically applied via four compliant hemispherical locators, acting on the central points of the cut-outs which define the cube diagonals, to hold the cavity cube 10 in place. The provision of symmetry-balancing elements 27 as disclosed herein can materially assist in providing better symmetry of the cube and its holding arrangement so as minimise the sensitivity of the structure to movement under vibration and acceleration.

[0246] Referring now to Figure 42, this shows another embodiment of spacer falling within the scope of the invention. The spacer 90 of this embodiment, instead of being cubic as in the preceding examples and embodiment, is a rectangular cuboid shape, in other words is longer in one dimension relative to its size in the other orthogonal directions. The embodiment of Figure 42 is square in transverse cross-section but in other embodiments it may be rectangular in every dimensional cross-section (that is in the x, y and z planes).

[0247] Save for its rectangular cuboidal shape, the cavity spacer 90 has a general form the same as that of the examples of Figures 35 to 40 described above, and the embodiment of Figure 41, with opposing mirrors 72, of which only one is visible in the view of Figure 43, and symmetry-balancing elements 27 as previously described. In order to adjust for the non-cubic shape of the spacer 90, the symmetry-balancing elements 27 may be adjusted in mass, typically in volume and dimensions, to balance the moment produced by each symmetry-balancing element 27 with the mirrors 12, 14.

[0248] The spacer 90 of Figure 42 has all the other features of the example of spacer 10 shown in Figures 35 to 40 and embodiment of Figure 41, and described above, so these will not be repeated here.

[0249] Referring now to Figure 43, this shows another embodiment of spacer falling within the scope of the invention. The spacer 100 of this embodiment, instead of being cubic or cuboidal as in the preceding examples and embodiments, is spherical in shape.

[0250] Save for its spherical shape, the cavity spacer 100 has a general form the same as that of the examples of Figures 35 to 40 described above, and the embodiments of Figures 41 and 42, with opposing mirrors 72, of which only one is visible in the view of Figure 43. Symmetry-balancing elements 27 are provided in pairs along orthogonal axes to the optical axis between the mirrors 72.

[0251] The spacer 100 of Figure 43 has all the other features of the previously described embodiments, so these will not be repeated here.

[0252] Another embodiment, not shown, has a spacer in the form of an oval spheroid rather than a sphere. The elements of such a spacer are the same or equivalent as those of the previously described embodiments so are not repeated here.

[0253] Referring now to Figure 44, there is shown a schematic plan view of a part of the surface of the spacer, showing a mirror 12 disposed over a bore 20 and which comprises a first embodiment of evacuation element.

[0254] The mirror 12, 14 comprises a substrate 120, which in this example has an external diameter of about 12 mm or about 25 mm. Disposed on the substrate 120 is a mirror coating, which is preferably round with an external diameter of, for example, about 6 mm, whereas the bore 20 has in this example a diameter of 5 mm.

[0255] The bore 20 is provided with an evacuation or access element 124, which in this embodiment is a slot 15 mm or 20 mm long cut into the polished spacer surface, and having a width typically of 5 mm and a depth of 5 mm. As will be apparent, both ends of the slot 124 extend beyond the perimeter of the mirror 12, 14, thereby providing evacuation paths for the bore 20. The narrow configuration of the slot 124 provides mirror optical contact areas 126 on the polished surface of the spacer, thereby supporting the mirror 12, 14.

[0256] The slot 124 allows evacuation of the bore 20 under the mirror outer annulus 120.

[0257] In another embodiment, the slot or slots 124 could be disposed under a symmetry-balancing element 27.

[0258] In the preferred embodiments, there are slots 124 under all of the facing mirrors 12 and symmetry-balancing elements 27, which helps maintain symmetry along a single axis, although it will be appreciated that in other embodiments there may be just one slot, one or two pairs of slots. Pairs of slots 124 may be parallel or disposed along orthogonal axes.

[0259] In this aspect, the or each access element 124 is exposed at an exterior surface of the spacer beyond a perimeter of at least one symmetry-balancing element or mirror. The access element 124 is preferably formed by a cut-out in the exterior surface, in the example shown being in the shape of a slot over which the symmetry-balancing element or mirror is disposed, and wherein at least one end of the slot extends beyond a zone or perimeter of the symmetry-balancing element or mirror. The slot 124 is preferably centrally positioned across the symmetry-balancing element or mirror and both ends of the slot extend beyond the zone or perimeter of the symmetry-balancing element or mirror.

[0260] The access element or slot allows evacuation of the bore under the symmetry-balancing element or mirror outer annulus. In some embodiments, there are provided access elements or slots on opposite spacer faces, thereby maintaining symmetry in at least one axis.

[0261] While Figure 45 shows the slots 124, that is the access elements, under the mirrors 12, 14, in other embodiments the slot or slots could instead be located under one or more of the symmetry-balancing elements 27. In other embodiments, slots 124 could be provided under both mirrors and symmetry- balancing elements, and in the preferred embodiment they may be provided under every mirror and symmetry-balancing element of the spacer.

[0262] Where facing slots 124 are provided, they may be parallel to one another or along orthogonal axes to one another.

[0263] Referring now to Figures 46 to 48, these show different views of the spacer of Figure 45. The mirrors 12, 14 and symmetry-balancing elements 27 at each face of the spacer cube can be seen and slots 124 disposed under each of the mirrors 12, 14 and symmetry-balancing elements 27. Figure 47 in particular shows the slots 124 arranged along orthogonal axes to one another, although this is not essential as they could be parallel to one another. Figure 48 shows how the slots 124 provide access to the bores 18 for evacuating the cavity 20 in the spacer 10, thereby acting as evacuation elements.

[0264] The slots 124 are just one way of creating evacuation elements to evacuate the bores 20. Another example is the provision of one or more bore side branches 134, extending at an angle so as to open at the surface of the spacer beyond the perimeter of the mirror 12, 14 or symmetry-balancing element 27 and which at the other end connect to the bore 20, as can be seen in Figures 49 to 52, which show the same configuration of spacer 10 as Figures 45 to 48 but with different evacuation elements. Bore side branches 134 provide for evacuation of the cavity 20. As will be seen in the embodiment of these figures, there is preferably provided a pair of side branches 134 per bore, arranged in orthogonal orientations for the sake of balance, should this be required. The side branches 134 may have the same, smaller or larger diameters than the diameters of the bores 18. Furthermore, while a pair of side branches 134 is shown per bore end, in other embodiments there may be provided only one or more pairs of side branches 134, and in others singular side branches rather than pairs, again for an end of one or more bores, for both ends of one or more bores or for all the bores.

[0265] It is to be understood that with 3 cavity axes covered with full mirrors and no evacuation slots, evacuation could take place via one, two, three or four bores from unused vertices in tetrahedral support arrangement to cube centre or beyond cube centre to close to opposite used vertex support point(s). Figures 53 to 56 show yet another embodiment of evacuation elements. In this embodiment the evacuation elements are in the form of evacuation bores 144 extending from the centre of the cavity 20 to the apices of the spacer 10 not used for the support elements, in other words the free truncated apices. As with the earlier disclosed embodiments, the evacuation bores 144 could be of the same, smaller or larger diameter than the diameters of the optical bores 18. There may be provided a single evacuation bore 144, or multiple. Preferably, there are provided sufficient evacuation bores 144 to ensure symmetry of the spacer.

[0266] The skilled person will be able to envisage other structures of evacuation element.

[0267] It will be appreciated that the provision of access elements avoids the need to have apertures in the symmetry-balancing elements, thereby enabling them to be even closer a match to the mirrors.

[0268] In other embodiments, evacuation of the bore or bores of the spacer can be accomplished by the provision of evacuation bores extending from an outer surface of the spacer and extending to the optical bore or bores of the spacer. Such evacuation bore or bores could be disposed at an angle (for example a diagonal) to the optical bores.

[0269] The above-described embodiments provide an etalon structure which is stable when fixed, to withstand high inertial forces and which is thus substantially insensitive to high inertial forces due to acceleration in all degrees of freedom (linear and rotational) as well as to differential forces. The etalon is also stable to temperature variations.

[0270] It is to be appreciated that the materials used in the preferred embodiments and the dimensions and proportions of the various elements described in the preferred embodiments may be varied, that is need not be the same as those given in connection with the above-described preferred embodiments.

[0271] The disclosure in British patent application numbers GB2416651.4, GB2500534.9 and GB2500529.9, from which this application claims priority, and in the Abstract accompanying this application are incorporated herein in their entirety by reference.

Claims

CLAIMS1. A Fabry-Perot etalon Including:a spacer element (70,80,100);first and second mirrors (72) spaced from one another by the spacer element (70,80,100), wherein the spacer element is provided with sides which provide support, respectively, for the first and second mirrors;the spacer element comprising an optical cavity (20) extending between the first and second mirrors (72) and having an optical axis and a centre; and a plurality of cut-out surfaces (74-80) located symmetrically about the optical axis of the cavity (20), the cut-out surfaces (74-80) being concave cut-outs of the spacer element (70,90,100); wherein said optical cavity (20) is provided by a bore (18) within the spacer element;a plurality of support elements (66) positioned about the spacer element (70,90,100) and configured to apply a compressive force at respective cut-out surfaces (74-80) of the spacer element (70,90,100) directed towards the centre of the cavity.

2. An etalon according to claim 1, wherein the concave cut-outs have a radius of curvature greater than a radius of curvature of the support elements (66).

3. An etalon according to claim 1 or 2, wherein the cut-outs (74-80) are part spherical.4 An etalon according to any preceding claim, wherein the cut-outs (74-80) have an apex directed to a centre-point of the spacer (70,90,100)5. An etalon according to any preceding claim, wherein all the cut-outs in the spacer (70,90,100) are equivalently concave.

6. An etalon according to any preceding claim, including one or more flat cut-outs.

7. An etalon according to any preceding claim, wherein the cut-outs have a concavity in the range of 50 -1000 mm ROC (radius of curvature).

8. An etalon according to any preceding claim, wherein the cut-outs have a concavity in the range of 200 mm ROC to 500 mm ROC.

9. An etalon according to any preceding claim, wherein the spacer is in the shape of a cube, a rectangular cuboid or sphere or an oval spheroid.

10. An etalon according to any preceding claim, wherein the support elements (74-80) are positioned tetrahedrally about the spacer element (70,90,100) and centred on vertex planes.

11. An etalon according to any preceding claim, wherein the cavity (20) of the spacer element (70, 90, 100) comprises three bores (18) extending orthogonally relative to one another through a centre-point of the spacer element (70,90,100) and to the centre of respective faces of the spacer element.

12. An etalon according to any preceding claim, wherein the or each bore (18) is cylindrical.

13. An etalon according to any preceding claim, including two, four, six or eight support elements (66) each provided at a respective cut-out (74-80) of the spacer element.

14. An etalon according to any preceding claim, wherein the support elements (66) are disposed tetrahedrally about the spacer element.

15. An etalon according to any preceding claim, wherein the support elements (66) include spherical or part-spherical holding members.

16. An etalon according to any preceding claim, wherein the first and second mirrors (72) are curved.

17. An etalon according to any preceding claim, wherein the first and second mirror (72) are provided on mirror substrates made from silica or ultra-low expansivity glass.

18. An etalon according to any preceding claim, including an annulus (50) between each of said mirrors (72) and their associated side of the spacer element.

19. An etalon according to claim 18, wherein each annulus (50) is made of silica or ultra-low expansion glass.

20. An etalon according to any preceding claim, comprising:three pairs of facing first and second mirrors spaced from one another by the spacer element, which spacer element is provided with sides which provide support, respectively, for the three pairs of first and second mirrors;optical cavities in the spacer element extending between the three pairs of first and second mirrors and having three orthogonal optical axes extending along the first, second and third bores within the spacer element, the bores crossing at a central point of the spacer, each pair of first and second mirrors being associated with a respective bore with the first and second mirrors disposed at opposing ends of the respective bore;at least one evacuation element connecting the cavities to an external surface of the spacer.

21. An etalon according to any one of claims 1 to 19, wherein:the spacer element includes a second bore, wherein the first and second bores extend along orthogonal axes; the etalon comprising:at least one pair effacing symmetry-balancing elements having a mass substantially the same or the same as a mass of the first and second mirrors, thesymmetry-balancing elements being disposed at and supported by sides of the spacer element adjacent ends of the second bore.

22. A structure comprising a plurality of Fabry-Perot etalons including: a spacer element;three pairs of facing first and second mirrors spaced from one another by the spacer element, which spacer element is provided with sides which provide support, respectively, for the three pairs of first and second mirrors;optical cavities in the spacer element extending between the three pairs of first and second mirrors and having three orthogonal optical axes extending along the first, second and third bores within the spacer element, the bores crossing at a central point of the spacer, each pair of first and second mirrors being associated with a respective bore with the first and second mirrors disposed at opposing ends of the respective bore;at least one evacuation element connecting the cavities to an external surface of the spacer;a plurality of cut-out surfaces for supporting the spacer element and being located symmetrically about the optical axes of the cavity; andsupport elements positioned about the spacer and configured to apply thereto a compressive force at the cut-out surfaces directed towards the centre of the cavities.

23. An etalon according to claim 22, wherein the bores are orthogonal to one another.

24. An etalon according to claim 22 or 23, wherein the bores are circular in transverse cross-section.

25. An etalon according to any one of claims 22 to 24, wherein the bores are identical in cross-section and / or in length.

26. An etalon according to any one of claims 22 to 25, wherein the mirrors are identical to one another.

27. An etalon according to any one of claims 22 to 25, wherein in at least one dimension, the mirrors differ in size and / or mass.

28. An etalon according to any one of claims 22 to 27, wherein the mirrors are formed as structures disposed on and outside of the spacer.

29. An etalon according to any one of claims 22 to 28, wherein the mirrors are formed of a plurality of coating layers arranged on a mirror substrate.

30. An etalon according to any one of claims 22 to 29, wherein the at least one evacuation element is a bore access element disposed at one of more ends of a bore and which at an exterior surface of the spacer beyond a perimeter of an associated mirror.

31. An etalon according to claim 30, wherein the or each access element is formed by an enlarged bore end at the exterior surface.

32. An etalon according to claim 31, wherein the enlarged bore end is in the shape of a slot over which the mirror is disposed, and in which at least one end of the slot extends beyond a zone or perimeter of the mirror.

33. An etalon according to claim 32, wherein the slot is centrally positioned across the mirror, and both ends of the slot extend beyond the zone or perimeter of the mirror.

34. An etalon according to claim 32 or 33, wherein there are provided access elements or slots on opposite spacer faces.

35. An etalon according to claim 32, 33 or 34, wherein there are provided access elements at a plurality of bore ends and / or at every bore end.

36. An etalon according to any one of claims 32 to 35, wherein the access elements, when in the form of slots, are disposed orthogonally to one another.

37. An etalon according to any one of claims 22 to 29, wherein the at least one evacuation element comprises one or more bore side branches extending from a respective bore to a surface of the spacer beyond a perimeter of the mirror.

38. An etalon according to claim 37, wherein there are provided a plurality of such bore side branches.

39. An etalon according to claim 38, wherein the side branches are disposed in a symmetrical arrangement in the spacer.

40. An etalon according to claim 38 or 39, wherein every bore is provided with one or more side branches.

41. An etalon according to any one of claims 22 to 29, wherein the at least one evacuation element comprises one or more evacuation bores extending from the cavity crossing point to a surface of the spacer.

42. An etalon according to claim 41, wherein the or each evacuation bore extends to a truncated surface of the spacer not used for support purposes.

43. An etalon according to any one of claims 22 to 42, wherein the support elements are positioned tetrahedrally about the spacer and are configured to have applied thereto a compressive force directed towards the centre of thecavity.

44. An etalon according to any one of claims 22 to 43, wherein the spacer is in the shape of a cube, a rectangular cuboid, sphere or an oval spheroid.

45. An etalon according to any one of claims 22 to 44, wherein both the mirror surfaces have a concave radius of curvature or the mirror combination comprises one concave radius of curvature and one flat surface.

46. An etalon according to any one of claims 22 to 45, wherein the mirror elements may be provided on substrates made from silica or ULE or other materials.

47. An etalon according to any one of claims 22 to 46, including an annulus between each of mirror and their associated side of the spacer element, the annuli made of silica or ultra-low expansion glass.

48. A Fabry-Perot etalon including:a spacer element;at least one pair of facing first and second mirrors spaced from one another by the spacer element, the spacer element being provided with sides which provide support, respectively, for the first and second mirrors;an optical cavity in the spacer element extending between the first and second mirrors and having an optical axis; the optical cavity comprising a first bore within the spacer element;the spacer element including a second bore, wherein the first and second bores extend along orthogonal axes;at least one pair effacing symmetry-balancing elements having a mass substantially the same or the same as a mass of the first and second mirrors, the symmetry-balancing elements being disposed at and supported by sides of the spacer element adjacent ends of the second bore;a plurality of cut-out surfaces in the spacer element for supporting the spacer element and being located symmetrically about the optical axis of the cavity; andsupport elements positioned about the spacer and configured to apply thereto a compressive force at the cut-out surfaces directed towards the centre of the cavity.

49. An etalon according to claim 48, wherein the spacer element includes a third bore, with the first, second and third bores extending along orthogonal axes; and comprising a second pair of facing mirrors or a second pair of symmetry-balancing elements disposed at respective ends of the third bore.

50. An etalon according to claim 48 or 49, wherein the mirrors are formed as structures disposed on and outside of the spacer, and the symmetry-balancing elements have the same or substantially the same structures as the mirrors.

51. An etalon according to any one of claims 48 to 50, wherein the mirrors are formed of a plurality of coatings in layered arrangement, at least one of the components being a mirror substrate, the symmetry-balancing elements being made of the same or similar components as the mirrors with the omission of the mirror coatings.

52. An etalon according to any one of claims 48 to 51, wherein the symmetry-balancing elements are formed of uncoated mirror substrates.

53. An etalon according to any one of claims 48 to 52, wherein the symmetry-balancing elements have the same structure and are formed of the same materials as the mirror elements.

54. An etalon according to any one of claims 48 to 53, wherein the symmetry-balancing elements and the mirror elements have the same or substantially the same dimensions.

55. An etalon according to any one of claims 48 to 54, wherein the symmetry-balancing elements are disposed over the ends of an orthogonal bore or bores of the spacer not used for light transmission.

56. An etalon according to any one of claims 48 to 55, wherein one or more of the symmetry-balancing elements is provided with an aperture therein, passing to an associated bore which the symmetry-balancing element overlies, the aperture allowing for passage of gas to and / or from the bore.

57. An etalon according to claim 56, wherein the aperture is disposed centrally to the associated bore.

58. An etalon according to claim 56 or 57, wherein the aperture is circular in cross-section.

59. An etalon according to any one of claims 48 to 55, wherein the spacer is provided with a bore access element at least at one end of the bore, which access element is exposed at an exterior surface of the spacer beyond a perimeter of at least one symmetry-balancing element or mirror.

60. An etalon according to claim 59, wherein the access element is formed by an enlarged bore end or cut-out at the exterior surface.

61. An etalon according to claim 60, wherein the enlarged bore end is in the shape of a slot over which the symmetry-balancing element or mirror is disposed, and wherein at least one end of the slot extends beyond a zone or perimeter of the symmetry-balancing element or mirror.

62. An etalon according to claim 61, wherein the slot is centrally positioned across the symmetry-balancing element or mirror and both ends of the slot extend beyond the zone or perimeter of the symmetry-balancing element or mirror.

63. An etalon according to any one of claims 48 to 62, including access elements or slots on opposite spacer faces, thereby maintaining symmetry in at least one axis.

64. An etalon according to any one of claims 48 to 63, wherein the spacer has three orthogonal bores and three pairs of access elements or slots at the bore ends, wherein the pairs of access elements or slots are arranged in orthogonal axes.

65. An etalon according to any one of claims 48 to 64, one or more access elements or slots are disposed under a mirror or a symmetry-balancing element, or both.

66. An etalon according to any one of claims 48 to 65, wherein the spacer is in the shape of a cube, a rectangular cuboid, sphere or an oval spheroid.

67. An etalon according to any one of claims 48 to 66, including support elements positioned tetrahedrally about the spacer and centred on vertex planes.

Images