Force-insensitive Fabry-Perot etalon
A Fabry-Perot etalon is provided with first and second partially reflecting mirrors ( 12, 14 ) attached to a cuboidal spacer ( 16 ). The spacer ( 16 ) has a cylindrical bore ( 18 ) through its center and the mirrors ( 12, 14 ) are bonded, on axis, to opposite ends of this bore ( 18 ). Four support elements ( 22 - 28 ) are arranged symmetrically with respect to the axis in a tetrahedral configuration and constrain the cuboidal spacer ( 16 ) in all degrees of freedom. The cuboidal spacer ( 16 ) is flattened at its vertices, which results in modified distortion of the spacer ( 16 ) under the action of a compressive force acting through the supports. This structure of etalon provides insensitivity to inertial forces due to acceleration in all six degrees of freedom (three linear and three rotational) simultaneously with insensitivity to differential forces.
1. A Fabry-Perot etalon including first and second partial mirrors spaced from one another by a spacer element in the shape of a cube, having six sides arranged in three opposed pairs of sides, the two sides of one of the opposed pairs providing support, respectively, for the first and second mirrors; an optical cavity extending between the first and second partial mirrors and having an optical axis; the vertices of the spacer element being truncated.
2. An etalon according to claim 1 , wherein said cavity is provided by a bore within the spacer element.
3. An etalon according to claim 2 , wherein there are 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.
4. An etalon according to claim 2 , wherein the or each bore is cylindrical.
5. An etalon according to claim 1 , including a plurality of support elements for supporting the spacer element and being located symmetrically about the optical axis of the cavity.
6. An etalon according to claim 5 , wherein a compressive force directed towards the centre of the cavity is applied at the supports.
7. An etalon according to claim 1 , including four support elements.
8. An etalon according to claim 7 , wherein the support elements are positioned tetrahedrally about the spacer element.
9. An etalon according to claim 7 , wherein the support elements include holding spheres.
10. An etalon according to claim 9 , wherein the spacer element is provided with flattened contact surfaces at the location of the holding spheres.
11. An etalon according to claim 1 , wherein the first and second mirrors are curved.
12. An etalon according to claim 11 , wherein the first and second mirrors are spherically curved.
13. An etalon according to claim 1 , wherein the spacer element is formed from ultra-low expansivity glass.
14. An etalon according to claim 1 , wherein the first and second mirror elements are provided on substrates made from silica.
15. An etalon according to claim 1 , including an annulus between each of said mirrors and their associated side of the spacer element.
16. An etalon according to claim 15 , wherein the annuli are made of silica.
17. An etalon according to claim 1 , including six mirrors contacted respectively to the six sides of the cube to form three orthogonal cavities.
18. A Fabry-Perot etalon including first and second partial mirrors spaced from one another by a spacer element in the shape of a cube, an optical cavity extending between the first and second partial mirrors and having an optical axis, and a plurality of support elements for supporting the spacer element and being located on the spacer element symmetrically about the optical axis of the cavity.
19. An etalon according to claim 18 , including six mirrors contacted to the spacer element to form three orthogonal cavities.
20. A Fabry-Perot etalon including first and second partial mirrors spaced from one another by a spacer element in the shape of a cube, which spacer element is provided with sides which provide support, respectively, for the first and second mirrors; an optical cavity extending between the first and second partial mirrors and having an optical axis; the vertices of the spacer element being truncated; and a plurality of support elements for supporting the spacer element and being located symmetrically about the optical axis of the cavity.