GB2426830A

GB2426830A - Optical interferometer with complementary pair of diffraction gratings

Info

Publication number: GB2426830A
Application number: GB0511146A
Authority: GB
Inventors: Anthony Gerald Booth
Original assignee: BTS Holdings PLC
Current assignee: BTS Holdings PLC
Priority date: 2005-06-01
Filing date: 2005-06-01
Publication date: 2006-12-06
Also published as: GB0511146D0

Abstract

An optical interferometer, for generating interference patterns descriptive of one or more characteristics of a target 109 indirectly illuminated by a source of coherent light 101, comprises an optically transmissive medium 105 for the transmission of source light rays and at least two diffraction gratings arranged to guide a light ray from the source to a target via the optically transmissive medium. A pair of the diffraction gratings 104,108 has a substantially equal diffracting characteristic, and the arrangement of said pair of diffraction gratings is complementary, so as to cancel unwanted variation in the diffracting characteristic. Optically absorptive regions 107,111,114 and detector 110 are shown.

Description

Optica' !nterferometer Optical interferorneters are typically used with a

coherent light source to illuminate a target. Reflection rays from the target have a phase that is dependent upon the distance to the target. When mixed with rays direct from the source, constructive and destructive interference provides precise distance information about the surface of the target. The precision is similar to or greater than the wavelength of the coherent light source. In an alternative application, the coherent light source, a laser, is frequency modulated. Patterns in the destructive and constructive interference may then provide a measure of the difference in time-of-travel of the direct source and target reflection beams. This method is used for precise measurement of displacements.

The precision of construction of optical interferometers is prohibitive in terms of the techniques and economics of their manufacture. Several topologies exist, these suffer, to greater or lesser degree, from the need for manual calibration during and also after manufacture. Although lasers are in widespread use, interferometers for visualisation or mapping of 3D surfaces cannot be manufactured in high volume. This also applies to the use of such devices for measuring distances and displacements, which also remain expensive The following documents provide a general background and examples of the state of the art in interferometry: Michelson interferorneter MachZehnder interferometer http://en.wikiped ia Diffracflon grating http://en wikipedia org/wiki/Diffraction grating Bragg thffraction (effect that occurs in diffraction gratings) http:I/en wikiped ia. org/wiki/Bragg diffraction Practical interferometer examples http://zone.ni.com/devzone/conceptd. nsf/webmain/9CD557B235A0770F8 625684E005F78B1 Reflection grating interferorneter topologies http:/iwww. ligo. ca Itech edu/docs/G/G05002000/G05002000 ppt "Reflective grating interferorneter" Applied Optics, 2002 Jan 10;41(2) :3427.

Pietro Ferraro et al. An rnproved topology for an optical interferometer is shown in Figure 1. A semiconductor laser light source 101 generates coherent light rays 102 that pass into a reflector 103. The source rays 102 are reflected at right angles to enter a first diffraction grating 104 The diffraction grating 104 causes source rays to ext at specific angles into an optically transmissive medium 105. Oniy one of these angles 106 is useful, and the other two direct rays are directed to an opticafly absorptive region 107. The preferred source rays 106 are directed towards a second, complementary diffraction grating 108, having the same angular refraction properties as the first diffraction grating 104. A first proportion of source rays 106 are directed towards a target object 109. A second proportion of the source rays 106 are reflected by the diffraction grating 108, directing them towards a detector 110. Other source rays are directed to another absorptive region 11 1.

Reflections from the target object 109 return along the same path as that taken by the source ray between the second grating 108 and the object 109. Upon exit from the second grating 108, the target rays are superimposed with source rays that are reflected from the second diffraction grating 108. Upon arrival at the detector 110, constructive and destructive interference between these rays occurs. The interference may be used to form a hologram, or e used n any of a number of applications for which interferometers are customarily used.

The optically transmissive medium is preferably glass plate, such as Schott(TM) BK7. The central plate 105 includes additional side plates 112 and 113, that are used to provide suitable nonrefracting interfaces at key points along the source ray paths. The reflector 103 and the side plates 112 and 113 are made from the same medium as the central plate 105.

Optically absorptive regions 107, 111 and 114 are formed using black paint.

The provision of complementary gratings 104 and 108 ensures that variations in angle of refraction due to temperature are largely cancelled out. This makes the interferometer robust, cheap and suitable for wide appcaton. Furthermore, key components of the interferometer are formed as part of a solid glass medium, making it simple to manufacture. The only significant variations in this system are those due to laser wavelength drift, particularly when laser diodes are used, and these are largely compensated, due to the complementary arrangement of the gratings 104 and 108.

The coherent tight source 101, must be directed correctly into the reflector I 03. However, this represents the only significant calibrated part of the interferometer's construction, which, in any case, shall remain stable after manufacture.

Claims

Claims 1. An optical interferorneter for generating interference patterns

descriptive of one or more characteristics of a target indirectly illuminated by a source of coherent light, comprising: an optically transrnissive medium for the transmission of source and target light rays, at least two diffraction gratings arranged to guide a light ray from said source to said target via said optically transmissive medium; wherein a pair of said diffraction gratings has a substantially equal diffracting characteristic; and the arrangement of said pair of diffraction gratings is complementary', so as to cancel unwanted variation in said diffracting characteristic.
2. An optical interferorneter according to claim 1, wherein the planes of said diffraction gratings are substantially parallel.
3. An opicai interferorneter according to claim 1, wherein said diffraction gratings are ocated on substantially parallel surfaces of said optically transmissive medium.
4. An optical interferometer according to claim 1, wherein the diffraction gratings in said pair are offset from each other.
5. An optical interferometer according to claim any of claims 1 to 4, including an optically absorptive region on a surface of said optically transrnissive medium, for the absorption of unwanted source rays.