Optics: measuring and testing – By light interference – For refractive indexing
Reexamination Certificate
2001-07-23
2004-01-13
Bruce, David V. (Department: 2882)
Optics: measuring and testing
By light interference
For refractive indexing
C359S485050, C359S732000, C359S834000, C359S839000, C359S857000, C359S861000
Reexamination Certificate
active
06678056
ABSTRACT:
BACKGROUND TO THE INVENTION
1. General Field of the Invention
This invention relates to a retroreflectors, to reflector systems, to optical apparatus, to methods of manufacturing reflector systems, to Jamin-type interferometers, to beam splitting blocks and to methods of manufacturing beam splitting blocks.
2. Particular Fields of the Invention and Description of the Prior Art
A first aspect of the invention relates to a retroreflector, and more particularly to a retroreflector having three mutually-orthogonal reflective surfaces arranged around an optical axis. Such retroreflectors are well known and have applications in, for example, vehicle rear reflectors and interferometers. In a typical Michelson-type interferometer, a reference beam and a measuring beam derived from a common beam are projected at right angles to each other and are then reflected back by respective reflectors; the reflected beams are superposed to form an interferogram. By contrast, in a typical Jamin-type interferometer (see, e.g., FIGS. 1 and 3 of patent document US-A-4571082 and FIGS. 16
a,b
of patent document US-A-5546184), a reference beam and a measuring beam derived from a common beam are projected parallel to each other, but spaced apart, and then back by a single reflector; again, the reflected beams are superposed to form an interferogram. Retroreflectors can usefully be used in such interferometers so that parallelism of the reflected and projected beams can be assured and so that the reflected beam can be offset from the projected beam. However, the use of a single conventional retroreflector for both the reflected and projected beams in a Jamin-type interferometer limits the applications of the interferometer.
A second aspect of the invention relates to a reflector system comprising first and second relatively movable reflectors.
A third aspect of the invention relates to an optical apparatus having an optical axis and a retroreflector.
Fourth and fifth aspects of the invention relates to a method of manufacturing a reflector or reflector system.
Sixth, seventh and eighth aspects of the invention relates to Jamin-type interferometers.
The Michelson-type interferometer mentioned above typically employs polarisation techniques to advantage, but consequently suffers from the disadvantage of the need to align the polarisation directions of the light beam source and polarisation-affecting components in the interferometer. By contrast a Jamin-type interferometer as described above typically does not use polarisation techniques and so does not suffer from the polarisation alignment problem. Indeed, alignment generally of a Jamin-type interferometer is less problematic than a Michelson-type interferometer. A further disadvantage of the typical conventional Jamin-type interferometer is that, because the reference and measuring beams share a common general path and a common reflector, there has been no widespread use of Jamin-type interferometers for displacement measurement— the Jamin-type interferometer of US-A4571082, for example, is limited to use in conjunction with a refractometer to measure refractive index rather than displacement and the Jamin-type interferometers of US-A-5546184 either are for use with a refractometer or employ a complex reflector arrangement to measure displacement.
The Jamin-type interferometer of the eighth aspect of the invention is concerned with the problem in the type of interferometer shown in FIG. 16
a
of US-A-5546184 that it is necessary to provide two separate phase-shifting means (the phase-shifting films 135,137 in US-A-5546184) in order to obtain a pair of interferogram beams in approximate phase quadrature.
The seventh and eighth aspects of the invention are concerned more particularly with a Jamin-type interferometer wherein: a beam splitter is arranged to split an incident beam of light into first and second generally-parallel, spaced-apart, projected beams; a reflector system is arranged to reflect the first and second projected beams to produce first and second return beams, respectively, which are spaced apart from and generally parallel to each other and the first and second projected beams; and the beam splitter is arranged to enable the first and second return beams to be superposed to produce at least one interferogram.
The ninth aspect of the invention relates to a method of manufacture of a beam splitting member, comprising the steps of: applying a thin-film, beam-splitting, metal coating to at least part of a surface of the member; and baking the member and coating so as to modify the phase shift produced by the coating.
Such a method is described in Vyskub VG, et al, 1977,
Pribory i Tekhnika Éksperimenta
, (Moscow Engineering Physics Institute), No 4, pp 210-211. That paper explains that members and coatings had been baked to achieve phase shifts of 90±3°.
SUMMARY OF THE INVENTION
An aim of the first aspect of the present invention is to provide a retroreflector which may increase the applications of a Jamin-type interferometer and which may have other uses.
The retroreflector of the first aspect of the invention is characterised in that the reflective surfaces stop short of the optical axis to provide a central region of the retroreflector which transmits incident light and a peripheral region of the retroreflector which retroreflects incident light. Accordingly, the retroreflector can retroreflect a beam which is incident on the peripheral region (with an offset between the incident beam and the parallel return beam), but can pass a beam which is incident on the central region.
In a first embodiment of the first aspect of the invention, the retroreflector may comprise a body of optical material which provides the reflective surfaces by internal reflection in the body. In this case, the retroreflector is preferably and conveniently in the form of a solid cube corner having a first transmitting surface for incident light and a second transmitting surface which truncates the cube corner. In this case, the first and second transmitting surfaces may be exactly parallel, but for some applications are preferably generally, but not exactly, parallel, so as to reduce the effects of stray reflections. Alternatively, the retroreflector may be in the form of a solid cube corner having a passageway extending therethrough generally in the direction of the optical axis to provide the central region.
In a second embodiment of the first aspect of the invention, the retroreflector may comprise three plane mirror elements arranged around the optical axis, each providing a respective one of the reflective surfaces. In this case, the retroreflector is preferably in the form of a hollow truncated cube corner.
The reflector system of the second aspect of the invention is characterised in that: the first reflector is a retroreflector according to the first aspect of the invention; and the reflectors are arranged so that light which is transmitted through the central region of the first reflector is reflected by the second reflector and transmitted back through the central region of the first reflector. When used as a reflector system for a Jamain-type interferometer, the enables to interferometer to be used to measure relative movement of the reflectors.
In a first embodiment of the second aspect of the invention, the second reflector is a retroreflector. In this case, the second reflector preferably has three mutually-orthogonal reflective surfaces arranged around a second optical axis, in which case each of the reflective surfaces of the second reflector is preferably arranged parallel to a respective one of the reflective surfaces of the first reflector. The second reflector may comprise a second body of optical material which provides the reflective surfaces by internal reflection in the second body, and in this case the second body may have a transmitting surface for incident light which may be exactly parallel, but for some applications is preferably generally, but not exactly, parallel, to the second transmitting surface of the first reflector, so as
Artman Thomas R
Bruce David V.
Downs Michael John
Sheridan Ross PC
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