Interferometers for precise determination of lengths and more particularly, a highly sensitive interferometer which can produce interference fringes one fringe separation of which corresponds to .lambda./2N (.lambda. is a wavelength of an incident light and N is an integer which is 2, 3, 4 . . . ). The interferometer comprises light emitting means at least one beam splitting means, retroreflecting optical means and bilateral reflecting optical means. Two embodiments of the beam splitting means are provided. The corresponding prisms in both preferred embodiments have the same effect of dividing an incident light into a reference light beam and a measuring light beam, but the configuration and construction of the equivalent prisms of the two embodiments differ slightly in order to provide mutually perpendicular light paths for the divided light beams. Rotator means are provided between the beam splitting means and the bilateral reflecting optical means in one embodiment, between the beam splitting means and the retroreflecting optical means in another embodiment, and between the beam splitting means on the one hand and the bilateral reflecting optical means and retroreflecting optical means on the other hand in a further embodiment. The light paths traversing the bilateral reflecting optical means are common to those traversing the reference and measuring retroreflecting optical means.
A multiple reflections interferometer (MRI) which eliminates the normal angle of incidence requirement of other interferometers, such as the standard Michelson interferometer (SMI), and allows convenient choices of interferometric sensitivities, such as exactly 100 nanometers per cycle or 10 nanometers per cycle using a helium-neon laser. The MRI has the mirror that usually receives the normal incidence beam replaced by a two-mirror wedge produce multiple reflections therein to provide greater displacement sensitivities.
An over-under double-pass interferometer in which the beamsplitter area and thickness can be reduced to conform only with optical flatness considerations is achieved by offsetting the optical center line of one cat's-eye retroreflector relative to the optical center line of the other in order that one split beam be folded into a plane distinct from the other folded split beam. The beamsplitter is made transparent in one area for a first folded beam to be passed to a mirror for doubling back and is made totally reflective in another area for the second folded beam to be reflected to a mirror for doubling back. The two beams thus doubled back are combined in the central, beam-splitting area of the beamsplitter and passed to a detector. This makes the beamsplitter insensitive to minimum-thickness requirements and selection of material.
A differential plane mirror interferometer system comprises a laser source (10) which emits an input beam (12) having two linear orthogonally polarized components, which may or may not be of the same optical frequency. A birefringent optical element (64) converts the input beam components (20,21) into two separated, parallel, orthogonally polarized beams (22,23). The birefringent element (64) together with a first quarter-wave phase retardation plate (66), a second quarter-wave phase retardation plate (62) with a pair of holes, and a retroreflector (60) with a pair of holes aligned with the holes in the second quarter-wave phase retardation plate (62), causes each of the separated, parallel, orthogonally polarized beams to be reflected twice by one of two plane mirrors (68,70), namely a first plane mirror (68) or a second plane mirror (70), respectively, to produce two separated, parallel, orthogonally polarized output beams (54, 55). The birefringent optical element (64) converts the two separated, parallel, orthogonally polarized outbeams (54, 55) into a single output beam (80) in which the phase difference between the two polarization components of the single output beam (80) is directly proportional to the optical path length between the two plane mirrors (b 68, 70). This phase difference is measured by passing the output beam (80) through a polarizer (81) which mixes the two orthogonally polarized components in beam (80) with the interference between these two components being detected by a photodector (83) which produces an electrical signal (85). An electronic module (80) extracts the phase variation from this signal (85).
A double-pass interferometer is provided which allows direct measurement of relative displacement between opposed surfaces. A conventional plane mirror interferometer may be modified by replacing the beam-measuring path cube-corner reflector with an additional quarter-wave plate. The beam path is altered to extend to an opposed plane mirrored surface and the reflected beam is placed in interference with a retained reference beam split from dual-beam source and retroreflected by a reference cube-corner reflector mounted stationary with the interferometer housing. This permits direct measurement of opposed mirror surfaces by laser interferometry while doubling the resolution as with a conventional double-pass plane mirror laser interferometer system.
A linear measurement interferometer 10 with a measurement axis directed toward a surface of a work piece 17 has a light source 11, a detection system 12, and a beam splitter 13 arranged to divide light from the source into test beams 21, 22, and 34 and reference beams 23, 24, and 33 that travel test and reference paths and recombine for detection by the detection system. Beam splitter 13 is arranged on the measurement axis with the work piece surface on one side of the beam splitter and the test beam path straddling the measurement axis on the opposite side of the beam splitter. A test beam retroreflector 25 mounted in the test path on the measurement axis reflects back the test beam from beam splitter 13 and is movable along the measurement axis without causing abbe error. A probe or focusing lens 15 movable along the measurement axis for measuring the work piece surface is interconnected with the test beam retroreflector 25 for movement together to determine a distance moved in measuring the work piece surface. A preferred way of arranging beam splitter 13 on the measurement axis while accommodating a probe is to bore a central aperture 20 in the beam splitter allowing the probe to pass through the beam splitter.
