Achromatic phase retarders constructed from right-angle prisms: design

Total internal reflection without change of polarization using a right-angle prism with half-wavelength-thick optical interference coating

Monochromatic light, which is polarized in an arbitrary state, is totally internally reflected at angle of incidence phi=45 degrees without change of polarization by a right-angle prism of refractive index n0=1+1/Square root of 2=1.70711 (e.g., N-LAK8 Schott glass at wavelength lambda=706 nm), which is coated with a transparent thin film of refractive index n1=(1+1/2)1/2=1.30656 (e.g., vacuum-deposited fluoride material) and of metric thickness equal to half of the vacuum wavelength of incident light, d=lambda/2. The ambient medium of evanescent refraction is assumed to be vacuum, air, or an inert gas. Wavelength shifts of +/-50 nm, or changes of the internal angle of incidence of +/-1 degrees around 45 degrees, cause phase errors of only a few degrees. The reflected and incident polarization states are nearly identical in the presence of such small phase errors.

In-line broadband 270 degrees (3lambda/4) chevron four-reflection wave retarders

The net differential phase shift Delta(t) introduced between the orthogonal p and s linear polarizations after four successive total internal reflections inside an in-line chevron dual-Fresnel-rhomb retarder is a function of the first internal angle of incidence phi and prism refractive index n. Retardance of 3lambda/4 (i.e., Delta(t)=270 degrees) is achieved with minimum angular sensitivity when phi=45 degrees and n=1.900822. Several optical glasses with this refractive index are identified. For Schott glass SF66 the deviation of Delta(t) from 270 degrees is < or = 4 degrees over a wavelength range of 0.55 < or = lambda < or = 1.1 microm in the visible and near-IR spectrum. For a SiC prism, whose totally reflecting surfaces are coated with an optically thick MgF(2) film, Delta(t)=270 degrees at two wavelengths: lambda(1)=0.707 microm and lambda(2)=4.129 microm. This coated prism has a maximum retardance error of approximately 5 degrees over > three octaves (0.5 to 4.5 microm) in the visible, near-, and mid-IR spectral range. Another mid-IR 3lambda/4 retarder uses a Si prism, which is coated by an optically thick silicon oxynitride film of the proper composition, to achieve retardance that differs from 270 degrees by < 0.5 degrees over the 3-5 microm spectral range.

Achromatic angle-insensitive infrared quarter-wave retarder based on total internal reflection at the Si-SiO2 interface

An achromatic infrared (lambda = 1.2-4 microm), Si-prism quarter-wave retarder (QWR) is described that uses total internal reflection at a buried Si-SiO2 interface at an angle of incidence phi near 33 degrees, where deltaDelta/deltaphi = 0. The retardance delta deviates from 90 degrees by < +/- 2 degrees within a field of view of +/- 10 degrees (in air) over the entire bandwidth. Because the SiO2 layer at the base of the prism is optically thick, this QWR is unaffected by environmental contamination.

Low-loss ultrabroadband 90 degree optical rotator with collinear input and output beams

An achromatic device to rotate optical polarization by 90 degrees is described. This is based on a series of reflecting surfaces that rotates incoming light about the optical axis and translates it such that the exiting light is collinear. Polarization rotation is achieved by rotation of the optical beam, as opposed to the more common approach of phase retardation by use of birefringent elements. For broadband operation from the UV to the near infrared, the device was constructed by use of total internal reflection in three fused-silica glass components. Losses are minimized with interstitial surfaces designed to be angled close to Brewster's angle.

Phase retarders highly insensitive to the input angle

In the few exceptional cases of total internal reflection phase retarders that are weakly sensitive to variations in the input angle of incidence i, the devices are of increasingly larger size; deviate or translate the emergent beam; are difficult to align in optical systems; or, in coated rhombs, have an impermanent stability of the retardance. Based on new compensating effects, novel devices of high retardance stability and advantageous characteristics are presented. One device is stable to within 0.09 degrees of 90 degrees for i ? 3 degrees . It is a two-reflection quarter-wave retarder that does not alter the path of the compact size light beam, is of compact size (one seventh the size occupied by the achromatic device AD-1 for equal aperture lengths), and can be easily aligned in optical systems or used as a rotating element. The path length of the beam inside the device is less than one third the corresponding length inside the achromatic device AD-1.

Achromatic wave retarder by phase subtraction

We analyze the method of phase subtraction in two identical optical structures to build an achromatic phase retarder. The two structures are made of right-angle prisms and are aligned orthogonal to each other. They are also made of materials of different refractive indices so that dispersion compensation can be taken advantage of. Essentially the phase retardation between the s and p waves in the first structure is subtracted from the phase retardation in the second structure. This can be done by reversing the roles of the s and p waves. By choosing the materials of the prisms properly, the phase retardation can be made to be constant over a broad spectral range. Indeed, calculations made with commercial optical glasses show that phase errors in the visible and near-infrared regions can be rather small. For example, for a 90° phase retarder (quarter-wave plate), a phase error of 0.35° can be obtained from 0.35 to 0.81 μm and from 0.59 to 1.26 μm.

Phase retarders with variable angles of total internal reflection

Results for three λ/4 phase retarders are reported. One of them is achromatic to within ± 0.06° of 90° in the visible region. It is shown that, under certain conditions, oblique incidence on the entrance face of total internal reflection phase retarders can greatly improve the achromaticity.

Achromatic prism retarder for use in polarimetric sensors

The optical resting point of a polarimetric sensor is normally reached by using a fixed retarder plate. It is shown that this retarder plate can be omitted by using 90 degrees prisms of selected glass materials. The sensitivity of the prism retarder toward changes in wavelength and temperature is mathematically analyzed, and it is shown how the stability of the retarder system is influenced by the selection of materials and production tolerances on prism angles. A practical example is given.