An infrared achromatic waveplate structure having a cadmium sulfide (CdS) plate and a cadmium selenide (CdSe) plate aligned with each other so that the fast axis of the plates are perpendicular to each other, this structure provides a desired retardance of a first orthogonal polarization component with respect to a second orthogonal polarization component of an incident light beam. The thickness of the plates are in a ratio between 0.8:1 and 0.9:1 (CdSe:CdS), an achromatic response with a substantially constant retardance is provided in a wavelength range from 3 to 11 microns. A desired amount of retardance is available by adjusting the thickness of the two plates as long as the ratio of the thicknesses is maintained within the recited value. In particular a quarter wave net retardance of an incident light beam operating between 3 and 11 microns is provided when the cadmium sulfide plate is 1.25 millimeters and the cadmium selenide plate is 1.0666 millimeters.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an infrared achromatic waveplate capable 
of improved response as a function of wavelength in a simple, small and 
inexpensive configuration of waveplates. 
2. Discussion of Background 
Many optical systems including those for spectropolarimetry, laser 
polarimetry, laser spectroscopy, and ellipsometry have a need for 
converting light between polarization states and a need to analyze 
polarized light. The design of polarimeters requires polarization elements 
whose properties satisfy a number of criteria including the very important 
criteria that the polarization properties need to be substantially 
constant over a range of wavelength of interest. Additional constraints in 
this area include a reasonable element size and proper positioning of the 
light beam exiting from the element and of course the cost of the element. 
Liquids and amorphous solids such as glass and crystalline solids have a 
cubic symmetry which normally show a behavior whereby the speed of light 
and the index of refraction is independent of the direction of propagation 
in the medium and is independent of the state of the polarization of the 
light. These types of elements are said to be optically isotropic. Other 
crystalline solids, which induce birefringent behavior, are optically 
anisotropic. Of course, solids may be anisotropic in many of their 
properties, but it is the optical anisotropy of a material which is used 
in order to provide the "double refraction" of a beam. The two emerging 
beams from an optically anisotropic material are plane-polarized beams 
with their planes of vibration at right angles to each other. 
The conversion of light between polarization states and the analysis of 
polarized light has traditionally involved the use of birefringent 
materials wherein a light beam incident on a birefringent material is 
divided into two orthogonal polarization components. A retarder can then 
shift the phase or in other words retard the phase of one of these two 
orthogonal polarization components with respect to the other component. In 
a birefringent material, the index of refraction depends on the 
polarization state of the light beam. 
The most appropriate way that these anisotropic or birefringent materials 
are used involves the exploiting of the dependency of the index of 
refraction on the polarization state of the incoming light beam. A phase 
shift is introduced between the polarization state aligned with the fast 
axis of the birefringent material, where the index of refraction is the 
smallest and the polarization state aligned along the slow axis of the 
material, where the index of refraction is at its highest value. 
When plane-polarized light falls at normal incidence on a slab or piece of 
anisotropic material so that the optic axis is parallel to the face of the 
slab, the two waves which emerge are plane-polarized at right angles to 
each other. Because the waves travel through the material at different 
speeds, there will be a phase difference between the two waves when they 
emerge from the material. If the material thickness is chosen so that for 
a particular frequency of light the phase angle or "phase change between 
the two waves" is 90.degree., the slab or piece of material is called a 
quarter-waveplate. If linearly polarized light is incident on this quarter 
waveplate with its plane of polarization oriented at .+-.45.degree. to the 
fast axis, the emerging light is said to be circularly polarized. 
Traditionally then, the proper thickness of the material was chosen in 
order to obtain the desired retardance. However, these prior art designs 
are very sensitive to small changes in wavelength of the incident beam and 
thus are not suitable for many optical systems where broadband light is 
used. 
While other designs have better response as a function of wavelength, they 
involve complicated, large and expensive devices as for example in a 
design utilizing a modified Fresnel rhomb which is 4 inches long which of 
course exceeds the requirements for size. 
Still other designs cause the exit beam to be shifted from the path of the 
incident beam which also makes these designs not appropriate for such 
polarimeter usage. 
Thus, there is a specific need for an achromatic infrared retarder in which 
the polarization properties of the element is substantially constant over 
a particular range of wavelength and wherein the elements are small in 
size and produce a light beam which is properly positioned upon emergence 
from the element. 
SUMMARY OF THE INVENTION 
It is the object of the present invention to provide a configuration of two 
plates made of cadmium sulfide and cadmium selenide respectively with the 
fast axis of the materials at right angles to each other in order to 
produce polarization of incident light which is substantially constant 
over a range of wavelengths. 
It is another object of the present invention to provide a retarder which 
can shift the phase or retard the phase of one of two orthogonal 
polarization components in such a way that the resultant device is 
inexpensive, insensitive to changes in wavelength and easy to manufacture. 
The structure which accomplishes these objects involves a Cadmium Sulfide 
(CdS) plate and a Cadmium Selenide (CdSe) plate oriented so that the fast 
axis of the plates are perpendicular to each other in order to provide a 
positive retardance from one plate and a negative retardance from the 
other plate with the net effect providing a desired retardance. 
The device according to the present invention provides achromatic response 
in the wavelength range of from 3-11 microns when the thickness of the 
plates have a ratio of between 0.8:1 and 0.9:1 (CdSe:CdS).

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, and more particularly to FIG. 1 thereof, 
there is shown an orientation of two plates 10 and 20. The plate 10 is 
made of Cadmium Sulfide (CdS) and plate 20 is made of Cadmium Selenide 
(CdSe). These plates have similar birefringent properties as a function of 
wavelength and the orientation of the two plates is such that the fast 
axis 11 of plate 10 is at a right angle with respect to the fast axis 21 
of plane 20. One of the plates produces a retardance of one polarization 
state (state 1) with respect to the other state (state 2) while the second 
plate retards state 2 with respect to state 1. The first plate 10 produces 
a positive retardance and the second plate 20 produces a negative 
retardance with the net result being a substantially constant retardance 
over a broad wavelength range. 
By choosing the proper thickness for each plate, a quarter wave retardance 
occurs over a broad wavelength range as illustrated in FIG. 2. Given the 
achromatic response for one quarter wavelength retardance, the ratio 
between the thicknesses of the two plates is computed. Waveplates of other 
retardances may be constructed by varying the thickness of the plates with 
the ratio being kept constant. 
In order to obtain retardances for various thickness, numerous experiments 
may be conducted or a program may be utilized to calculate the retardances 
of birefringent plates with the material properties of CdS and CdSe being 
entered into the program. For a net retardance of one quarter wave, the 
thickness of the CdS plate is 1.25 millimeters and the thickness of the 
CdSe plate is 1.0666 millimeters. This provides a ratio of the CdSe to CdS 
of 0.853. 
The thickness of each of these plates is ideal for fabrication because the 
single crystals of this size can be grown. Furthermore, the plates are 
thick enough to retain structural strength. 
This design utilizes the materials CdS and CdSe and is specifically used in 
the mid-infrared region. As a result the device, which is used as a 
retarder, is an optical component which converts light between the 
polarization states and by varying the thickness of the plates, while 
holding the ratio constant, the retardance of a quarter-or half-wave or of 
any arbitrary value may be fabricated. 
The response of these two materials, cadmium sulfide and cadmium selenide, 
when used together in this manner provided a remarkable improvement in the 
formation of an achromatic waveplate over a very broad wavelength range. 
The particular embodiment wherein the ratio of the thickness of the plates 
is between 0.8:1 and 0.9:1 (CdSe:CdS) provided achromatic response in the 
wavelength range from 3 to 11 microns. 
Although the discussed embodiment utilizes 100% cadmium sulfide and cadmium 
selenide respectively, composite materials having doped cadmium sulfide or 
doped cadmium selenide could be used. Materials such as zinc selenide, 
galium arsenide, mercury, indium, galium, arsenic and zinc could be 
utilized as additive materials to either or both of the cadmium sulfide 
and cadmium selenide waveplates. The criteria for selection and addition 
of additive materials is controlled by the birefringent characteristics of 
the resultant composite material. In other words, any material may be 
added in small amounts as a doping in order to form a composite material 
for the first and second waveplate as long as the birefringent 
characteristics of the resulting composite materials is substantially the 
same as cadmium sulfide alone or cadmium selenide alone for the first or 
second waveplates. 
In order to avoid any reflection losses which occur due to the passage of 
the light through the four surfaces of the two plates, an anti-reflection 
coating ma be applied to each of the surfaces of the plates 10 and 20. 
Furthermore, a computer program may be substituted for experimentation in 
order to calculate the retardances of the birefringent plates. The 
material properties of CdS and CdSe are entered into the program and the 
various retardances calculated for each material as a function of 
wavelength. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise then as specifically described herein.