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.