Polarization Interference Fringes
Polarization interference fringes are bands of light and dark strips produced when opposite linear polarizations of monochromatic light interfere. They are used to measure the optical retardation of transparent materials. These fringes can form in several ways. Depending upon the polarizer configuration used, they are: (1) anomalous fringes, (2) normal fringes, and (3) cross fringes.
Anomalous refinement occurs when a wave of one geometrical type is used with a right-handed circularly polarizer. This produces successive maximum and minimum fringes that move in opposite directions. Normal fringes are a result of linearly polarizing wave interference. The light waves have the same orientation of wavefronts, which produces maximum and minimum fringes that move in the same direction. Cross fringes are produced by the interference of two types of wave-fronts, one of which is circularly polarized and the other is linear or plane polarized.
When a plane wave of light falls on a transparent sample, it undergoes change in polarization due to its optical retardation, which is the time difference between two polarized wavefronts traversing the same physical distance. This difference is measurable with a polarimeter, an instrument which measures the plane of vibration of a light wave leaving the sample. This can be used to calculate the fast axis of retardation the material possesses.
Interference fringes formed using retardation samples provide a means to measure optical retardation. The basis of this is the optical phenomenon of polarization interferometry. When monochromatic light of right-handed circularly polarized wavefronts continuously illuminates a retardation sample, a pattern of interference fringe appears due to the interference between the two orthogonally polarized wavefronts.
The optical retardation of a transparent sample is calculated by measuring the positions of the interference fringes on the polarizer. This is done by rotating the polarizer, then counting the number of fringes that move up and down per each degree of rotation. The optical retardation of the sample is calculated as a function of the total number of fringes that move along the polarimeter per degree of rotation.
By measuring the optical retardation of a sample, it is possible to calculate refractive properties of the sample, such as its index of refraction or birefringence (optical difference between light wavefronts of orthogonal polarization directions.) Optical retardation is also used to measure and determine the strain, temperature and stresses in materials. Polarization interference fringes can also be used to measure the thickness of transparent materials accurately, since the sample thickness and optical retardation are inversely proportional.
Polarization interferometry using interference fringes is a powerful and commonly used technique in many aspects in optical wave measurements. It is used to analyze the nature of wavefronts of different polarizations as well as optical retardation of transparent materials. Such analysis is important to study optical activity, birefringence, and optical constants of materials which are necessary for the design of advanced optical systems. Polarization interference fringes offer an efficient way to study multiple wave-front parameters simultaneously. An intuitive understanding of principles of polarization interferometry and its applications could be helpful to scientists working in wave-measurement studies.
