Recent developments in the areas of image processing have guided a new paradigm for research work in communication of images over the internet. Hence, secure storage and retrieval of digital images is a demanding task for future communication services. A scheme to securely store and encrypt multiple images using sinusoidal phase grating is highlighted. In the proposed scheme, images are modulated using different spatial frequencies and orientation angles. In addition, images have been encrypted using random phase mask after filtering. In order to avoid any human intervention in the system, an intensity graph is plotted to retrieve the images by applying inverse Fourier transform. Using this method, it is possible to independently store, encrypt, and retrieve multiple images due to absence of aliasing problem.
Phase retarders normally exhibit strong wavelength dependence. However, for use with polychromatic light, achromatic retarders, which exhibit ideally nearly identical characteristics over a wide wavelength region, are in demand. Designing such a superachromatic retarder is a challenging task for an optical system designer where retardation will be almost constant within a wide range of wavelengths. The present investigation considers a system consisting of wave plates made of different materials, which focus on near-infrared wavelength range, viz. 800 to 2000 nm. Here, a flower pollination algorithm has been used for this nonlinear optimization problem. With the help of the mentioned optimization technique, the optimal values of thicknesses of wave plates are calculated for which the system act as a quarter-wave plate. The obtained result shows a significant improvement in terms of maximum deviation of retardation for the above-mentioned wavelength range. The proposed methodology holds promise for optimizing the design of optical systems.
In the present communication, a procedure for the synthesis of an optical finite impulse response (FIR) birefringent filter
generating arbitrary spectral output is presented. The basic filter consists of a cascaded system of n identical retarders
between two polarizers at the two ends. A mathematical model of the optical FIR filter is introduced using the FIR
theory of digital filter design. The parameters determined by the synthesis procedure are the angles of the optic axes of
the individual crystals and the angle of the output polarizer. Classical FIR filter design method along with the optical
backward transfer technique has been used. Two different arbitrarily specified spectral output profiles have been studied.
However, the method is equally applicable for any periodic transfer function whose corresponding impulse response is
real and causal.
Phase retarders usually exhibit strong wavelength dependence. For this paper, the design and characteristics of an achromatic cascaded system of birefringent plates were studied. The combination of three retarders in a series had been previously proposed by Pancharatnam, and he discussed the possibility of fabricating reasonably good achromatic quarter-wave plates with a suitable level of retardance. This combination, however, still shows substantial variation in terms of retardance within 500 to 700 nm. Here, the combination of four plates for the construction of an almost achromatic quarter-wave plate, which has a variation of only ± 1 degree over the wavelength spectrum of 500 to 700 nm, is proposed. Moreover, spherical trigonometric considerations have been used by Pancharatnam for obtaining the transmission characteristics of a cascaded system. We, however, used the succinct and relatively simple Jones matrix formalism to derive a general expression for the equivalent retardation of this combination. The proposed arrangement of four birefringent plates promises better achromatic combinations.
Birefringent wave plates, often referred to as retarders, generally exhibit a strong wavelength dependence. However, there is a real need for achromatic retarders that exhibit identical characteristics over a broad wavelength range. In this paper, we have studied the design and characteristics of a cascaded system of birefringent plates in the near infrared region. Here we have studied a cascaded system of three birefringent plates using matrix analysis and designed a system which, by suitable reorientation of one of the plates, may perform both as an achromatic quarter-wave plate and half-wave plate, over the wavelength range of 1000 to 1800 nm. An inexpensive design for such an achromatic combination using crystalline quartz is described. The new arrangement of three birefringent plates proposed has the promise of producing achromatic combinations with fairly good accuracy.
A technique is described for the continuous variation of the resultant retardance and effective privileged directions of a cascaded system of three optical retarders. The system consists of a rotatable retarder sandwiched between two fixed, identical, parallel retarders. The primary aim of the study was to develop a simple technique for a variable retarder for monochromatic light, but the robustness of the system for a narrow band on either side of the design wavelength was also studied. Simulated plots show that a linear variation of the effective retardance from 0 to 2 is achievable, as is a linear variation of the effective azimuth from 0 to . It is also shown that these two parameters may be varied independently of each other.