Ajay Ghosh

High focal depth with a quasi-bifocus birefringent lens.

The Strehl definition along the axis of a birefringent lens sandwiched between two polarizers is studied analytically. The optic axis of the birefringent lens made of a uniaxial crystal is perpendicular to the lens axis, and the system behaves like a bifocus lens for proper orientation of the polarizers. The Sparrow criterion is employed for designing an imaging system with high depth of focus. It is shown that, when the two foci are separated by the Sparrow limit of resolution, the focal depth is maximum and the intensity point-spread function remains almost identical within this limit. The resolution according to the Rayleigh criterion in this zone is more than that of an ideal lens.

Vector wave imagery using a birefringent lens

The imaging characteristics of a birefringent lens sandwiched between two polarizers is studied analytically. If the optic axis of the birefringent crystal is perpendicular to the lens axis, the system behaves as a bifocal lens for proper orientations of the polarizers. It is shown that the proposed system can be adapted either for apodization or for superresolution simply by rotating either of the two polarizers. It has greater depth of focus than conventional systems, as is evident from the results obtained. Expressions are obtained for the point spread function of such a system. Some specific cases are illustrated graphically.

Frequency-response characteristics of a perfect lens masked by polarizing devices

The imaging characteristics of a perfect lens partially masked by a linear polarizer can be varied continuously either by rotating an analyzer placed at the output side or by changing the state of polarization of the input beam. In this paper we study the frequency-response characteristics and the point-spread function of such a system. It is also shown that the proposed system can be adapted either for apodization or for superresolution simply by rotating the analyzer. The fact that the unpredictable phase introduced by the mask has no harmful effect on the performance of the system makes it convenient for several applications. Expressions are obtained for the optical transfer function and the point-spread function of such a system. Some specific cases are illustrated graphically, and some probable applications are discussed.

High tolerance to spherical aberrations and defects of focus with a birefringent lens

We report a systematic investigation of the imaging behavior of an optical system consisting of a lens from a uniaxial birefringent crystal sandwiched between two linear polarizers into which primary spherical aberration has been introduced. The proposed system has higher tolerance to primary spherical aberration and has a larger depth of focus than an imaging system found with an isotropic lens. Some specific cases are computed and illustrated graphically.

Age and sex variations in adiposity and central fat distribution among elderly Bengalee Hindus of Calcutta, India

Background : Although a sizeable portion of Indias population (13%, over 110 million) is elderly (aged > 55 years) very little information exists on their levels of adiposity and central body fat distribution. Aim : The present study seeks to investigate age and sex variations in adiposity and central fat distribution among urban elderly Bengalee men and women. Subjects and methods : A cross-sectional study of 410 (210 men and 200 women) elderly (> 55 years) urban Bengalee Hindu individuals resident in Calcutta, India, was undertaken utilizing various measures of adiposity and central fat distribution. Results : There existed significant sex differences in various anthropometric variables and indices. Age had significant negative association with most variables and indices in both sexes. In general, the associations were much stronger in men. Regression analysis demonstrated that age had significant negative effect on height, sitting height (SH), weight, body mass index (BMI), minimum waist (MWC), maximum hip (MHC) and mid upper arm (MUAC) circumferences and triceps skinfold (TSF), in both sexes. Age also had significant negative impact on conicity index (CI) in men. Conclusions : The present investigation revealed that there is a significant inverse age trend in adiposity among urban elderly Bengalee Hindus. Moreover, there existed sex differences in the effect of age on various anthropometric measures.

Simulation of effects of phase and amplitude coatings on the lens aperture with polarization masks

It is well known that circular symmetric phase and amplitude coatings on the lens aperture modify the imaging qualities of a lens. The present paper shows that the effects of such phase and amplitude coatings on the imaging properties of a lens can be achieved by using suitably oriented polarization masks on the lens aperture. The fact that the response of such a system can be continuously varied by changing the orientation of the polarizing devices included in the system lends it a versatility unobtainable by the use of conventional phase and amplitude coatings on the lens aperture.

Generation of an elliptic hollow beam using Mathieu and Bessel functions.

A new (to our knowledge) technique for the generation of a propagation-invariant elliptic hollow beam is reported. It avoids the use of the radial Mathieu function and hence is mathematically simpler. Bessel functions with their arguments having elliptic locus are used to generate the mask, which is then recorded using holographic technique. To generate such an elliptic beam, both the angular Mathieu function, i.e., elliptic vortex term, and the expression for the circular vortex are used separately. The resultant mask is illuminated with a plane beam, and the proper filtering of its Fourier transform generates the expected elliptic beam. Results with both vortex terms are satisfactory. It has been observed that even for higher ellipticity the vortices do not separate.

Frequency Response Characteristics of a Perfect Lens Partially Masked by a Retarder

Abstract The optical frequency response of a perfect lens partially masked by a retarder has been studied. The optical transfer function (OTF) of such a system depends upon the relative orientation of the analyser, placed at the output, relative to the slow and fast axes of the mask, and also on the relative aportioning of the lens aperture by the mask. The OTF of the system also depends upon the retardation introduced by the mask. In the present paper, we have computed the OTF of the masked lens for different values of the relevant parameters with and without the analyser at the output.

Sensitivity Enhancement of Turn-Around-Point Long Period Gratings By Tuning Initial Coupling Condition

Long period grating (LPG) at turn-around-point (TAP) has been studied with a view to enumerate the dependence of sensitivity of a particular resonant mode at the TAP to surrounding refractive index on the initial coupling strength. It has been shown theoretically and also validated experimentally that sensitivity can be enhanced significantly by tailoring the coupling strength of the cladding mode at the resonant wavelength near the TAP. Sensitivity characteristics have been studied for surrounding refractive index in the range 1.335-1.360, which is of interest in the field of biosensors, where the sensitivity of conventional LPGs is relatively small. We could attain a sensitivity of ~1850 nm/RIU using a TAP-LPG with ~3-dB attenuation at resonance.

Vector Wave Imagery with a Lens Masked by Polarizers

Abstract The optical transfer function (OTF) of an imaging system using polarization devices depends in general on the state of polarization of the input beam. In fact, the ‘amplitude point spread function’ (APSF) of such a system is different for different linearly polarized components of the input beam. This implies that the APSF of a system using polarizing devices behaves as a vector quantity. The nature of the final image is determined by the relative contribution of the two components of the vector APSF. The specific imaging system considered in the present study consists of a circularly symmetric polarization mask with linear polarizers having different orientations for the central circular and annular regions of an ideal lens aperture. If the light originating from an object point is in general elliptically polarized, the OTF of such a system with an analyser at the output is seen to have three additive components, which are, in fact, the Fourier coefficients of the OTF which varies periodically w...