Kallol Bhattacharya

Transmission digital holographic microscopy based on a beam-splitter cube interferometer

A new optical configuration for digital holographic microscopy is presented. Digital off-axis holograms are recorded by use of a single cube beam splitter in a nonconventional configuration to both split and combine a diverging spherical wavefront as it emerges from a single point source. Both the amplitude and the phase can then be reconstructed, yielding intensity and phase images with improved resolution. The novelty of the proposed configuration is its simplicity, minimal number of optical elements, insensitivity to vibration, and its inherent capability to compensate for the phase curvature that results from the illuminating wavefront in the case of microscopic samples.

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.

Possibility of an optical focal shift with polarization masks.

The polarization phase shift (PPS) has emerged as an important analytical tool in optical metrology. The present study utilizes the concept of controlling the polarization phase in applications such as focal shift and automatic focusing. When elliptically polarized light, in general, is incident upon a circularly symmetric polarization mask consisting of circular and annular zones with each zone having a unique linear polarizability, the polarization-phase difference introduced between the polarization-masked zones is also circularly symmetric. With the mask at the lens aperture, the polarization phase introduced is multiplicative with the lens function and is shown to result in a shift of the Gaussian focus plane. Because the polarization phase can be controlled by variation of the polarization parameters, the effective focal length of the imaging system can be varied within a small range. A study of the point-spread functions at the shifted focal planes has shown that the quality of the focal patch in these planes is comparable with that produced by a diffraction-limited imaging system at Gaussian focus. The shift of focus can be achieved by control of the polarization of the input beam. It is anticipated that this technique may find application in areas for which dynamic focusing within a small range is required.

Performance of a polarization-masked lens aperture in the presence of spherical aberration

This paper suggests the possibility of partial compensation of spherical aberration with a polarization-masked lens aperture. The principle is based on the fact that any pre-specified phase step can be introduced between two zones of a lens aperture masked by suitably oriented linear polarizers and employing an elliptically polarized imaging beam. By comparison of the point-spread function of such a polarization-masked imaging system with that of an ideal lens, it has been shown that the effect of spherical aberration can be appreciably compensated by making an appropriate choice of the polarization parameters involved.

Achromatic quarter-wave plate using crystalline quartz.

Achromatic wave plates are ideal components for use with tunable and multiline laser systems, broadband sources, and in astronomical instrumentation. The present study deals with the design and characteristics of two different quarter-wave achromatic retarders in the 500-700 nm range, using a cascaded system of two birefringent plates. The first of these shows a variation of less than ±0.5°, whereas the second system shows a variation of ±4° where the azimuth remains constant. Finally, a comparison between the two systems is made. The succinct and simple Jones matrix formalism has been used to derive the general expression for the equivalent retardation and azimuth of the combinations. It appears that the proposed arrangement has the promise of producing good achromatic combinations.

Self-referenced rectangular path cyclic interferometer with polarization phase shifting

A polarization phase shifting interferometer using a cyclic path configuration for measurement of phase nonuniformities in transparent samples is presented. A cube beam splitter masked by two linear polarizers is used to split the source wavefront into two counter propagating linearly polarized beams that pass through the sample. At the output of the interferometer, the two orthogonally polarized beams are rendered circularly polarized in the opposite sense through the use of a quarter wave plate. Finally, phase shifting is achieved by rotating a linear polarizer before the recording plane. In a rectangular path interferometer, although the two counter propagating wavefronts are laterally folded with respect to each other in the interferometer arms, the beams finally emerge mutually unfolded at the output of the interferometer. This phenomenon is utilized to create a reference if the sample is introduced in one lateral half of the beam in any one of the interferometer arms. The polarization phase shifting technique is used to generate four phase-shifted interferograms, which are utilized to evaluate the phase profile of the phase sample. Experimental results presented validate the proposed technique.

Reconfigurable achromatic half-wave and quarter-wave retarder in near infrared using crystalline quartz plates

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.

Polarization phase-shifting interferometric technique for complete evaluation of birefringence

A full-field technique for simultaneous measurement of the magnitude of birefringence and its orientation is presented. This is achieved using a monolithic birefringence sensitive interferometer where the interference fringes carry the information of both the birefringence phase and the orientation of the fast axis of an optically transmissive anisotropic material placed at the output of the interferometer. The interferometer consists of a suitably polarization-masked cube beam splitter, orientated as in the Gates interferometer, which serves to generate a pair of orthogonally polarized and collinearly propagating light beams. Experimental results are obtained through an algorithm incorporating eight polarization phase-shifted interferograms.

A holistic approach for Off-line handwritten cursive word recognition using directional feature based on Arnold transform

We present an off-line handwritten word recognition system.The system is based on extraction of directional features.Arnold transform followed by Hough transform is used to extract directional feature.A multi-class linear SVM is employed to recognize handwritten cursive words.Experiments are done on CENPERMI database of legal amounts written in English. This paper presents a holistic off-line handwriting recognition system based on extraction of directional features which depends on the stroke orientation distribution of cursive word. This stroke orientation distribution is estimated using Arnold transform followed by Hough transform. Besides this feature some other directional shape features are also used to form feature vector. Finally, a multi-class linear SVM is employed to recognize cursive word. Experiments are carried out on CENPARMI database of legal amount written in English and an overall accuracy of 87.19% is achieved. We have also compared our proposed method with the state-of-the-art methods for handwritten character recognition using C-Cube data-set.

Low-magnification polarization phase-shifting interference microscope for three-dimensional profilometry

An interferometric technique for three-dimensional phase measurement of optically transparent microscopic phase samples is presented. An obliquely aligned polarizer-masked cube beam-splitter, an infinity-corrected microscope objective, and a couple of simple polarization phase-shifting components serve as the setup for such a measurement. Surface phase profiles are then extracted using standard phase-shifting algorithms. The salient features of the proposed technique are its simple design, in-line configuration, possibility of integration with standard microscopic systems, and inherent compensation of the substrate phase. Experimental results are presented. The overall lateral magnification is restricted due to the low numerical aperture offered by the microscope objective and cube beam-splitter combination.