Sanjukta Sarkar

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.

Polarization phase shifting interferometric technique for phase calibration of a reflective phase spatial light modulator

Abstract. Calibration of phase in spatial light modulators is a prerequisite for applications where a prespecified phase distribution needs to be implemented over the surface of the modulator. The present work proposes a full-field polarization phase shifting interferometric technique, based on the Twyman-Green interferometer, for the purpose.

Polarization phase shifting cyclic interferometer for surface profilometry of non-birefringent phase samples

A rectangular path cyclic interferometer has the unique property that the counter-propagating wavefronts travelling in the interferometer arms are folded with respect to each other in the plane of the interferometer although the two wavefronts finally emerge from the interferometer unfolded. A phase disturbance introduced in one lateral half of the interferometer arm is therefore manifested in complementary lateral halves of the observed interference pattern. This phenomenon is utilized to evaluate the surface profile of a reflecting sample placed on one of the interferometer mirrors. The sample phase is retrieved using polarization phase shifting. Experimental results showing three-dimensional surface morphology of a small scale integrated circuitry directly etched on silicon are presented.

Application of a birefringent lens in photoelastic studies using a coherent moiré technique

Abstract In photoelastic investigations, one uses the property of stress induced birefringence for stress analysis. Normal polariscopes are used for such studies. However, the accuracy of the methods is low. For better accuracy, interferometric and holographic methods are generally used. Holography, along with a shearing technique, is also used to obtain partial derivatives of displacement with respect to spatial co-ordinates. In these methods, the resultant fringe pattern is obtained from the mutually shifted holograms and the generation of such patterns requires four exposures altogether. The present paper represents a method based on a coherent moire technique for studying the first derivative of a stress pattern using birefringence properties. The analysis seems to be very similar to holographic techniques as both produce second order fringes. The advantage of this method is that it requires only one exposure. The basic principle as well as some experimental results are presented.

Optical method of determination of stress at a point

A method of determining stress at a point is suggested here. The effect of bending of a wave front that is due to variations of the refractive index is used to measure different aspects of stresses. A Fourier lens with a cross slit at its front focal plane is used to form interference fringes at planes near its back focal plane. The sample, illuminated by a plane-parallel coherent beam of light, is placed close to a cross slit, and the change in fringe pattern due to axial shift of the spectrum planes of the slits is measured to relate it to the state of stress.

Digital holo-microscopy using a cube beam-splitter interferometer in parallel beam configuration

In this present study an attempt has been made to reduce the complexities of the digital recording and reconstruction processes through the use of a Gates interferometer employing a plane parallel beam. The use of a single cube beam-splitter for the recording process serves to eliminate phase noises associated with use of multiple optical elements. The proposed technique is discussed in detail and some reconstructions are presented.

Development of in-line laser interference microscope

Interference Microscope with exception of the widely used Mirau objective, involves a setup having Michelson or Mach-Zender Configuration. The present work proposes an in-line interference laser microscope configuration.

Edge enhancement in digital holo-microscopy

A simple edge enhancement technique in the digital holo-microscopy is presented here. In Digital Holo-microscopy (DHM) the intensity distribution of the CCD is produced by the interference of a plane reference wave and that scattered by the object. The reconstruction is accomplished by multiplication of the digitally stored hologram with a digital model of the reference wave and subsequent numerical determination of the diffracted field of the object in a defined image plane. Hence, a focused and a defocused version of the object may be reconstructed from only one recorded hologram by varying the reconstruction distance during numerical reconstruction. The edge enhancement of the object is possible by simply subtracting this numerically reconstructed defocused real image from the focused real image. It is interesting to note that using this technique edge enhancement technique is possible for amplitude and phase objects. The simulation and experimental results presented validate our theoretical expectations.

Microscopy of non-birefringent transmissive phase samples using Sagnac laser interferometer

A cyclic interferometer, appropriately combined with a long working distance microscope objective, is adapted for quantitative phase microscopy. In such an arrangement, the sample information, in terms of the diffracted orders emerging from the sample, is carried by both the counter propagating beams within the cyclic interferometer. However, positioning the sample close to the input/output cube beam splitter and use of a suitably converging laser beam of light as the input to the interferometer ensure that only one of the counter propagating beams carries the object information to the objective while the other beam, which serves as the reference, allows only the undiffracted component to contribute to the process of image formation. Use of suitable polarization optics renders the interferometer its polarization phase shifting property. Using the proposed arrangement, the experimental results showing the quantitative 3D phase rendering of polystyrene microspheres and micro-wells etched in glass are presented.

Digital holomicroscopy for nanometer depth resolution

We report a technique which provides the details of three dimensional phase profile of a transparent microscopic sample. The depth resolution achievable is of the order of nanometers. The physical dimensions of the sample are obtained from the numerically reconstructed phase of the object wave. The method involves digital holographic recording of interferograms produced by superposition of the object wave and the coherent reference wave. We sequentially record four different holograms by altering the phase differences between the object wave and the reference wave. To introduce different phase differences between the two interfering beams, we have made use of polarization phase shifting. Therefore, the configuration used is simple and provides accuracy as the phase is introduced by simply rotating a linear polarizer. Experimental results for a microhole etched on glass are presented.