Chandra Shakher

A review on refractive index and temperature profile measurements using laser-based interferometric techniques

In this review we have discussed laser-based interferometric techniques for measurement of refractive index, temperature and temperature profile of burners with special emphasis on laser speckle techniques and Talbot interferometric technique.

Talbot interferometer with circular gratings for the measurement of temperature in axisymmetric gaseous flames

A detailed study for measuring the temperature distribution in axisymmetric flames by using a Talbot interferometer with circular gratings is presented. We increased the sensitivity of the interferometer by optimizing the pitch of the grating and the Talbot plane. We compare the experimental results with the values that were measured with a thermocouple to an accuracy of ±0.2% of full scale ±4 digits. Good agreement is seen between the temperatures measured by use of a thermocouple and those measured by use of Talbot interferometry.

Fingerprint detection using full-field swept-source optical coherence tomography

We report the application of full-field swept-source optical coherence tomography for fingerprint detection. This system consists of a superluminescent diode as broadband light source and an acousto-optic tunable filter as wavelength-tuning device. The conventional optical coherence tomographic system was modified by coating aluminum oxide on one side of the beam splitter which is used as reference mirror and fingerprints on the glass slide as object. Low-coherence interferometry, nonmechanical scanning, and compactness are the main advantages of the proposed system over conventional fingerprint detection techniques. The present technique is noninvasive in nature and does not require any physical or chemical processing.

Measurement of temperature using speckle shearing interferometry

A laser speckle shearing interferometric technique is used for measuring the temperature profile inside a gaseous flame. The experimental results are compared with the values obtained by a thermocouple and also by speckle photography. Good agreement is seen among the temperatures measured by speckle shearing interferometry, speckle photography, and the thermocouple. Speckle shearing interferometry is easier to implement than speckle photography. This is because in speckle shearing interferometry the accurate positions of the fringes can be known without point-by-point analysis and correction for the halo effect.

Measurement of focal length with phase-shifting Talbot interferometry

Phase-shifting interferometry with a Fourier fringe analysis technique is implemented to analyze Talbot interferometric fringes and to evaluate the focal length of a lens. A four-step algorithm is used to obtain the phase map of the lens. The slope of the phase map is related to the focal length, and it is from this relationship that we evaluate the focal length. Experimental results are presented. Our experimental study suggests that phase-shifting Talbot interferometry combined with a Fourier fringe analysis technique can be advantageously used to improve the accuracy of measurement.

Application of wavelet filtering for vibration analysis using digital speckle pattern interferometry

We have investigated the utility of wavelet filters for enhanc- ing contrast in the digital speckle pattern interferometric fringes obtained from a vibrating loudspeaker diaphragm and from the vibrating bracket of an electric motor. Experimental results reveal that use of properly chosen other filters along with the wavelet filter gives better enhancement. Our investigations also show that if the data are preprocessed by median filtering before implementing the wavelet filtering, then a weakly diffusing object requires a higher-kernel filter and strongly diffusing object requires lower-kernel one.

Simultaneous topography and tomography of latent fingerprints using full-field swept-source optical coherence tomography

We demonstrate simultaneous topography and tomography of latent fingerprints using full-field swept-source optical coherence tomography (OCT). The swept-source OCT system comprises a superluminescent diode (SLD) as broad-band light source, an acousto-optic tunable filter (AOTF) as frequency tuning device, and a compact, nearly common-path interferometer. Both the amplitude and the phase map of the interference fringe signal are reconstructed. Optical sectioning of the latent fingerprint sample is obtained by selective Fourier filtering and the topography is retrieved from the phase map. Interferometry, selective filtering, low coherence and hence better resolution are some of the advantages of the proposed system over the conventional fingerprint detection techniques. The present technique is non-invasive in nature and does not require any physical or chemical processing. Therefore, the quality of the sample does not alter and hence the same fingerprint can be used for other types of forensic test. Exploitation of low-coherence interferometry for fingerprint detection itself provides an edge over other existing techniques as fingerprints can even be lifted from low-reflecting surfaces. The proposed system is very economical and compact.

Temperature measurement in laminar free convective flow using digital holography

A method for measurement of temperature in laminar free convection flow of water is presented using digital holographic interferometry. The method is relatively simple and fast because the method uses lensless Fourier transform digital holography, for which the reconstruction algorithm is simple and fast, and also the method does not require use of any extra experimental efforts as in phase shifting. The quantitative unwrapped phase difference is calculated experimentally from two digital holograms recorded in two different states of water--one in the quiescent state, the other in the laminar free convection. Unknown temperature in laminar free convection is measured quantitatively using a known value of temperature in the quiescent state from the unwrapped phase difference, where the equation by Tilton and Taylor describing the variation of refractive index of water with temperature is used to connect the phase with temperature. Experiments are also performed to visualize the turbulent free convection flow.

Measurement of temperature and temperature profile of an axisymmetric gaseous flames using Lau phase interferometer with linear gratings

Abstract In this paper we have investigated the utility of Lau phase interferometer with linear gratings and white light source to measure temperature and temperature profile of an axisymmetric flame. The temperature measured using Lau phase interferometer is in good agreement with the temperature measured with thermocouple and datalogger. Details of theoretical analysis and experimental investigation are presented.

Collimation testing with circular gratings

We investigate the testing of collimation by Talbot interferometry with circular gratings. Circular gratings are immune to misalignment owing to rigid body rotations in a plane perpendicular to the optic axis. Experimental results of the investigation are presented, and they are in good agreement with the theoretical predictions. Experimental results verify that collimation testing with circular gratings can be superior to that with linear dual-field gratings.