Tulsi Anna

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.

High-resolution full-field optical coherence microscopy using a Mirau interferometer for the quantitative imaging of biological cells

In this paper quantitative imaging of biological cells using high-resolution full-field optical coherence microscopy (FF-OCM) is reported. The FF-OCM was realized using a swept-source system, a Mirau interferometer, and a CCD camera (a two-dimensional detection unit). A Mirau-interferometric objective lens was used to generate the interferometric signal. The signal was analyzed by a Fourier analysis technique. Optically sectioned amplitude images and a quantitative phase map of biological cells such as onion skin and red blood cells (RBCs) are demonstrated. Further, the refractive index profile of the RBCs is also presented. For the 50× Mirau objective, the experimentally achieved axial and transverse resolution of the present system are 3.8 and 1.2 μm, respectively. The CCD provides parallel detection and measures enface images without X, Y, Z mechanical scanning.

Simultaneous tomography and topography of silicon integrated circuits using full-field swept-source optical coherence tomography

We demonstrate simultaneous tomography and topography of micro-electro-mechanical systems based on silicon integrated circuits using full-field swept-source optical coherence tomography. The optical set-up consists of a swept-source system, a compact Michelson interferometer, and an area detector. The swept-source system comprises a superluminescent diode as a broadband light source in conjunction with an acousto-optic tunable filter as a fast frequency tuning device. By means of sweeping the frequency of the light source, multiple interferograms were recorded and both the amplitude and the phase map of the interference fringe signal were reconstructed. Optically sectioned images of the silicon integrated circuits were obtained by selective Fourier filtering and the topography was retrieved from the phase map. The main advantages of the proposed system are completely non-mechanical scanning, ease of alignment, high stability because of its nearly common-path geometry, and compactness.

Full-field Hilbert phase microscopy using nearly common-path low coherence off-axis interferometry for quantitative imaging of biological cells

We demonstrate single shot low coherence quantitative Hilbert phase microscopy (HPM) for the reconstruction of the two dimensional (2D) phase map of biological cells. The system is based on a compact and nearly common-path high magnification Mirau-interferometric objective lens. The spatial carrier frequency of the interference fringes was increased by means of introducing tilt in one of the arms of the interferometer, thus making the system off-axis. The system is user friendly as the interference fringes and imaging of objects with high lateral and axial resolution can be obtained quickly using a low cost commercially available microscope. Experimental results for the 2D phase map of polystyrene spheres and human red blood cells (RBCs) are presented. Hilbert transform fringe analysis was used for reconstructing the phase map and refractive index (RI) of the objects. For dynamic substances which change rapidly, single shot low coherence interferometric microscopy is an important method for obtaining the phase. Experimental results with increased field-of-view and large tilt angle are also presented. It is well known that on increasing the tilt angle for improved spatial phase sampling the object remains focused in only a small area even though the field-of-view is large. This limitation was overcome by means of vertical scanning low coherence interferometry. Due to the low coherence properties of the light source the interference occurs only at the desired location of the object, i.e., where the object is sharply focused. The object was vertically scanned and the single shot interferograms were recorded for every scan and analyzed by Hilbert transform. In this way a large area of the sample can be imaged quantitatively.

Tomographic and volumetric reconstruction of composite materials using full-field swept-source optical coherence tomography

We demonstrate tomographic and volumetric reconstruction of composite material using a full-field swept-source optical coherence tomographic (FF-SS-OCT) system. The FF-SS-OCT system is based on a co-axial and common-path optical interferometric system based on Mirau objective lens. By means of sweeping the frequency of the broad-band light source, multiple spectral interferograms are recorded and stacked in the X–Y–λ axes and processed in a computer. Optically sectioned images of the composite material at different depths are reconstructed. From the spectral interference fringe signal, the phase maps at various depths are also reconstructed. Due to its common path configuration, the present FF-SS-OCT system is highly stable and less sensitive to external vibration and does not require lateral B-scan leading to a large area measurement of the sample.

Scientific and Engineering Applications of Full-field Swept-source Optical Coherence Tomography

We report the development of full-field swept-source optical coherence tomography (SS-OCT) in the wavelength range of 815-870 nm using a unique combination of super-luminescent diode (SLD) as broad-band light source and acousto-optic tunable filter (AOTF) as a frequency-scanning device. Some new applications of full-field SS-OCT in forensic sciences and engineering materials have been demonstrated. Results of simultaneous topography and tomography of latent fingerprints, silicon microelectronic circuits and composite materials are presented. The main advantages of the present system are completely non-mechanical scanning, wide-field, compact and low-cost.

Improvement of the dynamic range using background subtraction in single shot wide-field optical coherence tomography

We investigated on the signal in single shot wide-field optical coherence tomographic (SS-WF-OCT) system to improve the dynamic range (DR). The SS-WF-OCT system is based on two-dimensional (2D) polarization Michelson interferometer and superluminescent diode (center wavelength of 842.5 nm) as light source. Two π-phase-shifted interferograms were acquired simultaneously using a single CCD camera and after subtraction, the en-face OCT image (area (x) 4.0 mm × (y) 4.3 mm) is obtained using 2D Hilbert transform. The OCT signal including incoherent background noises was analyzed. To improve the DR, background noise subtraction has been introduced and its measurement process is presented. This method is valuable during the background noise is stable. Using the scattering samples, such as, grind metal and polymer sponge with background subtraction algorithm, a significant reduction in background noise and improvement in the DR was demonstrated .

Full-field swept-source optical coherence tomography with Gaussian spectral shaping

A swept source system was realized in the wavelength range of 810-875 nm with the combination of a broad-band superluminescent diode (SLD) and an acousto-optic tunable filter (AOTF) as a frequency-tuning device. SLD has two spectral centers at 820 nm and 845 nm with spectral bandwidth (FWHM) of around 40 nm. Gaussian spectral shaping was performed onto the original SLD spectrum while reconstructing OCT images for various test samples such as onion slice and fingerprint impression taken on a glass substrate. As a pulse can be considered a Gaussian distribution of frequencies, spectral shaping yields sharper Fourier peaks. Application of Gaussian spectrum facilitates in precisely locating the reflective boundaries within the sample that results in improved OCT images.

Transmission Mode Full-Field Swept-Source Optical Coherence Tomography for Simultaneous Amplitude and Quantitative Phase Imaging of Transparent Objects

A transmission mode full-field swept-source optical coherence tomographic system is demonstrated for simultaneous amplitude and quantitative phase imaging of transparent objects. The experimental setup consists of a swept-source system, fiber-optic Mach-Zehnder interferometer, and an area detector. By means of sweeping the wavelength of light source, multiple interefrograms were recorded and both amplitude as well as phase images of the multilayer object and onion skin were reconstructed. The main advantages of the present system are: light passes through the sample only once; high throughput; wide field; compact; and completely nonmechanical scanning.

Simultaneous Topography and Tomography of Micro‐structures using Full‐field Swept‐source Optical Coherence Tomography

A full‐field swept source optical coherence tomographic (FF‐SS‐OCT) system has been reported for the simultaneous topography and tomography of the microstructures. Our optical set‐up includes a swept‐source which is a combination of super‐luminescent diode (SLD) and acousto‐optic tunable filter (AOTF), a compact Michelson interferometer and an area detector (CCD). Interference fringe signal was recorded by tuning the wavelength in a regular interval and fringe analysis was done by using Fourier‐transform technique. By means of computing the inverse Fourier transform of the interference fringe signal both optically sectioned images (amplitude) and phase map of the microstructures were obtained. The present system is completely non‐mechanical scanning, highly stable and very cost effective due to which it can be explored for many scientific and engineering applications.