Sivakumar Narayanswamy

Optical design of a line-focused forward-viewing scanner for optical coherence tomography

We report an optical design of a line-focused forward-viewing optical coherence tomography (OCT) scanner for high-speed endoscopic imaging. To avoid a complex lens system, an off-axis cylindrical mirror is used for focusing the line illumination onto the sample surface. Because of its insensitivity to the broadband spectrum, the mirror-focused scanner improves the image quality compared to a lens-focused scanner. In this work, a feasibility study is carried out on the use of a reflective-optics-focused line scanner in OCT imaging, instead of the traditional refractive optics scanner. The Strehl ratio, chromatic focal shift, and field analysis were carried out for a plano-convex cylindrical lens, an achromatic cylindrical lens, and a cylindrical-mirror-focused scanner. ZEMAX optical modeling analysis showed that mirror-focused scanning provides better Strehl ratio in comparison to plano-convex cylindrical-lens-focused scanning, and that the Strehl ratio is comparable to achromatic cylindrical-lens-focused scanning. However, field analysis on the edges of the scanning elements within the scan range shows that mirror-focused scanning is more robust when compared to a cylindrical achromatic lens. Overall, a mirror-focused scanner shows better performance compared to lenses.

Design of spectrometer for high-speed line field optical coherence tomography

The quality of the spectrometer affects the sensitivity fall-off, axial resolution, and depth scan range, therefore overall performance of the spectral domain optical coherence tomography (SD-OCT) imaging. Chromatic aberration, optical resolution, and detector array resolution are the key design consideration for high-quality OCT spectrometer. Traditionally refractive optics spectrometer is used in SD-OCT. In the present work, the optical design of the reflective optics spectrometer and of the refractive optics spectrometers is reported for high-speed line field optical coherence tomography imaging. The performance of the spectrometers was compared by using the ZEMAX optical design software. The ZEMAX optical modeling analysis shows that the reflective optics spectrometer provides better performance by comparison with the refractive optics spectrometer.

Experimental investigation of texturing complex geometry using high repetition nano laser and comparison with the simulated COMSOL model

The effect of the laser and the optomechanical parameters in the micromachining process of the complex geometry is the challenging part in the manufacturing industry due to wide range of materials. There are limited ways to find the best process parameters for machining and texturing with specific depth, thickness and roughness. The COMSOL software was used to model all the laser parameters like laser power, sampling rate, and optomechanical parameters like pulse overlap. Presented simulation demonstrates the roughness, depth and thickness of machined path. In addition, from the simulation point of view, the laser and optomechanical parameters can be optimized for the specific depth and thickness. To validate the numerical model, experiments are conducted for different process parameters by changing the laser power, varying the sampling rate of the laser and data acquisition card, changing the pulse overlap and the results are tabulated. Also the same input parameters are given to numerical simulation and the results are in good agreement with experimental outcomes. In conclusion, the simulated model can be used to estimate the effect of the process parameters before the machining. So that the presented model has the control over the machined surface quality and the process can be optimized by giving different material properties in the simulation.

Innovative approaches for coordinate extraction of curved shape and analyzing the effect of process parameters on the quality of the laser micro-machined surface

Laser micro machining is one of the micro manufacturing processes. Since it has wide range of applications in Microelectronics, medical device, aerospace etc., the accuracy of the process is the most significant factor. In this study, the challenges and corresponding possible solutions that are encountered in machining complex geometries are addressed. Furthermore, the effect of process parameters on the overall quality of the manufacturing are discussed. The paper proposes mathematical function based and image processing based algorithms to find the machining coordinates. The former calculates the first and second derivatives of the functions to find the essential coordinates and the latter converts pixels of the image into coordinates. Achieving required overlap is one of the difficulty in the curved shapes, which can be solved by using both the algorithms. However, the function-based approach is more efficient as the image processing approach depends on the image resolution. Lower resolution results in reduction in smoothness of the extracted coordinates and higher resolution leads to increased computational cost. By changing the different laser parameters such as laser power and mechanical parameters like sampling rate of data acquisition card; different roughness, depth and overlaps can be attained. The study demonstrates precision micromachining and it establishes optimal relation between the process parameters and quality of the machined surface.

ANSYS simulation as a feasibility study for high repetition laser ultrasonic non-destructive evaluation (NDE)

The characterization of delamination in Carbon Fibre Reinforced Plastic (CFRP) composite laminates with laser generated ultrasonic shock waves is presented. This paper reports the understanding of shock-wave prompted free surface vibration which intern can help a user to estimate the frequency range for the material evaluation, wherein the model in the software contains an artificial internal delamination located at the middle in the thickness direction. The propagation characteristics of ultrasonic waves are in accordance with the pulsed laser in the nanosecond regime. The laser can produce a power density optimal enough for ultrasonic evaluation. The pressure deposited on the specimen would importantly be a function of the laser power density, and the acoustic impedance of the specimen. The comparison of free surface velocities and/or the deformation between the two models with and without any delamination, shows the change in the propagation of shock wave due to discontinuity. Also, the analysis is done by varying the location of the delamination in the model. The transducer or an interferometer can be selected based on the magnitude of the velocity and/or the deformation, for the ultrasound detection and evaluation.

Optimized off-axis cylindrical mirror-focused line-scanning system for optical coherence tomography imaging applications

The parameters of an off-axis cylindrical mirror-focused line-scanning system were studied to optimize the flatness of the 2 mm scan field. The scanning system parameters included the beam size, the distance between the scanning and the focusing mirror, the angle between the incident beam and the reflected beam, the optical scan angle, and the effective focal length of the cylindrical mirror. Because of the off-axis line-scanning system configuration, the scanning could be carried out either in the tangential (Y-scan) or in the sagittal (X-scan) plane. A 53 nm spectral bandwidth light source was used to evaluate the imaging performance of the scanning system. Since reflective optics is employed in this work for focusing, the scanning system could be used with a higher spectral bandwidth light source for optical coherence tomography applications. The effect of the angle between of the incident and reflected beams, the distance between the mirrors, the focal length of the cylindrical mirror and the scanning directions, on the flatness of the scan field were studied. It was proved that the sagittal scanning is least sensitive to variations in scanning system parameters and thus provides maximum flexibility in design.

Low power ultrasound inhibits cell proliferation and invasion of human cancer cells in vitro

Background: Applications of ultrasound in medicine for therapeutic purposes have been accepted, and they have several beneficial uses for many years. However, the outcome of low power ultrasound waves on cell proliferation, especially cell cycle progression and invasion as well as their associated genes on human breast and cervical cancer cells has not been investigated yet. Therefore, we examined the effect of low power ultrasound on BT20, BT20-E6/E7 and HeLa cell lines. Materials and Methods: BT20, BT20-E6/E7 and HeLa cell lines were used in this study. On the other hand, cell proliferation, cell cycle, and invasion assays were applied to study the effect of low ultrasound irradiation on these cell lines. Meanwhile, western blot was performed to study the expression patterns of some selected genes associated with this effect. Results: We found that low power ultrasound inhibits cell proliferation and provokes G0-G1 cell cycle arrest and reduction of S as well as an increase in the G2-M phase of HeLa cells in comparison with the untreated cells. This is accompanied by a down-regulation of Cdk-6 (cyclin dependent kinase) which is a major control switch for the cell cycle. Moreover, low power ultrasound inhibits cell invasion and consequently down-regulates the expression of Id-1, caveolin, and EGF-R which are widely considered as main regulators of cell invasion and metastasis of human cancer. Conclusion: These results suggest that application of low power ultrasound on human breast and cervical cancer could be an effective method to reduce cell proliferation and invasion of these cancers.