Abhijit Das

Optical sectioning microscope with a binary hologram based beam scanning.

We describe the development of a beam scanning microscope that can perform optical sectioning based on the principle of confocal microscopy. The scanning is performed by a laser beam diffracted from a dynamic binary hologram implemented using a liquid crystal spatial light modulator. Using the proposed scanning mechanism, unlike the conventional confocal microscopes, scanning over a two-dimensional area of the sample can be obtained without the use of a pair of galvo mirror scanners. The proposed microscope has a number of advantages, such as superior frame to frame repeatability, simpler optical arrangement, increased pixel dwell time relative to the time between two pixels, illumination of only the sample points without pulsing the laser, and absolute control over the amplitude and phase of the illumination beam on a pixel to pixel basis. The proposed microscope can be particularly useful for applications requiring very long exposure time or very large working distance objective lenses. In this paper we present experimental implementation of the setup using a nematic liquid crystal spatial light modulator and proof-of-concept experimental results.

High-speed zonal wavefront sensing

High speed wavefront sensing is important in real time profile analysis, analysis of fluid dynamics, ophthalmology and so on. Conventional Shack-Hartmann wavefront sensor uses an array of tiny lenses and a digital camera to record the focal spot array. Thus the frame rate of the sensor depends on the camera. In this paper we present a zonal wavefront sensor where the array of lenses is replaced by an array of gratings followed by a focusing lens. The gratings can be configured to generate just one array of focal spots. This reduction in row of the focal spot array leads to increase in the frame rate of the proposed wavefront sensor.

Zonal wavefront sensor with reduced number of rows in the detector array

In this paper, we describe a zonal wavefront sensor in which the photodetector array can have a smaller number of rows. The test wavefront is incident on a two-dimensional array of diffraction gratings followed by a single focusing lens. The periodicity and the orientation of the grating rulings of each grating can be chosen such that the +1 order beam from the gratings forms an array of focal spots in the detector plane. We show that by using a square array of zones, it is possible to generate an array of +1 order focal spots having a smaller number of rows, thus reducing the height of the required detector array. The phase profile of the test wavefront can be estimated by measuring the displacements of the +1 order focal spots for the test wavefront relative to the +1 order focal spots for a plane reference wavefront. The narrower width of the photodetector array can offer several advantages, such as a faster frame rate of the wavefront sensor, a reduced amount of cross talk between the nearby detector zones, and a decrease in the maximum thermal noise. We also present experimental results of a proof-of-concept experimental arrangement using the proposed wavefront sensing scheme.

Note: Laser beam scanning using a ferroelectric liquid crystal spatial light modulator

In this work we describe laser beam scanning using a ferroelectric liquid crystal spatial light modulator. Commercially available ferroelectric liquid crystal spatial light modulators are capable of displaying 85 colored images in 1 s using a time dithering technique. Each colored image, in fact, comprises 24 single bit (black and white) images displayed sequentially. We have used each single bit image to write a binary phase hologram. For a collimated laser beam incident on the hologram, one of the diffracted beams can be made to travel along a user defined direction. We have constructed a beam scanner employing the above arrangement and demonstrated its use to scan a single laser beam in a laser scanning optical sectioning microscope setup.

Note: A simple experimental arrangement to generate optical vortex beams

In this Note, we present a simple experimental arrangement to generate optical vortex beams. We have demonstrated how by taking print of an interferogram on a transparent sheet, vortex beams with various topological charges can be generated. Experimental results show that the vortex beam indeed carries the topological charge that is used to compute the interferograms. In addition to being simple and inexpensive, one major advantage of the arrangement is that it makes it possible to generate different vortex beams quickly, unlike using the photographic process to create the holograms.

Adaptive Packet Throttling Technique for Congestion Management in Mesh NoCs

Network on Chip is an emerging communication framework for multi-core systems. Due to increasing number of cores and complex workloads, congestion management techniques in NoC are gaining more research focus. Packet throttling is one of a cost effective technique for congestion management. It delays the packet injection into the network, thereby regulating traffic in network and hence provide ease of packet movement generated by other critical applications. Finding point of throttling and rate of throttling are two major design issues that can impact the performance and stability of any throttling algorithm. Existing state of the art throttling techniques use local throttling decision coordinated by a single central controller. We overcome the issues related with this by partitioning the network into number of subnetworks, each with a zonal controller. Our experiment results in 8 \(\times \) 8 2D mesh with real traffic workloads consisting of SPEC 2006 CPU benchmarks shows an average packet latency reduction of 6.2% than the state of the art packet throttling techniques.

Beam deflection by an aperiodic binary diffraction grating

In this article we investigate the beam deflection by a one-dimensional binary diffraction grating. We propose a simple theoretical expression that can be used to predict the beam deflection angle due to both periodic and aperiodic binary grating profiles. We show that the theoretically calculated beam deflection angles agree well with the experimentally obtained deflection angles for various grating patterns. Thus the expression can be used to precisely position the deflected beam at a predetermined location. Further, we show that the theoretical expression can be used to construct a map between the spatial frequency of the grating and the deflection angle which can be employed to deflect the beam at equal intervals by sequentially changing the spatial frequency in accordance with the map. We also demonstrate the superior beam repeatability of a binary grating based beam scanner.

Dynamic Control of Illumination Beam Phase Profile in a Scanning Optical Microscope

We propose a scanning optical microscope, for samples introducing spatially varying aberrations to the illumination beam. It is implemented with a microscope that has binary hologram based beam scanning mechanism where illumination beam phase profile is varied from pixel to pixel. Unlike a conventional scanning microscope, the scanning is achieved by the beam diffracted from a binary hologram written on the display panel of a liquid crystal spatial light modulator. The aberration correction is achieved without a separate wavefront sensor. For correcting the aberration in the illumination beam the signal is maximized by changing the shape of the binary hologram in terms of chosen Zernike mode coefficients.

Point Scanning Microscope with Adaptive Illumination Beam Intensity

In this paper we describe a point scanning optical microscope where the illumination beam can be programmably controlled in real time using a liquid crystal spatial light modulator (LCSLM). With an appropriate pattern displayed on the LCSLM, the device can be made to act as a binary diffraction hologram. In the proposed microscope the illumination beam is in fact the +1 order beam diffracted from the binary hologram. By displaying a sequence of binary holograms it is possible to make a beam scanning, similar to a conventional scanning microscope. Here we use a computer generated holography technique to compute the binary holograms which facilitate complete control of the amplitude and phase profile of the illumination beam. In a number of microscopy applications using reflected light, the reflectivity of the sample plane may differ from region to region. Therefore if a single illumination beam intensity is used for the whole sample plane, then the regions with less reflectivity will be imaged with poor si...