Bosanta R. Boruah

Stimulated emission depletion microscopy with a supercontinuum source and fluorescence lifetime imaging

We demonstrate stimulated emission depletion (STED) microscopy implemented in a laser scanning confocal microscope using excitation light derived from supercontinuum generation in a microstructured optical fiber. Images with resolution improvement beyond the far-field diffraction limit in both the lateral and axial directions were acquired by scanning overlapped excitation and depletion beams in two dimensions using the flying spot scanner of a commercially available laser scanning confocal microscope. The spatial properties of the depletion beam were controlled holographically using a programmable spatial light modulator, which can rapidly change between different STED imaging modes and also compensate for aberrations in the optical path. STED fluorescence lifetime imaging microscopy is demonstrated through the use of time-correlated single photon counting.

Dynamic manipulation of a laser beam using a liquid crystal spatial light modulator

This article describes a graduate level optics laboratory experiment on the manipulation of the wavefront of a laser beam using a spatial light modulator. A computer generated holography technique is employed to generate a custom defined wavefront, realized in the +1 diffraction order when a collimated laser beam is diffracted by a binary transmission hologram. The hologram is written on a liquid crystal spatial light modulator and can be updated at a video rate using a personal computer interface.

Susceptibility to and correction of azimuthal aberrations in singular light beams

We show how the effects of azimuthal optical aberrations on singular light beams can result in an intensity modulation in the beam waist or focal point spread function (PSF) that is directly proportional to the amplitude of the applied phase aberration. The resulting distortions are enough to significantly degrade the utility of the singular beams even in well corrected optical systems. However we show that pattern of these intensity modulations is related to the azimuthal order of the applied aberration and we suggest how this can be used to measure those aberrations. We demonstrate a closed loop system using a liquid crystal spatial light modulator as a programmable diffractive optical element to both generate the beam and correct for the sensed aberrations based on feed back from a CCD detected intensity image of the focal point spread function.

Improved wavefront reconstruction algorithm for Shack–Hartmann type wavefront sensors

In the present work, we propose an improved wavefront reconstruction algorithm for zonal wavefront estimation using Shack–Hartmann type wavefront sensors. We start with the well known W H Southwell reconstruction algorithm, where phase at a central point is described in terms of horizontal and vertical slope values. We develop the mathematical expressions to show that by incorporating the diagonal slope values in addition to the horizontal and vertical slope values, accuracy in phase estimation can be increased. We present here experimental results that demonstrate significant improvement in the wavefronts, estimated using the proposed algorithm, in comparison to the Southwell algorithm.

Lateral resolution enhancement in confocal microscopy by vectorial aperture engineering

This article reports the design and implementation of a lateral resolution-enhancement technique in confocal microscopy that can work, in principle, either in the reflection mode or in the fluorescence mode. Taking the difference between two images corresponding to two different vectorially (involving amplitude, phase, and polarization of light) engineered illumination pupils or apertures of a confocal microscope, high spatial frequency contents in the resultant image can be significantly enhanced. This can be realized by incorporating an extra vectorial beam-forming element into the illumination beam path of a conventional confocal microscope. The method of the proposed technique has been explained by giving it an analytical treatment supported by numerical simulation results. The technique has been implemented in a reflection mode confocal microscope and results obtained are presented.

Zonal wavefront sensing using an array of gratings

We describe a zonal-wavefront-sensing technique using an array of plane diffraction gratings. A spatially coherent beam, whose wavefront is to be measured, is incident on the array of gratings. The direction of a diffracted beam of a certain diffraction order is a function of the orientation and periodicity of the corresponding grating. Thus, by choosing the orientation and periodicity of each grating appropriately and by having a lens immediately behind the grating array, it is possible to get an array of focal spots. The profile of the incident wavefront can be estimated from the displacements of these focal spots relative to those due to an unaberrated beam. The arrangement makes it possible to increase the separation between two adjacent focal spots corresponding to two nearby gratings without effecting the areas of the gratings. Consequently, a relatively large dynamic range in wavefront measurement can be achieved without compromising the accuracy. With the arrangement it is also possible to use a photodetector array whose outline is independent of the grating array outline. The proposed wavefront-sensing technique is implemented experimentally using a liquid-crystal spatial-light modulator in conjunction with a CCD camera, and the obtained results are presented.

Laser scanning confocal microscope with programmable amplitude, phase, and polarization of the illumination beam

We describe the design and construction of a laser scanning confocal microscope with programmable beam forming optics. The amplitude, phase, and polarization of the laser beam used in the microscope can be controlled in real time with the help of a liquid crystal spatial light modulator, acting as a computer generated hologram, in conjunction with a polarizing beam splitter and two right angled prisms assembly. Two scan mirrors, comprising an on-axis fast moving scan mirror for line scanning and an off-axis slow moving scan mirror for frame scanning, configured in a way to minimize the movement of the scanned beam over the pupil plane of the microscope objective, form the XY scan unit. The confocal system, that incorporates the programmable beam forming unit and the scan unit, has been implemented to image in both reflected and fluorescence light from the specimen. Efficiency of the system to programmably generate custom defined vector beams has been demonstrated by generating a bottle structured focal volume, which in fact is the overlap of two cross polarized beams, that can simultaneously improve both the lateral and axial resolutions if used as the de-excitation beam in a stimulated emission depletion confocal microscope.

The generation of arbitrary vector beams using a division of a wavefront-based setup

In this paper, we introduce an arbitrary vector-beam-forming scheme using a simple arrangement involving only one liquid crystal spatial light modulator. An arbitrary vector beam can be obtained by overlapping two orthogonally polarized beams. In most of the existing vector-beam-forming schemes the two orthogonally polarized beams are essentially copies of a single incident wavefront. However, in the proposed scheme the two orthogonally polarized beams correspond to two separated parts of a single incident wavefront. Taking a cue from the two-beam interference phenomenon, the present scheme can be referred to as a division of a wavefront-based scheme. The proposed setup offers certain important advantages and is more suitable for the generation of higher average-power vector beams. We demonstrate the working of the vector-beam-forming scheme by generating various vector beams such as radially polarized, azimuthally polarized, and Bessel–Gauss beams and also a boat-shaped beam in the focal volume of a low-numerical-aperture focusing lens. The boat-shaped beam comprises a dark center surrounded by intense light from all but one direction. The beam is realized at the focus of an azimuthally polarized beam in the presence of a moderate amount of coma in the beam. The experimental results obtained using the proposed setup are verified by comparing them with the theoretical results.

Interferometry using binary holograms without high order diffraction effects

We describe a technique for a phase-stepping interferometer based on programmable binary phase holograms, particularly useful for optical testing of aspheric or free-form surfaces. It is well-known that binary holograms can be used to generate reference surfaces for interferometry, but a major problem is that cross talk from higher diffraction orders and aliasing can reduce the fidelity of the system. Here, we propose a new encoding technique which improves the accuracy of the technique and demonstrate its implementation using a binary liquid crystal spatial light modulator.

Programmable diffractive optics for laser scanning confocal microscopy

In this work, we describe the design and implementation of a laser scanning confocal microscope with active beam forming optics. We demonstrate dynamic control over intensity and polarization properties of the beam using the technique of programmable diffractive optics. This technique is used facilitate active aberration correction in the beam and to generate radially polarized pupil function in order to get an on-axis axially polarized point spread function. We also develop the high numerical aperture theory to calculate the focal point spread function for a radially polarized pupil function. We describe the design and implementation of a simple vector beam formation unit consisting of a polarizing beam splitter and two right angled prisms in conjunction with a ferro-electric spatial light modulator. We also describe the design and implementation of a beam scanning system comprising of a novel off-axis scanner mirror that maintains registration of the conjugate pupil planes in the system.