B. Raghavendra Prasad

High-contrast, all-optical switching in bacteriorhodopsin films

We report experiments with nonlinear-absorption-based, high-contrast, all-optical switching in photochromic bacteriorhodopsin (BR) films. The switching action is accomplished by control of the transmission of a weak probe beam through a BR sample with the help of strong pump beam illumination at 532 nm wavelength. We found that the switching properties of BR films depend on several experimentally controllable parameters such as probe wavelength, pump beam intensity, and excitation rate. A comparative study of the switching behavior and other parameters of practical use was carried out at three probe wavelengths (543, 594, and 633 nm) and various beam powers and pump excitation rates. The results are presented for commercially available wild-type and D96N variant BR films.

Multilayered temporally evolving phase screens based on statistical interpolation

Simulation of the dynamic effects of atmospheric turbulence assists in understanding, testing and effective implementation of adaptive optics systems. Statistical interpolation technique helps in retaining the spatial turbulence statistics when atmospheric phase screens are required to be moved by non-integer multiples of the grid spacing. We applied statistical interpolation in the simulation of temporally evolving phase screens using the multilayer model of atmospheric turbulence. A comparison of the statistical method with bilinear interpolator and random midpoint displacement method is presented. It is shown that underestimating Frieds parameter (r0) in the interpolation leads to large errors and hence it is appropriate to choose a little larger value of r0 than estimated from the phase screens.

Scatter studies for visible emission line coronagraph on board ADITYA-L1 mission

Abstract. Visible emission line coronagraph (VELC) on board ADITYA-L1 mission is an internally occulted mirror coronagraph designed for solar coronal observations over a field of view (FOV) of 1.05 Ro to 3 Ro. To achieve the proposed science goals, instrument background should be less than 5 ppm over the FOV of VELC. Major contributor toward instrument background is scatter from surface microroughness and particulate contamination over the primary mirror (M1). Hence, a detailed study of scatter through simulations is carried out to arrive at the surface microroughness specifications and surface cleanliness level requirements of M1. Estimation of RMS microroughness correction factor due to finite band width of the profilometer is very important while specifying the RMS microroughness of M1. This paper discusses in detail about scatter simulations, results, and analysis. All simulations are carried out using Advanced System Analysis Program, developed by Breault Research Organization.

Experimental evaluation of centroiding algorithms at different light intensity and noise levels

In this paper, we present results from an experimental evaluation of centroiding algorithms, simple center of gravity (CoG) and weighted center of gravity (WCoG) in a Shack Hartmann wavefront sensor at different light levels and background noise conditions. CoG performs better than WCoG at good light conditions. WCoG performs better than CoG algorithm at low light level conditions and in cases of high intensity.

Efficient Minimization of Servo Lag Error in Adaptive Optics Using Data Stream Mining

Prediction of the wavefronts helps in reducing the servo lag error in adaptive optics caused by finite time delays (~ 1-5 ms) before wavefront correction. Piecewise linear segmentation based prediction is not suitable in cases where the turbulence statistics of the atmosphere are fluctuating. In this paper, we address this problem by real time control of the prediction parameters through the application of data stream mining on wavefront sensor data obtained in real-time. Numerical experiments suggest that pixel-wise prediction of phase screens and slope extrapolation techniques lead to similar improvement while modal prediction is sensitive to the number of moments used and can yield better results with optimum number of modes.

Pixel size and pitch measurements of liquid crystal spatial light modulator by optical diffraction

We present a simple technique for the determination of pixel size and pitch of liquid crystal (LC) based spatial light modulator (SLM). The proposed method is based on optical diffraction from pixelated LC panel that has been modeled as a two-dimensional array of rectangular apertures. A novel yet simple, two-plane measurement technique is implemented to circumvent the difficulty in absolute distance measurement. Experimental results are presented for electrically addressed twisted nematic LC-SLM removed from the display projector.

Light-induced absorption in photorefractive BaTiO3 crystals

We present the experimental results on the measurement of temporal and steady-state light-induced absorption change in undoped and rhodium-doped barium titanate (Rh:BaTiO3) crystals at different wavelengths and intensities. Theoretical calculations based on a two-center charge transport model agree well with the experimental results which supplement the earlier studies carried out using photorefractive BaTiO3 crystals. We also report a wavelength-specific light-induced effect that modifies the absorption dynamics in a peculiar manner. This can be attributed to light-induced thermal effects. However, the possibility of additional photorefractive centers becoming active cannot be ruled out.

Dither-based sensor for improved consistency of adaptive optics system

The wavefront reconstruction accuracy in Shack Hartmann sensor based adaptive optics system depends on accurate centroiding and phase estimation from measured slope values. Monte Carlo simulations of vector matrix multiply and Fourier phase estimation methods show fluctuations in the value of wavefront reconstruction accuracy leading to inconsistency. In this paper, it is shown that these fluctuations can be minimized and high wavefront reconstruction accuracy can be maintained consistently by applying a dither signal on the Shack Hartmann lenslet array. The information of the dither signal to be applied can be obtained from the wavefronts of the past.

Intensity Weighted Noise Reduction in MEMS Based Deformable Mirror Images

The frames captured with a compact, high‐resolution, monochrome progressive scan CCD camera, of a continuous facesheet microelectromechanical systems (MEMS) deformable mirror (DM), include unwanted diffractive, background and readout noises; which significantly reduces the quality of imaging in many applications. Processing these images before passing them on to meet later experimental objectives can improve its performance greatly. In this paper, we propose a sequence of steps involving image intensity weighted noise removal and other smoothening techniques to minimize the noise and improve the phase production and correction capabilities of the DM.

Influence Function Measurement of Continuous Membrane Deformable Mirror Actuators Using Shack Hartmann Sensor

The characterization of a continuous membrane deformable mirror (CDM) involves the measurement of influence function, which quantifies the response of individual actuators to application of different voltages. In this paper, we present the results of influence function measurement for a 140‐actuator CDM using a Shack Hartmann sensor (SHS). The measured influence function was fitted using a Gaussian function and also decomposed into Zernike moments. The SHS‐CDM interaction matrix was also computed.