Adaptive optics based imaging and spectroscopy

This work is completely different from the OT work which is our main specialization area. However, we are an optics lab, and cannot remain confined only to one aspect in light-matter interactions! Thus, we have commenced work in classical optical interferometry using Fabry-Perot interferometers (FPI), and have taken up the challenging work of simultaneous spectroscopy and imaging using an FPI. Thus, our aim is to design imaging and spectroscopy modules to study the solar spectrum in collaboration with the Center for Excellence in Space Sciences (CESSI), IISER Kolkata. A proposal for an instrument called ‘SOMI’ (Solar Oscillation and Magnetic Imager) has already been put forward for ISRO’s Aditya L5 space mission concept. The aim of the spectroscope on-board SOMI would be to analyze the photospheric solar spectrum around the wavelength 617.3 nm, which is a neutral Fe I absorption line. The study of such magnetically sensitive spectral lines gives us a wealth of information about the magnetic properties (using Zeeman effect), velocity (using doppler shifts) etc. of the solar photosphere. This will enable generation of useful data products called ‘magnetograms’, ‘filtergrams’ and ‘dopplergrams’.

  • Construction of a Fabry-Perot cavity for the SOMI instrument

    Our efforts in the lab are aimed towards creating a high-finesse FPI (FWHM = 90mÅ, FSR = 3Å), which is able to scan the desired solar spectral line with extreme precision (see schematic here). FPIs work on the principle of multiple-beam interference, which makes them spectroscopic devices of extremely high resolving power. Normal spectrometers are not capable of observing the above-mentioned ‘Zeeman Effect’, where the spectral lines are far too closely spaced, and this is where FP cavities come to the rescue- by acting as filters of extremely narrow bandwidth. Once the working of the cavity is demonstrated, an ambitious next step is also to introduce a novel method to scan the cavity by tuning the voltage applied to SLMs (Spatial Light Modulators), instead of the regular methods of using a voltage-tunable crystal, such as Lithium Niobate (LiNBO3), to scan the cavity length. The expected advantage of this novel method is low voltage requirement among others. Another important work pertaining to this instrument happening in parallel is a feasibility check of whether the materials being used for making the spectroscope can be space qualified.

  • Development of a high resolution spectrometer for exploring the solar corona

    Myriad energetic manifestations such as flares and coronal mass ejections (CMEs) originate in the solar corona, which affect the space weather in the vicinity of the Earth. Understanding the dynamics of the corona requires sensitive spectrometers accompanied by high resolution imaging to resolve feeble emission lines emanating from this tenuous layer, which is very faint compared to the photosphere. Ground based solar observations are marred by seeing conditions and reduce the efficacy of these observations. To mitigate this problem, Adaptive Optics (AO) based imaging is deemed necessary. The development of a novel imaging spectrometer is underway, which is one of the modules of the Solar Hyperspectral Imaging Polarimeter (SHIP) instrument. (see schematic here). SHIP is envisaged to operate between 600-1100 nm and will provide 2D spectro-polarimetric investigations and wavefront corrected imaging of the solar corona, utilizing the phase/amplitude modulation properties of Spatial Light Modulators (SLMs).

  • Characterisation of phase modulation properties of a reflective Spatial Light Modulator (SLM)

    We use a Michelson interferometer setup to characterise the phase response of a reflective SLM. The conventional calibration method is time-consuming and we develop a new method using novel phase-masks which can enable threefold faster measurements without modifying any hardware in the experimental setup. To our knowledge, our method is the first endeavour directed towards enabling rapid phase characterisation of an SLM and can have very useful applications in settings which often require frequent and faster phase calibrations such as in astronomy. (see schematic animation here).

  • Development of automated pipelines for analyzing interferograms

    We have developed a semi-automated pipeline in python and shell from scratch to analyze all recorded interferograms obtained during our phase calibration experiment of an SLM. The first step in the analysis is to smoothen each interferogram and then make an appropriate 1-D cut in the interferogram at two locations, viz. one in the top half and the other in the bottom half of the interferogram. These values are kept fixed for all interferograms. A peak detection algorithm then detects the number of specified peaks in the fringe pattern and finds the relative shift between the upper half (variable graylevel) and lower half (reference graylevel) of the fringe pattern. We then compute the relative shifts between different graylevels and obtain the calibration curve for our SLM.

  • Generation of optimised phase masks using Iterative Fourier Transform Algorithms (IFTAs)

    Inherent defects in the SLM such as their pixelated nature, phase change according to the gray scale and edge effects limit the resolution and the diffraction efficiency of the SLM. IFTAs can be used to design the optimised phase distribution of the rectangular grating in 1D and 2D. SLMs are promising candidates to employ such optimised IFTAs wherein precise control and regulation of applied electrical voltages can enable us to manipulate the amplitude or phase of the incident beam. Another advantage of this method is that no additional changes need to be made in the SLM hardware to realise the goals. Thus, one can make up for the inherent limitations of the SLM and use it to maximum advantage. Besides, many of these optimised IFTAs are fast and computationally feasible. An array of 2D uniform spots can be useful for many applications such as optical trapping, adaptive wavefront sensing applications in an AO system etc. We have developed a code for generating optimised phases for 1D (see figure) or 2D (see figure) gratings and achieved experimental realisation of this method by generating an array of 1D/2D spots. We have already explored some of the synthesis applications of these iterative algorithms. We are currently exploring wavefront reconstruction applications of IFTAs using numerical studies and their putative uses in improving the resolution of aberrated images.

  • Improving diffraction efficiency of phase limited SLMs

    The diffraction efficiency of our SLM is reduced due to the limited phase depth (φ ∼ 0.8π) of our SLM. It has been shown that it is possible to improve the diffraction efficiency of phase limited SLMs by modifying the look up table (LUT) of the SLM, wherein the linear relationship between the grey value given by the user and that seen by the SLM (referred to as contrast) does not hold true. We have implemented this correction in our phase gratings displayed on the SLM and obtain slight improvement in efficiency (38%) using blazed gratings. We find that optimised phase masks constructed using IFTA gives higher efficiency of around 44%.

  • Generation of Laguerre-Gaussian (LG) beams using SLM

    We have generated Laguerre-Gaussian (LG) beams of higher orders using our reflective SLM. We obtain LG beams whose radial and azimuthal modes are upto (p,l)=(3,3) (see figure). We are currently trying to design experiments to use these beams for sensing applications.

  • Adaptive Optics (AO) using SLM

    Observations from ground based telescopes are marred by seeing effects and require adaptive optics systems. AO systems usually use deformable mirrors which are expensive and are used only for wavefront correction. SLMs can function as dynamical optical elements and can work both as an adaptive wavefront sensor as well as to compensate for wavefront abberation. AO using SLMs is an active area of research and we will develop an open-loop AO system using Zernike phase correction and then try to extend the system to closed-loop by developing appropriate control algorithms.

Amar is the PhD student leading this project, while Unnati is the MS student working parallelly. Dr. Nirmalya Ghosh and Dr. Dibyendu Nandi are our collaboratos.