Research
"Research is to see what everybody else has seen, and to think what nobody else has thought"Albert Szent-GyorgyiResearch Funding:
Awarded Ramanujan grant (RJF/2022/000106) of INR 1.19 Crores by SERB – DST, Govt. of India, for the project entitled, ‘Development of next generation adaptive optics system with programmable control parameters’, as a Principal Investigator (PI) for the duration 2023 – 2028.
An overview of the research findings during Postdoctoral studies at University of Oxford, UK:
Ultra-thin endo-microscopes are highly desired for many biomedical applications, and multimode fibers (MMF) are considered as a potential endoscopic tool due to their ability to carry a large number of modes and smaller cross-section. They help in providing access to unreachable locations with minimum disruption physiologically making them the tool of choice for many in vivo studies in neuroscience and other biomedical procedures. However, the utilization of the MMF to its full potential is limited due to mode dispersion among the spatial modes as a result of each mode travelling inside the fiber with their own propagation constants. Furthermore, the interaction of each mode field amongst themselves while propagating through the fiber may lead to exchange of energy, known as mode coupling. Thus, the presence of mode coupling and mode dispersion results in the formation of speckle pattern at the distal facet of the multimode fiber which bears no resemblance to the initial field distribution. The occurrence of such phase randomisation poses serious challenges to produce a diffraction limited spot for the purpose of imaging. One of the popular solutions to tackle such issues is by using wavefront shaping technique to spatially modulate the wavefront at the proximal end of the fiber through use of information from the transmission matrix (TM). The TM is measured interferometrically during the calibration step using an external phase reference that help to shape the wavefront, thereby producing a diffraction limited spot at the distal facet.
A. MMF Based Endo-Microscope
The schematic diagram of a MMF based endo-microscope system developed at Oxford is shown which consists of three modules: source module, calibration module and the beam shaping/imaging module. Below figure shows the experimental results for bead imaging after acquiring the TM to form a sequence of diffraction-limited focussed spots at a desired location.
The schematic diagram of a MMF based endo-microscope system developed at Oxford is shown which consists of three modules: source module, calibration module and the beam shaping/imaging module. Below figure shows the experimental results for bead imaging after acquiring the TM to form a sequence of diffraction-limited focussed spots at a desired location.
B. Compressive Imaging
A modelling study is performed to characterise the performance of computational super-resolution imaging in a MMF based endo-microscopy system and compressed image reconstruction algorithm - Basis Pursuit is used for different illuminations. In addition to the increased speed due to the reduced number of measurements, we characterized other potential benefits with respect to robustness to noise and resilience to fiber bending when using compressed imaging with optimized illuminations.
A modelling study is performed to characterise the performance of computational super-resolution imaging in a MMF based endo-microscopy system and compressed image reconstruction algorithm - Basis Pursuit is used for different illuminations. In addition to the increased speed due to the reduced number of measurements, we characterized other potential benefits with respect to robustness to noise and resilience to fiber bending when using compressed imaging with optimized illuminations.
C. Sensorless Optimisation - MMF
The applications of MMF based endoscopic imaging is very sensitive to bending of the fibers. Such occurrence of changes to the fiber’s physical state further changes the mode coupling that significantly changes the TM, degrading the imaging performance of the endo-microscope. In this work, fiber bending compensation is addressed by using sensorless optimisation technique without having to re-calibrate the system to update the TM from scratch (modelling study).
The applications of MMF based endoscopic imaging is very sensitive to bending of the fibers. Such occurrence of changes to the fiber’s physical state further changes the mode coupling that significantly changes the TM, degrading the imaging performance of the endo-microscope. In this work, fiber bending compensation is addressed by using sensorless optimisation technique without having to re-calibrate the system to update the TM from scratch (modelling study).
The human eye is a unique system which provides an opportunity for the thorough study of the retina and all other integral parts responsible for vision in human beings. Retinal imaging (ophthalmology) is an extremely important field of study which not only helps in the investigation and diagnosis of different eye diseases but also provides an insight into the fundamentals of human vision, changes in the neuronal and vasculature structures, etc. Accordingly, a number of technologies/instruments were developed in the last few decades to study the human retina. However, such technologies in their very initial stage could not provide retinal images of optimum quality due to the inherent ocular aberrations (particularly from the lens and the cornea), that posed a major challenge during the imaging process. Introduction of adaptive optics (AO) technique in retinal imaging systems has revolutionized the field of ophthalmology at an unprecedented scale. A basic AO system comprises of three important elements, namely, a wavefront sensor to measure the shape of the wavefront, a deformable mirror or SLM to correct the wavefront, and a real time computer to close the feedback loop of the system. More recently, the use of AO in ophthalmic imaging system has resulted in a very popular analytical tool, named as, adaptive optics scanning laser ophthalmoscope (AOSLO) which is capable of generating retinal images with high spatial and temporal resolution. The instrument can generate images of retinal substructures in the cellular level such as cone photoreceptors embedded within retinal tissue. The working of this high end instrument is based on the concept of confocal microscopy incorporated with adaptive optics. The eye’s optics is used as the microscope objective during the imaging process. A focussed spot is scanned across the human retinal surface and a detector is used to capture the back scattered light at every scan position. The out of focus light is eliminated by strategically placing a pinhole in front of the detector at a plane conjugate to the focussed spot on the retina. Moreover, incorporation of adaptive optics technique to the ophthalmic system helps to compensate the intrinsic aberrations of the ocular media, thereby forming a highly-focused spot of the light beam that can directly raster-scan onto the retina. Thus the AOSLO system, offers several advantages in comparison to the conventional imaging system, such as, optical sectioning of the retina, improved lateral resolution, improved light efficiency, aberration-free images, real time imaging, etc.
D. Adaptive Optics - Ophthalmology
A basic adaptive optics set-up (containing the three important elements as mentioned above) implemented in a retinal imaging system is shown. The images shown are confocal images (showing the photoreceptor mosaic) of my retina without AO correction and with AO correction, captured using the developed Oxford AOSLO setup.
A basic adaptive optics set-up (containing the three important elements as mentioned above) implemented in a retinal imaging system is shown. The images shown are confocal images (showing the photoreceptor mosaic) of my retina without AO correction and with AO correction, captured using the developed Oxford AOSLO setup.
E. AOSLO - Imaging Configurations
In this work both confocal as well as non-confocal (darkfield) imaging configurations are optimised simultaneously in an adaptive optics scanning laser ophthalmoscope (AOSLO) based on a reconfigurable aperture pattern. The reconfigurable aperture pattern is optimised based on the information collected from Shack Hartmann (SH) wavefront senor data and is realised by exploiting the programmable facility of the DMD. Experimental results are included from a human participant, demonstrating simultaneous optimisation of confocal and darkfield images.
In this work both confocal as well as non-confocal (darkfield) imaging configurations are optimised simultaneously in an adaptive optics scanning laser ophthalmoscope (AOSLO) based on a reconfigurable aperture pattern. The reconfigurable aperture pattern is optimised based on the information collected from Shack Hartmann (SH) wavefront senor data and is realised by exploiting the programmable facility of the DMD. Experimental results are included from a human participant, demonstrating simultaneous optimisation of confocal and darkfield images.
Demonstration of PSF estimation based on the quality metric and the residual phase error which eventually helps in the construction of an optimised aperture pattern to be displayed on the DMD. Experimental result from a healthy human participant showing a pair of simultaneous confocal and darkfield images with (a) d = 1.1 ADD and (b) D = 2 ADD, (c) d = 1.1 ADD and (d) D = 3 ADD and (e) d = 1.1 ADD and (f) D = 3.6 ADD, captured at close locations.
An overview of the research findings during Postdoctoral studies at IISER Mohali, India (Short Term):
Focusing of light beam passing through a scattering medium (like biological tissues) is highly desirable in a number of applications, such as optical microscopy, optical tweezing, photoacoustic, phototherapy, etc. However, such scattering medium puts restriction in the process of optical focussing and imaging that disrupt the formation of focus, poor quality of imaging, decorrelation, loss of spatial coherence, etc. There have been a number of efforts devoted to improve the focussing and imaging of incident light through scattering media, like iterative wavefront shaping, transmission matrix measurement and optical phase conjugation. Among the three methods, the transmission matrix approach is considered to be one of the most accurate and reliable method to focus light beam in a scattering medium. The transmission matrix is straightforwardly described as the relationship between the incident wavefront and the transmitted one. By using a wavefront modulating device, it is possible to divide the light beam at the back pupil plane into a large number of segments. Modulating the phase information with a large number of spatial degrees of freedom, it is possible to compensate the effect of scattering. Thus, using transmission matrix a relationship between various input modes and the desired output mode is established, that results in an enhanced focused spot by setting the phase mask to an optimal value. Such phase manipulation technique is also capable of generating multiple focussed spots corresponding to a single input beam passing through the scattering sample.
Focusing of light beam passing through a scattering medium (like biological tissues) is highly desirable in a number of applications, such as optical microscopy, optical tweezing, photoacoustic, phototherapy, etc. However, such scattering medium puts restriction in the process of optical focussing and imaging that disrupt the formation of focus, poor quality of imaging, decorrelation, loss of spatial coherence, etc. There have been a number of efforts devoted to improve the focussing and imaging of incident light through scattering media, like iterative wavefront shaping, transmission matrix measurement and optical phase conjugation. Among the three methods, the transmission matrix approach is considered to be one of the most accurate and reliable method to focus light beam in a scattering medium. The transmission matrix is straightforwardly described as the relationship between the incident wavefront and the transmitted one. By using a wavefront modulating device, it is possible to divide the light beam at the back pupil plane into a large number of segments. Modulating the phase information with a large number of spatial degrees of freedom, it is possible to compensate the effect of scattering. Thus, using transmission matrix a relationship between various input modes and the desired output mode is established, that results in an enhanced focused spot by setting the phase mask to an optimal value. Such phase manipulation technique is also capable of generating multiple focussed spots corresponding to a single input beam passing through the scattering sample.
A. Focusing - Scattering Media
Simulated speckle pattern having different speckle sizes and focusing of the light beam passing through a scattering medium, using the proposed experimental setup.
Simulated speckle pattern having different speckle sizes and focusing of the light beam passing through a scattering medium, using the proposed experimental setup.
An overview of the research findings during Doctoral studies at IIT Guwahati, India:
The Shack Hartmann wavefront sensor (SHWS) is a widely used zonal wavefront sensor, named after Johannes Franz Hartmann and Roland Shack. It consists of a 2D array of micro-lenses called lenslets along with a detector placed at the common focal plane of these micro-lenses. The SHWS comprises two essential processes, namely, the wavefront sensing and the wavefront estimation. However, it has been observed that a conventional SHWS encounters a number of issues in each of these processes that puts serious limitations in the performance of the sensor.
In view of the above issues related to wavefront sensing and wavefront estimation in a conventional SHWS, we proposed a grating array based zonal wavefront sensor that uses a two dimensional array of plane diffraction gratings and a single focusing lens. This arrangement thus replaces the lenslets array of the SHWS, thereby overcoming the limitations associated with the lenslets array. We call such an arrangement as grating array based zonal wavefront sensor (GAWS). The proposed sensor is implemented using a liquid crystal spatial light modulator (LCSLM), employing a computer generated holography technique. We introduced an improved wavefront estimation algorithm, applicable for Shack-Hartmann type wavefront sensors that shows significant improvement in the wavefront estimation. We also theoretically analyzed the performance of the proposed wavefront estimation algorithm in a comprehensive manner by quantifying the important sources of error associated with it. Further, we introduced a scheme to enhance the spatial resolution of the GAWS by using a sequence of laterally shifted binary grating array patterns, realised with the help of the 24 bit-planes display of a ferroelectric LCSLM. We also proposed a scheme to improve the accuracy of centroid detection and enhance the dynamic range of the sensor or reduce the possibility of crosstalk, in the GAWS by using more than one digital camera plane via a beam splitting mechanism. In addition, we described the use of a print of an array of binary diffraction gratings on a transparent polyester sheet as a cost effective means to implement the GAWS. The feasibility of each of the proposed schemes were verified with the help of proof-of-principle experiments. A few of the proposed schemes are illustrated below:
The Shack Hartmann wavefront sensor (SHWS) is a widely used zonal wavefront sensor, named after Johannes Franz Hartmann and Roland Shack. It consists of a 2D array of micro-lenses called lenslets along with a detector placed at the common focal plane of these micro-lenses. The SHWS comprises two essential processes, namely, the wavefront sensing and the wavefront estimation. However, it has been observed that a conventional SHWS encounters a number of issues in each of these processes that puts serious limitations in the performance of the sensor.
In view of the above issues related to wavefront sensing and wavefront estimation in a conventional SHWS, we proposed a grating array based zonal wavefront sensor that uses a two dimensional array of plane diffraction gratings and a single focusing lens. This arrangement thus replaces the lenslets array of the SHWS, thereby overcoming the limitations associated with the lenslets array. We call such an arrangement as grating array based zonal wavefront sensor (GAWS). The proposed sensor is implemented using a liquid crystal spatial light modulator (LCSLM), employing a computer generated holography technique. We introduced an improved wavefront estimation algorithm, applicable for Shack-Hartmann type wavefront sensors that shows significant improvement in the wavefront estimation. We also theoretically analyzed the performance of the proposed wavefront estimation algorithm in a comprehensive manner by quantifying the important sources of error associated with it. Further, we introduced a scheme to enhance the spatial resolution of the GAWS by using a sequence of laterally shifted binary grating array patterns, realised with the help of the 24 bit-planes display of a ferroelectric LCSLM. We also proposed a scheme to improve the accuracy of centroid detection and enhance the dynamic range of the sensor or reduce the possibility of crosstalk, in the GAWS by using more than one digital camera plane via a beam splitting mechanism. In addition, we described the use of a print of an array of binary diffraction gratings on a transparent polyester sheet as a cost effective means to implement the GAWS. The feasibility of each of the proposed schemes were verified with the help of proof-of-principle experiments. A few of the proposed schemes are illustrated below:
A. Grating Array Based Wavefront Sensor
Proposed grating array based zonal wavefront sensor (GAWS), implemented using a liquid crystal spatial light modulator (LCSLM), employing computer generated holography technique.
Proposed grating array based zonal wavefront sensor (GAWS), implemented using a liquid crystal spatial light modulator (LCSLM), employing computer generated holography technique.
B. Pathak - Boruah Algorithm
Proposed Pathak-Boruah wavefront estimation algorithm, applicable for Shack-Hartmann type wavefront sensors that shows significant improvement in the wavefront estimation.
Proposed Pathak-Boruah wavefront estimation algorithm, applicable for Shack-Hartmann type wavefront sensors that shows significant improvement in the wavefront estimation.
C. Enhanced Spatial Resolution - GAWS
Proposed spatial resolution enhancement scheme of the GAWS by using a sequence of laterally shifted binary grating array patterns, realised with the help of the 24 bit-planes display of a ferroelectric LCSLM.
Proposed spatial resolution enhancement scheme of the GAWS by using a sequence of laterally shifted binary grating array patterns, realised with the help of the 24 bit-planes display of a ferroelectric LCSLM.
