Browsing by Author "Kahro, Tauno"

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3D incoherent imaging using an ensemble of sparse self-rotating beams
(Optics Express, 2023) Bleahu, Andrei-ioan; Gopinath, Shivasubramanian; Kahro, Tauno; Angamuthu, Praveen Periyasamy; Rajeswary, Aravind Simon John Francis; Prabhakar, Shashi; Kumar, Ravi; Salla, Gangi Reddy; Singh, Ravindra P.; Kukli, Kaupo; Tamm, Aile; Rosen, Joseph; Anand, Vijayakumar
Interferenceless coded aperture correlation holography (I-COACH) is one of the simplest incoherent holography techniques. In I-COACH, the light from an object is modulated by a coded mask, and the resulting intensity distribution is recorded. The 3D image of the object is reconstructed by processing the object intensity distribution with the pre-recorded 3D point spread intensity distributions. The first version of I-COACH was implemented using a scattering phase mask, which makes its implementation challenging in light-sensitive experiments. The I-COACH technique gradually evolved with the advancement in the engineering of coded phase masks that retain randomness but improve the concentration of light in smaller areas in the image sensor. In this direction, I-COACH was demonstrated using weakly scattered intensity patterns, dot patterns and recently using accelerating Airy patterns, and the case with accelerating Airy patterns exhibited the highest SNR. In this study, we propose and demonstrate I-COACH with an ensemble of self-rotating beams. Unlike accelerating Airy beams, self-rotating beams exhibit a better energy concentration. In the case of self-rotating beams, the uniqueness of the intensity distributions with depth is attributed to the rotation of the intensity pattern as opposed to the shifts of the Airy patterns, making the intensity distribution stable along depths. A significant improvement in SNR was observed in optical experiments.
4D imaging using accelerating airy beams and nonlinear reconstruction
(2023) Bleahu, Andrei; Gopinath, Shivasubramanian; Anand, Vijayakumar; Rosen, Joseph; Juodkazis, Saulius; Tamm, Aile; Kukli, Kaupo; Rajeswary, Aravind Simon John Francis; Katkus, Tomas; Pristy, Agnes; Ng, Soon Hock; Praveen, P. A.; Kahro, Tauno; Smith, Daniel; Arokiaraj, Francis Gracy; Kumar, Ravi
Deep Deconvolution of Object Information Modulated by a Refractive Lens Using Lucy-Richardson-Rosen Algorithm
(2022) Praveen, P.A.; Arockiaraj, Francis Gracy; Gopinath, Shivasubramanian; Smith, Daniel; Kahro, Tauno; Valdma, Sandhra-Mirella; Bleahu, Andrei; Ng, Soon Hock; Reddy, Andra Naresh Kumar; Katkus, Tomas; Rajeswary, Aravind Simon John Francis; Ganeev, Rashid A.; Pikker, Siim; Kukli, Kaupo; Tamm, Aile; Juodkazis, Saulius; Anand, Vijayakumar
A refractive lens is one of the simplest, most cost-effective and easily available imaging elements. Given a spatially incoherent illumination, a refractive lens can faithfully map every object point to an image point in the sensor plane, when the object and image distances satisfy the imaging conditions. However, static imaging is limited to the depth of focus, beyond which the point-to-point mapping can only be obtained by changing either the location of the lens, object or the imaging sensor. In this study, the depth of focus of a refractive lens in static mode has been expanded using a recently developed computational reconstruction method, Lucy-Richardson-Rosen algorithm (LRRA). The imaging process consists of three steps. In the first step, point spread functions (PSFs) were recorded along different depths and stored in the computer as PSF library. In the next step, the object intensity distribution was recorded. The LRRA was then applied to deconvolve the object information from the recorded intensity distributions during the final step. The results of LRRA were compared with two well-known reconstruction methods, namely the Lucy-Richardson algorithm and non-linear reconstruction.
Enhanced design of multiplexed coded masks for Fresnel incoherent correlation holography
(Scientific Reports, 2023) Gopinath, Shivasubramanian; Bleahu, Andrei; Kahro, Tauno; Rajeswary, Aravind Simon John Francis; Kumar, Ravi; Kukli, Kaupo; Tamm, Aile; Rosen, Joseph; Anand, Vijayakumar
Fresnel incoherent correlation holography (FINCH) is a well-established incoherent digital holography technique. In FINCH, light from an object point splits into two, differently modulated using two diffractive lenses with different focal distances and interfered to form a self-interference hologram. The hologram numerically back propagates to reconstruct the image of the object at different depths. FINCH, in the inline configuration, requires at least three camera shots with different phase shifts between the two interfering beams followed by superposition to obtain a complex hologram that can be used to reconstruct an object’s image without the twin image and bias terms. In general, FINCH is implemented using an active device, such as a spatial light modulator, to display the diffractive lenses. The first version of FINCH used a phase mask generated by random multiplexing of two diffractive lenses, which resulted in high reconstruction noise. Therefore, a polarization multiplexing method was later developed to suppress the reconstruction noise at the expense of some power loss. In this study, a novel computational algorithm based on the Gerchberg-Saxton algorithm (GSA) called transport of amplitude into phase (TAP-GSA) was developed for FINCH to design multiplexed phase masks with high light throughput and low reconstruction noise. The simulation and optical experiments demonstrate a power efficiency improvement of ~ 150 and ~ 200% in the new method in comparison to random multiplexing and polarization multiplexing, respectively. The SNR of the proposed method is better than that of random multiplexing in all tested cases but lower than that of the polarization multiplexing method.
Enhanced design of pure phase greyscale diffractive optical elements by phase-retrieval-assisted multiplexing of complex functions
(2023) Gopinath, Shivasubramanian; Bleahu, Andrei; Kahro, Tauno; Rajeswary, Aravind Simon John Francis; Kumar, Ravi; Kukli, Kaupo; Tamm, Aile; Rosen, Joseph; Anand, Vijayakumar
Enhanced design of pure phase greyscale diffractive optical elements by phase-retrieval-assisted multiplexing of complex functions
(Society of Photo-Optical Instrumentation Engineers (SPIE), 2023) Gopinath, Shivasubramanian; Bleahu, Andrei; Kahro, Tauno; Rajeswary, Aravind Simon John Francis; Kumar, Ravi
Extending the Depth of Focus of Infrared Microscope Using a Binary Axicon Fabricated on Barium Fluoride
(2024) Han, Molong; Smith, Daniel; Kahro, Tauno; Stonytė, Dominyka; Kasikov, Aarne; Gailevičius, Darius; Tiwari, Vipin; Xavier, Agnes Pristy Ignatius; Gopinath, Shivasubramanian; Ng, Soon Hock; Rajeswary, Aravind Simon John Francis; Tamm, Aile; Kukli, Kaupo
Axial resolution is one of the most important characteristics of a microscope. In all microscopes, a high axial resolution is desired in order to discriminate information efficiently along the longitudinal direction. However, when studying thick samples that do not contain laterally overlapping information, a low axial resolution is desirable, as information from multiple planes can be recorded simultaneously from a single camera shot instead of plane-by-plane mechanical refocusing. In this study, we increased the focal depth of an infrared microscope non-invasively by introducing a binary axicon fabricated on a barium fluoride substrate close to the sample. Preliminary results of imaging the thick and sparse silk fibers showed an improved focal depth with a slight decrease in lateral resolution and an increase in background noise.
Holographic solution to a fundamental problem in diffractive optics: resolution beyond diffraction and lithography limits
(2023) Bleahu, Andrei; Gopinath, Shivasubramanian; Xavier, Agnes Pristy Ignatius; Kahro, Tauno; Reddy, Andra Naresh Kumar; Arockiaraj, Francis Gracy; Smith, Daniel; Ng, Soon Hock; Katkus, Tomas; Rajeswary, Aravind Simon John Francis; Angamuthu, Praveen Periyasami; Pikker, Siim; Kukli, Kaupo; Tamm, Aile; Juodkazis, Saulius; Rosen, Joseph; Anand, Vijayakumar
Implementation of a Large-Area Diffractive Lens Using Multiple Sub-Aperture Diffractive Lenses and Computational Reconstruction
(Licensee MDPI, 2022) Gopinath, Shivasubramanian; Praveen, Periyasamy Angamuthu; Kahro, Tauno; Bleahu, Andrei-Ioan; Arockiaraj, Francis Gracy; Smith, Daniel; Ng, Soon Hock; Juodkazis, Saulius; Kukli, Kaupo; Tamm, Aile; Anand, Vijayakumar
Direct imaging systems that create an image of an object directly on the sensor in a single step are prone to many constraints, as a perfect image is required to be recorded within this step. In designing high resolution direct imaging systems with a diffractive lens, the outermost zone width either reaches the lithography limit or the diffraction limit itself, imposing challenges in fabrication. However, if the imaging mode is switched to an indirect one consisting of multiple steps to complete imaging, then different possibilities open. One such method is the widely used indirect imaging method with Golay configuration telescopes. In this study, a Golay-like configuration has been adapted to realize a large-area diffractive lens with three sub-aperture diffractive lenses. The sub-aperture diffractive lenses are not required to collect light and focus them to a single point as in a direct imaging system, but to focus independently on different points within the sensor area. This approach of a Large-Area Diffractive lens with Integrated Sub-Apertures (LADISA) relaxes the fabrication constraints and allows the sub-aperture diffractive elements to have a larger outermost zone width and a smaller area. The diffractive sub-apertures were manufactured using photolithography. The fabricated diffractive element was implemented in indirect imaging mode using non-linear reconstruction and the Lucy–Richardson–Rosen algorithm with synthesized point spread functions. The computational optical experiments revealed improved optical and computational imaging resolutions compared to previous studies.
Implementation of a Large-Area Diffractive Lens Using Multiple Sub-Aperture Diffractive Lenses and Computational Reconstruction
(2023) Gopinath, Shivasubramanian; Angamuthu, Praveen Periysamy; Kahro, Tauno; Bleahu, Andrei; Arockiaraj, Francis Gracy; Smith, Daniel; Hock Ng, Soon; Juodkazis, Saulius; Kukli, Kaupo; Tamm, Aile; Anand, Vijayakumar
Direct imaging systems that create an image of an object directly on the sensor in a single step are prone to many constraints, as a perfect image is required to be recorded within this step. In designing high resolution direct imaging systems with a diffractive lens, the outermost zone width either reaches the lithography limit or the diffraction limit itself, imposing challenges in fabrication. However, if the imaging mode is switched to an indirect one consisting of multiple steps to complete imaging, then different possibilities open. One such method is the widely used indirect imaging method with Golay configuration telescopes. In this study, a Golay-like configuration has been adapted to realize a large-area diffractive lens with three sub-aperture diffractive lenses. The sub-aperture diffractive lenses are not required to collect light and focus them to a single point as in a direct imaging system, but to focus independently on different points within the sensor area. This approach of a Large-Area Diffractive lens with Integrated Sub-Apertures (LADISA) relaxes the fabrication constraints and allows the sub-aperture diffractive elements to have a larger outermost zone width and a smaller area. The diffractive sub-apertures were manufactured using photolithography. The fabricated diffractive element was implemented in indirect imaging mode using non-linear reconstruction and the Lucy–Richardson–Rosen algorithm with synthesized point spread functions. The computational optical experiments revealed improved optical and computational imaging resolutions compared to previous studies.
Realizing large-area diffractive lens using multiple subaperture diffractive lenses and computational reconstruction
(Society of Photo-Optical Instrumentation Engineers (SPIE), 2023) Gopinath, Shivasubramanian; Xavier, Agnes Pristy Ignatius; Angamuthu, Praveen Periyasamy; Kahro, Tauno; Tamm, Oskar
Realizing large-area diffractive lens using multiple subaperture diffractive lenses and computational reconstruction
(2023) Gopinath, Shivasubramanian; Xavier, Agnes Pristy Ignatius; Angamuthu, Praveen Periyasamy; Kahro, Tauno; Tamm, Oskar; Bleahu, Andrei; Arockiaraj, Francis Gracy; Smith, Daniel; Ng, Soon Hock; Juodkazis, Saulius; Kukli, Kaupo; Tamm, Aile; Anand, Vijayakumar
Self-wavefront interference using transverse splitting holography
(2023) Bleahu, Andrei-ioan; Gopinath, Shivasubramanian; Kahro, Tauno; Hock Ng, Soon; Kukli, Kaupo; Tamm, Aile; Juodkazis, Saulius; Rosen, Joseph; Anand, Vijayakumar
Manufacturing diffractive lenses with a high Numerical Aperture (NA) is a challenging task due to limitations in lithography methods and the inverse relation between the width and the radius of the zones. With low-resolution lithography techniques such as photolithography, the zone width reaches the lithography limit within a short radius, resulting in low-NA diffractive lenses. With high-resolution electron beam lithography, it is possible to manufacture high-NA diffractive lenses by prolonged writing. However, in this case, the width of the outermost zones becomes subwavelength, inducing undesirable polarization effects. In this proof-of-concept study, a holography solution has been demonstrated to enhance the imaging resolution of low-NA diffractive lenses. The light from an object is partly modulated by the low-NA diffractive lens and interfered with the remaining unmodulated light outside the area of the diffractive lens. This self-interference hologram of the object is processed in the computer with the point spread hologram to reconstruct the object with a resolution corresponding to the NA of the image sensor. This new imaging technique is called Self-Wavefront Interference using Transverse Splitting Holography (SWITSH). A resolution enhancement of ∼10 times has been demonstrated using a low-NA diffractive lens and SWITSH compared to direct imaging with the same low-NA diffractive lens.
Semiquantitative Classification of Two Oxidizing Gases with Graphene-Based Gas Sensors
(MDPI, 2022) Lind, Martin; Kiisk, Valter; Kodu, Margus; Kahro, Tauno; Renge, Indrek; Avarmaa, Tea; Makaram, Prashanth; Zurutuza, Amaia; Jaaniso, Raivo
Miniature and low-power gas sensing elements are urgently needed for a portable electronic nose, especially for outdoor pollution monitoring. Hereby we prepared chemiresistive sensors based on wide-area graphene (grown by chemical vapor deposition) placed on Si/Si3N4 substrates with interdigitated electrodes and built-in microheaters. Graphene of each sensor was individually functionalized with ultrathin oxide coating (CuO-MnO2, In2O3 or Sc2O3) by pulsed laser deposition. Over the course of 72 h, the heated sensors were exposed to randomly generated concentration cycles of 30 ppb NO2, 30 ppb O3, 60 ppb NO2, 60 ppb O3 and 30 ppb NO2 + 30 ppb O3 in synthetic air (21% O2, 50% relative humidity). While O3 completely dominated the response of sensors with CuO-MnO2 coating, the other sensors had comparable sensitivity to NO2 as well. Various response features (amplitude, response rate, and recovery rate) were considered as machine learning inputs. Using just the response amplitudes of two complementary sensors allowed us to distinguish these five gas environments with an accuracy of ~ 85%. Misclassification was mostly due to an overlap in the case of the 30 ppb O3, and 30 ppb O3 + 30 ppb NO2 responses, and was largely caused by the temporal drift of these responses. The addition of recovery rates to machine learning input variables enabled us to very clearly distinguish different gases and increase the overall accuracy to ~94%.