This research presents the WUTScope, a novel interferometric microscope developed by the Quantitative Computational Imaging group at Warsaw University of Technology. This system, leveraging Quantitative Phase Microscopy and Optical Diffraction Tomography, provides insightful three-dimensional reconstructions of the refractive index distribution in semi-transparent objects. The WUTScope is distinguished by its compact design and capability to operate under partially coherent illumination, using polarization diffraction gratings for beam splitting and recombination. This approach allows for efficient phase shifting and reduces speckle noise, enhancing image signal-to-noise ratio. The system's achromatic nature, due to the identical optical paths of the diffraction orders, facilitates the use of less coherent light sources, a distinct advantage over traditional holographic methods. Its effectiveness is demonstrated through tomographic reconstruction of a 3D-printed brain sample and analysis of refractive index changes in HeLa cells' lipid droplets, revealing the impact of cholesterol accumulation.
Two-photon vision is a newly discovered mechanism of the perception of pulsed near-infrared laser beams as a color stimulus corresponding to approximately half of the laser wavelength (PNAS 111(50), E5445–E5454). Based on this phenomenon, a new visual field test instrumentation – two-photon microperimetry, has been developed (BOE 10(9), 4551–4567). This study shows that two-photon perimetry gives more reproducible results than one-photon perimetry for standard threshold finding strategies. This unquestionable advantage of the nonlinear vision-based visual field testing technique may benefit the clinical assessment of retinal disease progression and treatment efficiency.
Two-photon vision relies on the perception of pulsed near-infrared laser beams as having colors like their half-wavelength counterparts. The phenomenon is due to two-photon absorption occurring in visual pigments [1]. This study is focused on methods to determine the contrast sensitivity function (CSF) for two-photon vision, which has not yet been investigated. CSF was measured for eight spatial frequencies using the tumbling E letter optotype. The optotype was projected on a white background by fast scanning the retina with a pulsed 1040 nm or 520 nm laser beam, both perceived as green. The contrast threshold was determined for the power of the beam corresponding to a minimum stimulus brightness for which the subject was able to state the correct letter orientation. Because a luminance curve for the two-photon stimulus is not available, expressing the brightness of the infrared stimulus in photometric units required finding a suitable method. Three approaches for determining contrast sensitivity for two-photon stimulus were proposed and tested to overcome this problem. The threshold contrast values, defined as Weber contrast, differ substantially between normal and two-photon vision mechanisms. Each tested method allowed qualitative comparison of the obtained contrast sensitivities. The results show that the two-photon CSF has a significantly broader range than the one-photon CSF. Determining the CSF for twophoton vision will help assess the applicability of this phenomenon to augmented reality displays.
In this paper, we present the preliminary results of the scotopic luminosity curve for two-photon vision measurements in the spectral range from 872 nm to 1027 nm. The results were obtained thanks to a newly-developed custom-build tunable femtosecond erbium-doped fiber laser that pulse train parameters and spectral width are close to constant while tuning. Such instrumentation enabled us to perform reliable measurements across the laser tuning range of over 150 nm.
As reported previously, OCT sources emit short infrared pulses that may be seen due to two-photon vision. In this work, we quantified visibility of OCT sources, which may be beneficial for eye imaging system designers.
Pulsed near-infrared (NIR) light sources can be successfully applied for both imaging and functional testing of the human eye, as published recently. These two groups of applications have different requirements. For imaging applications, the most preferable is invisible scanning beam while efficiently visible stimulating beam is preferable for functional testing applications. The functional testing of human eye using NIR laser beams is possible due to two-photon vision (2PV) phenomenon. 2PV enables perception of pulsed near-infrared laser light as color corresponding to approximately half of the laser wavelength. This study aims to characterize two-photon vision thresholds for various pulse lengths from a solidstate sub-picosecond laser (λc = 1043.3 nm, Frep = 62.65 MHz), either of 253 fs duration or elongated by Martinez- type stretcher to 2 ps, and fiber-optic picosecond laser (λc = 1028.4 nm, Frep = 19.19 MHz, τp = 12.2 ps).
The detection of molecules by surface-enhanced Raman spectroscopy (SERS) is dependent on the nanomaterial used to induce the enhancement effect. This depends on a variety of parameters of the substrate such as the metal used for their creation, their shape, size and size distribution, concentration, as well as the parameters of the solution, such as packing of the nanoparticles, the complexity of the sample, the solvent, etc. It is most crucial, that the parameters are kept constant to provide uniformity of the enhancement. this is crucial for the development of SERS as a reliable and quantitative technique for bioanalysis. Here, we have developed the silver-core and gold-shell nanoparticles, to serve as the enhancement material. The fabrication phase involved constant concentrations of chemicals stability of the solution physical parameters like stirring and heating, and differed only in the perturbation of the reagents addition kinetics. These nanoparticles were investigated further with their ability to measure the solutions of 2-naphtalenethiol in DMSO, as model for testing the variability of the signal due to the enhancement and the kinetics of the nanoparticle-sample solution during a routine Raman measurement procedure. The results indicate vast difference in the preference of the 2-naphthalenethiol to come into contact with the nanoparticles and the partial enhancement of DMSO in most cases, with an almost complete by-pass of the solvent and direct detection of the 2-naphthalenethiol in one case. Moreover, the kinetics of the measurement solution, or its stability during measurement, is provided.
Development of new microperimetric tools dedicated for imaging of early functional changes in the retina may help in the monitoring of various ocular diseases progression e.g. Age-Related Macular Degeneration. Recently described two-photon vision may be applied to microperimetric devices. Many subjects with well-known disease history could be investigated with newly developed instrumentation that tests ability of human eye to perceive near infrared radiation. The main limitation of this new method is a very high cost of the femtosecond laser. Facing this problem, we try to replace the femtosecond laser with lower cost fiber-optic picosecond light source. To compare these two lasers, we constructed dedicated measurement system. We performed measurements of two-photon vision threshold on healthy subjects for two different light sources - sub picosecond Kerr mode-locking solid-state laser and fiber-based picosecond laser. Experiments were conducted for an open circle flickering stimulus with 0.5 deg. diameter, for retinal locations varying from 0 deg. to 5.8 deg., using 4-2-1 threshold strategy that is well-known from classical microperimetry. Values of obtained thresholds are only 5 times higher for the fiber laser than that obtained by using the femtosecond laser, while it was expected to be about over 16 times higher. This fact requires further investigations. Nevertheless, the idea of replacement of the latter laser by relatively cheap fiber-optic one in ophthalmic devices for two-photon vision studies seems to be potentially promising.
In this paper, a study of a low-coherence fiber optic displacement sensor is presented. The sensor consisted of a broadband source whose central wavelength was either at 1310 nm or 1550 nm, a sensing Fabry-Pérot interferometer operating in reflective mode and an optical spectrum analyzer acting as the detection setup. All these components were connected by a single-mode fiber coupler. Metrological parameters of the sensor were investigated for different lengths of the fiber connecting the sensing Fabry-Pérot interferometer (1 m and 10 m). For each length of the fiber, displacement in the range of 0 μm to 500 μm, in increments of 50 μm were measured. Obtained results indicate that the developed sensor is not sensitive to changes in attenuation in the optical path, thus enabling remote measurement of the displacement on long distances while maintaining a satisfactory accuracy.
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