Optical imaging through highly scattering media such as biological tissue is limited by light scattering. Recently, it has been shown that wavefront shaping is a powerful tool to overcome this problem. In this work, wavefront shaping using spatial light modulators is used to compensate static scattering media (piece of translucent tape) to allow focusing of different intensity distributions. Light propagation is engineered into a specific region of interest. For this purpose, a sequential phase shape algorithm was implemented experimentally. This algorithm is used to encode a phase distribution on an incident beam to pre-compensate phase distortions acquired by the beam after propagating through the tape. The sequential algorithm combined with a spatial light modulator is used to synthesize a phase distribution required for redirecting light using wavefront shaping. The scattered light was re-directed at the detector plane, in order to be: i) focused at a single pixel, ii) at squared regions of 3×3 and 5×5 pixeles and iii) a line pattern of 41 pixels of the camera. Furthermore, the region of interest was placed outside the central area of the camera opening the possibility of image formation.
Conventional (analog) holographic interferometry (HI) has been used as a powerful technique in optical metrology since sixties of XX century. However, its practical applications have been constrained because of the cumbersome procedures required for holographic material development. Digital holography has brought significant simplifications due to digital capture of holograms and their further numerical reconstruction and manipulation of reconstructed phases and amplitudes. These features are the fundamentals of double exposure digital holographic interferometry which nowadays is used in such applications as industrial inspection, medical imaging, microscopy and metrology. However another very popular HI technique, namely real time holographic interferometry has not been demonstrated in its digital version. In this paper we propose the experimental-numerical method which allows for real-time DHI implementation. In the first stage a set of digital phase shifted holograms of an object in an initial condition is captured and the phase of an object wavefront in the hologram plane is calculated. This phase is used to address a spatial light modulator, which generates the initial object wavefront. This wavefront (after proper SLM calibration) propagates toward an object and interfere with an actual object wavefront giving real-time interference fringes. The procedure works correctly in the case when CCD camera and SLM LCOS pixel sizes are the same. Usually it is not the case. Therefore we had proposed two different methods which allow the overcome of this mismatch pixel problem. The first one compensates for lateral magnification and the second one is based on re-sampling of a captured phase. The methods are compared through numerical simulations and with experimental data. Finally, the implications of setting up the experiment with the object reference phase compensated by the two approaches are analyzed and the changes in an object are monitored in real time by DHI.
Fast optical self-focusing has been observed in a homeotropic nematic liquid crystal cell. This nonlinearity is induced by
an intensity modulated infrared laser having a peak power of 160mW, a pulse repetition rate of 150Hz, and a duty cycle
of 0.05 and launched with extraordinary polarization. During these experiments the illumination time is kept at 0.3msec
and the ambient temperature is controlled. We have observed that self-focusing propagation depends on ambient
temperature, laser power and duty cycle. Notably, when illuminating with a continuous beam having the same
corresponding average power, only diffraction can be observed. These results suggest that the nonlinearity is produced
by a combination of thermal effects and molecular reorientation that leads to changes in the order parameter. Further
optical experiments and thermal calculations have been conducted to identify the responsible mechanism for the self-focusing
of the laser beam. It has been found that soliton formation can be achieved if the parameters as ambient
temperature, pulse repetition rate and duty cycle of the laser are set to optimal conditions. Although, this nonlinearity in
a liquid crystal cell has been already demonstrated for transverse illumination, the presence of beam propagation with
self-focusing has not been reported yet. The fast nonlinearity reported in this work has the potential to generate a number
of new applications of liquid crystals.
Nonlinear phase contrast microscopy is an optical technique that uses an intensity-dependent refractive index material to
produce high-contrasted images of transparent specimens. Earlier proposal of liquid crystals as phase filters for phase
contrast applications used optically addressed spatial light modulators fabricated with photoconductive film. Here, we
propose the use of a simpler planar nematic liquid crystal cell doped with 1% wt methyl red. Owing to their polarization
dependent enhancement factor a tunable phase filter can be photoinduced efficiently. Thus, images of different degree of
contrast (and even contrast reversal) can be obtained either by rotating the polarization vector. All optical real-time
imaging of dynamic events can be performed and image processing such as edge enhancement is demonstrated.
The new astronomical instruments; spectrographs, cameras, focal reducers, telescopes, etc., requires to work in more and more wide spectral ranges and with very large fields of vision. Therefore, the chromatic aberrations and the field curvature are aberrations very difficult to correct and to balance in the process of optical design. For that problem, in the stage of optical design is necessary to add more optical components in the instrument, also we need to use more aspherical surfaces and we need more time of optical design, etc. In this work we propose to use the technique of wavefront coding using a cubic phase mask to obtain optical systems with an extended depth of field that corrects the chromatic aberration and the field curvature automatically. In this paper we present preliminary results of this technique.
Conference Committee Involvement (1)
21 August 2018 | San Diego, California, United States