Michael V. Vasil'ev

Deformed-helix ferroelectric liquid-crystal spatial light modulator that demonstrates high diffraction efficiency and 370-line pairs/mm resolution

New liquid-crystal media and photoconductor materials are being utilized in spatial light modulators to increase their resolution, diffraction efficiency, speed, and sensitivity. A prototypical device developed for real-time holography applications has shown an 8% diffraction efficiency from a holographic grating with a spatial frequency of 370 line pairs/mm (lp/mm). At 18 lp/mm the device has demonstrated a 31% diffraction efficiency with a 600-micros hologram write time using 400-nJ/cm(2) write beams.

Mitigation of aberration in a beam-shaping telescope and optical inhomogeinity in a free-space optical path using an extended light source coupled to the telescope

One important challenge to implementation of efficient free- space optical (FSO) systems is optical signal scintillation and fade caused by atmospheric turbulence and optical aberration in output beam shaping devices and windows. A new method for mitigation of these harmful effects to delivery of optical radiation to remote subscriber terminals, such as aberration and refraction index non-uniformity in a free- space path, has been developed and tested in field experiments. A known approach to damping optical signal scintillation caused by turbulence in a free-space path was based on forming several substantially parallel optical beams modulated by the same transmit signal and overlapping such beams on a receive optical aperture. The beams transmitted through different free-space paths with uncorrelated optical inhomogenity have different, uncorrelated, transverse distribution of light intensity. Their overlapping provides for averaging out the light intensity non-uniformity and efficient suppression of the signal scintillation. The existing approach to mitigation of optical aberration in atmosphere requires targeting several beam shaping telescopes at a subscriber. This is not always practical. For example, in point-multipoint FSO systems servicing multiple subscribers it is advisable to allocate one telescope per subscriber to achieve highest compactness and cost effectiveness of a system. Also the existing method has limitations in solving a problem of window glass optical inhomogenity and aberration in the telescope itself. A new method for optical aberration mitigation is based on using an extended light source with sufficiently large emitting surface and properly selected width of output radiation angular spectrum coupled to the telescope targeted at a subscriber terminal. The method has been implemented in a point-multipoint base terminal having multiple output beams that could be independently targeted at different subscriber terminals. Results of the trial are presented in this paper. The extended source with given light emitting surface diameter d and angular spectrum width (Theta) may be implemented with an optical fiber having core diameter d and numerical aperture NA equals sin((Theta) /2) installed in optical path between a light source with compact light emitting surface, such as a semiconductor laser, and the telescope. Exit end of such fiber coupled to the telescope acts as an extended light source with angular size (alpha) determined by the fiber core diameter and a focal length of the telescope via a formula (alpha) equals d/f. It has been proven in our field experiments, that by using the source with properly selected angular size and angular spectrum width the following results may be achieved with single telescope targeted at a subscriber terminal: (a) damping of optical signal scintillation at a remote photo-detector (the signal standard deviation has been decreased by several times for wide range of scintillation indexes); and (b) elimination of the signal fade caused by aberration in the telescope and output window (in our experiments the extended source provided 5 to 30 times increase in average signal power at the photodetector for a variety of window glass samples used in residential construction).

Dynamic correction for distortions in imaging optical systems using liquid crystal SLMs

Given are the results of experimental study on the quasi real time holographic correction for the lens distortions in the passive observational telescope in the visible range of spectrum, using the liquid crystal optically addressed spatial light modulator.

Experimental realization of the laser telescope with overall compensation for distortions via phase conjugation

For the first time was realized the laser telescope (beam director), in which the phase conjugation provides the comprensation for the distortions, caused by the primary and secondary mirrors. Nearly diffraction limited laser beam divergence was realized under the significant disadjustments and disalignments of the segmented mirror.

Holographic correction in mid-IR using OA LC SLM elements

Paper presents the results of experimental demonstration of dynamic holographic correction in mid-IR, using optically addressed liquid crystal spatial light modulators and method of two-wavelength holography, when the hologram is recorded at one wavelength and reconstructed at some shifted wavelength. On such a basis one can realize the dynamic interferometer, providing the arbitrary scaling of the wave front distortions and thus to record the dynamic hologram on the differential wavelength, which can be used for dynamic holographic correction of distortions in mid-IR. Method feasibility was confirmed in experiment.

Two-wavelength dynamic holography

Two-wavelength holography, when the hologram is recorded at one wavelength and reconstructed at some shifted wavelength, is an efficient tool for many applications. Optically addressed liquid crystal spatial light modulators are very convenient for recording thin dynamic holograms and, in particular, for the purposes of the dynamic two-wavelength holography. On such a basis one can realize the dynamic interferometer, providing the arbitrary scaling of the wave front distortions. Such an interferometer can be of use for solution of some of the tasks of the adaptive optics, namely, for simplification of the procedure of measuring of the robust wavefront distortions, for recording of the dynamic holographic correctors, working in spectral ranges, where the direct holographic record is impossible, in particular, in mid-IR range of spectrum, and for extension of the range of distortions, which can be corrected by means of the phase valve, mounted in the negative optical feedback loop. We report the experimental realization of such an interferometer.

Orientation dependencies of diffraction efficiency of the dynamic holographic corrector recorded in optically addressed spatial light modulator with the ferroelectric liquid crystal

The results are reported of theoretical and experimental studies of the dependence of diffraction efficiency of hologram-corrector upon an orientation of a diffraction grating vector and of the polarization of the reading-out radiation with respect to the normal to the smectic layers. The gratings recorded using DHF-effect and Clark-Lagerwall effect were studied. The conditions were determined when the dependence of diffraction efficiency vs. radiation polarization is weak. It was found out that in the hologram- corrector, using the polymer photoconductor, the diffraction efficiency is significantly dependent of the vector of grating orientation, while for the hologram-correctors using a-Si:C:H photoconductor this dependence is practically absent.

White-light imaging using large-numerical-aperture telescope with dynamic holographic correction for primary mirror distortions

Large numerical aperture telescope with nonlinear optical correction for distortions, designed for the remote self- luminous object imaging, was realized in experiment and investigated. Dynamic hologram, recorded in optically addressed liquid crystal spatial light modulator, was used as the corrector. Nearly diffraction limited performance of the system was demonstrated.

Experimental realization of a laser telescope with overall compensation for distortions via phase conjugation

For the first time was realized the laser telescope (beam director), in which the phase conjugation provides the compensation for the distortions, caused by the primary and secondary mirrors. Nearly diffraction limited laser beam divergence was realized under the significant disadjustments and disalignments of the segmented mirror.

Large numerical aperture imaging bypass system with dynamic holographic correction for primary mirror distortions

Large numerical aperture telescope with nonlinear optical correction for distortions, designed for the remote self- luminous object imaging, was realized in experiment and investigated. Dynamic hologram, recorded in optically addressed liquid crystal spatial light modulator, was used as the corrector. Nearly diffraction limited performance of the system was demonstrated.