Chromatic aberrations of diffractive waveplate optics for imaging applications can be corrected for different switchable states. Different opportunities for lenses and prisms, and their limitations are discussed.
The optical power of diffractive waveplate structures is limited not as much by fabrication technology issues as by the fundamental features of light propagation in complex anisotropic structures. The infinitely thin two-dimensional film approximation does not apply, and the efficiency of 4G lenses, prisms, etc., is reduced for geometries corresponding to sharp focusing lenses and large diffraction angles. Due to thin-film nature, these films can be combined to reduce effective focal length, increase effective diffraction angle, topological charge, etc. Along with this, we will discuss the opportunity of increasing optical power of 4G lenses, prisms, etc. without compromising efficiency.
The development history of polarization gratings (PGs), with origins in holography and Bragg gratings, accentuated and
reinforced their perception as gratings. We highlight their nature as waveplates - diffractive waveplates (DWs) - and
stress their family connection to vector vortex waveplates. This approach provides a straightforward understanding of
the unusual properties of PGs, such as nearly 100% diffraction in thin material layers, the presence of only one
diffraction order for a circularly polarized beam, wide diffraction bandwidth and the possibility of achromatic behavior.
With technology being ripe for applications such as beam steering, and optical switching, we characterize the resistance
of DWs to optical radiation, the effects of temperature and deformations. We also show that the boundary effects in the
manufacturing process make it necessary to use substrates larger than the desired aperture of the DW. The multicomponent
systems are discussed for developing normally transmissive switchable imaging systems, beam scanning, and
achromatic diffraction.
Absorption coefficients of highly transparent optical materials (glass) have been measured using a z-scan technique. Absorption coefficients as low as 10−5 cm−1 and lower can be measured by scanning a substrate of a transparent optical material in the focal region of a lens. Glass substrates with relatively high absorption coefficients are used for calibration and for verifying the results of measurements. The sensitivity of this technique proves to be high enough to reveal differences between absorption coefficients of different substrates made of the same material (BK7). We have suggested and used a novel procedure, scanning nonlinear lens profiling, for characterization of homogeneity of glass substrates and other transparent optical materials. Optical nonlinearity of glass substrates allows fast and high-precision measurement of the power and power density of high-power near-IR laser beams without large temperature increases and related problems.
We have tested a technique for characterizing optical substrates using high sensitivity of the constant of nonlinear refraction to the structural and compositional homogeneity of the material. The technique consists in two steps: first the substrate is positioned in the focal region of a lens where the signal of the nonlinear phase shift is maximal and, second, the substrate is scanned in directions transverse to the propagation direction of the probe laser beam. The
measured variations are proportional to variations in the nonlinear phase shift across the substrate and reflect distribution of parameters that contribute into the nonlinear phase shift, including the absorption coefficient and substrate thickness. This technique can be used for mapping trace amounts of impurities, dopants and inclusions as well as varying external/boundary conditions in glass substrates, liquid crystals, and other materials. As an example, we have visualized subtle changes inflicted on a holographic glass by UV exposure.
We have applied z-scan technique for measuring absorption coefficients of highly transparent optical materials (glass) and have used those for characterizing the power and power density of high power near IR laser beams. Self-phase modulation due to thermal indexing is the process underlying this technique. Glass plates with appreciable absorption coefficients are used for calibration purposes and for verifying the results of measurements. Fundamentally, as low absorption coefficients as 10-5 cm-1 can be measured by scanning a plate of a transparent optical material in the focal region of a lens. The sensitivity of this technique proved to be high enough to reveal strong variation between the absorption coefficients of optical windows made of the same material BK7 but obtained from different sources. We have suggested and used a novel procedure, scanning nonlinear lens profiling, for characterization of homogeneity of optical glasses and other transparent optical materials. Most importantly, the technique can be used for fast and high precision measurement of power of high power laser beams without evoking large temperature increase and related problems.
Nonlinear optical processes in liquid crystals (LC) can be used for construction of all-optical spatial light modulators (SLM) where the photosensitivity and phase modulating functions are integrated into a single layer of an LC-material. Such spatial light integrated modulators (SLIMs) cost only a fraction of the conventional LC-SLM and can be used with high power laser radiation due to high transparency of LC materials and absence of light absorbing electrodes on the substrates of the LC-cell constituting the SLIM. Recent development of LC materials the photosensitivity of which is comparable to that of semiconductors has led to using SLIM in schemes of optical anti-jamming, sensor protection, and image processing. All-optical processes add remarkable versatility to the operation of SLIM harnessing the wealth inherent to light-matter interaction phenomena.
The combination of liquid crystals (LCs) and reconfigurable nanoparticulate networks results in most versatile materials for controlling light beams. These material systems can be used for developing multi-functional reconfigurable photonics and opto-electronics components and spatial light modulators with ultimate light modulating capabilities. We review here our results in laser recording of one and two-dimensional diffraction gratings and gratings with variable pitch. Nonlinear optical properties of LC with nanoparticulate internal networks and diffraction gratings laser-recorded in these materials are discussed. Nanoparticulate networks are capable of stabilizing the thermodynamic relaxation of photoinduced cis-isomers of molecules in photosensitive azobenzene LCs used as host for the nanoparticulate network leading to bistability of the phase state (anisotropic and isotropic) of the material, and reversible all-optical switching between those states.
We report about applications of nonlinear optical processes for laser beam and optics characterization. The basic mechanism of the measurements consists in scanning a thin film of a liquid crystal in the focal region of a laser beam and processing the self-phase modulation signal. This technique allows precise and quick determination of the focal waist position and radius, which, in conjunction with the parameters of the focusing system allows determination of the laser beam divergence. We have demonstrated the capabilities of the technique for measuring submicron waist sizes and characterizing astigmatic optical systems. The technique is applicable to short laser pulses. The measurements were performed using the device implementation of the technique, the Crystal Scan Optical Multimeter.
We present the results of comprehensive characterization of capabilities of Crystal Scan device for measuring laser beams.
(1) As small as 0.7 mm waist radius was measured for a laser beam of λ = 409 nm wavelength. The measurement lasts about 1 s with actual scan time being about 300 ms.
(2) The focal waist position with accuracy better than 1% using focusing lenses of 6 mm to 50 mm focal length was determined.
(3) Laser beam divergence in the range of 10-5rad to 10-2rad was measured and compared both to theoretically anticipated values as well as to the results obtained by conventional techniques.
(4) We have identified the specific features of the signal received from astigmatic beams and measured the position and waist radii of both focuses.
(5) The technique was successfully applied to measuring of laser beams at 1064 nm, 1341 nm and 10.6 mm wavelengths.
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