Volume Bragg grating (VBG) with refractive index modulation (RIM) apodized with sinusoidal semiperiod profile is studied theoretically and experimentally. An apodized VBG of this type was fabricated with a sequential recording of two VBGs with slightly different resonant Bragg wavelengths in the same glass wafer. As a result, a moiré pattern was produced with a constant average refractive index and a slow sinusoidal envelope of RIM. Modeling showed that an apodized VBG with a sinusoidal semiperiod has provided a suppression of the sidelobes in the reflection spectrum. The experimental measurements are in good agreement with the theoretical predictions. This type of VBG is suitable for high-resolution spectroscopy applications due to a significant reduction of sidelobes.
Phase masks are important optical elements that have been utilized for several decades in a large variety of
applications. Recently, we demonstrated a new type of phase masks fabricated by encoding phase profiles into volume
Bragg gratings, allowing these holographic elements to be used as phase masks at any wavelength capable of satisfying
the Bragg condition of the hologram. Here, we present a new method of true achromatization of this type of phase masks
that removes the need for angle tuning and is implemented by combining this holographic phase masks approach with a
pair of surface diffraction gratings.
The theory of stretching and compression of short light pulses by chirped volume Bragg gratings (CBGs) is reviewed based on spectral decomposition of short pulses and on wavelength-dependent coupled wave equations. Analytic theory of diffraction efficiency of CBG with constant chirp and approximate theory time-delay dispersion is presented. Comparison of approximate analytic results with exact numeric coupled-wave modeling shows excellent agreement for smooth heterogeneities of CBGs.
We demonstrate the recording of volume phase masks in the bulk of photo-thermo-refractive glass. Recording was
produced by exposing the glass to UV radiation through binary amplitude masks. Depending on the profile of the
amplitude mask either a binary volume phase mask or a grayscale phase mask may be produced. Volume phase masks
have been used to generate Fresnel lenses, convert a Gaussian beam into higher order Hermite-Gauss and Laguerre-Gauss modes, to produce optical vortices, and to create aberration-correcting optical components.
Volume Bragg gratings have been successfully used in spectral beam combining of high power fiber lasers with narrow channel separation and in four channel passive coherent beam combining of fiber lasers. Future application of beam combining with kilowatt level lasers requires a more detailed understanding of how to cool the gratings without hurting beam quality. Forced air cooling blown across both surfaces of the grating is both easy and cheap, but has been avoided in the past due to concerns of how the air density fluctuations will hurt beam quality. It is now shown that forced air cooling has no adverse effect on the M2 parameter due to density fluctuations in the air, and can efficiently cool VBG’s such that no degradation in beam quality is seen due to thermal distortions.
Recent experiments demonstrated high conversion efficiency in the process of Orientational Stimulated Scattering (OSS) in nematics. This fact makes OSS attractive for beam combining and clean-up. We consider a scheme for such application and study it by numerically modeling the process. As the process of energy transfer goes, the reverse transfer also begins. However, the region of the back-transfer moves inside the cell with constant velocity in +z direction and given sufficient time, leaves the cell. By modeling the OSS of diffracting one-dimensional beams we show the possibility to obtain a maximum 94% fidelity and 96% power transfer in a numerical experiment with 6 individual overlapping pump beamlets. This means that 90% of the total pump power may be converted into the diffraction-limited output. Remarkably, this output suffers very little cross-phase modulation.
We consider a nematic liquid crystal (NLC) cell with planar structure of director sinusoidally modulated in transverse direction in the cell plane. If thickness of the cell is such that it locally acts as a half-wave plate, thin screen approximation predicts complete diffraction of normally incident light into +1st and −1st diffraction orders. We numerically study the case when this periodically aligned structure is made using anchoring at surfaces. Proper direction of anchoring can be achieved by illumination of photosensitive polymer coating by interference pattern of pair of circularly polarized waves. We found that the structure is unstable when its thickness exceeds a certain critical thickness. Calculations show that for a number of commonly used NLC's this critical thickness lies between 0.66 and 0.86 of the physical period. Such thickness allows for diffraction angles up to 21°. Friedericksz transition voltage of this structure depends on its thickness. This dependence can be well described by an ellipse in the voltage-thickness coordinates. Propagation of a plane wave through the periodically aligned NLC is described using coupled-mode approach. We estimate the contrast ratio versus the influence of walk-off effects and deviation of equilibrium structure from perfectly sinusoidal. Estimations for reasonable set of NLC's parameters show that contrast ratio can be 1000:1 and higher. As a result of this analysis, the transverse size of the cell can be estimated to be less or about 0.25 mm, which suggests it for use as a pixel in projection displays.
Teaching optical phenomena with the use of analogies to the motion of bi-frequency pendulum on a rotary platform is described. Adiabatic following in liquid crystals and in quantum mechanics is visualized. Particular examples include also diffraction by an edge and Fourier properties of a step function.
We demonstrate a pair of 90-45-45 prisms from binoculars appropriately attached to serve as Fresnel rhomb, i.e. achromatic quarter-wave plate. One and two Dove prisms with metallic reflection instead of TIR can work as half-wave plate and polarization rotator, respectively; both achromatic.
Effective operation of a space control system requires extremely accurate acquisition, tracking, pointing and discrimination (ATPD) capabilities, and should provide the ability to disrupt or degrade an adversary space operations if needed. We present a laser based system concept that will offer an innovative solutions to satisfy many space control mission needs. A long-range adaptive laser tracking system (ALTS) described here will provide the required capabilities in target tracking and characterization. It is based on an approach that uses the target as one of the mirrors of the laser resonator. Then, due to specificity of laser, the parameters of its emission allow for deriving the complete information on spatial-angular position of the target, its range, velocity, and flight direction. In this paper we discuss the architecture and operational principles of the ALTS capable in performing the required ATPD function for a remote target. A double-cavity laser scheme with its resonators coupled through the phase-conjugate mirror (PCM) is at the heart of the system. Four-wave mixing mechanism is applied here to form the PCM. Such a scheme allows for automatic adaptive operation of the laser with movable mirror. Both, the results of the theoretical analysis and experimental studies of the proposed ALTS system will be presented, as well as the methods of detecting the spatial-temporal characteristics of the target (its position, range, velocity) through analysis of the received signal. In addition the perspectives of using the proposed ALTS for remote target imaging are also discussed.
We performed architecture and design analyses of coupled-cavity laser systems to arrive at a concept for tracking distant objects. We also conducted an experimental study of such a system using a pulsed ruby laser as a prototype for the laboratory tests. Both laser cavities were coupled through the dynamic holographic grating. Special attention was paid to characterization of the coupled-cavity laser system and its operation. In particular, we studied the formation of holographic grating that serves to couple the cavity; the slope efficiency of the master and slave arms and their dependence upon cavity parameters. The resulted experimental data and analyses verified the tracking principle and proved the feasibility of the proposed phase-conjugate double cavity laser system for tracking moving object at distance with high accuracy.
We have demonstrated both experimentally and theoretically that applying nonlinear optical processes can enable new devices to be realized which allow direct measurement of the power density of laser beams, determination of focal spot location and the beam waist size, as well as visualization and analysis of complex beams' profiles, determination of the beam quality factor and the Strehl ratio. Materials with non-absorptive nonlinearity allow in-line non-distorting measurements. Furthermore, the absence of absorption makes the devices applicable to high power laser beams, and in the focal plane. The resolution of the method is extremely high and is principally limited by the wavelength making possible parallel measurements for a large number of beams. Use of 'intrinsic interference' allows one to obtain high accuracy with simple schemes. We have identified liquid crystals as a highly suitable medium for these purposes due to their giant, non-resonant, and non-absorptive orientational optical nonlinearity.
The effect of space phase self-modulation of light by a photorefractive crystal Ba2NaNb5O15 in an external alternating electric field was experimentally observed.
The influence of a longitudinal magnetic field on the behavior of the speckle-pattern of light, transmitted through an optical fiber, is investigated. The rotation of the speckle-pattern was observed. The angle of the rotation corresponds in order of magnitude and in sign to the Faraday rotation.
Here is theoretical research and experimental implementation of systems to change the polarization state of light with wave length tuning. The systems consist of several birefringent plates with different orientation.
Bifrequency pendulum on a rotary platform was tested and used for several years as a tool to teach various optical phenomena, mostly connected with the polarization of light waves. The following optical topics have found their mechanical analogs: linearity and superposition; damping; isotropy versus anisotropy; birefringence, uniaxial crystals; adiabatic following versus abrupt changes; liquid crystalline displays for watches, based on a twisted nematic cell; quantum-mechanical level anti-crossing; center-symmetric ((chi) (3)) nonlinearity and self- rotation of polarization ellipse; angular momentum conservation; non- center-symmetric ((chi) (2)) nonlinearity and second harmonic generation. This pendulum continues to serve as a source of ideas for new optical devices.
Investigations of interaction of laser beams with liquid crystals reveal new fundamental features of nonlinear dynamic systems. Those features are connected with the giant values of optical nonlinearity, multitude of control parameters, specific properties of light propagation in highly anisotropic and inhomogeneous media, and multidimensional (vector) character of light-induced modulations of liquid crystals. Exceptional opportunities to visualize complex spatio-temporal phenomena in real time, in simple experimental situations, and with utilization of low-power lasers and thin layers of materials, provide a great potential for the design of low-cost and portable set-ups for modeling, demonstrations, and general educational purposes.
Measurement principles of laser beams parameters such as intensity, power, the beam waist and its profile utilizing orientational phenomena in liquid crystals induced by electromagnetic fields are presented.
Optically induced second harmonic generation in glasses with large PbO content was studied. Tensor and phase properties of encoded (chi) (2) susceptibility indicate the presence of electrostatic field, which is proportional to E(EE) where E is a sum of real recording waves E(omega ) and E2(omega ). The model describing charge separation as a result of (chi) (3) optical rectification is discussed.
The mechanism of the recording of a static hologram by a rapidly moving interference pattern in a photorefractive crystal is investigated theoretically and experimentally. The difference between the frequencies of the writing beams strongly exceeds the inverse response time of the hologram and is equal or multiple to the frequency of the external ac field applied to the crystal. The experiment was performed in Bi12TiO20 crystal.
The peculiarities of a higher harmonics photorefractive response are considered theoretically for a grating recorded through a mechanism of phase-locked detection by external ac field. The conditions of existence of a higher harmonic grating are found analytically.
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