Accurate knowledge of the substrate optical properties is crucial for the theoretical designing and monitoring of optical coatings and characterization of produced optical coatings. Typically, substrate characterization is performed based on reflectance and transmittance data in the relevant spectral range. Measurement errors (offsets of spectral characteristics and noise) are inevitable. Neglecting scattering and assuming transparent spectral ranges, offset values of experimental data can be estimated as a difference between 100% and the sum of the measured transmittance and reflectance. It doesn't provide insights into which spectral characteristic(s), reflectance, transmittance, or both, contribute to the offset or to what extent. We suggest an approach that allows one to estimate the offset values in reflectance and transmittance separately, estimate the effect of these offsets on the determination of substrate optical constants and characterize the substrates reliably. We demonstrate the approach characterizing various substrates in the range 220-1700 nm based on PHOTON RT measurements (EssentOptics).
Phenanthrenequinone (PQ) doped poly(methyl methacrylate) (PMMA) photopolymer material has been studied
extensively due to the growing interest in application involving photopolymers. However, to progress the development
a more physical material model has become necessary. In this article, a kinetic model is developed, which includes: (i)
the time varying photon absorption, including the absorptivity of a second absorber, i.e., the singlet excited state of PQ,
(ii) the recovery/regeneration and the bleaching of the excited state PQ, (iii) the nonlocal effect, and (iv) the diffusion
effects of both the ground and excited state PQ molecules and of the methyl methacrylate (MMA). A set of rate
equations are derived, governing the temporal and spatial variations of each chemical component concentration. The
validity of the proposed model is examined by applying it to fit experimental data for PQ-PMMA layers containing three
different initial PQ concentrations, i.e., 1 mol.%, 2 mol.% and 3 mol.%. The effect of different exposure intensities is
also examined. Material parameters are extracted by numerically fitting experimentally measure normalized
transmission curves and the refractive index modulation growth curve using the theoretical models.
Phenanthreneauinone (PQ) doped poly(methyl methacrylate) (PMMA) photopoplymer material has been actively investigated in the literature. Based on the previously developed NPDD model and the analysis of the mechanisms, the behavior of the material is being further studied. The first harmonic refractive index modulation has been examined for both long time post-exposure and under thermal treatment. Twelve and four spatial concentration harmonics in the Fourier series expansions are applied respectively for comparison. Several effects, i.e., the non-local effect, the diffusion of both the ground state and excited states PQ molecules, which occur during and post-exposure in PQ-PMMA photopolymer materials, have been studied under thermal treatment. For long time post-exposure or when the heating treatment is applied, the formation of the photoproduct, PQ/PMMA, has become very important. The effects of nonlocality, diffusion and the different exposing intensities on the distribution of PQ/PMMA over space and higher harmonic PQ/PMMA concentration have been shown. The experimental results are presented, where no thermal treatment is applied.
The paper presents theoretical and experimental investigations of light beam self-trapping in a photorefractive
medium based on Plexiglas (polymethylmethacrylate, PMMA) with photosensitive phenanthrenequinone (PQ)-
molecules. It is shown that the self-trapping of a laser beam is generated due to the self-interaction of the propagating
light wave under the conditions of a well balanced concurrence of the effects of light diffraction and nonlinear focusing.
A new method for controlling the waveguide cross-section by changing the ratio of two competing mechanisms of the
nonlinear refractive-index variation (namely the formation of the photoproducts and the heating of the medium while
varying the power of the light beam) is proposed.
The recording of self-trapping structures implemented in PQ-PMMA layers has been realized with two laser sources
(405 nm and 514.5 nm) with an average power of several mW. It is shown that the photoattachment of the PQ-molecules
to the polymeric chains and the formation of the photoproduct play the decisive role for the light-induced increase of the
refractive index. Besides, the formation of the waveguide is strongly influenced by heating of the medium, which results
in an additional thermal defocusing of the light beam.
It has been established that the parameters of the waveguide (cross-section and length) are strongly dependent on the
wavelength and the power of the laser radiation, as well as on the concentration of the PQ-molecules. Waveguiding
structures with a diameter of 100 μm were recorded in samples with a high PQ-concentration (up to 4 mol.%) for the
wavelength of 514.5 nm. Reducing the dye-concentration by one order requires shorter (blue) wavelengths (405 nm).
The dependence of the waveguide parameters and the optimal laser wavelength on the concentration of PQ-molecules is
confirmed by the numerical calculation including a 3D-model of the light self-trapping.
We demonstrate a possibility to write efficient and thermally stable volume holographic gratings in a glassy polymer material based on PMMA and phenanthrenequinone with layers prepared by casting the liquid solution of ingredients on a substrate and drying to a solid state. A high concentration of phenanthrenequinone (up to 4 mol.%) makes it possible to use photosensitive layers of lower thicknesses (50 - 180 μm) for the recording of efficient holographic gratings. The exposing is followed by a thermal amplification of the grating due to diffusion of residual phenanthrenequinone molecules and fixation by an incoherent optical illumination. We present experimental temporal curves of the refractive index modulation and diffraction efficiency both under the exposure and the heating process. The behavior of the gratings under temperatures up to 140°C has been studied.
Self-trapping in photorefractive materials is a well-known effect and was widely investigated during the past decade. Usually, the light induced Pockels effect in photorefractive crystals compensates the natural divergence of the beam, therefore, creating a beam with constant diameter, a spatial soliton. The Pockels effect is based on the redistribution of charges in the crystal and can be forced by an external electric field. Due to the crystallographic structure of their elementary cell (tetragonal phase for temperatures below the Curie temperature) some crystals as BaTiO3 or SBN possess a preferential direction the so called c-axis. They are spontaneously poled but in a statistical manner. Applying an external electric field with temperatures higher than the Curie-temperature and then decreasing the temperature all elementary cells will be orientated in the same direction, the crystal will be poled. This poling remains even if the external field is switched off. This means that the crystal obtains a remanent internal electric polarization. The experiment shows that this internal polarization can be understood as an effective electric field and, therefore, the self-focusing of appropriate laser beams is possible. We show the self-focusing of beams of a HeNe-laser (Λ = 633 nm, P = 0.1 μW, 2w0 = 4 μm) in BaTiO3 without external electric fields and discuss the effect in dependence on the polarization and the intensity of the light beam at the entrance surface of the crystal. The experimental results are compared with numerical solutions of the stationary paraxial normalized wave equation.
We present some aspects of wave self-focusing and self-defocusing in a photorefractive Ba0.77Ca0.23TiO3 (BCT) crystal without external electric field and without background illumination. The effects depend on the cross-section of the input beam. We show that by decreasing of the diameter of the input beam from 730 μm the fanning effect disappears at 150 μm. A symmetrical self-focusing is observed for input diameters from 150 um down to 40 μm and a symmetrical self-defocusing for input diameters from 40 μm down to 20 μm. The 1D self-trapping is detected at 65 μm in BCT. Light power and wavelength are correspondingly 3 mW and 633 nm. The experimental results are supplemented with numerical calculations based on both photovoltaic model and model of screening soliton.
Efficient and thermally stable volume holographic gratings in glassy polymeric material based on PMMA and phenanthrenequinone have been recorded. Photosensitive layers were prepared by casting the liquid solution of ingredients on a substrate and drying to a solid state followed by a separation of the polymeric film. This technique was applied to create a possibility to write highly slanted gratings between prisms and to stick them to lightguides with glue. High diffractive efficiencies and moderate angle selectivity of the gratings were reached due to a high concentration of phenanthrenequinone (up to 4 mol.%) making it possible to use the photosensitive layers of lower thicknesses (60 - 150 μm) for the recording of the efficient holographic gratings. The exposing is followed by thermal amplification of the grating due to diffusion of unreacted phenanthrenequinone molecules and fixation by an incoherent optical illumination. The processes of generation, amplification and fixation are discussed for holographic gratings. The holographic gratings were written with an Ar-laser (wavelength 514,5 nm). The grating amplification was realized by heating up of the sample to 50-85°C.
Light-polarizing films are a basis of sheet polarizers, which are used in liquid crystal systems. A light-polarizing film is generally produced by the formation of a uniaxial stretched film made of polyvinyl alcohol (PVA) or its derivatives containing a light-polarizing element like iodine or dichroic dye. Films with iodine are excellent in their initial polarizing performance but poor in resistance to moisture and heat. In order to improve the durability of polarizers more stable dichroic element are used instead of iodine. Films with dichroic organic dyes first of all azodyes are superior in the resistance to moisture and heat as compared with iodine systems. Searching for thermally stable dyes, which do not change their colour under of the influence of high temperature and humidity and are suitable for the manufacturing of effective polarizers is an actual problem. In this paper we show the results of optical investigations of uniaxial PVA-films with iodine (with and without transparent layer) and dichroic azodye.
We present an experimental set-up for the optical detection of condensed vapours or liquids and their refractive indices using photorefractive crystals (e. g. Barium-Calcium-Titanate or Barium-Titanate). Hereby we use a self-pumped phase conjugating mirror along with the effect of total reflection. The modulation of the signal beam in order to avoid background illumination is not necessary due to the phase-conjugated reflection. The detection of different condensed vapours or liquids and their refractive indices is possible by the variation of the input angle of the signal beam.
We present investigations of self-focusing and screening soliton formation processes in photorefractive Bi12TiO20 (BTO) crystals for two different cuts in dependence on both the magnitude of the applied external voltage and additional illumination intensity. We study also temporal behavior of soliton growth and the possibility of stable soliton existence in BTO crystals of the cut [111][112][110].
We present an experimental setup for the optical storage of information via refreshing by inverse seeding on the basis of a photorefractive BCT crystal. The setup consists of two four-wave mixing processes with common amplified signal waves and phase-conjugated (pc) waves. This arrangement offers an opportunity to store optical information dynamically for at least 20 min with permanent strength of the hologram.
Ba0.77Ca0.23TiO3 (BCT) crystals have interesting photorefractive and electro optic properties as was recently demonstrated by several papers. Analyzing these results and our own experiments we have got the conclusion that there is an effective bulk electric field inside the crystals. In order to estimate the value of the suggested field inside the crystal we have developed an experimental method based on two- and four-wave mixings presented in this paper, whereas we are using a BCT crystal that has been grown by Ch. Kuper and co-workers from the University of Osnabrueck. Knowing the value of the bulk field we have estimated the value of the effective photoconductivity of the crystal.
We present a possibility of a rapid measurement of the shape of a human face for medical applications (e.g. jaw-measurement) using a method of color coding by stochastically generated fringe patterns. There are no synchronization problems between the source of the fringe patterns and the detectors within the time of measurement (<1s). The experimental error of the reconstructed 3D object is σ rms<0.1 mm.
We present a new experimental setup for optical storage of information via refreshing by inverse seeding (OSIRIS), which allows a sixfold increase of the storage time of holograms in a Ba0.77Ca0.23TiO3 crystal. The setup consists of two four-wave mixing processes with common amplified signal waves and phase-conjugated waves. Temporal behaviors of the amplified and pc-signal waves for the OSIRIS experiment as well as for the common four-wave mixing experiment are compared and discussed. The solutions of coupled equations under the depleted-pump approximation are obtained in order to estimate the pc-reflectivities and coupling gains of gratings inside the crystal.
We propose an experimental method for the determination of the electro-optic coefficients of photorefractive crystals. Using anisotropic properties of the crystal and using the response of the crystal to different polarization states of the incident light, it is possible to determine the ratio between various electro-optic coefficients by measuring the gain in a two wave mixing set-up. The accuracy of the measurements is estimated and the obtained electro-optic coefficients for a Ba0.77Ca0.23TiO3 crystal show good agreement with corresponding values already known from the literature.
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