Photopolymerization model for holographic gratings formation in photopolymers

This work describes the study of free-radical homopolymerization kinetics for a system based on acrylamide, triethanolamine, and methylene blue on a polyvinylalcohol matrix, by analyzing temporal variations of the diffraction efficiency. Due to the high viscosity of the material, it has been demonstrated that diffusion processes at the time of recording are negligible. So, the modulation index has been related to the parameters of the systems components, giving as a result, a method that can be used to estimate the chain-length of the polymer, the kinetic rate constants and absorption-scattering parameters. Depending on the termination step, two models of homopolymerization has been proposed – a bimolecular model (bimolecular termination) and a radicalic model (primary radical termination). Using these methods it was possible to compare the results obtained for the kinetic parameters of the photopolymerization process when the intensity and the concentration of each component were changed. Thus, holography can be used as holographic method to analyze the photopolymerization processes. PACS 42.70.Ln; 42.70.Jk; 42.40.Ht     

Pulse record of holographic transmission and reflection diffraction gratings in absorbent photopolymers. 1. Theoretical analysis

In the present paper, the process of formation of holographic diffraction gratings (HDG) in photopolymer materials in transmission and reflection geometry of record by an optical pulse train with duration comparable with characteristic times of the polymerization and diffusion processes is investigated with allowance for light-induced absorption. A solution of the problem of forming HDG with 2D spatially non-uniform amplitude profiles by light beams is derived in the approximation of zero and first harmonics of the monomer concentration and refractive index. 3D spatial distributions of the HDG amplitude profiles are presented in an analytical form of recursion relations. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 58–65, January, 2007.     

Pulse record of holographic transmission and reflection diffraction gratings in absorbent photopolymers. 2. Numerical modeling and experiment

The temporal dynamics of spatial profiles of the first grating harmonic is modeled numerically with allowance for the absorption in the transmission and reflection record geometry. The modeling is carried out for a theoretical model of record of spatially inhomogeneous holographic diffraction gratings by an optical pulse train constructed from a solution of the kinetic photopolymerization equations [1]. A dependence of the post-exposure gain on the recording conditions is analyzed numerically. The influence of the pulse train characteristics on the diffraction grating efficiency is also investigated. The model parameters of the experimentally investigated photopolymer material are estimated from a comparison of the experimental data on the kinetics of the diffraction efficiency with the results calculated for the model presented in [1]. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 34–39, April, 2007.     

Formation of blazed holographic gratings

Various methods for the recording of blazed holographic diffraction gratings are presented and the regions in the wavelength-groove density plane for which they can be used are shown. The gratings are made in the positive photoresist Shipley AZ-1350. The diffraction efficiency for s and p polarization was measured as a function of wavelength and angle of incidence. An absolute efficiency of 80% was achieved.     

Formation of holographic diffraction gratings in photoresist

The exposure-development process for the fabrication of holographic diffraction gratings for integrated optics in the positive photoresist Shipley AZ-1350 is theoretically and experimentally investigated. An analysis of the light intensity distribution is carried out, taking into account the reflectivity at the photoresist-substrate interface and the attenuation in the photoresist for two holographic exposure arrangements. The influence of the exposure energy, the development time for a fixed concentration of the developer and the initial photoresist thickness are described. Various grating profiles are calculated for photoresist films, coated on matched and reflective substrates. Scanning electron micrographs demonstrate very good agreement between the actual and calculated gratings profiles.     

Multiple holographic transmission gratings in silver halide emulsion

Holograms, recorded in silver halide emulsion by a plane reference wave andN cylindrical object waves, are measured as a function of the replay beam angle both for sequential and simultaneous recording. A theoretical model, based on coupled-wave differential equations, is presented. Numerical results for theN=3 case are compared with experiments and good agreement is found. Cross-modulation effects caused by simultaneous recording are discussed and are shown to be detrimental to the fidelity of reproduction of the original object beams. The results are relevant to more general hologram configurations, e.g., display holograms.     

Development of narrow-linewidth diode lasers by use of volume holographic transmission gratings

We present two diode laser setups that employ volume holographic transmission gratings to provide optical feedback. The advantage of this kind of grating is high diffraction efficiency and the possibility to place optical elements on both sides of the grating. This allows for advanced external cavities and adjustable feedback efficiency. The first setup is a diode laser in the Littman configuration with the transmission grating replacing the conventional reflection grating. The second setup improves the frequency selectivity by substituting the feedback mirror with a passive resonator. This grating-enhanced external cavity diode laser (GECDL) achieves excellent frequency stability. A prototype of the GECDL setup demonstrates an intrinsic linewidth of 7 kHz and an operation range that covers the full amplification profile of the laser diode.     

The production of primary radicals in photopolymers during holographic exposure

Recently, extensive studies have been carried out on the time varying absorption effects, which occur in photopolymer materials, during holographic exposure. The theoretical analysis presented in these studies, assumed that three primary mechanisms determined the dye kinetic behaviour. These mechanisms are: (i) photon absorption, (ii) the regeneration or recovery of absorptive photosensitiser, and (iii) photosensitiser bleaching. Based on this analysis, the generation of primary radicals were then described. In this article, we significantly extend these theoretical analyses. A detailed examination of the photochemical reactions, which take place during the photo-initiation mechanisms, is presented. Then a more accurate representation of the initiation mechanisms is developed. This enables more exact expressions for both the temporal and spatial variations of the photosensitiser and primary radical concentrations, to be generated.     

Study of holographic characteristics of photopolymers with two photoinitiator systems

The acrylamide-based photopolymer, one kind of holographic storage material that contains the photoinitiation triethanolamine and N-phenylglycine, was prepared and compared with the photopolymer material containing one photoinitiator. The results indicate that this material shows attractive recording characteristics, which include diffraction efficiency, refractive index modulation, sensitivity and so on. Temporal changes in transmission efficiency with various incident intensities are studied; the angular width of the photopolymer material is also measured.     

Profile formation in holographic diffraction gratings

A theoretical method is outlined for the calculation of the profiles of holographic diffraction gratings produced in photo-resist. This method is shown to give results in agreement with all experimental observations of such profiles. Consideration is then given to the flexibility of arrangements for the formation of holographic gratings and an experimental system is discussed which fully utilizes this flexibility. The wide range of profiles which may be generated with such a system is demonstrated. Finally a theoretical programme of investigation is suggested which may well make possible the formation of holographic gratings of greatly improved spectral performance.     

Amplification of holographic gratings in photorefractive crystals

In this work, we have theoretically and experimentally investigated the process of image amplification in photorefractive crystals as a result of the self-diffraction phenomenon. We have found that the amplification process is of the threshold type and is observed only for an angle of convergence of the light rays greater than a threshold value. The gain coefficient is rather high. For the LiNbO₃ crystal, at a wavelength of 0.63 µm the gain coefficient reached 15 dB; at a wavelength of 0.44 µm, the gain was greater than 50 dB.Tomsk State Academy for Control Systems and Radioelectronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 22–26, June, 1994.     

Second harmonic generation under Bragg diffraction conditions in crystals with light-induced index gratings

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 50, No. 3, pp. 380–385, March, 1989.     

丙烯酰胺基光致聚合物全息光栅的动力学研究

研究了光致聚合物的光化学反应和单体扩散反应的理论模型及动力学参数. 合成了以聚乙烯醇为粘结剂,三乙醇胺为引发剂,藻红B敏化的丙烯酰胺基光致聚合物全息存储材料,通过对测定的透过率和衍射效率进行曲线拟合,分析了染料浓度对光致聚合物的摩尔吸收系数ε、量子产率Φ和光化漂白速率常数k等光化动力学参数的影响规律,以及曝光强度对聚合反应速率k₀、扩散时间常数τ_D和最大折射率调制度Δn等扩散动力 关键词： 光致聚合物 透过率 衍射效率 动力学参数     

高质量全息光栅的制作

全息光栅由感光版记录高斯光束干涉条纹制得 ,干涉条纹的不等宽性降低了光栅品质。制取高质量全息光栅的关键是要用均匀平面波作光源。在分析干涉区内光强分布的基础上 ,给出了制作等周期正弦全息光栅的两种方法 :限制曝光区大小的方法和高斯光束强度均匀化的方法。     

Features of the dynamic self-action of light beams during holographic recording in photopolymers

The influence of changes in the average refractive index and thickness of a working medium layer on the holographic record of sloped transmission gratings in photopolymerizable compositions (PPCs) was studied. The parameters of holographic gratings change with increasing slope of grating lines. It is shown that shrinkage of the PPC layer is the main factor influencing the direction and rate of the energy exchange between recording beams and medium during the holographic recording in PPCs.     

Analysis of optical propagation in thick holographic gratings

A method of analyzing optical propagation in thick holographic gratings by decomposition of the thick material into thin gratings is discussed. The method is readily applicable to study propagation in multiple gratings of arbitrary spatial frequency and orientation recorded in the same thick emulsion. Applied to the double grating case, the method predicts strong cross-coupling between the two gratings for proper relative slope of the gratings. Results are given.     

A numerical study of blazed holographic gratings

The spectral performance of blazed holographic gratings is compared with that of triangular groove gratings.     

Diffraction efficiency of holographic blazed diffraction gratings

Holographic blazed diffraction grating is analysed as a periodic relief of an inclined sinusoidal curve in the shallow profile approximation. Under the condition that the rate of the development is a linear function of the development dose the evolution of the relief during the development is discussed. Theoretical curves of diffraction efficiency are compared with measurements on gratings made by direct holographic method for uv spectral region.