Holographic Transmission Gratings Research Articles

Experimental study of multiplexed holographic gratings recorded in a photopolymerizable silica glass

Recently the capability of photopolymerizable sol–gel materials to angular multiplex holographic gratings has been demonstrated [1, 2]. In this study, holographic transmission gratings were recorded using angular multiplexing in a silica glass doped photopolymer holographic recording material. The time interval between exposures and the angular selectivity were analyzed, in particular their effects on the dynamic range. By using the optimal parameters gratings of similar response were achieved with values of M/#=4.8, as well as good values of diffraction efficiency with high photosensitivity.

Design of Polysiloxane Materials for Electronic Applications

Dehydrocoupling reaction catalyzed by palladium was used to synthesize aromatic-substituted poly(carbosiloxane)s having potentiality to be used as thermo-resistant insulating materials. Well-defined transmission holographic gratings with high diffraction efficiency were fabricated by the introduction of siloxane components: 1) to mesogenic compounds, 2) to bifunctional epoxides as cationically polymerizable monomers, which polymerize slower than radically polymerizable multi-functional acrylate.

Transmission holographic gratings produced using networked polyurethane acrylates withvarious functionalities

Holographic gratings have been fabricated in networked polyurethaneacrylates with various cross-linking densities. It was found that low(f = 2) andhigh (f = 6) functionality monomers gave narrow and clean polymer–liquid crystal(LC) phase separation due presumably to the insufficient migration of LCmolecules out of the polymer domains. However, the mechanisms leadingto poor phase separation seem different, i.e., reaction at a slow rate forf = 2, and entrapment of LC molecules within highly networked polymer forf = 6. With intermediatemonomer functionality (f = 3), relatively large channels with large LC droplets gave the greatest diffractiondue to a large difference in refractive index between polymer and LC phases.

Low-threshold and narrow-linewidth lasing from dye-doped holographic polymer-dispersed liquid crystal transmission gratings

Optically pumped lasing with low threshold and narrow linewidth was observed in a 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran dye-doped holographic polymer-dispersed liquid crystal transmission grating structure. The results showed that the lasing peak centered at about 609 nm, with a full width at half maximum of about only 1.8 nm. The threshold pumping intensity was about 120μJ under the excitation of a frequency-doubled Nd:yttrium–aluminum–garnet laser operating at a wavelength of 532 nm. Theoretical calculation showed that the lasing from this structure happened at the band edge of the photonic band gap. The lasing modes were also investigated. The transmission grating investigated enjoys a much larger gain length compared to the reflection one.

High-Performance Transmission Holographic Gratings via Different Polymerization Rates of Dipentaerythritol Acrylates and Siloxane-Containing Epoxides

Well-defined transmission holographic gratings with high diffraction efficiency were fabricated by irradiating a mixture of photopolymerizable materials with different reaction rates consisting of penta-/hexafunctional acrylates and siloxane-containing glycidyl ether (88%) or cyclohexene oxide (85%) as radically and cationically polymerizable monomers, respectively, with Nd:YAG laser (λ = 532 nm). The ratio between the acrylate and epoxide was important to obtain gratings with high diffraction efficiency. They had smooth surface morphologies of well-regulated spacing (0.9 μm). Performance of holographic gratings was optimized by varying the chemical structures of epoxide under various experimental conditions. Introduction of siloxane component into epoxide induced easy diffusion and distinct separation of the epoxide from the polymerized matrix formed by the acrylate, by their low viscosity and incompatible property during the fabrication of the gratings. These gratings were actually used to store a real image.

Degenerate four-wave mixing experiments in Methyl green dye-doped gelatin film

Degenerate four-wave mixing (DFWM) was performed in Methyl green dye-doped gelatin films using continuous-wave laser radiation ( λ = 633 nm ) generated by a He–Ne laser of total power 35 mW. Various parameters which influence the phase-conjugate (PC) signal during the DFWM process were studied. The PC signal contributions from induced holographic transmission and reflection gratings were measured. We observed a maximum PC beam reflectivity of 0.13% in these dye-doped gelatin films.

Holographic optical elements as scanning lidar telescopes

We have developed and investigated the use of holographic optical elements (HOEs) and holographic transmission gratings for scanning lidar telescopes. Rotating a flat HOE in its own plane with the focal spot on the rotation axis makes a very simple and compact conical scanning telescope. We developed transmission and reflection HOEs for use at the first three harmonic wavelengths of Nd:YAG lasers. The diffraction efficiency, diffraction angle, focal length, focal spot size and optical losses were measured for several HOEs and holographic gratings, and found to be suitable for use as lidar receiver telescopes, and in many cases could also serve as the final collimating and beam steering optic for the laser transmitter. Two lidar systems based on this technology have been designed, built, and successfully tested in atmospheric science applications. This technology will enable future spaceborne lidar missions by significantly lowering the size, weight, power requirement and cost of a large aperture, narrow field of view scanning telescope.

Multiplexed holographic transmission gratings recorded in holographic polymer-dispersed liquid crystals: static and dynamic studies

The optimization of the experimental parameters of two multiplexed holographic transmission gratings recorded in holographic polymer-dispersed liquid crystals is investigated. Two methods are used to record the holograms: simultaneous and sequential multiplexing. These two processes are optimized to produce two multiplexed Bragg gratings that have the same and the highest possible diffraction efficiencies in the first order. The two methods show similar results when suitable recording parameters are used. The parameters of the recorded gratings (mainly the refractive-index modulation) are retrieved by use of an extension of the rigorous coupled-wave theory to multiplexed gratings. Finally, the response of the holograms to an electric field is studied. We demonstrate few coupling effects between the behavior of both gratings, and we expect a possibility of switching from one grating to the other.

Switchable holographic gratings formed in polymer-dispersed liquid-crystal cells by use of a He–Ne laser

We report on the formation of switchable holographic transmission gratings in polymer-dispersed liquid-crystal cells by use of the 633-nm wavelength of a He-Ne laser. We present results for the structure, diffraction efficiency, and switching characteristics of the gratings.

Transmission holographic gratings using (siloxane and mesogen)-containing compounds

(Siloxane and mesogen)-containing compounds were found effective in the formation of holographic gratings, over a wide concentration range from 3 wt% to 30 wt%. A fine grating with 40% diffractive efficiency was formed with 10 wt% 1-(4′-methoxyphenyloxycarbonylphenoxy)butyl-1,1,3,3,3-pentamethyldisiloxane. The diffraction efficiency and angle selectivity of the grating were also significantly affected by the structure of siloxane chain. 3-(4′-Methoxyphenyloxycarbonylphenoxy)butyl-1,1,1,3,5,5,5-heptamethyltrisiloxane gave high (approx. 90%) diffraction efficiency and showed very narrow angular selectivity of about 5°.

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.

A high throughput spectrometer system for helium ash detection on JET

Acquiring information about helium ash production and transport is fundamental for future burning plasma devices, such as International Thermonuclear Experimental Reactor, since the helium ash must be continuously removed from the plasma to prevent the dilution of the deuterium–tritium (DT) fuel. This diagnostic for future JET DT operation uses charge-exchange recombination spectroscopy (CXRS) in conjunction with the JET neutral heating beam to measure the helium density at 20 radial locations across the JET plasma via the 4686 Å He+ line and an array of heated 1 mm quartz fibers. The CXRS diagnostic utilizes a high throughput short focal length spectrometer with f/1.8 input optics, two entrance slits, a holographic transmission grating, and refractive optics. The detector is a thinned back-illuminated charge coupled device that has high quantum efficiency, a 10 MHz readout speed, and a time resolution of 5 ms.

Formation of Holographic Transmission Diffraction Gratings in Absorbing Photopolymer Media

The formation of holographic transmission gratings in absorbent photopolymer materials (PPM) is studied theoretically. Analytical model describing a spatiotemporal transformation of holographic grating field during record is developed with allowance for optical absorption and diffusion processes. It is demonstrated that optical attenuation during record causes the spatial inhomogeneity of refractive index grating profile and of the its growth kinetics. A degree of influence of optical attenuation on the integral diffraction characteristics is evaluated based on numerical simulation. The kinetics of record of high-efficient transmission holograms in PPM is experimentally investigated. Based on a comparison of theoretical and experimental time dependences of diffraction efficiency, physical parameters of the examined PPM are determined and adequacy and tendency in the development of the suggested mathematical model are evaluated.

Structural transitions in holographic polymer-dispersed liquid crystals.

Dynamic light scattering was used to analyze the structural and dynamic properties of nematic director field within liquid crystal domains formed in holographic polymer-dispersed liquid crystal transmission gratings. Samples prepared from two different types of prepolymer mixture: one curable with visible (VIS) and another curable with UV light were investigated. In both formulations a critical slowing down of thermal director fluctuations, signifying the second-order structural transition of the nematic director field was observed in the vicinity of some critical external electric field as well as close to some critical temperature. For VIS samples also the size and the shape of phase separated droplets and viscoelastic and surface anchoring parameters of the liquid crystalline (LC) material forming the droplets were deduced. The viscoelastic constants were found to significantly deviate from the viscoelastic parameters of the pure LC mixture.

Volume Phase Holographic Gratings: Polarization Properties and Diffraction Efficiency

ABSTRACTWe discuss the polarization properties and first‐order diffraction efficiencies of volume phase holographic (VPH) transmission gratings, which can be exploited to improve the throughput of modern spectrographs. The wavelength of peak efficiency can be tuned by adjustment of the incidence angle. We show that the variation of the Kogelnik efficiency versus Bragg angle depends only on one parameter, given by Ptune = (Δnd)/(nΛ), where Δn is semiamplitude of the refractive index modulation, n is the average index, d is the thickness of the active layer, and Λ is the grating period. The efficiency has a well‐defined dependence on polarization. In particular, it is possible to obtain theoretical 100% diffraction efficiency with one linear polarization at any angle, or to obtain 100% efficiency with unpolarized light at specific angles. In the latter case, high efficiency is the result of aligning the peaks of the s‐ and p‐polarization efficiency‐versus‐thickness curves. The first of these “s‐p–phased gratings” for astronomy is in use with the 6dF spectrograph. Consideration of polarization is particularly important for high spectral resolution, which requires large incidence angles. We also discuss the possibility of separating polarization states for improved throughput along the entire optical train of a spectrograph.

Monitoring of chemical reactions within microreactors using an inverted Raman microscopic spectrometer.

An inverted Raman microscope spectrometer has been used to profile the spatial evolution of reactant and product concentrations for a chemical reaction within a microreactor operating under hydrodynamic flow control. The Raman spectrometer was equipped with a laser source at wavelength of 780 nm, confocal optics, a holographic transmission grating, and a charge-coupled device (CCD) detector. The microreactor consisted of a T-shaped channel network etched within a 0.5 mm thick glass bottom plate that was thermally bonded to a 0.5 mm thick glass top plate. The ends of the channel network were connected to reagent reservoirs that were linked to a syringe pump for driving the solutions by hydrodynamic pumping within the channels. The microchannels were 221 micro m wide and 73 micro m deep. The synthesis of ethyl acetate from ethanol and acetic acid was investigated as a model system within the microreactor as Raman scattering bands for each reactant and product species were clearly resolved. Raman spectral intensities of each band were proportional to concentration for each species and hence all concentrations could be quantitatively measured after calibration. By scanning specific Raman bands within a selected area in the microchannel network at given steps in the X-Y plane, spatially resolved concentration profiles were obtained under steady-state flow conditions. Under the flow conditions used, different positions within the concentration profile correspond to different times after contact and mixing of the reagents, thereby enabling one to observe the time dependence of the product formation. Raman microscopy provides a useful complementary technique to UV/VIS absorbance and fluorescence methods for the in situ monitoring and analysis of chemical reaction species having their lowest S(0)-S(1) absorption bands too far in the UV to be of use, due to their probable overlap with the bands from other reactant, product and solvent molecules.

Mono-axial power spectrograph for a spectral imaging ellipsometer: design andexperimental results

A novel mono-axial power spectrograph for spectral imaging ellipsometry isdescribed in this paper. Spectral imaging ellipsometers employing theproposed mono-axial power spectrograph can measure information aboutthin films along the line of the entrance slit of the spectrograph. Themono-axial power spectrograph is simply composed of an entrance slit,a holographic transmission grating and a focusing cylindrical doublet.Several features of the proposed spectrograph give some benefits when usedwith an imaging ellipsometer. Above all, the holographic transmissiongrating guarantees polarization insensitivity of the incident wave to thespectrograph. Therefore, the polarization dependency and calibration factors inthe spectral imaging ellipsometer can be minimized. Also, the simplestructure of the spectrograph employing a cylindrical doublet allows opticalcompatibility with conventional ellipsometers and high adaptability to spectralimaging ellipsometers. The spectral resolution of the spectrograph is 5 nmand it has a spectral range from 400 to 800 nm. The focal length of themanufactured mono-axial power spectrograph is 25.4 mm and the total systemdimension is less than 120 mm(L) × 100 mm(W) × 100 mm(H).The design procedure and the results of experiments to measure the spatialtransmittance are discussed for some transmission samples.

Effect of Monomer Functionality on the Morphology and Performance of the Holographic Transmission Gratings Recorded on Polymer Dispersed Liquid Crystals

Transmission gratings were formed holographically in polymer dispersed liquid crystal (HPDLC) composite materials using commercially available UV-curable acrylate monomers and the nematic liquid crystal TL203. Monomer mixtures with average functionality ranging from 1.3 to 3.5 were used to prepare HPDLC transmission gratings. The phase-separated morphology and surface topology of the transmission gratings as observed through scanning electron microscopy and atomic force microscopy varied profoundly with the different monomer mixtures characterized by their average functionalities. The switching properties of the HPDLC gratings were correlated with the morphological changes.

Polarization and switching properties of holographic polymer-dispersed liquid-crystal gratings I Theoretical model

Polarization properties and electro-optical switching behavior of holographic polymer-dispersed liquid-crystal (HPDLC) reflection and transmission gratings are studied. A theoretical model is developed that combines anisotropic coupled-wave theory with an elongated liquid-crystal-droplet switching model and includes the effects of a statistical orientational distribution of droplet-symmetry axes. Angle- and polarization-dependent switching behaviors of HPDLC gratings are elucidated, and the effects on dynamic range are described. A new type of electro-optical switching not seen in ordinary polymer-dispersed liquid crystals, to the best of the author’s knowledge, is presented and given a physical interpretation. The model provides valuable insight to the physics of these gratings and can be applied to the design of HPDLC holographic optical elements.

Characterization of holographic polymer dispersed liquid crystal transmission gratings

We investigate physical properties of polymer dispersed liquid crystal transmission gratings which influence the light diffraction efficiency behavior. Scanning electron microscopy is used to determine the orientation and shape of nanometer-sized liquid crystal droplets in a polymer host. Dependence of diffraction efficiency on light polarization suggests that the orientation of a liquid crystal within the ellipsoidal droplets is axial. The distribution of droplet orientations appears to vary for different grating spacings. The switching properties of the gratings are investigated and analyzed with a sinusoidal dielectric tensor modulation model. The analysis also takes into account orientational distribution of average directors and spatial distribution of the droplets. Temperature dependence of diffraction efficiency is tentatively explained by a structural transition within the droplets and the presence of impurities in droplets.