Diffraction Efficiency Of Gratings Research Articles

The Promise of Diffractive Waveplates

Diffractive waveplates exhibit the high diffraction efficiency of Bragg gratings in micron-thick material layers.

Effect of recording-beam ratio on diffraction efficiency of polarization holographic gratings in dye-doped liquid-crystal films

The +1 order diffraction of polarization holographic gratings generated by two linear orthogonally polarized writing beams in dye-doped liquid-crystal films were increased by using two recording beams with unequal intensity. The maximum diffraction efficiency of the polarization holograms in our experimental sample can reach 23% when the intensity ratio of two recording beams is 7:1, and this value is almost triple that when the intensity ratio of two recording beams is 1:1.

An explanation for the non-uniform grating effects during recording of diffraction gratings in photopolymers

The recent results reported in reference 1 have produced an increased interest in explaining deviations from the ideal behavior of the energetic variation of the diffraction efficiency of holographic gratings. This ideal behavior occurs when uniform gratings are recorded, and the index modulation is proportional to the energetic exposure. As a result, a typical sin(2) curve is obtained reaching a maximum diffraction efficiency and saturation at or below this value. However, linear deviations are experimentally observed when the first maximum on the curve is lower than the second. This effect does not correspond to overmodulation and recently in PVA/acrylamide photopolymers of high thickness it has been explained by the dye concentration in the layer and the resulting molecular weight of the polymer chains generated in the polymerization process. In this work, new insights into these deviations are gained from the analysis of the non-uniform gratings recorded. Therefore, we show that deviations from the linear response can be explained by taking into account the energetic evolution of the index modulation as well as the fringe bending in the grating.

Effect of polarization on efficiency of volume Bragg grating filter

Diffraction efficiency of volume Bragg grating, whose period is in the same order as the incident wavelength, is related to the polarization direction of the incident linear polarized beam. When two linearly polarized recording beams with the same polarization direction are used for recording volume Bragg gratings in a photopolymer with diffusion amplification, the azimuth of polarization of the reconstruction beam influences the diffraction efficiency of the grating. When the probe beam is linearly polarized and oriented orthogonally to the grating vectors, the ?-order diffraction beams are also linearly polarized with polarization direction parallel to that of the probe beam. According to the results, a two-dimensional nonspatial optical filter consisting of the volume Bragg gratings would achieve significantly higher efficiency.

大模面积双包层光纤激光频谱组束的实验效果

Based on the design scheme of spectral beam combining using transmitting volume Bragg grating, the experimental results for spectral beam combining of two fiber lasers are reported. The gain mediums used in this experiment were two large mode area erbium-ytterbium co-doped double-cladding fibers, pumped by semiconductor lasers. The output powers of the two fiber lasers were 0.39 W and 0.53 W respectively. With grating diffraction efficiency lower than 60%, the maximal output power of 0.64 W with absolute combining efficiency of about 69.6% was achieved. The influencing factors of the combining efficiency in the experiment are analyzed.

Electrooptical properties of polarization holographic gratings formed in liquid-crystal composites

The results of experimental study of the electrooptical properties of polarization holographic gratings formed in a liquid-crystal composite by polarization holography are presented. The influence of the formation conditions, the parameters of the controlling ac electric field, and the composite geometry on the diffraction efficiency of such gratings is considered.

Spectral beam combining of fiber lasers based on a transmitting volume Bragg grating

Based on the principle of spectral beam combining using a transmitting volume Bragg grating, a simple experimental setup was designed, in which an approach to control the incidence angles of beams using a focusing lens is presented. The experimental results for the spectral beam combining of two fiber lasers are reported. The gain media used in this experiment were large-mode area Er 3+/Yb 3+ co-doped double-clad fibers, pumped by semiconductor lasers. The output powers of the two fiber lasers were 0.39 and 0.53 W, respectively. With a grating diffraction efficiency of about 60%, a maximum combined power of 0.64 W with an absolute combining efficiency of about 69.6% was achieved.

Increasing diffraction efficiency by heating phase gratings formed by femtosecond laser irradiation in poly(methyl methacrylate)

The diffraction efficiency of phase gratings formed by femtosecond laser irradiation in poly(methyl methacrylate) was increased by more than an order of magnitude by subsequent heating below the glass transition temperature (Tg). The first-order Bragg diffraction efficiency of a 10 μm period grating was 1.9% without heating, whereas it increased to 72% when heated at 70 °C for 500 h. This is because the induced refractive index changes (Δn) were increased by heating. From the Lorentz–Lorenz equation, one of the reasons why Δn increases by heating could be a slight volume contraction in the irradiated area.

Precise diffraction efficiency measurements of large-area greater-than-99%-efficient dielectric gratings at the Littrow angle

We have developed improved cavity-finesse methods for characterizing the diffraction efficiencies of large gratings at the Littrow angle. These methods include measuring cavity length with optical techniques, using a Michelson interferometer to calibrate piezoelectric transducer nonlinearities and angle-tuning procedures to confirm optimal alignment. We used these methods to characterize two 20 cm scale dielectric gratings. The values taken from across their surfaces collectively had means and standard deviations of micro=99.293% and sigma=0.164% and micro=99.084% and sigma=0.079%. The greatest efficiency observed at a single point on a grating was (99.577+/-0.002)%, which is also the most accurate measurement of the diffraction efficiency in the literature of which we are aware. These results prove that a high diffraction efficiency with low variation is achievable across large apertures for gratings.

Spectral dependence of the diffraction efficiency of light-induced gratings in YAlO 3:Mn crystals

The results of measurements of spectral dependence of diffraction efficiency of light-induced holographic gratings in YAlO 3:Mn(0.5%) crystal are reported. The experimental data are interpreted as an effect of interaction of both refractive index changes and light-induced absorption associated with optical transition within photo-induced Mn 5+ ions and intrinsic defect centers. Nature of the intrinsic traps involved in the process is discussed.

Holographic recording in acrylamide photopolymers: thickness limitations

Holographic recording in thick photopolymer layers is important for application in holographic data storage, volume holographic filters, and correlators. Here, we studied the characteristics of acrylamide-based photopolymer layers ranging in thickness from 250 microm to 1 mm. For each thickness, samples with three different values of absorbance were studied. By measuring the diffraction efficiency growth of holographically recorded gratings and studying the diffraction patterns obtained, the influence of scattering on the diffraction efficiency of thick volume holographic gratings was analyzed. It was found that, above a particular thickness and absorbance, the diffraction efficiency significantly decreased because of increased holographic scattering. From the diffraction efficiency dependence on absorbance and thickness it is possible to choose photopolymer layer properties that are suitable for a particular holographic application. This study was carried out to determine the highest layer thickness that could be used for phase code multiplexed holographic data storage utilizing thick photopolymer layers as a recording medium. Based on our studies to date we believe that the layer to be used for phase coded reference beam recording with 0.1 absorbance at 532 nm can have a thickness up to 450 microm. The potential use of thicker layers characterized by low scattering losses is part of our continuing research.

Diffraction study of volume holographic gratings in dispersive photorefractive material for femtosecond pulse readout

Based on the modified Kogelnik diffraction efficiency equation, the diffraction intensity spectrum and the total diffraction efficiency of volume gratings in photorefractive media are studied systematically. Taking photorefractive InP:Fe crystal as an example, the effect of the grating parameters and the pulse width on the diffraction properties are presented, in particular under the influence of crystal material dispersion. Under the combined effects, the diffraction pulse profiles and the total diffraction efficiency are shown. Also, the diffraction properties with and without crystal material dispersion are compared. These studies indicate that the properties of the diffraction beams can be controlled by the holographic grating parameters; this property can be used for pulse shaping applications.

Broadband multilayer-coated normal incidence blazed grating with ~10% diffraction efficiency through the 13-16 nm wavelength region

Diffraction gratings used in extreme UV are typically coated with periodic multilayer thin films. These coatings have a small bandwidth, thus leading to a narrow usable spectral region of multilayer gratings. Well-designed aperiodic multilayer coatings could provide high reflectivity over a much broader wavelength region, so they could broaden the usable spectral region of multilayer gratings. We designed and deposited an aperiodic Mo/Si multilayer coating onto a blazed grating substrate. At an incidence angle of 10 degrees, the -2nd-order diffraction efficiency of the multilayer grating is approximately 10% through the wavelength range of 13-16 nm.

Effect of Recording Time on Grating Formation and Enhancement in an Amphiphilic Diblock Liquid-Crystalline Copolymer

To precisely control the first-order diffraction efficiency of holographic gratings recorded in an amphiphilic diblock liquid-crystalline copolymer P40A37, effect of recording time on grating formation and enhancement was studied systematically. Several parameters such as surface relief, the first-order diffraction efficiency, the maximum transmittance under an optical polarizing microscope and order parameters of grating samples were evaluated before and after annealing. Then the enhancement coefficient with recording time was estimated. The best enhancement effect was obtained at 10-s recording upon nano-scaled phase separation. By precisely adjusting the recording time, the diffraction efficiency of holographic gratings in P40A37 can be finely controlled from 0.13% to 10%.

Bragg diffraction in thin 2D refractive index modulated semiconductor samples

A very simple model has been developed to describe the diffractive properties of a crossed grating structure of the refractive index formed by a thin transmission grating recorded in a Bragg reflector. When the Bragg condition of the transmission grating coincides with the band edge of the reflection grating seen as a one-dimensional photonic crystal, the diffraction efficiency and wavelength selectivity of the transmission grating are highly enhanced and a Bragg diffraction regime can be obtained, even in very thin samples. The model can be used to design micrometric very efficient new diffracting devices for optical signal processing.

Effect of saturation on the diffraction efficiency of holographically recorded gratings in azopolymer films

The diffraction efficiency of phase gratings recorded by two s-polarized waves in a polymer is modeled. The molecular orientation distribution along the grating period is seen to become homogeneous as a function of time due to saturation. Experimental data is presented, in agreement with the model.

Two-dimensional polarization holographic recordings in azobenzene liquid-crystalline polymer thin films

Two-dimensional （2D） polarization holographic recordings were accomplished in azobenzene side-chain liquid-crystalline polymer thin films. The 2D polarization grating makes up of two orthogonal 1D polarization gratings, which were written at the same place by two orthogonally linearly polarized 532 nm beams. It is found that the diffraction efficiency of 2D grating is lower than that of 1D grating, and the 2D grating acts as the integration of its componential 1D gratings. Furthermore, we find that the polarization dependency of diffraction efficiency and the polarization conversions arise from the collaboration of linear and circular birefringence.

Erasure mechanisms of polarization holographic gratings in spirooxazine‐doped polymer films

AbstractDynamic holographic recording under linear polarized writing beams at 632.8 nm was performed on two different new spirooxazine doped PMMA matrices assisted by UV light. Thermal rate constants of the two holographic storage medium were determined. It was found that diffraction efficiency and erasure dynamics of the holographic gratings in the two polymer films were dependent on the thermal stability and aggregation of spirooxazine molecules in PMMA matrices. During erasure of the holographic gratings, only in the polymer film with a lower thermal rate constant, a nearly instantaneous increase of diffraction efficiency was observed, resulting from the formation of a transient orientation grating. A theoretical description of the transient orientation grating, competing with isomerization gratings, agrees well with experimental results. The interaction between the chromophores and matrices was also discussed. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

The First Order Diffraction of Dye-Doped Polymer Film Assisted with Nematic Liquid Crystals

The dynamic behavior of surface relief grating of dye-doped polymer film has been investigated. The dependence of the first-order diffraction efficiency on the concentration of dye doped polymer film is studied. The first-order diffraction efficiency of surface relief grating of dye-doped polymer film can be enhanced through the use of nematic liquid crystals as the interface. The surface modulations of relief gratings measured by the atomic force microscopy were compared for both dye-doped polymer films in contact with or without liquid crystals.

회절격자의 변조방식에 따라 형성된 CGH의 재생 영상 분석

회절격자의 회절 효율은 회절격자의 내부에 형성되는 격자의 구조에 의해 영향을 받으며, 이러한 격자의 구조는 변조방식에 의해 결정된다. 컴퓨터 형성 홀로그램(CGH) 역시 기본적으로 회절격자의 원리와 같기 때문에 CGH의 내부에 기록된 패턴은 변조 방식에 따라 달라지며 이 때문에 재생 영상의 성능에 영향을 미치게 된다. 본 논문에서는 이러한 사실을 바탕으로 싸인파 변조 및 구형파 변조 방식에 따른 CGH를 제작하고 재생 영상의 특성을 분석하였다. 또한 각각의 변조방식에 대하여 진폭 및 위상형, 위상형 그리고 이진 위상형 CGH의 재생 능력을 분석하였다. The diffraction efficiencies of gratings are affected by the inner structure which is determined by modulation type. CGH (Computer generated Hologram) basically uses the principles of a diffraction grating. As like as the diffraction gratings the patterns recorded in the CGH are different according to modulation methods which result in the performances of the reconstructed images. In this paper, on this fact CGHs are recorded according to sinusoidal and rectangular modulation methods and the characteristics of reconstructed images are analyzed. Also the reconstruction performances of the amplitude and phase, phase-only and binary phase CGH are analyzed.