Holographic Gratings Research Articles

All-optically controlled beam splitting through asymmetric polarization-based holography.

All-optically controlled beam splitting is demonstrated by a tunable split ratio through polarization modulation. The beam splitters are essentially holographic gratings recorded by means of the interference of two 532nm beams with asymmetric polarization states in an azobenzene liquid crystal film. It is found that the intensity ratio between zero and the first diffraction order is adjustable through the polarization manipulation of the recording light. An arbitrary split ratio from 0 to 1 is obtained with the photoinduced birefringence of the film being set to an appropriate value, which is independent on the polarization state of the probe light. Furthermore, the formation processes of the recorded beam splitters are contributed to the dual-grating coupling, and the tunable split ratio under various polarization conditions is discussed in terms of the Fresnel theory and Jones matrices.

MINI OBSERVATORIO AUTÓNOMO PARA ESPECTRÓGRAFO DE METEOROS

We present a new autonomous system to study meteors’ emission spectra based on the image capture with CCDs, wide field cameras and a holographic diffraction gratings. The equipment contains two spectrographs with different configurations, so that unique and complementary images of the same event can be captured. The system is formed by an Autonomous Mini Observatory, which holds the two spectrographs, connected to a weather station and two computers where the data obtained nightly is stored in their hard drives.

Triggered Lightning Spectroscopy: 2. A Quantitative Analysis

AbstractIn the mid 1960s, researchers laid the foundation for modern quantitative lightning spectroscopy. They were the first to acquire time‐resolved return stroke spectra and the first to use spectroscopy as a diagnostic technique to characterize the physical properties of the lightning channel (Orville, 1968a, https://doi.org/10.1175/1520-0469(1968)025<0827:AHSTRS>2.0.CO;2, 1968b, https://doi.org/10.1175/1520-0469(1968)025<0839:AHSTRS>2.0.CO;2, 1968c, https://doi.org/10.1175/1520-0469(1968)025<0852:AHSTRS>2.0.CO;2; Salanave, 1961, https://doi.org/10.1126/science.134.3488.1395; Uman, 1966, https://doi.org/10.1109/MSPEC.1966.5217654). Now, almost 50 years later, technology, including high‐speed cameras, volume‐phase holographic gratings, and triggered lightning, has progressed to the point at which new studies in lightning spectroscopy are needed to verify and improve upon past measurements. In this study triggered lightning spectra were recorded at 673 kfps (1.5 μs per frame) with an exposure time of 1.1 μs. Temperature, number density, and pressure on a 1.5‐μs time scale are presented here for a five‐stroke‐triggered lightning flash. The analysis yields temperatures of the return stroke of greater than 40,000 K as well as electron number densities greater than 1019 cm−3 and peak pressures from 20–350 atm, significantly greater than previous studies. The analysis reveals that in the initial microseconds of the return stroke, the physical characteristics within the channel are rapidly changing and that submicrosecond resolution is needed to observe the initial ionized channel in greater detail.

LED-Cured Reflection Gratings Stored in an Acrylate-Based Photopolymer.

The storage of volume holographic reflection gratings in low-toxicity photopolymers represents a challenge at present since they can be used in many important applications such as biosensors and holographic optical elements. In this context, an acrylate-based photopolymer developed in our research group was employed to study the recording of unslanted holographic reflection gratings at high spatial frequencies. The optimal preparation conditions of the photopolymer layers were determinated. The diffraction efficiencies are measured in both recording and curing stage and a comparative study of these values was realized. In addition, a theoretical study using Kogelnik’s coupled wave theory was carried out with the aim of understanding the diffraction efficiency behaviour of both processes. In this work, a maximum diffraction efficiency of 14.1% was reached after a curing process in 150 µm layers at a recording wavelength of 488 nm. This value represents a good result compared to that reported in the literature and opens the way to reflection mode holography research using low-toxicity material.

Long-range in situ picometer measurement of the period of an interference field.

A scanning reference grating (SRG) method is proposed for high-precision in situ measuring and controlling the period of a long-range interference field. The reference grating is produced with the in situ interference field; then it is used to obtain phase shift signal when scanning in the interference field. With the phase shift signal collected by the SRG system, before the exposure process of the holographic grating fabrication, the period and the period uniformity of the holographic grating can be evaluated directly from the interference field; then optical adjustment can be applied until the grating period is tuned to any certain desired value. Experiments of measurement and adjustment are conducted, and an interference field with period value of 833.335 nm±10 pm in 60mm range is reached. The proposed method gives an efficient way to fabricate large gratings of an accurate period; furthermore, it provides a reliable tool that may lead us to picometer-level optical metrology and fabrication for the most advanced lithographic equipment and in other scientific fields.

Influence of buffer layer and grating layer on diffraction of multilayer volume holographic grating

In this paper, based on Kogelnik’s coupled-wave theory and transfer matrix method, we discussed the temporal diffraction properties of multilayer volume holographic gratings under femtosecond pulses readout. Results show that grating layers generate diffracted pulses, with each grating layer diffracting out one sub-pulse, whose waveform is determined only by parameters of the corresponding grating layer. The buffer layers placed on the right side of the grating layer translates the former diffracted sub-pulse along the negative time axis, and the translation is proportional to the total thickness of buffer layers placed on the right side of it, whereas the buffer layers placed on the left side of the grating layer has no effect on the diffracted sub-pulse. We also give explanations of these phenomena based on the diffraction efficiency of the grating layers and the phase shift of the buffer layers.

The Apache Point Observatory Galactic Evolution Experiment (APOGEE) Spectrographs

We describe the design and performance of the near-infrared (1.51–1.70 μm), fiber-fed, multi-object (300 fibers), high resolution (R = λ/Δλ ∼ 22,500) spectrograph built for the Apache Point Observatory Galactic Evolution Experiment (APOGEE). APOGEE is a survey of ∼105 red giant stars that systematically sampled all Milky Way populations (bulge, disk, and halo) to study the Galaxy’s chemical and kinematical history. It was part of the Sloan Digital Sky Survey III (SDSS-III) from 2011 to 2014 using the 2.5 m Sloan Foundation Telescope at Apache Point Observatory, New Mexico. The APOGEE-2 survey is now using the spectrograph as part of SDSS-IV, as well as a second spectrograph, a close copy of the first, operating at the 2.5 m du Pont Telescope at Las Campanas Observatory in Chile. Although several fiber-fed, multi-object, high resolution spectrographs have been built for visual wavelength spectroscopy, the APOGEE spectrograph is one of the first such instruments built for observations in the near-infrared. The instrument’s successful development was enabled by several key innovations, including a “gang connector” to allow simultaneous connections of 300 fibers; hermetically sealed feedthroughs to allow fibers to pass through the cryostat wall continuously; the first cryogenically deployed mosaic volume phase holographic grating; and a large refractive camera that includes mono-crystalline silicon and fused silica elements with diameters as large as ∼400 mm. This paper contains a comprehensive description of all aspects of the instrument including the fiber system, optics and opto-mechanics, detector arrays, mechanics and cryogenics, instrument control, calibration system, optical performance and stability, lessons learned, and design changes for the second instrument.

Spatial Frequency Response of Epoxy-Based Volume Holographic Recording Material.

Holographic volume phase gratings are recorded in an epoxy-based, free-surface, volume holographic recording material. Light-induced gratings are formed by photo-triggered mass migration caused by component diffusion. The material resolution enables a wide range of pattern spacings, to record both transmission and reflection holograms with many different spatial frequencies. An optimum spatial frequency response is found between the low spatial frequency roll-off and the high spatial frequency cut-off. The influence of the energy density of exposure on the spatial frequency response is investigated. Secondary volume holographic gratings (parasitic gratings) are observed in the high frequency range. The possibility of distinguishing the regular grating from the secondary grating is discussed in the form of probe wavelength detuning.

Characterization of Volume Gratings Based on Distributed Dielectric Constant Model Using Mueller Matrix Ellipsometry

Volume grating is a key optical component due to its comprehensive applications. Other than the common grating structures, volume grating is essentially a predesigned refractive index distribution recorded in materials, which raises the challenges of metrology. Although we have demonstrated the potential application of ellipsometry for volume grating characterization, it has been limited due to the absence of general forward model reflecting the refractive index distribution. Herein, we introduced a distributed dielectric constant based rigorous coupled-wave analysis (RCWA) model to interpret the interaction between the incident light and volume grating, with which the Mueller matrix can be calculated. Combining with a regression analysis with the objective to match the measured Mueller matrices with minimum mean square error (MSE), the parameters of the dielectric constant distribution function can be determined. The proposed method has been demonstrated using a series of simulations of measuring the volume gratings with different dielectric constant distribution functions. Further demonstration has been carried out by experimental measurements on volume holographic gratings recorded in the composite of polymer and zinc sulfide (ZnS) nanoparticles. By directly fitting the spatiotemporal concentration of the nanoparticles, the diffusion coefficient has been further evaluated, which is consistent to the result reported in our previous investigations.

Holographic Characteristics of Photopolymers Containing Different Mixtures of Nematic Liquid Crystals.

A holographic polymer dispersed liquid crystal (HPDLC) is used to record holographic diffraction gratings. Several mixtures of nematic liquid crystals (LC) are used as components of the HPDLC to evaluate their influence in static and dynamic basic properties. The diffraction efficiency obtained in the reconstruction of the holograms is evaluated to compare the influence of the different LC. Additionally, the samples are exposed to a variable electric field and the diffracted light intensity as a function of the applied voltage is measured to evaluate the influence of the LC. The results obtained show significant differences depending on the LC incorporated to the photopolymer.

On the estimation of absolute grating groove density and inter-grating groove density errors of laser pulse compression gratings

In this paper, experimental studies on estimation of absolute groove density of gratings and inter-grating groove density errors are reported with typical detector limited accuracies of ±0.23 lines mm−1 and ±0.005 lines mm−1, respectively at groove density of ~1740 lines mm−1 of holographic laser pulse compression gratings. A simple single detector based optical set-up with fixed optical elements to avoid mechanical eccentric errors, if any, due to goniometric movement of a rotatory stage, has been proposed to estimate absolute groove density of gratings. A modified Fizeau or a modified Michelson interferometer based optical set-up has been used to estimate inter-grating groove density errors of gratings. Various gratings from different manufacturers were examined for their absolute groove densities and inter-grating groove density errors.

Diffraction Efficiency Enhanced Volume Holographic Gratings via Coupling Ag Thin Film

Volume holographic gratings (VHGs) act as an in-coupler or out-coupler could be applied in holographic waveguide display system in which high diffraction efficiency is one of the key factors. Here, VHGs coupled with Ag thin film to enhance its diffraction efficiency is demonstrated. The first-order diffraction efficiency of the Ag coupled VHG increases by 34% when the uncoupled VHG is 42.8%. Numerical simulation results demonstrated that the Ag thin film could decrease the forward diffraction, corresponding with the first-order diffraction increasing in the coupled VHG. This paper provides useful information that Ag coupled VHGs may improve the diffraction efficiency of waveguide used in the head-mounted and near-eye display.

On-axis near-eye display system based on directional scattering holographic waveguide and curved goggle

The contradiction between the field of view (FOV), luminance uniformity (LU) and weight has always restricted the development of augmented reality display systems. An on-axis near-eye display (NED) system based on directional scattering holographic waveguide (DSHW) and curved goggle is proposed in order to realize a large FOV with high LU, light weight, and conformal design capability. The DSHW which consists of a linear volume holographic grating and holographic diffuser is used to deliver the virtual image and construct a transparent directional emission display screen with high LU. The curved goggle is used to project the image on the display screen into human eye and form a large FOV, with a suitable exit pupil diameter (EPD) and eye relief distance (ERF) and while keeping the external scene visible. Our proposed NED achieved an FOV of 44° horizontal (H) × 12° vertical (V) easily, which is almost consistent with the theoretical design. The EPD is 6 mm, ERF is 18.6 mm, and LU is about 88.09% at full viewing angle. The system is lightweight and flexible, which can be further applied in the next-generation, integrated protection-display helmet system through conformal optical design.

Demonstration of stimulated photon echo isolation using interferometric cancellation.

An alternative to phase-matched angled beam spatial-spectral holographic grating geometries for separating stimulated photon echoes (SPEs) from the probing pulse is proposed and demonstrated. By use of a Mach-Zehnder geometry with inhomogeneously broadened medium in both paths, the SPE can be interferometrically isolated from the generating probe pulse. This interferometric-based technique is well suited to waveguide geometries, which have benefits for future quantum and classical optical signal processing applications such as quantum memories, correlation, and efficient cryogenic microwave-to-optical conversion. Experimental demonstrations showing interferometric-based isolation in Tm3+-doped LiNbO3 at 3.2K are performed and analyzed.

Volume holographic spatial-spectral imaging systems [Invited

In this paper, we present an overview of the recent developments in applications of volume holographic imaging techniques in microscopy. In these techniques, three-dimensional imaging incorporates multiplexed volume holographic gratings, which are formed in phenanthrenequinone poly(methyl methacrylate) (PQ-PMMA) photopolymer and act as spatial-spectral filters, to obtain multiplane images from a volumetric object without scanning. We introduce recent major roles of volume holography in different imaging modalities, including large-capacity spatial-spectral multiplane microscopy, digital holographic microscopy, and structured Talbot (or speckle) illumination fluorescence imaging. Among various imaging applications of volume holography, simultaneous multiplane fluorescence microscopy for collecting spatial-spectral information is distinct and has great potential for hyperspectral imaging. Depth selective spatial-spectral information from an object is particularly useful for designing a high-resolution microscope in real-time operation. We further discuss volume holography in particle trapping and beam shaping. In addition, we investigate future prospects of volume holography in microscopy as well as endoscopy.

Compact high‐sensitivity symmetrical integrated optical system for the detection of Raman scattering with dual channels

AbstractWe present here a compact symmetrical integrated optical system for the high‐sensitivity handheld Raman spectrometer. This symmetric optical system is of higher sensitivity of Raman spectrum detection by coupling collection optics to the spectrometer directly, reflecting excitation laser incident at 45° while efficiently passing the longer Raman‐shift wavelength with two long‐pass dichroic laser‐flat beam‐splitters placed in parallel. Three aspherical collecting lens with high‐numerical aperture were used to collect Raman scattering. Four protected silver plane mirrors, two right‐angle mirrors, and an aspherized achromatic focusing lens were used to focus Raman scattering into the slit on the collecting optical path. The Raman scattering light from the entrance slit were collimated to a blazed holographic reflective grating by an achromatic doublet lens, and the two diffraction lights with different orders were focused onto the two different surface of charge‐coupled device with two broadband dielectric coating concave mirrors, respectively. The coma and astigmatism of this optical system are decreased by optimizing the optical structure parameters. The finer Raman spectrum of samples can be obtained through the second order diffraction spectrum acquisition channel, whereas the first order diffraction channel covers almost all the Raman spectrum range. The tested spectrum of Benzoyl peroxide indicates that this symmetric integrated optical system with three collecting channels and double‐detecting channels has a higher sensitivity, higher signal‐to‐noise ratio, and finer Raman spectrum than the conventional portable Raman spectrometer with independent probe and single detection channel.

Dynamic holographic liquid crystal device containing nanoscale CuPc film

ABSTRACTIn this paper, CuPc-film-containing nematic liquid crystal (NLC) cells were fabricated, and dynamic holographic gratings recorded in the NLC cells were studied. The dependence of the diffraction efficiency on the applied voltage, light intensity, and the thicknesses of CuPc and NLC film was investigated. The high diffraction efficiency about 30% and the moderate build-up and erasing time in the order of magnitude of 0.1 s were achieved. The asymmetric energy transferring in two-beam coupling indicates photorefractive (PR) nature of the grating. The results of the photoinduced change of birefringence experiments suggest that the surface charge at the interface between the CuPc film and the NLC film plays a crucial role in the PR effect and the CuPc–NLC interface is mainly responsible for the formation of modulated surface charge layer and enables the photoelectric function of the NLC device. Our research may provide a new material option to optimise and develop NLC device for optical information processing and storage.

Direct quantitative imaging of the writing stage in a photosensitive azopolymer by digital holography.

In this paper, we demonstrated that the gradual formation of a surface relief grating (SRG) in azopolymer thin films under continuous light exposure could be directly observed in situ and in real-time, allowing full-field characterization with high spatial resolution. We reported here for the first time, to the best of our knowledge, that digital holography (DH) can be adopted for investigating and monitoring an inscribed holographic surface relief grating (SRG) of azopolymers by two-beam laser interference lithography over a wide area. The writing process could be assessed through quantitative phase imaging (QPI). The reported results show that the proposed method is a truly valuable diagnostic tool that can be useful for investigating the spatial distribution of the writing process, which can eventually contribute to shedding light on the still unclear origin and related mechanism of SRG formation in azopolymers.

Generation of femtosecond dual pulses by a transverse standing wave in a volume holographic grating

Based on Kogelnik’s coupled-wave theory, it is found that when a femtosecond pulse is incident on a transmitted volume holographic grating, two transverse standing waves along the grating vector direction will be generated inside the volume holographic grating (VHG). Due to field localization of two standing waves, they have two different velocities along the propagation depth. On the output plane of the VHG, femtosecond dual pulses are generated in both the diffracted and transmitted directions. Results show that the pulse interval is determined by the refractive index modulation and thickness of the grating, while the waveform of the dual pulses is independent of the grating parameters.

Высокочастотные рельефно-фазовые голографические решетки на обработанном коротковолновым УФ-излучением бихромированном желатине

For the first time, the possibility of creating relief-phase holographic gratings with a high spatial frequency of 1500 mm-1 on dichromate gelatin (DCG) layers using the destructive effect of short-wave UV radiation on gelatin is shown.