Volume Gratings Research Articles

Polarization Volume Gratings for Near-Eye Displays and Novel Photonic Devices

Liquid crystal-based reflective polarization volume grating (PVG), also known as a linear Bragg–Berry phase optical element or a member of volume Bragg gratings (VBGs), is a functional planar structure with patterned orientation of optical axis. Due to the strong polarization selectivity, nearly 100% diffraction efficiency, large diffraction angle, and simple fabrication process, PVGs have found potential applications in novel photonic devices and emerging near-eye displays. In this review paper, we describe the operation principles, discuss the optical properties, present the fabrication methods, and provide promising applications of PVGs for near-eye displays and novel photonic devices.

Improved TDM scheme and data extracting algorithm for polymerization evaluation.

To quickly evaluate holographic photopolymers with different formulations, the most effective method is to record a volume holographic grating in the samples and detect the grating's diffraction in real time. Since the volume grating is highly sensitive to incident angle, existing schemes need to precisely control many space-related parameters. This study proposes an improved scheme, in which two different sized spots are used to reduce the requirements for the overlap of the two spots and the installation precision of the samples. Transmittances, diffractive efficiencies and diffractive asymmetries are obtained at a high sampling rate, through a specifically designed algorithm with the data from uncalibrated high-speed photodiodes. The experimental results show that the proposed scheme performance well in evaluating holographic photopolymer.

Femtosecond laser integration of volume grating in BK7 glass refractive lens

Femtosecond laser direct writing in glass is used to fabricate micro-optical integrated elements. To achieve high efficiency, volume gratings are fabricated inside glass by stacking layers. Maximum diffraction efficiency of 80% was achieved at 633 nm wavelength with a period of 10 μm in BK7 substrates. By embedding a diffraction grating inside the BK7 lens, an integrated element with two functions of light focusing and diffraction can be fabricated. This report describes optical elements of a volume grating embedded inside a planoconvex spherical lens and a cylindrical lens by showing light focusing of diffracted beams. Diffraction efficiency measurements and light-focusing characteristics of the fabricated integrated element were evaluated. Integrating a focus function with diffraction reduces system component count and facilitates miniaturization.

Femtosecond-laser direct-writing volume phase gratings inside Ge–As–S chalcogenide glass

In this study, volume-phase gratings (VPGs) were fabricated inside Ge–As–S chalcogenide glasses (ChG) with femtosecond-laser writing technology. VPGs with high thicknesses were obtained through single-step writing by exploiting the self-focusing effect of highly nonlinear ChGs. The clear diffraction patterns and high diffraction efficiency of the prepared gratings indicated their high optical performance. Testing results showed that the diffraction efficiency of VPGs first increased with laser pulse energies and then decreased when the laser damage threshold of the glass reached 9.42 nJ. Grating thickness also exhibited the same trend as laser pulse energies and reached the maximum value of 28 μm at 9.42 nJ. The changes in the refractive indexes of volume gratings fabricated under different laser pulse energies were carefully investigated. The changes in the refractive indexes of the gratings remained at the same level while laser pulse energy was increased. Meanwhile, grating thickness played a key role in diffraction efficiency. The maximum diffraction efficiency of the prepared VPGs reached 50% at 808 nm under optimized laser parameters.

Chirped polarization volume grating with ultra-wide angular bandwidth and high efficiency for see-through near-eye displays.

We report a reflective chirped polarization volume grating (CPVG) with a dramatically wider angular bandwidth and significantly higher first-order diffraction efficiency than the holographic volume grating and surface relief grating for large field-of-view (FOV) augmented reality (AR) displays. By introducing gradient pitch structure along the beam propagation direction, the angular bandwidth is extended from 18° to 54° while keeping over 80% diffraction efficiency. We also prepare a two-layer reflective PVG and compare its performance with the chirped structure. Based on the simulation and experimental results, CPVG is a strong contender for large FOV AR displays.

Recording of chirped volume gratings by a convergent beam and a divergent beam

Chirped volume gratings, which can be used for stretching and compressing ultrashort laser pulses, are widely used in the chirped pulse amplification systems. A method of recording chirped volume gratings by a convergent beam and a divergent beam is studied. Theoretical study of this recording method for chirped volume gratings is made and the expression of the period of chirped volume grating is derived. The influence of the parameters of the recording beams on the chirped volume grating is analyzed. Results show that linear chirped volume grating can be recorded by this method when the parameters of the recording beam meet the requirements. This method has the advantages of simple structure, easy operation, flexible adjustment of volume grating periods, no need to manufacture mask. With this method, chirped volume gratings can be produced at low cost.

Device simulation of liquid crystal polarization gratings.

Liquid crystal polarization gratings manifest several unique features, such as high diffraction efficiency, polarization selectivity, and fast switching time. However, few works address the chiral-doped liquid crystal alignment issue in such gratings. Here, we develop an improved relaxation method to analyze the liquid crystal director distribution in chiral-doped polarization gratings. Our simulation result agrees well with experimental data on a polarization volume grating. The criteria for forming planar or slanted polarization grating are discussed.

59‐4: Fast‐response polarization volume gratings for AR/VR displays

We demonstrate an active driving, self‐organized, high‐efficiency (~100%) polarization volume grating (PVG) for augmented reality and virtual reality displays. By using a dual‐frequency liquid crystal host with a chiral dopant, our reflective PVG exhibits a 5.54‐ms turn‐on time and 0.36‐ms turn‐off time.

Stretchable, flexible, and adherable polarization volume grating film for waveguide‐based augmented reality displays

AbstractWe report a stretchable, flexible, and adherable polarization volume grating (PVG) film and propose a compact optical system for augmented reality displays based on it. The Bragg reflection band shift, deflection angle change, and the mechanical robustness under stretch‐release cycles of the PVG film are also investigated.

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.

Stretchable, flexible, rollable, and adherable polarization volume grating film.

Volume Bragg gratings (VBGs) have many applications, including filters, wavelength multiplexing devices, and see-through displays. As a kind of VBGs, polarization volume gratings (PVGs) based on liquid crystal polymer have the advantages of nearly 100% efficiency, large deflection angle, and high polarization selectivity. However, previous reports regarding PVGs did not address high efficiency, tunable periodicity, and flexibility. Here, we report a stretchable, flexible, and rollable PVG film with high diffraction efficiency. The control of PVG by mechanical stretching is investigated, while the Bragg reflection band shift is evaluated quantitatively. Moreover, we quantified the deflection angle change's behavior, which has promising potential for laser beam steering applications. The mechanical robustness under stretch-release cycles is also scrutinized.

Optical properties of reflective liquid crystal polarization volume gratings

Polarization volume gratings are self-organized liquid crystal helical structures. They exhibit high diffraction efficiency and unique polarization selectivity. In this work, we investigate and compare two different configurations of polarization volume gratings: planar and slanted structures. We present the optical properties of polarization volume gratings with emphasis on their polarizing nature. Further experimental results reveal the existence of the slanted configuration.

Liquid-crystal-based polarization volume grating applied for full-color waveguide displays.

In this Letter, we demonstrate polarization volume grating (PVG)-based couplers for a double-layer waveguide display to realize a full-color near-eye display. The polarized interference exposure with photo-alignment methods was employed to generate a birefringent spiral configuration with two-dimensional periodicity in a chiral-dopant reactive mesogen material. Such a structure presents a unique highly efficient single-order Bragg diffraction with polarized selectivity. The prepared PVG couplers exhibited over 80% diffraction efficiency with large diffraction angles at spectra of blue (457nm), green (532nm), and red (630nm). The demonstrated waveguide prototype showed a full-color display with a diagonal field of view of around 35°. The overall optical efficiency was measured as high as 118.3 cd/m2 per lumen with a transparency of 72% for ambient light.

Diffraction Efficiency Distribution of Output Grating in Holographic Waveguide Display System

Planar waveguide structure has benefits to expand the exit pupil of the near-eye display. To achieve a uniform output luminance within the expanded exit pupil, a specific distribution of diffractive efficiency (DE) should be set for the out-coupling grating. In this paper, a physical model is built to study the DE distribution of out-coupling grating with consideration of the whole field of view. Compared to the previous works, which only considered the central view angle, this model is more consistent with the actual situations to realize a uniform imaging output for a waveguide-based near-eye display system. Also, a planar waveguide with two holographic volume gratings as the in- and out-coupling gratings is fabricated to verify the proposed DE distribution. In experiments, a spatial light modulator is applied as the spatial luminance modulator, and the coupling grating with the proposed DE distribution is fabricated by the holographic interference exposure setup. The results show a high consistency between the simulation and the experiment.

17‐5: High Efficiency Polarization Volume Grating for Waveguide‐based AR Displays

17‐5: High Efficiency Polarization Volume Grating for Waveguide‐based AR Displays

Holographic formation and diffractive properties of hybrid periodic structures.

The paper presents formation processes and diffraction properties of hybrid periodic structures such as volume gratings with refractive index modulation, on the surface of which relief gratings are formed. It is shown experimentally that relief gratings with significant diffraction properties in used photopolymer materials are formed in a narrow range of exposure parameters. It is established that the formation of a relief grating on the surface of a volume grating leads to significant changes in the selective properties of structures. We also consider possible applications of hybrid structures.

Bifurcation in reflection spectra of holographic diffraction grating recorded on dicromated pullulan

Volume diffraction gratings were created holographically on a dichromated pullulan (polysaccharide doped with chromium). Specific and very rare multi-peak structure in the reflection spectra is observed. It was found that multiple peaks are consequence of non-ideal structure of volume diffraction grating. Theoretical analysis and numerical simulations confirm that the observed behavior arise from a slight randomness of grating parameters: refractive index, grating period, film thickness. The results of calculation are in agreement with experimental results.

Ultrafast laser-inscribed mid-infrared transmission gratings in IG2: modelling and high-resolution spectral characterization

The ultrafast laser inscription technique was used to fabricate mid-infrared volume gratings in the bulk of chalcogenide glass IG2 (Ge33As12Se55). These gratings are spectrally characterized using a Fourier transform infrared spectrometer, by measuring the transmission spectrum over the entire transmission band of the IG2 substrate. The gratings exhibit first order diffraction across the entire 2–5 μm spectral band, with the specific transmission band tuned by changing the angle of incidence. Higher diffraction orders are also observed. High-resolution spectral data is presented alongside detailed modelling and analysis. This work provides the basis for future design of mid-infrared transmission gratings; ULI provides a low-cost, robust alternative to mid-infrared reflection gratings, with the added capability to engineer the grating response to a specific application.

Theoretical modeling and design of photonic structures in zeolite nanocomposites for gas sensing. Part II: volume gratings.

The suitability of holographic structures fabricated in zeolite nanoparticle-polymer composite materials for gas sensing applications has been investigated. Theoretical modeling of the sensor response (i.e., change in hologram readout due to a change in refractive index modulation or thickness as a result of gas adsorption) of different sensor designs was carried out using the Raman-Nath theory and Kogelnik's coupled wave theory. The influence of a range of parameters on the sensitivity of holographically recorded surface and volume photonic structures has been studied, namely, hologram geometry, hologram thickness and spatial frequency, reconstruction wavelength, and zeolite nanoparticle refractive index. From this, the optimum fabrication conditions for both surface and volume holographic gas sensor designs have been identified. Here in Part II, results from modeling of the influence of design on the sensor response of holographically recorded volume grating structures for gas sensing applications are reported.

Improvement of temperature-induced spectrum characterization in a holographic sensor based on N-isopropylacrylamide photopolymer hydrogel.

A novel thermo-sensitive N-isopropylacrylamide photopolymer was developed for improving the temperature and humidity responses of holographic sensors. Diffraction spectra of holographic volume gratings recorded in the materials were characterized to explore the sensing response capacity. A dependence of peak wavelength on the temperature was observed and provided a quantitative strategy for holographic sensing applications. Expansion of the humidity range induced a strong extension of wavelength shift. Finally, the temperature response reversibility was demonstrated experimentally. Our sensing results were completely different from the reported typical acrylamide polymer system. Compared with the former, we obtained a more sensitive temperature response and an evident shift expansion (>200 nm) at a relative humidity of 70% or higher. These results can obviously improve the thermo-sensitivity of a holographic sensor and expand the practical application area of the holographic sensing strategy.