Multiplexed Volume Holographic Gratings Research Articles

Selective Couplers Based on Multiplexed Volume Holographic Gratings for Waveguide Displays

Diffraction waveguides are widely used in augmented reality devices as information display systems for the introduction of data into the human visual field in order to supplement information about the world around us. This paper formulates the principles of radiation conversion in diffraction waveguides made of photo-thermo-refractive glass on the basis of multiplexed volume holographic gratings, and the advantages and disadvantages of high spectral-angular selectivity are analyzed. In the optical scheme, each of the superimposed volume holographic gratings in the diffraction waveguide forms a corresponding part of the composite angular field of view of the augmented reality device. A proposed mathematical model based on angular multiplexing made it possible to synthesize the diffraction optical element for a new type of diffraction waveguide made from photo-thermo-refractive glass and to create a prototype with an angular resolution of at least 3.0 ± 0.5′, with a brightness change in the image of less than 20% and with a composite angular field of view of 32°.

Multi-plane confocal microscopy with multiplexed volume holographic gratings [Invited

A volume holographic (VHG) grating-based multi-plane differential confocal microscopy (DCM) is proposed for axial scan-free imaging. Also, we briefly reviewed our previous works on volume holographic-based confocal imaging. We show that without degrading imaging performance, it is possible to simultaneously obtain two depth-resolved optically sectioned images with improved axial resolution using multi-plane DCM. The performance of our multi-plane DCM was evaluated by measuring the surface profile of a silicon micro-hole array with depths separation around 10 µm. The axial sensitivity of the system is around 25 nm. Our system has the advantages of multi-plane imaging with high axial sensitivity and high optical sectioning ability. Our method can be used for reflective surface profiling and multi-plane fluorescence imaging. The present methods may find important applications in surface metrology for label-free biological samples, as well as industrial applications.

Multiplane differential phase contrast imaging using asymmetric illumination in volume holographic microscopy.

.Significance: Differential phase contrast (DPC) is a well-known imaging technique for phase imaging. However, simultaneously acquiring multidepth DPC images is a non-trivial task. We propose simultaneous multiplane DPC imaging using volume holographic microscopy (VHM).Aim: To design and implement a new configuration of DPC-VHM for multiplane imaging.Approach: The angularly multiplexed volume holographic gratings (AMVHGs) and the wavelength-coded volume holographic gratings (WC-VHGs) are used for this purpose. To obtain asymmetric illumination for DPC images, a dynamic illumination system is designed by modifying the regular Köhler illumination using a thin film transistor panel (TFT-panel).Results: Multidepth DPC images of standard resolution chart and biosamples were used to compare imaging performance with the corresponding bright-field images. An average contrast enhancement of around three times is observed for target resolution chart by DPC-VHM. Imaging performance of our system is studied by modulation transfer function analysis, which suggests that DPC-VHM not only suppresses the DC component but also enhances high-frequency information.Conclusions: Proposed DPC-VHM can acquire multidepth-resolved DPC images without axial scanning. The illumination part of the system is adjustable so that the system can be adapted to bright-field mode, phase contrast mode, and DPC mode by controlling the pattern on the TFT-panel.

Two-photon fluorescence imaging of subsurface tissue structures with volume holographic microscopy.

.Significance: Two-photon (2P) fluorescence imaging can provide background-free high-contrast images from the scattering tissues. However, obtaining a multiplane image is not straightforward. We present a two-photon volume holographic imaging (2P-VHI) system for multiplane imaging.Aim: Our goal was to design and implement a 2P-VHI system that can provide the high-contrast optically sectioned images at multiple planes.Approach: A 2P-VHI system is presented that incorporates angularly multiplexed volume holographic gratings and a femtosecond laser source for fluorescence excitation for multiplane imaging. A volume hologram with multiplexed gratings provides multifocal observation, whereas nonlinear excitation using a femtosecond laser helps in significantly enhancing both depth resolution and contrast of images.Results: Standard fluorescent beads are used to demonstrate the imaging performance of the 2P-VHI system. Two-depth resolved optical-sectioning images of fluorescently labeled thick mice intestine samples were obtained. In addition, the optical sectioning capability of our system is measured and compared with that of a conventional VHI system.Conclusions: Results demonstrated that 2P excitation in VHI systems provided the optical sectioning ability that helps in reducing background noise in the images. Integration of nonlinear fluorescence excitation in the VHI provides some unique advantages to the system and has potential to design multidepth optical sectioned spatial–spectral imaging systems.

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.

Conventional volume holography for unconventional Airy beam shapes.

Utilizing multiplexed volume holography, a single optical element, enabling to shape structural variants of non-diffracting Airy wavefronts, from one-dimensional Airy mode to unconventional Airy modes such as vortex Airy and quad Airy modes, has been experimentally realized. Here, beam shaped angularly multiplexed volume holographic gratings (AMVHGs) are recorded in PQ: PMMA photopolymer, where five different spatial wavefronts of Airy beams have been sequentially recorded, for simultaneous reconstruction of different Airy modes, by a conventional Gaussian beam. Spatial and spectral mode selective properties of AMVHGs are demonstrated by narrow-band as well as by broadband light source. In addition, through wavelength degeneracy property, the maximum sensitivity wavelength of blue (488 nm) is used for recording in PQ: PMMA, but the AMVHGs are operated at a broad wavelength band of interest, all the way to longer wavelength in near infrared (850 nm). The K-sphere representation is used to explain the spectral properties of AMVHGs.

Speckle illumination holographic non-scanning fluorescence endoscopy.

Optical sectioning endoscopy such as confocal endoscopy offers capabilities to obtain three-dimensional (3D) information from various biological samples by discriminating between the desired in-focus signals and out-of-focus background. However, in general confocal images are formed through point-by-point scanning and the scanning time is proportional to the 3D space-bandwidth product. Recently, structured illumination endoscopy has been utilized for optically sectioned wide-field imaging, but it still needs axial scanning to acquire images from different depths of focal plane. Here, we report wide-field, multiplane, optical sectioning endoscopic imaging, incorporating 3D active speckle-based illumination and multiplexed volume holographic gratings, to simultaneously obtain images of fluorescently labeled tissue structures from different depths, without the need of scanning. We present the design, and implementation, as well as experimental data, demonstrating this endoscopic system's ability to obtain optically sectioned multiplane fluorescent images of tissue samples, with cellular level resolution in wide-field fashion, and no need for mechanical or optical axial scanning.(A) Schematic drawing of the SIHN endoscopy to simultaneously acquire multiplane images from different depths. (B) Uniform, and (C) SIHN illuminated images of standard fluorescence beads (25 μm in diameter) for the two axial planes. (D) Intensity profile on fluorescently labeled signal (ie, in-focus) and background (ie, out-of-focus) of microspheres.

Multiplexed holographic non-axial-scanning slit confocal fluorescence microscopy.

A non-axial-scanning multi-plane microscopic system incorporating multiplexed volume holographic gratings and slit array detection to simultaneously acquire optically sectioned images from different depths is presented. The proposed microscopic system is configured such that multiplexed volume holographic gratings are utilized to selectively produce axial focal points in two or more planes inside the sample, and then to use confocal slit apertures to simultaneously image these multiple planes onto corresponding detection areas of a CCD. We describe the design, implementation, and experimental data demonstrating this microscopic system's ability to obtain optically sectioned multi-plane images of fluorescently labeled standard micro-spheres and tissue samples without scanning in axial directions.

Spatial mode multiplexing using volume holographic gratings.

We experimentally demonstrate spatial mode multiplexing of optical beams using multiplexed volume holographic gratings (MVGHs) formed in phenanthrenquinone-poly (methyl methacrylate) (PQ-PMMA) photopolymer. Multiple spatial modes of Laguerre-Gaussian (LG) beams are recorded at the same pupil area of a volume hologram resulting in MVHGs, for simultaneous reconstruction of spatial modes. In addition, a helical phase beam, a non-diffracting beam with conical phase profile, and a parabolic non-diffracting beam with cubic phase profile have also been simultaneously recorded and reconstructed from MVHGs. Utilizing Bragg wavelength degeneracy property of volume hologram these multiplexed modes are reconstructed at multiple wavelengths ranging from blue (450nm) to red (635). Due to combined effect of three-dimensional pupil, Bragg wavelength degeneracy, angular selectivity, together with spatial mode properties these, MVHGs can act as spatial mode filter with spectral filtering property. Advantages of volume holography in beam shaping are discussed. Multiple first diffraction orders with desired beam shapes obtained from the single optical element (i.e. a volume hologram with MVHGs) may find important applications in optical communication experiments, and in volume holographic imaging and microscopy. Experimental results show solid evidence that MVGHs in beam shaping provide a simple, compact, single element, and direct way to multiplex spatial modes.

Talbot multi-focal holographic fluorescence endoscopy for optically sectioned imaging.

A wide-field multi-plane endoscopic system incorporating multiplexed volume holographic gratings and Talbot illumination to simultaneously acquire optically sectioned fluorescence images of tissue structures from different depths is presented. The proposed endoscopic system is configured such that multiple Talbot-illumination planes occur inside a volumetric sample and serve as the input focal planes for the subsequent multiplexed volume holographic imaging gratings. We describe the design, implementation, and experimental data demonstrating this endoscopic system's ability to obtain optically sectioned multi-plane fluorescent images of tissue samples in wide-field fashion without scanning in lateral and axial directions.

Speckle-based volume holographic microscopy for optically sectioned multi-plane fluorescent imaging.

Structured illumination microscopy has been widely used to reconstruct optically sectioned fluorescence images in wide-field fashion; however, it still requires axial scanning to obtain multiple depth information of a volumetric sample. In this paper, a new imaging scheme, called speckle-based volume holographic microscopy system, is presented. The proposed system incorporates volumetric speckle illumination and multiplexed volume holographic gratings to acquire multi-plane images with optical sectioning capability, without any axial scanning. We present the design, implementation, and experimental image data demonstrating the proposed system's ability to simultaneously obtain wide-field, optically sectioned, and multi-depth resolved images of fluorescently labeled microspheres and tissue structures.

Compact single mode tunable laser using a digital micromirror device

The wavelength tuning properties of a tunable external cavity laser based on multiplexed volume holographic gratings and a commercial micromirror device are reported. The 3x3x3 cm(3) laser exhibits single mode operation in single or multi colors between 776 nm and 783 nm with less than 7.5 MHz linewidth, 37 mW output power, 50 μs rise/fall time constant and a maximum switching rate of 0.66 KHz per wavelength. The unique discrete-wavelength-switching features of this laser are also well suited as a source for continuous wave Terahertz generation and three-dimensional metrology.

Simulations and experiments of aperiodic and multiplexed gratings in volume holographic imaging systems.

A new methodology describing the effects of aperiodic and multiplexed gratings in volume holographic imaging systems (VHIS) is presented. The aperiodic gratings are treated as an ensemble of localized planar gratings using coupled wave methods in conjunction with sequential and non-sequential ray-tracing techniques to accurately predict volumetric diffraction effects in VHIS. Our approach can be applied to aperiodic, multiplexed gratings and used to theoretically predict the performance of multiplexed volume holographic gratings within a volume hologram for VHIS. We present simulation and experimental results for the aperiodic and multiplexed imaging gratings formed in PQ-PMMA at 488nm and probed with a spherical wave at 633nm. Simulation results based on our approach that can be easily implemented in ray-tracing packages such as Zemax® are confirmed with experiments and show proof of consistency and usefulness of the proposed models.

Accurate diffraction efficiency control for multiplexed volume holographic gratings

We report the method of accurate diffraction efficiency control for multiplexed volume holographic gratings in dry photopolymer films (DuPont HRF-600). Based on the experimental evaluations of the grating formation characteristics in dry photopolymer films, we present the way to develop the practical recording schedules for the fabrication of holographic gratings under accurate diffraction efficiency control. Using this method, we obtained single holographic gratings with the desired diffraction efficiency (variation 2.5%) and high-efficiency equal-strength (47%/ 47%) double holographic gratings. As a practical application, we demonstrated the centralized optical backplane architecture with uniform fanouts using the single and equal-strength double holographic gratings we recorded.

Design of a new medium for volume holographic information processing

An optical analog of the neural networks involved in sensory processing consists of a dispersive medium with gain in a narrow band of wave numbers, cubic saturation, and a memory nonlinearity that may imprint multiplexed volume holographic gratings. Coupled-mode equations are derived for the time evolution of a wave scattered off these gratings; eigenmodes of the coupling matrix κ saturate preferentially, implementing stable reconstruction of a stored memory from partial input and associative reconstruction of a set of stored memories. Multiple scattering in the volume reconstructs cycles of associations that compete for saturation. Input of a new pattern switches all the energy into the cycle containing a representative of that pattern; the system thus acts as an abstract categorizer with multiple basins of stability. The advantages that an imprintable medium with gain biased near the critical point has over either the holographic or the adaptive matrix associative paradigms are (1) images may be input as noncoherent distributions which nucleate long-range critical modes within the medium, and (2) the interaction matrix κ of critical modes is full, thus implementing the sort of full connectivity needed for associative reconstruction in a physical medium that is only locally connected, such as a nonlinear crystal.