Beam shaping using Multiplexed 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.
Multiplane Holographic Non-scanning Endoscopy:
( Recent publication Chen Yen Lin, Wei-Tang Lin, Hsi-Hsun Chen, Jau-Min Wong, Vijay Raj Singh, and Yuan Luo, "Talbot multi-focal holographic fluorescence endoscopy for optically sectioned imaging," Opt. Lett. 41, 344-347 (2016))
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.
Multiplane Holographic Non-scanning microscopy:
Optical sectioning techniques offer the ability to acquire three-dimensional information from various organ tissues by discriminating between the desired in-focus and out-of-focus (background) signals. Alternative techniques to confocal, such as active structured illumination, exist for fast optically sectioned images, but they require individual axial planes to be imaged consecutively. In this article, an imaging technique (THIN), by utilizing active Talbot illumination in 3D and multiplexed holographic Bragg filters for depth discrimination, is demonstrated for imaging in vivo 3D biopsy without mechanical or optical axial scanning.
Transformation optics, a recent geometrical design strategy of light manipulation with both ray trajectories and optical phase controlled simultaneously, promises without precedent an invisibility cloaking device that can render a macroscopic object invisible even to a scientific instrument measuring optical phase. However, previous macroscopic cloaks only demonstrated the recovery of ray trajectories after passing through the cloaks, while whether the optical phase would reveal their existence still remains unverified. In this paper, a phase- preserved macroscopic visible-light cloak is demonstrated in a geometrical construction beyond two dimensions. As an extension of previous two-dimensional (2D) macroscopic cloaks, this almost-three-dimensional cloak exhibits three-dimensional (3D) invisibility for illumination near its center (i.e. with a limited field of view), and its ideal wide-angle invisibility performance is preserved in multiple 2D planes intersecting in the 3D space. Optical path length is measured with a broadband pulsed-laser interferometer, which provides unique experimental evidence on the geometrical nature of transformation optics.
Nano-SiO2 in PQ-PMMA for Holographic Filters in Imaging:
Holographic filters in imaging/data storage/communications are required to have high Bragg selectivity, namely narrow angular and spectral bandwidth, to obtain spatial-spectral information within a three-dimensional object. The holographic filters with optimized ratio of nano-SiO2 in PQ-PMMA can significantly improve the performance of Bragg selectivity and diffraction efficiency by 53% and 16%, respectively.
Images at two depths within a grapefruit obtained using two multiplexed holographic nano-SiO2 PQ-PPMA filters.
Wavelength-Coded Holographic Microscopy:
A wavelength-coded multifocal microscope incorporates multiplexed and wavelength-coded holographic gratings to generate wavelength-selective multifocal planes. The focal planes are longitudinally spaced on the object plane, and each focal plane is probed by a designated wavelength. The recording of the multiplexed gratings takes place at a single wavelength by utilizing the Bragg degeneracy property; thus the maximum sensitive wavelength of blue 488 nm is used for recording, but the device is operated at a broad wavelength band of interest, all the way to red 633 nm.
Figure on the left shows Two depth-resolved images of an onion obtained with wavelength-coded multifocal microscopy using both blue and red LEDs for illumination. Figure in the middle shows One of the two depth-resolved images obtained with wavelength-coded multifocal microscopy when the blue LED is on and red one is off. Figure on the right shows One of the two depth-resolved images obtained with wavelength-coded multifocal microscopy when the red LED is on and blue one is off.
Spatial-Spectral Fluorescence Imaging:
A three-dimensional imaging system incorporating multiplexed holographic gratings is able to visualize fluorescence tissue structures. Holographic gratings formed in volume recording materials such as PQ-PMMA photopolymer have narrowband angular and spectral transmittance filtering properties which enable obtaining spatial-spectral information within an object. The imaging system’s ability is demonstrated to obtain multiple depth-resolved fluorescence images.
Figure on the left shows the experimental system setup, and figure on the right shows two depth-resolved images of a mouse colon.
Parallel Optical Coherence Tomography (POCT):
Optical coherence tomography (OCT) shows great promise to produce micrometer-resolution images from deep with scattering media such as biological tissues. POCT system Compared to conventional OCT systems, the POCT system is a novel technology that replaces transverse scanning in the lateral dimension with electronic scanning. This will reduce the time required to acquire image data. The POCT system consists of a single mode fiber array with multiple reduced diameter (15µm) single mode fibers in the sample arm. The POCT imaging system can therefore be adapted to an endoscopic format for detecting cancerous structures in tissues.
Figure (a) Experimental setup of the POCT system. Figure (b) POCT image of tangerine (Citrus reticulate) flesh. The tangerine’s juice vacuoles are clearly visible.
Design and Fabrication of Micro-Diameter Single-Mode-Fiber Array for Endoscopic Probe Tip:
The single mode fibers in the array can be used in coherent imaging applications such as optical coherence tomography (OCT). Fiber to substrate and fiber to fiber coupling effects were studied using beam propagation and Monte Carlo techniques to determine the different design characteristics and the maximum length of the reduced diameter fiber that can be packaged in the probe tip. Single mode fibers are etched to reduce the cladding diameter from 125 microns to 15 microns. A 2 microns thick silica layer is grown in the silicon substrate to minimize the fiber-substrate coupling. Reduced diameter fibers are placed into a 5mm by 150 microns trench etched in a silicon-silica substrate and fixed with UV curable cement. Active alignment was used to ensure the correct alignment of fibers.
Picture of the etched fiber section next to a mask with 25 micron squares
Zoomed in on the fiber array with 8 channels and silicon trench.
Schematic of linear fiber array with 15 channels and silicon trench groove.