In this paper, we propose an advanced method to jointly optimize doublet metalens and deep learning-based postprocessing networks for wide-angle and full-color imaging with high fidelity. The optical image formation module in the spatially-variant system and a reconstruction network module are implemented in a differentiable manner. By premitigating coma aberration with doublet metalens, the proposed model outperforms both cases of singlet structure and analogous electronic implementations in terms of reconstruction accuracy.
A telephoto lens, which has a long effective focal length to obtain highly magnified images of objects, has been utilized in various imaging systems such as camera modules and astronomical telescopes. Nevertheless, the wasted space between the lens and the image sensor due to the long focal length and the huge volume of refractive lenses pose a major hurdle for system miniaturization. In this work, we propose a novel type of telephoto lens breaking the trade-off between system volume and magnification factor by arranging multiple metasurfaces on a glass substrate in a horizontal direction. This configuration obtains vastly extended optical path length leading to a high magnification factor within a downsized volume.
In general, the function of a passive nanophotonic device cannot be varied once its geometric and refractive index parameters are fixed. Thus, research on tunable optical nanostructures has been attracting much attention, and phasechange materials, whose optical properties can be actively controlled by thermal stimulus, have widely been utilized and integrated with nanophotonic devices. In this work, we propose a novel optimized design of an active metagrating transmission modulator composed of crossed ridge waveguides made of a VO2 film. The proposed device can provide not only near-unity modulation depth of the 0th order transmittance, but also polarization-independent operation with high efficiency and a broad bandwidth in the telecom wavelength region.
KEYWORDS: Projection systems, 3D image processing, Microlens array, 3D projection, Integral imaging, 3D displays, Displays, Prototyping, Lens arrays, Multilayers
In this paper, we present a novel approach for projecting a light field using multi-layer images and integral imaging technology. Here, the multi-layer images are used to enable the generation of natural parallax and view-dependent effects, and integral imaging is used to project the resulting light field on a large screen. We demonstrate the feasibility of the proposed method through a prototype and experimental results, and also discuss its potential advantages.
We present a numerical transformation technique for Computer-Generated Holograms (CGHs) using the deep learning method. Using the proposed technique, one can obtain CGHs for a user-defined holographic display system from given CGHs. The calculation speed of the proposed technique is about 20 times faster than that of the conventional free-space propagation algorithm. We verify through both numerical simulation and optical experiment that focal stacks produced with the CGHs obtained by the proposed technique are similar to those produced with the target CGHs.
Occlusion technology has grown its importance for enhancing immersive augmented reality experiences by improving mutual depth perceptions between the real and virtual scenes. Among various methods for implementing occlusion in augmented reality displays, the 4f system method has gained a lot of attention for its capability to produce a sharp occlusion effect. However, this method has a drawback of having a large form factor and difficulty in achieving sharp occlusion when implemented to display multiple-depth images. Previous studies have applied a lens array to a 4f system to reduce the form factor. In this work, we numerically and experimentally investigate the use of a pair of Focus-Tunable Lenses (FTLs) along with a lens array 4f system to achieve sharp occlusion at multiple levels of depths in addition.
Conventional models of Er/Yb co-doped fibers assume all ytterbium ions are equally involved in the energy transfer with erbium ions, governed by a singular transfer rate. This would predict output power clamping once ytterbium parasitic lasing starts, contrary to the observations that the output continued to grow albeit at a slower rate. One study explained this using elevated temperature at high powers. Our study, however, shows that elevated temperature and mode-dependent effects only play insignificant roles. A new model is developed based on the existence of isolated ytterbium ions, which can explain all the observed experimental behaviors.
We propose an optical method for uroflowmetry, exploiting the laser speckle contrast imaging (LSCI) technique onto an intermediate tubing apparatus having an elastic wall that can sensitively respond to flow-induced shedding vortices. Based on the method, we devised and fabricated an elastic-walled U-shaped tubing apparatus (EWUSTA), using the three-dimensional printing technique. We utilized the spatiotemporal contrast scheme for the LSCI as a fast and reliable computational algorithm. We investigated three different materials of flex-vinyl, ninja-flex, and natural rubber latex for the elastic wall of the EWUSTA in steady flow conditions, and verified that their optimal operational ranges could extend up to 7, 15, and 25 ml/s, respectively. We characterized the natural-rubber-latex-based EWUSTA in dynamic flow conditions in comparison with a commercial reservoir-weight-transducer-based gravimetric flowmeter, and verified its feasibility. We stress that the proposed method can offer precise and accurate information on flow dynamics. In addition, we found that the upper limit of the optimal operational range of the proposed apparatus had strong correlation with the tensile strength of the elastic-wall material. We reckon that the proposed and demonstrated method has great potential not only for uroflowmetry but also for other flow-related medical and industrial applications.
KEYWORDS: Acoustics, Fiber lasers, Cladding, Refractive index, Structured optical fibers, High power fiber lasers, Scattering, Solids, Signal attenuation, Control systems
This paper reviews the progress in active fibers suitable for power scaling, highlighting the advances in fiber design that
will enable the control of nonlinearities such as SRS and SBS in high power fiber lasers, as well as making feasible a
practical high power three-level system.
Master oscillator power amplifier (MOPA) is becoming the obvious choice in order to develop some high power single
frequency laser sources. Its simplicity, reliability, robustness have already allowed the demonstration of some
tremendous increase of the output power. In this paper we will report our latest results in the development of high power
single frequency, single mode and single polarization MOPA systems. We were able to obtain an output power as higher
as 500 W with still keeping the narrow linewidth proprieties of the source.
We report the generation of white light comprising red, green, and blue spectral bands from a frequency-doubled
fiber laser in submicron-sized cores of microstructured holey fibers. Picosecond pulses of green light are launched
into a single suspended core of a silica holey fiber where energy is transferred by an efficient four-wave mixing
process into a red and blue sideband whose wavelengths are fixed by birefringent phase matching due to a slight
asymmetry of the structure arising during the fiber fabrication. Numerical models of the fiber structure and
of the nonlinear processes confirm our interpretation. Finally, we discuss power scaling and limitations of this
white light source.
Over recent years, there has been a tremendous and rapid progress in power scaling Yb-doped fiber-based picosecond sources due to their high efficiency, excellent beam quality and immunity to thermo-optical effects. These remarkable properties are not only very attractive for many scientific and industrial applications but also for frequency doubling to generate green. Besides good beam quality, a high degree of polarization and a narrow linewidth, further increase in conversion efficiency requires high peak power and increased crystal length. High peak power can be obtained by employing a fiber master-oscillator power amplifier design (MOPA) where seed pulses with adequate duty cycle are amplified to high average powers. However in this arrangement minimizing nonlinear effects arising in the fiber amplifiers becomes a challenge. The amplification of picosecond pulses causes linewidth broadening and the spectral bandwidth of the crystal is reduced by a preferred longer length. This trade-off can result in lower frequency doubling efficiency. In this paper, as well as the benefits and limitations of fiber lasers applied to nonlinear frequency conversion, we will review the various design considerations for the development of a high average power picosecond green laser based on single-pass frequency doubling of a fiber MOPA system.
Fiber lasers and amplifiers offer unique characteristics that are derived from the use of a waveguide and the properties
of rare-earth doped silica glass. Their capability for high output power, with high efficiency, has been demonstrated
both in CW and pulsed regimes. Cladding-pumped Yb-doped fiber lasers have now reached beyond kW levels with
good beam quality. Advances in both fiber technology and high-power multimode diode pump sources, and the
inherent power scalability of cladding-pumped fibers, lie behind this power surge. However, there are still many
challenges to overcome in the high-power fiber laser area. These include, for example, single-mode output at higher
powers and power scaling of a three-level laser. This paper reviews novel W-type fiber and depressed clad hollow
optical fiber waveguide structures designed with distributed wavelength filter characteristics to achieve an efficient and
high power cladding-pumped three-level lasers such as Nd-doped fiber laser operating at 930 nm and Yb-doped fiber
laser at 980 nm. Moreover, such fiber geometries enable to scale up the output power in a small and single-mode core
for generating a single-mode output beam in a robust and reliable manner.
We propose a depressed clad hollow optical fiber with fundamental (LP01) mode cut-off suitable for high power short-wavelength, especially three-level, fiber laser operation by introducing highly wavelength dependent losses at longer wavelengths. The cut-off characteristic of such fiber structure was investigated. A Yb-doped depressed clad hollow optical fiber laser generating 59.1W of output power at 1046nm with 86% of slope efficiency with respect to the absorbed pump power was realised by placing the LP01 mode cut-off at ~1100nm.
Pulses at 1178 nm were produced by pulsed Raman pumping at 1060 nm in an ytterbium doped fiber. Single-pass frequency doubling of the Raman pulses generated 1 W of average power at 589 nm.
We discuss the dramatic development of high-power fiber laser technology in recent years and the prospects of kilowattclass
single-frequency fiber sources. We describe experimental results from an ytterbium-doped fiber-based multihundred-watt single-frequency, single-mode, plane-polarized master-oscillator power amplifier (MOPA) operating at 1060 nm and a similar source with 0.5 kW of output power, albeit with a degraded beam quality (M2 = 1.6) and not linearly polarized. Experiments and simulations aimed at predicting the Brillouin limit of single-frequency system with a
thermally broadened Brillouin gain are presented. These suggest that single-frequency MOPAs with over 1 kW of output power are possible. In addition, the power scalability of a simple single-strand fiber laser to 10 kW is discussed.
The effect of temperature variation along a high power fiber amplifier on the SBS threshold is considered theoretically. We show that for an end-pumped rare-earth doped double-clad fiber the inhomogeneous distribution of temperature, which is caused by absorption of pump radiation, may result in total suppression of SBS even for output powers well above 200 W.
We review recent advances in Yb fiber lasers and amplifiers for high power short pulse systems. We go on to describe associated recent developments in fiber components for use in such systems. Examples include microstructured optical fibers for pulse compression and supercontinuum generation, and advanced fiber grating technology for chirped-pulse amplifier systems.
We report power-scaling of an ytterbium-sensitized thulium-doped silica fiber laser generating up to 75 W of output power in the 2 μm wavelength range when cladding-pumped by a 975 nm diode stack. The slope efficiency is 32% with respect to launched pump power and the beam quality factor (M2) is 1.3. We also investigate the characteristics of this fiber in a tunable laser configuration, operating at ~10 W of output power with the tuning range extended from 2000 to 2080 nm at a launched pump power of 40 W.
We propose and characterize the remote multiplexing of holograms with random pattern references from a multimode fiber bundle. The random pattern reference is modeled by using the superposition and concatenation of propagation modes of multimode fibers and free space. We compare two laser-coupling methods to the fiber bundle, i.e., direct coupling and lens coupling, for angle, shift, and wavelength remote multiplexings. The optimum laser-coupling method for each multiplexing is analyzed based on the theoretical model and experimental results. Retrieved hologram images show that 1-D intensity variation of the image might occur for the 90-deg geometry holograms in relation to the random pattern properties. Environmental variation effects such as vibration and temperature variation are also discussed and examined. These remote multiplexing methods can be applied to general multimode waveguide arrays to construct compact and integrated optical systems, where multiplexing can be performed at remote places.
High Power Fiber Lasers (HPFLs) and High Power Fiber Amplifiers (HPFAs) promise a number of benefits in terms of their high optical efficiency, degree of integration, beam quality, reliability, spatial compactness and thermal management. These benefits are driving the rapid adoption of HPFLs in an increasingly wide range of applications and power levels ranging from a few Watts, in for example analytical applications, to high-power >1kW materials processing (machining and welding) applications. This paper describes SPI’s innovative technologies, HPFL products and their performance capabilities. The paper highlights key aspects of the design basis and provides an overview of the applications space in both the industrial and aerospace domains. Single-fiber CW lasers delivering 1kW output power at 1080nm have been demonstrated and are being commercialized for aerospace and industrial applications with wall-plug efficiencies in the range 20 to 25%, and with beam parameter products in the range 0.5 to 100 mm.mrad (corresponding to M2 = 1.5 to 300) tailored to application requirements. At power levels in the 1 - 200 W range, SPI’s proprietary cladding-pumping technology, GTWaveTM, has been employed to produce completely fiber-integrated systems using single-emitter broad-stripe multimode pump diodes. This modular construction enables an agile and flexible approach to the configuration of a range of fiber laser / amplifier systems for operation in the 1080nm and 1550nm wavelength ranges. Reliability modeling is applied to determine Systems martins such that performance specifications are robustly met throughout the designed product lifetime. An extensive Qualification and Reliability-proving programme is underway to qualify the technology building blocks that are utilized for the fiber laser cavity, pump modules, pump-driver systems and thermo-mechanical management. In addition to the CW products, pulsed fiber lasers with pulse energies exceeding 1mJ with peak pulse powers of up to 50kW have been developed and are being commercialized. In all cases reducing the total “cost of ownership” for customers and end users is our primary objective.
We demonstrate a high power erbium-ytterbium co-doped large-core fiber laser with narrow linewidth, an M2 value of 1.7 and a broad tuning range. The fiber was cladding-pumped by a diode stack emitting at 975 nm. The laser had a linewidth around 0.16 nm and was tuned from 1533 nm to 1566 nm by compression-tuning a fiber Bragg grating. Output powers in excess of 30 W were obtained over the entire laser tuning range which was limited by the low gain at wavelengths shorter than 1533 nm and by the grating fabrication wavelength at 1566 nm. The laser slope efficiency was ~30% and the threshold ~3.3 W. Our results underline the capability for efficient, broad-band, high-power operation of large-core Er-Yb doped fibers and demonstrate compatibility with telecom components like standard single-mode fibers and fiber Bragg gratings.
We propose remote-multiplexing methods of holograms. A multimode fiber bundle is used as a reference beam-guiding device. Coherent optical wave coupled into the fiber bundle propagates through the bundle and free-space at the output facet. The generated far-field pattern is a kind of random pattern and is used as a reference beam for hologram multiplexing. We model the random pattern by superposition and concatenation of the propagation modes of the multimode fiber and free-space. Based on this model, we analyze and optimize remote multiplexing methods of angle multiplexing, shift multiplexing, and wavelength multiplexing cases. The optimization comes from controlling the number of waveguiding multimode fibers and the transversal spatial spectral range of the fiber-coupled optical wave. The experimental results show the validity of our model. This method and model can be applied to general multimode wave-guide array for random pattern referencing and provide integrated optical system for holographic data storage system.
We assess different power limits of cladding-pumped fiber lasers. Despite recent advances in pump sources, these are still primarily limited by available pump power. We find that it should be possible to reach output powers beyond 1 kW in single-mode ytterbium doped fiber lasers. Experimentally, we have realized an ytterbium-doped fiber laser with 272 W of output power at 1080 nm, with an M2-value of 3.2, as well as an erbium-ytterbium co-doped fiber laser with 103 W of output power at 1565 nm, with an M2-value of 2.0. We believe these are the highest-power ytterbium and erbium-ytterbium fiber lasers ever reported.
Recently with the development of pickup and display devices, real-time integral photography or integral imaging has been an attractive method over other techniques for displaying three-dimensional images. However, previous methods basically belong to the transmission-type display. In this paper, a reflection-type integral imaging is studied and some experimental results are shown to prove its feasibility. This can be implemented by adopting a concave mirror array instead of a convex lens array. A concave mirror array in the reflection-type functions like a lens array in the conventional transmission-type. Thus, the 3D image is integrated by the rays that reflect from a mirror array. We could obtain autostereoscopic images with full parallax. In addition, if we combine the reflection-type system with another conventional transmission-type system, the feeling of depth can be more enhanced. A schematic of a concave mirror array on a curved surface is also proposed and discussed. The curved surface concentrates the rays to a small-sized display panel such as a beam projector. We expect this new form of integral imaging will contribute to realize a practical 3-D display system.
The Born approximation with paraxial assumption has often been utilized for a volume hologram analysis, which is a simple and useful method but has limits in the consideration of incident-wave depletion and multiple diffractions of both longitudinal and angular directions. In recent years, the random-phase code multiplexing has received considerable attention because it gives a sharp selectivity compared to other methods, such as angular multiplexing, wavelength multiplexing, etc. In this case, the image of the reference beam is randomly patterned that its spatial frequency bandwidth is widely spread. As the grain size of the random pattern decreases, its spatial frequency of the reference beam becomes more spread. As a result, the paraxial approximation may be insufficient with this case. In addition, the effect of multiple diffractions between different angular spectra can also be magnified because the structures of multiplexed volume holograms are more complicated than others. Here we analyze the volume holographic gratings based on the coupled-mode theory in discrete Fourier domain without assuming the paraxial approximation, in which the continuous spatial spectra of lights are discretized by discrete Fourier transform and the couplings among them are simultaneously considered into account. We propose two methodologies for the coupled-mode analysis of volume hologram: one is by discretization approach and the other by a first-order approximation. These approaches can be extended to any kind of volume hologram analysis, such as for the Fourier or Fresnel plane hologram that includes lenses or not. The selectivity and crosstalk of random-phase-multiplexed volume holograms are discussed by the two methods.
A theoretical study on a long-period liquid-crystal (LC) fiber grating is presented by means of an anisotropic waveguide analysis and the coupled-mode theory with the discretization method. Effective propagation and coupling constants are found for a hollow-core fiber filled with nematic liquid crystal, the optic axis of which is in the direction of the fiber axis. The results show that when uniaxial perturbations of the liquid-crystal director exist in the core by external electric fields, the mode couplings can occur between modes with different azimuth variations as well as between modes with the same. The cross couplings between the fundamental core mode and the transverse-magnetic cladding modes are most strong compared even to the self-coupling of the fundamental core mode, and thus the spectral shift with the growth of the director modulation becomes negligible in the weak-modulation regime. Using the derived theoretical results, we discuss numerical examples of long-period LC fiber gratings with respect to the modulation depth of the director and the polarization states of the fundamental core mode. The characteristics of the LC fiber can be modified in many ways based on the LC material, and the dynamic controllability of LC fiber gratings can have many applications in fiber-optic systems. Thus this theoretical analysis could be utilized for them.
We discuss an application of nonlinear characteristics of optical fiber gratings. A cascaded long-period fiber grating pair shows a very sharp interference fringe patterns in wavelength domain. This can be applied to the modulation of optical beam intensity by the use of cross-phase modulation. The core mode can be switched to the cladding mode by the method, and the pulse intensity needed in the switching is reduced by an order of magnitude compared to the previous method. We also discuss bidirectional wavelength add/drop multiplexers (B-WADM's) using fiber Bragg gratings (FBG's) and multi-port optical circulators. A further modification of this structure that uses two separate multiplexer (MUX) and demultiplexer (DEMUX) pairs and reflection-type FBG filters is also discussed. The proposed B-WADM alleviates the formerly stringent requirement on MUX/DEMUX crosstalk characteristics, while suppressing the power penalty induced by the Rayleigh backscattering or optical reflection. In experiment, the measured crosstalk induced by the backscattering was successfully suppressed to negligible level (-30 dB) while at the same time achieving high B-WADM span gain over 25 dB.
A self-referencing fiber-optic sensor has been demonstrated for macro-bending detection immune to temperature and strain perturbations using an assembly of a hollow-core splice and fiber Bragg gratings, which can detect the macro-bending curvature over hundreds of centimeters.
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