Applied optics最新文献

We report on the design and fabrication of nearly polarization-insensitive angular filters, which have been developed through the optimization of one-dimensional A g/M g F ₂ photonic crystals (PCs). We evaluate different initial systems for optimization and compare their results in terms of both the wavelength and angular selectivity. Our findings reveal that relaxing the strict periodic condition of initial photonic crystals with a small number of lattices has enabled improvement in the angular selectivity via Fabry-Perot resonances in dielectric layers, achieving a transmission as high as 81% at normal incidence by optimizing the dielectric layer thickness. The simulation results demonstrate that the transmitted beam through the angular filtering sample at 633 nm has allowable angles within 29° and 33° for TE and TM polarization, respectively, with a transmission over 80% at normal incidence. This proposed and demonstrated angular filter represents what we believe is a novel way to utilize 1D metal-dielectric PCs as polarization-insensitive angular filters, overcoming the main drawback of a low transmission. This angular filter will have significant applications in lighting, beam manipulation, optical coupling, and optical detectors.

Coded aperture compressive temporal imaging (CACTI) utilizes compressive sensing (CS) theory to compress three dimensional (3D) signals into 2D measurements for sampling in a single snapshot measurement, which in turn acquires high-dimensional (HD) visual signals. To solve the problems of low quality and slow runtime often encountered in reconstruction, deep learning has become the mainstream for signal reconstruction and has shown superior performance. Currently, however, impressive networks are typically supervised networks with large-sized models and require vast training sets that can be difficult to obtain or expensive. This limits their application in real optical imaging systems. In this paper, we propose a lightweight reconstruction network that recovers HD signals only from compressed measurements with noise and design a block consisting of convolution to extract and fuse local and global features, stacking multiple features to form a lightweight architecture. In addition, we also obtain unsupervised loss functions based on the geometric characteristics of the signal to guarantee the powerful generalization capability of the network in order to approximate the reconstruction process of real optical systems. Experimental results show that our proposed network significantly reduces the model size and not only has high performance in recovering dynamic scenes, but the unsupervised video reconstruction network can approximate its supervised version in terms of reconstruction performance.

The distance from the virtual image to the human eye is an important factor in measuring the comfort of a head-mounted display (HMD). However, accurately measuring their distance is challenging due to the dynamic changes in virtual presence and distance. In this paper, we proposed a virtual image distance measurement prototype based on a variable-focus liquid lens and derived a virtual image distance calculation model. We built a variable-focus liquid lens experimental platform to verify the method's correctness. In addition, we proposed an improved optimization algorithm that can efficiently and accurately search for the optimal focal length corresponding to the maximum sharpness moment of the virtual image within the focal length value space. Verified in an experimental scene of 0.5 m to 3.5 m, we observed that the error between the object image distance and the virtual image distance at the same focal length is about 5 cm. The proposed virtual image distance measurement method can accurately measure the distance value of the virtual image in the HMD. This method can be widely used in virtual and augmented reality, especially in the task of constructing realistic scenes.

A compact, low-loss, and high-polarized-extinction ratio TM-pass polarizer based on a graphene hybrid plasmonic waveguide (GHPW) has been demonstrated for the terahertz band. A ridge coated by a graphene layer and the hollow HPW with a semiround arch (SRA) Si core is introduced to improve structural compactness and suppress the loss. Based on this, a TM-pass polarizer has been designed that can effectively cut off the unwanted TE mode, and the TM mode passes with negligible loss. By optimizing the angle of the ridge, the height of the ridge, air gap height, and the length of the tapered mode converter, an optimum performance with a high polarization extinction ratio of 30.28 dB and a low insert loss of 0.4 dB is achieved in the 3 THz band. This work provides a scheme for the design and optimization of polarizers in the THz band, which has potential application value in integrated terahertz systems.

This paper introduces a new polarization-insensitive graphene-based frequency selective absorber (FSA) with a reflective notch designed for terahertz applications. The proposed structure features two absorption bands on either side of a central reflection band. The design composes a lossy frequency selective surface (FSS), a bandstop FSS with a metal backing, and an air spacer between. A wideband absorber structure is developed in the first step, leveraging graphene as an absorbent material in the lossy layer to achieve wideband absorptive characteristics. Subsequently, a reflection band is introduced by integrating a bandstop, lossless FSS layer into the absorber structure. The overall structure demonstrates two distinct absorption bands, characterized by absorptivity exceeding 80% within the frequency ranges of 0.30 to 0.57 and 0.67 to 0.90 THz. Simultaneously, a reflection notch is achieved at 0.60 THz. Extensive simulations assessed the performance of the designed FSA. The proposed structure exhibits stability under oblique incidence up to 40 deg and allows tunable absorption specifications by adjusting the chemical potential of graphene. It is noteworthy that the FSA reflector offers advantages such as eliminating the need for complicated, high-cost 3-D structures and welding of the lumped resistors.

Planar X e B r ^∗ and X e C l ^∗ excilamps emitting noncoherent narrowband UVB light (280-315 nm) are now widely used to cure psoriasis and vitiligo as well as to improve vitamin D synthesis. The two-dimensional integral formula has been deducted in this study, which is a good method and has great practical significance to calculate the total radiant power and assess the energy efficiency of a planar UV lamp. The measured radiant power of planar white LED lamps through a two-dimensional Keitz formula has been compared to that of gonio-photometer, verifying the applicability of the formula. The optimum measurement distance is dependent on the lamp length (1.5L≤D≤3.5L) for which the derivation from the two methods can be controlled within 10%. The planar X e B r ^∗ excilamps have been measured and compared to coaxial excilamps, which show similar patterns of change for the radiant characteristics. Since the planar radiant power formula only needs to measure normal illuminance at a certain distance from the symmetric center of the lamp, it is more convenient to use and is a low-cost method to promote the development of large-sized planar ultraviolet lamps.

Lateral resolving power is a key performance attribute of Fizeau interferometers, confocal microscopes, interference microscopes, and other instruments measuring surface form and texture. Within a well-defined scope of applicability, limited by surface slope, texture, and continuity, a linear response model provides a starting point for characterizing spatial resolution under ideal conditions. Presently, the instrument transfer function (ITF) is a standardized way to quantify linear response to surface height variations as a function of spatial frequency. In this paper, we build on the ITF idea and introduce terms, mathematical definitions, and appropriate physical units for applying a linear systems model to surface topography measurement. These new terms include topographical equivalents of the point-, line-, and edge-spread functions, as well as a complex-valued transfer function that extends the ITF concept to systems with spatial-frequency-dependent topography distortions. As an example, we consider the experimental determination of lateral resolving power of a coherence scanning interference microscope using a step-height surface feature to measure the ITF directly. The experiment illustrates the proposed mathematical definitions and provides a direct comparison to theoretical calculations performed using a scalar diffraction model.

We investigate terahertz time-domain spectroscopy using a low-noise dual-frequency-comb laser based on a single spatially multiplexed laser cavity. The laser cavity includes a reflective biprism, which enables generation of a pair of modelocked output pulse trains with slightly different repetition rates and highly correlated noise characteristics. These two pulse trains are used to generate the THz waves and detect them by equivalent time sampling. The laser is based on Yb:CALGO, operates at a nominal repetition rate of 1.18 GHz, and produces 110 mW per comb with 77 fs pulses around 1057 nm. We perform THz measurements with Fe-doped photoconductive antennas, operating these devices with gigahertz 1 µm lasers for the first time, to our knowledge, and obtain THz signal currents approximately as strong as those from reference measurements at 1.55 µm and 80 MHz. We investigate the influence of the laser's timing noise properties on THz measurements, showing that the laser's timing jitter is quantitatively explained by power-dependent shifts in center wavelength. We demonstrate reduction in noise by simple stabilization of the pump power and show up to 20 dB suppression in noise by the combination of shared pumping and shared cavity architecture. The laser's ultra-low-noise properties enable averaging of the THz waveform for repetition rate differences from 1 kHz to 22 kHz, resulting in a dynamic range of 55 dB when operating at 1 kHz and averaging for 2 s. We show that the obtained dynamic range is competitive and can be well explained by accounting for the measured optical delay range, integration time, as well as the measurement bandwidth dependence of the noise from transimpedance amplification. These results will help enable a new approach to high-resolution THz-TDS enabled by low-noise gigahertz dual-comb lasers.

Large-aperture telescopes based on optical synthetic aperture imaging are investigated for recent high-resolution spaceborne observations. An enabling technique of aperture synthesis is a cophasing method to suppress a piston-tip-tilt error between sub-apertures. This paper proposes a scene-based cophasing technique using the stochastic parallel gradient descent (SPGD) algorithm, assuming application to high-resolution Earth observation. A significant advantage of the SPGD algorithm is a model-less cophasing capability based on extended scenes, but the simultaneous scene-based piston-tip-tilt correction between multiple apertures has not been demonstrated. In this paper, we developed a tabletop synthetic aperture imaging system with 37 sub-apertures and demonstrated extended-scene-based piston-tip-tilt control by optimizing applied voltages to 111 actuators simultaneously. The demonstration experiments used not only static scenes but also a time-varying dynamic scene for observation targets. In every measurement, the proposed scene-based approach reduced the initially defined piston-tip-tilt errors, and the image sharpness significantly improved, although the correction rate in the dynamic scene observation was slower. Finally, this paper discusses the influence of scene dynamics on image-based cophasing.

We present, to our knowledge, a novel method to achieve experimental encryption using double random phase encoding with full complex modulation and a single phase-only spatial light modulator. Our approach uses double phase encoding to generate phase-only holograms containing complex-valued input planes for a joint transform correlator (JTC) cryptosystem. This approach enables users to independently manipulate both the phase and amplitude of the cryptographic keys and objects, thereby significantly enhancing the versatility of the optical cryptosystem. We validate the capabilities of our proposed scheme by generating optimized random phase masks and using them to experimentally encrypt various grayscale and binary objects. The experimental complex modulation obtained with the system detailed in this work, in conjunction with optimized random phase masks, results in an enhancement in the quality of the decrypted objects during reconstruction. Both numerical simulations and experimental findings corroborate the effectiveness of our proposal.