Pseudo-diffusive phenomenon is an abnormal transmission phenomenon both in electric system and photonic system which hold conical dispersion relations in their band structures. The phenomenon is: near a Dirac point, the transmittance of the system is inversely proportional to the sample’s thickness as if the material was a disordered medium. It is known that in photonic system, Dirac cones can be classified into three different types, which are standard single Dirac cone, double Dirac cones and Dirac-like cone. Some of them can be found either at the center and the corners of the first Brillouin zone. So it raises the questions that: Do the types of the Dirac cones affect the pseudo-diffusive phenomenon? And further, is there any connection between the pseudo-diffusive phenomenon and the locations of the Dirac cones? Through theoretical investigation, we found that the locations of the Dirac cones play a critical role on the pseudo-diffusive phenomenon. If the Dirac cone is located in the center of the Brillouin zone, the pseudo-diffusive phenomenon doesn't exist at the Dirac frequency. Besides, the shapes of the Dirac cones also affect the pseudo-diffusive phenomenon. The non-conical dispersion band of the Dirac-like cone makes the transmission quite different from linear decrease with the increase of the thickness.
A guided-mode resonance(GMR) filter which can work at multiple tunable frequencies is designed and obtained by using a readily available photonic crystal. The photonic crystal consists of several layers of dielectric rods with different radii in each layer. When light is incident upon this photonic crystal at a small incident angle, guided mode resonance in each rod layer will be excited. At the resonant frequencies, nearly 100% narrow reflection peaks with zero reflection sidebands, an excellent GRM filter, can be obtained. In addition, the resonant frequency of each layer is closely related to the radius of the rod and all the resonances of the whole structure almost don't interact with each other. Therefore, by adjusting the radii of the rods in each layer, the operating frequencies of the filter can be freely tuned both individually and simultaneously, a perfect tunable multichannel GMR photonic crystal filter is obtained. This optical device has apparent advantages, simple structure, small and compact size, easy fabrication and integration. Therefore, it has great application potential in modern optical network.
A key requirement to put terahertz (THz) imaging systems into applications is high resolution. Based on a self-developed THz quantum cascade laser (QCL), we demonstrate a THz inline digital holography imaging system with high lateral resolution. In our case, the lateral resolution of this holography imaging system is pushed to about 70 μm, which is close to the intrinsic resolution limit of this system. To the best of our knowledge, this is much smaller than what has been reported up to now. This is attributed to a series of improvements, such as shortening the QCL wavelength, increasing Nx and Ny by the synthetic aperture method, smoothing the source beam profile, and diminishing vibration due to the cryorefrigeration device. This kind of holography system with a resolution smaller than 100 μm opens the door for many imaging experiments. It will turn the THz imaging systems into applications.
Terahertz frequency range spans from 0.1 to 10 THz. Terahertz radiation can penetrate nonpolar materials and nonmetallic materials, such as plastics, wood, and clothes. Then the feature makes the terahertz imaging have important research value. Terahertz computed tomography makes use of the penetrability of terahertz radiation and obtains three-dimensional object projection data. In the paper, continuous-wave terahertz computed tomography with a pyroelectric array detectoris presented. Compared with scanning terahertz computed tomography, a pyroelectric array detector can obtain a large number of projection data in a short time, as the acquisition mode of the array pyroelectric detector omit the projection process on the vertical and horizontal direction. With the two-dimensional cross-sectional images of the object are obtained by the filtered back projection algorithm. The two side distance of the straw wall account for 80 pixels, so it multiplied by the pixel size is equal to the diameter of the straw about 6.4 mm. Compared with the actual diameter of the straw, the relative error is 6%. In order to reconstruct the three-dimensional internal structure image of the straw, the y direction range from 70 to 150 are selected on the array pyroelectric detector and are reconstructed by the filtered back projection algorithm. As the pixel size is 80 μm, the height of three-dimensional internal structure image of the straw is 6.48 mm. The presented system can rapidly reconstruct the three-dimensional object by using a pyroelectric array detector and explores the feasibility of on non-destructive evaluation and security testing.
Terahertz waves of which frequency spans from 0.1 to 10 THz bridge the gap between the infrared spectrum and microwaves. Owing to the special features of terahertz wave, such as penetrability and non-ionizing, terahertz imaging technique is a very significant and important method for inspections and detections. Digital holography can reconstruct the amplitude and phase distributions of a sample without scanning and it already has many successful applications in the area of visible and infrared light. The terahertz in-line digital holographic multi-plane imaging system which is presented in this paper is the combination of a continuous-wave terahertz source and the in-line scheme of digital holography. In order to observe a three dimensional (3D) shape sample only a portion of which appears in good focus, the autofocusing algorithm is brought to the data process. The synthetic aperture method is also applied to provide the high resolution imaging effect in the terahertz waveband. Both intrinsic twin images and defocused objective images confuse the quality of the image in an individual reconstructed plane. In order to solve this issue, phase retrieval iteration algorithm is used for the reconstruction. In addition, the reconstructed amplitude image in each plane multiplies the mask of which the threshold depends on the values of the autofocusing curve. A sample with simple artificial structure is observed which verifies that the present method is an authentic tool to acquire the multi-plane information of a target in terahertz waves. It can expect a wide application in terahertz defect detecting, terahertz medical inspection and other important areas in the future.
Terahertz (THz) continuous-wave digital holography is an advanced interference imaging technique that can reconstruct quantitative distributions of amplitude and phase of the sample in real time with high resolution. In this paper, a reflective off-axis holographic system is presented. A Gaussian fitting method is applied to enhance the hologram contrast and Laplacian of Gaussian filter is used to obtain the reconstructed distance automatically. Furthermore, spectrum filtering method and angular spectrum algorithm are used to obtain the complex amplitude of the one-yuan chinese metal coin. The results confirm the prospective application of terahertz digital holography in the surface morphology for reflective samples.
Terahertz computed tomography makes use of the penetrability of terahertz radiation and obtains three-dimensional (3-D) object projection data. Continuous-wave terahertz digital holographic tomography with a pyroelectric array detector is presented. Compared with scanning terahertz computed tomography, a pyroelectric array detector can obtain a large quantity of projection data in a short time. To obtain a 3-D image, in-line digital holograms of the object are recorded from various directions and reconstructed to obtain two-dimensional (2-D) projection data; then 2-D cross-sectional images and 3-D images of the internal structure of the object are obtained by the filtered back projection algorithm. The presented system can rapidly reconstruct the 3-D object and reveals the internal 3-D structure of the object. A 3-D reconstruction of a polyethylene straw is presented with a 6% error in retrieved diameter.
In this paper, the simulation experiments both of Abbe-Porter spatial filtering and of optical processing of image addition and subtraction with a grating filter are designed and performed. We realize the design and operation of optical information processing simulation experiments based on information optics theory and the experimental principle by using MATLAB programing language. The spatial filtering of Fourier spectrum, one of the main concepts in information optics, is intuitively described via the simulation experiments, and the experiment process is demonstrated step by step. The results show that the simulation experiments are really helpful for the filter's design and the image processing. These developed virtual experiments have been used in experimental teaching for undergraduate students majored in optics or optical engineering, which effectively assist students to understand concept and principle of optical information processing.
The terahertz (THz) imaging is an advanced technique on the basis of the unique characteristics of terahertz radiation. Due
to its noncontact, non-invasive and high-resolution capabilities, it has already shown great application prospects in
biomedical observation, sample measurement, and quality control. The continuous-wave terahertz in-line digital
holography is a combination of terahertz technology and in-line digital holography of which the source is a
continuous-wave terahertz laser. Over the past decade, many researchers used different terahertz sources and detectors to
undertake experiments. In this paper, the pre-process of the hologram is accomplished after the holograms’ recording
process because of the negative pixels in the pyroelectric detector and the air vibration caused by the chopper inside the
camera. To improve the quality of images, the phase retrieval algorithm is applied to eliminate the twin images. In the
experiment, the pin which terahertz wave can’t penetrate and the TPX slice carved letters “THz” are chosen for the samples.
The amplitude and phase images of samples are obtained and the twin image and noise in the reconstructed images are
suppressed. The results validate the feasibility of the terahertz in-line digital holographic imaging technique. This work
also shows the terahertz in-line digital holography technique’s prospects in materials science and biological samples’
We propose and experimentally demonstrate a shift-multiplexing complex spectral-domain optical coherence tomography (shift-multiplexing CSD-OCT) method, in which the maximum detection depth of SD-OCT can be greatly extended by incorporating the shift-multiplexing of detection positions with CSD-OCT. The tomographic imaging with twofold or threefold microscopic slides as the target sample is performed. The experimental results show that the tomographic imaging with more uniform brightness and clarity for the different depth regions in a thick sample can be achieved by the shift-multiplexing CSD-OCT system. In particular, even while the sample’s depth is beyond the maximum imaging depth of CSD-OCT system, the tomographic imaging of this sample can still be realized by using the shift-multiplexing CSD-OCT method without the need for any replacement of the equipment, such as high spectral capacity grating or high resolution of CCD. The shift-multiplexing CSD-OCT system can perform the imaging with the optimization and less reduction of sensitivity for the deeper detection position in the sample.
Optical Coherence Tomography (OCT) was successfully applied in the microstructure imaging of biological tissue
after being proposed firstly in 1991 by the researchers of MIT. As a novel optical imaging technology, it mainly uses
interference principles to achieve noninvasive and high resolution visualization of samples. OCT works analogously to
an ultrasound scanner, the major difference is that ultrasound pulses are replaced by broadband light. According to
whether need for mechanical axial scan in the depth direction, it can be classified into the time-domain OCT (TD-OCT)
and frequency-domain OCT (FD-OCT). The FD-OCT system overmatches the TD-OCT in imaging speed because of its
depth collection advantage. But in the reconstructive image of FD-OCT detection, the complex-conjugate ambiguity will
seriously deteriorate the imaging effect of tomogram. So the technique of removing the complex-conjugate image is
employed that is called complex FD-OCT. The complex FD-OCT has widely application in many fields, especially in
the refractive index measurement. The refractive index is an important parameter characterizing light propagation in the
medium. In the paper, we present a method to measure the average refractive index of the sample with substrate
calibration by using complex FD-OCT method, in which we can calculate it without depending on the parameters of
system such as spectral width of light source. Due to the measurement of average refractive index relative to the actual
thickness and optical length, it is necessary to obtain them of the sample experimentally. The complex FD-OCT method
can easily achieved the optical length via measuring the positions of the sample’s front and rear surfaces. In the
experiment, the coverslip (the borosilicate glass) is chosen as the sample and the calibration substrate. We make use of
the substrate to load the sample on it, and then the tomogram of the sample can be achieved by means of OCT’s lateral
scan in the edge of the sample and complex FD-OCT method. According to the experimental results, we can acquire the
sample’s tomographic information and position of the substrate. The ratio of actual thickness and optical length can be
indirectly calculated out with the pixel number obtained by analyzing the image data. So with only one time scan, we can
complete the measurement of average refractive index of the sample without aid of other instruments.
In this paper, a deformable spherical planet exploration robot has been introduced to achieve the task of environmental detection in outer space or extreme conditions. The robot imitates the morphology structure and motion mechanism of tumbleweeds. The robot is wind-driven. It consists of an axle, a spherical steel skeleton and twelve airbags. The axle is designed as two parts. The robot contracts by contracting the two-part axle. The spherical robot installs solar panels to provide energy for its control system.
The three-step or many steps phase shifting method is usually employed to resolve the complex-conjugate ambiguity in Spectral-domain optical coherence tomography (SD-OCT). However it reduces the image quality and also the imaging speed is slow. In this paper two steps phase-shifting is used in digital image processing to resolve the complex-conjugate ambiguity and improves the quality of reconstructed image in SD-OCT. In the two-step phase shifting method the phase shifting operation is used only once which simplified the experiment and also the effect of relative error in SD-OCT on image quality is eliminated.