A parallel measurement approach is presented to analyze multiple linear polarization (LP) modes using the spatial light modulator (SLM). The hologram loading onto SLM is generated by conjugate symmetric extension Fourier computer-generated holography. The mode array containing different LP modes is encoded into the hologram, which records the array amplitude and phase information. Demodulated by the SLM, the modal content and their relative powers of tested light are obtained by measuring the intensity and position of the demodulated light spot in the diffracted field. The experimental system is established and LP modes of single and few-mode step-index fibers are measured. Our method has some advantages, including simple optical setup and applicability of the higher-order modes detection. It is promising for the mode analysis of high-power fiber lasers and optical communication.
We propose an approach for three-dimensional (3-D) refractive index (RI) measurement of special optical fiber by phase-shifting digital holographic microscopy based on optical vortex. The experimental system consists of a modified Mach–Zehnder interferometer and related lab-developed software. The phase shifts are obtained by loading different computer-generated spiral phase holograms on a liquid-crystal spatial light modulator with digital blazed grating, which is more accurate than the traditional phase-shift methods. The optical vortex generated in +1 diffraction order of the diffraction field is utilized as the reference beam. Eight digital holograms with the phase-shift increments of 0, π / 2, π, and 3π / 2 are recorded by a CCD before and after the tested optical fiber is placed in the experimental system. Then, a four-step phase-shifting algorithm and phase unwrapping algorithm are adopted to extract the phase distribution from the holograms. Based on a filtered backprojection algorithm, the 3-D RI distribution of the tested fiber is obtained at high accuracy. Experimental results demonstrate the feasibility of the proposed method for 3-D RI measurement of special optical fiber.
We systematically study the effects of computer-generated hologram characteristics and spatial light modulator (SLM) performance on the quality of the optical vortex (OV). To improve the energy efficiency of the OV, the blazed fork gratings are introduced to transfer most of the energy to +1 diffraction order of SLM, and the relationship among three kinds of holograms is discussed. A mathematical model, based on the digital blazed grating theory, is proposed to analyze the diffraction angle and energy efficiency of +1 order. The theoretical analysis is further validated by experimental results. The research provides useful guidelines for OV-related applications.
An efficient adaptive simultaneous algebraic reconstruction technique (ASART) to calculate optical fiber refractive index profiles is proposed based on phase difference curves obtained by digital holography technique. We develop adaptive relaxation parameter (ARP) on simultaneous algebraic reconstruction technique (SART) to increase the convergence speed and improve the reconstruction accuracy. A formula of ARP is derived mathematically and multilevel scheme (MLS) is used to increase convergence speed in the first iteration. Experimental results show the proposed ASART convergences over 40% faster than SART and achieve significantly higher reconstruction accuracy. Experimental verification shows that ASART is more efficient than SART and filtered back projection in image reconstruction, especially with few projection views. The running time of ASART is much shorter than that of SART, and ASART needs fewer iteration numbers to obtain the same reconstruction effects. In addition, it can be used to measure optical fibers with various diameters that cannot be measured with S14 refractive index profiler (S14).
We propose the generation of double-ring vortex beams by coaxial superposition of Laguerre-Gaussian modes with
different topological charge numbers and different waist parameters. The formation and distribution characteristics of
double-ring vortex beams are analyzed in theory. A computer-generated holography based on conjugate-symmetric
extension is applied to encode the holograms, with which we reconstruct the double-ring vortex beams by optical
experiments. It is found that each ring of the double-ring vortex beams carries different orbital angular momentum.
Furthermore, when the internal topological charge remains unchanged, the distance between the two rings becomes
bigger with the increase of external topological charge. Excellent agreements between theoretical and experimental
results are observed. The double-ring vortex beams may find their potential application value in the fields of micro
particles manipulation and optical information encoding as different orbital angular momentum is respectively carried by
the internal and external rings.
A new method of synthesizing computer-generated hologram of three-dimensional (3D) objects is proposed from their projection images. A series of projection images of 3D objects are recorded with one-dimensional azimuth scanning. According to the principles of paraboloid of revolution in 3D Fourier space and 3D central slice theorem, spectra information of 3D objects can be gathered from their projection images. Considering quantization error of horizontal and vertical directions, the spectrum information from each projection image is efficiently extracted in double circle and four circles shape, to enhance the utilization of projection spectra. Then spectra information of 3D objects from all projection images is encoded into computer-generated hologram based on Fourier transform using conjugate-symmetric extension. The hologram includes 3D information of objects. Experimental results for numerical reconstruction of the CGH at different distance validate the proposed methods and show its good performance. Electro-holographic reconstruction can be realized by using an electronic addressing reflective liquid-crystal display (LCD) spatial light modulator. The CGH from the computer is loaded onto the LCD. By illuminating a reference light from a laser source to the LCD, the amplitude and phase information included in the CGH will be reconstructed due to the diffraction of the light modulated by the LCD.
The characteristics of optical fiber are quite important for improving the performance in various application of the optical fiber including communication and sensor systems. Based on optical Mach-Zehnder interferometer, a new measuring method of optical fiber refractive index profiling using digital holography is proposed in this paper, which simplifies the experimental setup compared to traditional holography. Several kinds of fiber samples such as multimode fiber, polarization-preserving fiber and special fiber are tested and their holograms are recorded by CCD. After filtering of the hologram, the phase distribution of fiber sample can be reconstructed and extracted. At last the experimental results of refractive index profiling of fiber are given.
An interrogation scheme for an in-series double cladding fiber (DCF) sensor is proposed and demonstrated, which can be
used for simultaneous refractive index (RI) and temperature measurement. It utilizes two commercial distributed
feedback lasers to match two cascade DCF sensors which have two band-rejected filtering spectra at different
wavelengths. The two lasers were intensity modulated by different frequencies and demodulated by a lock-in amplifier.
Experimental results indicated that a resolution of ±2×10-5 in RI and ±1.2°Cin temperature were achieved. Based on the
simple and low cost interrogation scheme, the dual parameters sensor system will find potential applications in chemical
sensors and biosensors.
A new method for synthesizing computer-generated holograms of 3D objects has been proposed with reduced number of
projections. According to the principles of paraboloid of revolution in 3D Fourier space, spectra information of 3D
objects is gathered from projection images. We record a series of real projection images of 3D objects under incoherent
white-light illumination by circular scanning method, and synthesize interpolated projection images by motion
estimation and compensation between adjacent real projection images, then extract the spectra information of the 3D
objects from all projection images in circle form. Because of quantization error, information extraction in two circles
form is better than in single circle. Finally hologram is encoded based on computer-generated holography using a
conjugate-symmetric extension. Our method significantly reduces the number of required real projections without
increasing much of the computing time of the hologram and degrading the reconstructed image. Numerical
reconstruction of the hologram shows good results.
KEYWORDS: Distortion, Projection systems, Imaging systems, Data modeling, 3D acquisition, 3D metrology, Charge-coupled devices, 3D modeling, Calibration, Structured light
The three-dimensional shape measurement has been widely used in a lot of applications such as traffic, entertainment, architecture design, manufacturing and archeology. The paper simplifies the principle of structured-light triangulation with the constraints of light-plane and takes the radial lens distortion during CCD imaging into account, which is able to improve the system accuracy. In order to release the limit of spatial and temporal stereo in the structured light system and improve the process rate and accuracy, the system utilizes the space-time stripe boundary coded patterns. ICP (Iterative Closest Points) is widely used for geometric alignment of three-dimensional models when an initial estimate of the relative pose is given, or the relative motion is small. According to the features of data from structured light acquisition system, the paper utilizes the advanced matching algorithm, which is based on projection. This algorithm is easer and more accurate than conventional ICP.
KEYWORDS: Local area networks, Video, Video compression, Analog electronics, Computer programming, Control systems, Embedded systems, Video coding, Microcontrollers, Video processing
In this work an embedded system is designed which implements MPEG-2 LAN transmission of CVBS or S-video signal. The hardware consists of three parts. The first is digitization of analog inputs CVBS or S-video (Y/C) from TV or VTR sources. The second is MPEG-2 compression coding primarily performed by a MPEG-2 1chip audio/video encoder. Its output is MPEG-2 system PS/TS. The third part includes data stream packing, accessing LAN and system control based on an ARM microcontroller. It packs the encoded stream into Ethernet data frames and accesses LAN, and accepts Ethernet data packets bearing control information from the network and decodes corresponding commands to control digitization, coding, and other operations. In order to increase the network transmission rate to conform to the MEPG-2 data stream, an efficient TCP/IP network protocol stack is constructed directly from network hardware provided by the embedded system, instead of using an ordinary operating system for embedded systems. In the design of the network protocol stack to obtain a high LAN transmission rate on a low-end ARM, a special transmission channel is opened for the MPEG-2 stream. The designed system has been tested on an experimental LAN. The experiment shows a maximum LAN transmission rate up to 12.7 Mbps with good sound and image quality, and satisfactory system reliability.
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