At present, motion recognition plays an important role in the field of computer vision, and the result of action recognition depends on the extracted feature mode and the expression type of action. Therefore, it is particularly important to propose a relatively fast and robust feature extraction method and apply it to action recognition. In this paper, we propose two feature extraction methods for motion recognition using the information provided by the depth camera, and achieve good recognition accuracy. The two feature extraction methods are as follows: The extraction of temporal and spatial information (ETS) and the clean extraction of temporal and spatial information (CETS). We compare the accuracy of the two feature extraction methods for action recognition, and find that the CETS method has a very good improvement compared with the ETS method. This will have a big significance to our exploration in the aspect of motion recognition.
A tunable passively mode-locked fiber laser with cylindrical vector beams (CVB) generation in a linear cavity was presented. The mode-locking mechanism is based on the nonlinear multimode interference effect of the graded-index multimode fiber (GIMF). The Sagnac loop mirror works as a filter. An ultra-broadband long-period fiber grating (UBLPFG) with a high conversion efficiency < 90% from 1004 nm to 1068 nm is used to covert fundamental mode to higher-order modes. The laser can achieve a continuously tunable wavelength range from 1032.344 nm to 1042.556 nm by changing the reflection spectrum of the Sagnac loop mirror thermally. When the laser operates at the central wavelength of 1042.556 nm, the output pulse trains have a period of 92.08 ns with pulse duration of 1.15 ns. The slope efficiency of the laser is about 4.73% with the mode-locking threshold of 167.7 mW. The purity of the radially and azimuthally polarized beams is measured to be higher than 93.8%.
KEYWORDS: Fiber lasers, Signal to noise ratio, Polarization, Linear polarizers, Telecommunications, Single mode fibers, Polarizers, Multiplexing, Multiplexers, Random lasers
We propose and experimentally demonstrate a Yb-doped random fiber laser with a controllable oscillating mode based on injection locking technology. An external signal light is used to realize the locking of the transverse mode. By simply adjusting the injected signal light power, random fiber laser can directly oscillate at the LP01 mode, hybrid mode, and LP11 mode, respectively. Besides, random lasing with a low threshold and high slope efficiency can be obtained.
All-fiber wavelength-tunable fiber laser is a desirable light source architecture in many fields owing to their low cost, compactness and easy integration. However, most of them have independent filter elements and saturated absorption elements, which will undoubtedly increase the redundancy of laser. In this paper, we realize an all-fiber wavelength-tunable Ytterbium (Yb)-doped laser with the tunable central wavelength from 1023 nm to 1076 nm based on fiber loop mirror (FLM). The fiber loop (FLM) fused with two segments of polarization-maintaining fibers (PMF) acts as both a comb filter and a saturable absorber (SA). We believe that lasers with a simple structure and wide wavelength tuning range will have potential applications in sensing, communication, spectroscopy, and medicine.
The human eye is more capable of color discrimination in a wide color gamut display. W. A. Thornton hypothesized that they may also enhance color discrimination for color-deficient observers. Compared with normal color vision observers (NCO), anomalous trichromats (AT) have color loss due to difference in cone sensitivity. The large color gamut display system provides a promising solution for solving this problem. We propose a new geometric method to compute the color stereo of AT based on different display color gamuts. The proposed method is fast and efficient to improve color perception for different types of AT.
We use the full vector beams propagation method (VFE-BPM) to simulate the nonlinear propagation process of the cylindrical vector beams (CVBs). We have shown that the CVBs have poor transverse stability which causes which causes the breakup due to the ring-shaped beam field. Then, we use the approximate analytic theory to qualitative analyze the variations of the numerically calculated instability growth rates, which shows excellent agreement with the simulated results.
KEYWORDS: Color and brightness control algorithms, Color management, 3D displays, Glasses, Displays, Stereoscopic displays, 3D vision, Light sources, Laser based displays, 3D image processing
Three-dimensional (3D) display is a very attractive research direction, and have potential in many areas. The cutting-edge autostereoscopic display technology allows glasses-free experience, but still be limited in the lab because of the small eye space and high cross-talk. The only commercially available technology is still polarization-interlaced stereoscopic display. The disadvantages are the bulk-cost of the polarization module, low-light efficiency, and the high crosstalk. Besides, the color gamut in two-dimensional (2D) display is very important to display systems, which represents the color rending ability, and at present limited by three primaries. Herein, we demonstrate a six-primary-laser projection system compatible with 3D and 2D display, achieve great 3D viewing experience with crosstalk lower than 1% by time-multiplexed stereoscopic display technology and spectral coating glasses. In addition, we study the volume color gamut of this system in 2D working mode. The color gamut is greatly increase to an amazing 178.4% NTSC, owing to the application of multiprimary color and narrow spectral line-width laser source. This system is also provide the possibility for us to study the color gamut involving binocular fusion in 3D working mode in future.
We have promoted and demonstrated a relationship between speckle contrast and the fluctuation of the screen surface in Laser Display Technology (LDT). Based on the speckle theory, we analyzed the speckle characteristics of diffuse scattering screen, and obtained the relationship between speckle contrast C and the standard deviation of surface fluctuation height σh. Meanwhile, we used the precision probe surface profilometer to measure the height and morphology of 12 kinds of screens. Finally, we built a speckle testing platform to simulate human eye, and measured the speckle contrast of 12 kinds of screens. By analyzing the gain and speckle contrast parameters of glass bead screen and metal screen, we could conclude that glass bead screen was more suitable for LDT.
We have experimentally demonstrated the generation of DSR in normal and anomalous dispersion regimes by using a chirped fiber Bragg grating (CFBG) for dispersion management. The dispersions of our proposed laser are calculated to be around +5.34 ps2 and -2.66 ps2 with different directions of CFBG. By properly adjusting the polarization state and pump power, we can achieve two kinds of square pulses with different shapes at the fundamental frequency of 3.58 MHz and 3.583 MHz, respectively. The width of square pulse can be tuned from 1.65 ns to 5.43 ns in normal dispersion regime. The width of square pulse can be tuned from 1 ns to 2.7 ns in anomalous dispersion regime.
A novel design for speckle reduction based on a rotating ball lens is investigated. In addition, a standardized speckle measurement method is also developed and the parameters mainly accord with characteristics of human eye. This method could open up an avenue to the practical speckle measurement application.
Color gamut is an important parameter for display system. Wider gamut means that the system can display more colors. In this article, we designed an experiment to measure the color discrimination ability of human eyes in a certain range under different light sources such as LCD, LED, OLED and laser display, which has different center wavelength and spectrum-width. In our experiment, a Xenon lamp projector and a RGB laser projector were used to run the testing program. Eight volunteers` results showed that the gamut of RGB laser display is 49.48% larger than that of ordinary Xenon lamp display, and the standard deviation of the result was 8.7%.
Many kinds of display technology such as LCD, OLED, quantum dot and laser display are developing rapidly. The final purpose of display technology is to achieve wider color gamut. Traditional method uses planar chromaticity diagram in CIEXYZ space to describe color gamut. This method did not use uniform color space and cannot deal with the situation of display systems with four or more primaries. In our previous work, we have already established a theory based on Macadam’s theory to calculate the stereoscopic color gamut for RGB laser display systems. In this letter, we extend this theory to display systems with four or more primaries. By using our algorithm, we calculate the stereoscopic color gamut of multi-primary display system. We set a four primary display system as an example. The center wavelength of the primary are set as 467nm, 520nm, 532nm and 630nm, white point is set as D65(x=0.3127, y=0.3290), and color gamut has been calculated in CIELAB color space. Results show that when the ratio of the intensity of four primary is 1.0000:0.7490: 0.4316:1.5000, the largest gamut is obtained, whose value is 2.1030*106. These results can be used in engineering to optimize the intensity of the primaries in four primaries display systems.
Laser display technology has developed rapidly in the past decade because it has advantages of high color saturation, high conversion efficiency, long life span and the capability to show real objects accurately. Many new laser technologies such as ALPD (Advanced Laser Phosphor Display) solution, Two Primary and three Primary Laser Display solutions have been studied for handling wide-gamut standard of Rec.2020 to reproduce the natural object colors faithfully. Different kinds of laser source including LD (laser diodes), diode-pumped solid-state laser, LD pumped Phosphor, VCSEL (Vertical-cavity surface-emitting laser) and fiber laser have been utilized in these solution. They have different center wavelengths and different linewidths which influence the color gamut and the speckle contrast of different display systems. In this paper, we calculate the color solid volume of laser display systems, and systematically investigate the color gamut conditions among display systems with different spectral linewidth selecting and different combination of three primaries. The color gamut of Rec.2020 is set as an object of comparison, in which the center wavelength of three primaries is set as Red=630nm, Green=532nm and Blue=467nm with spectral line width=1 nm. White balanced point is set as D65(x=0.3127, y=0.3291) and curve of color gamut as a function of spectral linewidth is calculated in CIELAB space. Our results would give researchers working on laser display a reference to choose suitable laser source with center wavelength and linewidth to keep a balance between laser speckle and color saturation.
We have proposed and demonstrated the generation of high power, wide band square-wave pulse in a figure-8 Yb-doped mode-locked fiber laser. The fiber laser operates at 1083.65 nm with 3 dBbandwidth of 6.5 nm, which can emit nanosecond square pulses with a maximum average output power of 1.31 W and peak power 40 W, respectively. The bandwidth, to the best of our knowledge, is the widest output bandwidth in a square-wave pulse fiber laser operating at 1 μm band.
A 978 nm single mode passively mode-locked all-fiber laser delivering square-wave pulses was demonstrated using a figure-8 cavity and a 75 cm commercial double-clad ytterbium-doped fiber. We found the three-level system near 978 nm was able to operate efficiently under clad pumping, simultaneously oscillation around 1030 nm well inhibited. The optimized nonlinear amplifying loop mirror made the mode locking stable and performed the square-pulses shaping. To the best of our knowledge, it is the first time to report the square-wave pulse fiber laser operating at 980 nm. The spectral width of the 978 mode-locked square pulses was about 4 nm, far greater than that of the mode-locked square pulses around 1060 nm reported before, which would be helpful to deeply understand the various square-wave pulses’ natures and forming mechanisms. Compared with modulated single-mode or multimode 980 nm LDs, this kind of 980 nm square-wave sources having higher brightness, more steeper rising and falling edge and shorter pulse width, might have potential applications in pumping nanosecond ytterbium or erbium fiber lasers and amplifiers.
We demonstrate a thulium-doped all fiber actively mode-locked laser by synchronously pumping without electronic modulator. A mode-locked fiber laser operating at 1550 nm based on nonlinear polarization rotation (NPR) is innovatively utilized as the pulsed pump. Through cavity length matching, stable mode-locking that operate at 1891.25 nm is achieved with a spectral width of 0.52 nm at 3 dB. The repetition rate is 11.59 MHz with an estimated pulse duration less than 125 ps.
Based on a few-mode fiber Bragg grating as polarization-selective output coupler and topological insulators Bi2Te3 as the saturable absorber, we propose a passively Q-switched fiber laser with cylindrical vector beam output. Both radially and azimuthally polarized beams can be readily generated, and the output polarization can be switchable through tuning the polarization controllers inside the laser cavity. The laser operates at the wavelength of 1557.5 nm with a 3 dB linewidth of less than 0.04 nm. The repetition rate of the Q-switched laser can be tuned from 31.54 kHz to 49.40 kHz when the pump power increases from 103.5 mW to 139.5 mW.
We demonstrated an all fiber actively mode-locked laser emitting cylindrical vector beam. A few-mode fiber Bragg grating is adopted to achieve mode selecting and spectrum filtering. An offset splicing of single-mode fiber with fourmode fiber is utilized as a mode coupler in the laser cavity. A LiNbO3 Mach-Zehnder modulator is used to achieve active mode locking in the laser. The laser operates at 1547nm with 30 dB spectrum width of 0.3nm. The emitted modelocked pulses have a duration of 1ns and repetition of 12.06MHz. Both radially and azimuthally polarized beams have been obtained with very good modal symmetry by adjusting the polarization in the laser cavity.
We have proposed and demonstrated a nanosecond square-wave fiber laser working in the 1060nm band. The passively
mode-locked fiber laser based on the nonlinear optical loop mirror has a peak power clamping effect which leads to the
generation of nanosecond square-wave pulses. To investigate the spectrum width of the nanosecond square-wave pulse
laser, we added couplers with different coupling ratio to the bidirectional ring of the figure-8 fiber laser and analyzed the
laser output. The results show that a higher output coupling ratio leads to stronger peak power clamping effect, and the
peak power of the square-wave pulse gets lower and the corresponding spectrum band width is narrower.
In this paper, the relationship between the spatial coherence of light field and the speckle contrast in a laser based projection display system is studied under the consideration of human visual percept. By using a varifocal liquid-crystal lens and a monochromatic CCD, a system which is used for simulating the human eye is set up to record the speckle pattern. An efficient method for controlling the spatial coherence by using a dielectric elastomer actuator (DEA) is proposed. The results show that the total efficiency for energy utilization is more than 60% during our experiment. When the distance between the observer and the screen is large enough (<3 meters), the speckle contrast can be eliminated well at last (<4%) and the observer won’t feel the speckle phenomenon.
A passively mode-locked ytterbium-doped all fiber laser has been demonstrated based on the nonlinear polarization
rotation technique in a all normal dispersion cavity. By optimizing the coupling ratio and position of the output coupler,
stable mode-locked pulses is generated with an average power of 200 mW at a repetition rate of 3.3 MHz, and
corresponding to single pulse energy of 60 nJ. The slope efficiency of power is as high as 68%.
KEYWORDS: Mode locking, Composites, Fiber lasers, Modulation, Picosecond phenomena, Signal to noise ratio, Polarization, Frequency combs, Signal attenuation, Optical isolators
We have demonstrated a passively mode-locked fiber laser with a composite cavity structure for repetition rate control.
An optical delay line is used to control the length so that the lengths of the main cavity and the sub-ring cavity are
accurately co-prime. Using this method we have obtained 46th harmonic pulses with a fundamental repetition of
17.39MHz. The fundamental mode-locking is substantially suppressed. The SNR of rf spectrum is higher than 45 dB.
Stable 800MHz repetition rate mode-locked pulses in duration of 14.27 ps are generated. The detuning phenomenon
appears when two cavity’s lengths are not matched. A larger pump power is required to maintain the oscillation.
Based on the few-mode fiber Bragg grating’s reflection characteristics, we propose and demonstrate a transverse modes switchable fiber laser fiber laser, fundamental mode and 1st higher order modes, and the states be switched by tuning the laser’s oscillating wavelength. The radial vector beam is also obtained by splitting the degenerated 1st higher order modes.
We present a numerical model for generating cylindrical vector beams (CVBs) through an ytterbium-doped few-mode
fiber laser. A practical numerical model based on propagation rate equations with consideration of propagating modes
competition between LP01 and LP11. To optimize the output power of LP11 while depress LP01, we analyze the
dependence of this competition on resonant wavelength, pump power, ytterbium doped fiber (YDF) length, doping
concentration and distribution. We also propose an effect method to completely depress the generation of LP01.
Direct laser driven ICF system requires a high degree of uniformity in laser energy in order to avoid Rayleigh-Taylor
instability. In all-fiber laser driven ICF system, millions of fibers are fixed around the target chamber. We suggest the
fibers are arranged in bundles with a single lens coupling the output of a bundle. If the arrangement is optimized, top-flat
intensity profile is acquired near the focus of the lens. Least square method is used here in determining the optimum
values of relative parameters. Factors on the irradiation nonuniformity are also analyzed here by numerical simulations.
We proposed and demonstrated an all-fiber laser to generate cylindrical vector beams at
1030 nm. The cylindrical vector beams were obtained by exciting high order mode operating in
the few-mode fiber inside the laser system, which was implemented through adjusting the angle
and transversal dimension of a couple of fiber collimators. The radial and azimuthal polarization
beams can be switchable conveniently just by applying twist and pressure to the few-mode fiber.
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