Linearly-polarized supercontinuum (SC) generated in an all-polarization-maintaining random fiber laser (RFL) structure is demonstrated. A visible to near-infrared SC spanning from 600 nm to 1900 nm is generated in a 100 m polarizationmaintaining germanium doped fiber (GDF). The measured polarization extinction ratio (PER) at all wavelengths from 800 nm to 1700 nm is greater than 18 dB, verifying the SC is well linearly-polarized. The results proved RFL can be a simple and cost effective linearly-polarized SC generation method for practical applications such as hyperspectral imaging and remote sensing.
A novel supercontinuum (SC) generation structure based on a random fiber laser is demonstrated in this paper. The effect of the passive fiber length for spectrum boarding is analyzed and discussed. A near-infrared SC with 20 dB bandwidth of more than 500 nm can be generated with 1 km length of passive fiber under 11.13 W pump power. The experimental results show that shorter passive fiber length is beneficial for the SC broadening to long wavelength region. The long wavelength edge of the output SC with 200 m and 1930 m passive fiber are 1680 nm and 1375 nm respectively when the pump power is 11.13W. The results proved that random fiber lasers can be a novel, simple, low cost, low coherence, and robust near-infrared SC generation source.
We present an experimental study on low-threshold supercontinuum generation mainly based on cascaded stimulated Raman scattering (SRS) effect in a four-stage fiber master oscillator power amplifier (MOPA) system. The cascaded SRS is achieved by using a long passive fiber pumped by a pulsed fiber laser centered at 1064 nm wavelength. The cascaded SRS seed is amplified in a double-clad ytterbium-doped fiber amplifier, generating supercontinuum ranging from 960 nm to 1700 nm with 13.6 W maximum average output power and approximately 17.7 mW/nm average spectral power density.
Supercontinuum generation directly from a nonlinear fiber amplifier, especially from a nonlinear ytterbium-doped fiber amplifier, attracts more and more attention due to its all-fiber structure, high optical to optical conversion efficiency, and high power output potential. However, the modeling of supercontinuum generation from a nonlinear fiber amplifier has been rarely reported. In this paper, the modeling of a tapered Ytterbium-doped fiber amplifier for visible extended to infrared supercontinuum generation is proposed based on the combination of the laser rate equations and the generalized nonlinear Schrödinger equation. Ytterbium-doped fiber amplifier generally can not generate visible extended supercontinuum due to its pumping wavelength and zero-dispersion wavelength. However, appropriate tapering and four-wave mixing makes the visible extended supercontinuum generation from an ytterbium-doped fiber amplifier possible. Tapering makes the zero-dispersion wavelength of the ytterbium-doped fiber shift to the short wavelength and minimizes the dispersion matching. Four-wave mixing plays an important role in the visible spectrum generation. The influence of pulse width and pump power on the supercontinuum generation is calculated and analyzed. The simulation results imply that it is promising and possible to fabricate a visible-to-infrared supercontinuum with low pump power and flat spectrum by using the tapered ytterbium-doped fiber amplifier scheme as long as the related parameters are well-selected.
Fused silica optics were treated by dynamic etching using buffered hydrofluoric acid (BHF) with different etching depths. The transmissivity of fused silica slightly increases in deep UV (DUV) range after dynamic etching. Surface qualities of fused silica were characterized in terms of surface roughness, surface profile and photoluminescence (PL) spectra. The results show that dynamic etching has a slight impact on surface RMS roughness.PL defects gradually reduces by dynamic etching, and laser damage resistance of fused silica continuously increases with etching depth extending. When removal depth increases to ~12μm, the damage threshold is the double that of the unetched surface. However, surface profile continuously deteriorates with etching depth increasing. Appropriate etching amount is very important for improving damage resistance and mitigating surface profile deteriorating of fused silica during etching process simultaneously. The study is expected to contribute to the practical application of dynamic etching for mitigating laser induced degradation of fused silica optics under UV laser irradiation.
Supercontinuum has great potential in defense applications due to its wide spectrum, high coherence and high brightness, and it has attracted more and more attention across the world especially in the visible and mid-infrared region like 3-5μm which is the atmospheric transparency window. Higher power, wider spectrum, and better spectrum flatness will be the dominant pursuit for the future development of supercontinuum. Currently silica based fiber are the dominant host for visible to near-infrared supercontinuum generation, and soft glass like fluoride fiber, chalcogenide fiber and tellurite fiber are widely used for mid-infrared supercontinuum generation due to their lower loss in the mid-infrared region. In this paper, the generalized non-linear Schrödinger equation is used to simulate the visible to mid-infrared supercontinuum generation in a tellurite fiber. A femtosecond laser at 1064 nm worked as the pump source. 1.5 μm and 2 μm lasers are generally first pump candidates to generate mid-infrared supercontinuum in tellurite glass because the zero-dispersion wavelength of the tellurite glasses is around 2.15 μm. However, 1064 nm laser has more advantages in application in terms of cost, structure, and power scaling, so it is meaningful to investigate whether 1064 nm laser can pump tellurite fiber to generate supercontinuum with wide bandwidth. The simulation results show that 500 nm-5000 nm supercontinuum can be generated in a tellurite fiber with less than 10 kW peak power for the pump laser, and the length of the tellurite fiber is only several millimeter. The simulation results provide important guidance for future supercontinuum development.
In order to study the damage characteristic of the contaminated resonating mirror in high power continuous wave (cw) laser system, we established a theoretical model based on the optical transmission theory with a gain medium. The optical propagation in the cavity is calculated utilizing a Fast Fourier Transform (FFT) repeatedly until the convergence of the calculations tend to a steady-state oscillation mode pattern. The influence of the contaminant size, the contaminant number and the cavity structure on the damage characteristic of the resonating mirror is studied in the theoretical model.
The laser induced damage is a troublesome issue in the application of optical mirrors, which is related to the robustness of the whole laser system. There are two types of mechanisms about the damage of optical mirrors: thermal effect and field effect, which are responsible for the high energy continuous wave (cw) laser induced damage and the high power pulsed laser induced damage, respectively. Under the irradiation of high energy laser, the contaminant on the mirror surface absorbs the laser energy and converts the laser energy to heat. With the heat accumulating, the optical mirror is likely to fuse and even be totally destroyed. The temperature of the contaminant was measured when it was irradiated by a cw high energy laser with power intensity 3.3kW/cm2. It is found that the contaminant achieves thermal equilibrium in a few seconds and then the temperature stays at ~1700K. A physical model was established to describe the process of the thermal equilibrium. The influence of the contaminant size on the thermal damage of the optical mirror was studied theoretically. The results show that the contaminant size plays an important role in the thermal damage of the optical mirror. Only when the contaminant size is smaller than a critical size (~10μm), the contaminant may reach thermal equilibrium and the optical mirror works well in the high energy laser system. If the contaminant size is quite large (<~100μm), the optical mirror will damage under the irradiation of high energy laser.
Photonic crystal fiber has been widely used in visible and near-infrared
supercontinuum generation due to its flexible dispersion control and high nonlinearity.
However, the maximum average output power of supercontinuum from a photonic crystal
fiber has not exceed one hundred watt owing to the small core diameter of the photonic
crystal fiber and the low coupling efficiency between the pump and the photonic crystal fiber.
Recently, supercontinuum generation directly from a nonlinear fiber amplifier attracts lots of
attention as a result of its simple structure and low splicing loss and many excellent results
have been achieved either in low and high average power, which is proved to be a promising
method to realize kilowatt level high power near-infrared supercontinuum. However, the
numerical study on high power near-infrared supercontinuum generation from a nonlinear
fiber amplifier has been rarely reported, so there is great necessity to carry out some
theoretical study on it. In this paper the complex Ginzburg-Landau equation is used to
describe the formation and propagation of high power near-infrared supercontinuum
generation in a nonlinear fiber amplifier. The chromatic dispersion of the ytterbium-doped
fiber is measured by a Mach-Zehnder interferometer. The roles of the small signal gain, input
pulse width and initial chirp of the input pulse played on the continuum formation are
analyzed in detail. The results are in good agreement with the experiments which can provide
some theoretical guidance on future optimization of the flatness and width of the
supercontinuum generation from a nonlinear fiber amplifier.
We achieve a surprisingly exact agreement between the simulation results and the experimental observations (J. C.
Travers et al. Opt. Express 16, 14435-14447(2008)) of visible supercontinuum (SC) generation pumped by a 400W
single mode continuous wave (CW) fiber laser. The propagation of CW field in photonic crystal fibers (PCF) is
numerically simulated by solving the generalized nonlinear Schrödinger equation with the adaptive split-step Fourier
method for high accuracy; moreover, we apply the method of averaging the generated spectra in many simulations to
approach the experimental results. Our simulations confirm that the extension of SC toward short wavelengths is due
to the trapping of dispersive waves by redshifted solitons.
Semiconductor saturable absorber mirror (SESAM) is widely used in both fiber and solid state lasers for passively
mode-locking in which two lenses are usually used to realize the coupling system between an SESAM and a fiber or
solid material. The method of optical propagation matrix is used to calculate the changes of the diameter of a Gaussian
beam in such a coupling system, the results show that the beam diameter could increase or decrease several times by
properly choose the focal lengths of the two lenses and the space between them. As a result, the change of the average
mode area inside the SESAM is fulfilled. This method has a great significance in reality and greatly increased the
flexibility of a given SESAM with fixed saturation fluence in different type of lasers and laser configurations.
we propose the improved dual-wavelengthpumped scheme based on the former work (C.Xiong et al, Journal of lightwave tech. 27, 1638-1643(2009)), which can obviously increase the power at the visible wavelengths and effectively improve the flatness of spectra by solving the Generalized Nonlinear Schrödinger Equation with the
adaptive split-step Fourier method and simulating
supercontinuum generation.
Retrieval of images containing a given object from a large image database has attracted much attention from various
researchers. These images may have been taken from different viewpoints under various imaging parameters with
different backgrounds, and the target objects might be occluded. In order to solve the problem, a new method has been
proposed in this paper. Firstly, some preliminary works of our method have been introduced. After features extracted
from images, a recursive self-organizing mapping tree can be used to save the similarities between them. And the classspecific
hyper-graphs have been used to index images belonging to same objects. When retrieving images, the RSOM
tree can be used to learn how many common features there are between the querying image and the images in the
database, and the response images could be sorted by these similarities. Using the response images from earlier queries
as the new querying inputs, all responses can be retrieved with the generic query expansion strategy recursively.
Subsequent re-queries can be simplified with all images indexed by CSHGs. There is innate concurrency in the method
proposed, which makes it suitable for large scale applications such as Internet services.
Numerous methods have been applied in automatic target recognition (ATR) systems now, and a lot of factors can impact the system robustness and the recognition ratio. There is always a need for a system structure to adapt different recognition algorithms and provide rapid performance evaluation and comparisons of these algorithms. In this paper, a hierarchical modular structure for automatic target recognition system is brought forward. With the applying of the hierarchical target recognition method, a complicated multi-class recognition problem can be disintegrated into simpler recognition problems with fewer classes in different layers. Meanwhile, an ATR system with the modular structure consists of relatively stand-alone modules that implement different functions such as data acquisition, feature extraction and target recognition. Different algorithms can be implemented, providing optional modules for the system construction. Designers can easily choose between algorithms, adjust and optimize these modules respectively and provide the optimal design for the whole system.
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