Knife-edge Method Research Articles

Corrections to the knife-edge based reconstruction scheme of tightly focused light beams

The knife-edge method is an established technique for profiling light beams. It was shown, that this technique even works for tightly focused beams, if the material and geometry of the probing knife-edges are chosen carefully. Furthermore, it was also reported recently that this method fails, when the knife-edges are made from pure materials. The artifacts introduced in the reconstructed beam shape and position depend strongly on the edge and input beam parameters, because the knife-edge is excited by the incoming beam. Here we show, that the actual beam shape and spot size of tightly focused beams can still be derived from knife-edge measurements for pure edge materials and different edge thicknesses by adapting the analysis method of the experimental data taking into account the interaction of the beam with the edge.

MTF Measurement Research on Satellite Remote Sensing Images Based on the Knife-Edge Method

In-orbit testing of Modulation Transfer Function (MTF) is an important index of monitoring in-oribit operation performance of the space optical remote sensors. How to grasp the change of remote sensor imaging system in operational process accurately concerns the use of remote sensing data reasonably and effectively. We analyse the knife-edge method for testing the principle of the MTF and the simulation algorithm, and tests the accuarcy of the algorithm. The analysis results demonstrate that the algorithm of this paper has higher precision for the calculation of MTF of in-oribit remote sensing image, and can resist noise effectively. Whats more, we focus on anlaysing the main factors of the precision of edge method influenced by edge fitting. The analysis results show that the edge data need to be interpolated non-parametrically when the edge is fitting.

Effect of polarization purity of cylindrical vector beam on tightly focused spot

The tightly focused spots of cylindrical vectors (CVs) are dependent on polarization composition. We experimentally demonstrate the effect of polarization purity (PP) of the CV beam on the tightly focused spot quantitatively, which should be strictly controlled for the effective applications of the CV beam. The focal spots measured by a knife-edge scanning method showed that the azimuthally polarized (AP) component increases the transverse field and the size of the focal spots, while the radially polarized component results in a nonzero intensity distribution at the center of the focus even in a high PP AP beam.

A Precision Grazing-incidence Angle Error Measurement of a Hard X-ray Condenser Mirror Using Single-grating Interferometry

Third-generation synchrotron radiation facilities enable us to use X-rays of extremely high brightness and low emittance, which are efficiently concentrated into a nanoscale area by a focusing device. Such progress has significantly enhanced the throughput of X-ray analyses based on scanning microscopy. On the other hand, the spatial resolution of scanning microscopy directly depends on the focal spot size. Many diffractive [1, 2], refractive [3, 4], and reflective [5, 6] focusing devices are currently available for producing small focal spots. Reflective optics realize the highest throughput with a sufficiently long working distance. Furthermore, among the above-listed focusing devices, reflective devices produce the smallest focal spot size in the hard X-ray region. A total reflection mirror may attain sub-30 nm focal spot size [7], while a multilayer mirror may attain sub-10 nm focal spot size [8]. The achievable focal spot size is significantly degraded by errors in the mirror shape and misalignment of grazing-incidence angle. In the hard X-ray region, the focusing mirror surface requires accuracies below 2 nm PV and 1 nm PV for sub-30 nm and sub-10 nm focusing, respectively. The error in the grazing-incidence angle must not exceed 0.5 × 10−6 rad and 0.1 × 10−6 rad for sub-30 nm and sub-10 nm focusing, respectively. Current advances in fabrication and mirror testing technologies have realized highly accurate deterministic figuring processes that can meet these requirements. Fabrication techniques include ion beam figuring [9, 10] and elastic emission machining (EEM) [11], while figure testing is achieved using optical interferometers [12, 13] or slope profilers [14, 15, 16]. In contrast, alignment accuracy is still evaluated by conventional beam profiling methods such as knife-edge scanning methods. The alignment is optimized by an iterative procedure of beam profiling and glazing-incidence angle adjustment. This procedure is very time-consuming and frequently introduces a significant profiling error from shape imperfections and/or vibration of the object to be scanned. Accordingly, the grazing-incidence alignment often determines the achievable focal spot size. The error in the grazing incidence angle generates a coma aberration, which is essentially a wavefront distortion with a cubic function. Hence, adjusting the grazing-incidence angle by monitoring the wavefront shape is possible. The wavefront shape can be characterized by several methods, such as phase retrieval [17] or ptychography [18, 19]. However, to obtain a wavefront shape of accuracy smaller than λ/4, the limit imposed by Rayleigh's criterion, large amounts of intensity information are required. In addition, these methods are unsuitable for the shot-by-shot measurements demanded in X-ray free-electron laser (XFEL) focusing.

A diode-end-pumped long-pulse-width acoustic–optical Q-switched Tm:LuAG laser at room temperature

We report a diode-end-pumped long-pulse-width acoustic–optical Q-switched Tm:LuAG laser at room temperature. The output energy for a single pulse of 27.5 mJ is obtained under incident pump energy of 208.0 mJ, corresponding to a slope efficiency of 20.4%. For the Q-switched regime, the maximum pulse energy of 3.7 mJ was achieved with the pulse width of 204.1 ns at repetition of 100 Hz. The wavelength of the Q-switched laser was 2.023 μm. A beam quality factor of M2 = 1.5 was measured using the traveling 90/10 knife-edge method.

Gaussian beam radius measurement with a knife-edge: a polynomial approximation to the inverse error function

A method for approximating the inverse error function involved in the determination of the radius of a Gaussian beam is proposed. It is based on a polynomial inversion that can be developed to any desired degree, according to an a priori defined error budget. Analytic expressions are obtained and used to determine the radius of a TEM(oo) He-Ne laser beam from intensity measurements experimentally obtained by using the knife edge method. The error and the interval of validity of the approximation are determined for polynomials of different degrees. The analysis of the theoretical and experimental errors is also presented.

An All Fiber 1.94 μm Nanosecond Pulse Laser Amplified by Highly Tm3+-Doped Silicon Fibers

We present an all-fiber 1.94 μm nanosecond pulse laser in master oscillation-power amplifier configuration. A dual-FBGs gain-switched Tm3+-doped fiber laser is built and is used as the seed laser. The output characteristics of the amplifier are studied at different pulse repetition rates, and the maximum output energy of 500 μJ is achieved under incident pump power of 39 W. The emitting spectra are checked at 5 and 10 kHz, and no obvious nonlinear effects are observed in the spectrum of the amplified laser. The beam quality factor is M2 = 2.1 ± 0.03 measured by the traveling knife-edge method, and the laser beam spot is also monitored by an infrared vidicon camera.

Modified knife-edge method for current density distribution measurements in e-beam writers

In this paper, the authors present a modified knife-edge method for two-dimensional current density mapping in electron-beam writing systems. This method is applicable in rectangular and in variable-shaped e-beam writers with a fixed mutual position of the e-beam and the 1st shutter during exposure. In contrast to other methods, the modified knife-edge method uses only native parts of the e-beam writer, giving it the advantage of easy implementation. The measurement error of our method is analyzed and the trade-off between the result accuracy, the current density resolution and the measurement duration is discussed. The presented method is demonstrated by adjusting an electron emitter in order to select a homogeneous e-beam pattern.

Experimental investigation of a diode-pumped powerful continuous-wave dual-wavelength Nd:YAG laser at 946 and 938.6 nm

In this paper, a diode-pumped high-power continuous-wave (cw) dual-wavelength Nd:YAG laser at 946 and 938.6 nm is reported. By using an end-pumped structure, comparative experiments indicate that a 5 mm-length Nd:YAG crystal with a Nd3+-doping concentration of 0.3 at.% is favorable for high-power laser operation, and the optimal transmissivity of the output coupler is 9%. As a result, a maximum output power of 17.2 W for a dual-wavelength laser at 946 and 938.6 nm is obtained at an incident pump power of 75.9 W, corresponding to a slope efficiency of 26.5%. To the best of our knowledge, this is the highest output power of a quasi-three-level dual-wavelength laser using a conventional Nd:YAG crystal achieved to date. By using a traveling knife-edge method, the beam quality factor and far-field divergence angle at 17 W power level are estimated to be 4.0 and 6.13 mrad, respectively.

A study on the minimum detectable energy density precise measurement method of STIL

This paper presents a precise measurement method of streak tube imaging LIDAR’s (STIL’s) minimum detectable energy density and the measurement result of a K008 streak tube. A Gaussian expression was used to approximate the one-dimensional intensity distribution of the laser spot. The laser pulse energy was measured with a sensitive photon counter while the diameter of the laser spot was measured by the single knife-edge method. After determining the parameters using the Gaussian expression and calculations, the authors concluded that the minimum detectable energy density of STIL in this experiment was 5.27 ± 0.11 × 108 J mm−2.

Three-dimensional measurement of a tightly focused laser beam

The spatial structure of a tightly focused light field is measured with a double knife-edge scanning method. The measurement method is based on the use of a high-quality double knife-edge fabricated from a right-angled silicon fragment mounted on a photodetector. The reconstruction of the three-dimensional structures of tightly focused spots is carried out with both uniform and partially obstructed linearly polarized incident light beams. The optical field distribution is found to deviate substantially from the input beam profile in the tightly focused region, which is in good agreement with the results of numerical simulations.

CW and AO Q-switched operation of a dual-crystal Tm, Ho:GdVO4 laser pumped by two diodes

Continuous wave (CW) mode and acousto-optic (AO) Q-switched mode operation of a dual-crystal Tm, Ho:GdVO4 laser is reported. The dual-crystal Tm, Ho:GdVO4 laser with output wavelength of 2.05 μm was pumped by two laser diodes (LDs). The Tm, Ho:GdVO4 crystals were cooled by liquid nitrogen and pumped by two fiber-coupled LDs with a center output wavelength of 801.0 nm. A 20.5 W output power was obtained at a 255 mm physical cavity length in CW mode operation, and a 19.6 W average power was obtained at a pulse repetition frequency (PRF) of 10 kHz with a 19 ns pulse duration. Also, the efficiency loss of the laser is not more than 4.4% from CW mode to Q-switch mode, and the M2 factor, which is measured by the traveling knife-edge method, does not exceed 1.2.

Space remote sensing systems transmission capabilities modeling

Space information, or information obtained by means of space remote sensing, is widely and effectively used by many countries to solve a lot of scientific, technical and applied problems. Most manufacturers of space remote sensing systems declared the high resolution values of their systems. However, these values are computed theoretically, without considering the various factors affected them. To determine the real resolution of the system, we have considered mathematical modeling which describes the influence of different factors on the satellite images resolution. Some of these factors are: atmosphere turbulence, image shift, residual defocusing, and diffraction. One of the most important characteristic of the images resolution is the modulation transfer function (MTF) which allows the estimation of different factors affected on the image resolution. The modulation transfer function (MTF) is a fundamental tool for assessing the performance of imaging systems. Various authors [Zhang et al. 2012, Hwang et al. 2008, Ryan et al. 2003] investigate different MTF assessment methods of high resolution satellite images: a slant-edge method, a knife-edge method, a sine wave method and a grill pattern. We propose a generalized approach for MTF assessment based on theoretical assumptions which allows to determine the influence of different factors. A comparative analysis of the modulation transfer function(s) for different space imaging systems shows that the image resolution depends mainly on the atmosphere turbulence and size of a sensor element. Additionally, we established that atmospheric turbulence significantly reduces the transmitting possibility of images. The parameters which describe the influence of turbulence required additional studies. The main goal of our researches is to show that real spatial image(s) resolution is much “inferior” than the value provided by the manufacturers of space remote sensing systems.

Production of quasimonochromatic X-ray microbeams using MeV-protons and a polycapillary X-ray half lens

In this paper, we have proposed monochromatic X-ray microbeams, produced by a proton-induced X-ray technique and a polycapillary X-ray half lens, as a tool for micro-X-ray fluorescence (XRF) analysis. A 30 μm thick planar Cu target was irradiated by a 2.5 MeV proton beam to produce Cu Kα X-rays (8.0 keV), and a polycapillary X-ray half lens was utilized to focus the X-rays emitted behind the Cu target. The focal spot size of the focused X-ray beam was 250 μm at full width at half maximum, which was verified using a knife-edge scanning method. The output focal distance and the depth of focus of the optics were measured to be 47 mm and 1 mm, respectively. A square grid pattern of Co thin films, formed on a thick Cu substrate by thermal evaporation, was used as a test sample for evaluation of the analytical performance of the micro-XRF setup. Two-dimensional mapping of the Co distribution on the Cu substrate was successful, and the spatial resolution was consistent with the beam spot size. For this Co layer, a minimum detection limit of 2.3 ng was achieved.

Foreign Object Detection by Sub-Terahertz Quasi-Bessel Beam Imaging

Food quality monitoring, particularly foreign object detection, has recently become a critical issue for the food industry. In contrast to X-ray imaging, terahertz imaging can provide a safe and ionizing-radiation-free nondestructive inspection method for foreign object sensing. In this work, a quasi-Bessel beam (QBB) known to be nondiffracting was generated by a conical dielectric lens to detect foreign objects in food samples. Using numerical evaluation via the finite-difference time-domain (FDTD) method, the beam profiles of a QBB were evaluated and compared with the results obtained via analytical calculation and experimental characterization (knife edge method, point scanning method). The FDTD method enables a more precise estimation of the beam profile. Foreign objects in food samples, namely crickets, were then detected with the QBB, which had a deep focus and a high spatial resolution at 210 GHz. Transmitted images using a Gaussian beam obtained with a conventional lens were compared in the sub-terahertz frequency experimentally with those using a QBB generated using an axicon.

Focusing of soft X-ray radiation and characterization of the beam profile enabling X-ray emission spectrometry at nanolayered specimens

For the analysis and speciation of nanoscaled materials, an efficient detection in high-resolution X-ray emission spectrometry is needed. Therefore, the focusing of monochromatized soft X-ray undulator radiation with a single bounce monocapillary down to micrometer range is reported. For a photon energy of 1060eV, spot sizes of 10μm–15μm retaining 34% of the incident beam radiant power could be obtained. The beam profile was characterized by two complementary methods, the knife-edge method and the so-called wire method, where a thin wire is scanned through the focused beam. With the focused beam, the efficiency of the wavelength-dispersive spectrometer could be significantly increased, which enables the X-ray emission spectrometry analysis of nanoscaled materials.

Measurement limitations in knife-edge tomographic phase retrieval of focused IR laser beams

An experimental setup to measure the three-dimensional phase-intensity distribution of an infrared laser beam in the focal region has been presented. It is based on the knife-edge method to perform a tomographic reconstruction and on a transport of intensity equation-based numerical method to obtain the propagating wavefront. This experimental approach allows us to characterize a focalized laser beam when the use of image or interferometer arrangements is not possible. Thus, we have recovered intensity and phase of an aberrated beam dominated by astigmatism. The phase evolution is fully consistent with that of the beam intensity along the optical axis. Moreover, this method is based on an expansion on both the irradiance and the phase information in a series of Zernike polynomials. We have described guidelines to choose a proper set of these polynomials depending on the experimental conditions and showed that, by abiding these criteria, numerical errors can be reduced.

High Power Q-Switched Dual-End-Pumped Ho:YAG Laser

We report the high power acousto-optically Q-switched operation of a dual-end-pumped Ho:YAG laser at room temperature. For the Q-swithched mode, a maximum pulse energy of 2.4 mJ and a minimum pulse width of 23 ns at the repetition rate of 10 kHz are achieved, resulting in a peak power of 104.3 kW. The beam quality factor of M2 ∼ 1.5, which is demonstrated by a knife-edge method. In addition, the Ho:YAG laser is employed as a pumping source of ZGP optical parametric oscillator, and its total average output power is 13.2 W at 3.9 μm and 4.4 μm with a slope efficiency of 68.4%.

Experimental study on the widely tunable performances of Tm:YAP laser under different output transmittance

Using by different output transmittances of 2%, 5%, and 10%, we have investigated the continuously tunable laser performance of c-cut Tm:YAP crystal. The 5-mm long c-cut crystal with 4 at.% thulium ion was used. Without a tuning element, the slope efficiency respect to the absorbed pump power reached 44.1% with transmittance of 5%, corresponding to the conversion efficiency of 35.3%. The laser operated at TEM00 mode with a beam quality factor of M2 ∼ 1.3, which was demonstrated by a 90/10 knife-edge method. A 3-mm thick birefringent plate was used as a tuning element. At transmittance of 5%, the widest tunable range of 159 nm extended from 1899 to 2058 nm. For transmittances of 2% and 10%, the tunable ranges of 150 and 68 nm were achieved, respectively.

탄소 나노섬유/PVDF-HFP 복합재로 코팅된 실크 직물의 물성

Abstract: Silk fabrics were coated with carbon nano fibers (CNFs)/poly(vinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP) composite solution by the knife edge method. The various contents of CNFs/PVDF-HFP coated silk fab-rics were investigated through their morphological, mechanical, and electrical properties. In the results regarding surfacemorphology, it was shown that the CNFs were evenly distributed in coated silk fabrics through SEM images; the exist-ence of CNFs in coated silk fabrics was confirmed. For the results regarding the mechanical properties, the tensile lin-earity and compression linearity improved as CNFs contents increased. Also, the electrical properties of CNFs/PVDF-HFP coated silk fabrics increased with 4% CNFs contents. Therefore, the results confirm the ability of conductive tex-tiles by using CNFs/PVDF-HFP composite-coated silk fabrics.Keywords: carbon nanofibers, poly(vinylidene fluoride-hexafluoropropylene), conductive silk fabrics, mechanical property,electrical property