Aiming at the precise measurement of numerical aperture which is a key optical parameter of gradient index (GRIN) lens. An incremental measurement method based on image recognition is proposed. Based on the definition of numerical aperture of GRIN lens by geometrical optics, the sinusoidal value of the maximum receiving angle is measured by imaging method. Incremental measurement means the GRIN lens images the light-emitting mesh twice, and calculates the sinusoidal value of the maximum receiving angle with the object heights of two images and the relative displacement. Two object heights are obtained by computer image recognition algorithm. According to the characteristics of mesh and the edge of field of view on the image, the complete object height in the longitudinal truncation is extracted which includes the central object height and the edge object height. The central height is determined by all the fringes on the truncation line, and the edge height that equals to the distance between the edge of field of view and the outermost fringe is determined by the extrapolation algorithm. The error analysis of the measurement system is carried out by establishing the error model, and the main source of the system error is determined. The experiment proves the stability of the sub-measurement method meets the practical needs. The measurement of the numerical aperture of a GRIN lens, which combines image recognition with incremental measurement, can reduce the measurement error caused by absolute measurement and discrimination of our eye, and it is simple, fast and accurate.
In order to study the spatial wavelength division diffraction effect, the Chirped Volume Bragg Gratings (CVBG) based on the Photo-thermo-refractive glass on the oblique incident light was analyzed by the fundamental matrix method. When diffracted by grating, the complex beam would be separated into multiple beams in space by wavelength. The diffraction efficiency of the separated optical wave can reach up to 90%. Furthermore, the diffraction efficiency, increases as the refractive index modulation depth increases and decreases as the chirp rate increases. The diffraction spectrum has the characteristics of flat-top band pass. To increase the spatial separation distance, multiple schemes of CVBGs in parallel combination are designed. By combining a self-focusing lens array, the proposed system can realize wavelength division multiplexing with bandwidth less than 0.4nm.
A collimating system is designed by two orthogonal aspheric cylindrical lenses, which is used to collimate the the output beam of the Laser Diode (LD). The theory of light transmission is used to analyze the optical collimating system. The theoretical expressions are derived and the numerical results are obtained. The nonlinear Levenberg-Marquardt (LM) fitting algorithm based on the trust-region rule is applied to the surface shape evaluation. Residual analysis is used to test the rationality of the model hypothesis, and a simple analysis is made on the source of the error and the data optimization process. And the optical simulation of model optimization results is established. Simulation results show that the semidiverging angles of 20° and 9° for the fast and slow axes of semiconductor laser beam are both reduced to 0.05 mrad after passing the collimation system which is much better than that of a normal cylindrical lens collimating system 3 mrad. A uniform circular spot can be formed in the far field by adjusting the distance between the two aspheric lenses.
An experimental method is used to measure the optical parameters of a GRIN lens. The intrinsic properties of the lens are well characterized by measuring the intercept values of the different GRIN lenses. Firstly, the intercept equation is derived from the transmission matrix of the GRIN lens, and the measurement method is described in detail. Secondly, we design and make the optical path measuring system. Finally, the error analysis of the experimental results is presented, which shows the feasibility of the working principle and the experiment operation. The principle and equipment of this measuring method are relatively effective, which affords great practical significance for the measurement of the GRIN lens.
The traditional measurement for Verdet constant of magneto-optic glass is to measure the Faraday rotation angle indirectly by using optical power meter, which is based on the Faraday rotation effect. There is a relatively large measurement error, because the method neglects the influence of the extinction ratio of polarizer and the axis angle. In our research, a method based on Jones matrix and simulation is studied and used to improve the measurement accuracy. The extinction ratio of polarizer and the axis angle are discussed during the measurement. The experiment system is built for measuring the Verdet constants of ZF7 glass and Ce3+/Tb3+ co-doped glass, by using the He-Ne laser light source, and different polarizers. The results show that the changes of the polarizer’s extinction ratio mainly affected the Verdet constant measurement. The lower extinction ratio, the larger measurement error. The extinction ratio more than 1:1000 for the polarizer is necessary for an accurate measurement, which has important value in terms of reference and guidance for the accurate measurement of Verdet constant of magneto optic glass.
In this paper, we have fabricated the tellurite glass (70TeO2-20ZnO-5Al2O3-5La2O3, mol%, TZAL) by using high temperature melting method. Considering the material dispersion, the dispersion properties of TZAL glass photonic crystal fiber (PCFs) for various structures are analyzed and precisely described based on the vector finite element method (FEM). A novel structure with three-ring TZAL Glass PCF is proposed by introducing large elliptical holes in the inner ring. The simulation results show that ZDW decreases from 1.586μm to1.485μm when the numbers of elliptical holes increases from two to six. ZDW of horizontally disposed ellipses of PCF is a litter lower than that of vertically positioned ellipses. Furthermore, with optimizing parameters of pitch period and diameter of air hole, ZDW is reduced to 1.396μm. The dispersion characteristics can be flexibly designed and adjusted. The approach and results can be guidance for design, manufacture the photonic crystal fibers.
Aiming to the rectangle beam shape in most high power laser, the influences of the several kinds of geometrical aberrations
in slit spatial filtering system have been simulated based on the diffraction theory of aberration. The aberration tolerances
of all kinds of geometrical aberrations in the lenses and incident beam are obtained.
Angular filtering with volume Bragg gratings in photothermorefractive glass was described. The coupled-wave theory was used to discuss the angular filtering based on Transmission Volume Bragg Gratings (TBGs). The results showed that the cutoff frequency was improved, the refractive index modulation had a greater impact on the output beam and the deviation from Bragg condition for incident beam should be less than 0.1mrad. The influence on output beam quality with TBGs parameters was analyzed. Experimental results showed that the near-field distribution of output beam through the angular filtering is better than that of the incident laser beam, and the low-frequency loss of near-field angular filtering is less than 1.8%.
Two-dimensional coupled wave theory is used to calculate spatial dispersion of the diffraction pulse through volume
Bragg grating (VBG). Results indicate that the spatial dispersion, which describes the spatial chirp and reflects globally
how far different frequency components separate, is influenced by the grating parameters, wavelength and the
propagation distance. To two ultrashort pulses with different durations (100fs, 20fs), the degree of spatial broadening
through the VBG and the propagation process is both presented. The approach provides a theoretical foundation to
design proper volume Bragg gratings for redressing the spatial chirp in the ultrashort-pulse laser system.
A novel structure of single-polarization single-mode photonic crystal fiber (SPSM-PCF) is proposed by introducing two large semicircular holes on both sides of the core region. The characteristics of SPSM fibers are numerically analyzed using the finite-element method. The SPSM operating range can be adjusted with the parameters of the elliptical ratio γ and the spacing Λ 1 between the semicircular hole and the adjacent circular hole. The SPSM bandwidth of about 0.7 μm and operating range from 1.0 to 1.7 μm for the PCF have been obtained with the silica–air holes structure. An all-solid PCF structure is proposed, and the SPSM bandwidth of about 0.30 μm is predicted. With the increase in refractive index difference between the substrate and filling materials the SPSM operating range is blue-shifted, and with the decrease in the refractive index difference the bandwidth of the SPSM is reduced.
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