Qinghua Lv

Extended depth of field through an axicon

One of the main disadvantages of imaging systems is the limited depth of field. We present the ability of a refractive axicon for imaging with an extended depth of field. We design an imaging system with a two-step imaging approach that uses a CCD camera to capture intermediate images and uses a digital process to obtain the final images. The depth of field is analyzed based on the condition of the focal segment. In particular, the point spread functions (PSFs) are discussed in simulation in detail. The performed experiments validate the effect and feasibility of the axicon for imaging with an extending depth of field.

Design of all-normal dispersion photonic crystal fiber for high coherent broadband supercontinuum generation in the telecommunication window

An all-normal dispersion photonic crystal fiber with nearly zero flattened dispersion at 1550 nm is designed for generating high coherent broadband supercontinuum. It is found that an all normal dispersion photonic crystal fiber with nearly zero flattened dispersion at 1550 nm can be obtained by appropriately designing geometrical parameters and optimizing the index of the first ring of air-holes with filling different index liquid of the photonic crystal fiber. The results show that the optimized design photonic crystal fiber for pumping at 1550 nm is suitable for flat broadband and high coherent supercontinuum generation with only 4 kW input peak power in a 40 cm long of the photonic crystal fiber. It is also found the weaker the dispersion effect is, the more advantage to the high coherent broadband supercontinuum generation due to the self-phase modulation.

Theoretical investigation on the effect of ASE noise for amplified fiber loop ring-down gas sensing

The effect of spontaneous emission (ASE) noise produced by the erbium-doped fiber amplifier (EDFA) on the performance of amplified fiber loop ring-down gas sensing systems is theoretically investigated. The results show that the EDFA placed after the gas cell with a smaller ASE noise can improve the measurement accuracy. The more narrower the noise equivalent optical bandwidth of the optical band pass filter following the EDFA is, the up shift of baseline of ring-down signal can be more effectively suppressed and thus improving the measurement accuracy. It is also found the longer ring-down time can be obtained by reducing the input power due to the smaller gain fluctuations of the EDFA.

Imaging characteristics of the axicon imaging system

The depth of an image system can be extended by an axicon which can generate line focus. According to physical optical theory, the diffracting patterns of the defocus point spread function (PSF) for the imaging system with axicon are analyzed through the generalize pupil function. The expressions of the PSF for the imaging system illuminated by white light are described as the superposition of the intensities in individual monochromatic patterns. Experimental results show that the central portion contains the most energy of the diffraction pattern from the PSF produced by the white light, and the contrast of secondary outside circular rings decreased rapidly. Furthermore, the central spot radius varied slowly with the increase of defocus parameter, and the depth of field of the imaging system is effectively extended with a shortcoming that the images need further processing.

Theoretical analyses of non-diffracting beams interference in a long distance

The intensity distribution of diffraction field of two non-diffracting beams which interfere in a long distance is derived. Non-diffracting beam is generated by an axicon, which is then split into two coherent beams by a beam splitting prism, one of the two beams is regarded as reference beam, which meet the other one through reflector and another beam splitting prism after a long distance. The distribution of interference field is the coherent superposition of each diffraction field. Results show that the locus of interference fringes can be generally assumed hyperbola, the shape of which is affected by interference instance. When the two beams remain parallel, the diffraction pattern is analyzed to be hyperbolic moire fringe,in a case where there is an oblique angle between two beams, the trend of hyperbolic moire fringe change with the interference fringe trend without axicon. Experimental results are compared with theoretical analysis results. Good agreements between them is obtained.

Displacement measurement based on TWAIN interface

The standard of technology without an interesting name (TWAIN) was introduced and a displacement measurement method was presented. Based on the theories of image capture and image measurement, a displacement measurement system was developed and subpixel and centroid technique were used to improve the measurement precision. The Experiment results show that the proposed method has low cost and high precision.

Calibration method with grating grooves based on precision vision measurement

This paper presents a new method of high-precision calibration with grating grooves for precision vision measurement system of micro-size part. After an image of high precision grating grooves are grabbed by CCD, a black-and-white striped picture may be gotten. A group of data being similar to sinusoid is gotten by calculating the sum of gray-value of pixels in each column. These calculated data are elaborately showed a Lissajous figure in one plane by translation for pixel. The size mapping coefficient between the grating groove and their image pixels of the optical measurement system is calculated by analyzing and processing the Lissajous figure. It is not necessary to calculate the interior and exterior parameters of the camera for the calibration of measurement system when this method is adopted. Finally, this paper gives the result of calibration and its error analysis for one optical system.

A subpixel edge detection algorithm for the high-precision automatic alignment system

A new sub-pixel edge detection algorithm has been presented in this paper, which can be used in some high precision automatic alignment systems, such as automatic industrial measurement systems; integrate circuit encapsulation systems, micro-electromechanical systems, micro-optical electromechanical systems, three-dimensional encapsulation systems and so on. This algorithm uses the spatial moment to detect the location of edge within sub-pixel resolutions, and gains an advantage over the traditional pixel edge detection algorithms. Moreover, an extraction technology of region of interest is applied to reduce the time of analysis after rough pose estimation.