Compression of Chinese document images based on morphologic analysis and pattern matching

We propose a highly efficient content-lossless compression scheme for Chinese document images. The scheme combines morphologic analysis with pattern matching to cluster patterns. In order to achieve the error maps with minimal error numbers, the morphologic analysis is applied to decomposing and recomposing the Chinese character patterns. In the pattern matching, the criteria are adapted to the characteristics of Chinese characters. Since small-size components sometimes can be inserted into the blank spaces of large-size components, we can achieve small-size pattern library images. Arithmetic coding is applied to the final compression. Our method achieves much better compression performance than most alternative methods, and assures content-lossless reconstruction. (c) 2006 Society of Photo-Optical Instrumentation Engineers.

Study of noncollinear quasi-phase matching optical parametric amplification with group-velocity matching based on periodically poled crystals

The properties of noncollinear optical parametric amplification (NOPA) based on quasi-phase matching of periodically poled crystals are investigated, under the condition that the group velocity matching (GVM) of the signal and idler pulses is satisfied. Our study focuses on the dependence of the gain spectrum upon the noncollinear angle, crystal temperature, and crystal angle with periodically poled KTiOPO4 (PPKTP), periodically poled LiNbO3 (PPLN), and periodically poled LiTaO3 (PPLT), and the NOPA gain properties of the three crystals are compared. Broad gain bandwidth exists above 85 nm at a signal wavelength of 800 nm with a 532 nm pump pulse, with proper noncollinear angle and grating period at a fixed temperature for GVM. Deviation from the group-velocity-matched noncollinear angle can be compensated by accurately tuning the crystal angle or temperature with a fixed grating period for phase matching. Moreover, there is a large capability of crystal angle tuning.

Investigation of near-collinear degenerated quasi-phase matching optical parametric amplification using PPKTP crystal

The gain properties of near-collinear degenerated phase-matched optical parametric amplification (OPA) using PPKTP crystal are investigated theoretically. The results indicate that the type-0 phase matching of PPKTP has larger accepted angle and better gain spectrum by tuning crystal temperature or rotating crystal angle.

Multiple quasi-phase-matching for enhanced generation of selected high harmonics in aperiodic modulated fibers

A technique for enhanced generation of selected high harmonics in a gas medium, in a high ionization limit, is proposed in this paper. An aperiodically corrugated hollow-core fiber is employed to modulate the intensity of the fundamental laser pulse along the direction of propagation, resulting in multiple quasi-phase-matched high harmonic emissions at the cutoff region. Simulated annealing (SA) algorithm is applied for optimizing the aperiodic hollow-core fiber. Our simulation shows that the yield of selected harmonics is increased equally by up to 2 orders of magnitude compared with no modulation and this permits flexible control of the quasi-phase-matched emission of selected harmonics by appropriate corrugation. (c) 2007 Optical Society of America.

Phase-matching control of high-order harmonic generation in a two-color laser field

The phase-matching condition of high-order harmonic generation driven by intense few-cycle pulses could be controlled by adding second-harmonic pulses to change the ionization fraction of the gaseous medium. The harmonic generation efficiency could be improved by moving the phase-matching point with an all-optical control of the ionization fraction or a proper change of the confocal parameter. A specific order of harmonics could be easily controlled to reach phase matching at a fixed higher gas pressure by adding second-harmonic pulses with a suitable intensity. Such an all-optical phase-matching control was demonstrated to be dependent upon the temporal delay between the fundamental-wave and second harmonic pulses.