Evolution of phase singularities is studied when a laser beam propagates through an inhomogeneous turbulent atmosphere. The simulation results reveal that, when the laser beam propagates along an uplink turbulent atmosphere path, the density of phase singularities (DPS) stars at zero and then grows rapidly with the increasing of the propagating height. When the beam propagates to a certain height, the DPS reaches its maximum and then begins to decrease. When the laser beam propagates along a downlink turbulent atmosphere path, the functional form of the DPS has a shape of monotone increasing cure with the decrease of the propagation height and reaches its maximum near the ground.
In the paper, a 45 degree TFBG was fabricated in photosensitive fiber successfully using phase mask technique. The polarization-dependent loss characteristic of the TFBG was experimentally researched in the paper using a special measurement system. The measurement results showed that the 45 degree TFBG could act as a polarization possession element. Based on the 45 degree TFBG, a linearly-polarized Yb-doped fiber laser was demonstrated. The polarization-extinction ratio of the output laser is about 30 dB. The output power was about 13 mW with the pump power of 100 mW. The central wavelength of the laser is 1064nm and the wavelength bandwidth was about 0.7nm. Being a polarization device, the TFBG has the advantages of in-fiber, compact, good polarization capability and low price.
A 974 nm laser diode(LD) pumped 1120 nm ytterbium(Yb)-doped fiber laser has been experimentally demonstrated, which will be subsequently used to pump 1178 nm Raman fiber laser based on stimulated Raman scattering(SRS) effect. The resonator composes a normal single-mode Yb-doped fiber and a pair of fiber Bragg gratings(FBG). The maximum output power of 8.4 mW is achieved when the launched pump power is 183.7 mW with an optical-to-optical conversion efficiency of 4.6 %. The central wavelength is 1120.93 nm with a linewidth as narrow as 0.02 nm.
Numerical experiments are carried out about the temporal and spatial variation of branch points by four-dimension code of laser propagating in atmosphere. The theory of branch-point detection and phase reconstruction is introduced. The act of branch points' creating and annihilating is emulated when the light wave propagating in atmosphere. The evolvement of branch points in some propagating range with time is emulated, too. The behavior of branch points in the distorted optical field is simulated when the main laser and beacon laser propagate in the atmosphere with opposite direction at the same time. The work could provide a reference for further study of laser propagation through atmosphere and adaptive optics system.
In this paper, a pulse stretcher based on multilayer volume holographic gratings(MVHG) is shown. The diffraction
properties of the pulse stretcher under ultrashort pulse are investigated based on the modified multilayer coupled wave
theory. The spectral intensity distributions of the diffracted beam are calculated. The diffraction bandwidth, pulse
duration, pulse expansion and the total diffraction efficiency of the pulse stretcher are also analyzed. The pulse
broadening is accomplished by adjusting the width of the intermediate layer of a system of MVHG. The calculation
results show that using this new pulse stretcher system to broaden pulse has many advantages: the efficiency of
diffraction is high, the structure of stretcher is adjustable to vary the amount of temporal broadening of the light pulse,
and the structure is also more compact than alternative approaches.
In this paper, the pulse shaping and diffraction properties of multilayer reflection volume holographic gratings under
ultrashort pulse with arbitrary shapes in time are investigated using the modified multilayer coupled wave theory. Bragg
diffraction of a system of multilayer reflection volume holographic gratings (MRVHG) is derived, and simple analytical
expressions for the spectrum and spatial profiles of the transmitted and diffracted beams are obtained. Numerical results
of the pulse shaping and diffraction properties of this system are also illustrated for several different temporal shapes.
Results show that the temporal shapes of the input pulsed beams have been found to be an important factor in the
analyses of the pulse shaping and diffraction properties of MRVHG. The analysis and observations of this paper will be
valuable for optimizing the design and for novel applications of optical elements based on multilayer volume gratings.
Exposure dose control technique in step-and-scan exposure lithography systems is discussed. The dose control principle for step-and-scan system is analyzed in depth. A dose control algorithm is proposed. Measurements of dose accuracy and repeatability are made on an experiment setup. Dose accuracy of 1.37% and dose repeatability of 0.31% are obtained using this dose control technique. Experiment results indicate that this dose control technique meets the requirement of sub-half-micron lithography.
An effective and simple method to determine the energy range of FOCAL is described in this paper. Relationship between the chop line width and defocus is analyzed. Simulated curves of the chop line width versus defocus are obtained by PROLITH. By choosing the curves which satisfy certain conditions, the energy range of FOCAL is determined off line. Independent of the lithographic tool, the method is time-saving and effective. The influences of some process factors, e.g. resist thickness, PEB temperature, PEB time and development time, on the energy range of FOCAL are analyzed.