Zhao Yuejin

Theory of noise in a kilo-Hz cascaded high-energy Yb-doped nanosecond pulsed fiber amplifier

A theoretical analysis of noise in a high-power cascaded fiber amplifier is presented. Unlike the noise theory in low power communication, the noise of a high power system is redefined as the leaked output energy between pulses with coherent beat noise uncounted. This definition is more appropriate for high power usage in which the pulse energy receives more attention than the pulse shape integrity. Then the low power pre-amplifying stages are considered as linear amplification and analyzed by linear theory. In the high-power amplification stages, the inversion is assumed to recover linearly in the time interval between pulses. The time shape of the output pulse is different from that of the input signal because of different gains at the front and back ends of the pulse. Then, a criterion is provided to distinguish the nonlinear and linear amplifications based on the signal-to-noise ratio (SNR) analysis. Then, an experiment that shows that the output SNR actually drops off in nonlinear amplification is performed. The change in the noise factor can be well evaluated by pulse shape distortion.

The Dynamics of Rubidium Atoms in THz Laser Fields

By using B-spline basis set combined with model potential, energy levels and wave functions of rubidium atoms are obtained. Using a time-dependent multi-level approach, we study the dependence of the population of rubidium atoms excited from a low lying state to a target state on the parameters of a single or two broadband terahertz laser pulses. The population redistribution between the states n = 23 and n = 24 due to the interaction with terahertz pulses is analyzed in detail. Population oscillation in the initial and final states as a function of the time delay between two half cycle pulses is shown, and the oscillation period is the same as the Kepler period of the selected states. The calculation results agree well with the experiment and can be explained by the semiclassical picture.

Power dependence of terahertz carrier frequency in a plasma-based two-color generation process

We conduct a frequency spectrum experiment to investigate terahertz (THz) emissions from laser-induced air plasma under different laser incident powers. The frequency spectra are measured using both air-biased-coherent detection and a Michelson interferometer. The red-shift of the THz pulse carrier frequency is observed as a response to increased pump power. These phenomena are related to plasma collisions and can be explained by the plasma collision model. Based on these findings, it is apparent that the tuning of the THz carrier frequency can be achieved through regulation of the pump beam.

Population Evolution of Rydberg Rubidium Atoms by Half-Cycle Pulses

Using the B-spline basis set method combined with model potential, the Stark energy level of rubidium atoms in the vicinity of n = 30 is presented. By using a using time-dependent multilevel approach, we calculate the population redistribution of high Rydberg rubidium atoms under the interaction of external time-dependent half-cycle pulses. Our numerical results show that the population of rubidium atoms can be driven to lower or higher n levels with a train of half cycle pulses, the final population distribution of all the l states for the same n is observed after these interactions.

Calculation of Multiphoton Transition in Li Atoms via Chirped Microwave Pulse

The position and width of avoided crossings of Li atom energy levels in a static electric field is presented by using the B-spline basis set method combined with the model potential. Using the time-dependent multilevel approach, the population of Li atoms is transferred to the target state completely by one-photon, two-photon or a single multiphoton adiabatic rapid passage, which requires only a small frequency sweep. The calculation results agree well with the experiment and novel explanations are given to understand the experimental results.

The application of the wavelet transform to the continuous wave Terahertz imaging

The wavelet transform is used for restoration the continuous wave THz images which have the appearance of sinusoidal waves covering the images with standing wave patterns for the THz imaging system.

Application of half-cycle phase-stepping algorithm in eliminating or diminishing errors of phase measurement

A half-cycle phase-stepping algorithm based on the character of trigonometric function is proposed for diminished the phase delay error and azimuth error of 1/4 plate and analyzer corner error in phase measurement.

A New Development Of The Method Of Calculating The Approximating Circle In Rotating Mirror Framing Camera

This paper deduces a formula which calculates two approximating circles simultaneously. With the help of computer and optimum technology, we calculate two approximating circles parameters under the condition of restricting the angle between optical axle of relay lens and reflecting optical axle of rotating mirror, the evenness of photographic frequency and the evenness of magnifying power of relay lens to certain limits. Rotating mirror high-speed cameras have many advantages such as high efficiency, high resolution and convenient for using. Thus the cameras are used in most of high speed process laboratories. In designing this camera, some calculation methods need to be improved. According to the effect of factors on image quality, this paper obsorbs the advantages of a few methods of selecting approximating circle parameters of framing camera and seeks more reasonable ways to determine approximating circle parameters of framing camera. It makes optical axle of relay lens consistant with reflecting optical axle of rotating mirror in working angle, and also makes the out-of-focus between approximating circle and practical image plane as small as possible. Thinking above two aspects and considering the evenness of photographic frequency and the evenness of magnification of relay lens, this paper deduces the relations between relay lens approximating circle and focal plane approximating circle by using relay lens imagery. By using the method of determining two approximating circles simultaneously, it increases image quality of image point in axle and off axle.

The Amount Of Information Of High-Speed Rotating Mirror Photography System

Based on the Schardins formula and the camera designing procedures, the expression of amount of information for high-speed rotating mirror photography system is obtained. This paper proposes a new concept of evenness of amount of information and takes the evenness of information as an additional standard which evaluates system quality. It shows that the amount of information depends on the solection of working angle scale and section shape of rotating mirror besides the maximum edge velocity of rotating mirror. As the wide application of high-speed camera, discussion of its designing method and evaluating criterion continually deepen. One of evaluating criterion of high-speed camera is amount of information. Many schlors discuss this problem from fundamental theory. Starting from the designing method, this paper discusses the amount of information and the evaluating criterion, therefore to find better evaluating criterion.