For the nonlinearity of Fabry—Perot interferometer (FPI) transmission spectrum, the measurement uncertainty of incoherent Mie Doppler wind lidar based on it increases evidently with the increase of backscattering signal Doppler shift. A method of repeating the use of the approximate linear part of FPI transmission spectra for reducing the high uncertainty of a big Doppler shift is proposed. One of the ways of realizing this method is discussed in detail, in which the characteristics of FPI transmission spectrum changing with thickness and incident angle are utilized simultaneously. Under different atmosphere conditions, it has been proved theoretically that the range of measurement uncertainty drops to one-sixth while its minimum has no serious change. This method can be used not only to guide the new system design, but also as a new working way for the fabricated system.
The delay theoretical analysis result of single-longitudinal-mode pulse injected in active Fabry-Perot cavity has been obtained. The gain in Fabry-Perot cavity has been calculated and the numerical simulation result of output pulse has been obtained. The results show that when the energy of the pulse decreases at a certain value, the gain produced by increased pump particle population can compensate the loss in the cavity and the delay pulse can achieve steady state. In experiment, the active Fabry-Perot cavity is a Nd: YAG laser sided pumped by laser diode. For the pulse inputed externally, we achieve 140 pulses within 2 ups. Finally, the reference light is delayed using the active Fabry-Perot cavity. And we show the heterodyne experiment of signal light whose frequency shift generated by an acousto-optic modulation (AOM), and the experiment results accord with the frequency shift of the AOM and the error is less than 4%.
In this work we investigate damages on graphite surface gercerated by bombardment with 530 KeV Au+ or 4.5 MeV Au++ at a dose of 1×1012 cm-2. The most significant damages are hillocks with a diameter around 1. 8 nm, as seen from our STM images. Very often they are surrounded by variable 31/2×31/2R30° superstructures of graphite, which die away within a distance of 1 to 3 nm. The density of the hillocks is in good agreement with the ion dose, indicating that they were induced by individual incident gold ions. Besides, in the STM images of higher magnifications there exist many smaller hillocks, which might be caused by the secondary ions or atoms.
The individual damage on high oriented pyrolytic graphite surface induced by gold ions of 530keV—4.5MeV was studied with scanning tunneling microscope. The hillock-like damage was observed and measured and then the average diameters of the hillocks were obtained. In terms of a modified nonlinear thermal spike model, the diameters of the damage were calculated and the results fit well with the experimental data. The possible mechanism of damage formation is discussed.
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