Yi Qu

Broadband FUV imaging spectrometer: advanced design with a single toroidal uniform-line-space grating.

Performances of a far-ultraviolet (FUV) imaging spectrometer in an advanced design are presented with a toroidal uniform-line-space (TULS) grating. It provides high spatial resolution and spectral resolution for a broadband and a wide field of view. A particular analysis for the grating aberrations, including all the high-order coefficients neglected by previous existing designs, was generated for indicating their significance. The analysis indicates that these high-order off-axis aberrations would have a remarkable influence on the design results. The transcendental equations composed of these aberration coefficients do not have analytic solutions in algebra. To solve the problem, the past designs always do some simplified calculation which only suits a narrow field of view and waveband. Thus, the optimization of the genetic algorithm is introduced to propose reasonable ranges of optical parameters. Then ZEMAX software is used to obtain the final optical system from these ranges. By comparing different design results of the same example, our advanced TULS design performs better than conventional TULS design and spherical varied-line-space grating design, and as well as the toroidal varied-line-space design. It is demonstrated that aberrations are minimized when the TULS design is operated by our method. The advanced design is low-cost, easy to fabricate, and more suitable for FUV observations.

Highly efficient four-wave mixing induced by quantum constructive interference in rubidium vapour

We demonstrate efficient four-wave mixing with an intensity conversion efficiency of nearly 100% in theory without considering the Doppler-broadening effect in a four-level double-Λ system of hot 87Rb gas. The corresponding experimental value of about 73% was reported in our earlier work under the same conditions. This dramatic efficiency is critically dependent on the constructive interference between two four-wave mixing processes relevant to the internally generated four-wave mixing signal.

Study on spectral broadband aberration-corrected imaging spectrometer for far-ultraviolet waveband

In order for the Czerny-Turner spectrometer to be utilized in the imaging spectrometer for far-ultraviolet waveband, we describe the elimination of the principal aberration coma and astigmatism for the arrangement by light-path function analysis. The modified structure is built by a spherical mirror, a toroidal mirror, and a fixed plane grating. Starting with coma and astigmatism corrected by adjusting the incident angle to the collimating mirror and using a toroidal mirror, we obtain the optimized distances between the grating to the focusing mirror and the focusing mirror to the image plane by geometric analysis and first order differential calculation, in order to acquire high imaging quality in broad waveband. The design result shows a simple optical imaging structure that can be used in the ionosphere detection (120 to 180 nm) that is superior and convenient.

Intensity-dependent effects on four-wave mixing based on electromagnetically induced transparency

We extend the study on a four-wave mixing (FWM) scheme of contiuous-wave lasers in a hot rubidium vapor when the probe and coupling fields work in the electromagnetically induced transparency (EIT) regime while the pump and signal fields work in the two-photon Raman regime. Our experimental results show that the generated signal field is well contained in an EIT dip of the incident probe field as a result of efficient FWM. We find, in particular, that an optimal FWM process can only be attained when the coupling and pump fields are well matched in intensity. If the probe intensity is far beyond the EIT condition, however, the nonlinear efficiency of energy transfer from the probe field to the signal field will be greatly reduced.

Measurements of air- and self-broadening shift coefficients of the methane R9 branch of the 2v (3) band by using a tunable diode laser

Room-temperature values for self-broadened and air-broadened coefficients of the methane R9 branch of the 2v3 band have been measured by using a tunable diode laser. The peak center position of the R9 branch shifted with a different ratio of methane and air was determined. The experimental results are compared with the calculated data from the Hitran Database 2004.

Multi-pathway all-optical wavelength conversion switching and routing via four- and six-wave mixing in hot rubidium vapour

We report an experimental demonstration of multi-pathway all-optical switching and routing via efficient wavelength conversion in hot rubidium vapour. We apply a coupling field and a control field to drive both 87Rb and 85Rb isotope atoms into either the double-Λ or triple-Λ configuration when a probe field is scanned across the D1 line of the rubidium atom. We find that this driving scheme facilitates the coherent generation of seven wave-mixing signals with different frequencies and polarizations based on four-wave mixing and six-wave mixing interactions. Also, each generated signal can be switched on and off simply by modulating the control field in frequency to optimize or suppress one specific wave-mixing interaction.

The second order diffraction efficiency measurements in the vacuum ultraviolet

A simple method for measuring the second order diffraction efficiency in the vacuum ultraviolet (VUV) is described. Spectral reflectance of a mirror will be influenced by the second diffraction order, measured in a spectrophotometer system without filters. The second order diffraction efficiency can be calculated from the different spectral reflectance values. A deuterium lamp and a scintillated photomultiplier are used in the measurement system. The second order diffraction efficiency can be determined from 120 nm to 165 nm. The result of 0.00579+/-11.9% is obtained at 161 nm.

The Research Review of the Stability of Grid-Connected Photovoltaic Generation System

The rapid development of large-scale photovoltaic power generation is an effective means to alleviate the energy crisis and saving haze. With the continuous expansion of photovoltaic grid-scale, grid access conditions are increasingly demanding. Given the inverter under different grid conditions, its parts are subject to a certain extent, and thus cause stability problems. This paper describes the influence of output filter, phase-locked loop (PLL), digital delay and weak grid on the stability of the grid-connected photovoltaic generation system, both from the inverter itself and connected grid.

Research on the Stability of Multi PV Inverters Connected to Weak Grid Based on Impedance Analysis

Abstract. In paralleled grid connected PV inverters system, when the design of the inverter control system is connected with the weak grid, the control performance will become poor, and even the operation is not stable. In this paper, a method of impedance analysis is presented, and the equivalent impedance of inverter is derived by using the small signal model of the inverter, which is used to analyze the stability of inverter parallel system. And this paper proposes that system can be stabilized by changing the phase locked loop design of the inverter. Simulation results verify the validity and feasibility of the theoretical analysis and the proposed method.