Yongsheng Zhang

Progress on high power excimer laser in NINT

Excimer laser has been shown to be efficient tools in plasma physics and material science. Recent progress on techniques of beam control in excimer laser system required for energy scaling are overviewed, Configuration and initial results of a 100J/10ns, 18 beam excimer laser system are given.

Progress on a short pulse excimer laser system with beam smoothing

A short pulse XeCl laser system is being developed for plasma physics and material science study. Partial spatial incoherence seed with short pulse is amplified by MOPA chain including three discharge pumped amplifiers and two electron beam pumped amplifiers for one beam. Final laser output of 5~10J in energy with pulse width of around 10ns has been achieved, which lays a good foundation for full scale construction.

Design and first tests of miniature K010X soft x-ray streak and single-frame camera

Description of a new K010X soft X-ray camera and the first results of it tests carried out in Russia and China are presented. In a streak mode the full sweep time for a 2 cm sweep route length on the image converter screen is from 2 ns up to 600 microseconds. In a single-frame mode the corresponding frame duration is from ~10 ns up to ~ 660 microseconds. Spatial resolution in a single frame mode was not less than 5 l.p./mm for soft X-ray radiation and not less 10 l.p./mm for UV radiation. Spatial resolution in a streak mode for soft X-ray radiation was from 5 up to 10 l.p./mm and for UV radiation on all the sweep ranges was not less than 10 l.p./mm, except for the range of 1 ns/cm where it was 5 l.p./mm. Limiting temporal resolution for UV radiation was near 10 ps and a dynamic range was 200 when full sweep time was 60 ns. The camera has small 430×115×200 mm dimensions, 5.0 kg weight and 10 VA power consumption.

Optically pumping XeF(C-A) laser and its properties

XeF(C-A) laser has been attracted attention nearly several years due to its higher saturable energy density (50 mJ/cm/sup 2/), 70/spl sim/80 nm broad gain bandwidth centered near 480 nm/sup 2/ (corresponds to the bandwidth of a transform-limited pulse of 10 fs duration), scalability to high energy, and a low level of the amplified spontaneous emission. XeF(C-A) laser can become one of the most suitable amplifier for a high-power femtosecond laser system.

The development and progress of XeCl Excimer laser system

A large angularly multiplexed XeCl Excimer laser system is under development at the Northwest Institute of Nuclear Technology (NINT). It is designed to explore the technical issues of uniform and controllable target illumination. Short wavelength, uniform and controllable target illumination is the fundamental requirement of high energy density physics research using large laser facility. With broadband, extended light source and multi-beam overlapping techniques, rare gas halide Excimer laser facility will provide uniform target illumination theoretically. Angular multiplexing and image relay techniques are briefly reviewed and some of the limitations are examined to put it more practical. The system consists of a commercial oscillator front end, three gas discharge amplifiers, two electron beam pumped amplifiers and the optics required to relay, encode and decode the laser beam. An 18 lens array targeting optics direct and focus the laser in the vacuum target chamber. The system is operational and currently undergoing tests. The total 18 beams output energy is more than 100J and the pulse width is 7ns (FWHM), the intensities on the target will exceed 1013W/cm2. The aberration of off-axis imaging optics at main amplifier should be minimized to improve the final image quality at the target. Automatic computer controlled alignment of the whole system is vital to efficiency and stability of the laser system, an array of automatic alignment model is under test and will be incorporated in the system soon.

ASE suppression of XeCl excimer laser MOPA system using UV electro-optical switch

In high power eximer laser system, amplified spontaneous emission (ASE) decreases the signal contrast ratio severely, leads to waveform broadening and distortion, and impacts on accurate physical experiments. In this article, based on principle of short pulse generation by electro-optical (E-O) switch, a method for ASE suppression of laser amplifiers chain was established. A series of studies on UV electro-optical switches were carried out, and electro-optical (E-O) switches with high extinction ratio were developed. In the waveform clipping experiments of the first pre-amplifier, the extinction ratio of the single and cascaded dual E-O switch reaches 103 and 104 order of magnitude, the laser pulse signal contrast ratio was promoted to 105 and 106 level, respectively. In the experiments of single channel MOPA (Master Oscillator Power Amplifier system), the cascaded dual E-O switch was adopted to suppress ASE of the whole system, and a fine narrow pulse was obtained on the target surface, which gives out one effective solution to the problem of waveform amplification of the high power eximer laser system.

Automatic alignment of double optical paths in excimer laser amplifier

A kind of beam automatic alignment method used for double paths amplification in the electron pumped excimer laser system is demonstrated. In this way, the beams from the amplifiers can be transferred along the designated direction and accordingly irradiate on the target with high stabilization and accuracy. However, owing to nonexistence of natural alignment references in excimer laser amplifiers, two cross-hairs structure is used to align the beams. Here, one crosshair put into the input beam is regarded as the near-field reference while the other put into output beam is regarded as the far-field reference. The two cross-hairs are transmitted onto Charge Coupled Devices (CCD) by image-relaying structures separately. The errors between intersection points of two cross-talk images and centroid coordinates of actual beam are recorded automatically and sent to closed loop feedback control mechanism. Negative feedback keeps running until preset accuracy is reached. On the basis of above-mentioned design, the alignment optical path is built and the software is compiled, whereafter the experiment of double paths automatic alignment in electron pumped excimer laser amplifier is carried through. Meanwhile, the related influencing factors and the alignment precision are analyzed. Experimental results indicate that the alignment system can achieve the aiming direction of automatic aligning beams in short time. The analysis shows that the accuracy of alignment system is 0.63μrad and the beam maximum restoration error is 13.75μm. Furthermore, the bigger distance between the two cross-hairs, the higher precision of the system is. Therefore, the automatic alignment system has been used in angular multiplexing excimer Main Oscillation Power Amplification (MOPA) system and can satisfy the requirement of beam alignment precision on the whole.

Generation of strongly coupled plasmas by high power excimer laser

(ultraviolet). To generate strongly coupled plasmas (SCP) by high power excimer laser, an Au-CH-Al-CH target is used to make the Al sample reach the state of SCP, in which the Au layer transforms laser energy to X-ray that heating the sample by volume and the CH layers provides necessary constraints. With aid of the MULTI-1D code, we calculate the state of the Al sample and its relationship with peak intensity, width and wavelength of laser pulses. The calculated results suggest that an excimer laser with peak intensity of the magnitude of 1013W/cm2 and pulse width being 5ns - 10ns is suitable to generate SCP with the temperature being tens of eV and the density of electron being of the order of 1022/cm-3. Lasers with shorter wavelength, such as KrF laser, are preferable.

Self-phase modulation of an ultra-short laser pulse from laser breakdown plasma

The detailed dynamic of an atom in a laser field with strength comparable to the atomic electric field is rich in physics and potential applications. Laser-breakdown plasma-induced spectral shifting in supersonic rare gases jet has been investigated with a sub-picosecond KrF excimer laser focused to peak intensity in the region of 1015W/cm2. A 1.4mm diameter gas jet target was used in the experiment to minimize the refraction of the laser beam and thus a well-defined focused region was obtained. The typical quasi-periodic spectral shifting structures for helium and argon have been measured at various gas densities. For gas densities below 1x1020cm-3,both spectral red-shift and blue-shift were observed, indicating the gas is partially ionized, in contrast to the predominantly blue shifted as the gas densities grows high and fully ionized. Compared to the other ultra-short pulse measurement methods, qualitative information about the pulse can be deduced by observing their spectrum after interacting with rare gas.

The development of XeF(C-A) lasers by optical pumping

The development of XeF(C-A) lasers by optical pumping in Northwest Institute of Nuclear Technology during recent 10 years is described in this paper. A joule level of XeF(C-A) laser optically pumped by a sectioned surface discharge was studied with the total efficiency of 0.1%. A more efficient XeF(C-A) laser was developed with the maximum outpup energy of 18.6 joules by two-side optically pumping and repetition mode operating.