1. INTRODUCTION
Researches using a high power lasers device for inertial confinement fusion (ICF) are widely explored (Perkins et al., Reference Perkins, Betti, LaFortune and Williams2009; Schmitt et al., Reference Schmitt, Bates, Obenschain, Zalesak and Fyfe2010; Ribeyre et al., Reference Ribeyre, Schurtz, Lafon, Galera and Weber2009; Atzeni, Reference Atzeni2009). Among them, the KrF excimer lasers, which uses the excimer gas as the excitable media have high gain coefficient, high repetition frequency, and shorter wavelength. It has become an important part of the ICF study and a focus point of research (Sethian et al., Reference Sethian, Friedman, Giuliani, Lehmberg, Myers, Obenschain, Wolford, Kepple, Hegeler and Swanekamp2003; Reference Sethian, Friedman, Giuliani, Lehmberg and Myers2001; Owadano et al., Reference Owadano, Okuda, Matsumoto, Matsushima, Koyama, Tomie and Yano1993; Obenschain et al., Reference Obenschain, Bodner, Colombant, Gerber, Lehmberg, McLean, Mostovych, Pronko, Pawley, Schmitt, Sethian, Serlin, Stamper, Sullian, Dahlburg, Gardner, Chan, Deniz, Hardgrove and Lehecka1996; Hegeler et al., Reference Hegeler, Myers, Wolford, Sethian, Burns, Friedman, Giuliani, Jaynes, Albert and Parish2008). The target of physical theoretical studies has shown that the shock ignition light pulses should have high power and specific distribution of time and spatial (Zvorykin et al., Reference Zvorykin, Didenko, Ionin, Kholin, Konyashchenko, Krokhin, Levchenko, Mavritshii, Mesyats, Molchanov, Rogulev, Seleznev, Sinitsyn, Tenyakov, Ustinovskii and Zayarnyi2007; Foldes et al., Reference Foldes and Szatmari2008; Lehmberg et al., Reference Lehmberg, Guiliani and Schmitt2009). In order to obtain a specific time distribution, the pulses should be shaped in time. In general, they begin with a low energy prepulse (picket) followed by a long (10 ns) low intensity foot pulse, which ramps up to an intermediate intensity pedestal (compression pulse) and ends with a short spike at much higher intensity. The picket and foot pulse shape the target adiabat profile and enhance the implosion stability. The stability of foot pulse and intermediate intensity pedestal (compression pulse) influence the stability of implosion. Hence, the mature pulse stacking technology is a feasible method to obtain those pulses (Ewing et al., Reference Ewing, Haas, Swingle, George and Krupke1979; Lowenthal et al., Reference Lowenthal, Ewing, Center, Mumola, Grossman, Olson and Shannon1981; Shaw et al., Reference Shaw, Bailly-Salins, Edwards, Harvey, Hirst, Hooker, Key, Kidd, Lister and Ross1993; Reference Shaw, Ross, Hooker, Dodson, Hirst, Lister, Divall, Kidd, Hancock, Damerell and Wyborn1999). But the difficulty of this technology is how to obtain stable shaping results and good beam distributions. This requires that the light pulses have stable waveforms, appropriate pulse width, uniform spatial distribution, and incoherence. Wherein, the technology of KrF laser pulse compression has high compression ratio and conversion efficiency (Takahashi et al., Reference Takahashi, Losev, Matsumoto, Okuda, Matsushima, Kato, Nakamurac, Kuwaharad and Owadano2003; Reference Takahashi, Losev, Matsumoto, Okuda, Kato, Aota and Owadano2005). However, there are some defects in the stability and non-coherent in the results of the pulses compression.
The amplified spontaneous emission (ASE) pulses outputting from the KrF excimer laser have advantages in the spatial energy distribution, incoherent, and natural bandwidth than the laser pulses (Allen et al., Reference Allen and Peters1971; Peters et al., Reference Peters and Allen1972). Therefore, the ASE pulses in the KrF excimer laser is studied as the important light source. Okuda et al. (Reference Okuda, Tomie and Owadano2000), Lehmberg et al. (Reference Lehmberg and Giuliani2003), and Hariri et al. (Reference Hariri and Sarikhani2012) theoretically discussed the amplifier mechanism of the ASE pulses in the KrF laser. Unfortunately, to our knowledge the truncated ASE pulses by using timeshare quenching have not been reported. In this paper, the timeshare quenching method was applied to truncate the ASE pulses in order to improve the ASE pulses shortening ratio and stability. By employing the 248 nm ASE pulse of the KrF excimer laser, the highly stable nanosecond level ASE pulses were obtained. This result could be used to study pulse stacking technology and to study ICF.
2. EXPERIMENTAL SETUP
The experimental setup is shown in Figure 1. A KrF excimer amplifier is used as the source, and the ASE pulses are all from this amplifier. The amplifier is an ultraviolet pre-ionization discharge pumped KrF excimer amplifier. M 1 and M 2 are the totally reflecting plane mirrors. θ1 and θ2 are the adjusting deviation angles. h 1 and h 2 are the distances from the main optical axis. d 1 and d 2 are the distances between the placed position and the window of the amplifier. Adjusting the totally reflecting mirror M 1 and M 2, the energy and the relative delay of the ASE pulses can be controlled. The diaphragm is a spatial filter. L 1 and L 2 constitute a 4F image transfer system. The photomultiplier is the product of Hamamatsu Company model for R1193U-02. The oscilloscope is the product of Tektronix Company model for DPO4052.
(Color online) The experimental schematic.
The output aperture of the amplifier of the laser is 7.0 mm × 20.0 mm. The length of the amplifier is d 0 = 1150 mm, the distance d 1 between the M 1 and the amplifier window should be small enough, subject to the limitation of the structural, selected 300 mm, the distance between the M 2 and the amplifier window d 2 = 765 mm. The diaphragm placed at the focal point of the convex lens L 1. The deflection angles of the adjustment range of the total reflection mirror M 1 and M 2 is 0–8.7 mrad. To protect the electrodes, the variable angles θ1 and θ2 of the mirrors M 1 and M 2 should be controlled in the range of 0–6.0 mrad. In the 4F image transfer system, the focal length of convex lens L 1 and L 2 are 1500 mm.
3. EXPERIMENTAL RESULTS AND ANALYSIS
3.1. Analysis of the ASE Pulses
The ASE pulse waveform before truncation is shown in Figure 2. Its rising time is 5.86 ns. The pulse width is 16.72 ns, and the pulse energy is about 110 mJ.
(Color online) The waveform of truncated ASE pulse.
Only M 1 was used in the first step of the experiment, when the ASE pulse reflected into the amplifier from M 1, the output truncated ASE pulse waveform is shown in Figure 3. The rising edge and the width of the first pulse peak are 2.52 ns and 2.97 ns, respectively. The time difference between the first crest and trough is 5.20 ns. The reason for this result is that the width of the reflected pulse is not enough. To obtaining a good waveform, the number of the reflected pulses should be increased.
(Color online) The truncation result of single reflected ASE pulse.
Therefore, the next step of the experiment is to use two reflected beams for truncating the ASE pulses. It means that M 1 and M 2 are all reflecting the ASE pulses. If the second pulse is delayed by less than 5.20 ns with the first pulse, only the first peak is observed and the truncation is obtained. Finally, the d 2 = 765 mm was selected. The second pulse is delayed by 3.10 ns than the first pulse. Because the long distance between the mirror and the main optical axis will reduce the energy of the reflected pulse. To reduce this affect, h 1 = 2.0 mm, h 2 = 4.0 mm were chosen in this paper. The variations of the truncated ASE pulses width with the angles of the reflection mirror M 1 are shown in Figure 4. It shows that when the deflection angles of the total reflection mirrors are in the adjustment range, the truncated pulse widths are distributed in a relatively small range. This shows that the method of timeshare quenching can be convenient for the ASE pulse truncation.
(Color online) The relationship of the pulse widths with the deflection angles.
For the high density of the reversal particle population, expanding the irradiated area of the amplifier central region by the reflected pulses, the consumption of the inversion particle can be effectively increased. It can better suppress the amplified ASE pulses, so that the truncation can be carried out. The ASE pulse has a certain divergence angle, which make the irradiation region greater than the theoretical size. Therefore, there exists an optimal value for the reflection mirror to obtain the best results. Meanwhile, by comparing, the best results were obtained when θ1 = 2 mrad, θ2 = 5 mrad.
The results with the optimal parameters are shown in Figure 5. The width and the rising time of the ASE pulse are 2.48 ns and 2.32 ns, respectively, and the shortening ratio is approximately 6.7. The pulse energy is about 5 mJ. The above experimental results have proved that the method of the timeshare quenching can achieve shorter nanosecond pulse width.
(Color online) The truncated ASE pulse waveform.
3.2. Stability Analysis of the Truncated ASE Pulses
Based on the above optimal parameters, the analysis of the truncated ASE pulse stability using the method of the timeshare quenching is following. The operating frequency of the KrF excimer amplifier is 1 Hz. The ASE pulse waveform data before truncation is recorded every 30 s. The data and the final results are shown in Table 1. The obtained results are 16.06 ± 0.42 ns.
The ASE pulse waveform data before truncation
The operating frequency of the KrF excimer amplifier is 1 Hz. The ASE pulse waveform data after truncation is recorded every 30s. The data and the final results are shown in Table 2. The obtained results are 2.59 ± 0.05 ns.
The ASE pulse waveform data after truncation
Compared the ASE pulse width variations before and after the truncation of the timeshare quenching method, it can be found that the ASE pulse width after truncation to stabilize. The above experimental results have proved that the method of the timeshare quenching can effectively improve the stability of the truncated pulse.
The reasons why this method can improve the ASE pulse truncation are as follows: Since all ASE pulses are derived from the same amplifier, the variations of the pulse width and the waveform are all the same. When the positions and deflection angles of M 1 and M 2 are fixed, the pulse relative delay and the energy are determined. Under the joint actions of the above factors, the truncated ASE pulses waveform and width have a high stability.
The principle of the timeshare quenching method to truncate the ASE pulses can be described as follow: In the amplifier, the reversal particles were consumed by the amplification of the ASE pulses. So the gain was reduced. In the process of the aimed ASE pulses (the output truncated ASE pulses) transmission amplification, a few injected ASE pulses (come from its own amplifier or other amplifier) were employed to increase the consumption of the reversal particle number. In the period of the pulse overlap, the gain rapid decrease, and the amplification of the aimed ASE pulses were suppressed so that the pulses truncation were obtained.
For shorter truncated ASE pulse width and higher stability, it is required to control the relative delay and the injected pulse energy. The smaller relative delay, the shorter the rising edge of the pulse waveform. The energy of the injected ASE pulse is inversely proportional to the fall time of the output ASE pulse. If the above two conditions are satisfied at the same time, the width of the truncated ASE pulse will shorter.
4. CONCLUSIONS
The method of timeshare quenching was proposed to truncate the KrF excimer amplifier ASE pulses in this paper. Short pulses of 2.48 ns were obtained from the 16.72 ns oscillator output, and the truncation ratio is approximately 6.7. The variations of the ASE pulses are 2.59 ± 0.05 ns. A comparing of the ASE pulses was proposed between the before and after truncation. The experimental results showed that our method can truncate the ASE pulse effectively and improve the stability of the truncated ASE pulse. This method is proved to be simple and effective. It could be extended to the other ultraviolet pre-ionization discharge pumped gas laser.
