We have operated a CW triply resonant OPO using a PPLN crystal pumped by a Nd:YAG laser at 1.06 µm and generating signal and idler modes in the 2-2.3 µm range. The OPO was operated stably in single mode operation over large periods of time with a pump threshold as low as 500 µW .

The development and study of various schemes of temporal population inversions in recombination processes of ions have shown the perspectives of transient gain amplification as one of the promising candidates for recombination pumped soft x-ray lasers. The point of interest here, what is the threshold of lasing in these schemes? Optimization of various experimental parameters (pulse delay, pulse energy ratio, target positioning, etc.) together with appropriate excitation scheme and pumping geometry might allow one to achieve the initial stages of lasing regime even for pulse energies below 1 Joule.A part of long pulse (300-ps) and a short pulse (475-fs) beams from a tabletop laser were sent into the target chamber. The long pulse was focused by cylindrical lens onto the MO target to fonn 100 pm x 2 min line focus, while the short pulse was sent through the laser-produced plasma from a longitudinal direction. The beam waist size and confocal parameter of the femtosecond radiation were 30 pm and 10 mm respectively. The maximal intensities of the long and short pulses were 1.5~10" and 2~1 0 '~ W cm-2 respectively. The spectrum of prolonged (2 mm) MO plasma dominated by the 3d94d'S, -+ 3d94p'PI laser line (18.9 nm). Our estimations of angular characteristics of x-ray laser spot have shown that the divergence was less than 10" rad. The energy of 18.9 nm spot was calculated to be 80 nJ and the conversion efficiency from the IR radiation to x-ray laser line was 2 x lo-' at full average energy of 350 mJ.We also used a spherical lens instead of a cylindrical one for generation of molybdenum plasma by a long pulse. The spot size of the MO plasma without longitudinal pumping in that case was 0.4 mm, and the intensity of 300-ps radiation was 3 x 10" W The spectrum of Mo plasma without longitudinal pumping of femtosecond radiation consisted of continuum in the range of 19 nin with a steep drop in intensity at the wavelength of cut-off transmission edge of aluminum filter. In the case of longitudinal pumping a 18.9 nm spot was generated. The divergence of this radiation was 3 x 10" rad. The bandwidth of x-ray laser line was measured to be 0.3 nm.The intensity of the x-ray laser radiation essentially depended on the delay between the long and short pumped pulses. Maximal signal was registered at 0.5 -1 -ns delay. A ratio between the heating pulses' energies (k = Ef,/ E,,J has changed from 0.3 to 10. The optimal conditions in this geometry were found to be at k = 3.Another important parameter is the distance between the target and focal point of femtosecond radiation. The maximal signal was registered at the distance of 0.2 -0.3 mm from the MO surface. The full energy of the pumping radiation was less than 0.5 J. 0-7803-7733-8/03/$20.00 2003 IEEE 44 I

A new quality in development of X-ray lasers (XRLs) has been demonstrated in many important experiments during the last years. This progress will be illuminated in the present contribution. Demonstration of table-top x-ray lasers in EUV region was the general vigorously pursued goal and activities pushed the field strongly forward especially in the case of collisionally pumped XRLs. Two pump schemes were deciding for this progress. Firstly, the transient inversion scheme in slab target geometry demonstrated in 1995 at MBI became a standard and was commonly applied it1 the experiments on the table-top (compact) lasers. This scheme applying combination of two pulses with different lengths (nano-/picosecond) allowed, if applied to Ni-like isoelectronic sequence, reduction in the pump energy up to 10 Joules. The MBI focussed its interest on a new single pulse laser pump scheme, and has recently demonstrated saturation in Ni-like Ag at 13.9 nm with less than 3 J pump energy. I h s