ИСТИНА

Neodymium-doped crystalline laser media are widely used to generate and amplify picosecond pulses due to their suitable gain bandwidth, typically a few units of 1/cm. Effective amplification aimed to obtaining pulsed-periodic radiation of high peak and average power usually involves the use of regenerative and then linear amplifiers. The overall efficiency of a laser system is determined primarily by the linear amplification output stages, which ought to operate in saturation mode. Up to the energy level of single output pulses of several mJ and repetition rates of ~1 kHz, the most attractive way is the use of amplification schemes with longitudinal pulse pumping which provide both the best overlap of the laser beam with the pump profile in the active crystal and the ability to work at high repetition frequencies. A significant limitation of such schemes is associated with the aberrational nature of the thermal lens, leading to additional losses and degradation of beam quality at high pump power. Nevertheless, up to repetition rates of the order of and above 1 kHz, such schemes prove to be effective. At a linear amplifier operating near the saturation condition, the optimal solution, from the point of view of pump conversion efficiency, is the use of two-pass amplifying schemes that provide uniform complete exhaustion of population inversion along the entire length of the pumped laser medium. In the present work, we develop and analyze operation of a picosecond Nd:YAG laser/ amplifier system based on longitudinal pumping with use of pulsed fiber coupled diode laser arrays with the peak power of up to 60 W. The scheme with two stages of two-pass amplifiers provides pulses of 25 ps duration with an energy not less than 5 mJ at the fundamental wavelength of 1064 nm with a repetition rate of 1 kHz. Following the scaling conditions such a design allows further increase in output energy and average power. Two-pass longitudinally pumped amplifiers operating close to or under saturated conditions are important structural parts of the scheme. In this regard, we compare the capabilities of amplifiers based on Nd:YAG and Nd:YLF. When implementing this approach, we pay attention to two significant points. Firstly, the amplification of picosecond pulses with a duration of 10-100 ps is a significantly non-stationary process, which actually follows a three-level scheme, in contrast to the case of nanosecond pulses corresponding to classical four-level scheme. Therefore, it is important that the time delay between two successive passes of a two-pass amplifier exceeded the lifetime of the lower laser level. In contrast to the lifetimes of the upper laser levels in the Nd:YAG and Nd:YLF, whose values are well known, there are significant differences in understanding regarding the lower levels lifetimes. The most detailed study [1] provides values of 170-225 ps for Nd:YAG and more than 10 ns for Nd:YLF. In our recent study based on direct measurements of gain dynamics at saturation it was found that in Nd:YAG the lifetime of the lower laser level is even shorter, that is, less than 100 ps. Thus, the differences between these two popular laser media must be adequately taken into account when developing the optimal design. The second important point is that in the case of picosecond pulses, the saturation fluence values turn out to be closely comparable with the laser crystal surface breakdown threshold, which is mainly determined by the dielectric coating. And the saturation energy densities in Nd:YAG and Nd:YLF differ approximately twice. When characterizing the optical breakdown threshold of laser dielectric coatings, there are usually indicated values related to nanosecond pulses. While there are practically no systematic data in relation to picosecond pulses. We fulfilled a series of measurements of the optical breakdown power density and energy fluence using high-power pulses with durations of 20 and 110 ps. The results obtained are ought to be taken into account at designing effective picosecond pulse amplification schemes based on Nd:YAG and Nd:YLF.