ИСТИНА

Compact and energy effective picosecond lasers providing single pulse output of a few millijoules at reasonably high repetition rates within kilohertz, are claimed by a number of applications. Such as satellite and lunar laser ranging, material processing, driving photocathode of electronic accelerators, OPO pumping, time-resolved laser spectroscopy, etc. Most common approaches to designing picosecond laser systems imply stages of pulse generation and regenerative amplification. Dynamical operation control scheme utilizing pulsed repetitive pumping, active and passive mode-locking, negative feedback, adjustable loss level in the oscillator cavity and switching to regenerative amplification regime [1,2], provides laser pulse formation in each laser shot. It takes several microseconds on the end of pump pulse. This approach actually allows shortest way to obtain, just at the laser output, near Fourier transform limited picosecond pulses of more than one millijoule energy with good pulse-to-pulse stability and low optical jitter value [3]. Both oscillator and regenerative amplifier can be based on the same single active crystal. Using Fabry-Perot etalons inside oscillator cavity allows significant varying output pulse width which can take values from 15 (with Nd:YLF) or 25 ps (with Nd:YAG) and up to 300 ps. Evolution of pulse energy, spectrum and time profile during a single generation cycle can be well illustrated using universal numerical calculation model [2] which describes pulse formation governed by the operation control and also taking into account the pulse profile modifying due to amplification. Utilizing diode end-pump geometry allows maximal overlapping of resonator mode and pumped volume, whereby providing optimal pump conversion efficiency into output radiation. As a result, using the described picosecond Nd:YAG laser scheme with one addition amplifying stage provides 25 ps pulses of up to 4,2 mJ at fundamental wavelength and 20 ps pulses of up to 2,5 mJ at second harmonic with repetition rates within ~400Hz. Both laser and amplifier use diode end-pumping by means of fiber coupled laser diode arrays of 70 and 120 W maximum peak powers respectively. Owing to pulse regime and end-pump geometry, thermal loading is not high and the system does not require liquid cooling and can be easily power scalable by means of an additional amplification stage. Further increase of output peak power and, respectively, single pulse energy by means of additional amplification stages implies operating near the saturation regime. However, saturation fluence value for Nd:YAG at pulse length of 20-30 ps is close to the damage threshold. Whereas, increase of the pump beam and laser mode diameter will result in reducing single pulse amplification, need for additional amplification trips and, finally, growth of reflective and diffraction losses and output falling. At high repetition rates and, respectively, average power values, operation conditions strongly depend on thermal lens induced in the laser crystal [4]. Increase of average pump power at longitudinal geometry principally results in aberrational lens formation. Spherical part of the thermal lens may be compensated using usual spherical optics, whereas aberrational part action is more complex. Along with the irretrievable aberrational losses such a lens exhibits a certain adaptive effect that may maintain, to some extent, operation steadiness. An adequate analysis is required for system developing with laser generation mode of acceptable quality and, at the same time, supporting mode locking regime. Detailed experimental and modeling results will be presented. [1] M.V.Gorbunkov, A.V.Konyashkin, P.V.Kostryukov, V.B.Morozov, A.N.Olenin, V.A.Rusov, L.S.Telegin, V.G.Tunkin, Yu.V.Shabalin, D.V.Yakovlev. Pulsed-diode-pumped, all-solid-state, electro-optically controlled picosecond Nd:YAG lasers. Quantum Electron., 35 (1), 2-6 (2005). [2] A.A.Karnaukhov, V.B.Morozov, A.N.Olenin and D.V.Yakovlev. J. Phys.: Conf. Ser. 414, 012027 (2013). [3] N.G.Mikheev, V.B.Morozov, A.N.Olenin, D.V.Yakovlev. Picosecond lasers with the dynamical operation control. Proc.of SPIE, 9917, 99170A1-9 (2016). [4] V.B.Morozov, A.N.Olenin, V.G.Tunkin, D.V.Yakovlev. Operation conditions for a picosecond laser with an aberration thermal lens under longitudinal pulsed diode pumping. Quantum Electron., 41 (6), 508–514 (2011).