ИСТИНА

Diode pumping is widely used in laser systems due to compactness, stability and reliability. For Nd-doped materials pulsed regime can be considered as preferable up to several kilohertz repetition rates owing to higher efficiency, and therefore, smaller heat generation. End-pump geometry is very attractive since it provides efficient amplification of lowest cavity mode and, as a result, output beam excellent spatial quality. At the same time, focused pump beam produces essentially inhomogeneous intensity distribution in radial direction inside active medium. It results usually in aberrational thermal lens formation and in inhomogeneous amplification profile even at low repetition rates. That is, the situation is very different from one in case of side pumping with flash lamps or diode arrays. Side pumping usually provides amplification profile almost uniform in transversal direction and thermal lens remains nearly spherical in a wide range of pump power variation, and can be easily compensated by conventional optical elements. Using pulsed pump regime usually implies energy accumulation during the pump pulse and subsequent fast amplification of pico- or nanosecond radiation pulse. Round trip amplification in this case is not so small, as in cw lasers; therefore relation between transversal sizes of pumped region in active medium and of laser mode affects not only pump power transfer efficiency, but the laser mode profile too. In our previous works [1, 2], we divided thermal lens manifestation into two steps: (i) diagrams of stable laser operation at variation of pump repetition rate were calculated taking into account parabolic part of thermolens; (ii) corrections to the diagrams were made by calculation of Strehl ratio [3] which included non-parabolic part of thermolens. Criterion of optimal resonator configuration was derived on the basis of this approach, but however, it did not allow estimating output radiation beam quality and pulse energy. In the present paper, we discuss experimental data on beam intensity distribution and modeling results of field distribution in picosecond laser at pulsed end-diode-pump regime. The model takes into account difference of inhomogeneous heat generation and population profiles difference arising as a result of pump radiation from diode bar fiber output focusing within active element. Transverse beam profiles and output pulse energies were calculated as functions of pump beam radius, resonator configuration and pump repetition rate. Aberrational thermal lens in active element results in a definite interrelated changes of laser mode radius, beam quality parameter and output pulse energy as a functions of pump repetition rate. On the basis of modeling results, we propose quality-beam generation criterion for end-diode-pump picosecond laser with aberrational thermal lens. The work was supported by Russian Foundation for Basic Research (10-02-01277-a)