3.2 Main oscillation power amplification technique

The main oscillating power amplifier consists of two parts: first, the main oscillator generates a seed light source with high beam quality, and the output power can be large or small; Then, the high power amplification of seed light source is realized through one or more power amplifiers. Therefore, using the main oscillation power amplification technique to obtain laser output can not only ensure the high quality of the output beam, but also amplify the output energy, i.e. it combines the advantages of high beam quality of low power seed light source and laser energy amplification of power amplifier.

3.2.1 Solid rod amplification technique

Solid rod-like laser crystal is still the most widely used gain medium because of its good uniformity and symmetry, which is suitable for a variety of pumping modes and can make pump light and oscillating light achieve good mode matching. It is proved that the rod-shaped laser crystal can obtain the laser output with high power and high beam quality under the condition of efficient thermal control. At present, solid rod-like laser crystals are still widely used as gain media in main oscillating power amplifiers.

In 2018, researchers developed a dual-wavelength laser with high efficiency and compact structure based on Nd:YVO₄ (www.wisoptic.com) crystal. The main oscillation power amplification structure was used in the laser system. The 808 nm end-pumped Nd:YVO₄ was used in the oscillation stage, and BBO E-O Q-switch (www.wisoptic.com) was adopted to obtain 1064 nm base transverse mode output with repetition rate of 5 kHz, single pulse energy of 400 μJ and pulse width of 1.1 ns. The amplification stage adopts 878.6 nm direct pumping Nd:YVO₄ two-stage amplification, and obtains fundamental optical output with single pulse energy of 2.72 mJ and pulse width of 1.03 ns. By using extra-cavity frequency doubling of LBO, laser pulse with duel wavelength of 532 nm and 1064 nm can be obtained. The 532 nm pulse has repetition rate of 5 kHz, single pulse energy of 1.54 mJ and pulse width of 910 ps. The 1064 nm pulse has repetition rate of 5 kHz, single pulse energy of 1.18 mJ and pulse width of 1.1 ns. The optical schematic of the MOPA and the Nd:YVO₄ cascade amplifier are shown in figure below.

Optical schematic of MOPA