A single pulse energy 6.77mJ, repetition rate of 1kHz, pulse width of 11ps at 532nm wavelength with the near top-hat intensity profile in the near filed picosecond laser amplification system is realized using a semiconductor laser side pumped Nd:YAG crystal. The seed pulse is generated in a home-built Nd:YVO4 oscillator, pumped with a 808 nm CW diode laser. The oscillator provides 2.8nJ, 11ps pulse width, 86 MHz repetition rate at 1064 nm wavelength. Pulse from the Nd:YVO4 oscillator is first amplified to 1.5mJ by a diode side-pump Nd:YAG regenerative amplifier. Then the pulse, increased in size by a negative lens, sequentially passed through a circular aperture and a spatial filter-image relaying system to produce a top-hat intensity profile in the modules, and an 8th order super-Gaussian beam is obtained, and total transmission of beam shaping set-up is about 30%. The beam, passed through a double-pass preamplifier of single rod and a double-pass main amplifier of single rod, is amplified up to 17.3mJ, corresponding peak power is 1.57 GW. A 4F relay-imaging system is used in the amplification stages to preserve the top-hat intensity profile and compensate the thermally induced birefringence of Nd:YAG rod. The amplified output beam leaving the double-pass Nd:YAG module is decreased in size and imaged on a 5×5×13 m^3 second-harmonic generation (SHG) crystal-LBO by a 4F relay-imaging system, finally a 532nm approximate top-hat intensity profile in the near filed, which single pulse energy is 6.77mJ, is obtained after doublefrequency. The second-harmonic generation efficiency is over 51%.
Azimuthal polarization beam amplification up to an average power of 11.08 W using the Nd:YAG amplifier structure was obtained with the relative purity of 93% at 1 kHz.The beam quality factor M2 is 3.29 and the beam wavelength is 1064 nm. The amplification factor of the amplified picosecond azimuthally polarized beam is 207.9%.
The effect of the gain medium planar shear stress, induced by non-radially symmetrical pumping during power amplification process, on the depolarization of radially polarized beam was analyzed in detail. For radially polarized beam, theoretical simulation had showed that the non-radial distribution of planar shear stress led to the different depolarization in different polarization directions, that the depolarization of the direction of 45° and 135° is five times more serious than that of 0° and 90°. In the following experiment, the radially polarized seed beam with a repetition rate of 1 kHz, an average power of 5 W and a pulse width of 100 ps was single-propagating double-rod Nd:YAG laser amplifier. Each rod was surrounded by three diode-pumped arrays with an angle of 120°. During the amplification process, the depolarization degree of radially polarized beam was 6.73%, which was obtained by measuring the difference between the power of the two beams transmitted and reflected by the analyzer TFP. The power of amplified radially polarized beam was 15.17 W with the purity of 89.3% and M2 of 3.95.
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