A picosecond, high power Nd:glass laser system using the chirped pulse amplification(CPA) technique was constructed. The CPA laser system technique consisted of an ultrashort laser oscillator, a four-pass pulse stretcher, a telescopic ring regenerative amplifier, a series of Nd:phosphate glass amplifiers, and a pulse compressor.
The front end, ultrashort Nd:YLF oscillator was pumped by a laser diode(LD) and modelocked by using the additive-pulse modelocking technique. The characteristics of a LD-pumped APM Nd:YLF laser such as the pulse width, spectrum, and temporal modulation were investigated with respect to the coarse cavity detuning to generate shortest and clean pulses. When the beam splitter commonly used to link the main and external cavities was utilized as an output coupler, the temporal modulation of the output pulse was observed from the autocorrelation trace, which also agreed with the calculation using an intensity-dependent reflectance. For the generation of short and clean pulses from an APM laser, the direct extraction of an output from the main cavity proved to be crucial and 1.7 ps pulses without any temporal modulation were obtained from the LD-pumped APM Nd:YLF laser. It was stabilized from external perturbations by an active stabilization unit and could last for several hours without interruption.
After stretching up to 170 ps in the four-pass grating stretcher, one modelocked pulse was trapped and amplified in the telescopic ring regenerative amplifier. The telescopic ring regenerative amplifier was designed for the reduction of optical components and the generation of high gain by maintaining a large mode volume. In the telescopic ring regenerative amplifier, the output energy could be obtained up to 33 mJ with an amplification of $4×10^8$. The beam size of the amplified pulse was 2.3 mm in width and 2.0 mm in height at 30 mJ output energy and its profile was well fitted to the Gaussian profile. The amplified spectrum with a spectral width of 6 Å was slightly shifted to the long wavelength region without a noticeable gain narrowing and agreed well with the calculated result.
After the regenerative amplifier, the laser pulse was injected into a Nd:phosphate amplifier chain. The Nd:phosphate amplifier chain consisted of two amplifiers with the diameter of 8 mm and one amplifier with the diameter of 20 mm. The image relay and spatial filtering system was installed at each amplifier to prevent the optical damage due to whole and small scale self-focusing. The amplified energy of the laser pulse in 8 mm amplifier was about 120 mJ. The laser pulse was then double-passed the 20 mm amplifer for high extraction efficiency without excessive thermal loading. The laser pulse could be amplified up to over 2 J in 20 mm amplifier.
The amplifier laser pulse was compressed by the pulse compressor consisted of grating pairs. The optimal distance between gratings was about 15.4 m for recompression to the range of oscillator pulse width. The compressed pulse width was measured with a single shot autocorrelator and the measured pulse width was 2 ps. This CPA Nd:glass laser system will be applied to experments, such as X-ray generation, laser-plasma interaction, and X-ray laser.
