A simple and compact four-pass Nd:glass laser amplifier with high gain was designed for a pre-amplifier followed by the multistage power amplifier in the high power laser system. And the characteristics of the four-pass amplifier were investigated by experiments and computer simulations.
The four-pass amplifier adopted the method of compensating the polarization distortion of a laser pulse induced by thermal birefringence of the optically pumped Nd:glass rod. The polarization distortion is a serious problem in the use of a four-pass amplifier since a part of the laser pulse with unwanted component of polarization remains in the amplifier even after the four passes through the Nd:glass rod and is repeatedly amplified to multiple post-pulses. The polarization distortion can be compensated if a Faraday rotator is inserted between the successive passes of the laser beam through the Nd:glass rod. The Faraday rotator rotates 90˚ of every component of the electric field of the laser beam such that the radial and tangential component of the polarizations are exchanged and the retardation induced in the former pass is reversed in the latter to maintain the linear polarization.
Using a Faraday rotator in the four-pass amplifier, the polarization distortion was considerably compensated and a stable amplification of a single laser pulse with high gain was performed. On the contrary, if a quarter-wave plate is used as a polarization rotating element, multiple pulses instead of a single amplified pulse were obtained and lasing occured when the amplifier gain is high. It is verified experimentally that the depolarization of a laser pulse in the four-pass amplifier with a Faraday rotator was only 1/10 of that with a quarter-wave plate.
The four-pass Nd:glass amplifier was characterized to have a high gain up to 2 × $10^5$ and an energy extraction efficiency of 20% which were limited by the optical damage. A stable single pulse amplification with gain of $10^4 - 10^5$ was performed in the range of the pumping energy between 700J and 1000J for an input laser pulse with 40 psec(FWHM) duration and 1.5 μJ of energy. Also the B-integral of the four-pass amplifier for the operation condition is reasonably small ( ＜ 1 ) so that the optical quality of the laser pulse is not seriously degraded in the amplifier to cause a self-focusing or a small scale ripple growth.
It can be concluded that the four-pass Nd:glass laser amplifier is suitable for high gain pre-amplifier in high-power laser systems.