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High-energy, high-power laser system for inertial confinement fusion (ICF), such as National Ignition Facility (NIF), is large in size and expensive in construction. The multi-pass amplifier (MA) with the large aperture amplifiers is widely used in high-power laser systems, such as ICF drivers. The systems with MA usually have four features: square beam, single aperture, single pulse and unidirectional propagation, and the expensive preamplifier systems are required to compensate for the limited gain of the main amplifiers. The gain and the extraction efficiency are limited in part by the number of passes that the beam can make through the amplifiers. Besides, the laser system in the multi-pass amplifier should operate at much higher fluence to achieve high extraction efficiency, which results in a technical challenge in the damage of optical components under high-power lasers irradiation.
To efficiently extract the stored energy with low injection energy at low laser fluence operation and make the system compact and reliable, a bidirectional ring amplifier (BRA) with twin pulses is proposed and discussed. The structure of the bidirectional ring amplifier is described. The characteristics of the bidirectional ring amplifier on extraction efficiency and output energy capability are simulated and discussed. The simulation results show that an extraction efficiency of 62.3% and the output energy of 13.4 kJ per pulse at the B integral limit can be obtained at low average fluence of 10.3 J/cm2 and the low injection energy of 3.9 mJ in the bidirectional ring amplifier. Compared with the multi-pass amplifier, the bidirectional ring amplifier is more compact and the extraction efficiency is much higher at low laser fluence operation, which is beneficial to reduce the effects of nonlinear phase shift. Furthermore, the preamplifier system for the bidirectional ring amplifier is simple, only a fiber oscillator and a regenerative amplifier can work.
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