3.2.1 Innoslab amplification technique

When adopting solid rod Master Oscillator Power-Amplifier (MOPA) technique, the amplification system usually has multiple passes, such as single-stage multi-pass, multi-stage single-pass and multi-stage multi-pass. The spot radius changes little as the seed laser power is continuously amplified, leading to the increase of peak power density. High laser peak power density is easy to cause damage to the optical components. In the system of the LD partially end-pumped laser amplification hybrid cavity (i. e. Innoslab laser), the seed laser passes back and forth through the slatted amplification gain medium. When the laser energy is amplified, the laser spot is also enlarged, so that the peak power density of the laser in the amplification process will not be too high to cause damage to the optical components. Therefore high-energy laser output can be generated from a laser source with high repetition frequency and low energy by using Innoslab technique via the design of reasonable structure.

In 2017, researchers used Innoslab technique to obtain laser output with repetition rate of 5 kHz, single pulse energy of 25 mJ, and pulse width of 3 ns. The seed source was an electro-optic Q-switched Nd:YVO₄ (wisoptic.com) laser with repetition rate of 5 kHz and single pulse energy of 0.3 mJ. The amplification stage uses a 40×14×1 mm³ Nd:YAG (wisoptic.com) slab with Nd-doping concentration of 0.6%. The seed laser was amplified by 12 passes to give laser beam with single pulse energy of 25 mJ and out power of 125 W. A schematic diagram of the Innoslab amplifier is shown in the figure bellow.

In 2020, a research team reported a Innoslab laser with repetition rate of 100 kHz, single pulse energy of 400 μJ, and average power of 40 W. The seed source was still an electro-optic Q-switched Nd:YVO₄ laser with an Innoslab amplifier. The gain medium of the amplification stage is a Nd:YAG slab with doping ratio of 0.6%, and size of 40×14×1.4 mm³. The seed light undergoes seven-pass amplification, and finally gives laser output with repetition frequency of 100 kHz,  single pulse energy of 2.1 mJ, and output power of 210 W.

To sum up, the electro-optic Q-switched laser is capable to give output with high repetition frequency and high energy, mainly by increasing the amount of gain media in the oscillator, or by using the MOPA technique outside the oscillator. The double-rod serial connection structure can effectively reduce the thermal effect in the laser crystal and improve the pulse energy. On this basis, by adopting in-band direct pumping and double-rod design with bonded crystal, the heat load of the gain medium will be further reduced, and high-energy laser output will be obtained consequently. The MOPA technique is an effective approach to obtain high-energy laser output, and the solid rod amplification technique is a classic pattern of the MOPA progress. In order to prevent the damage of the optical components caused by the high peak power density of the amplified laser, the solid slab amplification technique came into being, which further increased the output energy of the laser.