High repetition rate, high energy ultra-fast laser is indispensable in industrial application and scientific research. The most common way to obtain high power ultra-fast laser is through laser amplification. However, the small cross section of fiber amplifier poses a challenge in obtaining high energy ultra-fast pulse for nonlinear interaction. 

Meanwhile, the solid laser amplifiers need extra phase front correction to guarantee the beam quality of output pulse. Are these difficulties unsolvable?  

The answer is negative, a research team led by Prof. WANG Yishan from Xi'an Institute of Optics and Precision Mechanics (XIOPM) of the Chinese Academy of Sciences (CAS) gave their excellent solution: cryogenic technology and single crystal fiber (SCF). This work was published in the journal Optics & Laser Technology.

Solid gain material have better performance at cryogenic temperature, it is a revolution in power scalability for solid amplifier. The two-stage cryogenic chirped-pulse Yb:YAG amplifier is a good choice. But there is a problem that must be solved: these two cryogenic system cannot output sub-ps pulses because the emission spectrum of gain material is just 1.5 nm. 

Under their in-depth study of the subject, an emerging gain material, single crystal fiber(SCF), entered their vision. The seed laser travels freely in SCF and the pump laser is transmitted by total internal reflection. It makes the output of sub-ps pulses possible in this system.    

Finally, the researchers developed a chirped pulse amplification (CPA) system in which fiber laser is used as the seeder. Yb: YAG rod is employed as the first stage amplifier and SCF is used in the second stage to further scale the output power. An output power of 67.9 W has been achieved at 200 kHz after the second stage amplification, which corresponds to a pulse energy of 309 μJ.

"To the best of our knowledge, this is the highest power based on SCF at hundreds of kHz repetition rate", said Prof. WANG. 

Furthermore, the CPA system with high power and energy is cost saving, reliable and easy to integrate, which has a promising prospect in the industrial processing and scientific research.

Schematic diagram of the CPA system. (Image by XIOPM)