What is chirped pulse amplification technology?

On the afternoon of October 2, the Royal Swedish Academy of Sciences with a history of nearly 300 years announced the 2018 Nobel Prize in Physics. The three award-winning scientists have made groundbreaking inventions in the field of laser physics. Half of the award was awarded to Arthur Ashkin of Bell Laboratories in the United States for his invented optical tweezers technology and applied this technology to biological systems. The other half was shared by French scientist Gérard Mourou (Professor of the Ecole Polytechnique in Paris, France, Emeritus Professor of the University of Michigan, USA) and his student Donna Strickland (Associate Professor of the University of Waterloo, Canada). The Chirped Pulse Amplification (CPA) technology they proposed is an original method for generating ultra-strong and ultra-short pulse lasers.

Here, let’s talk about the chirp (zhōu jiū) pulse amplification technology that generates super-strong and ultra-short laser pulses. Since Mayman, a scientist at the Hughes Laboratory in California, announced that he had obtained the first laser in 1960, laser technology has long been integrated into daily life. Whether it is a laser pointer held by a teacher in class or a European free electron laser device that costs billions of euros and is more than 3 kilometers in length, a variety of lasers are used in industry, communications, science and entertainment. The ultrashort pulse defined in laser physics refers to an electromagnetic pulse whose time scale is less than the order of picoseconds (ps). The flash used by the camera has a flash time of approximately one hundredth of a second (0.01 s). Nowadays, the flashing time of ultra-short laser pulses has already reached the order of femtosecond (fs) or even attosecond (as).

As we all know, the unit of power is watt W, 1 W u003d 1 J / 1 s. When the energy of the laser pulse is larger, the time scale of the laser pulse is shorter, and the corresponding peak power is larger (that is, increase the numerator and reduce the denominator). In order to obtain extremely high peak power, scientists not only need to shorten the time scale of the laser pulse, but also need to continuously amplify the energy of the laser pulse. The innovation of ultra-strong and ultra-short laser technology has always promoted the development and development of a number of basic and cutting-edge interdisciplinary subjects such as high energy physics, fusion energy, precision measurement, chemistry, materials, information, and biomedicine.

Before the emergence of chirped pulse amplification technology, scientists have been able to reduce laser pulses from the order of milliseconds (ms) through ultra-fast laser technologies such as Q-switching and mode-locking. Increase to the order of nanoseconds (ns) and picoseconds (ps). The Kerr-Lens Mode-Locking (KLM) technology that emerged after the chirped pulse amplification technology has even directly compressed the time scale of the laser pulse to the femtosecond level, and the corresponding peak power has also been determined. Improvement.

However, directly amplifying the energy of the laser pulse to further increase the peak power has encountered an insurmountable bottleneck. Because in the direct amplification process, the ultra-high peak power density of the laser pulse (power density u003d power/focused spot area) is very easy to damage the gain medium and other transmissive optical components in the amplifier (the effect is similar to focusing sunlight with a magnifying glass Go to a small spot on the newspaper, it can easily be burnt down). Secondly, the time scale of the directly amplified laser pulse is too short, which is not conducive to efficiently absorbing all the energy in the amplified gain medium.