Chirped Pulse Amplification (CPA)

For many applications, the energies of ultra-short laser pulses delivered by mode-locked oscillators are not sufficient, and it is necessary to amplify them. However, the intensities obtained during direct amplification of short pulses quickly become very high and exceed the damage thresholds of optics. Chirped-Pulse Amplification (CPA) is an architectural concept developed to amplify short pulses without exceeding this damage threshold.

The general principle is to reduce pulse intensity during the amplification phase. The obvious solution of increasing beam diameter as peak power increases quickly reaches its limits, not only because of the size of the optics required, their bulk and cost, but also because decreasing fluence leads to laser systems operating far from the saturation fluence of the amplifying materials, making them highly inefficient. The CPA concept consists in increasing pulse duration, which only reduces pulse intensity while maintaining fluence.

Starting with a mode-locked oscillator, the first step is to temporally extend the extremely short laser pulses using a highly dispersive system. These are referred to as frequency-drift pulses, because dispersion introduces a quasi-linear relationship between optical frequency and group time. A pulse with a duration in the femtosecond to picosecond range is typically stretched into the nanosecond range. This process reduces the peak power of the pulse by several orders of magnitude, enabling it to be amplified to otherwise unattainable levels. Once the desired energy level has been reached, the pulse is reduced to a duration comparable to the original value by time compression using a dispersion element opposite to that of the stretcher.

It is the fundamental concept used for the realization of large ultra-high intensity laser installations, reaching peak powers in the petawatt range. The CPA method is also successfully used for lower peak powers, such as those delivered by commercial products, or when pulses are as short as a few optical cycles. There are also numerous applications in the field of fiber lasers, where high optical intensities are inherent to the confinement of light in the core of optical fibers.

The aim of this article is to describe and explain the concept, giving the basic elements and presenting the various technologies used to build the systems. We show how to manage dispersion and amplify ultra-short pulses, whether by stimulated emission or optical parametric amplification. Finally, significant examples of CPA systems illustrate the major advantages of this technology.

A glossary and table of acronyms are provided at the end of the article.