The Optics Letters paper from Xu and co-workers is an important step forward in the fascinating history of energy boosting, pulse compression and nonlinear interactions: in one word, of high power laser physics. Their ultimate goal is to study previously unexplored states of matter using light fields with peak powers at the remarkable level of petawatts. How large is such power? Well, it corresponds to the average sunlight power received by the whole of Australia. Now, there is a twofold strategy to reach high peak powers: to concentrate high energy over a very small time interval. Light is the best tool to convey high peak powers, since it is particularly suited to be compressed into ultrashort time scales. Like other groups in the world, the authors of this Optics Letters article are working to push the laser technology to the frontiers of high-energy and ultrashort light generation. To this aim, they follow the approach of combining techniques from high-power laser science and nonlinear optics, namely Chirped Pulse Amplification (CPA) and Optical Parametric Amplification (OPA); their contribution results in a very promising scheme that is capable of delivering high-peak-power, ultrashort pulses.
The main problem of any high-energy system is that the high peak-power of its signals may destroy the amplification medium - as the first ruby laser could drill holes in razor blades; and this becomes particularly challenging when handling short pulses. The solution came when CPA was introduced in laser science: the technique was known since the 1960s for amplification of radar power, but it was in 1985 that it was brilliantly transferred to lasers. With the CPA approach one avoids optical damage by stretching the short pulse before its amplification; after amplification the pulse is then re-compressed to its original shape. Since its first implementation, this technique has been successfully developed and applied to Titanium:sapphire-based amplifiers, so that CPA is nowadays associated with amplifiers based on the laser effect. In the 90s the technique was extended to a class of amplifiers exploiting nonlinear optics and called Optical Parametric Amplifiers; the scheme was then called Chirped Pulse Optical Parametric Amplification (OPCPA) and became part of the rush to high-power light generation. Since then, many schemes for combining CPAs and OPCPAs were suggested, requiring the parallel development of energetic pumping systems and large linear and nonlinear crystals of good quality.
The authors opted for a scheme based on a CPA block composed of a sequence of 3 Ti:sa-based amplifiers, followed by an OPCPA employing a large LBO crystal. Their scheme was designed to reach a good compromise between high energy and short pulse duration. Based on the principle that in order to obtain large results one needs large resources, high amplification gain called for the development of pump lasers which could deliver a pumping energy of more than 9 Joules to the CPA and 155 Joules to the OPCPA. For the OPA they also had to design the proper beam configuration and the crystal capable of broadband and efficient amplification; they opted for large LBO crystals. What is special about LBO crystals is that nowadays they can be grown into large sizes, and support the large amplification bandwidths required to obtain ultrashort pulses.. After initial testing, Xu and co-workers could demonstrate that their scheme, the first employing LBO as a nonlinear medium, could provide pulses with energy of about 20 Joules. Those who are not familiar with laser science may think that this is not much; but if you consider that this energy is released in only 34 femtoseconds, you can easily get that this corresponds to the remarkable peak power of 0.6 Petawatt. Ok, not the solar power shining over the whole surface of Australia, but still quite huge; few other groups in the world could do better in terms of peak power: the rush is still open.
By the way, how often are those light pulses delivered? The system is able to shine one pulse every 20 minutes. The rest of the time is used to charge the pumps. A nightmare for those who have to align the system; but as usual, there is always some price to pay.
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