Lichtgedanken 04

Rubrik 54 Half-speed ahead POLARIS is one of the most powerful laser systems there is. Jena re- searchers have already made several world records with POLARIS. Yet the laser only rarely reaches peak performance during normal operation. How come? BY UTE SCHÖNFELDER It is a good two years since Prof. Dr Malte Kaluza and his team announced their last world record: with the PO- LARIS laser, researchers from the Insti- tute of Optics and Quantum Electron- ics and from the Helmholtz Institute Jena have achieved the highest peak load energy for a fully diode-pumped short-pulse laser system. POLARIS ge­ nerated over 50 joules of pulse energy— in principle, each of these pulses could be compressed to 100 femtoseconds to thus provide an output of over 100 tera­ watts—equivalent to global electricity consumption many times over. Even though this places POLARIS in the league of huge lasers—in the peta­ watt class—in its routine operation it rarely does full justice to its name (PO- LARIS stands for Petawatt Optical La- ser Amplifier for Radiation Intensive experimentS). This is because, physi- cists generally just »shoot« with pulses containing 15 to 17 joules; i. e. around 170 terawatts, rather than the maximum achievable laser energy. Why? It is largely to do with the complex- ity of the laser, explains Kaluza: »PO- LARIS, like other laser systems, isn’t a device that you simply switch on and then use automatically in the same way each time.« It is actually an enormous technical challenge to keep the output of the laser at a consistent quality for a long enough period of time so it can be used for reproducible scientific mea­ surements. »And, at the end of the day, that is our main interest.« POLARIS is a scientific tool and peak performance is not an end in itself. Hundreds of optical components need to be calibrated and maintained Around 170 diode stacks, hundreds of mirrors and seven amplifying crystals or glasses must be carefully aligned to guarantee a defined beam profile and reproducible laser quality. The beam travels just under one kilometre through the 240 sqm system. POLARIS is based on the technique of »Chirped Pulse Amplification«. A low-energy start pulse is stretched in time and then amplified over several orders of mag- nitude. The amplified pulses are finally compressed to generate a pulse with an extreme peak power, »as the energy is released in an extremely short space of time«, explains Kaluza. The fact that PO- LARIS is currently only running at »half power« is owed to the sensitive optical components. The mirrors, gratings and amplification crystals can become dam- aged when the power is too high. »Even tiny defects on the coating of a mirror can lead to the next shot being the last«, summarizes Kaluza. Several researchers and technicians spend around half of the operating time adjusting, maintain- ing and servicing the optics to set them up to produce a laser beam profile that is as homogeneous as possible. After all: the more evenly the laser illu- minates the sensitive surfaces, the less likely it is that it will be damaged by the laser radiation. Without the perpetual testing of all optical components, it simply wouldn’t be possible to achieve very precise peak performance at any given time. And only with a uniform and even beam profile it is possible to achieve higher pulse energy with PO- LARIS and, therefore, to run the system »on full power«. »It is a really fiddly job. Often you just have to try and see«, says Kaluza. As a result, it is not just specialist know- ledge and patience that is required; but a playful nature too. But what are world records when you get to play with such an amazing »toy«? Behind the Scenes The final amplifier stage of the high-performance laser POLARIS.