Construction of a record-breaking laser gets off the ground

Most lasers amplify light by making use of classical technology with titanium ions doped sapphire crystals. An external laser is used to pump energy into the crystal where a part of the energy is subsequently taken over by a laser beam being amplified. Laser crystals have, however, numerous disadvantages, e.g., they warm up strongly and distort the cross section of the laser beam. An alternative is provided by parametric amplifiers that exploit non-linear optical effects. A laser with such an amplifier is being developed in the Laser Centre at the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) and the Faculty of Physics of the Warsaw University (FPWU). „Our goal is simple. We want to construct the most efficient and compact parametric amplifier in the world" -- says Dr Yuriy Stepanenko from IPC PAS.

The multi-pass optical parametric amplifier technology NOPCPA (Noncollinear Optical Parametric Chirped Pulse Amplifier) has been for several years developed in the Laser Centre in a group headed by Prof. Czesław Radzewicz (IPC PAS, FPWU). The method consists in an efficient energy transfer directly from the pumping laser beam to the beam being amplified. Combined with numerical modelling, theoretical tools developed by Polish researchers allow to optimize precisely the parameters of both beams and of the amplifier. These issues are non-trivial as field intensity distributions are inhomogeneous in time and space, and in addition the pulse being amplified has a time-dependent frequency (which the physicists call a chirp).

As no energy is being accumulated in a parametric amplifier, there are no damaging thermal effects, and the amplified pulses have excellent parameters. A NOPCPA amplifier has also compact dimensions: a length of several centimetres is enough to reach an amplification of hundreds of millions of times. Theoretical efficiency of such an amplifier is approximately 60% but it is difficult to get, and so far the best laser amplifier of this type reach below 30%. „Our minimum plan is to reach an efficiency of 40%, we will try, however, to overcome a barrier of 50%" -- says Dr Paweł Wnuk (IPC PAS).

The researchers expect to get the first 10 terawatt pulses with duration of dozen femtoseconds emitted by their laser next year. But this is only the beginning of the way. „We hope that already the present version of the parametric amplifier will allow us to generate over 100 TW pulses" -- stresses Prof. Radzewicz. The calculations indicate that 500 TW laser pulses could be used to accelerate protons to energies enabling them to be applied in medical therapies including anti-cancer treatment. The lasers with so high power can be found today only in a few research centres worldwide. „We have all the grounds to assume that our method of light amplification can in future help us to build relatively cheap lasers for proton acceleration, in addition with so compact size that they essentially would be considered portable devices" -- says Dr Stepanenko.

Under the research project being completed the new laser will be used to construct two demonstration setups. The first one, being developed in collaboration with the Military Academy of Technology (MAT) in Warsaw and the Institute of Physics of the PAS, will be used to construct x-ray sources with micrometric dimensions. Such sources are used in, e.g., x-ray microscopy, and in particular in non-destructive testing of structural materials. The second demonstrator will be a lidar for measurements of atmospheric pollution and will be developed with participation of the researchers from the Military Academy of Technology.