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Quantum cascade laser tests water for impurities on-site


esearchers at the Fraunhofer Institute for Applied Solid State Physics IAF in Freiburg have developed a spectrometer for testing drinking water for impurities based on a quantum cascade laser. The device allows water companies to identify impurities in the water on-site, at the waterworks. A demonstrator has undergone initial practical testing at the Kleine Kinzig waterworks in the Black Forest. Tests were conducted on various concentrations of sweetener as a simulant substance. Measurements were taken every three minutes over a period of six weeks, with the fully automated system collecting a total of 21,000 samples.

“ The system is only a little larger than a shoebox, and works automatically”

Research (BMBF). The chemical constituents of the water sample are examined using molecular spectroscopy under infrared light. Each chemical compound has a unique spectrum, since individual molecules vibrate and absorb light at characteristic frequencies. Water itself is a very strong absorber of infrared light; since the light sources employed to date have delivered little power, until now examinations of this sort have only been possible in a laboratory setting.

‘The main sticking point is

‘The equipment samples the water for dangerous substances at the waterworks itself in the course of routine operations, and allows for a rapid response,’ said Dr Frank Fuchs, coordinator for the IRLSENS project at Fraunhofer IAF. IRLSENS is funded by Germany’s Federal Ministry of Education and

the intensity of the light. In order to be in a position to employ molecular spectroscopy at the waterworks itself, we needed to find a more powerful light source,’ explained Fuchs. Fraunhofer IAF’s quantum cascade laser produces light that is up to 1,000 times more concentrated than the silicon carbide thermal emitters used in standard laboratory equipment. For molecular spectroscopy, analysts are interested in wavelengths between 7.3µm and 11µm. The

The device allows testing on-site, at the waterworks

measurement system is only a little larger than a shoebox, works automatically, and requires hardly any maintenance. Providing there is sufficient demand, project partner Bruker Optik, the company that built the demonstrator for the Kleine Kinzig waterworks, plans to develop the measurement system into a finished product.

Parametric amplifier generates terawatt light pulses

An optical amplifier has been constructed that is able to generate 10-terawatt light pulses. The parametric amplifier, built 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 (FUW), is extremely efficient and small enough to fit on a desktop.

The amplifier represents an

important step towards construction of compact, portable, relatively low-cost, high-power laser devices that could revolutionise anti-cancer therapies, for example. Dr Yuriy Stepanenko, the chief

constructor of the amplifier, said: ‘Theoretically, the efficiency of parametric amplifiers can reach 50 per cent. In practice, the best

amplifiers of this type are operated at an efficiency of about 30 per cent. We have reached this level already now, and what’s more, in a really compact device. In the coming months we are going to increase the amplifier’s efficiency by another a few per cent on the one hand, while on the other we intend to increase the power of laser pulses up to a few tens of terawatts.’ Most lasers generating ultrashort pulses amplify light using sapphire crystals doped with titanium ions. An external laser is used to pump energy into the crystal, and a fraction of the energy is subsequently taken over by a laser beam being amplified. The method has numerous disadvantages. One of the major ones is that the crystals warm up strongly, leading to adverse

distortions of the cross section of the laser beam. As a result, the crystals must cool down virtually after each laser shot.

Non-linear optical effects can be used to construct amplifiers of a different type. These parametric amplifiers transfer energy directly from the pumping laser beam to the beam being amplified. As the input energy is not stored anywhere, there are no adverse thermal effects, and the amplified pulses have excellent parameters. Parametric amplifiers can amplify light by hundreds of millions of times on an optical path of a few centimetres only. This means the system can be engineered in a relatively small package. The instrument from the Laser Centre of the IPC PAS and the FUW fits comfortably on half of a

typical desktop. The new amplifier will be used for construction of an x-ray source and to generate experimentally protons and secondary neutrons. One of the long-term objectives of the research on parametric amplifiers is to generate laser pulses with a power of 200TW and higher. Such powerful light pulses could be used for accelerating protons to energies that are useful in medical therapies, for instance to selectively kill cancer cells. The existing techniques for proton acceleration require construction of huge and high cost accelerators. High-power lasers would allow for significant increase in availability of the state-of-the-art proton therapies, with simultaneous radical reduction of treatment costs for cancer patients.


@electrooptics |

Martin Wagenhan / Fraunhofer IAF

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