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Space awaits thin-disc source

02 Aug 2002

A compact, thin-disc laser is to be installed in the International Space Station.

A research team in Germany has designed a diagnostic system for flame combustion based on a Yb:YAG thin-disc laser. Eventually it is hoped that the compact system will be installed aboard the International Space Station (ISS).

Wolfgang Triebel, head of the advanced disc laser (ADL) project, told OLE that the four-wavelength high-repetition-rate system is set for completion by mid-2003, and will be ready for microgravity applications by the following year.

The ADL consists of a two-stage oscillator–amplifier design. Narrow-bandwidth radiation, tunable between 1020 and 1060 nm, is first generated by a continuous-wave Yb:YAG seed laser. An electro-optic pulse-picker extracts pulses from the seed laser and these are amplified in a second Yb:YAG disk.

“We decided to use Yb:YAG as it offers pulse energies of several tens of millijoules and a high repetition rate in the infrared,” said Triebel. In addition, the ytterbium source offers a level of tunability that is far superior to that of a neodymium laser.

The ytterbium source is well-suited to fast repetition-rate emission thanks to the relatively long lifetime of its upper laser level, which is around 1 ms. Triebel told OLE that commercial laser systems could not offer the range of features needed for the combustion study.

Tunable excimer lasers only operate at up to 50 Hz, while copper-vapour lasers do not emit at the required wavelengths and an OPO system would need a high-repetition-rate pump laser.

LBO and BBO crystals generate tunable emissions in the UV-visible range via second, third and fourth-harmonic generation. The wavelengths generated can then be used to probe a variety of species found in flames.

The fourth-harmonic radiation, tunable between 255 and 265 nm, can be used to study oxygen, nitrous oxide (NO), hydroxyl radicals and HCO by means of laser-induced fluorescence. Nitrogen, oxygen and water molecules can be measured using Raman spectroscopy of the third and fourth-harmonic radiation, while the fundamental emission can be used for laser-induced incandescence probing of soot.

Currently, the laser has an average power of 5 W at the fundamental frequency, and 1 W at 515 nm. Kilohertz-rate pulse repetition enables image capture once every millisecond.

It is planned that the average power of the infrared beam will be increased 10-fold, extending the application potential of the system. In-flight microgravity tests in a Bremen drop tower and in parabolic flights will precede the installation of the ADL in the ISS combustion-integrated rack.

Michael Hatcher is technology editor of Opto and Laser Europe magazine.

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