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Pump it up: new proposals for X-ray lasers

12 Oct 2006

Scientists at Berlin's Max Born Institute have devised a new type of pump laser for X-ray lasers that produces a continuous, repetitive regime at 100Hz and energies exceeding 1J.

At the 10th International Conference on X-Ray Lasers in Berlin, organized by the Max Born Institute (MBI), novel designs for X-ray lasers and the associated pump lasers led many presentations.

MBI itself proposed a significant new solution to the challenge. Scientists worldwide are working on lasers with ever shorter wavelengths; the shorter the wavelength applied, the smaller the structures one can inspect, investigate and develop.

optics.org interviewed Peter Nickles, project coordinator at MBI, about the conference he chaired and about MBI's work on a novel DPSS pump laser.

optics.org: What was the purpose of the conference in Berlin?

Peter Nickles: MBI's X-Ray Laser group organized the 10th International Conference on X-Ray Lasers, to which we welcomed 115 mainly academic participants from 14 countries. The event was also supported by measurement and materials companies. We presented details of MBI's new pump laser, developed without input from industry.

o.o: What is your definition of an X-ray laser?

PN: The X-ray laser generates short-wavelength radiation by amplified spontaneous emission in a highly ionized active medium. The device is mirrorless, so no resonator is used to increase the interaction length between signal and medium. The latter is caused by an extremely short-lived (10-15 ps) gain.

The term X-ray laser is confusing because it was created at least 40 years ago and is still used by tradition, but current X-ray lasers generally work in the XUV or soft X-ray spectral range.

o.o: Why do we need new designs of pump laser to drive X-ray lasers?

PN: The majority of laser-plasma-based X-ray lasers until the development of the GRIP (grazing incidence pumping) geometry worked as single-shot devices. They delivered a high number of photons in a single shot. However, many applications require repetitive irradiation with a reasonable average photon flux. A typical repetition rate of 10 Hz (with GRIP and conventional Ti:sapphire laser) is too low and a new laser driver with a higher repetition rate and stability, compared with the common 10 Hz Ti:Sa laser, is needed. This is possible only with a diode-laser-pumped system.

o.o: How does MBI's design of pump laser operate?

PN: Our DPSS laser driver is the first and only pump laser that permits a continuous, repetitive regime at 100 Hz and energies exceeding 1 J. It could be a breakthrough in the pump technique for X-ray lasers.

o.o: What is the specification of the new MBI-pumped X-ray laser?

PN: We expect an average power of the new X-ray laser to be around 100 µW with an average brightness of 1016-1017 photon s-1mm-2mrad-2/0.1%Bw. at a wavelength of 13.9 nm. The new laser should have a conversion efficiency of ~30% and proportionally should increase the total conversion efficiency, achieving a value of about 10-5.

o.o: Did the conference discuss other models for pumping and designing X-ray lasers?

PN: New X-ray laser schemes, especially the so-called injector-amplifier arrangement, were discussed vigorously. These could offer a reduction in the pulse length of X-ray lasers to less than 1 ps. The conference also saw the first demonstration of a portable capillary discharge-based X-ray laser (by the Colorado State University group). This could find laboratory applications such as experiments on chemical catalysis.

o.o: What are the likely applications of the new MBI design of DPSS-pumped X-ray laser?

PN: Microlithography is less probable at the moment due to severe constraints put on the source by industry. However, such an application cannot be excluded with further development of the pump laser technology.

The X-ray laser in the form of a beamline could have broad application in short-wavelength metrology and inspection techniques, now that imaging with a resolution better than 38 nm has been demonstrated with an X-ray laser working around 13 nm. Many possible applications that are analogues to the visible and IR spectral regions (such as speckle-technique and diffraction microscopy) have not yet been explored.

Generally, conventional pumping lasers are not stable enough to ensure accurate scientific measurements. Particularly in sequences of measurements, when we average signals,the results can be "smeared" [unclear]. Diode lasers are far more stable and thus more suitable for the pumping process. They lead to more exact results and also allow high repetition rates, i.e. fast repeating pulses.

o.o: What is the commercialization potential of the MBI pump, who are likely customers, and what could be the market value?

PN: This pulsed pump laser would be a less repetitive but more energetic variant of an industrial laser. This is a rare combination of the output parameters and some specific material processing by energetic picosecond or nanosecond pulses could be applicable.

Our laser does not match the industrial standards known to us. However, it could be used for surface processing and as an efficient and more stable pump laser in specific high power lasers - such as Ti:Sa lasers - as well as for incoherent X-ray sources.

The MBI X-ray laser, which should be ready for use by the end of 2007, marks a milestone in the development of such lasers. The neighboring Ferdinand Braun Institute (FBH) is also involved in this research project, providing the special diode lasers. These sources are based on new designs of laminar structures and lateral structuring. The highly brilliant and efficient laser diodes emit at wavelengths around 935 nm and allow simple and reliable beam formation at low production costs.

o.o:What are the advantages of the new pump/X-ray laser design?

PN: One of the great advantages of such an X-ray laser is its comparatively small size. Furthermore, the diode-based pumping lasers require less energy than solid-state pumping lasers. A couple of standard-sized desks would be sufficient space to build such an X-ray laser. Thus, an intense short-wave light source can easily be moved - a feature that is especially interesting for industrial applications.

Their flexibility and easy transport would make the new design an interesting source of short-wave pulses complementary to short-wave free electrons lasers (FEL) which work as individual large-scale facilities based on particle accelerators.

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