When a high-powered laser hits a target, it generates recoil pressure from the target ablation. This recoil pressure could be used to propel objects into space. On the basis of theoretical considerations supported by model experiments, pulsed carbon-dioxide lasers with beam peak powers of 50MW and a pulse length of 20μs should be able to launch small satellites.1
To overcome limitations of ultrahigh power densities in a single laser, a new concept envisages a beam source consisting of several individual laser systems (see Figure 1). To increase the peak power, the lasers are ‘Q-switched’: a process in which the quality (Q) of the laser cavity is kept low while optical energy builds up, then the quality is switched to high to emit a short pulse with high peak powers. The lasers operate through radio-frequency excitation, and are characterized by a fast axial gas flow with excellent beam-power–to–volume ratio. Short laser pulses emitted by 16 Q-switched CO2 laser sources with more than 50MW total power in a coaxial electrode geometry are combined on a common optical beam path to form a longer single pulse as required. Coaxial lasers have already been built successfully, although without Q switching (see Figure 2).2
Figure 1. Overall layout of the 100kW laser beam and temporal structure of the pulsed radiation. The setup consists of 16 individual coaxial laser systems equipped with Q switches. Each laser delivers pulses of 1μs. Individual pulses are combined to form a pulse with a duration of 16μs.
Figure 2. CO2 laser with coaxial electrodes, radio-frequency (RF) excitation, and fast gas flow. It has a continuous-wave power of 6kW. The device is 2m long and 1.5m wide, including power supply, RF transmitters, and gas-mixing unit.
Although coaxial lasers are readily available, other components of the laser setup (such as the Q and beam-redirection switches) are not yet on the market. Their construction appears to pose a significant problem since they must be able to withstand extremely high beam powers. The feasibility of this new concept fully depends on the availability of optical switches that can swap between full transmission, full blocking, and full reflection.
In view of the extremely high beam power required, only plasmas seem to offer a useful solution. It is well known that very dense plasmas can fully reflect an incoming laser beam if the plasma frequency is equal to the device's radiation frequency at certain electron densities. Moreover, at lower electron densities, plasmas can either fully transmit or absorb CO2 laser radiation, depending on the electron density, because of the inverse-Bremsstrahlung effect.3
Aiming at achieving much-needed progress in the development of a plasma switch, we have investigated (theoretically and experimentally) the CO2 laser-beam absorption cross section and reflectivity of iron/argon plasma plumes generated by deep penetration welding of steel with argon as the protective gas. For the typical plasma electron densities of 1017cm−3 associated with this process nearly 100% of the beam power is absorbed, but the plasma frequency—and thus full reflectivity—cannot be reached since the electron density remains too low. These plasmas can therefore potentially be used for Q switching CO2 laser beams between full transmission and full blocking.
Unfortunately, thus far no solution can be offered for switching between full reflectivity and full transmission. The latter can only be achieved with much higher electron densities, for instance as found in plasmas generated during optical breakdown in atmospheric air ignited by Q-switched neodymium-doped yttrium-aluminum-garnet (Nd:YAG) lasers. Our future research efforts will focus on attaining the latter.
Dieter Schuöcker, Bernhard Holzinger, Gerald Humenberger
Department of Forming and High Power Laser Technology
Vienna University of Technology
Dieter Schuöcker is the director and founder of the Department of High Power Laser Technology. He is also head of the Austrian Laser Association, which he founded in the early 1980s.
Bernhard Holzinger was awarded his PhD by the Vienna University of Technology in May 2006.
Gerald Humenberger studied at the Faculty of Mechanical Engineering of the Vienna University of Technology.