A group of laser researchers led by Victor Apollonov at the Russian Academy of Sciences (RAS; Moscow) has developed a modification to high-power wide-aperture gas lasers that allows emission in a high-frequency pulse-periodic mode in which very short pulses are produced at a high rate without a sacrifice in average power.1 The improvement can be made to gas-dynamic lasers, hydrogen fluoride/deuterium fluoride chemical lasers, and chemical oxygen-iodine lasers. Potential uses include launching and propelling spacecraft with ground-based lasers.
At output powers exceeding several kilowatts, producing short pulses based on high-frequency resonator modulation runs into several problems, caused by the wide apertures of the resonator elements. Existing schemes for beam modulation, which include magnetic modulation of gain and physical chopping of the beam, all have problems that greatly reduce average power when compared with continuous-wave (CW) operation.
FIGURE 1. An experimental 10-kW CW gas-dynamic laser is converted to a high-frequency pulse-periodic mode with pulses 0.1 to 1 µs in length, frequencies of 25 kHz or greater, and peak powers of 100 kW (top). A portion of the laser's beam is passed through a modulator and fed back into the laser, causing the output beam to become pulsed. A scaled-up version of this laser could propel a so-called Lightcraft into space. A small Lightcraft prototype is placed in its launcher by Tregenna Myrabo (bottom).
In the scheme developed by the RAS researchers, a portion of the laser's output is extracted from the resonator, modified spatially and temporally, and then returned to the resonator (see figure). Injecting return light into the paraxial region of the resonator would require that the power of the injected beam be comparable with the output laser power to efficiently control the resonator of a continuously pumped laser—an impractical solution. Instead, the researchers inject the return light into the resonator periphery, resulting in a larger number of beam interactions within the resonator and thus good control with a smaller amount of return light.
Experimental results
To confirm theoretical calculations, an experiment was done on a carbon dioxide (CO2) gas-dynamic laser with a typical optical output of 50 kW. (The gas-dynamic laser is a powerful form of CO2 laser developed in the 1960s that also uses nitrogen and water vapor. It was and is used for military experiments such as the U.S. Air Force's Airborne Laser Laboratory, developed in the 1970s and 1980s to shoot down missiles.) The unstable resonator of the RAS laser consisted of two spherical mirrors with rectangular apertures and a geometrical amplification factor of 1.45. The laser gas flowed perpendicular to the resonator axis. In CW mode, the output was lowered to 10 kW to prevent damage to the mirrors. Because the test-bench components were uncooled, the laser was not operated for more than 3 seconds at a time. Full laser power was achieved after 0.3 seconds.
About 20% of the laser output was diverted by an inclined metallic mirror to the injection-beam-formation system, which consisted of two spherical mirrors with conjugate focal planes and a modulator placed at the beam waist formed by the mirrors.
The modulator was a rotating metal disk with holes machined along its perimeter. The experiments used disks containing either 150 or 200 holes with respective diameters of 4 and 2 mm and a 0.5 filling factor. The maximum modulation frequency was 33 kHz. To measure temporal characteristics of the laser, the output beam was attenuated and allowed to strike a photodetector hooked up to an oscilloscope. Power measurements were done with a water-cooled calorimeter.
For a modulation frequency of about 27 kHz and a modulation depth (relative to the beam within the system, not the output beam) of 2% to 3%, the laser radiation exhibits intensity fluctuations in time with the modulating signal, with the peak output power departing from the average power value by a factor of three. When the modulation depth was increased to 7% to 8%, the laser shifted to the pulse-periodic operating mode. In this case, lasing took place in the form of a package of five to ten pulses within one cycle of the opened modulator state. The duration of an individual pulse was about 200 ns (recorded pulse durations were limited by the 50-MHz bandwidth of the photodetector electronics). The amplitudes of individual pulses exceeded the average value by factors of 6.5 to 11. Pulse-periodic modulation with a pulse length of 0.1 to 1 ms, a peak output power greater than 100 kW, and an average output power equal to the CW 10-kW power was experimentally obtained for the gas-dynamic laser.
The experimental and theoretical data agreed well for frequencies ranging up to 30 kHz. It may be possible to increase the modulation frequency enough that a once-CW laser can be brought to the Q-switching regime, say the researchers.
Laser-propelled spacecraft
Because they are scalable to higher powers, pulse-periodic lasers may be useful for spacecraft propulsion. The Lightcraft, developed by Leik Myrabo of Lightcraft Technologies (Bennington, VT) and tested at White Sands Missile Range (White Sands, NM), is a craft that receives a ground-based laser beam, focusing it to create a detonating plasma from the air just behind it, propelling it upward (see Laser Focus World, September 2000, p. 29). A 10-kW CO2 laser pulsed at 28 Hz and with a pulse duration of a few microseconds has propelled a small Lightcraft to a height of 128 ft.
"Victor Apollonov's regenerative-amplifier gas-dynamic-laser experiments look very promising, and particularly so for applications that demand rapid scaling into the multimegawatt level, kilohertz pulse-repetition frequencies, and submicrosecond pulse durations—all attractive for the current laser Lightcraft engine design," says Myrabo. "Also, the physics appear to be well in hand with regard to realizing full theoretical efficiency from a 100 kW-class gas-dynamic laser. However, it should be noted that the demonstration of high beam quality with this setup has yet to be accomplished, nor has the power been extended up to the 100-kW level at the present time. This will require further development and funding equal to the task." For ambitious laser-propulsion projects such as this, Myrabo believes that government funding—perhaps by NASA—is the best approach. Apollonov notes that the RAS group is interested in investors in general for its pulse-periodic laser.
REFERENCE
- Quantum Electronics 33(9) 753.