Defence Research & Development Canada (DRDC) has recently initiated research with femtosecond lasers. One project focuses on the physics underlying self-focusing filaments in the atmosphere and their potential applications in other agency research areas. A second project is based on the use of femtosecond lasers to generate terahertz (THz) waves.
To facilitate this research, we decided to build on the scientific expertise of outside teams that have already carried out research and development in these areas.1–4 Their results proved useful, but it was important for us to continue research outside the laboratory under real operational conditions. To perform such field test experiments, we developed a portable laser system. The Terawatt & THz (T&T) mobile laboratory consists of a terawatt laser mounted in standard 20ft sea container. A schematic of the lab is shown in Figure 1. This project is similar in many respects to Teramobile, the Franco-German laser and detection system.5
Figure 1. Shown is a schematic of the T&T lab. The laser system is mounted in a class 100000 clean room that occupies about half of the container. The other half is filled with the air-conditioning system and the laser power supplies, chillers, and control computers.
Terawatt lasers are highly sensitive to shocks, vibrations, and variations in temperature and humidity. To install one in a portable container, we built a modular class 100000 clean room. There the laser system was mounted on two sets of pneumatic isolators, the first for use during operation and the second, more rigid set to be employed for transportation. Four 3in. openings in the clean room, which occupies half the container, enable the beam to propagate outside the laboratory. Even with one aperture open, ambient conditions are maintained, with an internal temperature of 20°C when temperatures outside range from -30 to 30°C
The second half of the container is occupied by the air-conditioning system, power supplies, chillers, and laser system controls. Total power consumption is under 45kW. The electrical system is designed to work from a 60Hz three-phase 208V supply, provided either by direct power line or a generator. Uninterruptible power supplies monitor the voltage and permit 10min of operation in case of power failure, allowing normal shutdown.
Due to winter weather conditions in Canada, we have adapted a building for use as a docking station for the lab. From this redoubt the laser beam can be directed to the adjacent laboratory or toward a confined 200m-long range fronting the building. Maintenance can also be performed here.
The laser system consists of a compact titanium:sapphire (Ti:Sa) oscillator and a chain of Ti:Sa amplifiers. The amplifier chain includes a regenerative amplifier and multipass amplifier pumped with a doubled neodymium yttrium aluminum garnet (Nd:YAG) laser. The laser system, shown in Figure 2 without protective covers, is mounted on a 1.5 × 2.5m optical table. The pulse energy of the beam can be attenuated stepwise to 100% in 1% increments with a motorized variable attenuator. One compressor grating, mounted on a motorized translation stage, allows control of pulse width and eventual introduction of a negative chirp to precompensate for propagation of the beam over long distances.
Figure 2. The terawatt laser, shown here without its protective covers, has a modest footprint.
With no optimization of the acousto-optic programmable dispersive filter (AOPDF), we measured a pulse energy of 270mJ and a pulse width of 47fs, resulting in peak power of 6TW. More characteristics can be found in Table 1. With optimization of the pulse spectrum using AOPDF, we believe that a width below 40fs will be achieved.
Table 1 Measured laser characteristics
Laser operations and monitoring are centralized on a computer with a user-friendly interface, as shown in Figure 3. Two spectrometer ports, four calibrated photodiodes, two CMOS cameras, and six motorized mounts are integrated in the laser system and allow us to monitor performance in real time and to carry out high-level alignments without having to open the protective covers. We are currently finalizing installation of the system and expect to be ready for our first mobile mission by spring 2007.
Figure 3. Laser monitoring and control systems are computer-controlled with a user-friendly interface.
Jacques Dubois joined DRDC in 1978 after obtaining his degree as an electrical engineer. As a defense scientist, he has worked on projects related to laser warning receivers, directed countermeasures, muzzle reference systems, laser seekers, combat identification systems, laser beam rider detection, and laser-guided hypervelocity missiles. He now leads the research team studying femtolaser applications and THz technology. As inventor or coinventor, he holds 12 patents.