Ruby laser captures munitions impacts
A high-energy, multiple-pulse Q-switched ruby laser has been developed by the U.S. Air Force to be used in high-speed photography and holographic movies.
by John Wallace
A high-energy, multiple-pulse Q-switched ruby laser has been developed by the U.S. Air Force to be used in high-speed photography and holographic movies. High-speed photography has many important applications, including ballistics, material evaluation, fluid combustion, and imaging flows. The Air Force Research Laboratory (Wright Patterson Air Force Base, OH) and the Joint Munition Test and Evaluation Project (Eglin Air Force Base, FL) teamed with Continuum (Santa Clara, CA) and Physical Sciences Inc. (PSI; Andover, MA) through the Air Force SBIR (Small Business Innovation Research) program to develop a new light source for the Terminal Effects Laser Camera Center (TELCC) at Eglin.
Terminal ballistic and munitions impact events are highly energetic and high speed, requiring a camera and lighting system that can both overpower the emitted incandescent light from the experiment and provide a very short pulse to freeze the motion of the particles, projectiles, and shock waves. Strobe lights and explosive-based "argon candles" have traditionally been the only source of light for high-speed photography, but nothing was readily available to both strobe and light an experiment for up to 70 continuous frames.
Although the laser system is based on novel techniques, it largely uses readily available and industrially rugged commercial components. This approach has promoted the ease of use and reliability, according to the Air Force scientists and engineers using the system. The laser provides high pulse energies, producing up to 80 pulses of 350-mJ energy at a 1 x 106 pulse-per-second rate. Use of an internal high-speed Pockels cell to Q-switch the oscillator and an external Pockels cell to shape and level the laser output envelope has led to a system that provides almost uniform illumination from pulse to pulse. The timing of the system was precisely set to promote lasing at the end of each Q-switch open time, critical when synchronizing to the high-speed digital cameras.
The system consists of an oscillator and two amplifier stages. The long oscillator cavity and thermal stability of the oscillator component has resulted in a stable and reliable laser, even when rocked by gunfire and shockwaves from projectile impact in the adjacent test cell.
The multipulse illumination system has been successfully integrated with a digital million-frame-per-second camera to achieve up to 1-MHz frame rates for imaging of ballistic events. Data have been captured of typical impact and fracture experiments.
Once integrated and working, the Air Force and Continuum/PSI team captured both holographic movies at 5 x 105 frames per second and stereo digital movies at up to 1 x 106 frames per second. The system is now being used at TELCC to capture fractures in composites after impact, aerosol sprays from containers, and small particles generated by projectile penetration of glass, armor plate, and ceramics. The combination of 10 billionth of a second pulse length and laser digital imaging acts to de-blur even the smallest particles while overcoming the impact flash and ionization. The three-dimensional imaging from the stereo or holographic cameras allows the researchers to assess vector motion and dispersion of the material at and after impact.
Based on the success of the system, a ruggedized eight-pulse Nd:YAG version of the system was developed at Continuum and fielded at Eglin for laser photography of warhead detonation at up to 1 x 108 frames per second, and a design completed for a compact and long-run-duration Nd:YAG version of the ruby system for field use.