To the two-dimensional world of photography, movies add a third dimension. Analysis of everything from a horse's gallop to the motion of atoms skittering across the surface of a crystal would be difficult without the ability to explore the dimension of time. Supersonic and hypersonic gas flows have been studied by freezing their motion at one or two instants using single- or double-pulse Q-switched Nd:YAG lasers; lacking a record of time, such studies cannot reveal the rapid evolution of effects in a flow. Pulsed lasers emitting at megahertz rates provide strobe illumination, making high-speed movies possible. But limits on the average power of such lasers keep the pulse energies low—in fact, a thousand times too low to allow the making of movies of supersonic flows.
Lasers that emit short bursts of high-power pulses skirt this problem. Researchers at Princeton University (Princeton, NJ) have constructed a next-generation laser system that emits bursts of 30 pulses at up to a 1-Mhz rate, with energy of 50 mJ per pulse. The same system can emit 100 pulses at lower energies or a single 3-J pulse. These 1064-nm pulses are then frequency doubled to 532 nm, providing illumination in the visible range.
The laser system is a master-oscillator and power-amplifier configuration that includes a continuous-wave (CW) master oscillator, a Pockels-cell-based pulse slicer to chop the CW light and generate a pulse burst, a five-stage amplifier, and a second-harmonic crystal. After passing through a flow within a wind tunnel, the laser light is imaged by a megahertz-rate charge-coupled device (CCD). Via laser-induced breakdown (ionization) of gas in the supersonic flow, the laser system itself can initiate disturbances that are then monitored as they evolve.
There is a tradeoff between the maximum number of pulses and the pulse energy. "The choice of the optimum number of pulses depends on the experiment," explains Richard Miles, one of the researchers. "In most of our experiments, we have used the laser in conjunction with a 30-frame CCD fast-framing camera that can run at up to a rate of 1 million frames per second; therefore, we have run with 30 pulses. The pulse rate is determined by the evolution speed of the phenomenon that is being imaged. In our case, this has been a high-speed boundary layer. Our flows are around 500 m/s and the image size is several centimeters, so a rate of 500,000 frames per second allows us to follow a feature all the way across the image plane at one pulse per millimeter of displacement."
The main application is the capture of rapidly evolving phenomena such as shock-wave/boundary-layer interactions or the transition from laminar to turbulent flow in supersonic and hypersonic environments, says Miles. In many of these cases, the air is seeded with 1% carbon dioxide, which freezes out into a low-density particle fog in the cold portions of the flow. Images show the interface between the cold core flow and the hot boundary layer near the wall. The researchers also have used the pulse-burst laser system to capture volumetric images of high-speed flow structures by illuminating a cross section as the flow passes by and then stacking the images to form a three-dimensional picture of the structure shape.
John Wallace | Senior Technical Editor (1998-2022)
John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.