A multi-pulsed ruby laser system has been developed utilizing repetitive Q-switching technology. The laser system has applications in fields where high speed dynamic events are studied, e.g., ballistics and non-destructive test evaluation, using laser imaging diagnostics such as photography, holography, and various interferometric techniques. The laser system is capable of producing more than 50 pulses at a repetition rate in excess of 500 kHz with a nearly constant pulse-to-pulse energy of several mJ. The individual laser pulses are approximately 50 ns FWHM and the envelope of multiple pulses is greater than 200 microseconds. The method for multiple Q-switching by modulating the Pockels cell's quarter wave voltage and the formation of an individual Q- switched pulse have been investigated and a computational model has been formulated. The energy within the individual pulses formed in the oscillator cavity has been successfully increased without degradation of the temporal or pulse-to- pulse amplitude stability by propagating through an amplification section. Data of an amplified pulse train at a repetition rate of 500 kHz is given. Etalons for longitudinal mode selection and an iris for spatial mode selection have been incorporated for increased coherence and an image of a reconstructed hologram is presented.
Improvements to solid rocket motor (SRM) nozzle designs and material performance is based on the ability to instrument motors during test firings to understand the internal combustion processes and the response of nozzle components to the severe heating environment. Measuring the desired parameters is very difficult because the environment inside of an SRM is extremely severe. Instrumentation can be quickly destroyed if exposed to the internal rocket motor environment. An optical method is under development to quantify the heating of the internal nozzle surface. A radiometric probe designed for measuring the thermal response and material surface recession within a nozzle while simultaneously confining the combustion products has been devised and demonstrated. As part of the probe design, optical fibers lead to calibrated detectors that measure the interior nozzle thermal response. This two color radiometric measurement can be used for a direct determination of the total heat flux impinging on interior nozzle surfaces. This measurement has been demonstrated using a high power CO2 laser to simulate SRM nozzle heating conditions on carbon phenolic and graphite phenolic materials.
Shadowgraphic holography allows imaging of small particles over a 180 degree field of view and with a large depth of field. The cylindrical holography technique developed by Hough and Gustafson has been modified to allow holograms of small hypervelocity impact generated simulated space debris particles to be successfully made by using a subnanosecond laser pulse length. The use of very short (135 ps) laser pulses with a corresponding short coherence length (4 cm) frees the motion of small high speed particles. With this system, shadowgraphic holograms of aluminum projectiles impacting aluminum and graphic epoxy plates have been achieved at hypervelocity. Results of these tests as well as low speed proof of concept tests are presented.
The light emission from the bow shock around the tip of a metal jet formed by the collapse of a shaped-charge linear was computed for tip speeds up to 15 km/s and the laser energy needed to overwhelm this emission for a front-lit photographic application has been determined. Upon approximating the nose of the jet as a hemisphere with a 2 cm radius, a 3D inviscid flow field code was used in conjunction with a nonequilibrium air radiation code to compute the shocked air properties including the temperature, pressure and emission. For comparison, an analytical calculation of the shocked air properties and visible radiation at the flow stagnation point was made. Both calculation methods yield results which indicate that at a tip velocity exceeding 10 km/s the emission from the bow shock is equivalent to blackbody radiation. Additional values for the emission at tip velocities below 10 km/s are also contained in the paper. These results specify that a laser pulse energy of 10 mJ would be required to match this background luminosity for the 10 km/s case assuming a 1000 cm2 illuminated object area, a 1.5 nm spectral bandpass and 50 ns exposure time for a camera.
Laser lithotripsy is now an accepted modality for the intracorporeal fragmentation of urinary tract and, to a lesser extent, biliary tract calculi. However, under conditions where constant direct vision is not possible or compromised, the risk of inadvertent laser damage to healthy soft tissue cannot be discounted. This is especially true at the higher laser pulse energies required to fragment the more recalcitrant stones. A series of in vitro and in vivo investigations are described which demonstrate a method and apparatus for the automatic feedback control of laser lithotriptors. In preliminary experiments, the control device, incorporated into a commercial flashlamp-pumped dye laser, is shown to significantly improve the margin of safety against laser tissue damage while still allowing effective stone fragmentation. The practical implications of our findings for the clinical possibility of `blind' laser lithotripsy are discussed.