Laser-Induced Thermal Acoustics (LITA) is a nonlinear optical technique for remotely measuring the sound speed and transport properties of a fluid with accuracy comparable to the best conventional intrusive methods. LITA excels in high pressure environments, where many other diagnostics fail. The development of the technique has addressed issues including the need to make single-pulse measurements in challenging environments.
Laser Induced Thermal Acoustics for Flow Diagnostics
The perturbations induced by the driver laser create perturbations in the susceptibility of the medium via acousto-optic effects. These perturbations therefore have the appearance of volume diffraction gratings. In the coherent scattering step, a CW or long-pulse source laser is trained on the sample volume at the Bragg angle of the gratings. Light from the source laser is scattered off the gratings into a coherent signal beam that is detected through large f-number optics. The signal is modulated in time by the evolution of the laser-induced perturbations. From this modulation, accurate physical properties of the medium may be inferred, including the sound speed, thermal diffusivity, and other properties that affect the evolution of the laser-induced gratings.
Gas property measurements are extracted from the recorded signals by a process of optimal nonlinear filtering. The nonlinear filter is a global least-squares fit between theory and experiment with the gas properties as adjustable parameters. This filtering vastly reduces the effect of signal noise and improves measurement accuracy.