Acompact shock-sensing device, which employs an innovative light sheet generator, has been created. The device may be used either as a solo aerodynamic shock detector or in combination with a scanning mode shock sensor. This permits easy detection and tracking of unstable and traveling shocks in supersonic inlets.
The device consists of a light source; a means to form the light emitted from the light source into a small-diameter, pencil-slim column of light; and a means to direct this column of light toward a diffracting element. Additionally, the diffracting element scatters the column of light in one or more sectors prior to the light entering the aerodynamic flow. Each of the sectors forms a sheet of light that is located in one plane. The thickness of the resultant light sheet is the same as the original diameter of the pencil-slim column of light. This device also has a means for changing the direction of the sectors of light in such a way that they propagate through a section of an inlet in a collimated or otherwise proscribed manner. A photodetector built into the device is capable of detecting the variations in the light intensity within original sectors caused by the presence of shocks. The device also contains a means to process signals from the photodetector in the form of signal processing apparatus and algorithms.
For verification of operability and functionality, the shock-sensing system based on this innovation used a HeNe laser source that produced an approximately 1.0-mm-diameter Gaussian beam measured at the exit aperture of the laser.
In the prior art (U.S. Patent 5,715,047), a small-diameter (pencil) laser beam was sent either toward a scanning (rotating) mirror positioned in such a way that the reflected beam was directed toward a lens or toward a light dispersion element, while the wavelength of the laser was changed or tuned. In both cases, the lens produced a collimated laser pencil beam that moved across the lens aperture while maintaining its small diameter. In the first embodiment described by the prior art, the rotation of the mirror provided for a laser pencil beam scan. In the second one, the scan of the laser beam was provided by a dispersion of the light with a varying wavelength on a grating. In both embodiments, the laser beam maintained its small diameter prior to entering the aerodynamic flow.
In the present embodiment, the time invariant spatial distribution of light intensity is obtained by diffraction of the incident laser beam by a wire. A sheet of light is the result. The wire is placed in the focal point of a collimating lens.
Because the classical diffraction patterns have “lobes” with bright and dark intensity zones with their locations depending on the number and geometry of diffracting wire(s), the diameter and position of the wire(s) are selected in a way that the resulting diffracted and collimated light sheet covers the entire area of interest in the inlet without producing dark zones.
This work was done by Grigory Adamovsky and Roger P. Tokars of Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Glenn Research Center
Refer to LEW-18586-1.