Patent ID: 6115127
Filing Date: 2000-09-05
Classification: G01B,G01H,G01N

Abstract:
An apparatus for non-contact measurement of ultrasonic waves in a moving paper web, comprising;a laser source for providing an incident beam and a pump beam having a coherence length sufficient for use in interferometry;an optical head for focusing the incident beam onto the surface of the paper and for collecting the resulting laser speckles scattered from the paper over a solid angle into a reflected beam, wherein the incident beam and the reflected beam are substantially coaxial and travel through the optical head with an angle of incidence for detecting ultrasonic waves moving in plane with the paper web and in directions outside the plane of the paper web;a photorefractive crystal for receiving the pump beam and the reflected beam, wherein the pump beam and the reflected beam interfere creating a grating for diffracting the pump beam into a reference beam having a wavefront substantially the same as the wavefront of the reflected beam; andapparatus for causing the reflected beam and the reference beam to interfere and for converting phase variations of the reflected beam caused by the ultrasonic waves into an electrical signal;wherein the electrical signal represents ultrasonic movement in the paper web.