US 5,680,212 AGranted

Sensitive and fast response optical detection of transient motion from a scattering surface by two-wave mixing

AssigneeNational Research Council of Canada

InventorsAlain Blouin, Philippe Delaye, Denis Drolet, Jean-Pierre Monchalin, Gerald Roosen

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Abstract

A method and apparatus are provided of optically detecting transient motion from the surface of a workpiece. After directing a laser beam onto a surface of a workpiece the beam is reflected from it and then interferes with a pump beam removed from the laser source, inside a real-time holographic material, so as to form a grating diffracting the pump beam into a reference beam, which interferes at the output of the real-time holographic material with the received light beam, so as to produce a signal representative of the surface transient motion. An electric field is applied to the holographic material of sufficient magnitude to increase substantially the intensity of said reference beam and to give to this beam a phase differing substantially from the phase of said received light beam. The electric field is applied prior to the transient motion to be detected, for a time interval sufficiently long so as to capture the transient motion but sufficiently short a duration so as to avoid excessive heating of the real-time holographic material. The pump beam is of sufficiently high intensity so as to cause the response time of the real-time holographic material to be substantially shorter than a characteristic time defined as the time during which the phase of the received light, on average across its spatial distribution or locally, is not substantially changed.

Claims23

Dependent claims

A method of optically detecting transient motion from the surface of a workpiece having a predetermined duration, comprising the steps of:

a) directing a beam generated by a laser onto said surface;
b) receiving a light beam that has been reflected or scattered by said surface;
c) allowing the received light beam to interfere with a pump beam removed from the laser, inside a real-time holographic material, so as to form a grating diffracting the pump beam into a reference beam, which interferes at the output of the real-time holographic material with the received light beam, so as to produce a signal representative of the surface transient motion;
d) applying onto said real-time holographic material, an electric field of sufficient magnitude to increase substantially the intensity of said reference beam and to give to this beam a phase differing substantially from the phase of said received light beam, wherein the step of applying the electric field is performed prior to the transient motion to be detected, for a time interval sufficiently long to capture the transient motion but sufficiently short to avoid excessive heating of the real-time holographic material;
wherein
said pump beam being of sufficiently high intensity to cause the response time of the real-time holographic material to be substantially shorter than a characteristic time defined as the time during which the phase of the received light, on average across its spatial distribution or locally, is not substantially changed;
and,
e) inducing surface transient motion at a predetermined time after electric field application onto the real-time holographic material, and wherein the pump beam intensity, applied field amplitude and applied field duration are such, as to avoid real-time holographic material catastrophic failure.

5 more elements

An apparatus for optically detecting transient motion from the surface of a workpiece having a predetermined duration, which comprises:

a) means for inducing said transient motion;
b) means for generating and directing a laser beam onto said surface;
c) means for receiving light reflected or scattered by said surface and phase modulated by its transient motion;
e) real-time holographic material element provided with electrodes and arranged to collect a received light beam;
f) means for removing directly from the laser beam a pump beam of sufficiently high intensity to cause the response time of the real-time holographic material element to be substantially shorter than a characteristic time defined as the time during which the phase of the received light, on average across its spatial distribution or locally, is not substantially changed;
g) means for causing the received light beam to interfere inside the real-time holographic material element with said pump beam, so as to form a grating diffracting said pump beam into a reference beam, which interferes at the output of the real-time holographic material with the received light beam, so as to produce a signal representative of the surface transient motion;
h) means for applying onto said real-time holographic material element electrodes a pulsed electrical voltage of sufficient magnitude to increase substantially said reference beam intensity with a phase also differing substantially from the phase of said transmitted received light beam;
i) means for timing the triggering of the excitation of the transient motion and the application of the electrical voltage, the voltage being turned on before the triggering of said means for inducing transient motion and turned off after end of transient motion, while keeping turn-on duration minimum to avoid heating of the real-time holographic material.

5 more elements

Dependencies are inferred from the claim text.