US 4,036,557 A · Laser doppler velocimeter

Abstract

A laser doppler velocimeter for measuring one or more velocity components of a moving material, fluid or solid, using a single set of optical focusing elements. A unique two component beam splitter provides multiple pairs of parallel beams, one pair from each input radiation beam from a given laser source. Each incident radiation input beam is separated by means of a selectively transmissive coating on one of the roof faces of an Amici prism into a first beam comprising substantially all of the polarized component parallel to the plane of incidence (P polarization) and one-half of the polarization component normal to the plane of incidence (S polarization), and a second beam comprising one-half of the S polarization component alone. At the measuring point, the two beams interfere, with the S components producing a differential doppler frequency superimposed on a non-doppler pedestal, while the P component contains only the non-doppler pedestal. The detector beam splitter directs the P polarization component to one radiation detector and the S polarization component to the remaining detector. The analog electrical equivalent signal to the optical P component signal is electrically subtracted from the electrical analog equivalent to the S component signal, the pedestal is removed, and this signal is divided by the P component signal to yield a signal having the pure doppler differential frequency information. The invention provides dual beam redundancy, extremely simple alignment, and two velocity measuring ranges.

Claims25

Dependent claims

A laser doppler velocimeter for measuring the velocity of material at a reference point, said velocimeter comprising:

a pulsed laser source for generating a pulsed coherent radiation input beam having first and second orthogonal polarization components;
first beam splitter means for generating a pair of parallel emergent beams of coherent radiation from said input beam, said parallel emergent beams having different polarization states;
said beam splitter means including a first optical axis, a radiation input face to which said input beam is directed, a pair of radiation output faces, said beams emerging from said radiation output faces parallel to said first optical axis, means for generating a first one of said pair of parallel emergent beams as a preselected fractional portion of said first polarization component and for generating the remaining one of said pair of parallel emergent beams as substantially all of said second polarization component and the remaining fractional portion of said first polarization component;
means for focussing said pair of beams to said reference point to produce reflected radiation having a first polarization component containing both doppler differential frequency information and background noise and a second polarization component containing substantially only background noise;
and
means for separating said reflected radiation into a pair of beams containing said first and second polarization components, respectively, so that the separated reflected beam pair may be directed to a measurement device.

3 more elements

A method for isolating differential doppler frequency information from background noise in a laser doppler velocimeter, said method comprising the steps of:

a. generating a coherent radiation input beam composed of mutually orthogonal polarization components;
b. deriving from said input radiation beam a first probe radiation beam comprising a predetermined fractional portion of one of said mutually orthogonal polarization components and a second probe radiation beam comprising the remaining fractional portion of said one of said mutually orthogonal polarization components and substantially all of the other one of said mutually orthogonal radiation components;
c. focussing said first and second probe beams to a measuring point to produce a reflected beam having a first orthogonal polarization component containing both said differential doppler frequency information and said background noise and a second orthogonal polarization component containing said background noise alone;
d. separating said reflected beam into said mutually orthogonal components thereof;
and
e. combining said mutually orthogonal components of said reflected beam to cancel said background noise.

3 more elements

A system for isolating differential doppler frequency information from background noise in a laser doppler velocimeter, said system comprising:

means for generating a coherent radiation input beam composed of mutually orthogonal polarization components;
means for deriving from said input radiation beam a first probe radiation beam comprising a predetermined fractional portion of one of said mutually orthogonal polarization components and a second probe radiation beam comprising the remaining fractional portion of said one of said mutually orthogonal polarization components and substantially all of the other one of said mutually orthogonal radiation components;
means for focussing said first and second probe beams to a measuring point to produce a reflected beam having a first orthogonal polarization component containing both said differential doppler frequency information and said background noise and a second orthogonal polarization component containing said background noise alone;
means for separating said reflected beam into said mutually orthogonal components thereof;
and
means for combining said mutually orthogonal components of said reflected beam to cancel said background noise.

3 more elements

Dependencies are inferred from the claim text.

Laser doppler velocimeter

Life of this patent

Laser doppler velocimeter

Life of this patent

Abstract

Claims25