US 4,854,706 A · Modal domain optical fiber sensors

Abstract

A modal domain optical fiber sensor for vibration monitoring and as a mechanical motion detector. A laser that focuses coherent light through a lens into one end of a multimode optical fiber. The fiber is secured to a mechanical mechanism for exciting vibrations. The other end of the optical fiber passes its output through a spatial filter and into a photodetector. A variable voltage signal from the photodetector is fed to a monitoring unit, such as an oscilloscope or a microcomputer, for analysis. Of particular significance is the intentional mismatch between the wavelength of the light 18 from the laser and the single mode cut-off wavelength of the glass optical fiber. In a preferred embodiment, the optical fiber has a wavelength of 850 nm and the coherent light has a wavelength of 633 mm.

Claims20

Dependent claims

A modal domain optical fiber sensor for detecting movements of a mechanical system, said sensor comprising:

(a) laser means for producing a coherent beam of light having a first wavelength;
(b) an optical fiber having a single mode cut-off at a second wavelength, said second wavelength being greater than said first wavelength;
(c) means for directing said beam of light into one end of said optical fiber;
(d) means for securing a portion of said optical fiber to said mechanical system for moving said portion in response to movement of said mechanical system;
(e) detecting means for detecting a portion of a speckle pattern created at the other end of said optical fiber as said beam emerges from said other end, said first and second wavelengths being chosen so that said speckle pattern has a plurality of lobes, the position of said speckle pattern changing in response to the movement of said portion of said optical fiber;
and
(f) means for filtering said speckle pattern so that said portion of said speckle pattern constitutes one of said lobes.

4 more elements

Dependencies are inferred from the claim text.

Modal domain optical fiber sensors

Life of this patent

Modal domain optical fiber sensors

Life of this patent

Abstract

Claims20