Distributed optical fiber sensor with controlled response

Details Distributed optical fiber sensor with controlled response Distributed optical fiber sensor with controlled response

1999-06-16

2001-03-20

Henry, Jon (Department: 2872)

Optical waveguides

Optical waveguide sensor

C436S805000, C436S527000, C356S073100, C356S445000, C250S227140

Reexamination Certificate

active

06205263

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to optical fiber sensors and more particularly to such sensors which include materials in the coatings and/or claddings of optical fibers which produce changes in the characteristics of the light carried by the fiber responsive to the presence of a material or field to be detected.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 4,834,496 issued May 30, 1989 discloses distributed fiber optic chemical sensors. Such a sensor comprises an optical fiber with a core and a cladding which is permeable. The cladding, or a coating on the cladding, includes a composition, the optical properties of which are altered in the presence of a material to be detected. The light (i.e. the wavelength or the intensity of the light) transmitted through the core of the fiber is a function of the change in optical properties caused by the interaction of the composition included in the permeable coating with the material to be detected. The change in optical properties may comprise (for example) a change in the index of refraction (or indices of refraction differential), or an increase or decrease in the optical absorbance or fluorescence of the composition.
The above-noted patent discloses materials, compositions, sensing parameters and examples of such sensors, all of which are useful in accordance with the principles of this invention. In addition, distributed fiber optic sensors for physical properties, such as temperature or pressure, can also be fabricated based on similar properties, e.g., temperature-induced or pressure-induced changes in the refractive index, optical absorbance, or fluorescence of a cladding or coating material applied to a light-guiding core.
Unfortunately, such optical fibers are characterized by spatial transients for transmitted light which causes the sensitivity of chemically or physically sensitive fibers to vary from point to point along the fiber. This is particularly true of multi-mode fibers with lossy (e.g. absorber-doped) coatings where light does not reach equilibrium for a considerable distance. Such a varying response is due to a spatial transient, associated with the existence of radiation modes, and a stronger attenuation of higher order bound modes. Accordingly, sensor response over the length of the fiber is not constant.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with the principles of this invention, the fraction of light lost per unit length may be controlled by controlling the physical design of the fiber. One use for this invention is to compensate for the spatial transient in distributed fiber optic sensors, wherein a constant sensitivity over the length of a distributed fiber sensor is achieved by introducing a change in the light-guiding characteristics of the fiber to compensate for the effects of spatial transients. Thus, for example, in one embodiment, a loss-compensated distributed optical fiber sensor comprises a core and a cladding where the core to cladding refractive index differential increases with length to compensate for the loss of higher order modes in the (multi-mode) fiber. As a result, a highly uniform, distributed sensor is realized.


REFERENCES:
patent: 4321057 (1982-03-01), Buckles
patent: 4834496 (1989-05-01), Blyler, Jr. et al.
patent: 5737472 (1998-04-01), Bernasson et al.

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