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Patent Issued for Optical Fiber Sensors Having Long Active Lengths, Systems, and Methods

March 5, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- A patent by the inventors Donlagic, Denis (Maribor, SI); Pevec, Simon (Podcetrtek, SI); Cibula, Edvard (Starse, SI), filed on March 11, 2011, was published online on February 18, 2014, according to news reporting originating from Alexandria, Virginia, by VerticalNews correspondents.

Patent number 8655117 is assigned to University of Maribor (Maribor, SI).

The following quote was obtained by the news editors from the background information supplied by the inventors: "Optical fiber strain sensors have been the topic of intense research during the last decade as they can be made very small, compact, immune to electromagnetic interference, biocompatible, and can be used at elevated temperature or in relatively harsh chemical environments. Applications for such sensors are, therefore, numerous and may range from structural monitoring to biomedical applications.

"Certain fiber optic strain sensors are known in the art. One presented solution includes a specially designed double core fiber. Optical power is exchanged between the two cores as a function of applied strain. However, considerable lengths of optical fiber and complex signal processing may be required to make such systems practical. Other solutions are based on bend loss phenomena in the optical fiber where the applied strain modulates the intensity at a sensor output. For those solutions a relatively simple detection scheme may be applied, but they may have drawbacks in terms of relatively low sensitivity, relatively large size, and relatively low absolute accuracy that is typical for intensity-based sensors based upon bend loss phenomena.

"Currently many fiber optic strain sensors are based on fiber Bragg gratings. Bragg grating sensors may however suffer from relatively high temperature sensitivity, require complex signal processing, and may not be miniature in their size (e.g., length). Other solutions to the optical fiber strain sensors rely on polarization effects in optical fibers and measurements of light pulse propagation time in a fiber that is exposed to a measured stress.

"Certain strain sensing solutions can be obtained in the form of fiber-based Fabry-Perot sensors. Such fiber-based Fabry-Perot sensors may be appropriate for practical sensing applications since they may be interrogated by variety of straightforward and cost effective, commercially available opto-electronic interrogation techniques. In some applications, a Fabry-Perot interferometer may be used for strain measurements. For example, in the prior art, two perpendicularly cleaved optical fibers may be placed in a glass capillary in such a way to form the short air cavity between the fiber ends. This cavity creates an optical Fabry-Perot resonator that changes its length proportionally to expansion of the glass capillary. One drawback of this approach is in the use of adhesive and not well-defined point where fiber adheres to the capillary as the adhesive randomly penetrates the gap between fiber and capillary. Furthermore, the adhesion between capillary and fiber may impose thermal, mechanical, and chemical stability limitations. The outer diameter of such sensor is always considerably larger than the fiber diameter, which increases its size and may limit its possible applications and packaging options. Friction between the fiber and the capillary may cause sensor hysteresis. Furthermore, the production process of such capillary sensors may be complex and may involve number of precision alignment steps.

"In another prior system, a hollow core optical fiber is used to create a spacer between two perpendicularly cleaved fibers. In this case, the sensor (hollow core optical fiber) has the same diameter as optical fibers but it relies on manufacturing and splicing of the hollow core optical fiber to a standard optical fiber which presents its own difficulties. In other embodiments, solutions including a concave cavity are used instead of hollow core fiber or capillary. In some embodiments, the creation of a strain sensitive cavity may be provided by an etching process. While such prior art optical sensors may eliminate some of the drawbacks of capillary-type sensors, they may suffer from limited sensitivity.

"In prior art all-fiber designs, the cavity length is varied and converted into cavity length change under influence of an applied strain. The sensor sensitivity to the strain can be incensed by increasing the cavity length. However, this may lead to high optical losses, low interference fringe visibility and overall sensor signal degradation. In one solution, an air cavity is replaced by a fiber that can guide the light. For example, a section of single mode fiber is inserted between two semi-refractive mirrors to create a Fabry-Perot interferometer. While such solution allows for arbitrary resonator length, it may suffer from high temperature sensitivity induced by fiber core refractive index temperature dependence and may require a complex manufacturing procedure that involves vacuum deposition of mirrors onto the individual fiber surfaces. Furthermore, long Fabry-Perot cavities may require more complex signal processing techniques as the free spectral range of long cavity becomes narrow, requiring higher resolution spectrometric sensor signal interrogation techniques.

"Furthermore, many, if not all, fiber Fabry-Perot strain sensors known in the art are made by complex and expensive production procedures that involve multiple production steps, and that are therefore not generally suitable for high volume, cost-effective production.

"Thus, it should be recognized that the performance of such optical sensor devices may be limited, and/or manufacturing of such optical sensor devices may be relatively complicated and not cost effective. Therefore, there is a long felt and unmet need for highly effective optical sensor and manufacturing methods thereof."

In addition to the background information obtained for this patent, VerticalNews journalists also obtained the inventors' summary information for this patent: "In one aspect, an optical fiber sensor is provided. The optical fiber sensor includes a lead-in optical fiber having an end surface at a forward end; a first optical element having a body with an outer dimension, Do, a front end surface coupled to the lead-in optical fiber, a pedestal including a retracted surface that is spaced from the front end surface, the retracted surface at least partially defining an optical cavity, a gutter surrounding the pedestal, the gutter having a gutter depth defining an active region of length, L, the first optical element further exhibiting L/Do.gtoreq.0.5.

"In another aspect, a strain measurement system is provided. The system includes an optical fiber sensor adapted to be coupleable to an object undergoing strain, the optical fiber sensor including a lead-in optical fiber and an optical element coupled thereto, the optical element having an outer dimension, Do, and a pedestal including a retracted surface, the retracted surface at least partially defining an optical cavity, a gutter surrounding the pedestal, the gutter having a gutter depth defining an active region of length, L, the first optical element further exhibiting L/Do.gtoreq.0.5; and a signal processor coupled to the optical fiber sensor, the signal processor operable to generate and pass a light signal into the lead-in optical fiber, receive a light signal reflected from at least the retracted surface of the optical element, and determine a strain applied to the object.

"In a method aspect, a method of manufacturing an optical fiber sensor is provided. The method includes providing a structure-forming fiber; and micromachining the structure-forming fiber to produce an outer dimension Do, a pedestal, and a gutter surrounding the pedestal having a gutter depth defining an active region of length, L, wherein L/Do.gtoreq.0.5.

"In a method aspect, a method of manufacturing an optical fiber sensor is provided. The method includes providing a structure-forming fiber having a central portion etchable at a first rate when exposed to an etching medium, a inner cladding portion surrounding the central portion having a second etching rate that is greater than the etching rate of the central portion when exposed to the etching medium, and an annular outer cladding portion surrounding the inner cladding portion having a third etching rate that is less than the etching rate of both the central portion and the inner cladding portion when exposed to the etching medium; and exposing an end of the structure-forming fiber to the etching medium for a sufficient time to produce an outer dimension Do, a pedestal, and a gutter surrounding the pedestal having a gutter depth defining an active region of length, L, wherein L/Do.gtoreq.0.5.

"In another method aspect, a method of manufacturing an optical fiber sensor is provided. The method includes providing a structure-forming fiber having a central portion having a composition of doped silica, an annular inner cladding portion surrounding the central portion having a composition of doped silica that is preferentially doped to provide an etching rate in an etching medium greater than the central portion, and an outer annulus portion surrounding the gutter portion and having a composition of substantially pure silica to provide an etching rate in the etching medium less than both the central portion and the annular inner cladding portion; and exposing an end of the first optical fiber segment to an etchant for a sufficient time to produce an outer dimension Do, pedestal, and a gutter surrounding the pedestal having a gutter depth defining an active region of length, L, wherein L/Do.gtoreq.0.5.

"In yet another method aspect, a method of using an optical fiber sensor is provided. The method includes providing an optical fiber sensor having a lead-in optical fiber having an end surface at a forward end, and a first optical element having a body with an outer dimension, Do, a front end surface coupled to the lead-in optical fiber, a pedestal including a retracted surface that is spaced from the front end surface, the retracted surface at least partially defining an optical cavity, a gutter surrounding the pedestal, the gutter having a gutter depth defining an active region of length, L, the first optical element further exhibiting L/Do.gtoreq.0.5; coupling at least a portion of the body of the first optical element to an object undergoing strain; passing a light signal into the lead-in optical fiber from a signal processor, the signal processor operable to receive a reflected light signal from at least the retracted surface of the first optical element and determine a strain applied to the object.

"In yet another method aspect, a method of using an optical fiber sensor is provided. The method includes providing an optical fiber sensor having a lead-in optical fiber having an end surface at a forward end, and a first optical element having a body with an outer dimension, Do, a front end surface coupled to the lead-in optical fiber, a pedestal including a retracted surface that is spaced from the front end surface, the retracted surface at least partially defining an optical cavity, a gutter surrounding the pedestal, the gutter having a gutter depth defining an active region of length, L, the first optical element further exhibiting L/Do.gtoreq.0.5; providing at least a portion of the body of the first optical element in a space undergoing pressure changes; passing a light signal into the lead-in optical fiber from a signal processor, the signal processor operable to: receiving a light signal reflected from at least the retracted surface of the first optical element, and determine a pressure applied based upon length changes in the active region of the first optical element.

"Numerous other aspects are provided in accordance with these and other aspects of the invention. Other features and aspects of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings. The drawings are not necessarily drawn to scale."

URL and more information on this patent, see: Donlagic, Denis; Pevec, Simon; Cibula, Edvard. Optical Fiber Sensors Having Long Active Lengths, Systems, and Methods. U.S. Patent Number 8655117, filed March 11, 2011, and published online on February 18, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=39&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=1902&f=G&l=50&co1=AND&d=PTXT&s1=20140218.PD.&OS=ISD/20140218&RS=ISD/20140218

Keywords for this news article include: Electronics, Signal Processing, University of Maribor.

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Source: Electronics Newsweekly


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