CA1304243C - Pressure or strain sensitive optical fiber - Google Patents

Abstract

PRESSURE OR STRAIN SENSITIVE OPTICAL FIBER

ABSTRACT

Pressure sensitive optical fiber having core means, at least a portion of which has a predetermined refractive index for transmitting light therethrough, cladding means adjacent the core means having a refractive index which is less than that of the core means and concentric light transmissive means adjacent the cladding means having refractive index which is greater than that of the cladding means and through which light passes in proportion to the amount of stress or strain induced in the fiber. Also, pressure sensors and methods for measuring forces or pertubations utilizing such fiber.

Description

13~2~

PRESSURE OR STRAIN SENSITIVE_OPTICAL FIBER

Techn c_l Field The invention relates generally to an optical waveguide having at least two ~oncentric cores arranged in such a manner that, upon an application of pre~sure on the waveguide, light is coupled between adjacent cores, t~ereby allowing the waveguide to act ~ a pressure sensor.
ack~round Art Optical waveguides are well known ln the art, and devices incorporating optical waveguides have been employed in many different field~ as communicators, sensors and 16 monitor~ An optical waveguide typically consists of a dielectric core fabricated from a material haviny a certain refractive index, aurrounded by a second material having a lower refractlve index. Thi~ surrounding material is generally known as the cladding. A beam o~ light is guidad by this composite structure as long as the refractive index of the core material exceeds the re~ractive index of the cladding material. A light beam within the core is guided generally along the core axis ~y reflection at the boundary between the core and cladding.
A number of different designs for optic~l waveguides have been develope~ inc~ding the multimode step index profile, the single mode ~rofile, and the multimode graded index profile. Where single mode transmission is desired, the slngle mode optical waveguide is used. Tn such a waveguide, the dia~eter of the core is typically less than l0 and the difference between the refractive indices of the cores and the cladding is on the order of lO 3 to l0 2. At ~31~Z~

wavelengths which are longer than a critical wavelength, called the LPl1 cuto~f wavelength, only the lowest order optical mode will be ~upported in such a waveguide.

Optical fibers have also been fabricated which include multiple cores disposed in numerous different arrays and positioned within a common claddincJ. one such disclosure is contained in U.S. Pat. No. 4,1~8,560. This disclosure is directed toward an assembly including a plurality of ~ibers tO embedded in an encapsulating materlal. This particular patent shows an optical bundle positioned between two reinforcing wires and embedded in a protective sheath of plastic material.

The phenomenon known as cross-talk between cores in a common cladding occurs when the light energy propagating along one core i3 coupled to an ad~acent core. This occurs because, as is known, the propagati~g optical energy is not totally confined by the boundary between the core and cladding but, in fact, it penetrates to a degree into the cladding.

U.S. Pat. No. 4,z95,738 discloses a pressure sensitive optical waveguide comprising multiple, non-csncentric cores formed in a manner that cross talk betwe~nadjacent cores is primarily a function of the strain or hydrostatic pressure applied to the waveguide.

Also known are multiple clad fibers such as those disclosed in U.S. Pat. No. 4,435,040; however~ all of the concentric, guidiny layers of t~is and similar patents ars configured to interact with the mode field diameter of the optical power distribution passing through the unperturbed ~ib~r. For this reason, these waveguides cannot be utilized for detecting pressure variations on the fiber. The intent and purpose of such mult~ply clad fibers is to alter the optical dispersion characteristics of the waveguide. Optical dispersion affects the information carrying capacity or bandwidth o~ a fi~er.

5 Summar of the Invention The invention relates to pressure sens~tive~Optical fiber comprising core means at least a portion of which having a predetermined refracti~e index for transmitting 10 light therethrough, cladding means adjacent the core ~eans having a refractive index which is less than that of the core means portion, and concentric light transmissive means adjacent the cladding means havi~g a reractive index which ~s greater than that of the cladding means and through which 15 light passes in propor~ion to the amount of stress or strain induced in the fiber.

The light transmission means preferably has an internal diameter greater than the unperturbed mode boundary 20 diameter of the core means, which can comprise single mode or multimode optical fiber, with or without means for adjusting or altering modal, waveguide and~or chromatic dispersion.
The core and cladding means can include step index, graded index, depressed clad, deeply depressed clad, W-type or 25 multiple clad single mode optical fib~r, and the fiber may include means for transferring analog and/or digital data.
If desired, the light transmissiv~ means may include a plurality of concentric light transmissive layers.

The invention also relates to a pressure sensor comprising the pressure sensitive optical fiber described a~ove, with means for introducing optical power into one end of the optical fiber, and means Por detecting the optical power at the opposite end of the optical fiber.

~3~gl2~3 The det~ating means can include means for measuring - the decrease ~n optical power in th~ core means, means for measuring the increase ~n optical power in the light trans~is~ive mean~, or both. The sensor may include means -; for converting or amplifying any external ~orce to bending forces on the fiber. Also~ the sensor ~ include at least one coating on the optical fiber to vary the sensitivity of the fiber.

The invention also relates to a method for detecting pressure or force pertubations which comprises preparing the pressure sensitiv~ optical fiber or pressure sensors describèd above, applying a light source at one end of the sensor, subjecting the sensor to hydrostatic or other pressure or strain, and mea6uri:~g at least one of core means optical power decrease, or light transmissive means optical power increase, thereby detecting pressure or strain variations on the fiber or sensor. The sensitivity of the sensor can be varied by changing at least one of attenuation coefficient, refractive index, or width of the core means or light transmissive means, or the distance between -e~ the light transmissive means and core means.

Brief Description of the Drawings The foregoing and other features and advantages of the present invention will become more apparent from the following description of the preferred embodiments illustrated by the accompa~ying drawings, where:

FI~ is a cross sectional view of the well known single and multimode step index optical fiber, along with their respective refractive index profiles;

~L3~

~ FIG. lB ls a cross sectional view of pressure sensitive optical flbers according to the invention, along with their respective refractive index profiles FIG. 2A illustrate~ the refractive index profile and design parameter3 for a single mode fiber of the invention; .

FIG~ 2~ illustrates refractive index profiles oP
the initial, perturbed~ and final conditions, respectively, and the corresponding optical power distributions of an optical fiber pressure sensor according to the present invention;

FIG. 2C is a graphical illustration o~ the interrelationship o~ the property parameters of the optical fiber of the inventlon;

FIGS. 3 through 10 are refractive index profiles 2~ for eight add~tional embodiments o~ the invention relating to single mode optical fiber pressure sensors:

FIGS. 11 and 12 are refractive index profiles ~or further embodiments of the invention relating to multimode optical ~iber pressure sensors;
FI~S. 13 through 15 are illustrations of various applications for the pressure sensitive optical fi~ers of the imrention; and FIG. 16 is a spectral attenuation curve of experimentally induced loss in the optical fiber prPssure sensor of FIG. 5.

~31~ 3 .

Detaile_ Descrip~ion of the Preferred Embodiments The pre~ent lnvention generally relates to a novel design ~or optical fiber~ which are sensitive to micro and macrobending forces. The design may ble applied to single mode as well as multimode fiber~. In the case o~ single mode fibers, it is possible to designate a pressure sensitlve passband and a relatively pressure-insensitive passband in the same fiber thus allowing si~ultaneous multi-functions (analog or dig~tal commun~cation~ or data trans~er) on one passband and pressure detection on the second passband.

The sensed pressure is proportional both to the fractional reduction in transmitted core power and the incre~sa in optical power in the concentric reglon. If desired, both the power reduction in the core and the increase in the concentric region can be measured for greater accuracy. Well defined design parameters can be adjusted to pre-select the dynamic range of these pressure sensors.

Referring initially to FIG. lA, there are illustrated cross sections of two well known optical fibers.
The cross section o~ a single mode step index fiber lO and of a multimode step index fiber 20 are shown along with their respective re~ractive index prof~les. In the case of the single mode fiber, a single narrow core 15 having a relatively hiqh index of refraction and being on the order of a few microns in diameter is provided within a cladding 25.
In the multimode fiber, a similar cladding (designated by the 3V same numeral 25) is utilized but the core material 30 is typically of a much larger diameter than that of single mode fiber. Thus, the multimode fiber enables a larger mode volume to pass through the fiber, ~4;i~3 ~7--FIG. lB illustrates t~he pressure sensitive optical fiber according to the present invention, ~or both ~ingle mode 11 and ~ultimode ~tep index fiber 21. The core and cladding for these fiber~ are the ~ame as those of FIG. lA;
however, as shown, the provision o~ a concentric light transmi~sive layer 22 enables pert~bations or other applications oE force on the fiber to be mea~ured, since light wlll pass through the concentric layer in proportion to the amount of stres~ or strain induced in the fiber.

FIG. 2A illustrates the major design parameters for an embodiment oP the single mode optical fiber of the invention. Thesa design parameters are ~ Na and ~ Nb, the index difference~ between concentric regions, respectively; a and w, the width of the core a"d the concentric re~ions, respectively; c, the separation between thè core and the conc~ntric region; and ~b the optical absorbance of the concentric region.

FIG. 2B illustrates the refractive index profile ~or an optical fiber pressure sensor according to the present invent~on which utilize~ a ~ingle mode fiber. The refractive index of the core material 35 is depicted and the dashed line 40 indicates the normalized distribution of the optical power passing therethrough. This normalized distribution is known as the mode field and it is generally accepted that the mode field diameter is measured from the -3dB down points of the peak of the power distribution. Thus, in the normal operation o~ the fiber, the distribution of light is substantially limited to the immediate vicinity ~f the high refractive index core material 35. When a pert~bation is applied to the fiber, such as by bending or other application of pressure, force or strain la macrobend is depicted with the left side of the drawing indicating that the ~iber is in compression while the r~ght side indicating that it is in 2~3 tension), the light distrlbution is partially shi~ted and a portion sn encounters the conc~ntric light transmissive layer 45. Now at the exit end of the pres~ure sen~or, the core contains an amount of light having a lower optical power than it doe~ at the entrance to the sensor with the difference appearing in the light transmissive layer 45 as optical power 50. Thi~ illustrates how the 3ensor can be used to measure the amount of pressure or strain on thle sensor because the reduction in optical power in the core or the gain in the light transmis~iv~ layer 45 can be measured and is representative of force applied to the sensor. Either the light loss ~f the core or the gain in the outer layer can be measured a~ noted above, or, for accuracy, both can be measured and compared.

In analogy with traditional sensors, the parameters ~Nb, W, and ~ b determine the "gain" of the fiber sensor and the parameter b determines the "zero" of the sensor. rhat is, the onset of power increase in the concentric region is determined by its proximity to the core region. The extent of power increase in the core for a given bending perturbation and given core parameters is determined by the index elevation, width, and attenuation characteristic~ of the concentric region. Linearity is a function of the shapes of the core and concentric regions.
The interrelation of some o:E these parameters is shown in FIG. 2C.

This fi~er sensor has three sp~ctral bands of particular interest. AboYe the LPll cutoff wavelength the fiber exhibits a pressure sensitive band wherein the attenuation o~ optical pow~r i~ proportional to the sensed pressur~. This is th~ regime wherein the fundamental mode may be biased, by bending perturbations, to couple into the concentric region. At or near the LPll cutoff wavelenyth, ~3~

the LPol mode is tightly bound and the "zero" of the sensor may be positioned such that ordinary bending perturbations are insufficient to force coupling between the fiber core and the concentric region. Thi~ spectral band is therefore relatively pressure ~nsensitivq. ~elow the LPll cutoff wavelength, the fiber is no lo~ger singlemode.
, .
In a wavelength divis~on multiplexed system, this fiber would allow analog or digital communications or data trans~er ~n the relatively pres~ure insensitive passband while simultaneously passing pressure information in the pressure sensitlve passband.

- In the single mode pressure sensor embodiment of the invention, the re~ractive index profile of any single mode fiber design (step index, W-type, depressed clad, dispersion shifted, etc.) ~s modified by the addition of a special concentric region. This secondary region comprises a refractive index which is greater than the average or effective index of that part o~ the fiber which lie~ between the core and the secondary region. The concentric region is situated radially such that the fundamental modes, which in the unperturbed condition are guided by the centr~l core region, can be induced to coupls into the concentric region by means of macro- and/or microbending of the fiber. The extent of coupling is a function of the extent of the induced macro- and microbending. Lateral pressure on optical fibers will produce macro- and microbending under a variety of conditions. Thus, this fiber may be used separately or in comhination with other components as a pressure sensor.

,s 1 o --FIGS. 3-5 illustrate different types of optical fiber which can be u~ed in accordance with the teaching of the invention. In thes~ ~igur~, the light tran~missive concentria layer o~ the inventi.on i~ designated as 60, with the ~ore generally indicated a! 70-FIGS. 6 and 7 illustrate optLcal s~nsors whichutiliz~ special cor~s that can be used to adjust the dispersion of the light passin~ therethrough. As illustrated therein, that special type of aore can also be used in accordance with the teachings of the present invention.

One skilled in the art would realize that the spacing o~ the light transmissive layer with respect to the core is important to determine the sensitivity of the fiber to pressure. Thus, it i8 possible to make extremely highly sensitive fiber by mai-ntaining the spacing ~c in FIG. 2~ of the pressure sensitive light transmissive layer at a minimum distance from the mode field diameter, while large spacings can be used for less sensitive pressure detection applications.

The concentxic regio~ is also situated radially such that the propagation of light within this concentric region may or may not be substantially attenuated along the length of the fiber, depending upon the application.
Attenuation of light coupl~d into the concentric region may be ~aximized by, for example, adding ab~orbing or scattering materials to that region ~r placing the concentric region in proximity with a lossy (absorbing or scattering) region.

FIGS~ 8-l0 illustrate further emhodiments of the invention. In FIG. 8, a high quality light transmissive concentric region is used in proximity with a high loss region, as mentioned above.

~3~ L3 FI~. 9 illustrates a highly absorbing or scattering concentric reg1On which can be used for minimizing the transmi~sion in the outer layer.

Alternat~vely, the attenuation of light coupled into the concentric region may be minimized by fabricating the region with the same high quality materials and the same design principles which are used to design low loss core region~. For example, the concentric region could comprise an elevated refractive index of limited radial extent bounded by a low loss cladding material, as lllustrated in FIG. lO.

For the multimode pressure sensor embodiment, the ` parameters b, ~ Nb, W, and ~b Pox the concentric region will contribute to a determination of the zero and gain of the sensor as described for the single mode 6ensor. The concentric region may also be either maximally attenuating or minimally attenuating, depending upon the design ~requirement~.
FIG. ll ~llustrates a step index multimode optical fiber having a pre3sure sensitlve concentric region, while FIG. 12 illustrates an optical fiber with an index profila of ad~usted higher order mode power ~raction. These alternate embodiments are also considere~ within the scope of the claims of this invention.

Modifications vf the cora refractive index profile which enhance the pressure sen~itivity ( i . e. enhance coupling of the fiber's higher order mocles into the concentric region) can be ach~eved by techniques that are well known in the art.
For example, the higher order mode power fraction may be boosted by designing a substantial central dip in the refractive index of the core, ~5 shown in FIG. 12.

2~L3 ~12-FIGS, 13-15 ~how applications for measuring pres~ure utilizing the pxessure sensitive optical fiber of the ~nv~ntion. In FIG. 13, as shown, the i~ostatic pressure on the ~iber induces ~icrobend~ at local non-uniformities in 5 the polymer coating. The polymer coating 80 can be used as a buffer to enable the sensor to on~y detect pressures above a certain threshsld value. The ~ardnes or non-ur.iformities of the coating can be increased (c3r decreased3 to raise (or lower~ this threshold value.
In FIG. 14, the optic:al ~iber pressure sensors of the invention 90 can be wound on a hollow ~lexible clrum, the interior of which i~ exposed.tc~ gas or fluid pressure. As the pressure on th~ interior oi' the drum exceeds that on the exterior or, alternatively, as the pressure on the exterior of the drum exceeds that on the interior~the~fiber will experience macro- and m.~crobending which then can be measured in the manner discussed above~ Thus, the sensor can be used to measure lnternal ~or external) pressure on a vessel.
Also, the sensor can be u~ed to prevent such p~essure from exceeding a certain predetermined value by triggering a shuto~ or bypass ~witch.

F~G. 15 illustrates ~ corrugated pressure plate lOo which can be used to translate a load or force F, into lateral bends on the optical fiber pressure sensor, which then can be measured in the ma~ner described above.

FIG. 16, is a graph qf an experlmentally induced 3a loss in the optical fiber pressure sen60r of FIG~ 5 when a two kilogram loading of flat parallel pressure plates is applied to two metexs of that pressure sensor. ~s illustrated in the FIG., there is a relatively pressure insensitive region of the fiber and a pressure sensitive ~L3~243 -~3-region. As mentioned above, those skllled in the art can adjust ~he width and sensitivity o~ these reglons to best suit the intended ne2ds and appllcations of the devlce.

While it ~ apparent that the invention herein disclosed ls well calculated to fulfill the objects above stated, it will be appreciated that numerous modifications and embodiments may be devised by those skilled in the art, and it is intended that the appended c;Laims cover all such modifications and embodiments as all within the true spirit and scope of the pre~ent inven~ion.

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Claims (17)

1. Pressure sensitive optical fiber comprising:
core means at least a portion of which having a predetermined refractive index for transmitting light therethrough:
cladding means adjacent said core means having a refractive index which is less than that of the core means portion; and concentric light transmissive means adjacent said cladding means having a refractive index which is greater than that of the cladding means and through which light passes in proportion to the amount of stress or strain induced in the fiber.

2. The fiber of claim 1 wherein the light transmission means has an internal diameter greater than the unperturbed mode boundary diameter of the core means.

3. The fiber of claim l wherein the core means comprises single mode or multimode optical fiber.

4. The fiber of claim 1 wherein the core means comprises means for adjusting modal, waveguide and/or chromatic dispersion.

5. The fiber of claim 1 wherein the core means and cladding means comprises step index, graded index, depressed clad, deeply depressed clad, W-type or multiply clad optical fiber.

6. The fiber of claim 1 which further comprises means for transferring analog and/or digital data.

7. The fiber of claim 1, wherein the light transmissive means comprises a plurality of concentric layers,

8. A pressure sensor comprising:
an optical fiber having:
core means at least a portion of which having a predetermined refractive index for transmitting light therethrough;
cladding means adjacent said core means having a refractive index which is less than that of the core means portion; and concentric light transmissive means adjacent said cladding means having a refractive index which is greater than that of the cladding means and through which light passes in proportion to the amount of stress or strain induced in the fiber:
means for introducing optical power into one end of the optical fiber; and means for detecting the optical power at the opposite end of the optical fiber.

9. The sensor of claim 8 wherein the detecting means comprises means for measuring the decrease in optical power in the core means.

10. The sensor of claim 8 wherein the detecting means comprises means for measuring the increase in optical power in the light transmissive means.

11. The sensor of claim 8 wherein the detecting means comprises means for measuring the decrease in optical power in the core means and means for measuring the increase in optical power in the light transmissive means.

12. The sensor of claim 8 further comprising means for converting or amplifying any external forces to micro- or macro- bending forces on the fiber.

13. The sensor of claim 8 further comprising at least one coating on the optical fiber.

14. A method for detecting pressure which comprises:
preparing pressure sensitive optical fiber comprising:
core means at least a portion of which having a predetermined refractive index for transmitting light therethrough:
cladding means adjacent said core means having a refractive index which is less than that of the core means portion: and light transmissive means adjacent said cladding means having a refractive index which is greater than that of the cladding means and through which light passes in proportion to the amount of stress or strain induced in the fiber:
applying a light source at one end of the sensor;
subjecting the sensor to hydrostatic pressure or strain: and measuring at least one of core means optical power decrease, or light transmissive means optical power increase, thereby detecting pressure or strain variations on the sensor.

15. A method for detecting force perturbations on an optical fiber which comprises:
preparing the pressure sensitive optical fiber of claim 1:

applying a light source at one end of the sensor: and measuring at least one of core means optical power decrease or light transmissive means optical power increase, thereby detecting pertubations applied to the fiber.

16. The method of claim 15 which further comprises modifying the sensitivity of the fiber by applying at least one coating thereupon.

17. The method of claim 16 which further comprises varying the sensitivity of the fiber by varying at least one of the light transmissive means attenuation coefficient; the width of the light transmissive means the core means attenuation coefficient; the width of the core means; the refractive index of the light transmissive layer; the refractive index of the core means, the distance of the spacing between the light transmissive means and the core means; or the shape of the refractive index profile.