NASA CASE NO. LAR 15050-1-SB

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NOTICE

The invention disclosed in this document resulted from research in aeronautical

and space activities performed under programs of the National Aeronautics and Space

Administration. The invention is owned by NASA and is, therefore, available for

licensing in accordance with the patent licensing regulations applicable to U.S.

Government-owned inventions (37 CFR 404.1 et seq.).

To encourage commercial utilization of NASA-owned inventions, it is NASA

policy to grant licenses to commercial concerns.

nonexclusive licensing to promote competition and

Although NASA encourages

achieve the widest possible

utilization, NASA will provide the necessary incentive to the licensee to achieve early

practical application of the invention.

Address informational requests for this invention to the Technology

Applications Team, NASA Langley Research Center, Code 200, Hampton, Virginia

23681-0001.

Address applications for license to the Patent Counsel Office, Mail Stop 212,

3 Langley Boulevard, Hampton, Virginia 23681-0001.

Serial No.: 08/636,453

Filed: 03/23/96 La RC
NASA Case No. LAR 1 5050- 1 -SB 

COLLECTION OF LIGHT FROM AN OPTICAL FIBER WITH A NUMERICAL 

APERTURE GREATER THAN ONE 

In an optical fiber having NA > 1 light may be internally reflected when it 

strikes the fiber end at a fiber-air interface. This problem may be overcome by 

modification of the fiber by reverse tapering the core. Light is redirected by the 

taper to strike the interface at an angle closer to normal. This allows light to 

exit the fiber end that would be internally reflected in an untapered fiber of 

NA > 1. 

The novelty of the present invention lies in the tapering of the fiber core for 

increased through transmission of light. Prior art devices have made use of 

fiber tapers to achieve mode control or fiber coupling. The problem of internal 

reflection has not been addressed as it is one that is not as important in fibers 

having NA < 1, which are more common. In chemical sensing it is 

advantageous to make use of fibers having higher NA due to an increased 

sensitivity. However the advantages in sensitivity are diminished due to the 

loss of signal at the fiber-air interface. The present invention overcomes the 

problem of loss at the interface, thus facilitating the use of high NA fibers for 

chemical sensing. 

Inventors: Claudio 0. Egalon Robert S. Rogowski 

Address: 2658 N. Armistead Ave. B-2 109 Wicomico Turn 

- -- ~mpton, VA 23666 Yorktown, VA 23693 

SSN 

Employer: AS&M NASA LaRC 

Initial Evaluator: Joe Heyman 

Serial No.: 081636,453 

Filed: 3/23/96 
Serial No."

Filed:

08/636,453

03/23/96 LaRC
LAR 15050-1-SB -1- PATENT APPLICATION

COLLECTION OF LIGHT FROM AN OPTICAL FIBERWITH A NUMERICAL

APERTUREGREATERTHAN ONE

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Origin of the Invention

The invention described herein was jointly made by an employee of

the United States Government and a contract employee during the

performance of work under NASA Contract No. NAS-1-19236. In

accordance with 35 U.S.C. 202, the grantee elected not to retain title.

Background of the Invention

1. Technical Field of the Invention

15 The present invention relates generally to the collection of light from

optical fibers and specifically to increased efficiency in the collection of light

from fibers having a high numerical aperture.

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2. Discussion of the Related Art

Optical fibers may be included in structures to act as chemical

sensors. Through use of special materials the fiber may be made to detect

the presence of particular chemical species. A coating may be chosen that

will fluoresce in the presence of certain chemical species. In this manner,

chemical concentration may be monitored in inaccessible portions of the

structure without requiring that the structure be dismantled. If the chemical

species are chosen to correspond to those that are expected to result from

corrosion of the structure, corrosion may be monitored.
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-2- PATENT APPLICATION

In general, fibers used for this purpose have a numerical aperture

(NA) of less than one. NA is defined by equation 1:

NA = (nl 2 - n2 2 )/2 nl 2, (1)

where n 1 is the index of refraction of the fiber core and n2 is the index of

refraction of the cladding. The critical angle, 8c, of a fiber is the angle at

which light traveling in the fiber core may strike the core/cladding interface

and still be totally internally reflected and thereby propagated down the

fiber. Light striking the core/cladding interface at an angle smaller than the

critical angle will refract through the interface into the cladding and be lost.

In a fiber of NA < 1, where the fiber end is in air, light that propagates

down the fiber at close to the critical angle of the fiber will refract through

the end of the fiber into the surrounding air, where it may be detected.

However in a fiber of NA > 1, such as a sapphire fiber, light traveling at

close to the critical angle will be internally reflected at the endface of the

fiber if the fiber is in air. Thus, use of a fiber with NA > 1 results in a

portion of the signal being lost due to total internal reflection at the end face

of the fiber. The disadvantage of using fibers with NA < 1 is that they are

not as sensitive as fibers with NA > 1, that is, the theoretical signal

produced in a system employing fibers with NA > 1 would be stronger than

that produced by a system using fibers with NA < 1 if the problem of

reflection at the air interface could be overcome.

To overcome the problem of total internal reflection, several solutions

have been suggested. The amount of light transmitted through the endface

of the fiber is dependant on n o, the index of refraction of the surrounding

medium. By altering n o one may allow more light to escape and be

available for detection. This may be achieved by the use of a graded index

lens at the fiber end or index matching fluids or gels. Another option is to

provide a fiber having a varying index of refraction in a region near the fiber

end. Alternatively, the fiber end may be hemispherically shaped to change

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LAR 15050-1-SB

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-3- PATENT APPLICATION

the angle of incidence of light rays striking the fiber end. Each of these

methods have limitations, both in implementation and on the amount of

light that they may untrap.

It is known that tapering of optical fibers may be employed to alter

the path traveled by the light that propagates down the fiber. Barnoski, et.

al. (U.S. 4,060,308, herein incorporated by reference) make use of a taper

in an optical fiber to control which modes of propagation are excited in the

core of an optical fiber when a laser beam impinges on the fiber. By

changing the angle at which the laser impinges the fiber, the mode may be

selected. In addition, the reverse holds true, that is, by placing a detector

at a particular angle, a particular mode may be selected to be detected.

Another use of fiber tapering is in coupling fibers to other fibers.

Darbon, et. al. (U.S. 5,337,380, herein incorporated by reference) make use

of a taper to couple two fibers having differing core diameters. Murphy, et.

al. (U.S. 5,21 6,731, herein incorporated by reference) fuses and draws to

a taper several fibers to couple multiple fibers in a bundle.

Summary of the Invention

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It is an object of the present invention to provide an optical fiber

having NA > 1 that has improved characteristics for through transmission

of light at a fiber-air interface by reducing the problem of total internal

reflection at the fiber end.

It is a further object of the present invention to make use of such a

fiber in chemical sensing.

To achieve the forgoing objects a fiber is employed that has a reverse

tapering of the core at one or both ends. The reverse tapering allows an

increased portion of the light traveling through the fiber to escape the fiber
LAR 15050-1-SB

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-4- PATENT APPLICATION

end by increasing the angle at which light strikes the interface so that the

light strikes the interface at an angle closer to the normal angle.

For chemical detection, the fiber is coated with a material that

fluoresces in the presence of preselected chemical species. At least one

end of the fiber is reverse tapered and is positioned so that light refracting

through the end will impinge on a photodetector indicating the presence of

the preselected chemical species.

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Fig. 1 is a drawing showing a fiber having NA < 1. A representative

light path is shown traveling at approximately the critical angle in the fiber

and refracting out of the end of the fiber.

Fig. 2 is a drawing showing a fiber having NA > 1. A representative

light path is shown traveling at approximately the critical angle in the fiber

and being reflected at the endface of the fiber.

Fig. 3 is a drawing showing a fiber having NA > 1 and a reversed

tapered end. Further it shows a light beam traveling out of the end of the

fiber. The path of the beam in a fiber without a reverse tapered end is

shown for comparison.

Description of Preferred Embodiments

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An optical fiber having NA < 1 is shown in Fig. 1, the view is

through the center line of the fiber, along its length. The fiber consists of

a core 10, surrounded by a cladding 12. At one end of the fiber is shown

an interface 14 between the fiber and air. A representative light ray 16 is

shown. This light ray is propagating along the length of the fiber, in the

core 10, and strikes the cladding 12 at an angle e c, the critical angle of the
LAR 15050-1-SB -5- PATENT APPLICATION

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fiber. Light traveling at close to the critical angle will strike the interface 14

at an angle Y1and refract through into the air, leaving at an angle Y2"

Fig. 2 represents an optical fiber having NA > 1. Otherwise the fiber

of Fig. 2 is similar to the fiber of Fig. 1. The fiber, shown in cutaway, has

a core 10 that is surrounded by a cladding 12. At one end of the fiber an

interface 14 between the fiber and air is shown. A representative light ray

16 is shown propagating along the length of the fiber, in the core 10, and

striking the cladding at an angle e c, the critical angle of the fiber. In this

case, light traveling at close to the critical angle will strike the interface 14,

and be totally internally reflected, remaining in the fiber core 10. Light rays

traveling at greater angles, and therefore striking the interface at angles

closer to the normal angle will refract through the interface.

The operation of the present invention is shown in Fig. 3. As in Figs.

1 and 2, the fiber is shown in cutaway, along its length. The fiber has a

core 10 surrounded by a cladding 12. At one end of the fiber an interface

14 between the fiber and air is shown. A representative light ray 16 is

shown propagating along the length of the fiber, in the core 10. The light

ray strikes the core cladding interface at a propagating angle, 0. Near to the

interface the core has a reverse taper 20, and thus has a larger diameter at

the interface than along its length. The core is tapered at an angle a,

defined as the angle between the tapered portion of the core and a line

normal to the interface. As the light ray 16 strikes the taper 20 at 18 it is

redirected along path 22, striking the interface at an angle, c, and refracting

through. After exiting the fiber it may be detected by detector 24. The

dashed lines represent the path 26 of a light ray in a fiber lacking taper 20

and correspond to the light path of totally internally reflected light as was

shown in Fig. 2.
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The optimum angle of taper (_ is dependant on n 1 and no, the index

of refraction of the core and medium, respectively. The optimum value of

a is found from the following inequality:

0< sin e < (no/n 1) (2)

where:

c = (n/2-e) when (n/2-e) _<cx (3)

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and,

and,

e = 2a- (n/2-E)) when (n/2-e) > cx (4)

8o _<e < n/2. (5)

Egalon and Rogowski (US app ser. no. 08/ , filed , herein

incorporated by reference) gives a further discussion of the derivation of

equations 2-4.

Another embodiment of the present invention makes use of a curved

reverse taper.

One application of a fiber having a reverse tapered core lies in the

detection of chemical species. A fiber may be coated with a sensitive

fluorescent substance, that is, in the presence of chemical species to which

the substance is sensitive, it will fluoresce. The light so produced will be

injected into the fiber and propagate in the fiber, primarily in the fiber core.

A detector aligned with the end of the fiber will detect this light indicating

the presence of the chemical species within. This type of device may be

used to interrogate the interior of structures for particular chemical species

without dismantling the structure. A structure may be built to include such

sensors. For example, in an aluminum aircraft lap joint a fiber may be

included that is coated with Morin hydrate, a sensitive substance that

fluoresces in the presence of aluminum ions, a product of the corrosion of

aluminum. As the aluminum corrodes, producing aluminum ions, the Morin
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hydrate will fluoresce, producing a light signal that may be detected at the

end of the fiber.

In a manner similar to the detection of chemical species temperatures

may be monitored. In lieu of a coating sensitive to chemical species a

coating may be applied that is sensitive to temperature.

Other applications and modifications of the present invention will be

apparent to those skilled in the art. The above embodiments are not

exhaustive but rather given by way of example. It is understood that the

present invention is capable of numerous modifications within the scope of

the following claims.
LAR 15050-1-SB PATENT APPLICATION

COLLECTIONOF LIGHT FROM AN OPTICAL FIBERHAVING NUMERICAL

APERTUREGREATERTHAN ONE

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Abstract of the Disclosure

In a fiber having NA > 1, in air, a large portion of light reaching

the fiber end is totally internally reflected instead of escaping at the

endface of the fiber. In the present invention a reverse taper is

introduced at the end of the fiber to alter the light path so that more of

the light in the fiber may escape to be collected by a detector. In

chemical sensors, fibers having NA > 1 are desirable due to increased

sensitivity. An application of the present invention is to allow the use of

fibers having NA > 1 in chemical sensors. Fiber sensors may further be

embedded into a structure so that corrosion may be monitored without

destruction or dismantling of the structure.
Serial No.:

Filed:

08/636,453

03/23/96 LaRC
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