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Description  |
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This invention relates to optical apparatus for providing a transition
between optical numerical apertures while conserving throughput, and more
particularly to improved optical apparatus for carrying out chemical and
biochemical assays.
Of the large variety of chemical and biochemical techniques used for
analysis or assay, a particularly useful and sensitive one is an optical
system employing the principles of attentuated total internal reflection
(ATR) spectroscopy. Particularly useful for immunoassays, such an optical
system employs a fiber optic wave guide, on a portion of the outer surface
of which can be covalently immobilized an antibody reactive with an
antigen in a test solution. A light beam introduced into one end of the
wave guide will be totally internally reflected in the dense medium of the
wave guide, and will generate an electromagnetic wave form, known as the
evanescent wave component. The latter characteristically extends only a
fraction of a wavelength across the interface between the wave guide and
test solution. This penetration, however, is sufficient to permit
substantial optical interaction between the evanescent wave component and
the immobilized antibody with which the antigen in the test solution will
complex, and only minimally with any bulk solution in which the antigen
was present. Such optical interaction then permits one to assay the
antigen. A number of such systems using internal total reflection
spectroscopy for an assay are known and have been described, for example,
in U.S. Pat. No. 4,133,639 in which is disclosed a system that measures
fluorescence induced by the optical interaction; U.S. Pat. No. 4,050,895
which decribes a system based on absorption of the evanescent wave by the
analyte; and U.S. Pat. Nos. 4,321,057 and 4,399,099 both of which disclose
systems that detect changes in the radiation transmitted through the
fiber; U.S. Pat. No. 4,447,546 which describes a fluorescence immunoassay
system; and others.
A number of factors determine the sensitivity of such systems, one of the
most important factors being that the sensitivity increases rapidly with
the numerical aperture (NA) of the fiber at the point of contact with the
surrounding medium being assayed. The sensitivity is a function with an
8th power dependence on NA at low values of the latter, and at a lower but
still significant power at high values of NA.
Numerical aperture (NA) can be defined as:
(1) NA=n.sub.2 sinB
where n.sub.2 is the refractive index of the medium (typically air) through
which the radiation is initially propagated so as to be incident upon an
input end of the fiber, and B is the maximum acceptance angle of radiation
at the input end of that fiber. Thus equation (1) defines the numerical
aperture at the fiber input.
Numerical aperture can also be defined as:
(2) NA=(n.sub.0.sup.2 -n.sub.1.sup.2).sup.1/2
where n.sub.0 is the refractive index of the fiber core, and n.sub.1 is the
refractive index of the medium around the fiber (e.g. essentially the
sample or bulk solution in which the antigen is disposed). Equation (2)
thus may be used to define the numerical aperture at the point of contact
between the fiber and the fluid being assayed. For such fiber, the
numerical aperture where the surrounding medium contacts the fiber is
highest when the fiber core material has a very high index and the medium
surrounding it has a very low index, or n.sub.0 >>n.sub.1. For example,
satisfactory sensitivities can be obtained where a glass fiber of ordinary
index of refraction is surrounded by an aqueous solution that typically
has an index of refraction in the vicinity of 1.33-1.35.
If the numerical aperture at the input of the fiber is less than that at
the point of contact with the surrounding solution, then advantage is not
being taken of the larger NA at the point of contact, and the system will
not be nearly as sensitive as it could be, considering the eighth power
dependence of the latter. Should the NA at the input be greater than that
at the point of contact with the fiber, input radiation will spill out of
the fiber at the interface and may vastly and undesirably increase
background fluorescence.
While it is important to provide mounting means for the fiber so that at
least that end of the fiber into which radiation is projected will be
accurately positioned, contact between the fiber and the mounting will
tend to reduce the numerical aperture inasamuch as the refractive index of
the mounting material is generally higher than n.sub.1. To reduce this
problem, it has been customary to clad the fiber, at least near the end of
the fiber into which radiation is propagated, with a coating, typically of
a high molecular weight polymer, disposed to provide an interposed,
low-refractive index medium between the mounting and the fiber. Such
coating may also be opaque and exhibit preferably low reflectivity. The
portion of the fiber intended to contact the analyte solution or sample to
be assayed is left uncoated. Ideally, any radiation that can go through
the fiber can be usefully employed if the index of the cladding is the
same as the index of the sample. Unfortunately, the refractive index of
most cladding obtainable is around 1.40 to 1.43, and such indices limit
the maximum numerical aperture to a value much lower than the one that
might be obtained if a lower index cladding were available.
Additionally, the numerical aperture can be improved by providing a clad
fiber with a higher index glass core, but very high refractive index
glasses are not currently commercially available as fibers with plastic,
strippable cladding of low refractive index. Improvement in numerical
aperture also can be achieved by insuring that radiation flux is as high
as possible over the maximum solid acceptance angle of the system, and
precisely mounting the clad end of the fiber becomes particularly
important when the off-axis angle of incoming radiation is very large.
This mounting requirement is particularly difficult to meet with respect
to the very small diameter (e.g. ca. <350 microns) telecommunication
fibers presently available. For example, in U.S. Pat. No. 4,050,895, there
is shown the use of a number of hemispherical lenses and an annular
aperture to couple large angle rays into a fiber. However, to obtain very
high numerical apertures in this manner requires highly corrected lenses
with shallow depth-of-field. Such lenses are difficult to fabricate, quite
expensive, difficult to keep aligned, and have reduced transmission
because of their typically multi-element structure. Immersion systems must
currently be used for high numerical aperture illumination, but tend to be
unwieldy and are also expensive.
A principal object of the present invention is therefor to provide an
improved optical system which minimizes the need for immersion optics,
improves the optical transmission of the fiber, permits the illuminating
lenses with larger depth of focus, all with the benefit of an improved
numerical aperture. Other important objects of the present invention are
to provide a novel optical fiber with a substantially increased numerical
aperture; and to provide such an optical fiber for total internal
reflective transmission, which fiber permits the use of cladding material
of ordinary refractive index to be used adjacent the portion where the
fiber is mounted or held.
Yet other important objects of the present invention are to provide an
improved optical system for detecting analytes at a solid-liquid interface
with ATR techniques; to provide such an optical system using fiber optics
with improved numerical aperture and thus an increased sensitivity; to
provide such a system in which the numerical aperture achieved may be
substantially as high as is allowed by the refractive indices of the
sample and the fiber; to provide such a system in which the effect on the
system response due to varying the refractive index of the analyte sample
is reduced; to provide such a system which permits one to employ fibers of
larger diameter than could heretofore be reasonably employed, and
therefore make it easier to mount and align the fiber and to provide a
more rugged system; to provide such a system in which the entrance area is
substantially increased, thereby allowing greater light collection but in
which the fiber diameter at the sampling region is reduced thereby
providing greater sensitivity; to provide such a system in which the
tolerance requirements for both transverse and axial alignment of the
fiber are reduced; and to provide a method of improving the numerical
aperture in a fiber-optic assay system.
These and other objects are realized by employing, for purposes of the
present invention, a fiber tapered gradually from a relatively large
diameter entrance pupil to a substantially smaller diameter at a position
longitudinally displaced from the entrance pupil. Such a tapered fiber
will exhibit conservation of throughput because there will be a gradual
increase in the incidence angle of the radiation as the latter travels
down the fiber from the entrance pupil toward the smaller diameter portion
section, the numerical aperture of the radiation in the smaller
cross-sectional portions becoming higher by the inverse ratio of the
diameters. In other words, the light beam traversing the fiber is
angularly compressed by the taper of the medium in which it is confined;
the ratio of the diameters of the large and small portions of the tapered
fiber is the exact inverse of the ratio of numerical apertures.
Other objects of the present invention will in part be obvious and will in
part appear hereinafter.
The invention accordingly comprises the apparatus possessing the
construction, combination of elements and arrangement of parts, and the
method comprising the several steps and relation and order of one or more
of such steps with respect to the others, all of which are exemplified in
the following detailed disclosure, and the scope of the application of
which will be indicated in the claims.
For a fuller understanding of the nature and objects of the present
invention, reference should be had to the following detailed description
taken in connection with the accompanying drawing wherein
FIG. 1 shows, in idealized, enlarged cross-section, a fiber incorporating
the principles of the present invention;
FIG. 2 illustrates, in an enlarged longitudinal cross-section, an assay
device incorporating an idealized optical fiber embodying the principles
of the present invention; and
FIG. 3 is an elevational view of the input end of the assay device of FIG.
2;
FIG. 4 illustrates, in idealized, enlarged cross-section, an alternative
form of a fiber structure incorporating the principles of the present
invention;
FIG. 5 illustrates, in an enlarged longitudinal cross-section an
alternative assay device incorporating an optical fiber of the present
invention; and
FIG. 6 shows a schematic system employing the fiber of the present
invention as a transition element in a typical solid state optical
transmission system;
Referrring to FIG. 1, there is shown an embodiment of the present invention
exemplified by fiber 20. The latter is an elongated body extending from
one end or entrance face 22 to an opposite or terminal end 24, fiber 20
preferably having a substantially circular cross-section. At face 22 the
fiber surface typically is planar, is disposed normally to the
longitudinal axis of the fiber and is preferably highly polished to
minimize any blemishes or surface defects that would tend to scatter
incident excitation radiation. Alternatively, face 22 of the fiber may be
configured in other desired optical shapes to serve, for example as a
magnifying or matching optical surface. Fiber 20 is adapted to propagate
along its length, by multiple total internal relection, optical excitation
radiation entering entrance face 22 within a conical acceptance angle (B)
substantially symmetric with the long axis of the fiber and defined
hereinbefore, as well known to those skilled in the fiber optics art, in
equation (1). Fiber 20 may be any of a very large number of substantially
homogeneously materials optically transparent to the excitation radiation,
e.g. glassy materials such as glass, crystalline materials such as quartz,
sapphire and the like; synthetic polymers such as polyolefins,
polypropylenes and the like. Where fiber 20 is to be used in fluid assays
as described hereinafter, the index of refraction (n.sub.1) of the
material forming fiber 20 must be greater than n.sub.2, the index of
refration of the fluid being assayed. The latter index is typically aboout
1.3 for an aqueous solution.
In one form, fiber 20 is shaped to provide a preferably gradual, smooth,
longitudinal taper from entrance face 22 to terminal end 24. This gradual
transition serves to increase the input beam convergence gradually without
exceeding the critical angle for the fiber. Ideally the taper of the fiber
should not exceed 5.degree.. Typically for a fiber of several millimeters
in length, the diameter would then taper smoothly (i.e. not necessarily
linearly but with substantially no discontinuities or abrupt angular
changes in the taper angle) from an diameter of 1 mm at face 22 to a few
hundred microns at end 24. If the diameter of the fiber at end 24 finally
becomes smaller than some limiting value at which maximum numerical
aperture is achieved, the radiation will spill or escape from fiber at
that point.
Referring to the embodiment shown in FIG. 2, there is shown an exemplary
apparatus 26 for assaying a fluid, which apparatus incorporates the
principles of the present invention. Apparatus 26 includes optical fiber
20, enclosure 28 and mounting means 30, and is similar in many respects to
the system shown in U.S. Pat. No. 4,447,546 issued May 8, 1984 to one of
the inventors of the present invention.
Fiber 20 of FIG. 2 has essentially the features described in connection
with the embodiment of FIG. 1 except that for purposes of maximizing
excitation by input radiation, tapered portion 32 thereof extends between
input portion 34 and an elongated output portion 36. Both of the latter
have preferably substantially constant diameters. The diameter of input
portion 34 matches the larger diameter of tapered portion 32 and the
diameter of output portion 36 matches the diameter of the smaller diameter
of tapered portion 32. Because the surface of output portion 36 is to be
used as a sampling or sensing zone for the assay, all sampling is then
achieved at the highest numerical aperture of the system, i.e. at the
smaller diameter end of the fiber. For purposes of an immunoassay
apparatus, fiber 20 will typically be about 25 mm in length, it being
understood however, that such length is merely exemplary and not limiting.
Mounting means 30 is shown simply as short sleeve, cladding or ferrule 38
surrounding a short portion of fiber 20 adjacent face 22 and extending
radially therefrom into contact with a portion of the internal surface of
enclosure 28 adjacent one end of the latter. As shown particularly in FIG.
3, ferrule 38 is preferably provided with one or more perforations 40
extending substantially parallel to the axis of fiber 20 so as to enable
fluid communication between the volumes adjacent each end of the ferrule.
The primary function of ferrule 38 is to position fiber 20 so that input
radiation can be directed accurately onto face 22, and also to maintain
fiber 20 in spaced relation to the internal surface of enclosure 28.
Because ferrule 38 necessarily is in contact with a portion of the surface
of fiber 20, it may adversely affect the numerical aperture of the fiber,
so it is highly desirable to limit the contact between the ferrule and
fiber to a minimum commensurate with the ferrule's mechanical role, and to
fabricate it of a material, such as siloxane, having a low index of
refraction preferably near or matching that of the fluid to be assayed.
In an exemplary embodiment, it is intended that an operative portion such
as elongated output portion 36 of the fiber surface be defined as an
activated region at which the assay is to be performed. Portion 36 can be
delimited by additional cladding added at opposite ends of the fiber
surface so that only the desired portion of the fiber remains unclad. The
dimensions of the activated region can, of course be controlled by other
techniques and indeed, substantially the entire length of the fiber beyond
the ferrule can constitute the activated region. However, as noted, it is
desireable to configure the activated portion as a cylinder of constant
diameter of the highest obtainable numerical aperture, thereby conferring
the greatest sensitivity upon the system. To activate the surface of
portion 36, the latter is typically coated or treated with a reagent such
as those described in detail in U.S. Pat. No. 4,447,546 and incorporated
herein by reference.
Enclosure 28 is a tube, preferably but not necessarily optically
transparent, but formed of a material that is relatively insoluble and
chemically non-reactive with the fluid being assayed. Typically enclosure
28 is simply a glass tube having an inside diameter greater than the
maximum outside diameter of fiber 20, and preferably dimensioned to
delimit a predetermined volume surrounding at least the activated surface
of fiber portion 36.
Manufacturing of the tapered fiber can be accomplished quite simply by
starting, for example, with a commercially available, constant diameter
(e.g. 500 microns) glass fiber core, heating the fiber locally as with a
torch or electrical heater until a local portion of the fiber becomes
plastic, and then drawing the fiber at a rate and with a temperature
distribution as to cause the fiber to taper to a reduced diameter, e.g.
300 microns. The minimum acceptable taper angle is governed by such
practical considerations as the acceptable length of fiber, but the
maximum taper angle should not exceed that necessary to preserve the
necessary critical angular relationship necessary to maintain transmission
by total internal reflection, and thus depends upon the index of
refraction of the fiber material. For example, for a fiber core of fused
quartz, the maximum taper angle should preferably be kept below about
5.degree.. The tapered fiber may then be severed where desired. The choice
of time, temperatures, drawing rates and temperature distribution of
course depend largely upon the physical characteristics of the particular
material chosen for the fiber. Any of a number of other known methods for
drawing or forming a tapered fiber may be used. The present invention
lends itself surprisingly well to formation of a double-passing fiber
(i.e. one capable of transmitting light from the entrance face to the
distal end and then reflecting the light back through the fiber to the
entrance face if desired). Such a distal reflecting surface, located at
end 42 in FIG. 2, can easily be formed simply by heating the fiber at that
point and drawing the fiber at a rate high enough to rupture the fiber and
form a high-angled termination which serves as a mirror capable of
reflecting light in the opposite direction. Termination 42 thus can be
similar in effect to a prism reflector, but can be much more simply formed
without the problems attendant on formation of conventional mirrors such
as grinding, plating, polishing and the like. Of course, termination 42
can simply be shaped to spill out the light, if desired, inasmuch as one
can observe the fluoresence at input face 22 without interferece from the
exciting radiation.
In operation of the embodiment of FIG. 2, portion 36 of fiber 20 is
provided with coat 43 of any of a number of activating reagents (such as a
constituent of an antibody-antigen complex that includes a fluorescent
tag) and essentially subjected to the same procedure as are described in
U.S. Pat. No. 4,447,546. Briefly, interspace 44 between enclosure 28 and
fiber 20 is filled with a liquid sample of the material to be assayed, the
sample allowed to incubate if necessary. As shown in FIG. 2, the apparatus
is employed with fluorimeter 45 which provides an excitation radiation
source, preferably a solid-state radiation emitter or a laser so that the
wave length of the radiation can be precisely specified. Entrance face 22
is illuminated with the excitation radiation, the latter being typically
capable of exciting or inducing fluorescence in the volume traversed
adjacent the surface of portion 36 by an evanescent wave accompanying the
transmission of the radiation down the fiber. The excitation radiation is
angularly compressed from the input end of the fiber to the coated portion
by the taper of the fiber, thereby increasing the numerical aperture
considerably. Because the numerical aperture of portion 36 is so much
greater than that of face 22, the reagent being excited by the evanescent
wave is subjected to a considerably greater excitation intensity that
would be experienced by an untapered fiber with the same diameter entrance
face. The induced fluorescence then tunnels back into the fiber from the
excited material to be read by photometer 45. Alternatively, one can
positionthe detection portion of the photometer at the distal end of the
fiber to measure the fluoresence emitted therefrom, but in such case
provision should be made to discriminate between the excitation radiation
and the fluoresent radiation by filtering.
For maximum sensitivity in an assay system of the present invention it is
highly desirable to achieve as nearly as possible the maximum numerical
aperture. As previously noted, too little loses signal as the eighth
power; too large spills power out, losing some signal and increasing
background fluorescence. To reproduce fibers exhibiting maximum numerical
aperture requires accurate reproduction from fiber to fiber of the same
ratio of the diameter of the input face to the sensitive region of the
fiber, implying unrealistic manufacturing tolerances.
This problem can be obviated by building the fiber with an entrance
diameter somewhat larger than necessary, i.e. to provide a diameter ratio
d.sub.i /d.sub.f slighly too large, d.sub.i being the diameter at the
input and df being the diameter at the final or sensitive point on the
fiber. With such as structure, input radiation will start spilling out of
the fiber when it reaches the sample covered portion of the fiber, the
lower surrounding refractive index of air up to that point preventing
energy loss through the fiber surface. This serves as a constraint on the
maximum useable diameter ratio. The latter should be larger than can be
accomodated by the fiber/sample interface, but smaller than that which
insures total internal reflection at the fiber/air interface. This
relation can be expressed as
##EQU1##
where n.sub.0 is the refractive index of the fiber, n.sub.1 the refractive
index of the sample, and NAi is the numerical aperture at the input
surface of the fiber. Under these conditions, the extra light collected by
the larger input area will remain in the fiber until it reaches the point
or zone where the surrounding medium has the same index as the sample.
Because the larger d.sub.i implies larger energy input, the loss of energy
will leave the net energy transfer the same as in a system of smaller
d.sub.i without spilled radiation, but the spilled energy should not be
permitted to create background interference. To this end, as shown in FIG.
4 (wherein like numerals denote like parts with repect to the embodiment
of FIG. 2, at or adjacent the junction between portions 32 and 36, at
which any radiation spilling out of the fiber will tend to cause
background problems, the fiber is surrounded with radiation absorbent
collar 47 in contact with the fiber around the periphery thereoof and
formed of a material having an index of refraction matched to that of the
sample solution as nearly as is expedient. For example, for aqueous
samples, collar 47 may be a gel filled with an absorbent dye or carbon
bench, or may be simply a ring of radiation absorbent plastic such as
black polytetrafluoroethylene.
Not only does the increase in diameter ratios lower the manufacturing
tolerance requirements, but it also serves to reduce the positioning
tolerances required, both axially and transversely, for the fiber in an
assay system. The increase in the area of the input face obiously
decreases the transverse positioning tolerance requirement because in such
case a focussed beam has more distance to wander transverely before
leaving the input face. Because the increase in area also reduces the
acceptance angle B required to obtain the same numerical aperture, the
permissible depth of focus of the input optics is enlarged and the axial
tolerance required for the input face of the fiber to remain within the
depth of focus is relaxed.
The present system allows one to provide an optical fiber assay apparatus
with as high a numerical aperture as may be achieved subject to the
constraints imposed by the refractive index of the sample and the index of
the fiber core. Since one may start with a fairly substantial glass "rod"
rather than the fine fibers such as are disclosed in U.S Pat. No.
4,447,546, one is not limited to the type of glass that may be used, i.e.
telecommunication glasses, and therefore one may use very high index
glasses which further enhances the maximum numerical aperture that can be
obtained at the fiber portion in contact with the sample. In fact, the
maximum numerical aperture at the sample can now be larger than unity. To
achieve this without tapering the fiber, the illuminator would have to be
an immersion system. In other words, since the input numerical aperture of
a tapered fiber can be lower than the numerical aperture that exists
inside the tapered fiber at the point of contact with the sample, one may
dispense with immersion by using a fiber which has an entrance diameter
large enough so that the entrance numerical aperture stays below unity.
Further, use of the tapered fiber of the present invention permits one to
use input lenses of lower numerical aperture. Such lower numerical
aperture lenses are cheaper, easier to build, better corrected, tend to
have higher transmission, and also have better depths of field so that
focusing them is less critical.
Because the tapered fiber itself (at least at the end where it is mounted
or held) may be larger in diameter than in prior art assay devices, one
can use the tapered fiber to construct a system that is more rugged and
has much more relaxed tolerances in positioning. And because the entrance
or wide end of the taper fiber need not have a numerical aperture as large
as that required by the prior art, cladding of any reasonable refractive
index can be accommodated and the problem of holding or mounting the fiber
is trivialized in this respect.
In assay systems using optical fibers, small uncontrollable variations in
the refractive index of the sample desirably should nevertheless have a
minimum effect on the readings. Small variations in the refractive index
of the surface layer are also hard to control because they are dependent
on the manner in which the reagent, such as an antibody layer, has been
applied to the surface of the fiber. Because the tapered fibers of the
present invention so improve the effective numerical aperture and one can
use fibers of high refractive index in such assay devices, the importance
of controlling the surface film and background refractive indices is thus
much reduced and better measurements are possible. For example, in an
assay device using a tapered fiber with a refractive index around 1.76,
measurements made respectively using samples of water and serum showed a
response variation of about 10% as contrasted with a factor of
approximately 2 for a system using a standard cylindrical untapered fiber.
The advantages of the use of a tapered fiber in an fluoresence assay device
are quite considerable. The limit of numerical aperture (for liquid
samples) for normal fibers is roughly 0.3 if one uses extended cladding or
0.4 if only short segments of cladding are used and some loss into the
cladding can be tolerated. In a tapered fiber of the present invention
numerical apertures in excess of 1.0 are easily obtainable. To optimize
for the particular sample being assayed, the taper ratio should be
selected such that the ratio of the numerical apertures gives the wanted
final numerical aperture at the sample. The maximum numerical aperture
obtainable in this manner is the square root of the fiber index squared
minus the sample index squared. For example, the maximum numerical
aperture is 1.12 for a high index 1.76 fiber and index 1.351 sample. This
high numerical aperture implies a factor of about 500 times greater
sensitivity than is obtainable with standard cylindrical fibers.
The improvement in the assay apparatus signal with numerical aperture is
due optimally to four factors: a square factor in the light-gathering
power of the system, a square factor in the efficiency of exciting the
evanescent wave, a square factor in the efficiency of collecting the
fluorescence produced by the evanescent wave and a square factor in the
solid angle of collection of the fluorescence. However, there is intrinsic
reduction in the strengths of coupling with an increase in the index of
the fiber so that at high numerical apertures, one cannot expect an 8th
power enhancement. Thus, in assay apparatus with tapered fibers of high
numerical aperture one only finds an improvement over the prior art by a
maximum factor of roughly 500 instead of a 10,000 fold theoretical
improvement. This improvement occurs partially because the thickness of
the evanescent zone becomes less at high numerical aperture and therefore
a smaller volume is being sampled. There is a thickness effect for the
bulk sample, and where the thickness effect is absent as in a small fiber,
there is a small further effect due to index mismatch and a variation in
coupling strengths of radiation across the interface.
In the description of the fiber of the present invention heretofore,
portions 32 and 36 have been considered to be homogeneous, at least by
implication, but such is not necessary and in some cases, not particularly
desirable. As shown particularly in FIG. 5, fiber 20 is formed of two
abutting sections, 46 and 48, corresponding respectively to portions 32
and 36, but tapered section 46 is formed of a clear, synthetic polymer
such as polymethylemethacrylate, and section 48 is formed of an optical
glass, due care being taken to configure the joint between the sections to
provide maximum transmission of the propagated radiation along the fiber.
As shown in FIG. 6, the fiber of the present invention also finds
substantial utility as a transition element to match diameter of
input-output optics in transmission lines. In FIG. 6, fiber 20 is coupled
at input face 22 to solid-state optical source 50 such as a laser or
light-emitting diode that cannot normally be reduced in size, and distal
end 24 of fiber 20 is coupled to the input of transmission-quality optical
fiber 52. The output of the latter in turn is connected to photoelectric
sensor 54. Alternatively, if a further transition is desired, the output
of fiber 52 may be connected to the smaller end of another like fiber 20,
the larger end of which is then coupled to sensor 54.
Since certain changes may be made in the above apparatus and method without
departing from the scope of the invention herein involved, it is intended
that all matter contained in the above description or shown in the
accompanying drawings shall be interpreted as illustrative and not in a
limiting sense.
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