Fiber optic transducers with improved sensitivity

Fiber optic transducers are constructed from a single optical fiber wherein the geometry of the fiber is altered such as by introducing a bend or curve area in the fiber adjacent to the exit end of the fiber or by forming a canted or angled reflective surface at the exit end of the fiber. The alteration in the geometry of the fiber at or adjacent to the exit end thereof causes the light exiting from the fiber to project therefrom in a modified or expanded cone enabling more sensitive measurement capabilities.

BACKGROUND OF THE INVENTION 
The present invention relates generally to fiber optic transducers and, 
more particularly, to single fiber displacement transducers suitable for 
detecting linear and rotational movement of a deformable reflective 
surface. 
An example of the state of the technology up to the point of this invention 
is taught in the J. H. Porter et al U.S. Pat. No. 4,071,753. The fiber 
optic transducers taught in the Porter patent consist of separate input 
and output optical fibers. The output fiber is optically positioned 
relative to the input fiber so that a predetermined portion of the optical 
power carried by the input fiber is coupled into the output fiber and the 
modulated optical power received by the output fiber is carried away from 
the transducer through the output fiber. 
Other examples of multi-fiber optic transducers are taught in U.S. Pat. 
Nos. 3,394,976; 3,789,667; 3,789,674 and 3,961,185. Each of such 
transducers operate on the same general principle as those taught in the 
4,071,753 patent wherein light is introduced through an input fiber and is 
reflected off of a movable or deformable reflective surface to produce a 
modulated beam of light which is coupled into an output fiber through 
which the modulated light is carried away from the transducer to a 
detector. 
The problem with these prior art devices is that they do not provide the 
requisite sensitivity in detection of displacement of the reflective 
surface under certain conditions and, since they are multi-fiber 
arrangements, they are not suited for the production of micro-miniaturized 
assemblies. Therefore, it has been an object of on-going investigation to 
develop single fiber optical transducers which would provide enhanced 
sensitivity for detecting minor deflections or movements of a movable or 
deformable reflective surface but are readily adaptable to 
micro-miniaturized constructions. 
However, in prior single fiber assemblies, it has been standard to align 
the fiber axially with its face perpendicular to the axis of the fiber and 
essentially parallel to the movable or deformable surface. With such 
assemblies, a cone of light symmetric with the axis of the fiber is 
projected from the fiber onto the reflective surface and a cone symmetric 
with the axis is reflected back toward the fiber face where a fraction of 
the returning light reenters the fiber for transmission back through the 
fiber. The sensitivity of these assemblies to displacement or deflection 
of the reflective surface (i.e., the change in the intensity of light 
reentering the fiber as a function of a change in the distance of the 
reflective surface from the fiber face as the surface moves) has been 
found to be acceptable when the initial distance of the fiber face from 
the reflective surface is relatively short (i.e., up to about 50% of the 
diameter of a fiber having a numerical aperture of about 0.5). However, at 
greater distances, the sensitivity to deviation in distance of the 
reflective surface from the fiber face is not adequate for practical use. 
Thus, it is an object of the present invention to provide single fiber 
optical displacement transducers which have the requisite sensitivity to 
changes in distance of the reflective surface from the fiber face over a 
relatively broad range of distances and are amenable to fabrication in 
micro-miniaturized assemblies. 
It is another object of this invention to provide single fiber transducers 
exhibiting a sensitivity curve such that any small movement of the 
reflective surface relative to the face of the fiber will result in a 
significant change in the intensity of reflected light reentering the 
fiber. 
It is a further object of this invention to provide single fiber optical 
transducers which may be used to sense linear and/or angular displacement 
caused by changes in pressure, force, torque, acceleration, temperature, 
electric or magnetic fields applied to a reflective surface. 
It is a still further object of this invention to provide single fiber 
optical transducers which are simple in construction and which may be 
utilized in a wide variety of potential applications. 
SUMMARY OF THE INVENTION 
In accordance with the above objects, the invention is accomplished by a 
transducer constructed from a single optical fiber or waveguide. The fiber 
or waveguide may be a single-mode or multimode fiber. At or adjacent to 
the end of this fiber, the geometry of the fiber is altered in a manner 
such that light exiting from the fiber projects onto a movable or 
deformable reflective surface in an expanded cone. A preferred means for 
achieving this expanded cone of projected light is to introduce a bend or 
curve area in the fiber adjacent its exit end. Another preferred means is 
to form a canted or angled reflective surface on the exit end face of the 
fiber. 
The advantage of this construction is that the expanded cone of light will 
be projected onto the reflective surface from the end face of the fiber 
and will be reflected off of this reflective surface back toward the fiber 
as a further expanded cone. A portion of this reflected cone of light will 
reenter the fiber and be transmitted back through the fiber with the 
intensity of light reentering being mainly dependent on the ratio of the 
area of the face or surface of the fiber which will receive the reentering 
light to the area of the cone of reflected light in the plane defined by 
the receiving face or surface of the fiber. Thus, since the area of the 
face or surface of the fiber is constant, the sensitivity of the system 
will be largely dependent on the variance in the area of the cone of 
reflected light and in view of the expansion of this cone provided herein, 
sensitivity to displacement of the reflective surface will be enhanced and 
more sensitive displacement sensing will be accomplished in accordance 
with the present invention. 
The foregoing and other objects, features and advantages of this invention 
will be apparent from the following detailed description of the preferred 
embodiments taken together with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A description of the invention follows referring to the drawings in which 
like reference numerals denote like elements of structure in each of the 
several figures. In general, the terms "optical fiber" or "optical 
waveguide" will be used herein to refer to a glass or plastic transmission 
line having a core member with cladding members concentrically surrounding 
the core for transmission by internal reflection at the core-cladding 
interface of electromagnetic radiation which lies in the optical portion 
of the electromagnetic spectrum between microwaves and X-rays including 
the ultra-violet, visible and infra-red regions. 
Reference is now made to FIG. 1 of the drawings which shows a fiber optic 
assembly referred to generally by the reference numeral 10 in which an 
optical transducer 12 according to the present invention is employed. The 
optical transducer 12 is completely optical in nature and employs no 
electrical connections or signals therein. The assembly 10 operates on the 
principle of producing an output optical signal whose amplitude is 
proportional to the input information variation with an optical fiber 
being utilized to convey the output optical power variation from the 
transducer 12 to a suitable destination. 
A light source 14 such as an incandescent lamp, a laser or a fiber optic 
illuminator emits a beam of light which is coupled into a glass or plastic 
optical fiber or optical waveguide 16, preferably employing a fiber optic 
coupling assembly as described in my copending U.S. patent application, 
Ser. No. 506,839. The light is then transmitted in a forward direction 
through the core of the fiber 16 to the transducer where the light is 
projected out of the fiber 16 at its exit end onto a reflective surface 
(such as a movable or deformable mirrored surface) the displacement of 
which is to be measured. Contemplative measurements which can be made 
relative to the displacement of the reflective surface are linear or 
angular displacements caused by variances in pressure, force, torque, 
temperature, acceleration, magnetic fields or electric fields acting on 
the reflective surface and inherent curvature measurements of the surface 
itself. 
Of the light exiting fiber 16 which is reflected off of the reflective 
surface, only a small amount will actually reenter the core of the fiber 
16 via transducer 12. The reentering light is then transmitted in the 
reverse direction through the fiber 16 to the initial transmission end of 
the fiber 16 where the returning light which constitutes a modulated 
optical information wave or light signal is collected by a photodector 18 
such as a photodiode for measurement of its intensity and comparison with 
a reference light signal. 
Referring now to FIG. 2, there is shown a preferred embodiment of the 
optical transducers (identified generally by reference numeral 12) 
according to the present invention which can be utilized in a variety of 
assemblies such as the system shown in FIG. 1. In this embodiment, 
depicting the transducer 12 utilized as a displacement transducer, the 
exit end 22 of fiber 16 is supported and fixed in a transducer housing 24. 
The fiber 16 adjacent exit end 22, and preferably not more than about 5 or 
6 fiber diameter lengths from its end face 26, is bent in a manner such 
that light exiting from the fiber 16 through face 26 will project 
therefrom in an expanded cone 28 which is assymetric to the axis of the 
fiber 16. The fiber 16 is positioned in housing 24 so that the cone of 
light 28 will project from the face 26 onto a reflective surface 30 of a 
movable or deformable member 32 which may be mounted, for example, on 
biasing springs 34 and 35 in a manner such that a force applied to the 
member 32 will cause the member to be displaced relative to the end face 
26 and in a plane parallel to the plane of the face 26. 
In operation, as the reflective surface 30 moves in response to a force 
acting on the surface 30 which is sufficient to overcome the biasing force 
of springs 34 and 35, the area of the surface 30 which will be illuminated 
by the cone of light 28 will expand or contract as a function of the 
distance (designated x in the drawing) between fiber face 26 and the 
reflective surface 30 and the resulting reflected cone of light 36 which 
will project back toward face 26 of fiber 16 will expand or contract 
similarly. However, only a small proportion of the light in reflected cone 
36 will actually reenter the fiber 16 for transmission back through the 
fiber 16 to the photodetector and the amount of reflected light that will 
reenter the fiber 16 in this embodiment will depend mainly on the ratio of 
the area of the face 26 which will receive the light to the total area of 
projection of the reflected cone 36 in a plane defined by the face 26. 
Thus, the sensitivity of the assembly in detecting movement of the 
reflective surface will depend primarily on the extent of change in the 
projected area of reflected light as a result of changes in the distance x 
since the area of face 26 which will couple with the reflected light is 
constant. 
In this regard, it should be noted that a modification in the sensitivity 
curve of the system is achieved utilizing the assembly of FIG. 2 wherein 
an expanded cone of light 28 assymetric to the axis of the fiber 16 is 
projected as compared with prior single fiber assemblies such as those 
discussed at page 2 hereinabove wherein a normal symmetric cone of light 
is projected. From my experimentation, I have found that sensitivity of 
the prior assemblies changes linearly with changes of distance, in 
accordance with a sensitivity curve which is based on the intensity of 
reentering light as a function of the distance of the face from the 
reflective surface, up to a distance of about 50% of the diameter of a 
fiber having a numerical aperture of about 0.5. At further distances, the 
sensitivity curve for these prior assemblies is too flat to enable 
practical discrimination of changing distances of the reflective surface 
from the fiber face. However, utilizing the assembly of FIG. 2, the 
sensitivity curve has been modified to an extent that even minor movement 
of the reflective surface from the fiber face can be detected over a wide 
range of distances. Accordingly, it has been found that utilizing the 
assembly of FIG. 2, any movement of the reflective surface away from a 
given distance will result in a significant change in the intensity of 
reflected light that will reenter the fiber and, thus, provide a more 
effective means for sensing movement of the reflective surface. 
Turning now to the embodiment of the invention shown in FIG. 3, an optical 
transducer designated generally by the reference numeral 12 is shown in 
which the optical fiber 16 is supported and fixed within a transducer 
housing 24. The fiber 16 terminates in an end face 38 which has been 
bevelled by any suitable means such as polishing so that a canted surface 
is formed relative to the axis of the fiber 16. Additionally, this end 
face 38 may be coated with a reflective material, if desired. The slope of 
the canted end face 38 is defined by an angle .alpha. as shown in FIG. 3. 
Furthermore, the angle .alpha. is complementary to an angle .beta. which 
lies between the axis of the fiber 16 (i.e., the general direction of the 
light transmitted through the fiber 16) and a vector 40 normal to the 
canted end face 38. The angle .beta. thereby defining the angle of 
incidence for the light transmitted along the axis. The lower limit of 
this angle .beta. (and, accordingly, the upper limit of angle .alpha.) is 
established such that the angle of incidence of light transmitted in a 
forward direction through fiber 16 impacting the canted end face 38 will 
equal or exceed the critical angle of reflection so that the light will 
not pass through the end face 38 but, rather, will reflect off of the 
face. When the face 38 is coated with a reflective material, the upper 
limit of angle .alpha. will not exceed an angle such that the light 
transmitted in a forward direction through fiber 16 will reflect off of 
the mirrored end face in the reverse direction without having been 
reflected toward the periphery of the fiber for projection onto the 
movable or deformable reflective surface 30. 
Operation of the transducer 12 illustrated in FIG. 3 depends on the light 
transmitted through the fiber 16 reflecting off of end face 38 and being 
projected out of fiber 16 in a cone 28 onto the reflective surface 30 of a 
movable or deformable member 32. As depicted in FIG. 3, the angle .alpha. 
defining the slope of the canted end face 38 equals 45.degree.. 
Accordingly, angle .beta. will likewise equal 45.degree. and the light 
transmitted through the fiber 16 along the axis will reflect off of end 
face 38 at an angle normal to the axis. This light will then be projected 
along this path normal to the axis of the fiber and through the curved 
surface presented by the circumference of the fiber whereby the projected 
cone of light will be expanded. As previously described with regard to the 
embodiment of this invention illustrated in FIG. 2, in view of the cone of 
light which is projected, the sensitivity of the assembly in detecting 
movement of the reflective surface will be enhanced as a result of the 
modification in the sensitivity curve resulting from the projection of the 
expanded cone of light 28 onto the reflective surface 30. Furthermore, it 
should be recognized that the sensitivity of transducers constructed in 
accordance with this embodiment of the invention will be further enhanced 
in view of the change in the sensitivity curve which will result from the 
fact that the reflected light 36 reentering the fiber core for 
transmission back through the fiber 16 also will be reintroduced into the 
fiber 16 through the curved circumferential surface of the fiber. 
In FIG. 4, a detailed enlargement is shown of the light path in a further 
embodiment of the transducer 12 wherein the angle .alpha. defining the 
slope of the canted end face 38 exceeds 45.degree.. In this embodiment, it 
can be seen that the light reflected off of end face 38 is projected back 
through the fiber toward the curved circumferential surface of the fiber 
and impacts the fiber surface at an angle of incidence which is shown as 
angle .theta.. For this reflected light to be refracted out of the fiber 
core, it is necessary for this angle of incidence .theta. (which 
corresponds to the formula .theta.=90.degree.-2.beta. or 
2.alpha.-90.degree.) to be less than the critical angle of reflection so 
that the light will not be internally reflected but, rather, will be 
projected out of the fiber. Furthermore, as the light exits from the 
denser medium of the fiber 16 into air before impacting the reflective 
surface 30, the emergent light will be displaced in accordance with the 
law of refraction (Snell's Law) so that angle .alpha. will be greater than 
angle .theta. resulting in a further modulation of the projected cone of 
light in addition to the modulation caused by the projection of light 
through the circumferential surface of the fiber. 
FIG. 5 is a detailed enlargement of the light path in another embodiment of 
the transducer 12. In this embodiment, angle .alpha. is less than 
45.degree. and angle .theta. corresponds to the formula 
.theta.=2.beta.-90.degree. or 90.degree.-2.alpha.. As in the embodiment of 
FIG. 4, angle .theta. must be less than the critical angle of reflection 
in order to enable the light to be projected out of the fiber. Also, angle 
.gamma. will exceed angle .theta. as discussed hereinabove as a result of 
the refraction of the light passing from the denser fiber medium to the 
less dense medium of air whereby further modulation of the projected cone 
of light is achieved. 
In the above figures, the representations have been schematic to aid 
understanding of the invention. It will be appreciated by one skilled in 
the art that the dimensions being dealt with are extremely small and can 
only be schematically illustrated. While the invention has been 
particularly shown and described with reference to preferred embodiments 
thereof, it will be understood by those skilled in the art that the 
foregoing and other changes in form and details may be made therein 
without departing from the spirit and scope of the invention. One such 
variation might include the choice of configuration of the end face of the 
fiber in order to alter the shape of the cone of exiting light such as, 
for example, by forming the end face 26 in a concave shape as illustrated 
in FIG. 6 or a convex shape as illustrated in FIG. 7. Another contemplated 
variation would be to alter the position and orientation of the transducer 
relative to the reflective surface in order to accomplish alternative 
measurements in the displacement or structure of the reflective surface.