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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to the measurement of temperature using optical
techniques and more particularly relates to an apparatus for determining
temperature utilizing a sensor material having a thermally dependent
refractive index.
2. Description of the Prior Art
Present technology utilizes thermistor beads having a temperature dependent
resistance characteristic, wherein the thermistor's resistance is
monitored to yield an indication of temperature. Such sensors, however,
suffer from various disadvantages including susceptibility to
electromagnetic radiation interference and the noise produced thereby.
Furthermore, the flow of sensing current through the thermistor element
may cause a temperature rise in the sensor, thus rendering such sensors
inaccurate. These sensors are also bulky, slow responding, and costly to
produce.
Optical sensors have the advantage of being relatively insensitive to
electromagnetic interference as well as being small in size and adaptable
to inexpensive manufacturing techniques. Optical sensors known in the art
include birefringent digital temperature sensors which exploit temperature
dependent birefringence effects of various crystals. In such a sensor, a
source spectrum is transmitted by a fiber bundle through a polarizer. The
polarization vector of the resulting light is then rotated by the
birefringence cell through an angle which is dependent upon the
temperature. A second polarizing filter is then used to convert the
polarization changes into intensity changes. Such a sensor, however,
requires the use of costly birefringent crystals and has a limited dynamic
range. A further optical temperature sensor known in the art exploits the
temperature dependent phosphorescence decay time of phosphors. In sensors
of this nature a phosphor bonded to one end of a fiber optic light guide
is illuminated with pulsed laser light. By examining the decay time of the
phosphorescence emitted by the phosphor, a temperature measurement may be
made. Such a sensor, however, requires complex electronics and has a slow
response time.
The present invention provides a compact, simple, and economical apparatus
having fast response for measuring temperature at remote and non-remote
locations.
SUMMARY OF THE INVENTION
In a preferred embodiment of the first surface temperature sensor, a layer
of sensor material having first and second surfaces and a temperature
dependent index of refraction n.sub.2, such as amorphous hydrogenated
silicon, is coupled to an optical fiber core having an index of refraction
n.sub.1, forming a dielectric interface therebetween. The intensity of
light reflections resulting from light incident to the dielectric
interface will depend on the index of refraction n.sub.2, which in turn is
temperature dependent, thereby providing a light signal whose intensity is
temperature dependent. The intensity of such a light signal may be
detected and utilized to provide an indication of temperature.
In another embodiment of the first surface optical temperature sensor, the
ratio of hydrogen to silicon in amorphous hydrogenated silicon is smoothly
varied to provide a changing light absorption characteristic across a
cross-section of the layer of amorphous hydrogenated silicon, thereby
becoming increasingly opaque to light incident thereon and consequently
substantially reducing reflections from the second surface of the layer.
The method for fabricating the optical temperature sensor including
amorphous hydrogenated silicon having a smoothly varying ratio of hydrogen
to silicon comprises varying the partial pressure of hydrogen in the
atmosphere in which silicon is r.f. sputtered onto a substrate.
In the preferred embodiment of the interferometric temperature sensor, a
layer of sensor material having a temperature dependent index of
refraction n.sub.2, such as amorphous hydrogenated silicon is coupled to
an optical fiber core having an index of refraction n.sub.1 to form an
etalon type interferometer in which the reflections of light resulting
from light having approximately normal incidence to the first and second
surfaces are utilized. The light reflected from the first and second
surfaces resulting from light having substantially normal incidence
thereupon will constructively and destructively interfere and therefore
the light intensity will vary depending upon the index of refraction of
the sensor material n.sub.2, which in turn depends upon temperature,
thereby providing a light signal, the intensity of which is temperature
dependent. The intensity of such a light signal may be detected and
utilized to provide an indication of temperature.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of a first surface temperature sensor in accordance
with the present invention.
FIG. 2 is a graph useful in explaining the operation of the present
invention.
FIGS. 3A through 3E are diagrams of alternative embodiments of the
temperature sensor in accordance with the present invention.
FIG. 4 is an interferometric temperature sensor in accordance with the
present invention.
FIG. 5 is a graph useful in explaining the operation of the present
invention.
FIGS. 6A and 6B are alternative embodiments of the temperature sensor
according to the present invention.
FIG. 7 and FIG. 8 are block diagrams of the temperature detector systems in
accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, a fiber optic first surface optical temperature sensor
includes an optical fiber 11 coupled to a film of sensor material 12
forming dielectric interface 13, sensor material 12 being in turn coupled
at interface 14 to reflection inhibiting material 15 which has a surface
16. Optical fiber core 11 may be surrounded by cladding 17. In the
preferred embodiment, optical fiber core 11 is a multimode optical fiber
and sensor material 12 comprises amorphous hydrogenated silicon, a-(Si:H).
The hydrogenated silicon sensor material may be deposited by r.f.
sputtering in a gas mixture of hydrogen, argon and phosphorous. In
addition, other useful sensor materials would include Si, Ge, a-(Se),
a-(Ge:H), CdS, CdSe, AlSb, CdSSe, GaAs, GaAlAs, InP and other substances
selected from the 3-5 and 2-6 compounds of the periodic table of the
elements.
The sensor material may be deposited by r.f. sputtering, thermal
evaporation, inductively coupled r.f. plasmas or by E-beam evaporation
depending on the material selected. Alternatively, the sensor material may
be deposited on a film of glass and polished, the glass cut to a small
disc, and the small disc thereafter attached to the fiber using
transparent cement. Although any optical fiber core material may be used,
silica, is preferred. The fiber may be single mode or multimode and may
have any diameter, any numerical aperture, and may have either a graded or
step index profile.
It is well known in the optical art that light incident on the interface
between two dielectrics having differing indices of refraction n.sub.1 and
n.sub.2 will be reflected, transmitted, or both reflected and transmitted
depending upon the indices of refraction and the angle of incidence. For
light incident upon an interface between the first and second dielectrics
having indices of refraction n.sub.1 and n.sub.2, respectively, the
percentage of reflected power reflected from the interface will be
##EQU1##
Where .theta..sub.1 equals the angle of incidence and .theta..sub.2 equals
the angle of the refracted light propagated through the dielectric having
an index of n.sub.2. Referring again to FIG. 1, light rays 20 having
normal incidence at the dielectric interface 13 between optical fiber core
11 and sensor material 12 having index of refractions n.sub.1 and n.sub.2,
respectively, will be partially transmitted as shown by light ray 21 and
partially reflected as shown by light ray 22. Since light rays emerging
from optical fiber core 11 emerge in a cone having a small angle such
light approximates the normal incidence on interface 13. Substituting
0.degree. for .theta..sub.1 and .theta..sub.2 corresponding to light
impinging upon the dielectric interface 13 for normal incidence, equation
(1) becomes:
##EQU2##
Thus, if the indices of refraction of fiber core 11 or sensor 12 change
with respect to one another, then the reflectance power R will also
change.
In the instant invention, temperature dependent changes in the n.sub.2, of
sensor material 12 in response to temperature changes, .DELTA.n.sub.2
/.DELTA.T, are utilized to sense temperature changes. Many semiconductors,
including those mentioned hereinabove, exhibit such a temperature
dependent index of refraction. It has been suggested that .DELTA.n.sub.2
/.DELTA.T for amorphous silicon is approximately the same as that for
crystalline silicon since the retention of the short range order in the
amorphous phase provides an average interband separation similar to that
occurring in crystalline silicon. Thermally induced index of refraction
changes in crystalline silicon have been found to be related to two
effects: (1) volume changes, that is expansion and contraction of the
crystalline lattice, and (2) electron-phonon scattering effects at
constant volume. Considering the crystalline lattice as composed of
individual single oscillators, volume changes will lead to an increase or
decrease in the number of oscillators per unit volume, since temperature
changes cause an increase in volume but do not affect the mass of the
material.
It is well known that the energy band gap of a semiconductor is temperature
dependent as related by the equation
E.sub.g (T)=E.sub.g (0)-K.sub.1 T.sup.2 /(T+K.sub.2) (3)
where E.sub.g (0)=the band gap energy at 0.degree. Kelvin;
T=temperature in degrees Kelvin; and
K.sub.1 and K.sub.2 are material constants.
If a single oscillator model is used to approximate the behavior of
crystalline silicon, then the value of the index of refraction n in terms
of energy band gap and frequency may be related by the following equation:
n.sup.2 =1+A/(E.sub.g.sup.2 -(h.gamma.).sup.2) (4)
where A is proportional to the electron density,
.gamma.=the frequency of the illuminating light,
h=Planck's constant.
Then,
##EQU3##
where .beta.=the thermal expansion coefficient for the material. The first
term is dependent upon the energy band gap and the second term is
dependent upon the thermal expansion coefficient for the crystal.
Although the index shift .DELTA.n.sub.2 /.DELTA.T for the sensing material
may be small, the change in reflectance power R may be large. Selecting an
optical fiber core 11 having a negligible .DELTA.n.sub.1 /.DELTA.T,
differentation of equation (2) shows that:
##EQU4##
For amorphous hydrogenated silicon at 22.degree. C. for an incident light
ray 20, having a wavelength of 633 nanometers, sensor material 12 having
an index of refraction n.sub.2 of 3.095, and a band gap edge at 585
nanometers, coupled to an optical fiber 11 having an index of refraction
n.sub.1 =1.47, equation (2) yields R=0.127. In silicon's transparency
region (approximately 1,500 nanometers) .DELTA.n.sub.2 /.DELTA.T is thus
approximately 2.4.times.10.sup.-4 /C.degree.., yielding a reflectance
power change .DELTA.R/.DELTA.T of 0.4% per degree centrigrade. Such a
change in reflectance is easily detected utilizing methods known in the
art.
The selection of the wavelength of the incident light is not critical; the
peak wavelength can satisfactorily be chosen in the range from
.lambda.=500 nanometers to 1,500 nanometers for a-Si:H. In practice, one
may either choose the light source to approximately match the sensor
material or tailor the sensor material to match the particular source
chosen, as discussed below.
The composition of the hydrogenated silicon can be tailored during the r.f.
sputtering process by adjusting the partial pressure of the hydrogen
present during sputtering. By varying the partial pressure of hydrogen
present during sputtering, compositions ranging from 20% hydrogen/80%
silicon, to pure silicon may be obtained using this method. The
fundamental energy band gap edge of hydrogenated silicon may be adjusted
over a range of approximately 550 to 1100 nanometers. The optical
properties of such compositions vary smoothly from one composition to the
next as shown in FIG. 2, wherein are shown four a-Si:H samples having
varying hydrogen/silicon ratios and their resulting characteristics shown
by curves 30 to 33. Selection of the appropriate composition, thus
provides a convenient method for tailoring the sensor composition to the
light source and fiber optic material utilized.
Using this sputtering process the composition of a hydrogenated silicon
film 12 may be graded from the interface 13 to interface 14 with an
accompanying variation of thermooptic properties. This is accomplished by
removing hydrogen gas from the system as the sputtering process proceeds.
For example, a film comprising 20% hydrogen and 80% silicon may be formed
at the fiber optic interface 13 (FIG. 1) with a gradual change in the film
until it becomes pure silicon as one moves farther from the interface 13
to interface 14. Such a graded composition film has important properties
as will be further described.
The first surface optical temperature sensor described thus far utilizes
only changes in reflection at the optical fiber-sensor film interface 13.
It is thus necessary to suppress reflections that occur at interface 14 of
sensor material 12. This can be accomplished in several ways as shown in
FIGS. 3A through 3E. In FIG. 3A, anti-reflection coating 41 is shown
aligned with sensor material 12. Anti-reflection coating 41 is a
dielectric well known in the art selected so that all reflections at
interface 14, which is adjacent to anti-reflection coating 41, are
inhibited. In FIG. 3B a black, absorbing film is placed in alignment with
sensor material 12 which will absorb substantially all light incident
thereupon. In FIG. 3C, a material whose refractive index closely matches
that of the sensor material 12 is aligned therewith. Material 43 is
further selected to be opaque at the optical source wavelength. Since the
refractive index of material 43 substantially matches that of sensor
material 12, there will be little or no reflection at the interface 44
therebetween. Furthermore, as the light propagates through material 43, it
is absorbed since material 43 is selected to be opaque at the frequency of
interest; there is therefore substantially no reflection from surface 45.
In FIG. 3D, a structure utilizing the graded composition hydrogenated
silicon discussed above is shown. When the graded composition material is
used for sensor film 46, the index of refraction and the optical
absorption coefficient may be smoothly varied from interface 13 where it
comprises an a-Si:H mixture to a second surface 47, where it is nearly
pure silicon. As the light propagates through graded material 46, it is
absorbed so that there is substantially no reflection from surface 47.
FIG. 3E shows a basic structure comprising sensor material 12 in which the
wavelength of the incident light source is approximately 500 Angstroms
less than the wavelength of the band edge gap. Since the incident light
will thus fall within the absorbing region of the sensor material 12,
there will be little or no reflection from surface 48.
In the embodiment of the invention shown in FIG. 4, the second surface
reflections are not suppressed but are utilized to provide an
interferometric sensor similar in operation to that of a Fabry-Perot
Etalon. In FIG. 4, the optical fiber core 50 having an index of refraction
n.sub.1 is coupled at surface 51 to sensor material 52 that has a
temperature dependent index of refraction n.sub.2, and is in turn coupled
at surface 53 to dielectric 54 which has an index of refraction n.sub.3
and a surface 55. Sensor material 52 has a thickness D and may be of the
same semiconducting sensor material as previously described, for example,
amorphous hydrogenated silicon, while dielectric 54 may be any material
having an index of refraction different from the sensor material,
preferably lower, and including, for example, air.
Refer again to FIG. 4. In operation, light ray 56 propagates through
optical fiber core 50 and has approximately normal incidence to surface 51
(light ray 56 is shown deviating from normal incidence for clarity). At
surface 51, a portion of the light will be reflected as shown by light ray
57 and a portion will continue to propagate through sensor material 52 as
shown by light ray 58 until it strikes second surface 53 whereat a portion
will be reflected as light ray 59. Additional light rays will be reflected
and propagated between the first surface 51 and second surface 53 as
shown. Emerging light rays 57, 60 and 62 will interfere with one another
in constructive and destructive interference. The optical performance of
thin films such as those utilized herein is well known in the art. The
reflectance power R on a non-absorbing substrate such as optical fiber
core 50 for light rays having normal or nearly normal incidence is:
##EQU5##
where .beta.=(2.pi.n.sub.2 D)/.lambda. at normal incidence,
.beta.=(2.pi.n.sub.2 D cos .phi.)/.lambda. at slight deviations from
normal incidence.
Refer now to FIG. 5 which shows the reflectance power R versus wavelength
for a smooth, parallel 1100 nanometer thick amorphous hydrogenated silicon
film in accordance with equation (7). Curve 70 shows an oscillatory
interference pattern at a temperature T.sub.1, whereas curve 71 shows the
interference pattern found at a temperature T.sub.2 which has the same
oscillator pattern shifted along the abscissa. From equation (7), it is
clear that if n.sub.2 is temperature dependent, the reflectance power will
change for different temperatures as shown in FIG. 5. It is equally clear
that in order to insure best results, optical fiber core 50 and
superstrate 54 should have indices of refraction n.sub.1 and n.sub.3,
respectively, that are relatively independent of temperature changes.
A substantially monochromatic light source selected to have its central
wavelength near the midpoint of a side of one of the sinusoidal cycles in
the sensor material's transparent range, as shown for example by point 72,
is preferably used to illuminate the etalon sensor. Since such a
wavelength lies on curve 71 in its region of maximum linearity and slope,
changes in R, such as .DELTA.R in FIG. 5 may be used in a temperature
sensor having high resolution and linearity.
Those skilled in the art will recognize that changes in the thickness D of
sensor film 52 (FIG. 4) due to thermal expansions and contractions will
also affect the reflectance power R and may need to be included in the
calibration of the interferometric sensor of the present invention.
Reflectance changes due to thermal expansion and contraction of a sensor
material such as a-Si:H are substantially smaller than the reflectance
changes due to the change in the index of refraction, and may in many
instances be ignored.
It is well known in the art that an etalon type interferometer will yield a
series of circular fringes in the well known interference pattern.
Equation (7) describes the case in which one observes primarily the
central fringe at normal incidence or at slight deviations therefrom. In
practice, the light from the fiber fore 50 having index of refraction
n.sub.1, does not emerge in a parallel-on axis beam. The light will emerge
from the fiber core 50 in a cone of rays with a maximum half angle of
.theta..sub.1 relative to the fiber axis. When entering sensor material 52
having index of refraction n.sub.2 greater than n.sub.1, this angle is
decreased in accordance with Snell's Law of refraction .theta..sub.2
=n.sub.1 /n.sub.2 .times..theta..sub.1. The angle .theta..sub.1 may be
determined from the numerical aperture (N.A.) according to the relation
sin .theta..sub.1 =N.A. For a sensor where n.sub.1 =1.5 and n.sub.2 =3.8,
then .theta..sub.2 =1.5/3.8, then .theta..sub.2
=1.5/3/8.times..theta..sub.1 = 0.39 .theta..sub.1. If, for example, the
numerical aperture equals 0.20, then .theta..sub.1 =11.5.degree. and
.theta..sub.2 will equal 4.5.degree., which provides light rays having an
angle of incidence upon sensor material 52 that is close enough to normal
so that light from substantially only the central fringe is returned and
equation (7) is predictive of the sensor performance. Furthermore,
integrating equation (7) over the full range of light ray angles (in this
example 4.5.degree.), as occurs in a multimode optical fiber averages the
nonnormal incident light. This averaging in effect occurs over the
4.5.degree. cone and yields a response substantially the same as if the
incident light were restricted to purely normal incidence.
Typically, the thickness of the sensor film D, would approximate the
wavelength of the light used to illuminate the sensor material, for
example, 1000 to 3000 nanometers.
In addition to the basic structure described in FIG. 4, an optical absorber
75 as shown in FIG. 6A may be added to surface 55 to minimize interference
from external light sources. In FIG. 6B a metal film 76 has been added at
surface 53 to provide reflection in lieu of dielectric 54 while
simultaneously minimizing interference from external light sources and
protecting the dielectric surface.
In operation, it is necessary to collect light reflected from either the
first surface reflective sensor or the etalon type sensor and channel it
to a detector, while segregating the return light from the light incident
to the sensor material. Two structures useful for accomplishing this
function are shown in FIGS. 7 and 8.
In FIG. 7, a fused fiber coupler comprising fibers 80 and 81 are physically
fused at junction 82 which forms an optical directional coupler having
input ports 83 and 84 and output ports 85 and 86. Optical light source 90
is coupled to input port 83 and sensor 91 is coupled to input port 84.
Optical detector 92, which is used for detecting the intensity of light
reflected from the sensor 91, is coupled to an output port 85, the
remaining end of optical fiber 80 terminates at optical port 86 into an
absorber 93 which absorbs substantially all light incident thereon. In
operation, light incident from optical light source 90 propagates through
fiber 80 and at junction 82 couples into fiber 81 propagating in a
direction towards sensor 91. At sensor 91, in accordance with the
principles previously disclosed, light will be reflected depending upon
the temperature of sensor 91 and propagate through fiber 81 to detector 92
where the intensity of the reflected light maybe determined. Since a
portion of the light from optical source 90 will continue to propagate in
optical fiber 80 past junction 82, it may be absorbed by absorber 93 to
prevent unwanted reflections and thereby decrease noise.
If fiber 81 has a larger core diameter than fiber 80, the ratio of light
reflected from sensor 91 remaining in fiber 81 will be greater than light
coupling from fiber 81 into fiber 80, and propagating towards optical
source 90, thus maximizing signal-to-noise ratio by maximizing the power
available at detector 92.
FIG. 8 shows a differential or subtractive system that minimizes intensity
variations in the detected signal due to optical source fluctuations. The
sensor system of FIG. 8 utilizes the fused fiber coupler of FIG. 7 and
comprises optical source 90 coupled to input port 83 and sensor 91 coupled
to the remaining input port 84. First and second optical detectors 92 and
93 are coupled to output ports 85 and 86, respectively, the output signals
of optical detectors 92 and 93 being coupled to difference circuit 94
whose output may in turn be coupled to signal processing means 95 and is
in turn coupled to utilization means 96. In operation, light signals
generated in optical source 90 propagate through fiber 80, the power
therefrom dividing at junction 82 part of which propagates towards sensor
91 in fiber 81, the remainder of which propagates to the second optical
detector 93 as a reference signal. Light which propagates towards optical
detector 91 will be reflected therefrom in accordance with the temperature
detected and propagates back towards the first optical detector 92.
Optical detectors 92 and 93 detect the intensity of light received from
their respective output ports 85 and 86 and generate a signal
corresponding in amplitude thereto. The output signals of optical
detectors 92 and 93 may then be fed to a difference circuit 94 which may
be a differential amplifier that generates an output signal representative
of the difference between first and second optical detectors output
signals. The reference signal appearing at optical detector 93 and the
return signal appearing at optical detector 92 are thus subtracted from
one another and instantaneous time variations in the optical source
strength will be subtracted out, thereby providing more accurate and noise
free signals. The output signal of difference circuit 94 may be further
processed in signal processing circuit 95, which may be a preamplifier or
other circuit needed to condition the signal, the output of which is in
turn coupled to utilization device 96, which may be a meter or other
device useful for displaying an electrical voltage or current.
While the invention has been described in its preferred embodiments, it is
to be understood that the words which have been used are words of
description rather than limitation and that changes may be made within the
purview of the appended claims without departing from the true scope and
spirit of the invention in its broader aspects.
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