Thermal measuring and testing – Temperature measurement – Nonelectrical – nonmagnetic – or nonmechanical temperature...
Reexamination Certificate
2000-03-03
2001-08-07
Gutierreez, Diego (Department: 2859)
Thermal measuring and testing
Temperature measurement
Nonelectrical, nonmagnetic, or nonmechanical temperature...
C374S120000
Reexamination Certificate
active
06270254
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, in general, to temperature sensors, and in particular to a new and useful etalon/fiber optic based temperature sensor.
2. Description of the Related Art
A Fabry-Perot etalon consists of two parallel planar reflecting surfaces separated by a distance &tgr;. Due to interference among the multiply reflected beams from the two reflecting surfaces, the reflectance of an etalon is a strong function of wavelength and the optical path length, n&tgr;; where n is the index of refraction of the medium between the two surfaces.
Mathematically the reflectance can be written as:
R
=
4
r
Sin
2
(
δ
/
2
)
+
(
r
1
1
/
2
-
r
2
1
/
2
)
2
(
1
-
r
)
2
+
4
r
Sin
2
(
δ
/
2
)
Where
r
1
is the reflectance of the first surface
r
2
is the reflectance of the second surface
r=(r
1
•r
2
)
½
and &dgr;=the phase difference between two successive beams and is given by
δ
=
4
π
n
τ
λ
o
when the illuminator is a collimated beam of wavelength=&lgr;
o
, incident perpendicular to the reflective surfaces.
FIGS. 1 and 2
show how the reflectance changes as a function of and &tgr; and &lgr;
o
, respectively.
The three references L. Schultheis, H. Amstutz, and M. Kaufmann, “Fiber Optic Temperature Sensing With Ultrathin Silicon Etalons,”
Optics Letters
13, No. 9, Sep. 1988, p. 782; J. W. Berthold, S. E. Reed, and R. G. Sarkis “Simple, Repeatable, Fiber Optic Intensity Sensor for Temperature Measurement,”
SPIE OE Fibers '
89
Proceedings
, Vol. 1169; and J. C. Hartl, E. W. Saaski, and G. L. Mitchell, “Fiber Optic Temperature Sensor Using Spectral Modulation,”
SPIE
Vol. 838, Fiber Optic & Laser Sensors V (1987), p. 257; describe temperature sensors that use a thin silicon etalon deposited on the end of an optical fiber. Components of those sensors are shown schematically in FIG.
3
.
These sensors consist of:
a narrow band light source
10
; an optical fiber
12
that carries the light to the thin film (&tgr;=500 to 1000 nm) silicon etalon
14
; a 2×2 fiber optic coupler
16
which serves to divert half of the outgoing light to a reference detector
18
and half of the light reflected from the etalon to a second detector
20
; and electronics
22
that ratios the reflected signal I to the reference signal I
o
to determine the reflectance of the etalon R=I/I
0
.
The index of refraction of silicon decreases with temperature causing a change in phase, &dgr;. The resultant effect on the reflectance is shown in
FIG. 4
for an etalon with room temperature thickness of 785 nm. The temperature also changes the thickness (thermal expansion) of the etalon but the magnitude is negligible in comparison to the index change.
It can be seen in
FIG. 4
that the measured reflectance is a single valued function only over the restricted range from −100° C. to about 4000° C. Actually, the limits of this range can be shifted by changing either the wavelength or the thickness, but the upper and lower limits move together yielding a fixed range of about 5000C.
The reference J. C. Hartl, E. W. Saaski, and G. L. Mitchell, “Fiber Optic Temperature Sensor Using Spectral Modulation,”
SPIE
Vol. 838, Fiber Optic & Laser Sensors V (1987), p. 257 describes a similar sensor where instead of using a reference detector to compensate for source variations, the signal beam reflected from the etalon is separated into two wavelength bands (both within the narrow 100 nm band of the LED source) by a dichroic beam splitter. The two bands are detected and the ratio of the two provides a signal that is dependent on temperature, but insensitive to changes in source intensity, fiber transmission or connector loss. This approach also yields a range of about 5000° C.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an etalon/fiber optic based temperature sensor with increased temperature range, in particular, a sensor having a temperature range of more than 500° C.
According to the present invention, a temperature range of upwards of 1,000° C. is possible.
This is achieved by using two separate light sources which are combined into a single fiber using an appropriate coupler, the light sources being distinguishable by either wavelength or time when they are applied to the fiber optic, or both. The opposite end of the fiber is connected to a thin film etalon. Somewhere intermediate, the light sources and the etalon, a 2×2 coupler splits off half of the light from the fiber and directs it at one end to a pair of reference detectors, and at an opposite end to a pair of signal detectors. This is in the wavelength domain embodiment of the invention. Two pairs of detectors are each serviced by a wavelength-division-multiplexer, and the output signals of the four detectors are supplied to a multiplexer which reads the signals and supplies them to an A/D converter and thereafter to a computer or CPU, for processing.
In the time domain embodiment of the invention, only a single reference and a single signal detector are needed which are likewise serviced by a multiplexer, A/D converter and CPU, for signal processing. The CPU also drives a switching signal that alternates activation of the two light sources for applying first one and then the other light wavelength to the optical fiber.
In both embodiments, one of the wavelengths is utilized to measure temperatures in a lower temperature range, e.g. −100 to +400° C. and the other is used to measure temperatures in a higher temperature range, e.g. 400° C. to 900° C.
The present invention has the advantages of robust fiber optic design plus a broad temperature range, with only a relatively small increase in complexity.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.
REFERENCES:
patent: 5408091 (1995-04-01), Perraud et al.
patent: 5474381 (1995-12-01), Moslehi
patent: 5498867 (1996-03-01), Senuma et al.
patent: 5564832 (1996-10-01), Ball et al.
patent: 5639162 (1997-06-01), Sai
patent: 5645351 (1997-07-01), Nakata et al.
patent: 5741070 (1998-04-01), Moslehi
Berthold John W.
Jeffers Larry A.
Edwards Robert J.
Guadalupe Yaritza
Gutierreez Diego
Marich Eric
McDermott Technology Inc.
LandOfFree
Extended range fiber-optic temperature sensor does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Extended range fiber-optic temperature sensor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Extended range fiber-optic temperature sensor will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2443134