A temperature sensor is described which consists of a silicon etalon that is sputtered directly onto the end of an optical fiber. A two-layer protective cap structure is used to improve the sensor's long-term stability. The sensor's output is wavelength encoded to provide a high degree of immunity from cable and connector effects. This sensor is extremely compact and potentially inexpensive

Azar, Massood Tabib

Beheim, Glenn

Flatico, Joseph M.

Fritsch, Klaus

English

NASA Technical Reports Server

NASA Technical Memorandum 102389
,,,__ __:_Silicon-Etalon Fiber-Optic
Temperature Sensor
Glenn Beheim
Lewis Research Center
Cleveland, Ohio
Klaus Fritsch ....................
John Carroll University
Cleveland, Ohio
• r
and
Joseph M. Flatico and Massood Tabib Azar
Case Western Reserve UnDersity
Cleveland, Ohio - -_- - -
Prepared for the
Fiber Optic and Laser Sensors VII Conference
of the OE/Fibers '89Symposium
sponsored by the Society of Photo'Optica! Instrumentation Engineers
Boston, Massachusetts, September 5-8, 1989
(NASA-TM-IO2389) SILiCON-ETALJN
TEMPERATURE S_NSOR (NASA) 8 p
FI_ER-OPTIC
CSCL 010
N90-13381
Unc] _s
0243255G3/O6
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https://ntrs.nasa.gov/search.jsp?R=19900004065 2020-03-19T23:43:08+00:00Z

SILICON-ETALON FIBER-OPTIC TEMPERATURE SENSOR
Glenn Behelm
National Aeronautlcs and Space Adminlstratlon
Lewls Research Center
Cleveland, Ohio 44135
Klaus Frltsch
Department of Physics
John Carroll University
Cleveland, Ohio 44118
Joseph M. Flatico
Department of Physics
Case Western Reserve University
Cleveland, Ohio 44106
and
Massood Tabib Azar
Department of Electrlcal Englneering and Applied Physics
Case Western Reserve University
Cleveland, Ohio 44106
0
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I
SUMMARY
A temperature sensor Is descrlbed which conslsts of a silicon etalon that
is sputtered directly onto the end of an optical fiber. A two-layer protective
cap structure is used to improve the sensor's long-term stability. The sen-
sor's output is wavelength encoded to provide a high degree of Immunity from
cable and connector effects. This sensor is extremely compact and potentlally
inexpensive.
INTRODUCTION
Conslderable research has been performed to develop an accurate, compact
and inexpensive fiber-optic temperature sensor. The prlnclple advantages of
fiber-optic sensors are immunity to electrical interference, small slze, llght
welght, intrinslc safety, and chemical inertness. A disadvantage of fiber-
optic sensors is the high variability of the fiber link's transmlssivity,
which necessitates the use of Information encodlng schemes that are signal-
level insensitlve.
Fabry-Perot etalons are a particularly promlslng type of temperature sen-
sor because the measurement information is wavelength encoded and Is therefore
highly resistant to degradation due to changes in the properties of the fiber-
optic components (refs. I to 6). An etalon's reflectance is a minimum at each
of its resonant wavelengths, which, for an uncoated solid etalon, are given by
2nd/m, where n Is the etalon's refractive Index, d is its thickness, and
m is an integer. Temperature can be sensed uslng an etalon constructed of a
materlal, such as silicon, whose refractive index Is a strong function of tem-
perature. The positions of the mlnlma in the etalon's spectral reflectance
can then serve as a sensitive temperature indicator.
A Fabry-Perot etalon made from an approximately l-Nm thick piece of
single-crystal sl]Icon is used In a commercial fiber-optlc temperature sensor
(ref. 4). Manufacture of thls type of sensor requires that relatively thick
silicon wafers be etched to the desired thickness and then bonded to the end
of an optical fiber. By sputtering a silicon film directly onto the fiber
end, reduced cost, smaller slze and greater ruggedness may be obtalned.
Schulthels, et al., have described a sensor that consisted of a sputtered sill-
con fllm which was protected by a polylmlde coating (ref. 6). This paper
describes a sensor that uses a two-layer sputtered cap structure which pro-
vldes greater stability than can be obtained when an organic coating is used.
SENSOR DESCRIPTION
A critical issue in the development of a practical sputtered-film tempera-
ture sensor is its long-term stability. Due to changes in its physlcal struc-
ture, the sllIcon film's optical properties can be expected to change after
exposure to elevated temperatures. A silicon-film temperature sensor would
need to be annealed, prlor to use, at a temperature significantly higher than
the maximum temperature it will encounter in service.
A practlcal silicon-film temperature sensor also requires some type of
protective coating on its exterior surface. This protective cap structure
should provide reasonably high reflectivlty and at the same tlme block out any
stray light. To prevent long term drift due to chemical reactions, the sensor
structure must be highly stable; at the maximum temperature to which the sen-
sor will be exposed, the protective cap materlal(s) should not appreciably
react with or diffuse into the underlying silicon. The protective cap should
also block external reactants; in particular, It should act as barrier to oxy-
gen in order to prevent oxidation of the silicon.
Figure l shows, schematically, our temperature sensor, which has the fol-
1owlng three-film structure"
(I) 1.4-Nm silicon,
(2) O.14-pm silicon dloxide,
(3) l-pm FeCrAl (77.5:10.6"11.9, by weight).
The Intermediate oxlde layer is intended to prevent the constituents of the
outer metal film from diffusing into the silicon and irreversibly altering its
refractive index. The oxide film's thickness is chosen to be I/4-wave at the
sensor's deslgn wavelength of 850 nm. This causes the reflections from the
silicon-oxide and oxide-metal Interfaces to interfere constructively, thereby
maximizing the amount of light reflected back through the silicon film.
FeCrAl, like stainless steel, forms a protective scale on exposure to an oxi-
dizlng ambient, and is therefore expected to act as a barrier to oxygen to pre-
vent oxidation of the underlying silicon.
This sensor was fabricated by sputtering onto the ends of step-index mul-
timode fibers having lO0-pm core diameters and 140-pm cladding diameters.
These fibers have an approximately lO-pm thick polyimide buffer coating which
protects the fiber surface. This buffer materlal permlts these fibers to be
exposed to temperatures as high as 350 °C. In order to measure temperatures
higher than 350 °C, fibers with gold buffer layers can be used (to 650 °C),
however, these fibers are expensive.
The fiber ends were prepared for sputtering by first removlng the buffer
material from the last l In. of a l-m long piece of fiber. The polyimlde buf-
fer was first turned to ash using a butane flame, then the ash was wiped off
uslng a methanol-soaked wipe. The fiber was cleaved approximately ] mm from
the end of the remalnlng buffer material, and the cleaved ends were then
Inspected for smoothness and cleanliness using a microscope. Prlor to inser-
tion In the sputtering system, the fibers and thelr holder were ultrasonlcally
cleaned uslng delonlzed water and detergent, then rinsed under runnlng
delonlzed water, and finally blown dry with nitrogen.
The films were deposited in an RF sputtering system having three 6-In.
diameter targets. A fiber holder was used to hold the fiber ends perpendicu-
lar to the target. Without breaking vacuum, all three films were sputtered at
a power level of 200 W. The silicon and FeCrAl were sputtered in argon, while
the oxlde was sputtered in an 80:20 mixture of argon and oxygen. The fibers
were then spliced to fused-fiber couplers, as shown in figure I, so that the
reflectlvlty of the sputtered films could be monitored.
EXPERIMENTAL RESULTS
Figure 2 shows the sensor's spectral transmissivlty which was measured,
at room temperature, Immedlately after fabrication and again after the sensor
was annealed for 19 hr at 310 °C. As shown by the data plotted In figure 2,
annealing has the effect of reducing the silicon film's absorption coefficient
and refractive index. Figure 3 shows the transmlsslvity of the annealed sen-
sor at 25, 125 and 230 °C (changes In transmlsslvlty from the previous figure
are caused by the use of different fibers and connectors). As shown by fig-
ure 3, increasing the sensor's temperature Increases the silicon film's refrac-
tive Index, thereby shifting the minima In the sensor's spectral transmlsslvlty
to longer wavelengths.
In order to assess the stability of this sensor, its transmlsslvity was
monitored over the course of a 700-hr long exposure to a temperature of 230 °C.
Figure 4 shows, as a functlon of time, the position of one of the minima in
the sensor's spectral transmisslvity. The observed variations in the resonant
wavelength were determined to be caused by instabilities in our measurement
apparatus. More accurate measurements need to be performed before the long-
term stability of this sensor can be determined.
This temperature sensor is intended to be used in conjunction with an LED
source and a mlcro-optic spectrometer (ref. 3). The spectrometer uses a
2400 line/mm prism grating and a 5-mm diameter GRIN lens to disperse the sen-
sor's output spectrum across a 12-element photodiode array. The array's
active elements have a width of 80 _m and a center-to-center spacing of 140 _m.
The spectrometer's channel separation is 7.2 nm and each channel's width (FWHM)
Is 12.4 rim. Thls type of spectrometer permits the development of a compact
multi-channel sensor system in which one spectrometer analyzes the outputs
from a number of different kinds of spectrum-modulatlng fiber-optic sensors
(ref. 3).
Figure 5 shows the sensor's input and output spectra when an LED source
is used. Figure 6 shows the spectrum from the LED-powered sensor that is
Incident on each of elght spectrometer channels. These data show that the
mlcro-optic spectrometer has sufflclent resolution to analyze the sensor's
output.
A simple method of determining the sensed temperature Is to use the ratio
of the outputs from two channels. Figure 7 shows the ratio of the channel 3
and 5 outputs as a function of temperature. Alternatively, a more sophistl-
cated signal processing method, which uses the outputs from all the channels,
might be used (ref. 1). Such an approach may be able to compensate for changes
In the LED spectrum, eliminating the need for precise control of the LED's tem-
perature and permitting aged LEDs to be readily replaced.
CONCLUDING REMARKS
The three-film sensor structure described here has been shown promising,
however, its long-term stabllity has not yet been determined. Considerable
work also needs to be performed to develop the electronic hardware and signal
processing software that are needed to produce a practical instrument.
REFERENCES
I. W.H. Quick, K.A. James, and J.E. Coker, "Fibre Optics Senslng Tech-
nlques,"In First International Conference on Optical Fiber Sensors, IEE
CP-221, 6-9 (1983).
2. G. Behelm, "Fibre-Optlc Thermometer Uslng SemIconductor-Etalon Sensor,"
Electron. Lett. 22(5), 238-239 (1986).
3. G. Behelm and K. Frltsch, "Spectrum-Modulating Fiber-Optlc Sensors for Air-
craft Control Systems," NASA TM-88968 (1987).
4. J.C. Hartl, E.W. Saaskl, and G.L. Mitchell, "Fiber Optic Temperature Sen-
sor Using Spectral Modulation," in Fiber Optic and Laser Sensors V,
R.P. DePaula and E. Udd, eds., Proc, SPIE 838, 257-262 (1988).
5. G. Beheim, K. Frltsch, and D.J. Anthan, "Fiber Optic Temperature Sensor
Using A Spectrum-Modulating Semiconductor Etalon," in Fiber Optic and
Laser Sensors V, R.P. DePaula and E. Udd, eds., Proc. SPIE 838, 238-246
(1988).
6. L. Schulthels, H. Amstutz, and M. Kaufmann, "Fiber-Optlc Temperature Sens-
Ing With Ultrathin Silicon Etalons," Opt. Lett. 13(9), 782-784 (1988).
4
SILICON DIOXIDE-_
SILICON _ x F FeCrA|
FIBER CORE___ AIR
_' F SENSOR
SPLICE -7 _/
INPUT FIBER / //
"', _J"'--H]GH TEMPERATUREINPUT/
\
_ \'-COUPLER
/
LOUTPUT FIBER
FIGURE I. - THIN-FILM TEMPERATURE SENSOR.
.035 ,
•030
fff'ER -, \BEFORE
•025
i .020
,015
.010
.005
%0 900 950
WAVELENGTH, nm
FIGURE 2. - TRANSMISSIVITY BEFOREAND AFTER ANNEALING•
863
857
TIME, hr
FIGURE 4. - RESONANT WAVELENGTH VERSUS TIME AT 230 °C.
861
E
l-
if-
. 859
>_
.O2O
.015
.010
.005
•:125 °C
i i ! f __0
600 650 700 750 800 850 900 950
WAVELENGTH, nm
FIGURE 3. - TRANSMISSIVITY AT 25, 125, AND 230 °C.
.015 .4
,010
_ .005
0
780 800 820 840 850 880 900
WAVELENGTH,nm
FIGURE 5. - SENSOR INPUT AND OUTPUTSPECTRA.
_U I OUTPUT
92O
.2 --
¢
.1E
81.0
.S
0
800 900
_-CH S_..
",, _LH b_
, /
'/'_' CH 77
"'N/ MCH 3_ \ CH 8;"
820 840 8GO 880
WAVELENGTH,nm
FIGURE 6. - SPECTRAINCIDENT ON EACH PHOTODIODE
ARRAYELEMENT.
83
5
q
%
i i l
30 80 130 180 230
TEMPERATURE, °C
FIGURE 7. - RATIO OF CHANNEL 3 AND 5 OUTPUTS VERSUS
TEMPERATURE.
6
Report Documentation PageNational Aeronautics and
Space Administration
1. Report No. 2, Government Accession No. 3. Recipient's Catalog No.
NASA TM- 102389
5. Report Date4. Title and Subtitle
Silicon-Etalon Fiber-Optic Temperature Sensor
7. Author(s)
Glenn Beheim, Klaus Fritsch, Joseph M. Flatico, and Massood Tabib Azar
6. Performing Organization Code
8. Performing Organization Report No,
E-5130
9. Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135-3191
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546-0001
10. Work Unit No.
505-62-01
11. Contract or Grant No.
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementary Notes
Prepared for the Fiber Optic and Laser Sensors VII Conference of the OE/Fibers '89 Symposium sponsored by
the Society of Photo-Optical Instrumentation Engineers, Boston, Massachusetts, September 5-8, 1989. Glenn
Beheim, NASA Lewis Research Center; Klaus Fritsch, Dept. of Physics, John Carroll University, Cleveland,
Ohio 44118; Joseph M. Flatico, Dept. of Physics, Case Western Reserve University, Cleveland, Ohio 44106;
Massood Tabib Azar, Dept. of Electrical Engineering and Applied Physics, Case Western Reserve University.
16, Abstract
A temperature sensor is described which consists of a silicon etalon that is sputtered directly onto the end of an
optical fiber. A two-layer protective cap structure is used to improve the sensor's long-term stability. The
sensor's output is wavelength encoded to provide a high degree of immunity from cable and connector effects.
This sensor is extremely compact and potentially inexpensive.
17. Key Words (Suggested by Author(s))
Fiber-optic sensors
Temperature sensors
18. Distribution Statement
Unclassified - Unlimited
Subject Category 06
19. Security Classif. (of this report) 20, Security Classif. (of this page) 21, No of pages
Unclassified Unclassified 8
NASAFORMlS_6oc'r es *For sale by the National Technical Information Service, Springfield, Virginia 22161
22, Price*
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