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NASA Technical Memorandum 102282 

Compensation for Effects of Ambient 

Temperature on Rare-Earth Doped 

Fiber Optic Thermometer 

G. Adamovsky , J. L. Sotomayor, 

and M.J. Krasowski 

Lewis Research Center 

Cleveland, Ohio 

and 

J.G. Eustace 

John Carroll University 

Cleveland, Ohio 

Prepared for 

Fiber Optic and Laser Sensors VI1 

sponsored by the Society of Photo-Optical Instrumentation Engineers 

Boston, Massachusetts, September 5-8, 1989 

_- H89-27998 -\ 

;msa-~a-102282) CORPES ATION- POP EFFECTS 

OH ABBIENT TEHPEElATURE ON fl?ARE-EELBTH DOPED 

PIBE8 OPTIC TBERBOBETBR (SASA. ,' WU%S Besearch Center) 14 p CSCL 14B unclas 

G3/35 0224115 
Compensation for effects of ambient temperature on 

rare-earth doped fiber optic thermometer 

G. Adamovsky, J.L. Sotomayor, and M.J. Krasowski 

NASA Lewis Research Center 

Cleveland, Ohio 44135 

and 

J.G. Eustace 

John Carroll University 

Physics Department 

Cleveland, Ohio 44118 

ABS TRACT 

Variations in ambient temperature have a negative effect on the performance of 

any fiber optic sensing system. A change in ambient temperature may alter the 

design parameters of fiber optic cables, connectors, sources, detectors, and other 

fiber optic components and eventually the performance of the entire system. The 

thermal stability of components is especially important in a system which employs 

intensity modulated sensors. Several referencing schemes have been developed to 

account for the variable losses that occur within the system. However, none of these 

conventional compensating techniques can be used to stabilize the thermal drift of 

the light source in a system based on the spectral properties of the sensor material. 

The compensation for changes in ambient temperature becomes especially important in 

fiber optic thermometers doped with rare earths. Thus, the purpose of this work is 

to search and analyze different approaches to solving this problem. 

1. INTRODUCTION 

Spectral sensors are those that respond to the measured parameter by modulating 

the spectrum of an optical signal according to spectral characteristics of the sensor 

material. The spectral sensors are governed by different physical phenomena, among 

which absorption is the most commonly used. The measured parameter (temperature, 

pressure, etc.) alters the absorption spectrum of the sensor material and changes the 

transmission of the system accordin 1 . Various spectral sensors have already been 

discussed in numerous publications. 9-i 

The inherently selective optical properties of the spectral sensors make the 

entire system heavily dependent on the stability of the light sources used. Light 

emitting diodes (LED) have relatively narrow spectral bandwidths. A change in the 

ambient temperature causes the LED to change its central wavelength, spectral bandwidth, and emitted power. This results in changes in the return signal that are not 

a result of changes in the measurand. The signal processing electronics do not discriminate between transmission variations due to changes in the measured parameter 

and those due to changes in the ambient conditions. Thus the ambiguity results in 

an increased error and limits applicability of spectral sensors. 
Ambient temperature has the greatest effect on the spectral and power characteristics of the light sources. The spectral stability of the sources becomes 

especially important in airborne fiber optic sensing systems. Optical sources for 

aircraft applications are expected to operate over a temperature range from -55 to 

125 OC. In this work we will discuss different ways to compensate for or at least 

minimize the effects of a changing ambient temperature on the performance of a fiber 

,~ptic thermometer based on absorption in rare-earth doped materials. 

Due to a specific electron configuration of the rare earths, all elements of 

this group exhibit a strong temperature dependent absorption after being incorporated 

into a host material as impurity ions. The absorption spectrum of the rare earth ion 

varies from element to element and also depends on the host medium.5,6 

is one of the most attractive materials to use in temperature sensing systems, mainly 

because its absorption spectrum lies in the same spectral region as commercially 

available high power GaAlAs LEDs. 

can be achieved over a broad range of temperatures. 

Neodymium 

At the same time accurate temperature measurement 

2. EFFECT OF TEMPERATURE ON LED PERFORMANCE 

The spectral distribution of energy radiated from an LED during an electronhole recombination process is governed by the Boltzmann relation and is therefore 

temperature dependent. The theory of this phenomenon is explained in detail in 

numerous ublications on semiconductors, solid state physics, and optical communi- 

~ation.~-?o It has also been shown that the observed emission spectra of LEDs are 

much more symmetrical around the central wavelength than one would expect from theoretical predictions. For reasons of simplicity, it is conventionally accepted to 

treat the emission spectrum of an LED as a Gaussian distribution. The central wavelength of the emission spectrum is determined largely by a direct band gap in a 

binary semiconductor material. GaAs, InSb, and InP are examples of 111-V binary compounds widely used for manufacturing LEDs. Tuning of the central wavelength can be 

obtained by varying the composition of components in more complex configurations like 

ternary (GaAlAs) or quaternary (InGaAsP, InGaAsSb) systems. In all cases, the temperature has a direct effect on the band gap and its increase causes the central 

wavelength to shift toward longer wavelengths. The spectral half-width of the radiation is also affected by the temperature. Theoretical and experimental studies done 

on optical characteristics of semiconductor light sources have shown that their temperature dependent properties obey some exponential relationship. A prolonged operation of LEDs at elevated temperatures also causes degradation in their performance. 

The degradation manifests itself in a quasi-exponential decrease in output power with 

time. The change in the spectral characteristics of an LED operated at 100 mA continuous current and ambient temperature Ta = 90 OC is demonstrated in Fig. 1. The 

figure shows spectra measured immediately after the ambient temperature reached 90 OC, 

after 72 hours, and after 168 hours of operation at this temperature. 

Figure 2 shows LED spectra measured at different ambient temperatures. To characterize the thermal sensitivity of LEDs such incremental parameters as a thermally 

induced changes in the central wavelength AXc/AT, spectral bandwidth A(AX)/AT, 

and emitting power AP/AT are often used. These parameters are sometimes given by 

LED manufacturers in product specifications, and their values depend on both the 

material and the fabrication process used. 

2 
 

3. COMPENSATION FOR CHANGES IN EMITTED POWER 

Among the parameters mentioned above, the temperature effects on LED emitted 

OIfferent techniques for compensation have been proposed.11 

power is the easiest one for which to compensate. Variations in the emitted power 

propagate through the system and affect its performance the same way variable losses 

do. The most commonly 

used compensating technique is based on a two wavelength referencing scheme.l2313 

In this compensating technique, one wavelength lies in the range of spectral sensitivity of the sensor. The other wavelength is outside this range. Thus, the ratio 

of signals coming back from the sensor is independent of variable losses through the 

system. If the parameter APIAT has the same value at both wavelengths, this ratio 

would also be independent of the variations in the emitted power. However, in a system with two LEDs the parameter AP/AT is usually different for each source. In 

this case additional referencing may be used. 

An example of a sensing system with dual wavelength referencing and power level 

The LEDs are driven by a 

compensation is shown in Fig. 3. Two LEDs with different wavelengths, 11 and 

X2, are used to generate a signal and a reference channel. 

pulse generator in such a way that pulses of radiation emitted by the LEDs are multiplexed in time. This permits using the same photodetector for both the signal and 

the reference channels. An initial signal that consists of two pulses enters the 

system through a 2 by 2 coupler. The coupler splits the signal through two ports. 

One port directs the pulses towards the sensor and the return signal is detected by a 

photodetector (PDs). The signal that reaches this photodetector consists of a periodic double pulse, with one pulse at the wavelength 11 having an amplitude Vsl 

and the other one at the wavelength 12 having an amplitude Vs2. The other port 

is used to direct a portion of the initial signal towards another photodetector, PDr, 

for the power level compensation. This photodetector sees pulses of amplitudes Vrl 

and Vr2 at wavelengths 11 and 12 , respectively. The resultant signal used 

to retrieve information about the measured parameter can be written in the form: 

V = VslVr2lVs2Vrl 

where Vsl and Vs2 are amplitudes of signals coming from the sensor at wavelengths 11 and 12 respectively, and Vrl and Vr2 are amplitudes of power 

level compensating signals at wavelengths 11 and 12 respectively, and is independent of variations in LED power levels. Experimental testing of this scheme has 

shown that an arbitrary decrease in the output power of any of the LEDs used to 70 

percent of the initial power results in less than a 2 percent error in the final 

result. 

An alternate approach involves time domain referencing.14-16 The principle 

of this technique is based on separation of the signal and reference channels in the 

time domain over the same fiber optic link. This is achieved by pulsing the source 

and comparing the amplitude of a reference pulse with the one returned from the sensor. The ratio of the pulse amplitudes is independent of the initial power level. 

Another compensating scheme that can be used involves contrast modulation of an 

interference pattern by an i ntensi ty sensor. 17 

4. COMPENSATION FOR CHANGES IN SPECTRAL DISTRIBUTION 

Because of the specific nature of spectral sensors, power level referencing 

alone is not sufficient for constructing a thermally stable sensing system. 

3 
Therefore, in addition to power referencing, it is necessary also to stabilize the 

iystem against changes in the optical spectrum ,3f the light source used. fiber 

optic sensing systems stabilized against changes in spectral characteristics of LEDs 

due to changes in the ambient temperature can be grouped into three categories: systems with active compensation, systems with passive compensation, and systems withmt Compensation. 

:.l. __ Systems with active compensation 

In systems with active compensation the spectral characteristics of the source 

dre maintained by controlling the temperature of the junction directly. Two parameters are mainly involved in a thermodynamic process of heating an LED, an average 

current that passes through the junction and the ambient temperature of the device. 

dotn the current and the ambient temperature contribute jointly to the thermal stability of the light sources. LED spectra at room temperature and different dc currents are shown in Fig. 4. It is obvious that in order to stabilize the spectrum at 

room temperature the a v e r a g e c u r r e n t should be small. This can be achieved by pulsing the source instead of employing continuous dc. Lower pulse duty cycles yield 

lower average curr-ents and lessen the thermodynamic disturbance (heat) .I8 This 

results in reduced spectral changes in the LED radiation. Thus, the simplest form 

of active compensation could involve changing the average current through the junction in accordance with the change in the ambient temperature. However, this 

approach has restrictions because of limited dynamic range of the current-wavelength 

dependency. 

Another active compensation technique involves heating or cooling the device to 

temperatures which are basically outside the reach of ambient temperature excursions 

or holding it at a desired set point temperature anywhere within the operating range 

of the device. 

Heating of the LED is achieved with either resistive or thermal electric 

devices. Resistive heaters fall into two general categories: resistive strip and 

positive temperature coefficient (PTC) thermistors. In the resistive strip the current passes through a resistive element which dissipates heat generated in the process. The LED assembly is thermally connected with the strip and is in turn heated. 

PTC thermistors are passive devices whose electrical resistance rises with temperature. The device dissipates the power as heat. The heat in turn raises the resistance of the thermistor. As a result the dissipated power decreases. This internal 

feedback allows stable operation without external control. 

Another form of active compensation involves thermal electric devices (TED). 

The principle of these devices is based on the dissimilar thermodynamic properties 

of two materials configured in such a way that they form two junctions. A voltage 

applied to these junctions creates a temperature difference between the two junctions. This phenomenon is known as the Peltier effect. The direction of heat flow 

through the device is dependent on the polarity of the input electrical current. 

Therefore heating can be achieved by applying current in one direction, cooling by 

applying it in the other. In the heating mode a large portion of the heat produced 

by the device is due to its resistive component. The remainder must be pumped from 

the environment and this requires a great deal of electric power; more so than the 

schemes mentioned above. Also, the device loses efficiency as the temperature of the 

outside environment drops. 

4 
The efficiency of the TED as a cooling element is even lower because the resistive component in the device is producing heat regardless of the direction of the 

current flow. Therefore in the cooling mode the power applied to the TED must overcome its self heating and also cool the attached LED. However for its size and given 

the fact that it has no moving parts or restrictions on physical orientation, the TED 

is an ideal cooling device for electronic components. 

4.2. Systems with passive compensation 

In systems wfth passive compensation the average current through the junction 

remains the same and the compensation is achieved in the signal processing hardware. 

The signal processing hardware provides compensation based on knowledge of values of 

LEDs spectral characterlstlcs, such as the central wavelength and bandwidth, at dlfferent temperatures. The information about the spectral parameters can be obtained 

either indirectly by measuring the ambient temperature and predicting these parameters or directly by measuring the position of the central wavelength and the 

bandwidth. 

A schematic of an indirect passive compensation scheme is presented in Fig. 5. 

A look up table (LUT) which contains the values of the spectral parameters for the 

specific LED used versus temperatures is positioned in the read only memory (ROM). 

The signal that comes back from the sensor is detected by the photodetector (PD), 

analyzed in the preprocessor (PPC), and sent to the microprocessor (PPI. Values of 

the spectral parameters of the LED at the measured ambient temperature Ta are read 

from the LUT. 

another one could lead to a reprogramming of the ROM and to a recalibration of the 

entire sensing system. Despite the fact that variations in the source intensity can 

be compensated by the power referencing technique as mentioned above, any other 

changes In the source spectral characteristics due to aging and degradation are not 

eliminated by this technique. 

A major problem lies in the fact that a replacement of one LED by 

A direct passive compensation technique involves the continuous measurement of 

the spectral parameters of the LEDs used and processing these data with a microprocessor. A schematic diagram of a sensing system using direct passive compensation 

is shown in Fig. 6. A portion of the radiation emitted by the light source is spread 

in the wavelength domain by an appropriate dispersing element (DE). A photodetector 

array (PDA) sees this dispersion as a spectral intensity distribution along the 

array. Thus the shape of the distribution and its position can be easily detected. 

A signal conditioner (SC) takes the information from the array and converts it into 

data acceptable to a microprocessor (PPI. The data from the sensor is also sent 

to the microprocessor via a photodetector (PO> and a preprocessor (PPC). This technique permits an accurate tracking of the entire spectral intensity distribution. 

Thus, additional power level referencing can be avoided. Obviously, this technique 

provides a better compensation than the previous one. However a sensing system with 

direct passive compensation would be more complex, and therefore, possibly less 

re1 iable. 

4.3. Systems without compensation 

variations in the spectral distribution of the LED radiation over a specified range 

of ambient temperatures. 

Fiber optic sensing systems without compensation are designed to tolerate all 

5 
The best known configuration of a Nd-based temperature sensing system without 

cmpensation involves restricting the optical spectrum of the source using optical 

filters.19 This technique provides effective stabilization and has been used 5UCcessfully on several occasions. However the employment of filters also has dra+ 

backs. Transmisbion of the filters Is low and usually does not exceed 60 percent. 

This places demands on the power budget and establishes a requirement either to use 

more powerful LEDs or to increase the operating current. In addition to this, the 

spectral window of the filter itself is affected by the ambient temperature. This 

thermal sensitivity is greater for filters with narrower spectral windows. Among 

the other disadvantages of using optical filters are possible alignment problems 

and reduced reliability due to use of additional components. 

Another approach to building a Nd-doped thermometer without compensation 

involves using parts and components that are insensitive to changes in the ambient 

temperature. The sensor itself and the LED used are the primary targets for engineering efforts to build a system insensitive to ambient temperatures. 

The sensor could be designed to tolerate some variations in the LED emission 

spectrum. The absorption,spectrum of Nd-doped material depends on both the concentration of Nd3+ ions and the host used. An increase in the concentration leads 

to overlapping of individual absorption lines and the formation of solid absorption 

bands. 

glass doped with 3.5. 6.0. and 8.0 wt % Nd2O3. Broader absorption bands and 

sharper boundaries between the bands are clearly associated with a larger concentration. 

range of ambient temperatures because the middle points of the neighboring absorption and transmission bands in the spectral region of interest (700 to 900 nm) are 

separated by approximately 30 nm. Thus, for an LED with AXc/AT = 0.1 nm/OC. 

PA = 20 nm, dnd AtAA>/AT = 0 , compensation would be possible for an approximate 

range of ambient temperatures Ta = To 5 5 OC. This prediction has been made under 

the assumptions that boundaries between the bands have a rectangular shape and the 

LED central wdvelength at room temperature To lies in the middle of the absorption 

band. 

Figure 7 presents measured transmission spectra of 6 mm thick samples of 

However the described approach permits only partial stabilization over a small 

Host materials also assist in the broadening of the absorption bands. Hosts 

with crystalline structures of lower symmetry would exhibit broadening and overlapping of the absorption bands. On the other hand, fine structures of absorption 

asscciated transitions could be observed in host materials with highly symmetrical 

structures. Recent reports on such materials with low symmetry hosts as Nd:YVO and 

Nd:BeL have shown significant broadening of the absorption bands in 1 percent 

Nd-doped crystal s.20-24 

glass would be one of the best candidates as a host material. 

Because amorphous materials have the lowest symmetry, 

As regards the source it is obvious that the sensitivity of the system to 

ambient temperature can be reduced if the spectral bandwidth is significantly greater 

than the spectral region of interest where the absorption bands of Nd3+ ion lie. 

To show the effect of the spectral bandwidth, two LEDs with the same central wavelength of 810 nm and different spectral bandwidths of 30 and 100 nm have been chosen 

for the study. Then assuming that the initial emitted power at room temperature for 

both LEDs is the same, 

power P that reaches a photodetector has been computed as a function of the 

sensor temperature Ts for ambient temperatures Ta = -40 OC, Ta = 25 OC, and 

Ta = 75 OC. The results of the computations are shown in Table I. It is seen from 

the table that the deviation in the computed power for the LED with A1 = 100 nm 

AAc/AT = 0.2 nm/OC, and A(AA)IAT = 0.1 nm/OC, a total 
-- 

at Ta = -40 OC and Ta = 75 OC does not exceed 1 percent of the power at 

Ta = 25 OC over the range of Ts 

with ah = 30 nm the corresponding changes in the computed power rapidly become 

intolerable. 

from 0 to 300 OC. On the other hand, for the LED 

5. CONCLUSION 

The preliminary results of the study have shown that despite a variety of compensating techniques, not all of them can be used in airborne applications due to 

either complexity, size, or power budget considerations. The most promising approach 

would involve an LED with a broad spectral bandwidth. A reference channel in this 

case could be constructed by either using another LED with a central wavelength in 

the spectral region outside the absorption bands of the Nd3+ ion, or employing one 

of the time domain or RF modulation referencing techniques. The current nonavailability of LEDs with sufficiently broad spectral bandwidths can be overcome by 

combining several LEDs with different central wavelengths and narrower bandwidths. 

This approach leads to a more complex system. However, the complexity can be reduced 

if, in addition, one of the forms of either active or passive compensation is used. 

This would permit the utilizatipn of commercially available LEDs with narrower bandwidths. Experimental tests are planned in order to analyze fully the advantages and 

disadvantages of the compensating techniques discussed in this study. 

J.G. Eustace acknow 

Grant NAG-3-984). 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

6. ACKNOWLEDGMENTS 

edges the support of the NASA Lewis Research Center (NASA 

7. REFERENCES 

K. Kyuma, S. Tai, T. Sawada. and M. Nunoshita, "Fiber-ODtic Instrument for 

Temperature Measurement," IEEE J. Quantum Electron, vol'. QE-18, pp. 676-679, 

Apr. 1982. 

E. Theocharous, "Displacement and Temperature Mu1 timeasurant Transducer," in 

Conference Proceedings OFS '84 - Second Internatlonal Conference on Optical 

Fiber Sensors, R.T. Kersten and R. Kist, eds., Proc. SPIE 514, pp. 199-203, 

1984. 

E. Snitzer, W.W. Morey, and W.H. Glenn, "Fiber Optic Rare Earth Temperature 

Sensors," in First International Conference on Optical Fiber Sensors, Conf. 

Publ. No. 221, Institution of Electrical Enqineers, London, U.K., pp. 79-81, - 

1983. 

M. Bacci, M. Brenci, G. Conforti, R. Falciai, A.G. Mignani, and 

"Thermochromic Transducer Optical Fiber Thermometer ,I8 Appl . Opt 

pp. 1079-1082, Apr. 1, 1986. 

G.H. Dieke and H.M. Crosswhite, "The Spectra of the Doubly and 

Rare Earths," Appl. Opt., vol. 2, pp. 675-686, July 1963. 

A.M. Scheggi, 

, vol . 25, 

riply Ionized 

J.R. Thornton, W.D. Fountain, G.W. Flint, and T.G. Crow, "Properties of 

Neodymium Laser Materials," Appl. Opt., vol. 8, pp. 1087-1102, June 1969. 

J.I. Pankove, Optical Processes in Semiconductors, Prentice-Hall, Englewood 

Cliffs, NJ, pp. 229-234, 1971. 

7 
8. J. Gowar, Optical Communication Systems, Prentice-Hall, London, pp. 234-236, 

1984. 

I 9. A. Chiari, M. Colocci, F. Fermi, Y. Li, R. Querzoli, A. Vinattieri, and 

I W. Zhuang, "Temperature Dependance of the Photoluminescence in GaAs-GaAlAs 

Multiple Quantum Well Structures," Phys. Stat. Sol. B, vol. 147, pp. 421-429, 

1988. I 

I 

10. R.C. Goodfellow and R. Davis. "Optical Source Devices," in Optical Fibre 

Communications, M.J. Howes and D.V. Morgan, eds., John Wiley a Sons, pp. 

27-106, 1980. 

I 11. G. Adamovsky, "Referencing in Fiber Optic Sensing Systems," in Optical 

Techniques for Sensing and Measurement in Hostile Environments, C.H. Gillespie 

and R.A. Greenwell, eds., Proc. SPIE 787, pp. 17-23, 1987. 

12. B.E. Jones and R.C. Spooncer, "Two-Wavelength Referencing of an Optical Fibre 

I Intensity-Modulated Sensor," J. Phys. E: Sci. Instrum., vol. 16, pp. ~ 1124-1126, NOV. 1983. 

13. B.E. Jones and R.C. Spooncer, "An Optical Fibre Pressure Sensor Using a 

Holographic Shutter Modulator with Two-Wavelength Intensity Referencing," in 

Conference Proceedings OFS '84 - Second International Conference on Optical 

Fiber Sensors, R.T. Kersten and R. Kist, eds., Proc. SPIE 514, pp. 223-226, 

1984. 

i 14. G. Adamovsky, "Time Domain Referencing in Intensity Modulation Fiber Optic 

Sensing Systems," in Optical Testing and Metrology, C.P. Grover, ed., Proc. 

SPIE 661, pp. 145-151, 1986. 

15. W.B. Spillman, Jr. and J.R. Lord, "Self-Referencing Multiplexing Technique for 

Fiber-optic Intensity Sensors," J. Liqhtwave Technol, vol. LT-5. pp. 865-869, 

I 

1 July 1987. 

16. G. Adamovsky and N.D. Piltch, "Fiber-Optic Thermometer Using Temperature 

Dependent Absorption, Broadband Detection, and Time Domain Referencing," Appl. 

Opt., vol. 25, pp. 4439-4443, Dec. 1, 1986. 

I 17. G. Adamovsky, T.N. Sherer, and D.J. Maitland, "Intensity-Based Fibre-Optic 

I 

I 

Sensing System Using Contrast Modulation of Subcarrier Interference Pattern," 

Electron. Letters, vol. 25, pp. 325-326, Mar. 2, 1989. 

18. W. Nakwaski and A.M. Kontkiewicz, "Temperature Distribution in a Light-Emitting 

Diode During a Pulse Operation," Electron. Letters, vol. 20, pp. 984-986, 

Nov. 8, 1984. 

19. W.H. Glenn, W.W. Morey, and R.M. Elkow, "Design, Fabrication, and Testing of an 

Optical Temperature Sensor for High Temperature Control Applications," NASA I CR-174698, 1984. 

20. R.A. Fields, M. Birnbaum, and C.L. Fincher, "Highly Efficient Nd:YVO4 DiodeLaser End-Pumped Laser," Appl. Phys. Lett., vol. 51, pp. 1885-1886, dec. 7, 

1987. 

I 8 
21. H.P. Jenssen, R.F. Begley, R. Webb, and R.C. Morris, "Spectroscoplc Properties 

and Laser Performance of Nd3+ in Lanthanum Beryl late," J. Appl . Phys., 

vol. 47, pp. 1496-1500, Apr. 1976. 

xC = 820 nrn, AX = 30 nm 

Ta = -40 OC Ta = 25 OC Ta = 75 OC 

22. J.R. O'Connor, "Unusual Crystal-Field Energy Levels and Efficient Laser 

Properties of YV04:Nd," Appl. Phys. Lett. vol. 9, pp. 407-409, Dec. 1, 1966. 

X, = 820 nm, AX = 100 nm 

Ta = -40 OC Ta = 25 OC Ta = 75 OC 

TABLE I 

0 

100 

200 

300 

0.222 0.369 0.543 0.501 0.506 0.501 

.184 .306 .481 .453 .457 .454 

.160 .267 .438 .419 .422 .419 

.144 .241 .407 .392 ' .394 .391 

AFTER THE MBIENT TEMPERATURE REACHES 90°C 

_-__ AFTER 72 HR Of OPERATION I 

I AFTER 168 HR OF OPERATION 

WAVELENGTH. NM 

I 

la 

OC I 

! 

1 

~ 

E' 

r"l 

PI fl 

9' :i 

:I 

r 

W PI 

, 

I 

660 720 890 900 960 1020 

WAVELENGTH. NM 

FIGURE 1. - RELATIVE SPECTRAL INTENSITY DISTRIBUTIONS OF LED FIGURE 2. - RELATIVE SPECTRAL INTENSITY DISTRIBUTIONS OF LED AT 

OPERATED AT 100 MA CONTINOUS CURRENT AND APIHIENT TEPIPERATURE 1, = 90°C. 

DIFFERENT APIBIENT TERERATURE la. 

9 
n 

JL 

M 

A1 A2 

A1 A2 TO THE SENSOR 

COUPLER 

V M 

4 A2 

Al A2 

PDS 4 

Vr, vr2 

nn. 

PDr 

FIGURE 3. SCHEMATIC OF SENSING SYSTEM WITH DUAL WAVELENGTH REFERENCING AND 

POWER LEVEL COMPENSATION. 

I I I 1 1 I 

7AO 800 820 840 860 880 900 920 1 

WAVELENGTH, NM 

FIGURE 4. - RELATIVE SPECTRAL INTENSITY DISTRIBUTIONS OF LED 

OPERATED AT ROOn TEMPERATURE AND DIFFERENT CONTINOUS 

CURRENTS i. 

INITIAL - SIGNAL 

RESPONSE FROM THE SENSOR 

- 

TS 

w 

FIGURE 5. - SCHEMATIC OF INDIRECT PASSIVE COMPENSATION 

TECHNIQUE. 

10 
INITIAL - SIGNAL 

+ 

RESPONSE FROM THE SENSOR 

FIGURE 6. - SCHEMATIC OF DIRECT PASSIVE COMPENSATION 

TECHNIQUE. 

Nd203 

PERCENT 

3.5 

6.0 ____ 

. _. 

WAVELENGTH. NH 

FIGURE 7. - RELATIVE TRANSMISSION SPECTRA OF GLASS SAMPLES 

WITH DIFFERENT CONCENTRATIONS OF MgO3 AT ROOM TEMPERATURE. 

11 
I Fi~i nna Aeronautics and 

I 2. Government Accession No. 

Report Documentation Page 

3. Recipient's Catalog No. 

.- __I7 Author@) 

G. Adamovsky, J.L. Sotomayor, M.J. Krasowski, and J.G. Eustace 

I I 1 NASA TM-102282 

8. Performing Organization Report No. 

E-4946 ._____ 

10. Work Unit No. 

5. Report Date 

Compensation tijr Effects of Ambient Temperature on Rare-Earth 

17. Key Words (Suggested by Author@)) 

Fiber optics; Sensors; Temperature sensors; 

'Thermal effect; LED 

1 Doped Fiber Optic Thermometer 

18. Distribution Statement 

Unclassified - Unlimited 

Subject Category 35 

6. Performing Organization Code t------ 

19. Security Classif. (of this report) 20. Security Classif. (of this page) 

Unclassified Unclassi tied 

21. No of pages 22. Price' 

12 A03 

9 Performing Organization Name and Address 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135-3191 

505-62-01 

Technical Memorandum ___. 

I 2. Sponsoring Agency Name and Address 

National Aeronautics and Space Administration 

Washington, D.C. 20546-0001 1 14. Sponsoring Agency Code 

I 

15 Supplementary Notes 

Prepared for the Fiber Optic and Laser Sensors VI1 sponsored by the Society of Photo-Optical instrumentation 

Engineers, Boston, Massachusetts, September 5-8, 1989. G. Adamovsky, J.L. Sotomayor, and M.J. Krasowski, 

NASA Lewis Research Center; J.G. Eustace, John Carroll University, Physics Department, Cleveland, Ohio 441 18 

(work funded by NASA Grant NAG3-984). __ 16 Abstract 

Variations in ambient temperature have a negative effect on the performance of any fiber optic sensing system. A 

change in ambient temperature may alter the design parameters of fiber optic cables, connectors, sources, detectors, 

and other fiber optic components and eventually the performance of the entire system. The thermal stability of 

components is especially important in a system which employs intensity modulated sensors. Several referencing 

schemes have been developed to account for the variable losses that occur within the system. However, none of 

these conventional compensating techniques can be used to stabilize the thermal drift of the light source in a 

system based on the spectral properties of the sensor material. The compensation for changes in ambient temperature becomes especially important in fiber optic thermometers doped with rare earths. Thus, the purpose of this 

work is to search and analyze different approaches to solving this problem. 

'For sale by the National Technical Information Service, Springfield, Virginia 22161 NASA FORM 1626 OCT 86