The variation in the green intensity ratio 12H11@2 and 4S3@2 energy levels to the ground state2 of Er ions in silica fibers has been studied as a function of ...
Erbium-doped silica fibers for intrinsic fiber-optic temperature sensors E. Maurice, G. Monnom, B. Dussardier, A. Saı¨ssy, D. B. Ostrowsky, and G. W. Baxter
The variation in the green intensity ratio 12H11@2 and 4S3@2 energy levels to the ground state2 of Er ions in silica fibers has been studied as a function of temperature. The different processes that are used to determine the population of these levels are investigated, in particular 800-nm excited-state absorption in Er-doped fibers and 980-nm energy transfer, in Yb–Er-codoped fibers. The invariance of the intensity ratio at a fixed temperature with respect to power, wavelength, and doped fiber length has been investigated and shown to permit the realization of a high-dynamic-range 1greater than 600 °C2, autocalibrated fiber-optic temperature sensor. Key words: Fiber-optic sensors, rare-earth-doped fiber, fluorescence. r 1995 Optical Society of America
1.
Introduction
The development of fiber-optic point temperature sensors has attracted considerable interest over the past 15 years with fully developed products now commercially available.1 Such interest is motivated by the relatively high immunity of optical-fiber technology to electromagnetic interference and certain forms of chemical attack when compared with conventional temperature-measurement techniques. Currently available fiber-optic point-temperature-sensing systems have been based on a variety of optical-sensing techniques that include pyrometric intensity ratio, fluorescence decay time, and luminescence intensity. We present here a detailed investigation of the possibility of realizing point temperature sensing by using the green fluorescence intensity ratio of Erdoped silica fiber. We previously demonstrated the physical principle of the thermalization of the two emitting levels involved in the green emission in Er-doped silica fibers,2 and we have proposed a temperature sensor based on this effect in Yb–Er-codoped
E. Maurice, G. Monnom, B. Dussardier, A. Saı¨ssy, and D. B. Ostrowsky are with Laboratoire de Physique de la Matie`re Condense´e, Centre National de la Recherche Scientifique 190, Universite´ de Nice, Sophia Antipolis, Parc Valrose, 06108 Nice Cedex 2, France. G. W. Baxter is with Optical Technology Research Laboratory, Victoria University of Technology, P.O. Box 14428, Melbourne 3000, Australia. Received 17 March 1995; revised manuscript received 18 July 1995. 0003-6935@95@348019-07$06.00@0. r 1995 Optical Society of America.
fibers pumped at 980 nm 1Ref. 32 by an energytransfer process. In this paper, we study essentially a prototype sensor, using Er-doped fibers pumped at 800 nm by excited-state absorption 1ESA2, which could be obtained with semiconductor lasers. We focus mainly on the sensitivity of the proposed sensor to pump-parameter variations and show that such a sensor is feasible and efficient. We also consider the possibility of extending these results to the 980-nmpumped, Yb–Er-codoped fiber sensor. In Section 2 we recall the physical principle of the sensor, the thermalization between the two levels involved in the green emission. Section 3 is devoted to the self-absorption process, which is not necessary for those interested only in the performance of the sensor that we propose, but some observed dependencies, described below, are explained, such as the fluorescence-intensity ratio with fiber length and the fluorescence intensity with the pumping process. The effects of parameters such as pump power, pump wavelength, fiber length, and pumping scheme on the green intensity ratio are investigated in Section 4, showing that there is a quasi-independence of the fluorescence-intensity ratio with a variation in these parameters, as desired for the sensor. In Section 5, we present the performance of a laboratory-based temperature sensor, using the proposed technique, and discuss the development of a practical device. 2.
Sensor Principle
Thermalization, which is not a priori evident for the isolated 2H11@2 and 4S3@2 energy levels of the Er ion, has been observed in a variety of hosts. However, in 1 December 1995 @ Vol. 34, No. 34 @ APPLIED OPTICS
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a silica host the levels have short lifetimes 1,1 µs2, and as a result only recently has the thermalization of populations of the 2H11@2 and 4S3@2 energy levels been proposed4 and later confirmed.2 The application of the observed thermalization to temperature sensing has been investigated in heavy metal fluoride optical fibers,5 but this host material has disadvantages, such as a reduced temperature-sensing range and increased fragility compared with silica fibers. With the thermalization of the population of the 2H 4 11@2 and S3@2 energy levels and ignoring the effects of self-absorption of the fluorescent light, we may represent the ratio of the integrated fluorescence intensities, 2H11@2 to 4I15@2 and 4S3@2 to 4I15@2, by6
R5
IH IS
5
cH1n2 pHr gH hnH cS1n2 pSr gS hnS
1
exp 2
2,
DE kT
112
where IH and IS are the measured intensities, gH and gR are the degeneracies 12 J 1 12 for a given 2S11LJ level, and pHr and pSr are the total spontaneous emission rates of the 2H11@2 and 4S3@2 levels, respectively. The responses of the detection system in the frequency range of the 2H11@2 and 4S3@2 levels are given by cH1n2 and cS1n2, respectively. The photon energy is hn, DE is the energy difference between the levels, with k the Boltzmann constant and T the temperature in degrees kelvin. In the current investigation, which is only a temperature study, and with the hypothesis that the spontaneous emission rates are temperature independent, it is sufficient to rewrite this equation as
1
R 5 a exp 2
2
DE kT
122
or
ln1R2 5 c 2
b
,
T
132
where a, b, and c are constants for a particular fiber. Consequently, we may determine the temperature from a measurement of the green intensity ratio based on two calibration points, using
T5
3.
b c 2 ln1R2
.
142
Self-absorption
This section is devoted to the self-absorption mechanism. This is the physical phenomenon that limits sensor length, forces the use of relatively high pump power, and imposes the pump wavelength. Figure 1 shows the simple process of self-absorption resulting from two absorptions and two emissions 1not necessarily radiatives2. First, a pump photon of frequency np is absorbed by the atom, leading to an excited-state electron 1the first stage2. A photon of frequency ne, which is produced by deexcitation of this excited-state elec8020
Fig. 1. Self-absorption mechanism: dashed arrows, nonradiative transition; continuous arrows, radiative transition. Prad, Pnonrad, np, and ne designate the radiative and nonradiative emission probabilities and the frequency of the pump and emitted photons, respectively.
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tron, is then absorbed by another atom 1the second stage2. The electron in this atom then deexcites 1the third stage2 through one of many possible mechanisms with the reproduction of a photon of frequency ne having a small probability. The value of the ratio Prad@Pnonrad is approximately 1@1000 for transitions starting from the 2H11@2 and 4S3@2 levels in Er-doped silica fibers in which nonradiative transitions dominate 1because of the high phonon energy in this host2. Prad and Pnonrad designate the radiative and nonradiative emission probabilities, respectively. This process, known as self-absorption, occurs along the length of the doped section of fiber reducing the intensity of the green signal 1in the case of Er-doped fiber2 at the end of the fiber. Equation 112 has been established without self-absorption, but if this phenomenon is important, the equation must be corrected by a term including the different absorption coefficients for the two signal frequencies, nH and nS. This term can be calculated only numerically, because it includes a variation of the two absorption coefficients 1a2 with pump power all along the fiber. This correction factor leads to lowering R with increasing fiber length 3a12H11@22 . a14S3@224, and the fact that we neglect it induces a change in the c coefficient 3Eq. 1324 without slope coefficient b being modified. At low pump powers, one may predict absorption through the fiber for a particular wavelength, using a previously measured absorption coefficient 1the cutback technique2. However, at higher pump powers the number of ground-state electrons able to absorb an incident photon decreases, reducing the magnitude of the absorption coefficient. This effect, known as saturated absorption, is due to the increasing population inversion of Er31 ions 1the 4I13@2–4I15@2 gain transition2 because higher pump powers sustain more electrons in the 4I13@2 excited state of the atom. Such changes in absorption are not equivalent at all wavelengths because of the different absorption cross sections and therefore affect the 2H11@2–4I15@2 and 4S 4 3@2– I15@2 fluorescence intensities nonuniformly: 2 The H11@2 level saturates at a pump power that is lower than the 4S3@2 level. This explains why R is more sensitive to pump power at low launched power
because the absorption coefficient ratio is higher. This effect also explains the drastic variation of R with pump wavelength on the wings of the 4I15@2–4I9@2 pump transition. In this wavelength range, the absorption coefficient being lower than that at the top of the absorption curve, the population inversion is obtained for higher values of pump power: The transition can be saturated at the top of the curve, whereas it continues to absorb on the wings. From the discussion above we see that selfabsorption leads to dependencies of the green intensity ratio on such parameters as pump power and wavelength or fiber length, which are undesirable for sensing applications. Consequently these dependencies are investigated experimentally in detail in the sections below. 4.
Green-Intensity-Ratio Dependencies
The doped fibers used for this research were fabricated with a modified-chemical-vapor-deposition process together with the solution-doping technique. Relevant data concerning each fiber, including the doping and codoping concentrations, are summarized in Table 1. The experimental arrangement used to test the dependency of the green intensity ratio on the pump scheme, power, and wavelength is shown in Fig. 2. Generally a cw Ti:sapphire laser tuned to the required wavelengths was used to excite particular levels of the Er31 ion. The mirror shown in the diagram had a significant transmission for wavelengths between 750 and 1000 nm and a significant reflection for wavelengths between 500 and 600 nm. We detected the counterpropagating fluorescence by collecting the returned light in a multimode fiber connected to an optical spectrum analyzer. A.
Fig. 2. Experimental setup used to measure the dependency of the green intensity ratio with pump-parameter variations.
lates the 4I13@2 metasable level through nonradiative transitions. Other pump photons are then absorbed by electrons in this level, directly populating the 2H 4 2 Figure 11@2 and S3@2 thermalized energy levels. 31c2 illustrates another ESA process whereby strong pumping at 980 nm populates the short-lived intermediate 4I11@2 energy level. Electrons in this level then absorb another pump photon, thereby populating the 4F 4 Figure 31d2 depicts an energy-transfer 7@2 level. process in which the host glass has been heavily doped with Er ions together with Yb ions. In this case the short interionic distances permit transferring the energy stored in the 4F5@2 metastable pumped level of Yb first to the 4I11@2 and then to the 4F7@2 energy levels of the Er ions. These two energy transfers are very efficient because of the resonances of the transitions:
DE12F7@2–2F5@22 5 DE14I15@2–4I11@22 5 DE14I11@2–4F7@22.
Pump Schemes
Excitation of the 2H11@2 and 4S3@2 energy levels of the Er31 ions may be achieved in a number of ways. Presented in Fig. 3 are four pumping schemes: some of them have been reported previously. Figure 31a2 shows direct pumping 1at 488 nm with an Ar laser2 to the 4F7@2 energy level. Electrons in this level then decay nonradiatively to the 2H11@2 and 4S Figure 31b2 depicts an ESA process in 3@2 levels. which pumping in the 800-nm region leads to excitation of the 4I9@2 level and quasi-instantaneously popu-
Table 1.
Optogeometrical Characteristics and Doping Levels of the Studied Fibers
Sample Name Parameter
Er1
Er2
YbEr
Core radius 1µm2 Numerical aperture Rare-earth-doping level 1ppm2 Al2O3-doping level 1wt. %2 P2O5-doping level 1wt. %2 GeO2-doping level 1wt. %2
4.6 0.16 100
3.0 0.2 2500
3.1 0.16 2500@2500
0 2 10
0.8 2 17
1.5 2 10
Fig. 3. Different schemes permitting the population of the 2H11@2 and 4S3@2 energy levels of Er31 ions in silica fibers and the observation of the green fluorescence. Only levels of Er and Yb ions taking a part into the green emission are shown: continuous arrows, radiative transitions; dashed arrows, nonradiative transitions. 1 December 1995 @ Vol. 34, No. 34 @ APPLIED OPTICS
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The strong absorption, resulting from the high doping level and the high-absorption cross section of the 2F5@2 level of the Yb ions, permits use of much shorter lengths of fiber than the above-mentioned processes with the same upconversion efficiency. For practical sensor development, two of the pumping schemes above may be eliminated 3Figs. 31a2 and 31c24. Pumping at 488 nm 3Fig. 31a24, to populate directly the 4F7@2 energy level, is achieved with an Ar-ion laser source that is not practical for sensors. Furthermore the 488-nm pump wavelength is spectrally close to the wavelengths of the fluorescence lines resulting from the 2H11@2 and 4S3@2 to groundstate transitions, which increases the difficulty of detecting such signals because they, even for the counterpropagating signal, are several orders of magnitude less intense than the pump. Pumping an Er-doped fiber at 980 nm 3Fig. 31c24 relies on the ESA process from a relatively short-lived intermediate 4I 11@2 level 1of some microseconds2; consequently such a process is significantly less efficient than one relying on an ESA process from a long-lived metastable level, such as the 4I13@2 energy level 110 ms2. However, the population of the 4I11@2 level, and the energy available to permit an ESA process to occur, can be increased 1for a given 980-nm pump intensity2 by codoping the fiber core with Yb. Hence, for Yb–Er-codoped fibers, the population of the 2H11@2 and 4S3@2 energy levels is enhanced, as is the total green fluorescence intensity. But, even in this case, the total green intensity coming from the 980-nm ESA process is ,1 order of magnitude less than that obtained with the 800-nm ESA process. Figure 4 shows the excitation spectrum 1total green intensity versus pump wavelength2 at a constant absorbed pump power for pump wavelengths ranging from 770 to 1000 nm in a Yb–Er-codoped fiber 1Yb– Er2. The peak in the green intensity level occurring near 800 nm corresponds to the 800-nm ESA 3Fig. 31b24 4I 2 The minor peak visible in 13@2– H11@2 transition. the spectrum near 850 nm corresponds to the photon energy at which an ESA event occurs between the 4I 4 This is, however, significantly 13@2 and S3@2 levels. less than the corresponding 4I13@2–2H11@2 excitation because absorption from the ground state to the transient 4I9@2 level is relatively small near 850 nm because this wavelength is on the wing of the 4I9@2
Whatever the pump process is, when the fiber is cooled to liquid N2 temperature, only the low-energy radiation 14S3@22 is seen, according to Boltzmann’s law. When the fiber is heated the high-energy-level emission 12H11@22 appears, and its intensity increases with the temperature as that of the lower energy level decreases.2 The natural logarithm of the measured intensity ratio 1 R2, determined from the ratio of intensities for 530 and 555 nm, versus pump power for the three pumping schemes 1488-nm-direct, 800-nm-ESA, and 980-nm energy-transfer pumpings2 plotted against the inverse of temperature 1in K212 is shown in Fig. 5. This graph clearly demonstrates a thermalization process between the two levels; a straight-line fit to these data yields an energy difference between the two competing levels of 780 cm21 for the Er fiber, when pumped at 488 or 800 nm. This value is the same as that obtained from the absorption spectra of the fiber, calculated as the difference between the maximum of the two absorption peaks. The data for the Er–Yb fiber pumped at 980 nm yield an apparent energy separation of 800 cm21, greater than expected but in the uncertainty range of our measurements. The difference between the Er1 and Yb–Er curves comes from the magnitude of R, that of Er-doped fiber being higher than that of Yb–Er-codoped fiber. This effect is due to selfabsorption of the counterpropagating green fluores-
Fig. 4. Green fluorescence excitation spectrum of the Yb–Ercodoped fiber.
Fig. 5. Natural logarithm of the measured intensity ratio plotted against the inverse of temperature. Continuous lines are a linear fit to the data.
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absorption band. The behavior of the ESA process across the 4I9@2 absorption band was described previously.2 The peak near 920 nm corresponds to the double-energy-transfer process between Er and Yb ions 3Fig. 31d24, whereas the 980-nm peak is due to both energy transfer and 980-nm ESA phenomena 3Figs. 31c2 and 31d24. The total maximum measured green intensity when pumping is done at 980 nm is approximately half of that near 800 nm for the fiber measured. However, an improved energy transfer has been observed in fibers with a different ratio of Er to Yb ions so that some enhancement of the total green intensity may be expected. Laser-diode sources are available at wavelengths near 800 and 980 nm, so that either 800-nmpumped Er-doped or 980-nm-pumped Yb–Er-codoped fibers may be suitable for further development. B.
Temperature Behavior with the Pump Process
cence along the fiber; the 980-nm-pumped fiber had a shorter optimal length caused by the higher-absorption cross section of Yb ions in the double-energy transfer pump scheme. In nonsaturated absorption conditions, the strong self-absorption of Er for the 2H 4 11@2 = I15@2 transition reduces the fluorescence intensity of this band, lowering R. C.
Pump Power and Wavelength Dependence
To determine the temperature uncertainty induced by pump-parameter variations, we first calculate the sensitivity S of the doped fiber from Eq. 122:
S 5
1 dR R dT
5
DE kT 2
.
142
This results in an absolute temperature uncertainty of
DT 5
D R@ R S
.
152
At room temperature and for a 780 6 70-cm21 energy gap between the 2H11@2 and 4S3@2 energy levels, the sensitivity is 1.3 6 0.1%@°C. Ideally a fiber-optic sensing device should be independent of fluctuations commonly experienced in its optical power source. However, as stated above, some dependency on both the pump power and wavelength might be expected. Such a situation might lead to dependencies in the measured ratio with pump power for unsaturated pump powers where the exponential variation of the pump intensity along a long length of fiber results in significant changes in the self-absorption of the fluorescent light. However, for most temperature-sensor applications, the length of the doped fiber is relatively short 1hence self-absorption is small2, and by pumping with moderate-power diode-laser sources, quasi-total depletion of the fundamental level can be reached, leading to the transparency of the fiber at all wavelengths. This results in a reduction in the level of self-absorption of the green fluorescence along the doped fiber. Shown in Fig. 6 is R versus launched powers 1Pp2 between 15 and 250 mW as measured from the green intensity spectra acquired by the optical spectrum analyzer with 10-nm resolution for cw pumping near 800 nm and for a 30-cm length of fiber Er1. At low launched power R is more sensitive to pump power. The absorption coefficients being higher, the effect of fiber length is more important than that at high launched power as the saturated absorption takes place. From the slope of this curve, we obtain a 1 3
Fig. 6. Intensity ratio as a function of launched pump power at 800 nm.
1023 value for D R@ R for a 1% variation in pump power in the 100–240-mW range, yielding a 0.07 °C@% error in temperature. The ratio of the emission of light from the 2H11@2 and 4S 3@2 to ground-state transitions should not depend on the mechanism by which the atoms involved were excited and, as such, should be independent of the pump wavelength. However, because the magnitude of the self-absorption of the two emissions of interest varies greatly with this parameter, any change in the absorption of the pump light affects disproportionately the self-absorption of 2H11@2 to ground-state emission and therefore the green intensity ratio. Shown in Fig. 7 is R as a function of pump wavelengths 1lp2 near 800 nm for a 30-cm length of fiber Er1 and for 200-mW launched pump power. A quite dramatic variation in R with pump wavelength—D R@ R 5 1.7 3 1022@nm fluctuation, leading to a 1.3 °C@nm error—is observed on the wings of the curve. The behavior of R with pump wavelength is the same as that of the absorption curve in the 800-nm range. This is explained by the fact that at the top of the absorption curve the saturation power is less than that on the wings of the curve, and as a result the absorption at the top can be lower at high pump power than that on the wings. However, the magnitude of these changes should not be significant for temperature-stabilized laser-diode sources operating near 805 nm at the top of the curve. D.
Fiber-Length Dependence
The application of short lengths of Er-doped fiber to point temperature sensing may be achieved with high doping concentration. However, the application of this technology to average temperature sensing by the use of much greater lengths of doped fiber should not be overlooked. Figure 8 shows R plotted against length for fiber Er1. The relative decrease in the intensity of light at 525 nm over that at 555 nm, when the length of the fiber is increased, is another example of the influence of self-absorption on the measured green intensity ratio. Figure 9 shows the total green intensity plotted against the fiber length. It is evident that increasing the length of the fiber results in a complementary increase in the total fluorescence intensity. However, the relationship is nonlinear because of the nonuniform self-absorption
Fig. 7. Intensity ratio as a function of pump wavelength for 200 mW of absorbed power. 1 December 1995 @ Vol. 34, No. 34 @ APPLIED OPTICS
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Fig. 8. Intensity ratio as a function of fiber length when the doped fiber is pumped at 800 nm by the ESA process.
characteristics of the doped fiber at green wavelengths. Because of these effects a longer fiber is more sensitive to power and wavelength fluctuations of the pump, whereas its fluorescence level is higher. So a trade-off between higher fluorescence intensity and the self-absorption of the doped fiber section is necessary in the design of a practical sensor. 5.
Fig. 10.
Schematic of the experimental sensor arrangement.
Laboratory Prototype Sensor Performance
Here we report preliminary data concerning the sensitivity of a laboratory prototype and discuss probable further developments. A schematic of the experimental arrangement is shown in Fig. 10. The beam coming from the Ti:Al2O3 laser, tuned at 800 nm 11.2-W maximum output power2, was chopped at a frequency of 80 Hz. After passing through dichroic mirror 1, the pumping beam was launched by a microscope objective into a 20-m-long telecommunication fiber with a coupling efficiency of ,30%. Note that one could obtain 250 mW of launched power by using available pigtailed semiconductor lasers. The doped and undoped sections of fiber were fusion spliced with the full 2.0-cm doped section 1Er22 and ,3 cm of the undoped fiber housed together with the thermocouple tip in a 3.0-cm-diameter and 8.5-cm-long metal cylinder. The cylinder helped to stabilize the thermal environment near the temperature sensors and was located in the oven center. The total green intensity loss resulting from absorption of the pump and fluorescence counterpropagative signal through the telecommunication fiber and the fiber-to-fiber splice was estimated to be 10 dB. The counterpropagating green light after reflection on dichroic mirror 1 was divided into two beams by dichroic mirror 2. The dichroic properties of this second mirror, along with filters 1 and 2 placed before the detectors, isolated the contributions from either of the 4S3@2 and
Fig. 9. Total green intensity as a function of fiber length when the doped fiber is pumped at 800 nm by the ESA process. 8024
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2H
The 11@2 to ground-state transitions to a detector. electrical signals produced by the detectors were monitored by two lockin amplifiers at a frequency of 80 Hz. The potential produced at the thermocouple was initially amplified 131002 by a dc amplifier close to the oven. All three signals were monitored simultaneously 1within 32 µs2 and stored with IEEE standard interfacing with a PC. R is plotted against temperature as measured by the thermocouple in Fig. 111a2 at temperatures between ambient and 640 °C. The logarithm of R is plotted against the inverse of the absolute temperature in Fig. 111b2 and has been fitted by a straight line. The temperature difference between the measured ratio and the fit to the data has been plotted against temperature in the inset.
Fig. 11. Behavior of 1a2 R with temperature; 1b2 the natural logarithm of R with the inverse of the temperature. The inset shows the difference between data and the fitted values.
The standard deviation between the data and the fit was determined to be 1.1 °C; scatter in the data caused by noise in the measured thermocouple voltage has an effect on the magnitude of this value, so the true uncertainty caused by the fiber sensor is much smaller than that quoted above. The increased error at high temperature is due to homogeneous line broadening of the two bands; in this case the detectors do not measure a single emission line. The dynamic obtained was 11 dB over the temperature range shown, leading to a mean rate of change of the green intensity ratio of ,0.016 dB@°C. According to Boltzmann’s law the slope of the line fitted to the data in Fig. 111b2 may be related to the energy difference between the two thermalizing levels. The current data yield an energy gap of 762 cm21, which is in good agreement with the 780-cm21 value obtained in absorption measurements. Several temperature cycles have been carried out, and we have observed good repeatability. As for the stability, no modifications have been observed on the two intensities when the fiber was heated for several hours at temperatures to as high as 600 °C. Nevertheless a systematic lowering of the green amplitudes has been measured when the higher value of the cycle exceeded 650 °C. This behavior can be explained by some permanent structural modifications of the fiber, such as the diffusion of the dopants from the core to the cladding, leading to a lowering of the numerical aperture and an increase in the core radius. However, because the observed decrease in the two intensities is the same, the ratio between them is constant, even in this case.
6.
Conclusion
We have experimentally demonstrated the possibility of realizing a novel fiber-optic temperature sensor. The data show promise for the future development of such a sensor with competitive performance both in mean sensitivity 10.016 dB@°C2 and in the measurement range 1from room temperature to 600 °C2. If
the mean sensitivity is the same as in fluoride fibers,5 the maximum measurable temperature is increased from 160 to 600 °C because of the silica host. The most unfavorable variation observed was 1.3 °C@nm of pump wavelength fluctuation, but this value would be decreased by at least 1 order of magnitude by pumping the fiber at 805 nm. We have demonstrated that the pumping schemes of this device are compatible with emission wavelengths and the power of standard IR laser diodes, with both Er-doped and Yb–Er-codoped fibers, and that there is little dependence on parameter variations commonly experienced in such diodes. Further investigation will be directed toward the realization of a laser-diode pumped device, where optimal reliability and compactness are obtained by replacing the dichroic mirrors with wavelength-selective couplers and by choosing more selective filters. References 1. K. A. Wickersheim and W. D. Hyatt, ‘‘Commercial applications of fiberoptic temperature measurement,’’ in Fiber Optic Sensors IV, R. T. Kersten, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 1267, 84–96 119902. 2. E. Maurice, G. Monnom, A. Saı¨ssy, D. B. Ostrowsky, and G. W. Baxter, ‘‘Thermalization effects between upper levels of green fluorescence in Er-doped silica fibers,’’ Opt. Lett. 19, 990–992 119942. 3. E. Maurice, G. Monnom, D. B. Ostrowsky, and G. W. Baxter, ‘‘High-temperature point sensor using green fluorescence intensity ratio in Er-doped silica fibres,’’ in Tenth International Conference on Optical Fiber Sensors, B. Culshaw and J. D. Jones, eds., Proc. Soc. Photo-Opt. Instrum. Eng. 2360, 219–222 119942. 4. P. A. Krug, M. G. Sceats, G. R. Atkins, S. C. Guy, and S. B. Poole, ‘‘Intermediate excited-state absorption in erbium-doped fiber strongly pumped at 980 nm,’’ Opt. Lett. 16, 1976–1978 119912. 5. H. Berthou and C. K. Jo¨rgensen, ‘‘Optical-fiber temperature sensor based on upconversion-excited state fluorescence,’’ Opt. Lett. 15, 1100–1102 119902. 6. M. D. Shinn, W. A. Sibley, M. G. Drexhage, and R. N. Brown, ‘‘Optical transitions of Er31 ions in fluorozirconate glass,’’ Phys. Rev. B 27, 6635–6648 119832.
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