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| EXPLORING THE INTERFACE OF LIGHT AND BIOMEDICINE

  • Editors: Andrew Dunn and Anthony Durkin
  • Vol. 7, Iss. 2 — Feb. 1, 2012
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Whispering gallery modes in a glass microsphere as a function of temperature

L. L. Martín, C. Pérez-Rodríguez, P. Haro-González, and I. R. Martín  »View Author Affiliations


Optics Express, Vol. 19, Issue 25, pp. 25792-25798 (2011)
http://dx.doi.org/10.1364/OE.19.025792


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Abstract

Microspheres of Nd3+ doped barium titano silicate glass were prepared and the whispering gallery mode resonances were observed in a modified confocal microscope. A bulk sample of the same glass was calibrated as temperature sensor by the fluorescence intensity ratio technique. After that, the microsphere was heated by laser irradiation process technique in the microscope and the surface temperature was estimated using the fluorescence intensity ratio. This temperature is correlated with the displacement of the whispering gallery mode peaks, showing an average red-shift of 10 pm/K in a wide range of surface temperatures varying from 300 K to 950K. The limit of resolution in temperature was estimated for the fluorescence intensity ratio and the whispering gallery mode displacement, showing an improvement of an order of magnitude for the second method.

© 2011 OSA

1. Introduction

Optical sensors are an area of strong interest for applications. Conventional solid state temperature sensors are mainly based in thermoelectric materials like thermistors and thermocouples, the goal of optical sensors has significant advantages compared to them in terms of their properties like electrical passiveness, greater sensitivity, freedom from electromagnetic interference, wide dynamic range, point and distributed configurations and multiplexing capabilities [1

1. V. K. Rai, “Temperature sensors and optical sensors,” Appl. Phys. B 88(2), 297–303 (2007). [CrossRef]

]. Generally, accordingly to which property of the light beam changes when interacts with the optical sensors, they can be classified in two groups: interferometers in which optical phase is affected and intensity based devices in which optical intensity is modulated.

In the past decades, a number of optical temperature sensors have been presented and are principally based on the fluorescence intensity ratio (FIR) technique [1

1. V. K. Rai, “Temperature sensors and optical sensors,” Appl. Phys. B 88(2), 297–303 (2007). [CrossRef]

11

11. M. A. R. C. Alencar, G. S. Maciel, C. B. de Araujo, and A. Patra, “Er3+-doped BaTiO3 nanocrystals for thermometry: Influence of nanoenvironment on the sensitivity of a fluorescence based temperature sensor,” Appl. Phys. Lett. 84(23), 4753–4756 (2004). [CrossRef]

]. In this method, the fluorescence intensities of two closely spaced energy levels are recorded as a function of the temperature in order to be analyzed in a simple three-level system.

Therefore, in the present study both techniques are combined to study the viability as temperature sensors of micro-sphere type resonators made of Nd3+ doped barium titano-silicate (BTS) glass. This is a glass with high refractive index and high melting point. Moreover, the BTS glass produces transparent glass-ceramic by thermal treatment [18

18. N. Maruyama, T. Honma, and T. Komatsu, “Enhanced quantum yield of yellow photoluminescence of Dy3+ ions in nonlinear optical Ba2TiSi2O8 nanocrystals formed in glass,” J. Solid State Chem. 182(2), 246–252 (2009). [CrossRef]

,19

19. L. L. Martin, P. Haro-González, and I. R. Martín, “Optical properties of transparent Dy3+ doped Ba2TiSi2O8 glass ceramic,” Opt. Mater. 33(5), 738–741 (2011). [CrossRef]

] where the glassy and nanocrystalline phases are present; this property can be useful in future developments. One remarkable advantage of the microspheres reported is that due to their fabrication method they are not coupled to fibers or substrates, a fact that avoids geometrical irregularities that can affect to the WGM. Moreover the detection can take place remotely without the need of coupling the modes to a waveguide. As a result, the proposed technique allows to measure inside physical systems with a small perturbation by just introducing the microsphere and detecting a few centimeters away.

FIR measurements depend only of the temperature and the host matrix of the ions and so can be correlated very easily with the temperature of the glass. In the paper by Ma et al [14

14. Q. Ma, T. Rossmann, and Z. Guo, “Temperature sensitivity of silica micro-resonators,” J. Phys. D Appl. Phys. 41(24), 245111 (2008). [CrossRef]

], a thermal insulated cell and a thermocouple near the microsphere was used is a small temperature range (297-310 K). However in the range of temperatures studied in this work (300-950 K), the FIR (although it is not a direct method as used by Ma et al [14

14. Q. Ma, T. Rossmann, and Z. Guo, “Temperature sensitivity of silica micro-resonators,” J. Phys. D Appl. Phys. 41(24), 245111 (2008). [CrossRef]

]) is the only method that can ensure that the temperature is correctly estimated without air convection or other problems associated with our non coupled set-up. Thereafter it is possible to calibrate the WGM as function of the FIR and thus as function of temperature of the air-microsphere interface even when the variations of refractive index and radius of the glass with temperature are unknown.

2. Theoretical models

The FIR technique is a widely studied technique [1

1. V. K. Rai, “Temperature sensors and optical sensors,” Appl. Phys. B 88(2), 297–303 (2007). [CrossRef]

11

11. M. A. R. C. Alencar, G. S. Maciel, C. B. de Araujo, and A. Patra, “Er3+-doped BaTiO3 nanocrystals for thermometry: Influence of nanoenvironment on the sensitivity of a fluorescence based temperature sensor,” Appl. Phys. Lett. 84(23), 4753–4756 (2004). [CrossRef]

] where the relative luminescence of two radiative transitions is studied. One transition is between the electronic energy levels E2 to E1 and the other transition is from E3 to E1 (See Fig. 1a
Fig. 1 a) Simplified scheme of Nd3+ energy levels involved in the experiment. b) Experimental values for the ratio of the intensities obtained with the emission spectra of the Nd3+:BTS doped sample inside an electrical furnace (squares) and fit curve to Eq. (1) as described in text (red line). The inset in Fig. 1b shows the spectra obtained at RT and 950 K.
).

The small energy gap between the two close electronic levels E2 and E3 allows populating the upper level from the lower level by thermal excitation. The ratio of these intensities is independent of the source power intensity, since it is proportional to the population of each level involved. The relative population between the two levels, R, follows a Boltzmann-type population distribution given by [8

8. S. F. Collins, G. W. Baxter, S. A. Wade, T. Sun, K. T. V. Grattan, Z. Y. Zhang, and A. W. Palmer, “Comparison of fluorescence-based temperature sensor schemes: Theoretical analysis and experimental validation,” J. Appl. Phys. 84(9), 4649–4655 (1998). [CrossRef]

,9

9. S. A. Wade, Ph.D. thesis (Victoria University, Melbourne, Australia, 1999).

]:
R=I31I21=ω31Rg3hν3ω21Rg2hν2exp(E32KT)=Cexp(E32KT)
(1)
where K is the Boltzmann constant, E32 is the energy gap between these two excited levels, g3 and g2 are the degeneracies (2J +1) of the levels and ωR31 and ωR21 are the spontaneous emission rates of the E3 and E2 levels to the E1 level, respectively.

Regarding the WGM micro-resonators, the resonance condition of a given resonance in the wavelength λ is approximately described by the following equation [15

15. G. Adamovsky and M. V. Otugen, “Morphology-dependent resonances and their applications to sensing in aerospace environments,” J. Aerosp. Comp. Inf. Commun. 5(10), 409–424 (2009).

]
λ=2πnrl
(2)
where n is the refractive index and r is the radius of the sphere, l is the polar mode number that is the number of wavelengths that fits into the resonator. This geometrical approximation is valid only for high values of l [13

13. G. Schweiger and M. Horn, “Effect of changes in size and index of refraction on the resonance wavelength of microspheres,” J. Opt. Soc. Am. B 23(2), 212–217 (2006). [CrossRef]

].

One common parameter on optical temperature sensors is the “sensitivity” (S), defined as the variation of the measured parameter (MP) with the temperature.

S=1MPdMPdT
(3)

Using this definition, it is straightforward to obtain the sensitivities of the FIR and WGM displacement that are respectively,

SFIR=δRRδT=E32kT2
(4)
SWGM=δλλδT=(1nδnδT+1rδrδT)
(5)

Many lanthanide ions are feasible for FIR experiments. As example, the Nd3+ ions used in the systems studied in this work are good candidates for the FIR technique [1

1. V. K. Rai, “Temperature sensors and optical sensors,” Appl. Phys. B 88(2), 297–303 (2007). [CrossRef]

,10

10. S. A. Wade, S. F. Collins, and G. W. Baxter, “Fluorescence intensity ratio technique for optical fiber point temperature sensing,” J. Appl. Phys. 94(8), 4743–4756 (2003). [CrossRef]

,20

20. P. Haro-González, I. R. Martín, L. L. Martín, S. F. León-Luis, C. Pérez-Rodríguez, and V. Lavín, “Characterization of Er3+ and Nd3+ doped Strontium Barium Niobate glass ceramic as temperature sensors,” Opt. Mater. 33(5), 742–745 (2011). [CrossRef]

]. These ions have many upper levels (not shown in Fig. 1) that can be easily excited by common lasers lines as green lines of Ar+ or 532 nm line of doubled Nd3+ lasers. From these levels the nonradiative relaxation processes populate the 4F5/2 and 4F3/2 close levels.

Moreover, the temperature resolution ΔTmin in both methods can be estimated by [14

14. Q. Ma, T. Rossmann, and Z. Guo, “Temperature sensitivity of silica micro-resonators,” J. Phys. D Appl. Phys. 41(24), 245111 (2008). [CrossRef]

]
ΔTmin=ΔMPminMPS
(6)
where ΔMPmin is the limit of detection of the measured parameter in each technique.

3. Experimental

A glass with the composition of 40%BaO–20%TiO2–40%SiO2 and doped with 1.5% of Nd2O3 (in the molar ratio) was prepared using a conventional melt-quenching method. Commercial powders of ACS reagent grade (purity ≥99.9%) BaCO3, TiO2, SiO2, and Nd2O3 were mixed and melted in a platinum-rhodium crucible at 1500 °C for 1 hour in an electric furnace. After that, the melt was poured between two bronze plates. Obtaining a bulk glass from which microspheres have been made.

Microspheres can be made by different methods; these ones include polishing, chemical etching and rapid quenching of liquid droplets [21

21. V. Lefèvre-Seguin, “Whispering-gallery mode lasers with doped silica microspheres,” Opt. Mater. 11(2-3), 153–165 (1999). [CrossRef]

,22

22. G. R. Elliott, D. W. Hewak, G. S. Murugan, and J. S. Wilkinson, “Chalcogenide glass microspheres; their production, characterization and potential,” Opt. Express 15(26), 17542–17553 (2007). [CrossRef] [PubMed]

]. In this letter, the microspheres are fabricated by the method exposed by Gregor R. Elliott et al. [22

22. G. R. Elliott, D. W. Hewak, G. S. Murugan, and J. S. Wilkinson, “Chalcogenide glass microspheres; their production, characterization and potential,” Opt. Express 15(26), 17542–17553 (2007). [CrossRef] [PubMed]

] from the glass mentioned above. Using this technique, microspheres of diameters ranging from 5 μm to 100 μm can be obtained.

The BTS microsphere and bulk samples were excited with a commercial continuous wave 532 nm Diode Pumped Solid State laser. The Nd3+: 4F5/24I9/2 (810 nm) and 4F3/24I9/2 (880 nm) transitions were recorded using a CCD spectrograph.

The spectral measurements to compute the FIR, were performed by placing the BTS bulk glass sample inside an electric furnace to increase the temperature from room temperature to 950 K at a rate of 2 K/min.

In our experiment, while the polar mode number l is in the order of 300 [23

23. L. L. Martín, P. Haro-González, I. R. Martín, D. Navarro-Urrios, D. Alonso, C. Pérez-Rodríguez, D. Jaque, and N. E. Capuj, “Whispering-gallery modes in glass microspheres: optimization of pumping in a modified confocal microscope,” Opt. Lett. 36(5), 615–617 (2011). [CrossRef] [PubMed]

] so the geometrical approximation is applicable, Eq. (3) is not fully satisfied because the temperature of the microsphere is non homogeneous due to the laser heating nature. As the microsphere is heated by the pumping laser, the heating is non-homogeneous yielding a temperature gradient from high temperature in the centre to low temperature in the surface of the sphere.

4. Results and discussion

From spectral measurements completed in the bulk sample inside an electric furnace, the areas of the emission bands associated to the 4F5/24I9/2 (810 nm) and 4F3/24I9/2 (880 nm) transitions are obtained and fitted to Eq. (1) giving a value of 887 cm−1 for the energy gap E32 and a pre-exponential parameter C with a value of 1.982. The experimental values and the fit curve are shown in Fig. 1b. The E32 energy gap value is similar to the one obtained from absorption spectrum, that is 944 cm−1 which is also in good agreement with other Nd3+ doped matrices [19

19. L. L. Martin, P. Haro-González, and I. R. Martín, “Optical properties of transparent Dy3+ doped Ba2TiSi2O8 glass ceramic,” Opt. Mater. 33(5), 738–741 (2011). [CrossRef]

,24

24. M. M. Mann and L. G. DeShazer, “Energy levels and spectral broadening of neodymium ions in laser glass,” J. Appl. Phys. 41(7), 2951–2957 (1970). [CrossRef]

26

26. P. W. France, in Fluoride Glass Optical Fibers, P. W. France, ed. (Blackie, 1990), pp. 165–167.

]. This calibration will be employed to estimate the temperature of the microsphere surface in the laser heating process.

When the surface temperature increases until 950 K, the WGM peaks experiences a wavelength increase of 7 nm. As consequence, the estimated variation of the wavelength results an average 10 pm/K shift in our BTS glass, which is similar to 11 pm/K observed at room temperatures in silica microspheres [17

17. Q. Ma, T. Rossmann, and Z. Guo, “Whispering-gallery mode silica microsensors for cryogenic to room temperature measurement,” Meas. Sci. Technol. 21(2), 025310–025317 (2010). [CrossRef]

].

In Fig. 4
Fig. 4 Temperature resolutions for the FIR technique in the glass (solid line), WGM displacements in the measured microsphere (red line) and the calculated ones for a homogeneous heated fused silica microsphere (dashed line).
, it is shown the temperature resolution ΔTmin, computed as described in Eq. (6) for the FIR and WGM displacement. The estimated WGM displacement temperature resolution of a homogeneous heated fused silica microsphere is also shown evaluated using Eqs. (5) and (6) with equal radius to the measured microsphere (30 µm) and constant parameters δn/δT = 1.28x10−5/K and δr/δT = 5.5x10−7/K obtained from literature [27

27. C. V. I. Melles Griot, “Technical Guide: Material Properties” (2009), Vol. 4.9, Iss. 9.

]. As can be seen in this figure, the temperature resolution obtained in this work with the BTS microsphere is similar to the calculated values for a silica microsphere.

The FIR sensitivity of the Nd3+ ions in the BTS glass can be calculated by Eq. (3) and it reaches a maximum of 0.01 in the measured range which is similar to the sensitivity of FIR technique achieved in other host glasses [1

1. V. K. Rai, “Temperature sensors and optical sensors,” Appl. Phys. B 88(2), 297–303 (2007). [CrossRef]

,10

10. S. A. Wade, S. F. Collins, and G. W. Baxter, “Fluorescence intensity ratio technique for optical fiber point temperature sensing,” J. Appl. Phys. 94(8), 4743–4756 (2003). [CrossRef]

]. This yields a temperature resolution about 1 K using Eq. (6) and an estimated error of 5%. This error is due to the inaccuracy of the areas due to the overlap between the thermalized bands.

On the other hand, in the WGM the limit of detection of the displacements is about 0.01% of the line-width of the resonances [28

28. F. Vollmer and S. Arnold, “Whispering-gallery-mode biosensing: label-free detection down to single molecules,” Nat. Methods 5(7), 591–596 (2008). [CrossRef] [PubMed]

]. Therefore, the achievable resolution limit on temperature is about 0.1 K (independently of the calibration technique) that probes an increase near one order of magnitude in the detection limits respect to the FIR technique. However, using a narrow line tunable laser or a high resolution spectrograph, the resolutions obtained can be as high as [14

14. Q. Ma, T. Rossmann, and Z. Guo, “Temperature sensitivity of silica micro-resonators,” J. Phys. D Appl. Phys. 41(24), 245111 (2008). [CrossRef]

] and [17

17. Q. Ma, T. Rossmann, and Z. Guo, “Whispering-gallery mode silica microsensors for cryogenic to room temperature measurement,” Meas. Sci. Technol. 21(2), 025310–025317 (2010). [CrossRef]

] specifies.

The conjunction of both techniques, allows a coarse temperature estimation by the FIR and a fine temperature estimation by the WGM and avoid the penalty that can be caused by rapid heating cooling proceses, where a incorrect temperature sampling can lead to a jump in the WGM peak.

5. Conclusions

Microspheres made from Nd3+ doped BTS glass were prepared, and the WGM resonances were observed in a modified confocal microscope. A bulk sample of the Nd3+:BTS glass was used to calibrate as temperature sensor by the FIR technique. The microsphere was heated by laser irradiation process and the surface temperature was computed using the FIR technique. This temperature is correlated to WGM peak displacement, showing an average red-shift of 10 pm/K. The limit of resolution on temperature measurements was estimated about 1 K for the FIR technique and about 0.1 K for the WGM shift technique.

Acknowledgments

The authors are grateful to Comisión Interministerial de Ciencia y Tecnología (MAT2010-21270-C04-02), Malta Consolider Ingenio 2010 (CSD2007-0045) and FPI of Gobierno de Canarias for financial support.

References and links

1.

V. K. Rai, “Temperature sensors and optical sensors,” Appl. Phys. B 88(2), 297–303 (2007). [CrossRef]

2.

H. Kusama, O. J. Sovers, and T. Yoshioka, “Line Shift method for phosphor temperature measurements,” Jpn. J. Appl. Phys. 15(12), 2349–2358 (1976). [CrossRef]

3.

O. Svelto, Principles of Lasers, 3rd ed. (Plenum, 1989), pp. 70–71.

4.

H. Berthou and C. K. Jörgensen, “Optical-fiber temperature sensor based on upconversion-excited fluorescence,” Opt. Lett. 15(19), 1100–1102 (1990). [CrossRef] [PubMed]

5.

W. J. Miniscalco, in Rare Earth Doped Fiber Lasers and Amplifiers, M. J. F. Digonnet, ed. (Marcel Dekker, 1993), p. 35.

6.

E. Maurice, G. Monnom, B. Dussardier, A. Saïssy, D. B. Ostrowsky, and G. W. Baxter, “Thermalization effects between upper levels of green fluorescence in Er-doped silica fibers,” Opt. Lett. 19(13), 990–992 (1994). [CrossRef] [PubMed]

7.

P. V. dos Santos, M. T. de Araujo, A. S. Gouveia-Neto, J. A. Medeiros Neto, and A. S. B. Sombra, “Optical temperature sensing using upconversion fluorescence emission in Er3+/Yb3+-codoped chalcogenide glass,” Appl. Phys. Lett. 73(5), 578–581 (1998). [CrossRef]

8.

S. F. Collins, G. W. Baxter, S. A. Wade, T. Sun, K. T. V. Grattan, Z. Y. Zhang, and A. W. Palmer, “Comparison of fluorescence-based temperature sensor schemes: Theoretical analysis and experimental validation,” J. Appl. Phys. 84(9), 4649–4655 (1998). [CrossRef]

9.

S. A. Wade, Ph.D. thesis (Victoria University, Melbourne, Australia, 1999).

10.

S. A. Wade, S. F. Collins, and G. W. Baxter, “Fluorescence intensity ratio technique for optical fiber point temperature sensing,” J. Appl. Phys. 94(8), 4743–4756 (2003). [CrossRef]

11.

M. A. R. C. Alencar, G. S. Maciel, C. B. de Araujo, and A. Patra, “Er3+-doped BaTiO3 nanocrystals for thermometry: Influence of nanoenvironment on the sensitivity of a fluorescence based temperature sensor,” Appl. Phys. Lett. 84(23), 4753–4756 (2004). [CrossRef]

12.

A. B. Matsko, A. A. Savchenkov, D. Strekalov, V. S. Ilchenko, and L. Maleki, “Review of Applications of Whispering-Gallery Mode Resonators in Photonics and Nonlinear Optics,” IPN Progress Report 42-162 (2005), pp. 1–51.

13.

G. Schweiger and M. Horn, “Effect of changes in size and index of refraction on the resonance wavelength of microspheres,” J. Opt. Soc. Am. B 23(2), 212–217 (2006). [CrossRef]

14.

Q. Ma, T. Rossmann, and Z. Guo, “Temperature sensitivity of silica micro-resonators,” J. Phys. D Appl. Phys. 41(24), 245111 (2008). [CrossRef]

15.

G. Adamovsky and M. V. Otugen, “Morphology-dependent resonances and their applications to sensing in aerospace environments,” J. Aerosp. Comp. Inf. Commun. 5(10), 409–424 (2009).

16.

T. Carmon, L. Yang, and K. J. Vahala, “Dynamical thermal behavior and thermal self-stability of microcavities,” Opt. Express 12(20), 4742–4750 (2004). [CrossRef] [PubMed]

17.

Q. Ma, T. Rossmann, and Z. Guo, “Whispering-gallery mode silica microsensors for cryogenic to room temperature measurement,” Meas. Sci. Technol. 21(2), 025310–025317 (2010). [CrossRef]

18.

N. Maruyama, T. Honma, and T. Komatsu, “Enhanced quantum yield of yellow photoluminescence of Dy3+ ions in nonlinear optical Ba2TiSi2O8 nanocrystals formed in glass,” J. Solid State Chem. 182(2), 246–252 (2009). [CrossRef]

19.

L. L. Martin, P. Haro-González, and I. R. Martín, “Optical properties of transparent Dy3+ doped Ba2TiSi2O8 glass ceramic,” Opt. Mater. 33(5), 738–741 (2011). [CrossRef]

20.

P. Haro-González, I. R. Martín, L. L. Martín, S. F. León-Luis, C. Pérez-Rodríguez, and V. Lavín, “Characterization of Er3+ and Nd3+ doped Strontium Barium Niobate glass ceramic as temperature sensors,” Opt. Mater. 33(5), 742–745 (2011). [CrossRef]

21.

V. Lefèvre-Seguin, “Whispering-gallery mode lasers with doped silica microspheres,” Opt. Mater. 11(2-3), 153–165 (1999). [CrossRef]

22.

G. R. Elliott, D. W. Hewak, G. S. Murugan, and J. S. Wilkinson, “Chalcogenide glass microspheres; their production, characterization and potential,” Opt. Express 15(26), 17542–17553 (2007). [CrossRef] [PubMed]

23.

L. L. Martín, P. Haro-González, I. R. Martín, D. Navarro-Urrios, D. Alonso, C. Pérez-Rodríguez, D. Jaque, and N. E. Capuj, “Whispering-gallery modes in glass microspheres: optimization of pumping in a modified confocal microscope,” Opt. Lett. 36(5), 615–617 (2011). [CrossRef] [PubMed]

24.

M. M. Mann and L. G. DeShazer, “Energy levels and spectral broadening of neodymium ions in laser glass,” J. Appl. Phys. 41(7), 2951–2957 (1970). [CrossRef]

25.

A. A. Kaminskii, Laser Crystals: Their Physics and Applications (Springer, 1981), pp. 121–147.

26.

P. W. France, in Fluoride Glass Optical Fibers, P. W. France, ed. (Blackie, 1990), pp. 165–167.

27.

C. V. I. Melles Griot, “Technical Guide: Material Properties” (2009), Vol. 4.9, Iss. 9.

28.

F. Vollmer and S. Arnold, “Whispering-gallery-mode biosensing: label-free detection down to single molecules,” Nat. Methods 5(7), 591–596 (2008). [CrossRef] [PubMed]

OCIS Codes
(230.5750) Optical devices : Resonators
(280.6780) Remote sensing and sensors : Temperature
(130.3990) Integrated optics : Micro-optical devices

ToC Category:
Sensors

History
Original Manuscript: July 21, 2011
Revised Manuscript: October 13, 2011
Manuscript Accepted: October 21, 2011
Published: December 2, 2011

Virtual Issues
Vol. 7, Iss. 2 Virtual Journal for Biomedical Optics

Citation
L. L. Martín, C. Pérez-Rodríguez, P. Haro-González, and I. R. Martín, "Whispering gallery modes in a glass microsphere as a function of temperature," Opt. Express 19, 25792-25798 (2011)
http://www.opticsinfobase.org/vjbo/abstract.cfm?URI=oe-19-25-25792


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References

  1. V. K. Rai, “Temperature sensors and optical sensors,” Appl. Phys. B88(2), 297–303 (2007). [CrossRef]
  2. H. Kusama, O. J. Sovers, and T. Yoshioka, “Line Shift method for phosphor temperature measurements,” Jpn. J. Appl. Phys.15(12), 2349–2358 (1976). [CrossRef]
  3. O. Svelto, Principles of Lasers, 3rd ed. (Plenum, 1989), pp. 70–71.
  4. H. Berthou and C. K. Jörgensen, “Optical-fiber temperature sensor based on upconversion-excited fluorescence,” Opt. Lett.15(19), 1100–1102 (1990). [CrossRef] [PubMed]
  5. W. J. Miniscalco, in Rare Earth Doped Fiber Lasers and Amplifiers, M. J. F. Digonnet, ed. (Marcel Dekker, 1993), p. 35.
  6. E. Maurice, G. Monnom, B. Dussardier, A. Saïssy, D. B. Ostrowsky, and G. W. Baxter, “Thermalization effects between upper levels of green fluorescence in Er-doped silica fibers,” Opt. Lett.19(13), 990–992 (1994). [CrossRef] [PubMed]
  7. P. V. dos Santos, M. T. de Araujo, A. S. Gouveia-Neto, J. A. Medeiros Neto, and A. S. B. Sombra, “Optical temperature sensing using upconversion fluorescence emission in Er3+/Yb3+-codoped chalcogenide glass,” Appl. Phys. Lett.73(5), 578–581 (1998). [CrossRef]
  8. S. F. Collins, G. W. Baxter, S. A. Wade, T. Sun, K. T. V. Grattan, Z. Y. Zhang, and A. W. Palmer, “Comparison of fluorescence-based temperature sensor schemes: Theoretical analysis and experimental validation,” J. Appl. Phys.84(9), 4649–4655 (1998). [CrossRef]
  9. S. A. Wade, Ph.D. thesis (Victoria University, Melbourne, Australia, 1999).
  10. S. A. Wade, S. F. Collins, and G. W. Baxter, “Fluorescence intensity ratio technique for optical fiber point temperature sensing,” J. Appl. Phys.94(8), 4743–4756 (2003). [CrossRef]
  11. M. A. R. C. Alencar, G. S. Maciel, C. B. de Araujo, and A. Patra, “Er3+-doped BaTiO3 nanocrystals for thermometry: Influence of nanoenvironment on the sensitivity of a fluorescence based temperature sensor,” Appl. Phys. Lett.84(23), 4753–4756 (2004). [CrossRef]
  12. A. B. Matsko, A. A. Savchenkov, D. Strekalov, V. S. Ilchenko, and L. Maleki, “Review of Applications of Whispering-Gallery Mode Resonators in Photonics and Nonlinear Optics,” IPN Progress Report 42-162 (2005), pp. 1–51.
  13. G. Schweiger and M. Horn, “Effect of changes in size and index of refraction on the resonance wavelength of microspheres,” J. Opt. Soc. Am. B23(2), 212–217 (2006). [CrossRef]
  14. Q. Ma, T. Rossmann, and Z. Guo, “Temperature sensitivity of silica micro-resonators,” J. Phys. D Appl. Phys.41(24), 245111 (2008). [CrossRef]
  15. G. Adamovsky and M. V. Otugen, “Morphology-dependent resonances and their applications to sensing in aerospace environments,” J. Aerosp. Comp. Inf. Commun.5(10), 409–424 (2009).
  16. T. Carmon, L. Yang, and K. J. Vahala, “Dynamical thermal behavior and thermal self-stability of microcavities,” Opt. Express12(20), 4742–4750 (2004). [CrossRef] [PubMed]
  17. Q. Ma, T. Rossmann, and Z. Guo, “Whispering-gallery mode silica microsensors for cryogenic to room temperature measurement,” Meas. Sci. Technol.21(2), 025310–025317 (2010). [CrossRef]
  18. N. Maruyama, T. Honma, and T. Komatsu, “Enhanced quantum yield of yellow photoluminescence of Dy3+ ions in nonlinear optical Ba2TiSi2O8 nanocrystals formed in glass,” J. Solid State Chem.182(2), 246–252 (2009). [CrossRef]
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