Abstract

A temperature-independent fiber-Bragg-grating strain-sensing system, based on a novel optical-power-detection scheme, is developed and analyzed. In this system a pair of fiber Bragg gratings with reflection spectra either partially or substantially overlapping is placed side by side to form a temperature-independent strain-sensor unit. Conventional wavelength-interrogation techniques are not used here, and instead an optical-power-detection scheme is proposed to directly calibrate the measurand, i.e., the strain. Unlike the conventional approach in a multiplexed sensing system, the presented power-detection-based interrogation method does not need the fiber-Bragg-grating sensors to be spectrally separate. The only requirement is that the spectra of the two fiber Bragg gratings of each sensor unit in a multiplexed system be identical or slightly separate (slightly overlapping spectra would also work in the sensing scheme) and the source’s optical power be sufficient for sensitive measurement. Based on a three-sensor-unit system, we demonstrate simple strain measurements of high linearity (±0.4%), good sensitivity [2 microstrains (µS)], high thermal stability (±0.8%), and zero cross talk. The effects of light source spectral flatness and fiber bending loss on measurement accuracy are also discussed.

© 2002 Optical Society of America

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References

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  1. A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putnam, E. J. Friebele, “Fiber grating sensor,” J. Lightwave Technol. 15, 1442–1463 (1997).
    [CrossRef]
  2. J. Mora, A. Diez, J. L. Cruz, M. V. Andres, “A magnetostrictive sensor interrogated by fiber gratings for dc current and temperature discrimination,” IEEE Photon. Technol. 12, 1680–1682 (2000).
    [CrossRef]
  3. G. A. Johnson, M. D. Todd, B. L. Althouse, C. C. Chang, “Fiber Bragg grating interrogation and multiplexing with a 3 × 3 coupler and a scanning filter,” J. Lightwave Technol. 18, 1101–1105 (2000).
    [CrossRef]
  4. A. D. Kersey, T. A. Berkoff, W. W. Morey, “High-resolution fiber-grating-based strain sensor with interferometric wavelength-shift detection,” Electron. Lett. 28, 236–238 (1992).
    [CrossRef]
  5. A. Ezbiri, A. Munoz, S. E. Kanellopoulos, V. A. Handerek, “High resolution fiber Bragg grating sensor demodulation using a diffraction grating spectrometer and CCD detection,” in IEE Colloquium on Optical Techniques for Smart Structures and Structural Monitoring, Digest 1997/033 (Institute of Electrical Engineers, London, U.K., 1997).
  6. A. Arie, B. Lissak, M. Tur, “Static fiber-Bragg grating strain sensing using frequency-locked lasers,” J. Lightwave Technol. 17, 1849–1854 (1999).
    [CrossRef]
  7. L. A. Ferreira, E. V. Diatzikis, J. L. Santos, F. Farahi, “Frequency-modulated multimode laser diode for fiber Bragg grating sensors,” J. Lightwave Technol. 16, 1620–1630 (1998).
    [CrossRef]
  8. D. A. Jackson, A. B. Lobo Ribeiro, L. Reekie, J. L. Archambault, “Simple multiplexing scheme for a fiber-optic grating sensor network,” Opt. Lett. 18, 1192–1194 (1993).
    [CrossRef] [PubMed]
  9. K. P. Koo, A. D. Kersey, “Bragg grating-based laser sensors systems with interferometric interrogation and wavelength division multiplexing,” J. Lightwave Technol. 13, 1243–1249 (1995).
    [CrossRef]
  10. M. A. Davis, D. G. Bellemore, M. A. Putnam, A. D. Kersey, “Interrogation of 60 fiber Bragg grating sensors with microstrain resolution capability,” Electron. Lett. 32, 1393–1394 (1996).
    [CrossRef]
  11. R. W. Fallon, L. Zhang, A. Gloag, I. Bennion, “Multiplexed identical broadband chirped grating interrogation system for large strain sensing application,” IEEE Photon. Technol. Lett. 9, 1616–1618 (1997).
    [CrossRef]
  12. M. G. Xu, J. L. Archambault, L. Reekie, J. P. Dakin, “Discrimination between strain and temperature effects using dual-wavelength fiber grating sensors,” Electron. Lett. 30, 1085–1087 (1994).
    [CrossRef]
  13. B. O. Guan, H. Y. Tam, X. M. Tao, X. Y. Dong, “Simultaneous strain and temperature measurement using a superstructure fiber Bragg grating,” IEEE Photon. Technol. Lett. 12, 675–677 (2000).
    [CrossRef]
  14. W. C. Du, X. M. Tao, H. Y. Tam, “Fiber Bragg grating cavity sensor for simultaneous measurement of strain and temperature,” IEEE Photon. Technol. Lett. 11, 105–107 (1999).
    [CrossRef]
  15. S. Kim, J. Kwon, S. Kim, B. Lee, “Temperature-independent strain sensor using chirped grating partially embedded in a glass tube,” IEEE Photon. Technol. Lett. 12, 678–680 (2000).
    [CrossRef]

2000 (4)

J. Mora, A. Diez, J. L. Cruz, M. V. Andres, “A magnetostrictive sensor interrogated by fiber gratings for dc current and temperature discrimination,” IEEE Photon. Technol. 12, 1680–1682 (2000).
[CrossRef]

G. A. Johnson, M. D. Todd, B. L. Althouse, C. C. Chang, “Fiber Bragg grating interrogation and multiplexing with a 3 × 3 coupler and a scanning filter,” J. Lightwave Technol. 18, 1101–1105 (2000).
[CrossRef]

B. O. Guan, H. Y. Tam, X. M. Tao, X. Y. Dong, “Simultaneous strain and temperature measurement using a superstructure fiber Bragg grating,” IEEE Photon. Technol. Lett. 12, 675–677 (2000).
[CrossRef]

S. Kim, J. Kwon, S. Kim, B. Lee, “Temperature-independent strain sensor using chirped grating partially embedded in a glass tube,” IEEE Photon. Technol. Lett. 12, 678–680 (2000).
[CrossRef]

1999 (2)

W. C. Du, X. M. Tao, H. Y. Tam, “Fiber Bragg grating cavity sensor for simultaneous measurement of strain and temperature,” IEEE Photon. Technol. Lett. 11, 105–107 (1999).
[CrossRef]

A. Arie, B. Lissak, M. Tur, “Static fiber-Bragg grating strain sensing using frequency-locked lasers,” J. Lightwave Technol. 17, 1849–1854 (1999).
[CrossRef]

1998 (1)

1997 (2)

A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putnam, E. J. Friebele, “Fiber grating sensor,” J. Lightwave Technol. 15, 1442–1463 (1997).
[CrossRef]

R. W. Fallon, L. Zhang, A. Gloag, I. Bennion, “Multiplexed identical broadband chirped grating interrogation system for large strain sensing application,” IEEE Photon. Technol. Lett. 9, 1616–1618 (1997).
[CrossRef]

1996 (1)

M. A. Davis, D. G. Bellemore, M. A. Putnam, A. D. Kersey, “Interrogation of 60 fiber Bragg grating sensors with microstrain resolution capability,” Electron. Lett. 32, 1393–1394 (1996).
[CrossRef]

1995 (1)

K. P. Koo, A. D. Kersey, “Bragg grating-based laser sensors systems with interferometric interrogation and wavelength division multiplexing,” J. Lightwave Technol. 13, 1243–1249 (1995).
[CrossRef]

1994 (1)

M. G. Xu, J. L. Archambault, L. Reekie, J. P. Dakin, “Discrimination between strain and temperature effects using dual-wavelength fiber grating sensors,” Electron. Lett. 30, 1085–1087 (1994).
[CrossRef]

1993 (1)

1992 (1)

A. D. Kersey, T. A. Berkoff, W. W. Morey, “High-resolution fiber-grating-based strain sensor with interferometric wavelength-shift detection,” Electron. Lett. 28, 236–238 (1992).
[CrossRef]

Althouse, B. L.

Andres, M. V.

J. Mora, A. Diez, J. L. Cruz, M. V. Andres, “A magnetostrictive sensor interrogated by fiber gratings for dc current and temperature discrimination,” IEEE Photon. Technol. 12, 1680–1682 (2000).
[CrossRef]

Archambault, J. L.

M. G. Xu, J. L. Archambault, L. Reekie, J. P. Dakin, “Discrimination between strain and temperature effects using dual-wavelength fiber grating sensors,” Electron. Lett. 30, 1085–1087 (1994).
[CrossRef]

D. A. Jackson, A. B. Lobo Ribeiro, L. Reekie, J. L. Archambault, “Simple multiplexing scheme for a fiber-optic grating sensor network,” Opt. Lett. 18, 1192–1194 (1993).
[CrossRef] [PubMed]

Arie, A.

Askins, C. G.

A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putnam, E. J. Friebele, “Fiber grating sensor,” J. Lightwave Technol. 15, 1442–1463 (1997).
[CrossRef]

Bellemore, D. G.

M. A. Davis, D. G. Bellemore, M. A. Putnam, A. D. Kersey, “Interrogation of 60 fiber Bragg grating sensors with microstrain resolution capability,” Electron. Lett. 32, 1393–1394 (1996).
[CrossRef]

Bennion, I.

R. W. Fallon, L. Zhang, A. Gloag, I. Bennion, “Multiplexed identical broadband chirped grating interrogation system for large strain sensing application,” IEEE Photon. Technol. Lett. 9, 1616–1618 (1997).
[CrossRef]

Berkoff, T. A.

A. D. Kersey, T. A. Berkoff, W. W. Morey, “High-resolution fiber-grating-based strain sensor with interferometric wavelength-shift detection,” Electron. Lett. 28, 236–238 (1992).
[CrossRef]

Chang, C. C.

Cruz, J. L.

J. Mora, A. Diez, J. L. Cruz, M. V. Andres, “A magnetostrictive sensor interrogated by fiber gratings for dc current and temperature discrimination,” IEEE Photon. Technol. 12, 1680–1682 (2000).
[CrossRef]

Dakin, J. P.

M. G. Xu, J. L. Archambault, L. Reekie, J. P. Dakin, “Discrimination between strain and temperature effects using dual-wavelength fiber grating sensors,” Electron. Lett. 30, 1085–1087 (1994).
[CrossRef]

Davis, M. A.

A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putnam, E. J. Friebele, “Fiber grating sensor,” J. Lightwave Technol. 15, 1442–1463 (1997).
[CrossRef]

M. A. Davis, D. G. Bellemore, M. A. Putnam, A. D. Kersey, “Interrogation of 60 fiber Bragg grating sensors with microstrain resolution capability,” Electron. Lett. 32, 1393–1394 (1996).
[CrossRef]

Diatzikis, E. V.

Diez, A.

J. Mora, A. Diez, J. L. Cruz, M. V. Andres, “A magnetostrictive sensor interrogated by fiber gratings for dc current and temperature discrimination,” IEEE Photon. Technol. 12, 1680–1682 (2000).
[CrossRef]

Dong, X. Y.

B. O. Guan, H. Y. Tam, X. M. Tao, X. Y. Dong, “Simultaneous strain and temperature measurement using a superstructure fiber Bragg grating,” IEEE Photon. Technol. Lett. 12, 675–677 (2000).
[CrossRef]

Du, W. C.

W. C. Du, X. M. Tao, H. Y. Tam, “Fiber Bragg grating cavity sensor for simultaneous measurement of strain and temperature,” IEEE Photon. Technol. Lett. 11, 105–107 (1999).
[CrossRef]

Ezbiri, A.

A. Ezbiri, A. Munoz, S. E. Kanellopoulos, V. A. Handerek, “High resolution fiber Bragg grating sensor demodulation using a diffraction grating spectrometer and CCD detection,” in IEE Colloquium on Optical Techniques for Smart Structures and Structural Monitoring, Digest 1997/033 (Institute of Electrical Engineers, London, U.K., 1997).

Fallon, R. W.

R. W. Fallon, L. Zhang, A. Gloag, I. Bennion, “Multiplexed identical broadband chirped grating interrogation system for large strain sensing application,” IEEE Photon. Technol. Lett. 9, 1616–1618 (1997).
[CrossRef]

Farahi, F.

Ferreira, L. A.

Friebele, E. J.

A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putnam, E. J. Friebele, “Fiber grating sensor,” J. Lightwave Technol. 15, 1442–1463 (1997).
[CrossRef]

Gloag, A.

R. W. Fallon, L. Zhang, A. Gloag, I. Bennion, “Multiplexed identical broadband chirped grating interrogation system for large strain sensing application,” IEEE Photon. Technol. Lett. 9, 1616–1618 (1997).
[CrossRef]

Guan, B. O.

B. O. Guan, H. Y. Tam, X. M. Tao, X. Y. Dong, “Simultaneous strain and temperature measurement using a superstructure fiber Bragg grating,” IEEE Photon. Technol. Lett. 12, 675–677 (2000).
[CrossRef]

Handerek, V. A.

A. Ezbiri, A. Munoz, S. E. Kanellopoulos, V. A. Handerek, “High resolution fiber Bragg grating sensor demodulation using a diffraction grating spectrometer and CCD detection,” in IEE Colloquium on Optical Techniques for Smart Structures and Structural Monitoring, Digest 1997/033 (Institute of Electrical Engineers, London, U.K., 1997).

Jackson, D. A.

Johnson, G. A.

Kanellopoulos, S. E.

A. Ezbiri, A. Munoz, S. E. Kanellopoulos, V. A. Handerek, “High resolution fiber Bragg grating sensor demodulation using a diffraction grating spectrometer and CCD detection,” in IEE Colloquium on Optical Techniques for Smart Structures and Structural Monitoring, Digest 1997/033 (Institute of Electrical Engineers, London, U.K., 1997).

Kersey, A. D.

A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putnam, E. J. Friebele, “Fiber grating sensor,” J. Lightwave Technol. 15, 1442–1463 (1997).
[CrossRef]

M. A. Davis, D. G. Bellemore, M. A. Putnam, A. D. Kersey, “Interrogation of 60 fiber Bragg grating sensors with microstrain resolution capability,” Electron. Lett. 32, 1393–1394 (1996).
[CrossRef]

K. P. Koo, A. D. Kersey, “Bragg grating-based laser sensors systems with interferometric interrogation and wavelength division multiplexing,” J. Lightwave Technol. 13, 1243–1249 (1995).
[CrossRef]

A. D. Kersey, T. A. Berkoff, W. W. Morey, “High-resolution fiber-grating-based strain sensor with interferometric wavelength-shift detection,” Electron. Lett. 28, 236–238 (1992).
[CrossRef]

Kim, S.

S. Kim, J. Kwon, S. Kim, B. Lee, “Temperature-independent strain sensor using chirped grating partially embedded in a glass tube,” IEEE Photon. Technol. Lett. 12, 678–680 (2000).
[CrossRef]

S. Kim, J. Kwon, S. Kim, B. Lee, “Temperature-independent strain sensor using chirped grating partially embedded in a glass tube,” IEEE Photon. Technol. Lett. 12, 678–680 (2000).
[CrossRef]

Koo, K. P.

A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putnam, E. J. Friebele, “Fiber grating sensor,” J. Lightwave Technol. 15, 1442–1463 (1997).
[CrossRef]

K. P. Koo, A. D. Kersey, “Bragg grating-based laser sensors systems with interferometric interrogation and wavelength division multiplexing,” J. Lightwave Technol. 13, 1243–1249 (1995).
[CrossRef]

Kwon, J.

S. Kim, J. Kwon, S. Kim, B. Lee, “Temperature-independent strain sensor using chirped grating partially embedded in a glass tube,” IEEE Photon. Technol. Lett. 12, 678–680 (2000).
[CrossRef]

LeBlanc, M.

A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putnam, E. J. Friebele, “Fiber grating sensor,” J. Lightwave Technol. 15, 1442–1463 (1997).
[CrossRef]

Lee, B.

S. Kim, J. Kwon, S. Kim, B. Lee, “Temperature-independent strain sensor using chirped grating partially embedded in a glass tube,” IEEE Photon. Technol. Lett. 12, 678–680 (2000).
[CrossRef]

Lissak, B.

Lobo Ribeiro, A. B.

Mora, J.

J. Mora, A. Diez, J. L. Cruz, M. V. Andres, “A magnetostrictive sensor interrogated by fiber gratings for dc current and temperature discrimination,” IEEE Photon. Technol. 12, 1680–1682 (2000).
[CrossRef]

Morey, W. W.

A. D. Kersey, T. A. Berkoff, W. W. Morey, “High-resolution fiber-grating-based strain sensor with interferometric wavelength-shift detection,” Electron. Lett. 28, 236–238 (1992).
[CrossRef]

Munoz, A.

A. Ezbiri, A. Munoz, S. E. Kanellopoulos, V. A. Handerek, “High resolution fiber Bragg grating sensor demodulation using a diffraction grating spectrometer and CCD detection,” in IEE Colloquium on Optical Techniques for Smart Structures and Structural Monitoring, Digest 1997/033 (Institute of Electrical Engineers, London, U.K., 1997).

Patrick, H. J.

A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putnam, E. J. Friebele, “Fiber grating sensor,” J. Lightwave Technol. 15, 1442–1463 (1997).
[CrossRef]

Putnam, M. A.

A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putnam, E. J. Friebele, “Fiber grating sensor,” J. Lightwave Technol. 15, 1442–1463 (1997).
[CrossRef]

M. A. Davis, D. G. Bellemore, M. A. Putnam, A. D. Kersey, “Interrogation of 60 fiber Bragg grating sensors with microstrain resolution capability,” Electron. Lett. 32, 1393–1394 (1996).
[CrossRef]

Reekie, L.

M. G. Xu, J. L. Archambault, L. Reekie, J. P. Dakin, “Discrimination between strain and temperature effects using dual-wavelength fiber grating sensors,” Electron. Lett. 30, 1085–1087 (1994).
[CrossRef]

D. A. Jackson, A. B. Lobo Ribeiro, L. Reekie, J. L. Archambault, “Simple multiplexing scheme for a fiber-optic grating sensor network,” Opt. Lett. 18, 1192–1194 (1993).
[CrossRef] [PubMed]

Santos, J. L.

Tam, H. Y.

B. O. Guan, H. Y. Tam, X. M. Tao, X. Y. Dong, “Simultaneous strain and temperature measurement using a superstructure fiber Bragg grating,” IEEE Photon. Technol. Lett. 12, 675–677 (2000).
[CrossRef]

W. C. Du, X. M. Tao, H. Y. Tam, “Fiber Bragg grating cavity sensor for simultaneous measurement of strain and temperature,” IEEE Photon. Technol. Lett. 11, 105–107 (1999).
[CrossRef]

Tao, X. M.

B. O. Guan, H. Y. Tam, X. M. Tao, X. Y. Dong, “Simultaneous strain and temperature measurement using a superstructure fiber Bragg grating,” IEEE Photon. Technol. Lett. 12, 675–677 (2000).
[CrossRef]

W. C. Du, X. M. Tao, H. Y. Tam, “Fiber Bragg grating cavity sensor for simultaneous measurement of strain and temperature,” IEEE Photon. Technol. Lett. 11, 105–107 (1999).
[CrossRef]

Todd, M. D.

Tur, M.

Xu, M. G.

M. G. Xu, J. L. Archambault, L. Reekie, J. P. Dakin, “Discrimination between strain and temperature effects using dual-wavelength fiber grating sensors,” Electron. Lett. 30, 1085–1087 (1994).
[CrossRef]

Zhang, L.

R. W. Fallon, L. Zhang, A. Gloag, I. Bennion, “Multiplexed identical broadband chirped grating interrogation system for large strain sensing application,” IEEE Photon. Technol. Lett. 9, 1616–1618 (1997).
[CrossRef]

Electron. Lett. (3)

A. D. Kersey, T. A. Berkoff, W. W. Morey, “High-resolution fiber-grating-based strain sensor with interferometric wavelength-shift detection,” Electron. Lett. 28, 236–238 (1992).
[CrossRef]

M. G. Xu, J. L. Archambault, L. Reekie, J. P. Dakin, “Discrimination between strain and temperature effects using dual-wavelength fiber grating sensors,” Electron. Lett. 30, 1085–1087 (1994).
[CrossRef]

M. A. Davis, D. G. Bellemore, M. A. Putnam, A. D. Kersey, “Interrogation of 60 fiber Bragg grating sensors with microstrain resolution capability,” Electron. Lett. 32, 1393–1394 (1996).
[CrossRef]

IEEE Photon. Technol. (1)

J. Mora, A. Diez, J. L. Cruz, M. V. Andres, “A magnetostrictive sensor interrogated by fiber gratings for dc current and temperature discrimination,” IEEE Photon. Technol. 12, 1680–1682 (2000).
[CrossRef]

IEEE Photon. Technol. Lett. (4)

R. W. Fallon, L. Zhang, A. Gloag, I. Bennion, “Multiplexed identical broadband chirped grating interrogation system for large strain sensing application,” IEEE Photon. Technol. Lett. 9, 1616–1618 (1997).
[CrossRef]

B. O. Guan, H. Y. Tam, X. M. Tao, X. Y. Dong, “Simultaneous strain and temperature measurement using a superstructure fiber Bragg grating,” IEEE Photon. Technol. Lett. 12, 675–677 (2000).
[CrossRef]

W. C. Du, X. M. Tao, H. Y. Tam, “Fiber Bragg grating cavity sensor for simultaneous measurement of strain and temperature,” IEEE Photon. Technol. Lett. 11, 105–107 (1999).
[CrossRef]

S. Kim, J. Kwon, S. Kim, B. Lee, “Temperature-independent strain sensor using chirped grating partially embedded in a glass tube,” IEEE Photon. Technol. Lett. 12, 678–680 (2000).
[CrossRef]

J. Lightwave Technol. (5)

Opt. Lett. (1)

Other (1)

A. Ezbiri, A. Munoz, S. E. Kanellopoulos, V. A. Handerek, “High resolution fiber Bragg grating sensor demodulation using a diffraction grating spectrometer and CCD detection,” in IEE Colloquium on Optical Techniques for Smart Structures and Structural Monitoring, Digest 1997/033 (Institute of Electrical Engineers, London, U.K., 1997).

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Figures (9)

Fig. 1
Fig. 1

Proposed power-detection scheme for temperature-independent strain measurement: FC, fiber coupler; FBG, fiber Bragg grating. The fiber coupler, FC 3, is used to tap out part of the input power for the purpose of power referencing. All unused fiber ends are immersed in index-matching fluids.

Fig. 2
Fig. 2

Simulated result of the strain measurement: (a) Reflection spectra of both FBGs in super-Gaussian shape. (b) Calculated detected optical power as a function of strain. Here both spectra are assumed to be 1.5 nm wide. A strain sensitivity of 0.001 nm/µS for FBG 2 is assumed. (b) Results for the case of Gaussian-shaped spectra for comparison are also shown.

Fig. 3
Fig. 3

Multipoint strain-sensing system using the power-detection scheme. The broken-line frames represent the power reading unit of Fig. 1.

Fig. 4
Fig. 4

Normalized individual reflection spectra of (a) sensor unit 1, (b) sensor unit 2, and (c) sensor unit 3.

Fig. 5
Fig. 5

Detected optical power versus the axial strain applied to (a) sensor unit 1, (b) sensor unit 2, and (c) sensor unit 3, while the other two are kept strain free. Solid squares, circles, and triangles represent the data points of sensor units 1, 2, and 3, respectively.

Fig. 6
Fig. 6

Three overlapping spectra measured at the detection port of sensor unit 1 when the sensor unit is kept at (left) 22 °C, (middle) 50 °C, and (right) 80 °C, in the condition of zero axial strain.

Fig. 7
Fig. 7

Optical powers measured at the power-detection ports of solid squares, sensor unit 1; solid squares, sensor unit 2; and, solid triangles, sensor unit 3, as a function of temperature when each sensor unit is heated alone.

Fig. 8
Fig. 8

Spectra of the broadband light source with injection currents of 27.9 and 80 mA.

Fig. 9
Fig. 9

Measured optical power versus radius of curvature of the bend of a section of fiber lead.

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