Abstract

A fiber Bragg grating (FBG) vibroacoustic sensor exploiting an intensity-based interrogation principle is presented. The optical system is complemented by signal processing techniques that allow disturbances to be mitigated and improve the spectral estimation. The sensor is capable of performing frequency analysis of sounds up to 3kHz, with top sensitivity in the 100500Hz frequency range, and of dynamically tracking pulsed phenomena that induce a strain to the FBG. The sensor has been applied to the detection of voice, showing a great intelligibility of the speech despite the low-quality environment, and to the monitoring of the heartbeat rate from the wrist.

© 2008 Optical Society of America

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  1. A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlane, K. P. Koo, C. G. Askins, M. A. Putnam, and E. J. Friebele, “Fiber grating sensors,” J. Lightwave Technol. 15, 1442-1463 (1997).
    [CrossRef]
  2. Y. J. Rao, “In-fibre Bragg grating sensors,” Meas. Sci. Technol. 8, 355-375 (1997).
    [CrossRef]
  3. L. Mohanty, L. M. Koh, and S. C. Tjin, “Fiber Bragg grating microphone system,” Appl. Phys. Lett. 89, 161109 (2006).
    [CrossRef]
  4. T. Iida, K. Nakamura, and S. Ueha, “A microphone array using fiber Bragg gratings,” in 15th Optical Fiber Sensors Conference Technical Digest, OFS 2002 (IEEE, 2002), Vol. 1, pp. 239-242.
    [CrossRef]
  5. D. Gurkan, D. Starodubov, and X. Yuan, “Monitoring of the heartbeat sounds using an optical fiber Bragg grating sensor,” in 2005 IEEE Sensors (IEEE, 2005).
    [CrossRef]
  6. F. A. Bezombes, M. J. Lalor, and D. R. Burton, “Contact microphone using optical fibre Bragg grating technology,” J. Phys. Conf. Ser. 76, 012017 (2007).
    [CrossRef]
  7. A. Cusano, S. D'Addio, A. Cutolo, M. Giordano, S. Campopiano, M. Balbi, and S. Balzarini, “Plastic coated fiber Bragg gratings as high sensitivity hydrophones”, in 5th IEEE Conference on Sensors, 2006 (IEEE, 2007), pp. 166-169.
  8. S. Haykin, Adaptive Filter Theory (Prentice Hall, 2001).
  9. S. Benedetto and E. Biglieri, Principles of Digital Transmission with Wireless Applications (Kluwer Academic, 1999), pp. 381-426.
  10. W. W. Morey, “Distributed fibre grating sensors,” in Proceedings of the 7th Optical Fiber Sensor Conference (Institution of Radio and Electronics Engineers, 1997), pp. 285-288.
  11. A. Wilson, S. W. James, and R. P. Tatam, “Time-division-multiplexed interrogation of fibre Bragg grating sensors using laser diodes,” Meas. Sci. Technol. 12,181-187 (2001).
    [CrossRef]
  12. A. Othonos and K. Kalli, “Properties of fiber Bragg gratings,” in Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and Sensing (Artech House, 1999), pp. 95-147.
  13. G. Meltz and W. W. Morey, “Bragg grating formation and germanosilicate fiber photosensitivity,” Proc. SPIE 1516, 185-199 (1991).
    [CrossRef]
  14. D. Tosi, M. Olivero, and G. Perrone, “Self-mixing based interrogation techniques for high-reflectivity fibre Bragg grating sensors,” Electron. Lett. 44, 405-406 (2008).
    [CrossRef]
  15. A. Zaknich, Principles of Adaptive Filters and Self-Learning Systems (Springer, 2005).
  16. A. V. Oppenheim and R. W. Schafer, “Power spectrum estimation,” in Digital Signal Processing (Prentice Hall, 1997), pp. 532-571.
  17. J. Capon, “High-resolution frequency-wavenumber spectrum analysis,” Proc. IEEE 57, 1408-1418 (1969).
    [CrossRef]
  18. T. Ekman, A. Jakobsson, and P. Stoica, “On efficient implementation of the Capon algorithm,” in Proceedings of the Tenth European Signal Processing Conference (European Association for Signal Processing, 2000), pp. 1221-1224.
  19. EMusic Institute, Inc., “Metronome online,” http://www.metronomeonline.com.
  20. D. Seidner, “Polyphase vs. classical aliasing analysis in enlargements,” in Proceedings of the Eighth International Symposium on Signal Processing and Its Applications, 2005 (IEEE, 2005), Vol. 1, pp. 171-174.
    [CrossRef]

2008 (1)

D. Tosi, M. Olivero, and G. Perrone, “Self-mixing based interrogation techniques for high-reflectivity fibre Bragg grating sensors,” Electron. Lett. 44, 405-406 (2008).
[CrossRef]

2007 (1)

F. A. Bezombes, M. J. Lalor, and D. R. Burton, “Contact microphone using optical fibre Bragg grating technology,” J. Phys. Conf. Ser. 76, 012017 (2007).
[CrossRef]

2006 (1)

L. Mohanty, L. M. Koh, and S. C. Tjin, “Fiber Bragg grating microphone system,” Appl. Phys. Lett. 89, 161109 (2006).
[CrossRef]

2001 (1)

A. Wilson, S. W. James, and R. P. Tatam, “Time-division-multiplexed interrogation of fibre Bragg grating sensors using laser diodes,” Meas. Sci. Technol. 12,181-187 (2001).
[CrossRef]

1997 (2)

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

Y. J. Rao, “In-fibre Bragg grating sensors,” Meas. Sci. Technol. 8, 355-375 (1997).
[CrossRef]

1991 (1)

G. Meltz and W. W. Morey, “Bragg grating formation and germanosilicate fiber photosensitivity,” Proc. SPIE 1516, 185-199 (1991).
[CrossRef]

1969 (1)

J. Capon, “High-resolution frequency-wavenumber spectrum analysis,” Proc. IEEE 57, 1408-1418 (1969).
[CrossRef]

Askins, C. G.

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

Balbi, M.

A. Cusano, S. D'Addio, A. Cutolo, M. Giordano, S. Campopiano, M. Balbi, and S. Balzarini, “Plastic coated fiber Bragg gratings as high sensitivity hydrophones”, in 5th IEEE Conference on Sensors, 2006 (IEEE, 2007), pp. 166-169.

Balzarini, S.

A. Cusano, S. D'Addio, A. Cutolo, M. Giordano, S. Campopiano, M. Balbi, and S. Balzarini, “Plastic coated fiber Bragg gratings as high sensitivity hydrophones”, in 5th IEEE Conference on Sensors, 2006 (IEEE, 2007), pp. 166-169.

Benedetto, S.

S. Benedetto and E. Biglieri, Principles of Digital Transmission with Wireless Applications (Kluwer Academic, 1999), pp. 381-426.

Bezombes, F. A.

F. A. Bezombes, M. J. Lalor, and D. R. Burton, “Contact microphone using optical fibre Bragg grating technology,” J. Phys. Conf. Ser. 76, 012017 (2007).
[CrossRef]

Biglieri, E.

S. Benedetto and E. Biglieri, Principles of Digital Transmission with Wireless Applications (Kluwer Academic, 1999), pp. 381-426.

Burton, D. R.

F. A. Bezombes, M. J. Lalor, and D. R. Burton, “Contact microphone using optical fibre Bragg grating technology,” J. Phys. Conf. Ser. 76, 012017 (2007).
[CrossRef]

Campopiano, S.

A. Cusano, S. D'Addio, A. Cutolo, M. Giordano, S. Campopiano, M. Balbi, and S. Balzarini, “Plastic coated fiber Bragg gratings as high sensitivity hydrophones”, in 5th IEEE Conference on Sensors, 2006 (IEEE, 2007), pp. 166-169.

Capon, J.

J. Capon, “High-resolution frequency-wavenumber spectrum analysis,” Proc. IEEE 57, 1408-1418 (1969).
[CrossRef]

Cusano, A.

A. Cusano, S. D'Addio, A. Cutolo, M. Giordano, S. Campopiano, M. Balbi, and S. Balzarini, “Plastic coated fiber Bragg gratings as high sensitivity hydrophones”, in 5th IEEE Conference on Sensors, 2006 (IEEE, 2007), pp. 166-169.

Cutolo, A.

A. Cusano, S. D'Addio, A. Cutolo, M. Giordano, S. Campopiano, M. Balbi, and S. Balzarini, “Plastic coated fiber Bragg gratings as high sensitivity hydrophones”, in 5th IEEE Conference on Sensors, 2006 (IEEE, 2007), pp. 166-169.

D'Addio, S.

A. Cusano, S. D'Addio, A. Cutolo, M. Giordano, S. Campopiano, M. Balbi, and S. Balzarini, “Plastic coated fiber Bragg gratings as high sensitivity hydrophones”, in 5th IEEE Conference on Sensors, 2006 (IEEE, 2007), pp. 166-169.

Davis, M. A.

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

Ekman, T.

T. Ekman, A. Jakobsson, and P. Stoica, “On efficient implementation of the Capon algorithm,” in Proceedings of the Tenth European Signal Processing Conference (European Association for Signal Processing, 2000), pp. 1221-1224.

Friebele, E. J.

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

Giordano, M.

A. Cusano, S. D'Addio, A. Cutolo, M. Giordano, S. Campopiano, M. Balbi, and S. Balzarini, “Plastic coated fiber Bragg gratings as high sensitivity hydrophones”, in 5th IEEE Conference on Sensors, 2006 (IEEE, 2007), pp. 166-169.

Gurkan, D.

D. Gurkan, D. Starodubov, and X. Yuan, “Monitoring of the heartbeat sounds using an optical fiber Bragg grating sensor,” in 2005 IEEE Sensors (IEEE, 2005).
[CrossRef]

Haykin, S.

S. Haykin, Adaptive Filter Theory (Prentice Hall, 2001).

Iida, T.

T. Iida, K. Nakamura, and S. Ueha, “A microphone array using fiber Bragg gratings,” in 15th Optical Fiber Sensors Conference Technical Digest, OFS 2002 (IEEE, 2002), Vol. 1, pp. 239-242.
[CrossRef]

Jakobsson, A.

T. Ekman, A. Jakobsson, and P. Stoica, “On efficient implementation of the Capon algorithm,” in Proceedings of the Tenth European Signal Processing Conference (European Association for Signal Processing, 2000), pp. 1221-1224.

James, S. W.

A. Wilson, S. W. James, and R. P. Tatam, “Time-division-multiplexed interrogation of fibre Bragg grating sensors using laser diodes,” Meas. Sci. Technol. 12,181-187 (2001).
[CrossRef]

Kalli, K.

A. Othonos and K. Kalli, “Properties of fiber Bragg gratings,” in Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and Sensing (Artech House, 1999), pp. 95-147.

Kersey, A. D.

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

Koh, L. M.

L. Mohanty, L. M. Koh, and S. C. Tjin, “Fiber Bragg grating microphone system,” Appl. Phys. Lett. 89, 161109 (2006).
[CrossRef]

Koo, K. P.

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

Lalor, M. J.

F. A. Bezombes, M. J. Lalor, and D. R. Burton, “Contact microphone using optical fibre Bragg grating technology,” J. Phys. Conf. Ser. 76, 012017 (2007).
[CrossRef]

LeBlane, M.

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

Meltz, G.

G. Meltz and W. W. Morey, “Bragg grating formation and germanosilicate fiber photosensitivity,” Proc. SPIE 1516, 185-199 (1991).
[CrossRef]

Mohanty, L.

L. Mohanty, L. M. Koh, and S. C. Tjin, “Fiber Bragg grating microphone system,” Appl. Phys. Lett. 89, 161109 (2006).
[CrossRef]

Morey, W. W.

G. Meltz and W. W. Morey, “Bragg grating formation and germanosilicate fiber photosensitivity,” Proc. SPIE 1516, 185-199 (1991).
[CrossRef]

W. W. Morey, “Distributed fibre grating sensors,” in Proceedings of the 7th Optical Fiber Sensor Conference (Institution of Radio and Electronics Engineers, 1997), pp. 285-288.

Nakamura, K.

T. Iida, K. Nakamura, and S. Ueha, “A microphone array using fiber Bragg gratings,” in 15th Optical Fiber Sensors Conference Technical Digest, OFS 2002 (IEEE, 2002), Vol. 1, pp. 239-242.
[CrossRef]

Olivero, M.

D. Tosi, M. Olivero, and G. Perrone, “Self-mixing based interrogation techniques for high-reflectivity fibre Bragg grating sensors,” Electron. Lett. 44, 405-406 (2008).
[CrossRef]

Oppenheim, A. V.

A. V. Oppenheim and R. W. Schafer, “Power spectrum estimation,” in Digital Signal Processing (Prentice Hall, 1997), pp. 532-571.

Othonos, A.

A. Othonos and K. Kalli, “Properties of fiber Bragg gratings,” in Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and Sensing (Artech House, 1999), pp. 95-147.

Patrick, H. J.

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

Perrone, G.

D. Tosi, M. Olivero, and G. Perrone, “Self-mixing based interrogation techniques for high-reflectivity fibre Bragg grating sensors,” Electron. Lett. 44, 405-406 (2008).
[CrossRef]

Putnam, M. A.

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

Rao, Y. J.

Y. J. Rao, “In-fibre Bragg grating sensors,” Meas. Sci. Technol. 8, 355-375 (1997).
[CrossRef]

Schafer, R. W.

A. V. Oppenheim and R. W. Schafer, “Power spectrum estimation,” in Digital Signal Processing (Prentice Hall, 1997), pp. 532-571.

Seidner, D.

D. Seidner, “Polyphase vs. classical aliasing analysis in enlargements,” in Proceedings of the Eighth International Symposium on Signal Processing and Its Applications, 2005 (IEEE, 2005), Vol. 1, pp. 171-174.
[CrossRef]

Starodubov, D.

D. Gurkan, D. Starodubov, and X. Yuan, “Monitoring of the heartbeat sounds using an optical fiber Bragg grating sensor,” in 2005 IEEE Sensors (IEEE, 2005).
[CrossRef]

Stoica, P.

T. Ekman, A. Jakobsson, and P. Stoica, “On efficient implementation of the Capon algorithm,” in Proceedings of the Tenth European Signal Processing Conference (European Association for Signal Processing, 2000), pp. 1221-1224.

Tatam, R. P.

A. Wilson, S. W. James, and R. P. Tatam, “Time-division-multiplexed interrogation of fibre Bragg grating sensors using laser diodes,” Meas. Sci. Technol. 12,181-187 (2001).
[CrossRef]

Tjin, S. C.

L. Mohanty, L. M. Koh, and S. C. Tjin, “Fiber Bragg grating microphone system,” Appl. Phys. Lett. 89, 161109 (2006).
[CrossRef]

Tosi, D.

D. Tosi, M. Olivero, and G. Perrone, “Self-mixing based interrogation techniques for high-reflectivity fibre Bragg grating sensors,” Electron. Lett. 44, 405-406 (2008).
[CrossRef]

Ueha, S.

T. Iida, K. Nakamura, and S. Ueha, “A microphone array using fiber Bragg gratings,” in 15th Optical Fiber Sensors Conference Technical Digest, OFS 2002 (IEEE, 2002), Vol. 1, pp. 239-242.
[CrossRef]

Wilson, A.

A. Wilson, S. W. James, and R. P. Tatam, “Time-division-multiplexed interrogation of fibre Bragg grating sensors using laser diodes,” Meas. Sci. Technol. 12,181-187 (2001).
[CrossRef]

Yuan, X.

D. Gurkan, D. Starodubov, and X. Yuan, “Monitoring of the heartbeat sounds using an optical fiber Bragg grating sensor,” in 2005 IEEE Sensors (IEEE, 2005).
[CrossRef]

Zaknich, A.

A. Zaknich, Principles of Adaptive Filters and Self-Learning Systems (Springer, 2005).

Appl. Phys. Lett. (1)

L. Mohanty, L. M. Koh, and S. C. Tjin, “Fiber Bragg grating microphone system,” Appl. Phys. Lett. 89, 161109 (2006).
[CrossRef]

Electron. Lett. (1)

D. Tosi, M. Olivero, and G. Perrone, “Self-mixing based interrogation techniques for high-reflectivity fibre Bragg grating sensors,” Electron. Lett. 44, 405-406 (2008).
[CrossRef]

J. Lightwave Technol. (1)

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

J. Phys. Conf. Ser. (1)

F. A. Bezombes, M. J. Lalor, and D. R. Burton, “Contact microphone using optical fibre Bragg grating technology,” J. Phys. Conf. Ser. 76, 012017 (2007).
[CrossRef]

Meas. Sci. Technol. (2)

Y. J. Rao, “In-fibre Bragg grating sensors,” Meas. Sci. Technol. 8, 355-375 (1997).
[CrossRef]

A. Wilson, S. W. James, and R. P. Tatam, “Time-division-multiplexed interrogation of fibre Bragg grating sensors using laser diodes,” Meas. Sci. Technol. 12,181-187 (2001).
[CrossRef]

Proc. IEEE (1)

J. Capon, “High-resolution frequency-wavenumber spectrum analysis,” Proc. IEEE 57, 1408-1418 (1969).
[CrossRef]

Proc. SPIE (1)

G. Meltz and W. W. Morey, “Bragg grating formation and germanosilicate fiber photosensitivity,” Proc. SPIE 1516, 185-199 (1991).
[CrossRef]

Other (12)

T. Ekman, A. Jakobsson, and P. Stoica, “On efficient implementation of the Capon algorithm,” in Proceedings of the Tenth European Signal Processing Conference (European Association for Signal Processing, 2000), pp. 1221-1224.

EMusic Institute, Inc., “Metronome online,” http://www.metronomeonline.com.

D. Seidner, “Polyphase vs. classical aliasing analysis in enlargements,” in Proceedings of the Eighth International Symposium on Signal Processing and Its Applications, 2005 (IEEE, 2005), Vol. 1, pp. 171-174.
[CrossRef]

A. Othonos and K. Kalli, “Properties of fiber Bragg gratings,” in Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and Sensing (Artech House, 1999), pp. 95-147.

A. Zaknich, Principles of Adaptive Filters and Self-Learning Systems (Springer, 2005).

A. V. Oppenheim and R. W. Schafer, “Power spectrum estimation,” in Digital Signal Processing (Prentice Hall, 1997), pp. 532-571.

T. Iida, K. Nakamura, and S. Ueha, “A microphone array using fiber Bragg gratings,” in 15th Optical Fiber Sensors Conference Technical Digest, OFS 2002 (IEEE, 2002), Vol. 1, pp. 239-242.
[CrossRef]

D. Gurkan, D. Starodubov, and X. Yuan, “Monitoring of the heartbeat sounds using an optical fiber Bragg grating sensor,” in 2005 IEEE Sensors (IEEE, 2005).
[CrossRef]

A. Cusano, S. D'Addio, A. Cutolo, M. Giordano, S. Campopiano, M. Balbi, and S. Balzarini, “Plastic coated fiber Bragg gratings as high sensitivity hydrophones”, in 5th IEEE Conference on Sensors, 2006 (IEEE, 2007), pp. 166-169.

S. Haykin, Adaptive Filter Theory (Prentice Hall, 2001).

S. Benedetto and E. Biglieri, Principles of Digital Transmission with Wireless Applications (Kluwer Academic, 1999), pp. 381-426.

W. W. Morey, “Distributed fibre grating sensors,” in Proceedings of the 7th Optical Fiber Sensor Conference (Institution of Radio and Electronics Engineers, 1997), pp. 285-288.

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Figures (12)

Fig. 1
Fig. 1

Schematic of the FBG interrogation setup: DAQ, data acquisition; PD, photodiode.

Fig. 2
Fig. 2

Application of signal processing techniques to the optical sensor.

Fig. 3
Fig. 3

Preliminary feasibility analysis of adaptive filters: (a) time plot of the autocorrelation r x ( t ) of a typical input signal x ( t ) in the absence of sounds; (b) modulus of the eigenvalues of the size 80 covariance matrix estimate R ̲ X X of the input process.

Fig. 4
Fig. 4

Time plot of the acquired x ( t ) , filtered y ( t ) , and desired d ( t ) signals during the training phase (i.e., without input sounds), applying a RLS filter with 60 taps.

Fig. 5
Fig. 5

Estimated PSD of a 370 Hz pure tone, applying a 5-tap and a 60-tap RLS adaptive filter, normalized to the 370 Hz peak value.

Fig. 6
Fig. 6

Performance of the MVE estimator. (a) Comparison of MVE, zero-padding FFT, and Welch periodogram applied to the 370 Hz reference tone. (b) Estimated PSD of a sinusoidal signal with numeric frequency 0.3 embedded into Gaussian noise with different SNRs, computed with the MVE filters with 500 taps. (c) Computation time (CT) of the MVE algorithm as a function of the number of taps, input signal length (N) and number of frequency points (F).

Fig. 7
Fig. 7

(a) Time plot of the signals acquired and processed by the FBG interrogator, for five different frequencies; (b) estimated spectra, in linear units.

Fig. 8
Fig. 8

Frequency response of the combination of the optical microphone and the loudspeaker, in the 0 2500 Hz frequency range.

Fig. 9
Fig. 9

Detection of vocal sound through the optical sensor. The graph reports the 100 1300 Hz MVE spectrum estimation of a sample voice saying “Hello” and the correspondent sound acquired and RLS-filtered by the FBG sensor. The input sound has been also recorded by a standard microphone in 8   bit quality.

Fig. 10
Fig. 10

Estimated spectrum of a sample speech (gray dashed curves) is compared with the corresponding FBG sensor output (black solid curves); the spectrum is divided into three bandwidths ( 300 700 , 700 1100 , 1100 1500 Hz ).

Fig. 11
Fig. 11

Estimated spectrum of the metronome repetition frequency with 92, 112, and 144 bpm .

Fig. 12
Fig. 12

Estimated spectrum of the heartbeat measured with the FBG sensor.

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