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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 19 — Sep. 23, 2013
  • pp: 22799–22807
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A time- and wavelength-division multiplexing sensor network with ultra-weak fiber Bragg gratings

Zhihui Luo, Hongqiao Wen, Huiyong Guo, and Minghong Yang  »View Author Affiliations


Optics Express, Vol. 21, Issue 19, pp. 22799-22807 (2013)
http://dx.doi.org/10.1364/OE.21.022799


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Abstract

A time- and wavelength-division multiplexing sensor network based on ultra-weak fiber Bragg gratings (FBGs) was proposed. The low insertion loss and the high multiplexing capability of the proposed sensor network were investigated through both theoretical analysis and experimental study. The demodulation system, which consists of two semiconductor optical amplifiers and one high-speed charge-coupled device module, was constructed to interrogate 2000 serial ultra-weak FBGs with peak reflectivity ranging from −47 dB to −51 dB and a spatial resolution of 2 m along an optical fiber. The distinct advantages of the proposed sensor network make it an excellent candidate for the large-scale sensing network.

© 2013 OSA

1. Introduction

Large-scale fiber Bragg grating (FBG) sensor networks have attractive prospects for major engineer monitoring because of their low cost and high multiplexing capability [1

1. W. Jin, “Multiplexed FBG sensors and their applications,” Proc. SPIE 3897, 468–479 (1999). [CrossRef]

, 2

2. C. S. Kim, T. H. Lee, Y. S. Yu, Y. G. Han, S. B. Lee, and M. Y. Jeong, “Multi-point interrogation of FBG sensors using cascaded flexible wavelength-division Sagnac loop filters,” Opt. Express 14(19), 8546–8551 (2006). [CrossRef] [PubMed]

]. Wavelength-division multiplexing (WDM) and time-division multiplexing (TDM) are two major multiplexing techniques for the expansion of the sensor network capacity [3

3. J. Ou and Z. Zhou, “Optic fiber Bragg-grating-based sensing technologies and their applications in structural health monitoring,” Proc. SPIE 6595, 01–08 (2007).

, 4

4. G. Gagliardi, M. Salza, P. Ferraro, and P. De Natale, “Fiber Bragg-grating strain sensor interrogation using laser radio-frequency modulation,” Opt. Express 13(7), 2377–2384 (2005). [CrossRef] [PubMed]

]. For the WDM method, the maximum number of FBGs is restricted by the ratio of the source spectral width over the dynamic wavelength range of an individual FBG sensor (i.e., typically a few nanometers). The TDM method utilizes different time delays between reflected pulses to distinguish sensors even with an identical wavelength and to relieve the spectral bandwidth issue. Several types of TDM networks with resonant cavity based on a semiconductor optical amplifier (SOA) have been proposed [5

5. W. H. Chung and H. Y. Tam, “Time- and wavelength-division multiplexing of FBG sensors using a semi- conductor optical amplifier in ring cavity configuration,” IEEE Photon. Technol. Lett. 17(12), 2709–2711 (2005). [CrossRef]

8

8. Y. B. Dai, Y. J. Liu, J. S. Leng, G. Deng, and A. Asundi, “A novel time- division multiplexing fiber Bragg grating sensor interrogator for structural health monitoring,” Opt. Lasers Eng. 47(10), 1028–1033 (2009). [CrossRef]

]. Limited by the gain of a single SOA, the interrogated FBGs with reflectivity of more than 1% are required. However, the multiplexing capacity is seriously limited by signal crosstalk among these normal FBGs [9

9. C. C. Chan, W. Jin, D. J. Wang, and M. S. Demokan, “Intrinsic crosstalk analysis of a serial TDM FBG sensor array by using a tunable laser,” Proc. LEOS36, 2–4(2000).

]. A newly proposed optical time-domain reflectometry–fiber Bragg grating (OTDR–FBG) network with ultra-weak FBG array has been reported [10

10. Y. M. Wang, J. M. Gong, D. Y. Wang, B. Dong, W. Bi, and A. Wang, “A quasi-distributed sensing network with time-division-multiplexed fiber Bragg gratings,” IEEE Photon. Technol. Lett. 23(2), 70–72 (2011). [CrossRef]

, 11

11. Y. M. Wang, J. M. Gong, D. Y. Wang, T. J. Shilig, and A. Wang, “A large Serial time-division multiplexed fiber Bragg grating sensor network,” J. Lightwave Technol. 30(17), 2751–2756 (2012). [CrossRef]

]. This design employs a tunable laser (T-LD) as light source and distinguishes different FBGs via high-speed data acquisition. With the high-power tunable laser, the system has powerful detection of ultra-weak FBGs. However, due to slow speed of T-LD wavelength scanning and mass redundant data from no grating zones, the system could be inferred to subject a low speed for the precise interrogation.

2. Operation principle

Figure 1
Fig. 1 Sensor network with an ultra-weak TDM + WDM-FBG array.
illustrates the TDM + WDM ultra-weak FBG sensor network. A 2-6 nm light source consists of an amplified spontaneous emission source and a band-pass filter. The light is modulated and amplified into nanosecond pulses by the first SOA (SOA1, INPHENIX IPSAD1502). The pulses pass through the circulator and are launched into the serial TDM + WDM-FBG array (G11, …, G1n, …, Gm1,…, Gmn), where G are FBGs; m is the number of sub-arrays in the entire array, each sub-array includes n FBGs with an identical wavelength, and all sub-arrays are different at the center wavelength. The total number of FBGs is m × n in such a large-scale array. Each FBG reflects part of the incident pulses back to the second SOA (SOA2, INPHENIX IPSAD1502) at different times. When the SOA2 is “on,” the pulses arriving at the SOA2 are amplified. The pulses that pass through SOA2 are detected and then demodulated by a high-speed CCD module (IBSEN I-MON 80D) to obtain their wavelength shift and intensity directly. Both SOAs are driven by two trains of homogeneous pulses from a two-channel generator programmed to adjust the time delay between pulse trains. The SOA extinction ratio of above 40 dB can improve the optical signal-to-noise ratio (OSNR) [13

13. A. V. Xabier, M. L. Sonia, C. Pedro, and G. H. Miguel, “100 km BOTDA temperature sensor with sub-meter resolution,” Proc. SPIE 8421, 842117, 842117-4 (2012). [CrossRef]

]. Furthermore, the two SOAs have gain of more than 20 dB, which can increase the signal power. The Erbium-doped fiber amplifier (EDFA) is used to amplify further the pulses. The data from the CCD detector are processed by an embedded computer and then are uploaded to a remote PC, which also controls the CCD module and the generator synchronously.

Given the different spatial positions of FBGs along the fiber, each FBG can be addressed separately by changing the time delay. The relation between the time delay (τi, 1≤in*m) and the distance (Li) of FBGi from SOA is
τi=2neLic
(1)
(τr+τf)w2nedc,
(2)
where ne is the effective refractive index of the fiber; c is the speed of light in vacuum; w is the pulse width; d is the separation distance between FBGs; τr and τf are the rise-time and the fall-time of the modulated pulse, respectively. To eliminate signal overlapping, the time delay between two adjacent sensors should be more than the pulse width. During the initialization of the sensor network, the generator scans the time delay at a step of 1 ns, which corresponds to a resolution of 0.1 meter along the fiber. The CCD module detects the reflected signal and judges whether the reflected pulses are from FBGs according to the signal intensity. The CCD module then records the time delay at the peak power of the reflected signals from each FBG. When the sensor network runs, the generator reads these recorded delays and adjusts the time delay between pulse trains one by one. All FBGs are addressed in turn. The CCD module demodulates the reflected signal from every FBG and uploads the peak wavelength. The remote PC calculates the variation of the Bragg wavelength and infers the value of the monitored measurand. On the basis of the recorded time delay, the spatial position of each FBG can be calculated by using Eq. (1).

3. Experimental results and discussion

3.1 Optical Power Budget

The reflectivity of the ultra-weak FBG is generally less than −30 dB, and its reflected signals is weak, which may cause the sensor network to work improperly or as expected. Therefore, the optical power budget for the entire sensor network is introduced. The average light power refers to the radiation energy of all wavelengths in a second. Considering the narrow bandwidth of the reflected signal from the FBG (typically less than 0.2 nm), the average power of the reflected signal is far less than the broadband noise power of the SOA. When measuring the average power, the sensing signal will be submersed by the noise. The power spectral density (PSD) describes the distribution of signal energy in the wavelength axis and expresses the relative power of the sensing wavelength directly without the effect of other wavelengths. Therefore, the PSD is proposed to analyze the sensor network. Given that P0 is the PSD of the sensing wavelength from the source, the PSD of the modulated pulses Pm is expressed as
Pm=P0+10log(w/T)=P0+10logfw
(3)
where P0 = −25 dBm; f is the modulation frequency (f = 40 KHz), and w is the pulse width (w = 16 ns). According to Eq. (3), the PSD of the modulated pulses will be 32 dB lower than the source. Figure 2
Fig. 2 PSD of the light source, modulated signal, and reflected signal.
shows the PSD of the signals before and after modulation. A 28 dB decline is observed by using an optical spectra analyzer (OSA, YOKOGAWA AQ6370), which includes a 4 dB gain from SOA1. Therefore, the tested result is in good agreement with the theoretical calculation. The detection circuit of the OSA has integral effect and can continuously accumulate the signals in the response period. Thus, the PSD of the sensing signals is given as
POSAPm+ASOA1+AEDFAPc+PFBG+ASOA2Pother,
(4)
where ASOA1 and ASOA2 are the gain of SOA1 and SOA2, respectively (ASOA1 = 4 dB; ASOA2 = 17 dB); AEDFA is the gain of EDFA (AEDFA = 28 dB); Pc is the insertion loss of the circulator (Pn = 2 dB); PFBG is the peak reflectivity of the ultra-weak FBG; Pother includes the loss of connectors (Pother = 1 dB), and POSA is the sensitivity of the OSA. When the modulated pulses with 40 kHz launch into the sensor network and the peak reflectivity of the FBGs range from −47 dB to −51 dB, the measured PSD of the reflected pulses is from −59 dBm to −64 dBm, which is close to the theoretical calculation of −58 dBm to −62 dBm. The consistent result confirms the feasibility of the proposed optical power budget.

The CCD module works similar to the OSA. Given that the sensitivity of the proposed CCD module is −60 dBm, the detectable minimum power within the response time of 20 ms can be down to −107 dBm. Therefore, the minimum reflectivity of the FBG is −96 dB as power budget analysis. However, limited by the spontaneous noise of about −70 dBm from the SOA2, the reflectivity of the FBG should be more than −59 dB. Moreover, the dynamic range of the CCD detector is about 40 dB, and the response time can be adjusted from 20 ms to 1 s to improve further the dynamic range. Therefore, the theoretical dynamic range can reach up to 87 dB.

3.2 Crosstalk and Multiplexing Capability

3.3 Interrogation Experiment

With the automated on-line writing system, two sub-arrays were fabricated in SMF-28 fiber and then spliced together. Every sub-array contained 1000 FBGs with center wavelengths of 1551.17 nm and 1552.05 nm. After 20 d of aging under the temperature of 80 °C, the peak reflectivity of the array was stable in the range of −47 dB to −51 dB. The separation distance between FBGs was 2 m. The bandwidth at −0.5 dB of the source was 2.92 nm. The modulation frequency was 40 kHz and the “on” time of SOA was 16 ns.

Single point and multi-point scanning measurement were performed to demonstrate the outstanding interrogation capability of the sensing network. The 1000th FBG was installed on the metal cantilever beam, and the strain resulting from the deflection of the free end was monitored by keeping the time delay of the 1000th FBG (see Fig. 4
Fig. 4 Dynamic monitoring of the deflection of the free end.
). The results show a measurement rate of 100 Hz for the 1000th FBG. Moreover, 2000 ultra-weak FBGs were interrogated in turns by employing the PC remote control, and the peak wavelengths of these FBGs were achieved (see Fig. 5
Fig. 5 The peak wavelengths of 2000 serial ultra-weak FBGs measured by PC remote control.
). The interrogation speed based on the PC remote control was up to 100 FBGs per second. Considering that the control signal is transmitted via a general purpose interface bus, the interrogation time was mainly spent on the instruction generation of the operating system software and the conversion of data communication protocol. In the new design, the generator will be integrated into the CCD module to eliminate the operating system and low-speed communication. The interrogation speed is expected to be up to 5000 FBGs per second, which is limited by the maximum response speed of the CCD module.

Distributed temperature was measured to investigate the sensing performance of the network. Two sections of the sensor array, containing 20 FBGs, were heated in a high-low temperature test chamber (SIDA TEMI300), whereas the rest of the FBG sensors were kept at a room temperature of 25 °С [14

14. M. L. Zhang, Q. Z. Sun, Z. Wang, X. Li, H. Liu, and D. Liu, “A Large Capacity Sensing Network with Identical Weak Fiber Bragg Gratings Multiplexing,” Opt. Commun. 285(13-14), 3082–3087 (2012). [CrossRef]

, 15

15. Z. Wang, Q. Z. Sun, and M. L. Zhang, “A Distributed Sensing System Based on Low-Reflective-Index Bragg Gratings,” in Proceedings of Photonics and Optoelectronics (SOPO), Wuhan, 1-3(2011).

]. The chamber temperature was increased from 25 °С to 90 °С at a step of 5 °С and accuracy of 0.1 °С. At each step, the temperature was first kept for half an hour to ensure the accuracy of 0.1 °С and was then measured. Figure 7
Fig. 7 Temperature measurement results of the 2000-sensor array.
shows the result of temperature measurement. The peak wavelengths of the heated FBGs shifted with increasing temperature. Figure 8
Fig. 8 Wavelength shift versus temperature change.
shows the relationship between the wavelength shift and the temperature. The measurement data of each FBG was fitted to obtain its temperature sensitivity, and the fitted coefficients are within the range of 10.2 pm/°С to 10.8 pm/°С. This finding is similar to the temperature characteristic of the ordinary grating. The variation is mainly caused by the measurement errors determined by the SNR of ultra-weak FBGs.

4. Conclusion

A novel large-scale ultra-weak FBG sensor network with two SOAs and one high-speed CCD module has been proposed and demonstrated. Results show that the proposed sensor network can interrogate the large-scale array at the speed of 100 FBGs per second and contain over 2000 FBGs by TDM + WDM. The pioneering design has low cost, weak crosstalk and good sensing characteristic. Therefore, the proposed design has a promising future for sensor application, in which large numbers of FBGs are needed.

Acknowledgments

This work was supported in part by the National Science Foundation of China, NSFC (Grant No. 61205070), and the Major Program of the National Natural Science Foundation of China, NSFC (Grant No. 61290311).

References and links

1.

W. Jin, “Multiplexed FBG sensors and their applications,” Proc. SPIE 3897, 468–479 (1999). [CrossRef]

2.

C. S. Kim, T. H. Lee, Y. S. Yu, Y. G. Han, S. B. Lee, and M. Y. Jeong, “Multi-point interrogation of FBG sensors using cascaded flexible wavelength-division Sagnac loop filters,” Opt. Express 14(19), 8546–8551 (2006). [CrossRef] [PubMed]

3.

J. Ou and Z. Zhou, “Optic fiber Bragg-grating-based sensing technologies and their applications in structural health monitoring,” Proc. SPIE 6595, 01–08 (2007).

4.

G. Gagliardi, M. Salza, P. Ferraro, and P. De Natale, “Fiber Bragg-grating strain sensor interrogation using laser radio-frequency modulation,” Opt. Express 13(7), 2377–2384 (2005). [CrossRef] [PubMed]

5.

W. H. Chung and H. Y. Tam, “Time- and wavelength-division multiplexing of FBG sensors using a semi- conductor optical amplifier in ring cavity configuration,” IEEE Photon. Technol. Lett. 17(12), 2709–2711 (2005). [CrossRef]

6.

M. Y. Jeon, J. Zhang, Q. Wang, and Z. Chen, “High-speed and wide bandwidth Fourier domain mode-locked wavelength swept laser with multiple SOAs,” Opt. Express 16(4), 2547–2554 (2008). [CrossRef] [PubMed]

7.

G. D. Lloyd, L. Bennion, L. A. Everall, and K. Sugden, “Novel resonant cavity TDM demodulation scheme for FBG sensing,” in Proceedings of Lasers and Electro-Optics, San Francisco, CA, CWD4(2004).

8.

Y. B. Dai, Y. J. Liu, J. S. Leng, G. Deng, and A. Asundi, “A novel time- division multiplexing fiber Bragg grating sensor interrogator for structural health monitoring,” Opt. Lasers Eng. 47(10), 1028–1033 (2009). [CrossRef]

9.

C. C. Chan, W. Jin, D. J. Wang, and M. S. Demokan, “Intrinsic crosstalk analysis of a serial TDM FBG sensor array by using a tunable laser,” Proc. LEOS36, 2–4(2000).

10.

Y. M. Wang, J. M. Gong, D. Y. Wang, B. Dong, W. Bi, and A. Wang, “A quasi-distributed sensing network with time-division-multiplexed fiber Bragg gratings,” IEEE Photon. Technol. Lett. 23(2), 70–72 (2011). [CrossRef]

11.

Y. M. Wang, J. M. Gong, D. Y. Wang, T. J. Shilig, and A. Wang, “A large Serial time-division multiplexed fiber Bragg grating sensor network,” J. Lightwave Technol. 30(17), 2751–2756 (2012). [CrossRef]

12.

H. Y. Guo, J. G. Tang, X. F. Li, Y. Zheng, and H. F. Yu, “On-line writing weak fiber Bragg gratings array,” Chin. Opt. Lett. 11(3), 030602–030605 (2013). [CrossRef]

13.

A. V. Xabier, M. L. Sonia, C. Pedro, and G. H. Miguel, “100 km BOTDA temperature sensor with sub-meter resolution,” Proc. SPIE 8421, 842117, 842117-4 (2012). [CrossRef]

14.

M. L. Zhang, Q. Z. Sun, Z. Wang, X. Li, H. Liu, and D. Liu, “A Large Capacity Sensing Network with Identical Weak Fiber Bragg Gratings Multiplexing,” Opt. Commun. 285(13-14), 3082–3087 (2012). [CrossRef]

15.

Z. Wang, Q. Z. Sun, and M. L. Zhang, “A Distributed Sensing System Based on Low-Reflective-Index Bragg Gratings,” in Proceedings of Photonics and Optoelectronics (SOPO), Wuhan, 1-3(2011).

OCIS Codes
(050.2770) Diffraction and gratings : Gratings
(060.4230) Fiber optics and optical communications : Multiplexing
(060.4250) Fiber optics and optical communications : Networks
(060.3738) Fiber optics and optical communications : Fiber Bragg gratings, photosensitivity

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: July 17, 2013
Revised Manuscript: September 12, 2013
Manuscript Accepted: September 16, 2013
Published: September 20, 2013

Citation
Zhihui Luo, Hongqiao Wen, Huiyong Guo, and Minghong Yang, "A time- and wavelength-division multiplexing sensor network with ultra-weak fiber Bragg gratings," Opt. Express 21, 22799-22807 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-19-22799


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References

  1. W. Jin, “Multiplexed FBG sensors and their applications,” Proc. SPIE3897, 468–479 (1999). [CrossRef]
  2. C. S. Kim, T. H. Lee, Y. S. Yu, Y. G. Han, S. B. Lee, and M. Y. Jeong, “Multi-point interrogation of FBG sensors using cascaded flexible wavelength-division Sagnac loop filters,” Opt. Express14(19), 8546–8551 (2006). [CrossRef] [PubMed]
  3. J. Ou and Z. Zhou, “Optic fiber Bragg-grating-based sensing technologies and their applications in structural health monitoring,” Proc. SPIE6595, 01–08 (2007).
  4. G. Gagliardi, M. Salza, P. Ferraro, and P. De Natale, “Fiber Bragg-grating strain sensor interrogation using laser radio-frequency modulation,” Opt. Express13(7), 2377–2384 (2005). [CrossRef] [PubMed]
  5. W. H. Chung and H. Y. Tam, “Time- and wavelength-division multiplexing of FBG sensors using a semi- conductor optical amplifier in ring cavity configuration,” IEEE Photon. Technol. Lett.17(12), 2709–2711 (2005). [CrossRef]
  6. M. Y. Jeon, J. Zhang, Q. Wang, and Z. Chen, “High-speed and wide bandwidth Fourier domain mode-locked wavelength swept laser with multiple SOAs,” Opt. Express16(4), 2547–2554 (2008). [CrossRef] [PubMed]
  7. G. D. Lloyd, L. Bennion, L. A. Everall, and K. Sugden, “Novel resonant cavity TDM demodulation scheme for FBG sensing,” in Proceedings of Lasers and Electro-Optics, San Francisco, CA, CWD4(2004).
  8. Y. B. Dai, Y. J. Liu, J. S. Leng, G. Deng, and A. Asundi, “A novel time- division multiplexing fiber Bragg grating sensor interrogator for structural health monitoring,” Opt. Lasers Eng.47(10), 1028–1033 (2009). [CrossRef]
  9. C. C. Chan, W. Jin, D. J. Wang, and M. S. Demokan, “Intrinsic crosstalk analysis of a serial TDM FBG sensor array by using a tunable laser,” Proc. LEOS36, 2–4(2000).
  10. Y. M. Wang, J. M. Gong, D. Y. Wang, B. Dong, W. Bi, and A. Wang, “A quasi-distributed sensing network with time-division-multiplexed fiber Bragg gratings,” IEEE Photon. Technol. Lett.23(2), 70–72 (2011). [CrossRef]
  11. Y. M. Wang, J. M. Gong, D. Y. Wang, T. J. Shilig, and A. Wang, “A large Serial time-division multiplexed fiber Bragg grating sensor network,” J. Lightwave Technol.30(17), 2751–2756 (2012). [CrossRef]
  12. H. Y. Guo, J. G. Tang, X. F. Li, Y. Zheng, and H. F. Yu, “On-line writing weak fiber Bragg gratings array,” Chin. Opt. Lett.11(3), 030602–030605 (2013). [CrossRef]
  13. A. V. Xabier, M. L. Sonia, C. Pedro, and G. H. Miguel, “100 km BOTDA temperature sensor with sub-meter resolution,” Proc. SPIE8421, 842117, 842117-4 (2012). [CrossRef]
  14. M. L. Zhang, Q. Z. Sun, Z. Wang, X. Li, H. Liu, and D. Liu, “A Large Capacity Sensing Network with Identical Weak Fiber Bragg Gratings Multiplexing,” Opt. Commun.285(13-14), 3082–3087 (2012). [CrossRef]
  15. Z. Wang, Q. Z. Sun, and M. L. Zhang, “A Distributed Sensing System Based on Low-Reflective-Index Bragg Gratings,” in Proceedings of Photonics and Optoelectronics (SOPO), Wuhan, 1-3(2011).

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