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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 2 — Jan. 23, 2006
  • pp: 556–561
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Novel strain- and temperature-sensing mechanism based on dynamic grating in polarization-maintaining erbium-doped fiber

Xinyu Fan, Zuyuan He, and Kazuo Hotate  »View Author Affiliations


Optics Express, Vol. 14, Issue 2, pp. 556-561 (2006)
http://dx.doi.org/10.1364/OPEX.14.000556


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Abstract

The first experimental observation of a dynamic grating in polarization-maintaining erbium-doped fiber (PM-EDF) is reported and a novel fiber-optic strain- and temperature-sensing mechanism based on the dynamic grating in PM-EDF is demonstrated experimentally. The dynamic grating is written with light beams in one primary polarization axis of the PM-EDF, and read with a light beam in the other primary polarization axis. The readout Bragg reflection wavelength of the grating differs from the writing wavelength and the wavelength difference is proportional to the birefringence between the two polarization axes. Making use of the dependence of the birefringence on strain or temperature, strain- and temperature-sensing is realized by measuring the Bragg reflection wavelength (frequency) shift. In order to detect the weak reflection from the dynamic grating, a dual-stage synchronous detection scheme is adopted in the experiment. The results show a strain-sensitivity of 1.4 MHz/με and a temperature-sensitivity of 60 MHz/°C, respectively.

© 2006 Optical Society of America

1. Introduction

Distributed fiber-optic strain sensors and temperature sensors have become important devices for monitoring the condition of structures and structural materials with their capabilities of spatially continuous measurement. A typical technique for distributed fiber optic sensors is based on the stimulated Brillouin scattering (SBS) process, such as the Brillouin optical time-domain analysis (BOTDA) [1

1. T. Horiguchi, K. Shimizu, T. Kurashima, M. Taneda, and Y. Koyamada, “Development of a distributed sensing technique using Brillouin scattering,” J. Lightwave Technol. 13, 1296–1302 (1995). [CrossRef]

] and the Brillouin optical correlation-domain analysis (BOCDA) [2

2. K. Hotate and M. Tanaka, “Distributed fiber Brillouin strain sensing with 1-cm spatial resolution by correlation-based continuous-wave technique,” IEEE Photonics Technol. Lett. 14, 179–181 (2002). [CrossRef]

]. Recently, we have proposed a novel scheme based on forming a dynamic grating in polarization-maintaining erbium-doped fiber (PM-EDF) [3

3. X. Fan, Z. He, and K. Hotate, “Novel distributed fiber-optic strain sensor by localizing dynamic grating in polarization-maintaining erbium-doped fiber: proposal and theoretical analysis,” Jpn. J. App. Phys. 44, 1101–1106 (2005). [CrossRef]

]. In this scheme, the dynamic grating is written with light beams in one primary polarization axis of the PM-EDF, and read with a light beam in the other primary polarization axis. The readout Bragg reflection wavelength of the grating differs from the writing wavelength and the wavelength difference is proportional to the birefringence of two polarization axes. Strain- and temperature- sensing can be realized by measuring the Bragg reflection wavelength (frequency) shift, since the birefringence is sensitive to the strain or temperature on the fiber. The dynamic grating is localized and swept along the fiber by using the technique of synthesis of optical coherence function (SOCF) [4

4. Z. He and K. Hotate, “Distributed fiber optic stress location measurement by arbitrary shaping of optical coherence function,” J. Lightwave Technol. 20, 1715–1723 (2002). [CrossRef]

] to realize fully distributed sensing. It has been predicted by simulation that the performance of strain-sensitivity and temperature-sensitivity comparable or even better than SBS-based technology is expected with the new scheme [3

3. X. Fan, Z. He, and K. Hotate, “Novel distributed fiber-optic strain sensor by localizing dynamic grating in polarization-maintaining erbium-doped fiber: proposal and theoretical analysis,” Jpn. J. App. Phys. 44, 1101–1106 (2005). [CrossRef]

].

However, dynamic gratings reported up to date are all formed in single mode EDF [5–8

5. S. J. Frisken, “Transient Bragg reflection gratings in erbium-doped fiber amplifiers,” Opt. Lett. 17, 1776–1778 (1992). [CrossRef] [PubMed]

]. These gratings’ Bragg reflection wavelength is just the same as the wavelength of the lightwaves that form the grating. In this letter, we report, for the first time to the best of our knowledge, a successful experimental observation of the dynamic grating in PM-EDF, and demonstrate a novel fiber-optic strain- and temperature-sensing mechanism based on the grating.

2. Principle

As shown schematically in Fig. 1, when two counter-propagating coherent light beams (referred to as writing beams hereafter) are launched into a pumped erbium-doped fiber (EDF), they interfere to each other and form stationary interference fringes in the fiber. The interference fringes create a periodical gain structure per the phenomenon of gain saturation and hence produce a dynamic grating in the EDF [5–8

5. S. J. Frisken, “Transient Bragg reflection gratings in erbium-doped fiber amplifiers,” Opt. Lett. 17, 1776–1778 (1992). [CrossRef] [PubMed]

]. The period of the grating is the half of the writing beam’s wavelength in the fiber. When a third beam is launched into the fiber (referred to as reading beam hereafter), it is reflected by the dynamic grating when its optical frequency (wavelength) is the same as the writing beams’. In other words, the reading beam is reflected when it satisfies the Bragg condition of the dynamic grating.

Here, we form the dynamic grating in PM-EDF. As shown in Fig. 2, the polarization direction of the writing beams is in one primary polarization axis (x-axis) of the PM-EDF, and the reading beam’s in the other primary polarization axis (y-axis). Due to the birefringence between these two primary polarization axes, the Bragg reflection frequency is different from that of the writing beams. The Bragg frequency is given by

Fig. 1. Schematic of dynamic grating. (a) Writing of dynamic grating. (b) Interference pattern along the EDF.
Fig. 2. Writing and reading the dynamic grating in PM-EDF. I1, I2, the writing beams; I3, the reading beam; I4, the reflection from the dynamic grating. x and y denote the polarization directions.
fBragg=fx(nxny),
(1)

where fx denotes the writing beams’ frequency, and nx and ny the refractive indices of the two primary polarization axes, respectively.

When a strain or temperature change is applied to the fiber, the refractive indices change because of the photo-elastic effect, which results in a change in the birefringence. Consequently, the detected Bragg reflection frequency shifts as shown in Fig. 3. The shift of the Bragg frequency is in proportion to the birefringence change, which is proportional to the strain or temperature:

Δf=(fxΔB)ny,
(2)

where ∆f and ∆B denote the shift of the Bragg frequency and the birefringence change caused by strain and temperature variation, respectively. Therefore, the magnitude of strain or temperature change can be calculated by measuring the ∆f:

ε=(nyΔf)(αfx),
(3)
ΔT=(nyΔf)(βfx),
(4)

where ε and ∆T denote the strain and temperature change, and α and β the strain coefficient and temperature coefficient of the birefringence, respectively. Our simulation has shown a strain sensitivity of 0.426 MHz/με under a conservative assumption in the parameters [3

3. X. Fan, Z. He, and K. Hotate, “Novel distributed fiber-optic strain sensor by localizing dynamic grating in polarization-maintaining erbium-doped fiber: proposal and theoretical analysis,” Jpn. J. App. Phys. 44, 1101–1106 (2005). [CrossRef]

].

3. Experimental setup

The experimental setup is shown in Fig. 4. Two light beams from LD1, a distributed feedback laser diode (DFB-LD), are launched into the PM-EDF (50-cm Nufern PM-ESF-7/125 high-doped PM-EDF) to write the dynamic grating in polarization direction X. The PM-EDF is pumped with a 980-nm pump laser diode. A light beam from LD2, a wavelength-tunable DFB-LD, is used to read the grating in polarization direction Y. Three adjustable attenuators are used for obtaining optimum writing and reading intensities. The intensity of the writing beams is around 0 dBm, while the reading beam about -5 dBm, respectively. Three polarizers are used to enhance the polarization extinction ratio of the writing/reading beams, and a polarization beam splitter/combiner (PBS/PBC) is used to introduce the reading beam to the PM-EDF and to output the reflection from the dynamic grating in polarization direction Y.

Fig. 3. Strain- and temperature-sensing by measuring the shift in the Bragg frequency of the dynamic grating. (a) Original spectrum of the dynamic grating; (b) the spectrum when strain applied or temperature changed.
Fig. 4. Experimental setup. LD, laser diode; PC, polarizer controller; IM, intensity modulator; PD, photo-detector; PM-WDM, polarization-maintaining wavelength division multiplexer; PBS/PBC, polarization beam splitter / polarization beam combiner; GPIB, general purpose interface bus.

Since the PM fiber and the polarization components have only a limited extinction ratio, the light beam in one primary polarization axis can couple to the other primary polarization axis. Therefore, when we measure the reflection from the dynamic grating in polarization direction Y, the detector receives not only the reflection of the reading beam but also a part of writing beam that coupled to polarization direction Y. In order to remove the leakage of the writing beam from the reflected reading beam, we chop the reading beam and use a lock-in amplifier LIA1 for synchronous detection. A LiNbO3 electro-optic intensity modulator (IM) is used as a chopper, where the input polarization is controlled by a polarization controller (PC). The chopping frequency is selected as high as 37 MHz to avoid the gain modulation effect in EDF [6

6. B. Fischer, J. L. Zyskind, J. W. Sulhoff, and D. J. DiGiovanni, “Nonlinear four-wave mixing in erbium-doped fiber amplifiers,” Electron. Lett. 29, 1858–1859 (1993). [CrossRef]

].

Fig. 5. Noise origins related to reading beam I3. IARBS, noise caused by the amplified Rayleigh backscattering; I3ara, noise related to the reflection of I3 at the splicing or connection points: I3 is amplified and reflected at splicing points or connection points,

4. Experimental results

With the dual-stage synchronous signal processing method, we observed the reflection from the dynamic grating in PM-EDF successfully. To measure the reflection spectrum of the dynamic grating, we tune the wavelength of the reading beam by changing the injection current to LD2 and keep its intensity constant with a saturated EDFA (not drawn in Fig. 4) after LD2. Figure 6 shows the reflection spectrum of the dynamic grating. The peak is 47-GHz deviated from the frequency of the writing beams, corresponding to a birefringence of 3.6×10-4. Gaussian fitting shows a bandwidth of 140 MHz. The bandwidth is in inverse proportion to the length of the grating [8

8. X. Fan, Z. He, Y. Mizuno, and K. Hotate, “Bandwidth-adjustable dynamic grating in erbium-doped fiber by synthesis of optical coherence function,” Opt. Express 13, 5756–5761 (2005). http://www.opticsexpress.org/abstract.cfm?URI=OPEX-13-15-5756 [CrossRef] [PubMed]

], which is here the full length of the PM-EDF.

The experiment of strain-sensing is performed as we apply a strain to the whole PM-EDF. The PM-EDF is fixed on two translation stages with epoxy adhesive and stretched by adjusting one of the stages. On the other hand, the temperature sensing experiment is performed when we put the PM-EDF into a temperature-controllable water bath. Figures 7 and 8 show the strain dependence and temperature dependence of the Bragg frequency shift, respectively. These results exhibit good linearity and demonstrate a strain-sensitivity of 1.4 MHz/με and temperature-sensitivity of 60 MHz/°C, which are 28 and 60 times higher than those of SBS technique, respectively.

Fig. 6. Reflection spectrum of the dynamic grating. The horizontal axis is the frequency deviation from the frequency of the writing beams.
Fig. 7. Strain dependence of Bragg frequency shift of the reflection peak.
Fig. 8. Temperature dependence of Bragg frequency shift of the reflection peak.

Another factor to compare the performance of the two sensing mechanisms is the strain-or temperature-resolution, which is basically defined as the spectral bandwidth divided by the sensitivity. SBS has a fixed 3-dB bandwidth of about 30 MHz. For the dynamic grating, there is a tradeoff relationship between the dynamic grating’s length and its bandwidth: the longer the dynamic grating is, the smaller its bandwidth [8

8. X. Fan, Z. He, Y. Mizuno, and K. Hotate, “Bandwidth-adjustable dynamic grating in erbium-doped fiber by synthesis of optical coherence function,” Opt. Express 13, 5756–5761 (2005). http://www.opticsexpress.org/abstract.cfm?URI=OPEX-13-15-5756 [CrossRef] [PubMed]

]. In the experiments described above, the length of the dynamic grating is the whole length of the PM-EDF. When the PM-EDF is longer than 8 cm for strain sensing, or longer than 4 cm for temperature sensing, respectively, the dynamic grating method has a better resolution than SBS-based method.

As described in Eq. (2), the Bragg frequency shift is due to the change in birefringence. Our measured strain- and temperature-sensitivity correspond to 1.06×10-8/με and 4.56×10-7/°C in birefringence change, respectively. Using a PM-EDF with higher birefringence change is favorable for better sensing performance.

5. Summary

In this paper, we report the first experimental observation of a dynamic grating in PM-EDF and demonstrate a novel fiber-optic strain- and temperature-sensing mechanism based on the grating. Our experimental results show a strain-sensitivity of 1.4 MHz/με and temperature-sensitivity of 60 MHz/°C, which are 28 and 60 times higher than those of SBS-based technique, respectively. By using the technique of SOCF [4

4. Z. He and K. Hotate, “Distributed fiber optic stress location measurement by arbitrary shaping of optical coherence function,” J. Lightwave Technol. 20, 1715–1723 (2002). [CrossRef]

] to localize and scan the position of the dynamic grating along the PM-EDF, we expect this scheme can be developed as a new technique for distributed strain and temperature sensors.

References and links

1.

T. Horiguchi, K. Shimizu, T. Kurashima, M. Taneda, and Y. Koyamada, “Development of a distributed sensing technique using Brillouin scattering,” J. Lightwave Technol. 13, 1296–1302 (1995). [CrossRef]

2.

K. Hotate and M. Tanaka, “Distributed fiber Brillouin strain sensing with 1-cm spatial resolution by correlation-based continuous-wave technique,” IEEE Photonics Technol. Lett. 14, 179–181 (2002). [CrossRef]

3.

X. Fan, Z. He, and K. Hotate, “Novel distributed fiber-optic strain sensor by localizing dynamic grating in polarization-maintaining erbium-doped fiber: proposal and theoretical analysis,” Jpn. J. App. Phys. 44, 1101–1106 (2005). [CrossRef]

4.

Z. He and K. Hotate, “Distributed fiber optic stress location measurement by arbitrary shaping of optical coherence function,” J. Lightwave Technol. 20, 1715–1723 (2002). [CrossRef]

5.

S. J. Frisken, “Transient Bragg reflection gratings in erbium-doped fiber amplifiers,” Opt. Lett. 17, 1776–1778 (1992). [CrossRef] [PubMed]

6.

B. Fischer, J. L. Zyskind, J. W. Sulhoff, and D. J. DiGiovanni, “Nonlinear four-wave mixing in erbium-doped fiber amplifiers,” Electron. Lett. 29, 1858–1859 (1993). [CrossRef]

7.

B. Fischer, J. L. Zyskind, J. W. Sulhoff, and D. J. DiGiovanni, “Nonlinear wave mixing and induced gratings in erbium- doped fiber amplifiers,” Opt. Lett. 18, 2108–2110 (1993). [CrossRef] [PubMed]

8.

X. Fan, Z. He, Y. Mizuno, and K. Hotate, “Bandwidth-adjustable dynamic grating in erbium-doped fiber by synthesis of optical coherence function,” Opt. Express 13, 5756–5761 (2005). http://www.opticsexpress.org/abstract.cfm?URI=OPEX-13-15-5756 [CrossRef] [PubMed]

OCIS Codes
(060.2370) Fiber optics and optical communications : Fiber optics sensors
(060.2410) Fiber optics and optical communications : Fibers, erbium
(060.2420) Fiber optics and optical communications : Fibers, polarization-maintaining

ToC Category:
Fiber Optics and Optical Communications

Citation
Xinyu Fan, Zuyuan He, and Kazuo Hotate, "Novel strain- and temperature-sensing mechanism based on dynamic grating in polarization-maintaining erbium-doped fiber," Opt. Express 14, 556-561 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-2-556


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References

  1. T. Horiguchi, K. Shimizu, T. Kurashima, M. Taneda, and Y. Koyamada, "Development of a distributed sensing technique using Brillouin scattering," J. Lightwave Technol. 13, 1296-1302 (1995). [CrossRef]
  2. K. Hotate and M. Tanaka, "Distributed fiber Brillouin strain sensing with 1-cm spatial resolution by correlation-based continuous-wave technique," IEEE Photonics Technol. Lett. 14, 179-181 (2002). [CrossRef]
  3. X. Fan, Z. He, and K. Hotate, "Novel distributed fiber-optic strain sensor by localizing dynamic grating in polarization-maintaining erbium-doped fiber: proposal and theoretical analysis," Jpn. J. App. Phys. 44, 1101-1106 (2005). [CrossRef]
  4. Z. He and K. Hotate, "Distributed fiber optic stress location measurement by arbitrary shaping of optical coherence function," J. Lightwave Technol. 20, 1715-1723 (2002). [CrossRef]
  5. S. J. Frisken, "Transient Bragg reflection gratings in erbium-doped fiber amplifiers," Opt. Lett. 17, 1776-1778 (1992). [CrossRef] [PubMed]
  6. B. Fischer, J. L. Zyskind, J. W. Sulhoff, and D. J. DiGiovanni, "Nonlinear four-wave mixing in erbium-doped fiber amplifiers," Electron. Lett. 29, 1858-1859 (1993). [CrossRef]
  7. B. Fischer, J. L. Zyskind, J. W. Sulhoff, and D. J. DiGiovanni, "Nonlinear wave mixing and induced gratings in erbium- doped fiber amplifiers," Opt. Lett. 18, 2108-2110 (1993). [CrossRef] [PubMed]
  8. X. Fan, Z. He, Y. Mizuno, and K. Hotate, "Bandwidth-adjustable dynamic grating in erbium-doped fiber by synthesis of optical coherence function," Opt. Express 13, 5756-5761 (2005), <a href="http://www.opticsexpress.org/abstract.cfm?URI=OPEX-13-15-5756">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-13-15-5756</a>. [CrossRef] [PubMed]

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