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Applied Optics

Applied Optics

APPLICATIONS-CENTERED RESEARCH IN OPTICS

  • Editor: James C. Wyant
  • Vol. 47, Iss. 15 — May. 20, 2008
  • pp: 2835–2839
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Pressure sensor realized with polarization-maintaining photonic crystal fiber-based Sagnac interferometer

H. Y. Fu, H. Y. Tam, Li-Yang Shao, Xinyong Dong, P. K. A. Wai, C. Lu, and Sunil K. Khijwania  »View Author Affiliations


Applied Optics, Vol. 47, Issue 15, pp. 2835-2839 (2008)
http://dx.doi.org/10.1364/AO.47.002835


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Abstract

A novel intrinsic fiber optic pressure sensor realized with a polarization-maintaining photonic crystal fiber (PM-PCF) based Sagnac interferometer is proposed and demonstrated experimentally. A large wavelength–pressure coefficient of 3.42 nm / MPa was measured using a 58.4 cm long PM-PCF as the sensing element. Owing to the inherently low bending loss and thermal dependence of the PM-PCF, the proposed pressure sensor is very compact and exhibits low temperature sensitivity.

© 2008 Optical Society of America

1. Introduction

Optical fiber Sagnac interferometers have been developed for gyroscopes and other sensor applications due to their unique advantages, such as simple design, ease of manufacture, and lower susceptibility to environmental pickup noise in comparison to other types of fiber optic sensors [1

1. V. Vali and R. W. Shorthill, “Fiber ring interferometer,” Appl. Opt. 15, 1099–1103 (1976). [CrossRef] [PubMed]

, 2

2. S. Knudsen and K. Blotekjaer, “An ultrasonic fiber-optic hydrophone incorporating a push–pull transducer in a Sagnac interferometer,” J. Lightwave Technol. 12, 1696–1700 (1994). [CrossRef]

]. Polarization-maintaining fiber (PMF) is usually used in Sagnac interferometers to introduce optical path difference and cause interference between the two counterpropagating waves in the fiber loop [3

3. A. N. Starodumov, L. A. Zenteno, D. Monzon, and E. De La Rose, “Fiber Sagnac interferometer temperature sensor,” Appl. Phys. Lett. 70, 19–21 (1997). [CrossRef]

, 4

4. E. De La Rose, L. A. Zenteno, A. N. Starodumov, and D. Monzon, “All-fiber absolute temperature sensor using an unbalanced high-birefringence Sagnac loop,” Opt. Lett. 22, 481–483 (1997). [CrossRef]

, 5

5. Y. J. Song, L. Zhan, S. Hu, Q. H. Ye, and Y. X. Xia, “Tunable multiwavelength Brillouin–erbium fiber laser with a polarization-maintaining fiber Sagnac loop filter,” IEEE Photon. Technol. Lett. 16, 2015–2017 (2004). [CrossRef]

, 6

6. M. Campbell, G. Zheng, A. S. Holmes-Smith, and P. A. Wallace, “A frequency-modulated continuous wave birefringent fibre-optic strain sensor based on a Sagnac ring configuration,” Meas. Sci. Technol. 10, 218–224 (1999). [CrossRef]

, 7

7. Y. Liu, B. Liu, X. Feng, W. Zhang, G. Zhou, S. Yuan, G. Kai, and X. Dong, “High-birefringence fiber loop mirrors and their applications as sensors,” Appl. Opt. 44, 2382–2390 (2005). [CrossRef] [PubMed]

]. However, conventional PMFs (e.g., Panda and bow-tie PMFs) have a high thermal sensitivity due to the large thermal expansion coefficient difference between boron-doped stress-applying parts and the cladding (normally pure silica). Consequently, conventional PMFs exhibit temperature-sensitive birefringence. Therefore, conventional PMF based Sagnac interferometer sensors exhibit relatively high temperature sensitivity, which is about 1 and 2 orders of magnitude higher than that of long-period fiber grating (LPG) and fiber Bragg grating (FBG) sensors [3

3. A. N. Starodumov, L. A. Zenteno, D. Monzon, and E. De La Rose, “Fiber Sagnac interferometer temperature sensor,” Appl. Phys. Lett. 70, 19–21 (1997). [CrossRef]

, 4

4. E. De La Rose, L. A. Zenteno, A. N. Starodumov, and D. Monzon, “All-fiber absolute temperature sensor using an unbalanced high-birefringence Sagnac loop,” Opt. Lett. 22, 481–483 (1997). [CrossRef]

]. When they are used for sensing other measurements rather than temperature, such as pressure, the temperature change and fluctuation will cause serious cross-sensitivity effects and would affect the measurement accuracy significantly.

Recently, a PM-PCF-based Sagnac interferometer employed as a strain sensor that demonstrated high sensitivity of 0.23pm/με and measurement range of up to 32 has been reported [9

9. X. Dong, H. Y. Tam, and P. Shum, “Temperature-insensitive strain sensor with polarization-maintaining photonic crystal fiber based Sagnac interferometer,” Appl. Phys. Lett. 90, 151113 (2007). [CrossRef]

]. A PM-PCF-based pressure sensor with polarimetric detection has also been proposed and demonstrated [10

10. H. K. Gahir and D. Khanna, “Design and development of a temperature-compensated fiber optic polarimetric pressure sensor based on photonic crystal fiber at 1550nm,” Appl. Opt. 46, 1184–1189 (2007). [CrossRef] [PubMed]

]. Polarimetric sensors are complicated and are generally not preferred in most applications. In this paper, we propose and demonstrate a pressure sensor based on a PM-PCF Sagnac interferometer. The Sagnac loop itself acts as a sensitive pressure sensing element, making it an ideal candidate for pressure sensor. Other reported fiber optic pressure sensors generally required some sort of modification to the fiber to increase their sensitivity [14

14. Y. Zhang, D. Feng, Z. Liu, Z. Guo, X. Dong, K. S. Chiang, and B. C. B. Chu, “High-sensitivity pressure sensor using a shielded polymer-coated fiber Bragg grating,” IEEE Photon. Technol. Lett. 13, 618–619 (2001). [CrossRef]

]. The proposed pressure sensor does not require polarimetric detection and the pressure information is wavelength encoded.

The theoretical analysis for the pressure-induced spectral shift is briefly presented in this paper. Pressure measurement results show a sensing sensitivity of 3.42nm/MPa, which is achieved by using a 58.4cm PM-PCF-based Sagnac interferometer. The demonstrated measurement range is 0.3MPa, which is limited by the test apparatus available in our laboratory. Important features of the pressure sensor are the low thermal coefficient and the exceptionally low bending loss of the PM-PCF, which permits the fiber to be coiled into a 5mm diameter circle. This allows the realization of a very small pressure sensor.

2. Experimental Setup and Operating Principle

Figure 1 shows the experimental setup of our proposed pressure sensor with the PM-PCF based Sagnac interferometer. It includes a conventional 3dB single-mode fiber coupler and a 58.4cm PM-PCF. The PM-PCF (PM-1550-01, Blaze Photonics) has a beat length of <4mm at 1550nm and a polarization extinction ratio of >30dB over 100m. The scanning electron micrograph (SEM) image of the transverse cross section of the PM-PCF is shown in the inset of Fig. 1. Mode field diameters for the two orthogonal polarization modes are 3.6 and 3.1μm, respectively. The combined loss of the two splicing points is 4dB. Low splicing loss could be achieved by repeated arc discharges applied over the splicing points to collapse the air holes of the small-core PM-PCF [15

15. L. Xiao, W. Jin, and M. S. Demokan, “Fusion splicing small-core photonic crystal fibers and single-mode fibers by repeated arc discharges,” Opt. Lett. 32, 115–117 (2007). [CrossRef]

]. Collapsing enlarges the mode field at the interface of the PM-PCF so as to match the mode field of the single-mode fiber (SMF). The Sagnac interferometer is laid in an open metal box and the box is placed inside a sealed air tank. The tank is connected to an air compressor with adjustable air pressure that was measured with a pressure meter. The input and output ends of the Sagnac interferometer are placed outside the air tank. When a broadband light source (amplified spontaneous emission source with pumped erbium-doped fiber) is connected to the input, an interference output, as shown in Fig. 2, can be observed. By measuring the wavelength shift of one of the transmission minimums with an optical spectrum analyzer (OSA), the applied pressure to the PM-PCF can be determined.

In the fiber loop, the two counterpropagating lights split by the 3dB SMF coupler interfere again at the coupler and the resulting spectrum is determined by the relative phase difference introduced to the two orthogonal guided modes mainly by the PM-PCF. Ignoring the loss of the Sagnac loop, the transmission spectrum of the fiber loop is approximately a periodic function of the wavelength and is given by
T=[1-cos(δ)]/2.
(1)
The total phase difference δ introduced by the PM-PCF can be expressed as
δ=δ0+δP,
(2)
where δ0 and δP are the phase differences due to the intrinsic and pressure-induced birefringence over the length L of the PM-PCF and are given by
δ0=2π·B·Lλ,
(3)
δP=2π·(KPΔP)·Lλ.
(4)
B=nsbf is the birefringence of the PM-PCF; ns and nf are effective refractive indices of the PM-PCF at the slow and fast axes, respectively. ΔP is the applied pressure and the birefringence-pressure coefficient of PM-PCF can be described as [10

10. H. K. Gahir and D. Khanna, “Design and development of a temperature-compensated fiber optic polarimetric pressure sensor based on photonic crystal fiber at 1550nm,” Appl. Opt. 46, 1184–1189 (2007). [CrossRef] [PubMed]

]
KP=nsP-nfP.
(5)
The spacing S between two adjacent transmission minimums can be approximated by
S=λ2/(B·L).
(6)
The pressure-induced wavelength shift of the transmission minimum is Δλ=S·δP/2π. Thus, the relationship between wavelength shift and applied pressure can be obtained by
Δλ=(KP·λB)·ΔP.
(7)
Equation (7) shows that for a small wavelength shift the spectral shift is linearly proportional to the applied pressure.

3. Experimental Results and Discussions

Figure 2 shows the transmission spectrum of the PM-PCF-based Sagnac interferometer at atmospheric pressure, i.e., at zero applied pressure. The spacing between two adjacent transmission minimums is 5.3nm and an extinction ratio of better than 20dB was achieved. The intrinsic birefringence of the PM-PCF used in our experiment is 7.8×104 at 1550nm.

The air compressor is initially at one atmospheric pressure (about 0.1MPa). In our experiment, we can increase air pressure up to 0.3MPa; thus, the maximum pressure that can be applied to the PM-PCF-based Sagnac interferometer sensor is 0.4MPa. At one atmospheric pressure one of the transmission minimums occurs at 1551.86nm and shifts to a longer wavelength with applied pressure. When the applied pressure was increased by 0.3MPa, a 1.04nm wavelength shift of the transmission minimum was measured, as shown in Fig. 3. Figure 4 shows the experimental data of the wavelength–pressure variation and the linear curve fitting. The measured wavelength–pressure coefficient is 3.42nm/MPa with a good R2 value of 0.999, which agrees well with our theoretical prediction. From Eq. (7), the birefringence–pressure coefficient is 1.7×106MPa1. The resolution of the pressure measurement is 2.9kPa when using an OSA with a 10pm wavelength resolution. Because of the limitations of our equipment, we have not studied the performance of this pressure sensor for high pressure at this stage. However, we found that the PM-PCF can stand pressure of 10MPa without damage to its structure. This part of the work is ongoing and will be reported in our further studies.

Although the length of PM-PCF used in our experiment is 58.4cm, it is important to note that the PM-PCF can be coiled into a very small diameter circle with virtually no additional bending loss so that a compact pressure sensor design can be achieved. The induced bending loss by coiling the PM-PCF fiber into 10 turns of a 5mm diameter circle, shown in the inset of Fig. 4, is measured to be less than 0.01dB with a power meter (FSM-8210, ILX Lightwave Corporation). The exceptionally low bending loss will simplify sensor design and packaging and fulfills the strict requirements of some applications where small size is needed, such as in down-hole oil well applications. To investigate the effects of coiling, we have studied two extreme cases in which the PM-PCF was wound with its fast axis and then its slow axis on the same plane of the coil. There were no measurable changes for either the birefringence or the wavelength–pressure coefficient when the fiber was coiled into 15 and 6mm diameter circles with both of the orientations coiling. The coiling of the PM-PCF into small diameter circles makes the entire sensor very compact and could reduce any unwanted environmental distortions, such as vibrations.

The wavelength–pressure coefficient is independent of the length of the PM-PCF, as described in Eq. (7). Figure 5 shows the wavelength-pressure coefficients are 3.46 and 3.43nm/MPa for PM-PCFs with lengths of 40 and 79.6cm, respectively. After comparing the two wavelength–pressure coefficients with that of the pressure sensor with a 58.4cm PM-PCF (Fig. 4), we observed that the wavelength–pressure coefficient is constant around 1550nm; this agrees well with our theoretical prediction. However, the length of the PM-PCF cannot be reduced too much because this would result in broad attenuation peaks in the transmission spectrum and that would reduce the reading accuracy of the transmission minimums.

Temperature sensitivity of the proposed pressure sensor is also investigated by placing the sensor into an oven and varying its temperature. Figure 6 shows the wavelength shift of a transmission minimum versus temperature linearly with a good R2 value of 0.9984. The measured temperature coefficient is -2.2pm/°C, which is much smaller than the 10pm/°C of fiber Bragg grating. The temperature may be neglected for applications that operate over a normal temperature variation range.

Based on the small size, the high wavelength– pressure coefficient, the reduced temperature sensitivity characteristic, and other intrinsic advantages of fiber optic sensors, such as light weight and electromagnetically passive operation, the proposed pressure sensor is a promising candidate for pressure sensing even in harsh environments. Considering the whole pressure sensing system, we can also replace the light source with laser and use a photodiode for intensity detection at the sensing signal receiving end. Since the power fluctuation is very small even when the PM-PCF is bent, intensity detection is practical for real applications. Because of the compact size of the laser and photodiode, the entire system can be made into a very portable system. Furthermore, the use of intensity detection instead of wavelength measurement would greatly enhance interrogation speed and consequently makes the system much more attractive.

4. Conclusion

A novel fiber Sagnac interferometer pressure sensor realized by using a PM-PCF as the sensing element has been proposed and demonstrated. Experimental results and simplified theoretical analysis of the pressure sensor have been presented. The sensitivity of the pressure sensor is 3.42nm/MPa. The proposed pressure sensor exhibits the advantages of high sensitivity, compact size, low temperature sensitivity, and is potentially low cost.

The authors thank Dr. Limin Xiao and Dr. S. Y. Liu for fruitful discussions and timely help. This work was supported in part by the Hong Kong Polytechnic University under project G-U263 and in part by the Central Research Grant of the Hong Kong Polytechnic University under project G-YX77.

Fig. 1 Schematic diagram of the proposed pressure sensor constructed with PM-PCF based Sagnac interferometer.
Fig. 2 Transmission spectrum of the PM-PCF based Sagnac interferometer.
Fig. 3 Measured transmission spectra under different pressures.
Fig. 4 Wavelength shift of the transmission minimum at 1551.86nm against applied pressure with variation up to 0.3MPa based on one atmospheric pressure.
Fig. 5 Wavelength shift of the transmission minimum against applied pressure for PM-PCFs with length of 40 (circles) and 79.6cm (triangles); the wavelength–pressure coefficients are 3.46 and 3.43nm/MPa, respectively.
Fig. 6 Wavelength shift of the transmission minimum at 1551.86nm against temperature.
1.

V. Vali and R. W. Shorthill, “Fiber ring interferometer,” Appl. Opt. 15, 1099–1103 (1976). [CrossRef] [PubMed]

2.

S. Knudsen and K. Blotekjaer, “An ultrasonic fiber-optic hydrophone incorporating a push–pull transducer in a Sagnac interferometer,” J. Lightwave Technol. 12, 1696–1700 (1994). [CrossRef]

3.

A. N. Starodumov, L. A. Zenteno, D. Monzon, and E. De La Rose, “Fiber Sagnac interferometer temperature sensor,” Appl. Phys. Lett. 70, 19–21 (1997). [CrossRef]

4.

E. De La Rose, L. A. Zenteno, A. N. Starodumov, and D. Monzon, “All-fiber absolute temperature sensor using an unbalanced high-birefringence Sagnac loop,” Opt. Lett. 22, 481–483 (1997). [CrossRef]

5.

Y. J. Song, L. Zhan, S. Hu, Q. H. Ye, and Y. X. Xia, “Tunable multiwavelength Brillouin–erbium fiber laser with a polarization-maintaining fiber Sagnac loop filter,” IEEE Photon. Technol. Lett. 16, 2015–2017 (2004). [CrossRef]

6.

M. Campbell, G. Zheng, A. S. Holmes-Smith, and P. A. Wallace, “A frequency-modulated continuous wave birefringent fibre-optic strain sensor based on a Sagnac ring configuration,” Meas. Sci. Technol. 10, 218–224 (1999). [CrossRef]

7.

Y. Liu, B. Liu, X. Feng, W. Zhang, G. Zhou, S. Yuan, G. Kai, and X. Dong, “High-birefringence fiber loop mirrors and their applications as sensors,” Appl. Opt. 44, 2382–2390 (2005). [CrossRef] [PubMed]

8.

G. Kakarantzas, A. Ortigosa-Blanch, T. A. Birks, P. St. Russell, L. Farr, F. Couny, and B. J. Mangan, “Structural rocking filters in highly birefringent photonic crystal fiber,” Opt. Lett. 28, 158–160 (2003). [CrossRef] [PubMed]

9.

X. Dong, H. Y. Tam, and P. Shum, “Temperature-insensitive strain sensor with polarization-maintaining photonic crystal fiber based Sagnac interferometer,” Appl. Phys. Lett. 90, 151113 (2007). [CrossRef]

10.

H. K. Gahir and D. Khanna, “Design and development of a temperature-compensated fiber optic polarimetric pressure sensor based on photonic crystal fiber at 1550nm,” Appl. Opt. 46, 1184–1189 (2007). [CrossRef] [PubMed]

11.

C.-L. Zhao, C. Lu, W. Jin, and M. S. Demokan, “Temperature-insensitive interferometer using a highly birefringent photonic crystal fiber loop mirror,” IEEE Photon. Technol. Lett. 16, 2535–2537 (2004). [CrossRef]

12.

D.-H. Kim and J. U. Kang, “Sagnac loop interferometer based on polarization maintaining photonic crystal fiber with reduced temperature sensitivity,” Opt. Express 12, 4490–4495 (2004). [CrossRef] [PubMed]

13.

T. P. Hansen, J. Broeng, S. E. B. Libori, E. Knudsen, A. Bjarklev, J. R. Jensen, and H. Simonsen, “Highly birefringent index-guiding photonic crystal fibers,” IEEE Photon. Technol. Lett. 13, 588–590 (2001). [CrossRef]

14.

Y. Zhang, D. Feng, Z. Liu, Z. Guo, X. Dong, K. S. Chiang, and B. C. B. Chu, “High-sensitivity pressure sensor using a shielded polymer-coated fiber Bragg grating,” IEEE Photon. Technol. Lett. 13, 618–619 (2001). [CrossRef]

15.

L. Xiao, W. Jin, and M. S. Demokan, “Fusion splicing small-core photonic crystal fibers and single-mode fibers by repeated arc discharges,” Opt. Lett. 32, 115–117 (2007). [CrossRef]

OCIS Codes
(060.2370) Fiber optics and optical communications : Fiber optics sensors
(060.2420) Fiber optics and optical communications : Fibers, polarization-maintaining
(060.5295) Fiber optics and optical communications : Photonic crystal fibers

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: March 3, 2008
Manuscript Accepted: April 16, 2008
Published: May 14, 2008

Citation
H. Y. Fu, H. Y. Tam, Li-Yang Shao, Xinyong Dong, P. K. A. Wai, C. Lu, and Sunil K. Khijwania, "Pressure sensor realized with polarization-maintaining photonic crystal fiber-based Sagnac interferometer," Appl. Opt. 47, 2835-2839 (2008)
http://www.opticsinfobase.org/ao/abstract.cfm?URI=ao-47-15-2835


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References

  1. V. Vali and R. W. Shorthill, “Fiber ring interferometer,” Appl. Opt. 15, 1099-1103 (1976). [CrossRef] [PubMed]
  2. S. Knudsen and K. Blotekjaer, “An ultrasonic fiber-optic hydrophone incorporating a push-pull transducer in a Sagnac interferometer,” J. Lightwave Technol. 12, 1696-1700 (1994). [CrossRef]
  3. A. N. Starodumov, L. A. Zenteno, D. Monzon, and E. De La Rose, “Fiber Sagnac interferometer temperature sensor,” Appl. Phys. Lett. 70, 19-21 (1997). [CrossRef]
  4. E. De La Rose, L. A. Zenteno, A. N. Starodumov, and D. Monzon, “All-fiber absolute temperature sensor using an unbalanced high-birefringence Sagnac loop,” Opt. Lett. 22, 481-483 (1997). [CrossRef]
  5. Y. J. Song, L. Zhan, S. Hu, Q. H. Ye, and Y. X. Xia, “Tunable multiwavelength Brillouin-erbium fiber laser with a polarization-maintaining fiber Sagnac loop filter,” IEEE Photon. Technol. Lett. 16, 2015-2017 (2004). [CrossRef]
  6. M. Campbell, G. Zheng, A. S. Holmes-Smith, and P. A. Wallace, “A frequency-modulated continuous wave birefringent fibre-optic strain sensor based on a Sagnac ring configuration,” Meas. Sci. Technol. 10, 218-224 (1999). [CrossRef]
  7. Y. Liu, B. Liu, X. Feng, W. Zhang, G. Zhou, S. Yuan, G. Kai, and X. Dong, “High-birefringence fiber loop mirrors and their applications as sensors,” Appl. Opt. 44, 2382-2390 (2005). [CrossRef] [PubMed]
  8. G. Kakarantzas, A. Ortigosa-Blanch, T. A. Birks, P. St. Russell, L. Farr, F. Couny, and B. J. Mangan, “Structural rocking filters in highly birefringent photonic crystal fiber,” Opt. Lett. 28, 158-160 (2003). [CrossRef] [PubMed]
  9. X. Dong, H. Y. Tam, and P. Shum, “Temperature-insensitive strain sensor with polarization-maintaining photonic crystal fiber based Sagnac interferometer,” Appl. Phys. Lett. 90, 151113 (2007). [CrossRef]
  10. H. K. Gahir and D. Khanna, “Design and development of a temperature-compensated fiber optic polarimetric pressure sensor based on photonic crystal fiber at 1550 nm,” Appl. Opt. 46, 1184-1189 (2007). [CrossRef] [PubMed]
  11. C.-L. Zhao, X, Yang, C. Lu, W. Jin, and M. S. Demokan, “Temperature-insensitive interferometer using a highly birefringent photonic crystal fiber loop mirror,” IEEE Photon. Technol. Lett. 16, 2535-2537 (2004). [CrossRef]
  12. D.-H. Kim and J. U. Kang, “Sagnac loop interferometer based on polarization maintaining photonic crystal fiber with reduced temperature sensitivity,” Opt. Express 12, 4490-4495 (2004). [CrossRef] [PubMed]
  13. T. P. Hansen, J. Broeng, S. E. B. Libori, E. Knudsen, A. Bjarklev, J. R. Jensen, and H. Simonsen, “Highly birefringent index-guiding photonic crystal fibers,” IEEE Photon. Technol. Lett. 13, 588-590 (2001). [CrossRef]
  14. Y. Zhang, D. Feng, Z. Liu, Z. Guo, X. Dong, K. S. Chiang, and B. C. B. Chu, “High-sensitivity pressure sensor using a shielded polymer-coated fiber Bragg grating,” IEEE Photon. Technol. Lett. 13, 618-619 (2001). [CrossRef]
  15. L. Xiao, W. Jin, and M. S. Demokan, “Fusion splicing small-core photonic crystal fibers and single-mode fibers by repeated arc discharges,” Opt. Lett. 32, 115-117 (2007). [CrossRef]

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