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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 25 — Dec. 11, 2006
  • pp: 12445–12450
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Photonic sensing based on variation of propagation properties of photonic crystal fibres

John H. Rothwell, Dónal A. Flavin, William N. MacPherson, Julian D. C. Jones, Jonathan C. Knight, and Philip St. J. Russell  »View Author Affiliations


Optics Express, Vol. 14, Issue 25, pp. 12445-12450 (2006)
http://dx.doi.org/10.1364/OE.14.012445


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Abstract

We report on a low-coherence interferometric scheme for the measurement of the strain and temperature dependences of group delay and dispersion in short, index-guiding, ‘endlessly-single-mode’ photonic crystal fibre elements in the 840 nm and 1550 nm regions. Based on the measurements, we propose two schemes for simultaneous strain and temperature measurement using a single unmodified PCF element, without a requirement for any compensating components, and we project the measurement accuracies of these schemes.

© 2006 Optical Society of America

1. Introduction

Photonic crystal fibres (PCF’s) have attracted much recent research interest for their significance in a range of fields including non-linear optics, optical data transmission, biomedical imaging, and optical frequency metrology [1

1. P. St. J. Russell, “Photonic crystal fibers,” Science 299, 358–362 (2003). [CrossRef] [PubMed]

]. These fibres are formed in a single material—typically undoped silica—by incorporating a periodic array of microscopic holes running parallel to the fibre axis along the complete fibre length.

The optical properties of these fibres are strongly controlled by the geometry of the ‘holey’ region. In particular, it has been demonstrated that it is possible to engineer dispersive properties which are dramatically different from those of conventional fibre waveguides formed from the same material [2

2. J. C. Knight, J. Arriaga, T. A. Birks, A. Ortigosa-Blanch, W. J. Wadsworth, and P. St. J. Russell, “Anomalous dispersion in photonic crystal fiber,” IEEE Photon. Technol. Lett. 12, 807–809 (2000). [CrossRef]

]. For a specific geometry, the propagation properties are further modulated by external parameters such as strain and temperature, or even by fluids introduced into the microstructured holes [3

3. C. Kerbage, R. S. Windeler, B. J. Eggleton, P. Mach, M. Dolinski, and J. A. Rogers, “Tunable devices based on dynamic positioning of micro-fluids in micro-structured optical fiber,” Opt. Commun. 204, 179–184 (2002). [CrossRef]

]. It is therefore desirable to investigate schemes which can measure the variation of group velocity and dispersion induced by such external measurands in PCF’s, with a view to developing new applicable sensing techniques for these measurands. The fact that the fibre geometry can also be controlled to produce endlessly single-mode operation suggests the further value of undertaking measurements at widely separated wavelengths [4

4. T. A. Birks, J. C. Knight, and P. St. J. Russell, “Endlessly single-mode photonic crystal fiber,” Opt. Lett. 22, 961–963 (1997). [CrossRef] [PubMed]

].

In this paper we demonstrate the interferometric approach of dispersive Fourier Transform Spectroscopy (DFTS) for high-resolution measurement of small-scale variation of group velocity and dispersion in PCF elements [5

5. D. A. Flavin, R. McBride, and J. D. C. Jones, “Dispersion of birefringence and differential group delay in polarization-maintaining fiber,” Opt. Lett. 27, 1010–1012 (2002). [CrossRef]

], and we report on the first measurements of the strain and temperature dependences of group velocity and dispersion in PCF elements. The same PCF element is shown to support interferometric demodulation at widely spaced wavelengths—in both the 800 nm and 1550 nm regions. We discuss the potential for simultaneous measurement of strain and temperature using PCF elements, without need for modification or for associated sensing elements exposed to only one of the measurands.

2. Measurement principle

Consider the case of a generic two-beam amplitude-dividing interferometer, represented in Fig. 1, illuminated by a low-coherence beam of spectrum G(ω). One of the arms is composed of a minimally dispersive, scanned delay line; the light in the other arm is directed to a waveguide and sequentially reflected from two sites (S1 and S2) of low reflectivity separated by a distance L. In our experiment S1 and S2 correspond to the cleaved ends of a test fibre. As the delay in the first arm is scanned, two low-coherence interferograms are detected, in sequence, in the regions where the scanned delay matches the delays associated with the two reflection sites in the second arm.

Fig. 1. Generic diagram for the experimental approach.

Mathematically, these two interferograms can be written in terms of the complex mutual coherence function on γ˜(τ) as follows [5

5. D. A. Flavin, R. McBride, and J. D. C. Jones, “Dispersion of birefringence and differential group delay in polarization-maintaining fiber,” Opt. Lett. 27, 1010–1012 (2002). [CrossRef]

]:

Ij(τ)Re[γ˜i(τ)]=Re[G(ω)exp{iϕj(ω)}exp{iωτ}dω]
(1)

where the subscript j=1, 2 relates to the first and second reflection site respectively. The phase difference ϕj(ω) represents the phase imbalance—across the spectral range of the source—between the optical path in the scanned arm and the optical path to the j th reflection site in the second arm. From Eq. (1), the value of ϕj(ω) can be directly determined from the inverse Fourier transform of the interferogram Ij(τ). We can express this phase in terms of the Taylor expansion about the reference frequency ω r,

ϕj(ω)=ϕj(ωr)+ϕj(ωr)(ωωr)+ϕj(ωr)(ωωr)22+
(2)

It is clear that a polynomial fit to the phase curve generates, unambiguously, the values of the first and second-order derivatives of ϕj(ωr), i.e. ϕj(ωr) and ϕj(ωr).

The differences between the values of the first and second derivatives for the two reflection sites, yield the normalised values of group delay τg(ωr) and the group delay dispersion τg(ωr) respectively for the waveguide section:

(τg(ωr)τg(ωr))=(β(ωr)β(ωr))=({ϕ2(ωr)ϕ1(ωr)}{ϕ2(ωr)ϕ1(ωr)})2L
(3)

3. Experiment and signal processing

Figure 2 shows the full experimental configuration. The delay in the bulk-optic Michelson interferometer is scanned by translating the retro-reflector. An air-track is used to eliminate position jitter that has been observed with motor driven translation stages. The PCF sample is placed in the stationary interferometer arm; the sample’s front and rear facets act as the reflection sites (Fresnel reflections at the cleaves) in this arm.

Fig. 2. Experimental setup, incorporating ability to interrogate the PCF element at 840 nm or 1550 nm.

The PCF used, shown in Fig. 3, was a solid core all-silica fibre designed for operation at 1550 nm, and suitable for our purpose of single mode operation from at least 800 to 1600 nm. To maintain accurate tracking of the delay, light from a wavelength stabilized 633 nm HeNe laser is launched into the interferometer and the PCF interferogram data acquisition is triggered from the zero-crossing of the resulting high-coherence HeNe interferogram.

Fig. 3. (Left) Micrograph of the microstructured region of the PCF used in these experiments. (Right) Typical interferograms I1 and I2 due to fibre end facet reflections S1 and S2

To enable us to make measurements in both the 840 nm and 1550 nm regions, the configuration allows illumination from two superluminescent sources: a SuperLum diode with a central wavelength at 843 nm, FWHM bandwidth of 49.6 nm and a maximum output power of 1.6 mW; or an Exalos diode with central wavelength of 1534 nm, FWHM of 60 nm, and maximum power of 2.3 mW. The source illuminating the interferometer can be interchanged by the simple adjustment of a mirror on a kinematic base plate.

The PCF was subject to either strain or temperature changes. Strain was applied by attaching the fibre to a precision translation stage and a rigid fixed support using epoxy adhesive. The length of fibre between the support and translation stage was subject to strain, which was independently measured by accurately measuring the stage displacement. Two techniques of temperature control were employed: simple resistive heating, and closed-loop Peltier heating and cooling. An array of thermocouples allowed the average temperature to be determined.

The two critical interferograms were captured in a single scan of the reference arm. Optical path scans in the range of 80 mm to 200 mm were used in the experiments; the length of the scan is determined by the length of fibre under test. The interferograms were processed, using the procedure described above, by customised DFTS-based software implemented in MATLAB; this yielded the values of the phases ϕ 1(ω) and ϕ 2(ω) across the spectral ranges of the source used. From third-order polynomial fits to each set of phase values, the group delay τg(ωr) and the group delay dispersion τg(ωr) of the waveguide section were determined following the procedures inherent in Eq. (2) and Eq. (3). The values of group delay dispersion are readily converted to the normalised dispersion parameter D using

D(λr)={ωr22πc}τg(ωr)
(4)

4. Results and discussion

4.1 Strain and temperature dependences of group delay and dispersion.

Figure 4 illustrates the measurement of the dispersion parameter D across the wavelength ranges of the individual superluminescent diodes. The Superlum source supported measurement in the range 820 nm to 870 nm, while the Exalos source supported measurement in the range 1490 nm to 1570 nm. It is clear that the geometry and air-filling fractions of the PCF combine to produce dispersion values significantly different from those in conventional single-mode fibre (SMF) in both spectral regions—typically D values for single-mode fibre at 840 nm and 1550 nm are in the regions of -100 ps/nm/km and +17 ps/nm/km respectively.

Fig. 4. Measurements of the normalised dispersion parameter D across the spectral range of the two superluminescent diodes.

Using the DFTS approach, we obtain the strain and temperature dependences of both the group delay and dispersion at two specific wavelengths, 845 nm and 1530 nm, close to the maximum power wavelength of the individual sources. Typical results obtained at 845 nm are given in Fig. 5.

Fig. 5. The measured variations of normalised group delay and dispersion with strain and temperature at a wavelength of 845 nm.

Table 1 indicates the measured variations of the group delay and dispersion parameters at 845 nm and 1530 nm. Also included, for comparison, are the variations in these parameters for two standard fibres, each single-mode only for the measurement wavelength. The most notable features are that the magnitudes of the dependences of dispersion (∂D/∂T) on temperature are significantly higher than those occurring in SMF fibre and the small differences between the strain dependences of group delay at 845 nm and at 1530 nm in the PCF.

Table 1. Temperature and strain sensitivity of group delay and dispersion

table-icon
View This Table

For simplicity in implementing the PCF as a sensing element, we have chosen to normalise these values to initial fibre length L0. The increase in length, ΔL, due to the applied temperature and strain is not taken into account. Including ΔL in the calculations would yield values consistent with those obtained in [6

6. W. J. Bock, W. Urbanczyk, and J. Wójcik, “Measurements of sensitivity of the single-mode photonic crystal holey fibre to temperature, elongation and hydrostatic pressure,” Meas. Sci Technol 15, 1496–1500 (2004). [CrossRef]

,7

7. W. N. MacPherson, M. J. Gander, R. McBride, J. D. C. Jones, P. M. Blanchard, J. G. Burnett, A. H. Greenaway, B. Mangan, T. A. Birks, J. C. Knight, and P. St. J. Russell, “Remotely addressed optical fibre curvature sensor using multicore photonic crystal fibre,” Opt. Commun. 193, 97–104 (2001). [CrossRef]

] for the group index in a different PCF.

4.2 Potential for simultaneous measurement of strain and temperature.

PCF elements coupled with the DFTS-based measurement technique suggest the potential for two approaches for the simultaneous and independent measurement of temperature and strain. The first approach is based on the measurement of the induced variation of group delay, in each of the two wavelength regions. Specific PCF elements offer a unique opportunity for this approach, given their support of ‘endlessly single-mode’ propagation. The DFTS approach allows both group delay variations to be determined from a single sweep of the optical path delay (OPD), when the system is illuminated by both sources. On first inspection, the differences between the strain and temperature dependences of group delay at 845 nm and at 1530 nm do not suggest an obvious potential for a strain-temperature discrimination; for strain and temperature, the dependences differ only by about 0.5% and 5%, respectively, between these two wavelengths. However, our group delay measurement exhibits a resolution of 0.001 ps/m, obtained from the standard deviation of ten repeated measurements at each strain value. Using the matrix-inversion technique [8

8. J. D. C. Jones, “Review of fibre sensor techniques for temperature-strain discrimination,” in Twelfth International Conference on Optical Fibre Sensors, Vol. 16 of OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1997), pp. 36–39.

], this resolution is projected to yield independent strain-temperature measurement with rms errors of ~10 με and 1°C for a 100 mm PCF element. Cross-sensitivity has not been considered in this initial work. However, for small strain and temperature changes in conventional fibres, cross-sensitivity is minimal [9

9. F. Farahi, D. J. Webb, J. D. C. Jones, and D. A. Jackson, “Simultaneous measurement of temperature and strain: Cross-sensitivity considerations,” J. Lightwave Technol. 8, 138–142 (1990). [CrossRef]

]. Further work is required to determine any systematic cross-sensitivity which could then be accounted for in the analysis.

The second feasible approach for simultaneous strain-temperature measurement is based on the variation of both group delay and dispersion, under interrogation by a single source. In this approach the strain-temperature measurement is based on the simultaneously measured variation of group delay and dispersion at a wavelength within the spectral range of the source. In our current arrangement, the dispersion measurement resolution is 0.01 ps/nm/km, for a 100 mm PCF element. Based on this resolution the predicted rms errors for the recovery of strain and temperature are ~7 με and ± 0.7°C respectively for the 845 nm source. The transfer matrix obtained from the dependences in the 1550 nm region exhibits a similarly acceptable degree of error. In all cases, measurement resolution will, of course, increase proportionately with the sensor length used.

5. Conclusion

We have demonstrated the application of DFTS to the measurement of small-scale variation of group delay and dispersion at widely separated wavelength ranges for ‘endlessly singlemode’ photonic crystal fibre. This has allowed us to report the first measurements of the strain and temperature dependences of both group delay and dispersion in short PCF elements. The approach has been shown to support interferometric interrogation in both the 800 nm and 1550 nm regions. The measured dependences have further allowed us to estimate the feasibility of two schemes for the simultaneous measurement of strain and temperature using a single unmodified PCF element, without any requirement for compensating components. The schemes are respectively based on the variation of the group delay in two wavelength ranges, or of group delay and dispersion within either wavelength range. Based on a 100 mm PCF element, the projected rms errors for strain and temperature, for the two schemes are respectively ~10 με and 1°C, and ~7 με and ± 0.7°C.

Acknowledgments

*This paper is dedicated to the memory of Dr. Dónal A. Flavin (1952–2005). W. N. MacPherson wishes to acknowledge the UK EPSRC for funding via the Advanced Fellowship Scheme. J. H. Rothwell and D. A. Flavin acknowledge travel assistance received under the Enterprise-Ireland International Collaboration Program.

References

1.

P. St. J. Russell, “Photonic crystal fibers,” Science 299, 358–362 (2003). [CrossRef] [PubMed]

2.

J. C. Knight, J. Arriaga, T. A. Birks, A. Ortigosa-Blanch, W. J. Wadsworth, and P. St. J. Russell, “Anomalous dispersion in photonic crystal fiber,” IEEE Photon. Technol. Lett. 12, 807–809 (2000). [CrossRef]

3.

C. Kerbage, R. S. Windeler, B. J. Eggleton, P. Mach, M. Dolinski, and J. A. Rogers, “Tunable devices based on dynamic positioning of micro-fluids in micro-structured optical fiber,” Opt. Commun. 204, 179–184 (2002). [CrossRef]

4.

T. A. Birks, J. C. Knight, and P. St. J. Russell, “Endlessly single-mode photonic crystal fiber,” Opt. Lett. 22, 961–963 (1997). [CrossRef] [PubMed]

5.

D. A. Flavin, R. McBride, and J. D. C. Jones, “Dispersion of birefringence and differential group delay in polarization-maintaining fiber,” Opt. Lett. 27, 1010–1012 (2002). [CrossRef]

6.

W. J. Bock, W. Urbanczyk, and J. Wójcik, “Measurements of sensitivity of the single-mode photonic crystal holey fibre to temperature, elongation and hydrostatic pressure,” Meas. Sci Technol 15, 1496–1500 (2004). [CrossRef]

7.

W. N. MacPherson, M. J. Gander, R. McBride, J. D. C. Jones, P. M. Blanchard, J. G. Burnett, A. H. Greenaway, B. Mangan, T. A. Birks, J. C. Knight, and P. St. J. Russell, “Remotely addressed optical fibre curvature sensor using multicore photonic crystal fibre,” Opt. Commun. 193, 97–104 (2001). [CrossRef]

8.

J. D. C. Jones, “Review of fibre sensor techniques for temperature-strain discrimination,” in Twelfth International Conference on Optical Fibre Sensors, Vol. 16 of OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1997), pp. 36–39.

9.

F. Farahi, D. J. Webb, J. D. C. Jones, and D. A. Jackson, “Simultaneous measurement of temperature and strain: Cross-sensitivity considerations,” J. Lightwave Technol. 8, 138–142 (1990). [CrossRef]

OCIS Codes
(060.2370) Fiber optics and optical communications : Fiber optics sensors
(060.2400) Fiber optics and optical communications : Fiber properties

ToC Category:
Photonic Crystal Fibers

History
Original Manuscript: October 12, 2006
Revised Manuscript: November 23, 2006
Manuscript Accepted: November 23, 2006
Published: December 11, 2006

Citation
John H. Rothwell, Dónal A. Flavin, William N. MacPherson, Julian D. Jones, Jonathan C. Knight, and Philip St. J. Russell, "Photonic sensing based on variation of propagation properties of photonic crystal fibres," Opt. Express 14, 12445-12450 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-25-12445


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References

  1. P. St. J. Russell, "Photonic crystal fibers," Science 299, 358-362 (2003). [CrossRef] [PubMed]
  2. J. C. Knight, J. Arriaga, T. A. Birks, A. Ortigosa-Blanch, W. J. Wadsworth, and P. St. J. Russell, "Anomalous dispersion in photonic crystal fiber," IEEE Photon. Technol. Lett. 12, 807-809 (2000). [CrossRef]
  3. C. Kerbage, R. S. Windeler, B. J. Eggleton, P. Mach, M. Dolinski, and J. A. Rogers, "Tunable devices based on dynamic positioning of micro-fluids in micro-structured optical fiber," Opt. Commun. 204, 179-184 (2002). [CrossRef]
  4. T. A. Birks, J. C. Knight, and P. St. J. Russell, "Endlessly single-mode photonic crystal fiber," Opt. Lett. 22, 961-963 (1997). [CrossRef] [PubMed]
  5. D. A. Flavin, R. McBride, and J. D. C. Jones, "Dispersion of birefringence and differential group delay in polarization-maintaining fiber," Opt. Lett. 27, 1010-1012 (2002). [CrossRef]
  6. W. J. Bock, W. Urbanczyk, and J. Wójcik, "Measurements of sensitivity of the single-mode photonic crystal holey fibre to temperature, elongation and hydrostatic pressure," Meas. Sci Technol 15, 1496-1500 (2004). [CrossRef]
  7. W. N. MacPherson, M. J. Gander, R. McBride, J. D. C. Jones, P. M. Blanchard, J. G. Burnett, A. H. Greenaway, B. Mangan, T. A. Birks, J. C. Knight and P. St. J. Russell, "Remotely addressed optical fibre curvature sensor using multicore photonic crystal fibre," Opt. Commun. 193, 97-104 (2001). [CrossRef]
  8. J. D. C. Jones, "Review of fibre sensor techniques for temperature-strain discrimination," in Twelfth International Conference on Optical Fibre Sensors, Vol. 16 of OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1997), pp. 36-39.
  9. F. Farahi, D. J. Webb, J. D. C. Jones and D. A. Jackson, "Simultaneous measurement of temperature and strain: Cross-sensitivity considerations," J. Lightwave Technol. 8,138-142 (1990). [CrossRef]

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