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Optics Letters

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  • Editor: Alan E. Willner
  • Vol. 33, Iss. 6 — Mar. 15, 2008
  • pp: 563–565
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Highly sensitive refractometer with a photonic-crystal-fiber long-period grating

Lars Rindorf and Ole Bang  »View Author Affiliations


Optics Letters, Vol. 33, Issue 6, pp. 563-565 (2008)
http://dx.doi.org/10.1364/OL.33.000563


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Abstract

We present highly sensitive refractometers based on a long-period grating in a large-mode-area photonic crystal fiber (PCF). The maximum sensitivity is 1500 nm refractive index unit at a refractive index of 1.33, to our knowledge the highest reported for any fiber grating. The minimal detectable index change is 2 × 10 5 . The high sensitivity is obtained by infiltrating the sample into the holes of the PCF to give a strong interaction between the sample and the probing field.

© 2008 Optical Society of America

Optical fiber sensors are attracting increasing interest. Fiber grating sensors are being used for a variety of purposes, including temperature, strain, and refractive-index sensing. The sensors possess high sensitivity as well as low susceptibility to interferences. In long-period fiber gratings (LPGs) the core mode is coupled resonantly to a cladding mode. In standard optical fibers the cladding mode probes the surroundings of the fiber, and in this way the resonance wavelength may be shifted. The shift in resonance wavelength is used as the indicator of the refractometer [1

1. V. Bhatia and A. M. Vengsarkar, Opt. Lett. 21, 692 (1996). [CrossRef] [PubMed]

]. At a refractive index of 1.33, typical for aqueous environments, the sensitivity of an LPG in a standard telecom fiber is typically 50nmrefractive index unit (RIU) [2

2. J. H. Chong, P. Shum, H. Haryono, A. Yohana, M. K. Rao, C. Lu, and Y. Zhu, Opt. Commun. 229, 65 (2004). [CrossRef]

].

LPGs can also be realized in photonic-crystal fibers (PCFs) [3

3. B. J. Eggleton, P. S. Westbrook, R. S. Windeler, S. Spälter, and T. A. Strasser, Opt. Lett. 24, 1460 (1999). [CrossRef]

]. PCFs have an array of air holes running along the fiber axis that confine the light to the core. The propagating wave inside the PCF has a particularly strong evanescent wave compared with a standard optical fiber owing to a much-closer proximity of the electromagnetic wave and the holes than the exterior of the fiber. PCFs are characterized by their hole diameter, d, and the pitch of their structure, Λ. Fini [4

4. J. M. Fini, Meas. Sci. Technol. 15, 1120 (2004). [CrossRef]

] has shown that the probing of the holes is strong when the structure has a large air-filling fraction, and the feature size is comparable with the wavelength, i.e., for small pitch and for large air holes. The probing also increases when the contrast between the refractive index of silica and the refractive index of the holes is small. Huy et al. [5

5. M. C. P. Huy, G. Laffont, V. Dewynter-Marty, P. Ferdinand, P. Roy, J.-L. Auguste, D. Pagnoux, W. Blanc, and B. Dussardier, Opt. Lett. 32, 2390 (2007). [CrossRef]

] recently studied the sensitivity of a PCF with a Bragg grating to refractive index. By fabricating a PCF with a very small core and thus a large evanescent field fu, the sensitivity was increased by 2 orders of magnitude with respect to a large core design.

In this Letter we show that the refractive-index sensitivity for a PCF-LPG is almost 2 orders of magnitude larger than the sensitivity for a LPG for standard optical fibers. The sensitivity is enhanced by a factor of 3 by choosing longer wavelengths. Finally, guidelines for designing PCF-LPGs with even surpassing sensitivity are discussed.

In the LPG, the incident core (co) mode is coupled resonantly to a cladding (cl) mode by a periodic perturbation that equals the beat length between the two modes. According to coupled-mode theory [6

6. A. Yariv, IEEE J. Quantum Electron. QE-9, 919 (1973). [CrossRef]

] the resonance condition is λr=[nco(λ)ncl(λ)]ΛG. The effective indices, nco and ncl, also depend on wavelength, and this must be taken into consideration. The PCF used in our work is a large-mode-area PCF with a mode diameter of 10μm from Crystal Fibre A/S [7

7. Crystal Fibre A/S, http://www.crystal-fibre.com.

]. The structure parameters were Λ=7.12μm and dΛ0.478 (Fig. 1 ), making the PCF almost single mode. The LPGs were inscribed with the CO2 laser method [8

8. G. Kakarantzas, T. A. Birks, and P. St. J. Russell, Opt. Lett. 27, 1013 (2002). [CrossRef]

]. We used a Synrad Fenix CO2 laser with a maximum output power of 75W, which is set to 3%. There was no collapsing of the holes, which has also been observed by others [9

9. Y. Zhu, P. Shum, J.-H. Chong, M. K. Rao, and C. Lu, Opt. Lett. 28, 2467 (2003). [CrossRef] [PubMed]

]. In our experimental setup the PCF is mounted in a stage on top of two linear stages. The stages move the fiber in and out of the CO2 laser beam, which is kept fixed. The stages are controlled by a Labview program, and the inscription is fully automated. The inscription progress is monitored in situ in the transmission spectrum. The setup gives a high degree of freedom by the choice of the number of grating periods and the grating period ΛG.

Four PCF-LPGs are fabricated, with the grating periods 820, 740, 620, and 580μm. The number of periods is 60, to our knowledge the largest number reported to date for a CO2-laser-inscribed PCF-LPG, making the total lengths 49, 44, 37, and 35mm, respectively. The spectra and the resonance wavelength are presented in Fig. 2 and Table 1 . The spectra show only a single resonance in contrast to LPGs in standard optical fibers, which have multiple resonances. The PCF-LPGs were 3040cm in length. One end was directly connected to an ANDO AQ-6315A optical spectrum analyzer, and the other end was connected to a broadband halogen light source (Ocean Optics HL-2000).

Methanol has a refractive index close to that of water. Their refractive indices are displayed in Fig. 2 along with the refractive index of silica calculated from a Sellmeier expression. The refractive indices of water and methanol are found from empirical Cauchy expressions [10

10. H. El-Kashef, Physica B 279, 295 (2000). [CrossRef]

], n=A+Bλ2, with the fitted parameters (400800nm): methanol, A=1.29461±1×105 and B=12706.1±0.1nm2; water, A=1.3242±1×105 and B=3063.799±0.031nm2. The thermo-optic coefficient of methanol is dependent on the wavelength but can be found using the equation of Murphy and Alpert [10

10. H. El-Kashef, Physica B 279, 295 (2000). [CrossRef]

].

The methanol is infiltrated into the PCFs by immersing one end in the methanol inside a pressure chamber, with the other end outside. A 200kPa overhead was applied for 1h, after which no bubbles were observed at the exit facet. The methanol is degassed 30min in vacuum prior to infiltration to avoid the formation of air bubbles inside the PCF. The resulting spectra are seen in Fig. 2. The magnitude of the shifts clearly increases with the resonance wavelength. The resonance wavelengths are obtained by interpolating the transmission (on a linear scale) around the resonance dip with a second-order polynomial. Large redshifts of 48, 72, 97, and 127nm in the resonance wavelength are seen. With such large wavelength shifts linearity cannot be expected, since the sensitivity itself increases with increasing refractive index. To obtain the correct sensitivities at a refractive index of the liquid we tune the refractive index by temperature through the thermo-optic coefficient of the liquid. The temperature response of the PCF-LPG with air in the holes is negligible; a mere 6pm°C was measured.

The PCF-LPGs were mounted onto a heater stage with temperature control (MC60 & TH60, Linkam Scientific Instruments). The temperature was increased in steps up to 60°C. The refractive index of the methanol decreases with temperature, since the thermo-optic coefficient is negative (Table 1), thereby blueshifting the resonance wavelengths (Fig. 3 ). In each experiment the temperature was decreased to 30°C to estimate the hysteresis. The hysteresis was 1.4, 0.4, 0.05, and 1.0nm for ΛG=580, 620, 740, and 820μm, respectively. The sensitivity increases (inset in Fig. 3, and Table 1) from 420nmRIU at λr=671nm to 1500nmRIU at λr=1050nm, a more than threefold enhancement. We will allude to this dramatic increase in the following. Taking the minimal detectable wavelength shift to be λFWHM100, 30pm of the resonant dip at 1050nm, the minimal detectable refractive index change is 30pm(1500nmRIU)=2×105. It may also seem surprising that the resonance wavelength is redshifted rather than blueshifted when the methanol is infiltrated into the PCF-LPG. One would suspect that the cladding index was increased more than the core index, and that the resonance wavelength should be blueshifted according to the resonance condition. The apparent anomaly originates in the wavelength dependence of the effective indices. The wavelength dependence causes the resonance wavelength to decrease with increasing grating period, as seen in Table 1. A consistent treatment of the resonance condition with the chain rule [11

11. X. W. Shu, L. Zhang, and I. Bennion, J. Lightwave Technol. 20, 255 (2002). [CrossRef]

, 12

12. L. Rindorf and O. Bang, J. Opt. Soc. Am. B 25, 310 (2008). [CrossRef]

] yields the surprising result for the resonant wavelength shift (dλr)(dnr)=λr(ng,cong,cl)[d(nconcl)]dnr, where nr is the refractive index of methanol and ng,i=niλλni is the group index of mode i. The group-index mismatch ng,cong,cl can be calculated from the reso nance wavelengths and the grating period by ng,cong,cl=λrΛGλrλ(λrΛG), to obtain the curves in Fig. 4 . Clearly, the group-index mismatch is negative, and this accounts for the redshifting of the resonance wavelength for increasing refractive index, since [d(nconcl)]dnr is negative, giving an overall positive sign. Using perturbation theory it is possible to derive an analytical expression for the sensitivity [12

12. L. Rindorf and O. Bang, J. Opt. Soc. Am. B 25, 310 (2008). [CrossRef]

]
dλrdnrd(nconcl)dnr2nconr(fu,cofu,cl),
(1)
where fu,i is the fraction of field inside the holes of the PCF of mode i. The fraction for the core and cladding modes has been calculated using a commercial implementation of the finite-element method [13

13. Cosmol Multiphysics, http://www.comsol.com.

]. The fraction increases, as expected, with wavelength. The cladding fraction is much larger than the core fraction, Fig. 4. Thus the contribution to the wavelength shift is exclusively determined by the perturbation of the cladding mode.

It is enticing to design a LPG-PCF with a high sensitivity by adjusting the PCF structure. One such opportunity is obtaining group-index matching ng,cong,cl0, since the resonance wavelength shift is inversely proportional, as mentioned earlier. Such a sensor could thus have extremely high sensitivity. Unfortunately, the FWHM of the resonance dip also depends on the group-index mismatch, and such a PCF-LPG will have very wide resonance dips [12

12. L. Rindorf and O. Bang, J. Opt. Soc. Am. B 25, 310 (2008). [CrossRef]

]. Taking this fact into account it is still possible to enhance the sensitivity by designing the PCF structure.

The optimized small-core PCF used by Huy et al. [5

5. M. C. P. Huy, G. Laffont, V. Dewynter-Marty, P. Ferdinand, P. Roy, J.-L. Auguste, D. Pagnoux, W. Blanc, and B. Dussardier, Opt. Lett. 32, 2390 (2007). [CrossRef]

] achieved an increase in sensitivity by 2 orders of magnitude with respect to a large core design. We anticipate that a similar increase could be realized for a PCF-LPG. Indeed, theoretical considerations predict a minimal detectable refractive index change of 107 RIU [12

12. L. Rindorf and O. Bang, J. Opt. Soc. Am. B 25, 310 (2008). [CrossRef]

], comparable with the best surface-plasmon-resonance biosensors. This could lead to competitive PCF-LPGs biosensors [14

14. L. Rindorf, J. B. Jensen, M. Dufva, L. H. Pedersen, P. E. Hoiby, and O. Bang, Opt. Express 14, 8824 (2006). [CrossRef]

].

In conclusion, we have demonstrated highly sensitive refractometers in a PCF with an LPG by infiltrating the fluid into the fiber. The sensitivity is increased by a factor of 3 from the resonance wavelengths 6711050nm.

We acknowledge Crystal Fibre A/S [7

7. Crystal Fibre A/S, http://www.crystal-fibre.com.

] for providing the PCF.

Table 1. Resonant Wavelengths for Methanol-Filled PCF-LPGs and the Corresponding Material Parameters nr and (dλr)(dnr) for Methanol at the Resonance Wavelengtha

table-icon
View This Table
Fig. 1 Scanning-electron-microscope pictures of the LMA10 PCF [7] used. The parameters are dΛ=0.478, Λ=7.12μm.
Fig. 2 Spectra of the PCF-LPG filled with air and methanol. Each pair is offset by 5dBm for clarity. Inset, the refractive index of silica glass, water, and methanol.
Fig. 3 Shifts for different resonance wavelengths for methanol-filled LPGs. Inset, the refractive index coefficient for the different resonant wavelengths.
Fig. 4 Group-index mismatch calculated from the resonance wavelengths and grating periods for air- and methanol-filled PCF-LPG. The field fraction as a function of wavelength for the core and cladding mode is shown.
1.

V. Bhatia and A. M. Vengsarkar, Opt. Lett. 21, 692 (1996). [CrossRef] [PubMed]

2.

J. H. Chong, P. Shum, H. Haryono, A. Yohana, M. K. Rao, C. Lu, and Y. Zhu, Opt. Commun. 229, 65 (2004). [CrossRef]

3.

B. J. Eggleton, P. S. Westbrook, R. S. Windeler, S. Spälter, and T. A. Strasser, Opt. Lett. 24, 1460 (1999). [CrossRef]

4.

J. M. Fini, Meas. Sci. Technol. 15, 1120 (2004). [CrossRef]

5.

M. C. P. Huy, G. Laffont, V. Dewynter-Marty, P. Ferdinand, P. Roy, J.-L. Auguste, D. Pagnoux, W. Blanc, and B. Dussardier, Opt. Lett. 32, 2390 (2007). [CrossRef]

6.

A. Yariv, IEEE J. Quantum Electron. QE-9, 919 (1973). [CrossRef]

7.

Crystal Fibre A/S, http://www.crystal-fibre.com.

8.

G. Kakarantzas, T. A. Birks, and P. St. J. Russell, Opt. Lett. 27, 1013 (2002). [CrossRef]

9.

Y. Zhu, P. Shum, J.-H. Chong, M. K. Rao, and C. Lu, Opt. Lett. 28, 2467 (2003). [CrossRef] [PubMed]

10.

H. El-Kashef, Physica B 279, 295 (2000). [CrossRef]

11.

X. W. Shu, L. Zhang, and I. Bennion, J. Lightwave Technol. 20, 255 (2002). [CrossRef]

12.

L. Rindorf and O. Bang, J. Opt. Soc. Am. B 25, 310 (2008). [CrossRef]

13.

Cosmol Multiphysics, http://www.comsol.com.

14.

L. Rindorf, J. B. Jensen, M. Dufva, L. H. Pedersen, P. E. Hoiby, and O. Bang, Opt. Express 14, 8824 (2006). [CrossRef]

OCIS Codes
(050.2770) Diffraction and gratings : Gratings
(060.0060) Fiber optics and optical communications : Fiber optics and optical communications
(060.2370) Fiber optics and optical communications : Fiber optics sensors
(230.3990) Optical devices : Micro-optical devices
(060.3735) Fiber optics and optical communications : Fiber Bragg gratings
(060.5295) Fiber optics and optical communications : Photonic crystal fibers

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: November 21, 2007
Revised Manuscript: December 12, 2007
Manuscript Accepted: January 29, 2008
Published: March 11, 2008

Virtual Issues
Vol. 3, Iss. 4 Virtual Journal for Biomedical Optics

Citation
Lars Rindorf and Ole Bang, "Highly sensitive refractometer with a photonic-crystal-fiber long-period grating," Opt. Lett. 33, 563-565 (2008)
http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-33-6-563


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