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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 22 — Oct. 29, 2007
  • pp: 14376–14381
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Optical liquid ring resonator sensor

M. Sumetsky, R. S. Windeler, Y. Dulashko, and X. Fan  »View Author Affiliations


Optics Express, Vol. 15, Issue 22, pp. 14376-14381 (2007)
http://dx.doi.org/10.1364/OE.15.014376


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Abstract

We demonstrate a robust and highly responsive optical microsensor, which probes the refractive index of liquids flowing along a ~ 100 μm radius channel formed in a polymer matrix. Sensing is based on measurement of the transmission spectrum of the whispering gallery modes, which are excited across the liquid channel by an optical microfiber imbedded into the polymer. The achieved sensitivity is 800 nm/RIU. Potentially, it is straightforward to assemble the sensing elements of this type into a lab-on-the-chip imbedded in a solidified optical material.

© 2007 Optical Society of America

1. Introduction

2. Fabrication of the LRROS

Fig. 1. Illustration of an LRROS

3. Measurement of the LRROS sensitivity

If the refractive index of the tested liquid, nc, is greater than that of the polymer np, then, with thinning of the CF wall, the WGMs move from the CF wall into the tested liquid. As a result, the sensitivity of the LRROS grows dramatically. However, for nc < np and in the absence of the CF, the WGMs no longer exist and oscillations in transmission spectrum disappear. For this reason, in our first experiment the CF was not removed completely. This allowed us to create a highly sensitive LRROS, which worked both for nc > np and for nc < np. In pratice, we periodically interrupted the etching process in order to measure the transmission spectrum of the LRROS for the low refractive index liquid with nc = 1.296 and to ensure that the transmission spectrum still had noticeable oscillations. The etching was stopped when the oscillations at 1.296nc= were as depicted by the black curve in Fig. 2(b). The red curve in Fig. 2(a) shows that at this stage of etching the transmission oscillations for the empty core (i.e. for nc = 1) disappeared. We measured the sensitivity of our device using the Cargille Labs optical refractive index matching liquids. At the wavelength of 1.54 μm, the refractive indices of these liquids were calculated using the Cauchy equations provided by the Cargille Labs. The liquids were delivered into the CF with a vacuum pump. Figures 2(b) and 2(c) show the representative transmission spectra of liquids with refractive indices 1.296, 1.390, 1.444, 1.496, and 1.606. Most of the curves in Fig. 2 have a characteristic double-dip periodic structure. We suspect that two dips correspond to two polarizations of light similar to the ring resonator (see e.g. [6

6. M. Sumetsky, Y. Dulashko, J. M. Fini, A. Hale, and D. J. DiGiovanni, “The Microfiber Loop Resonator: Theory, Experiment, and Application,” IEEE J. Lightwave Technol. 24, 242–250 (2006). [CrossRef]

]). Figure 2(d) shows an example of our measurement of the sensitivity. The spectrum of the LRROS was recorded for close refractive indices n 1 = 1.444 and n 2 = 1.446. The sensitivity is expressed through the shift of the dips in these spectra, Δλ , as S = Δλ/Δn. For the spectra depicted in Fig. 2(d), Δλ ≈ 1 nm and Δn = n 2 ? n 1 = 0.002 resulting in S ≈ 500 nm/RIU. It is seen that the transmission spectrum of the matching liquid with refractive index 1.444 (blue curve in Fig. 2(b)) well matches the transmission spectrum of unetched silica CF (black curve in Fig. 2(a)) with the same refractive index. A small shift between these curves of 0.35 nm can be explained by inaccuracy of the refractive index of glass and the matching liquid, which are given here with 0.001 RIU accuracy. Actually, with the sensitivity of 500 nm/RIU, the 0.35 nm inaccuracy in wavelength corresponds to the inaccuracy in refractive index of 0.35/500=0.0007 RIU. The plot of sensitivity of the LRROS is shown in Fig. 3, black curve. For the test liquids with larger refractive indices, nc > 1.5, the sensitivity was as high as ~ 700 nm/RIU. For smaller indices, the sensitivity decreased and was ~ 25 nm/RIU at nc = 1.296. The sensitivity achieved at larger indices was comparable with the maximum possible sensitivity at refractive index nc and wavelength λ, Smax = λ/nc [15

15. H. Zhu, I. M. White, J. D. Suter, P. S. Dale, and X. Fan, “Analysis of biomolecule detection with optofluidic ring resonator sensors,” Opt. Express 15, 9139–9146 (2007); [CrossRef] [PubMed]

]. The dotted curve in Fig. 3 shows this dependence for λ = 1.54 μm.

Fig. 2. Transmission spectrum of the LRROS for different indices of the tested liquids. (a) - Unetched CF (black) and etched CF filled with air (red); (b),(c) - Examples of transmission spectra for different refractive indices of liquids; the values of indices are shown on the figures. (d) - Illustration of measurement of the LRROS sensitivity.
Fig. 3. The LRROS sensitivity measured when the CF was not removed completely (black curve) and for the removed CF (red curve). The dotted violet curve is the maximum theoretical limit of sensitivity. The dashed blue curve is the sensitivity suspected for the Teflon AF matrix.
Fig. 4. Transmission spectra of the LRROS with the removed CF at refractive indices shown in the figure.

In order to further increase the sensitivity of the LRROS, we continued etching the CF down to what we believed corresponded to complete removal of the silica capillary. We stopped etching when the transmission spectrum of the device at refractive indices below the index of the polymer matrix, np = 1.384, did not have the WGM oscillations. The corresponding spectra are depicted in Fig. 4. Interestingly, at lower indices, the transmission exhibited significant attenuation with the change of the refractive index. Thus, at indices below nc ~ 1.4, our device performed as an attenuation (nonresonant) sensor similar to the microfiber sensor of Ref. [18

18. P. Polynkin, A. Polynkin, N. Peyghambarian, and M. Mansuripur, “Evanescent field-based optical fiber sensing device for measuring the refractive index of liquids in microfluidic channels,” Opt. Lett. 30, 1273–1275 (2005). [CrossRef] [PubMed]

]. For the empty channel (nc = 1), the transmission spectrum (black curve in Fig. 4) showed more than 4 dB increase in transmission power compared to the similar spectrum of the under-etched device (red curve in Fig. 2(a)). This may be due to the reduction of the power loss caused by radiation from the MF into the silica wall of the CF (the mechanism of radiation is similar to that described in Ref. [20

20. M. Sumetsky, Y. Dulashko, J. M. Fini, A. Hale, and J. W. Nicholson, “Probing optical microfiber nonuniformities at nanoscale,” Opt. Lett. 31, 2393–2395 (2006). [CrossRef] [PubMed]

]). At higher indices, the WGMs oscillations re-appear as illustrated by the transmission spectra at nc = 1.41 and 1.43 in Fig. 4. The corresponding sensitivity of the LRROS is given by the red curve in Fig. 3 obtained by the measurement of the shifts in WGM transmission spectra as described above. This curve demonstrates the increase of the sensitivity at larger indices up to ~ 800 nm/RIU and also a significant growth of sensitivity at lower indices. It should be noted that the resonant transmission spectrum of light propagating in a polymer is by an order of magnitude more sensitive to temperature variations than the light propagating in glass [14

14. X. Fan, I. M. White, H. Zhu, J. D. Suter, and H. Oveys, “Overview of novel integrated optical ring resonator bio/chemical sensors,” Proc. SPIE 6452, 6452M, 1–20 (2007).

]. For this reason, for highly accurate measurement of the refractive index, the temperature variations should be controlled and suppressed.

4. Discussion and summary

The LRROS, which is introduced in this paper, shows very high sensitivity enabling very accurate measurement of refractive index variations. For example, for the sensitivity S = 800 nm/RIU and with 1 pm wavelength measurement resolution, the accuracy of (800 nm/RIU)/pm ~ 10-6 can be achieved. The shapes of the transmission spectra measured for liquids with different refractive indices were qualitatively different as illustrated in Fig. 2. For this reason, the computer pattern recognition would allow us to achieve very high accuracy in measuring variations of the refractive index as well as its absolute values.

Using the developed fabrication technique, it is straightforward to create a lab-on-the-chip device consisting of many LRROS elements. In application to the LCORRS, this concept has been discussed previously [15

15. H. Zhu, I. M. White, J. D. Suter, P. S. Dale, and X. Fan, “Analysis of biomolecule detection with optofluidic ring resonator sensors,” Opt. Express 15, 9139–9146 (2007); [CrossRef] [PubMed]

]. The advantage of our design is that it has the significantly increased sensitivity and robustness, and that it ensures protection from contamination and corrosion.

It is possible that the sensitivity of LRROS can be increased even more by further modifications. In particular, the polymer matrix with refractive index 1.384 np = used in our experiments can be replaced by a very low index Teflon AF with np = 1.291. We anticipate that, then, the plot of sensitivity, the red solid curve in Fig. 3, will be shifted to the left towards lower refractive indices by approximately the difference of these indices, 1.384 – 1.291 ~ 0.1. This would significantly increase the sensitivity at lower refractive indices as illustrated by a dashed blue curve in Fig. 3. In the regime of attenuation sensing (Fig. 4) the sensitivity could be increased by using a thinner MF which causes greater radiation loss [20

20. M. Sumetsky, Y. Dulashko, J. M. Fini, A. Hale, and J. W. Nicholson, “Probing optical microfiber nonuniformities at nanoscale,” Opt. Lett. 31, 2393–2395 (2006). [CrossRef] [PubMed]

] and stronger dependence on the refractive index nc.

References and links

1.

M. Noto, F. Vollmer, D. Keng, I. Teraoka, and S. Arnold, “Nanolayer characterization through wavelength multiplexing of a microsphere resonator,” Opt. Lett. 30, 510–512 (2005). [CrossRef] [PubMed]

2.

I. M. White, N. M. Hanumegowda, and X. Fan, “Subfemtomole detection of small molecules with microsphere sensors,” Opt. Lett. 30, 3189–3191 (2005). [CrossRef] [PubMed]

3.

Ashkenazi, C.-Y. Chao, L. J. Guo, and M. O’Donnell, “Ultrasound detection using polymer microring optical resonator,” Appl. Phys. Lett. 85, 5418–5420 (2004). [CrossRef]

4.

A. Ksendzov and Y. Lin, “Integrated optics ring-resonator sensors for protein detection,” Opt. Lett. 30, 3344–3346 (2005). [CrossRef]

5.

C-Y. Chao, W. Fung, and L. J. Guo, “Polymer Microring Resonators for Biochemical Sensing Applications,” IEEE J. Sel. Top. Quantum Electron. 12, 134–142 (2006). [CrossRef]

6.

M. Sumetsky, Y. Dulashko, J. M. Fini, A. Hale, and D. J. DiGiovanni, “The Microfiber Loop Resonator: Theory, Experiment, and Application,” IEEE J. Lightwave Technol. 24, 242–250 (2006). [CrossRef]

7.

A. Yalçin, K.C. Popat, J. C. Aldridge, T. A Desai, J. Hryniewicz, N. Chbouki, B. E. Little, O. King, V. Van, S. Chu, D. Gill, M. Anthes-Washburn, M. S. Ünlü, and B. B. Goldberg, “Optical Sensing of Biomolecules using Microring Resonators,” IEEE J. Sel. Top. Quantum Electron. 12, 148–155 (2006). [CrossRef]

8.

R. W. Boyd and J. E. Heebner, “Sensitive disk resonator photonic biosensor,” Appl. Opt. 40, 5742–5747 (2001). [CrossRef]

9.

E. Krioukov, D. J. W. Klunder, A. Driessen, J. Greve, and C. Otto, “Integrated optical microcavities for enhanced evanescent-wave spectroscopy,” Opt. Lett. , 27, 1504–1506 (2002). [CrossRef]

10.

T. A. Birks, J. C. Knight, and T. E. Dimmick, “High-resolution measurement of the fiber diameter variations using whispering gallery modes and no optical alignment,” IEEE Photon. Technol. Lett. 12, 182–184 (2000). [CrossRef]

11.

M. Sumetsky and Y. Dulashko, “Sensing an optical fiber surface by a microfiber with angstrom accuracy,” in Optical Fiber Communication conference, paper OTuL6, (Anaheim, 2006).

12.

I. M. White, H. Oveys, and X. Fan, “Liquid Core Optical Ring Resonator Sensors,” Opt. Lett. 31, 1319–1321 (2006). [CrossRef] [PubMed]

13.

I. M. White, H. Oveys, X. Fan, Terry L. Smith, and J. Zhang, “Integrated multiplexed biosensors based on liquid core optical ring resonators and antiresonant reflecting optical waveguides,” Appl. Phys. Lett. 89, Art. 191106 (2006). [CrossRef]

14.

X. Fan, I. M. White, H. Zhu, J. D. Suter, and H. Oveys, “Overview of novel integrated optical ring resonator bio/chemical sensors,” Proc. SPIE 6452, 6452M, 1–20 (2007).

15.

H. Zhu, I. M. White, J. D. Suter, P. S. Dale, and X. Fan, “Analysis of biomolecule detection with optofluidic ring resonator sensors,” Opt. Express 15, 9139–9146 (2007); [CrossRef] [PubMed]

16.

W. Liang, Y. Huang, Y. Xu, R. K. Lee, and A. Yariv, “Highly sensitive fiber Bragg grating refractive index sensors,” Appl. Phys. Lett. 86, Art. 151122 (2005). [CrossRef]

17.

N. Chen, B. Yun, Y. Wang, and Y. Cui, “Theoretical and experimental study on etched fiber Bragg grating cladding mode resonances for ambient refractive index sensing,” J. Opt. Soc. Am. B 24, 439–445 (2007). [CrossRef]

18.

P. Polynkin, A. Polynkin, N. Peyghambarian, and M. Mansuripur, “Evanescent field-based optical fiber sensing device for measuring the refractive index of liquids in microfluidic channels,” Opt. Lett. 30, 1273–1275 (2005). [CrossRef] [PubMed]

19.

M. Sumetsky, Y. Dulashko, and A. Hale, “Fabrication and study of bent and coiled free silica nanowires: Self-coupling microloop optical interferometer,” Opt. Express 12, 3521–3531 (2004). [CrossRef] [PubMed]

20.

M. Sumetsky, Y. Dulashko, J. M. Fini, A. Hale, and J. W. Nicholson, “Probing optical microfiber nonuniformities at nanoscale,” Opt. Lett. 31, 2393–2395 (2006). [CrossRef] [PubMed]

OCIS Codes
(060.2370) Fiber optics and optical communications : Fiber optics sensors
(120.1880) Instrumentation, measurement, and metrology : Detection
(170.4520) Medical optics and biotechnology : Optical confinement and manipulation
(230.5750) Optical devices : Resonators
(060.4005) Fiber optics and optical communications : Microstructured fibers

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: September 6, 2007
Revised Manuscript: October 9, 2007
Manuscript Accepted: October 11, 2007
Published: October 16, 2007

Virtual Issues
Vol. 2, Iss. 11 Virtual Journal for Biomedical Optics

Citation
M. Sumetsky, R. S. Windeler, Y. Dulashko, and X. Fan, "Optical liquid ring resonator sensor," Opt. Express 15, 14376-14381 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-22-14376


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References

  1. M. Noto, F. Vollmer, D. Keng, I. Teraoka, and S. Arnold, "Nanolayer characterization through wavelength multiplexing of a microsphere resonator," Opt. Lett. 30, 510-512 (2005). [CrossRef] [PubMed]
  2. I. M. White, N. M. Hanumegowda, and X. Fan, "Subfemtomole detection of small molecules with microsphere sensors," Opt. Lett. 30, 3189-3191 (2005). [CrossRef] [PubMed]
  3. Ashkenazi, C.-Y. Chao, L. J. Guo, and M. O’Donnell, "Ultrasound detection using polymer microring optical resonator," Appl. Phys. Lett. 85, 5418-5420 (2004). [CrossRef]
  4. A. Ksendzov and Y. Lin, "Integrated optics ring-resonator sensors for protein detection," Opt. Lett. 30, 3344-3346 (2005). [CrossRef]
  5. C-Y. Chao, W. Fung, and L. J. Guo, "Polymer Microring Resonators for Biochemical Sensing Applications," IEEE J. Sel. Top. Quantum Electron. 12, 134-142 (2006). [CrossRef]
  6. M. Sumetsky, Y. Dulashko, J. M. Fini, A. Hale, and D. J. DiGiovanni, "The Microfiber Loop Resonator: Theory, Experiment, and Application," IEEE J. Lightwave Technol. 24, 242-250 (2006). [CrossRef]
  7. A. Yalçin, K.C. Popat, J. C. Aldridge, T. A Desai, J. Hryniewicz, N. Chbouki, B. E. Little, O. King, V. Van, S. Chu, D. Gill, M. Anthes-Washburn, M. S. Ünlü, and B. B. Goldberg, "Optical Sensing of Biomolecules using Microring Resonators," IEEE J. Sel. Top. Quantum Electron. 12, 148-155 (2006). [CrossRef]
  8. R. W. Boyd and J. E. Heebner, "Sensitive disk resonator photonic biosensor," Appl. Opt. 40, 5742-5747 (2001). [CrossRef]
  9. E. Krioukov, D. J. W. Klunder, A. Driessen, J. Greve, and C. Otto, "Integrated optical microcavities for enhanced evanescent-wave spectroscopy," Opt. Lett.,  27, 1504-1506 (2002). [CrossRef]
  10. T. A. Birks, J. C. Knight, and T. E. Dimmick, "High-resolution measurement of the fiber diameter variations using whispering gallery modes and no optical alignment," IEEE Photon. Technol. Lett. 12, 182-184 (2000). [CrossRef]
  11. M. Sumetsky and Y. Dulashko, "Sensing an optical fiber surface by a microfiber with angstrom accuracy," in Optical Fiber Communication conference, paper OTuL6, (Anaheim, 2006).
  12. I. M. White, H. Oveys, and X. Fan, "Liquid Core Optical Ring Resonator Sensors," Opt. Lett. 31, 1319-1321 (2006). [CrossRef] [PubMed]
  13. I. M. White, H. Oveys, X. Fan, T. L. Smith, and J. Zhang, "Integrated multiplexed biosensors based on liquid core optical ring resonators and antiresonant reflecting optical waveguides," Appl. Phys. Lett. 89, 191106 (2006). [CrossRef]
  14. X. Fan, I. M. White, H. Zhu, J. D. Suter, and H. Oveys, "Overview of novel integrated optical ring resonator bio/chemical sensors," Proc. SPIE 6452, 6452M, 1-20 (2007).
  15. H. Zhu, I. M. White, J. D. Suter, P. S. Dale, and X. Fan, "Analysis of biomolecule detection with optofluidic ring resonator sensors," Opt. Express 15, 9139-9146 (2007); [CrossRef] [PubMed]
  16. W. Liang, Y. Huang, Y. Xu, R. K. Lee, and A. Yariv, "Highly sensitive fiber Bragg grating refractive index sensors,"Appl. Phys. Lett. 86, Art. 151122 (2005). [CrossRef]
  17. N. Chen, B. Yun, Y. Wang, and Y. Cui, "Theoretical and experimental study on etched fiber Bragg grating cladding mode resonances for ambient refractive index sensing," J. Opt. Soc. Am. B 24,439-445 (2007). [CrossRef]
  18. P. Polynkin, A. Polynkin, N. Peyghambarian, and M. Mansuripur, "Evanescent field-based optical fiber sensing device for measuring the refractive index of liquids in microfluidic channels," Opt. Lett. 30, 1273-1275 (2005). [CrossRef] [PubMed]
  19. M. Sumetsky, Y. Dulashko, and A. Hale, "Fabrication and study of bent and coiled free silica nanowires: Self-coupling microloop optical interferometer," Opt. Express 12, 3521-3531 (2004). [CrossRef] [PubMed]
  20. M. Sumetsky, Y. Dulashko, J. M. Fini, A. Hale, and J. W. Nicholson, "Probing optical microfiber nonuniformities at nanoscale," Opt. Lett. 31, 2393-2395 (2006). [CrossRef] [PubMed]

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