Ultrahigh sensitivity is achieved in a new active sensor structure: coupled optofluidic ring laser. The sensor consists of one ring laser and one optofluidic tube. The emission intensity of the multimode whispering gallery resonance from the coupled ring laser is strongly modulated. By using the optofluidic tube as the sensing element, and monitoring the envelope shift of the modulated lasing spectrum, we achieved a sensitivity of 5930 nm/RIU, which is two orders of magnitude higher than a conventional ring resonator sensor.
©2011 Optical Society of America
Whispering gallery (WG) mode resonators are good candidates for high sensitive bio and chemical sensors [1–4]. When bio or chemical analytes enter the evanescent field area of the WG mode, the effective refractive index of the resonant mode (neff) changes a little bit, which subsequently leads to a tiny shift of the resonant wavelength. Sensing ability is normally characterized by resonance wavelength shift per refractive index unit (nm/RIU). Like many other optical guided mode sensors, WG mode sensor responses to the change of neff, which varies along with the real environment refractive index change Δn. The ratio of the values, S = Δneff /Δn, is determined by the proportion of evanescent field in the whole mode volume. In a tightly confined WG mode, majority of optical field is in the cavity, not in the environment, thus S<<1. This limits the sensitivity of WG mode sensors in a range of 10-100 nm/RIU [5,6]. Many attempts have been made to produce larger proportion of evanescent field. As a result, much higher sensitivities (several hundreds nm/RIU) have been achieved [7–9]. One good example is the optofluidic ring resonator (OFRR) sensor. The evanescent wave in an OFRR sensor was manipulated by precisely controlling the fluidic tube wall thickness, leading to a substantial improvement of detection sensitivity (570 nm/RIU) and detection limit (2.8 × 10−7 RIU) .
Up to date, almost all WG mode sensors are passive sensors. Only a few approaches have been made in active WG mode sensing . Active sensors do not need sophisticated setup (e.g. critical coupling with tapered fiber), they generate intense signals and allow parallel detection. In our previous works, we demonstrated that coupled ring laser (CRL) is an excellent sensor structure [11,12]. The CRL has two coupled resonators that have a little different sizes. As a result, the lasing spectrum is strongly modulated. Spectral modulation can be used to generate single mode lasing  and mode hopping sensing . More recently, we found that modulated spectral envelope shift measurement can be a very sensitive sensing scheme . Suppose that only one of the two coupled cavities responses to environmental refractive index change, comparing with the resonance wavelength shift, the modulated spectral envelope shift is amplified by a factor of M = FSR/ΔFSR, so called Vernier effect. Here FSR and ΔFSR are the average free spectral range of the cavities and their difference. If the two cavities have slightly different sizes, FSR>>ΔFSR, the amplification factor can be very large. As a simple estimation, if the two rings in a CRL are 125 μm and 129 μm in diameter, we have M = 31, which means that sensitivity can be orders of magnitude higher.
In this paper, we report on ultrahigh sensitivity of a coupled optofluidic ring laser (CORL). The new sensor consists of one ring laser (master resonator) and one optofluidic tube (slave resonator). The optofluidic tube also serves as sensing element. Majority portion of the WG mode distributes in the tube, therefore S~1. Combining large M and S, very high sensitivity is expected. Experimentally, a sensitivity of 5930 nm/RIU was achieved, which is two orders of magnitude higher than conventional ring resonator sensors, and one order of magnitude higher than evanescent field engineered ring resonator sensors.
2. Experiment method and results
The fabrication process of CORL is basically the same as in our earlier works . Two commercial glass fibers (125 μm diameter, 10 cm long) were used. One fiber was coated with a thin film (refractive index n = 1.52, thickness 2 μm). The coating materials were rhodamine B (RhB) dye doped organic/inorganic hybrid glass materials. The other fiber was treated in Trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane to form a hydrophobic surface. The two fibers were then stacked in parallel with each other and sealed in polydimethylsiloxane (PDMS, refractive index n = 1.42), and annealed at 120 °C for 1 hour. After annealing, the hydrophobic treated fiber was pulled out from the PDMS, leaving in the matrix a hollow tube which serves as the optofluidic channel. Figure 1(a) is the schematic fabrication process of the coupled cavity. Figure 1(b) shows the schematic cross-section of the coupled cavity. When the fluidic channel was connected with a syringe pump and dimethyl sulfoxide (DMSO, refractive index n = 1.477) flows through the channel, the RhB coated fiber and the fluidic channel form a CORL. The two cavities in the device are 129 μm and 125 μm in diameter. Fluid flow rate was kept at 2 μl/minute in experiment.
The CORL was pumped by a 532 nm mode-locked laser (30 ps pulse width, 10 Hz repetition rate). The emitted laser light was collected by a multimode fiber (1 mm in diameter) and was transmitted to a spectrometer which was equipped with a scientific CCD detector.
The emission spectra from a CORL are shown in Fig. 2 . When the optofluidic channel is empty (air filled), it does not support WG mode resonance, the RhB ring works as a circular oscillator, and its multimode lasing spectrum forms a single broad band envelope. When DMSO flows through the optofluidic channel, WG modes exist in both the RhB ring and the optofluidic tube, the device works as a coupled ring laser. As a result, we saw strong spectrum modulation. The emission envelope can be fitted well with a Lorentz shape, envelope center can thus be derived.
We examined the lasing spectrum change when different fluid passed through the fluidic channel. We used mixture of DMSO/water to adjust the refractive index of the fluid in a step of ~0.0004. When the fluid refractive index increases, the modulated spectrum envelope substantially shifts to shorter wavelength (see Fig. 3 ). Figure 4 plots the changes of envelope center wavelength versus refractive index of the fluid. A linear fitting gives a giant refractive index sensitivity of 5930 ± 360 nm/RIU.
We also calculated the modulation spectra of the coupled cavity by using the same theoretical treatment that was described in our previous work . In the calculation, we used the geometry that is shown in Fig. 1(b). Cavity sizes are 129 μm and 125 μm, ring thickness is 2 μm. From Mie scattering calculation , we have S = 0.90. Note again that the large S value comes from large proportion of mode field in the fluidic tube. The calculated modulation spectrum and the changes of envelope center as a function of real fluid refractive index are plotted in Fig. 5 and Fig. 6 . A sensitivity of 5935 nm/RIU was obtained. This result agrees well with the experimental measurement.
The ultrahigh sensitivity of the coupled optofluidic ring laser comes from two parts: Vernier effect induced significant amplification of spectral shift (M) and large ratio of effective refractive index/fluid refractive index change (S). In Ref. , S was optimized by controlling the capillary wall thickness, consequently, sensitivity rose to 570 nm/RIU. In the present work, by using coupled cavity structure, sensitivity was further improved by one order of magnitude when envelope center of the modulated spectrum was monitored.
The detection limit (DL) of the CORL is determined by accuracy in the modulated spectrum envelope center measurement. In experiment, for each refractive index of fluid, we take one spectrum every minute. As it is shown in Fig. 7 , the fitted envelope center fluctuates slightly, because the pump laser power fluctuation is over 10% therefore the resonant mode intensities vary. The mean standard deviation, δ, is 0.16 nm, leading to a bulk refractive index noise equivalent detection limit (NEDL) of 2.7 × 10−5 RIU. Lower DL is expectable, if the modulation spectrum becomes more stable by using more stable pump light, or by generating passive modulation spectrum instead of active modulation spectrum which depends nonlinearly on pump power. If intensity fluctuation can be lowered down efficiently, the accuracy in determining modulated spectrum envelope center will be only limited by spectral resolution. For a spectrometer resolution of 0.01 nm (as in our experiment), envelope center uncertainty is 0.004nm after multi-peak fitting. In that case, a DL of 7 × 10−7 RIU can be reached. Better spectrometer resolution will help in reducing DL furthermore, nevertheless, we would like to emphasize that rapid and convenient detection with high sensitivity are the key advantages of a CORL sensor.
In the present work, the fluidic channel forms in PDMS (n = 1.42), thus the coupled cavity works only for fluids that have higher refractive indices. However, an alternative coupled cavity structure is a ring laser coupling with a fluidic glass capillary tube. In that case, the device can be exposed in air, and water can be used as the fluid. Hence CORL can be a high sensitivity bio-sensor.
In summary, very large sensitivity was achieved in a coupled optofluidic ring laser. Sensitivity as high as 5930 nm/RIU is obtained experimentally and agrees well with theoretical calculation. The new sensing scheme and senor structure open a new way in achieving ultrahigh sensitive bio and chemical sensing.
This work is supported in part by National Natural Science Foundation of China (grant # 10874033, 60977047, 60907011, 61078052, 11074051), National Basic Research Program of China (973 Program) (grant # 2011CB921802) and Natural Science Foundation of Shanghai (grant # 09ZR1402800).
References and links
2. J. G. Zhu, S. K. Ozdemir, Y.-F. Xiao, L. Li, L. He, D.-R. Chen, and L. Yang, “On-chip single nanoparticle detection and sizing by mode splitting in an ultrahigh-Q microresonator,” Nat. Photonics 4(1), 46–49 (2010). [CrossRef]
4. A. M. Armani, R. P. Kulkarni, S. E. Fraser, R. C. Flagan, and K. J. Vahala, “Label-free, single-molecule detection with optical microcavities,” Science 317(5839), 783–787 (2007). [CrossRef] [PubMed]
5. N. M. Hanumegowda, C. J. Stica, B. C. Patel, I. White, and X. D. Fan, “Refractometric sensors based on microsphere resonators,” Appl. Phys. Lett. 87(20), 201107 (2005). [CrossRef]
6. I. M. White, H. Zhu, J. Suter, N. M. Hanumegowda, H. Oveys, M. Zourob, and X. D. Fan, “Refractometric sensors for lab-on-a-chip based on optical ring resonators,” IEEE Sens. J. 7(1), 28–35 (2007). [CrossRef]
7. I. Teraoka and S. Arnold, “Enhancing the sensitivity of a whispering-gallery mode microsphere sensor by a high-refractive-index surface layer,” J. Opt. Soc. Am. B 23(7), 1434–1441 (2006). [CrossRef]
9. H. Li and X. D. Fan, “Characterization of sensing capability of optofluidic ring resonator biosensors,” Appl. Phys. Lett. 97(1), 011105 (2010). [CrossRef]
10. A. Francois and M. Himmelhaus, “Whispering gallery mode biosensor operated in the stimulated emission regime,” Appl. Phys. Lett. 94(3), 031101 (2009). [CrossRef]
12. H. Li, L. Shang, X. Tu, L. Y. Liu, and L. Xu, “Coupling variation induced ultrasensitive label-free biosensing by using single mode coupled microcavity laser,” J. Am. Chem. Soc. 131(46), 16612–16613 (2009). [CrossRef] [PubMed]
13. X. W. Zhang, H. Li, X. Tu, X. Wu, L. Y. Liu, and L. Xu, “Suppression and hopping of whispering gallery modes in multiple-ring-coupled microcavity lasers,” J. Opt. Soc. Am. B 28(3), 483–488 (2011). [CrossRef]
14. C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles (John Wiley & Sons, Inc., 1998).