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Highly-sensitive silicon-on-insulator sensor based on two cascaded micro-ring resonators with vernier effect
Optics Communications 284 (2011) 156–159
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Highly-sensitive silicon-on-insulator sensor based on two cascaded micro-ring resonators with vernier effect Lei Jin, Mingyu Li ⁎, Jian-Jun He Centre for Integrated Optoelectronics, State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, Hangzhou, PR China 310027
a r t i c l e
i n f o
Article history: Received 1 July 2010 Received in revised form 13 August 2010 Accepted 15 August 2010 Keywords: Optical sensor Silicon-on-insulator Cascaded micro-rings resonator
a b s t r a c t A highly-sensitive integrated optical biosensor based on two cascaded micro-rings resonator (MRR) is investigated theoretically and experimentally. The free spectral ranges (FSRs) of two cascaded micro-rings are designed to be slightly different in order to generate Vernier effect. A preliminary investigation of our sensor with a Q factor of 2 × 104 using different ethanol concentrations shows that the Vernier effect can improve the sensitivity to 1300 nm per refractive index unit (RIU), compared to 62 nm/RIU for a single ring sensor. The sensor also has a large measurement range of refractive index change up to 1.15 × 10− 2 RIU. It can be useful for low-cost and highly-sensitive optical biosensor system. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Because of its low-cost, highly-sensitive and ultra-compact features, much research effort has recently been devoted to the development of optical label-free biosensors [1–4]. Surface plasmon resonances in the noble-metal nano-structures and evanescent field (EF) waveguides sensing are the two optical techniques which are most commonly used in label-free biosensing applications such as environmental monitoring, biological recognition, medical diagnostics, food quality and safety analysis [1–8]. Significant effort is put into the development of EF sensors due to their potential for increased sensitivity and suitability for implementing into multi-channel detection. The waveguide EF sensors rely on monitoring the perturbation of the waveguide mode effective refractive index neff due to the various concentrations of an analyte in the solution that is in contact with the waveguide surface. People have developed all kinds of structures and mechanisms such as interferometer-based biosensors [5,6], high-Q optical ring resonator based biosensors [7], and optical fiber based biosensors [8]. Among them, micro-ring resonator can provide a higher sensitivity due to the sharp resonance peak. There are two typical interrogation approaches which have been used in a high-Q optical micro-resonant cavities sensing: intensity interrogation and wavelength interrogation [7]. Intensity interrogation is the simplest way for sensing, which measures the output intensity change at a given wavelength. Although the intensity interrogation has a high limit of detection, the measurement range is very small because of a high-Q resonant cavity. For the wavelength
⁎ Corresponding author. Tel.: +86 571 87953340; fax: +86 571 87953875. E-mail address: [email protected] (M. Li). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.08.035
interrogation, the change in the ambient refractive index can be detected by the magnitude of the wavelength shift in the resonant mode. To detect the small shift of resonant wavelength and improve the sensitivity, the combination of a very narrow line width tunable laser and power meter or a broad band source with a high wavelength resolution optical spectrum analyzer (OSA) can be utilized. But these instruments are very expensive and complex and cannot be integrated with optical sensor on one chip. However, a low wavelength resolution micro-OSA based on array waveguide grating (AWG) or etched diffraction grating (EDG) is low-cost and can be integrated with optical sensor on one chip [9,10]. This requires the sensor to produce a large resonant wavelength shift with a small refractive index change of analyte. In this paper, we investigate an EF waveguide sensor based on two cascaded micro-rings resonator with slightly different diameters to introduce Vernier effect. Although the Vernier effect was used in other applications such as optical filters [11,12] and tunable lasers [13,14], it was only recently proposed for sensor application [15]. By using the Vernier effect we can increase the sensitivity of the effective refractive index measurement without requiring a narrow line width tunable laser or a high-resolution OSA. The experimental results of the cascaded micro-ring resonator sensor based on SOI waveguide will be presented. A wavelength shift sensitivity as high as 1300 nm/RIU (Refractive Index Unit) is obtained. The sensor also has a large measurement range of refractive index change up to 1.15 × 10− 2 RIU. The SOI based biosensor is compatible with standard CMOS processing and potentially allows for cheap mass production and integration with electronics. This technology is readily combined with microfluidic components, and is easily extendable to a multi-array biosensor with thousands of sensing spots for real lab-on-chip applications.
L. Jin et al. / Optics Communications 284 (2011) 156–159
157
Δnminwhich produces a detectable change in the sensor output Δλmin is Δnmin = neff ΔλFSR −ΔλFSR = λB × Δnc = Δneff
ð2Þ
B
A
Here Δneff / Δnc is the ratio of the change in the waveguide effective refractive index to the change in the analyte refractive index, which is determined by the structure of the waveguide. The sensor sensitivity S is the ratio of the change in sensor's output (e.g., resonant wavelength) to the change in the quantity to be measured (e.g., the refractive index of analyte). h i S = Δλ = Δnc = ΔλB = Δneff Δneff = Δnc ΔλFSR = ΔλFSR −ΔλFSR ð3Þ A
Fig. 1. Schematic of the two cascaded micro-rings resonator sensor based on silicon-oninsulator. The reference ring A is covered by SU8 upper cladding layer while the sensing ring B is exposed to analyte in the sensing window.
2. Device structure and operation principle Two micro-rings resonators based on SOI were cascaded by a ridge waveguide to form a high-sensitive optical sensor, as shown in Fig. 1. The whole optical sensor is covered by SU8 upper cladding layer except the sensing ring B, which is exposed to a fluid sample to be analyzed. The free spectral ranges (FSRs) of the two ringsΔλFSRA,ΔλFSRB are designed to be slightly different to employ the Vernier effect by choosing different radii (RA and RB) for ring A and ring B, respectively. To describe the principle of our sensor, the transmission spectra TA and TB for ring A and B are plotted in Fig. 2. In our design, RA b RB results in ΔλFRSA N ΔλFRSB. The total transmission spectrum T equals to TA × TB, as shown in Fig. 2(c). The maximal peak of transmission T is located at the mutual resonant wavelength of ring A and ring B. The variation of the refractive index of the analyte fluid in the sensing window Δnc results in a change in the effective refractive index of ring B Δneff, consequently the resonant wavelength of the ring B changes by ΔλB = λB(Δneff / neff). According to the Vernier effect, the change Δλ in the resonant wavelength of the two cascaded miro-rings resonator is h i Δλ = ΔλB × ΔλFSR = ΔλFSR −ΔλFSR A
A
B
ð1Þ
Since the resonant wavelength of the reference ring A is fixed, the minimal change of Δλmin is ΔλFSRAwhen ΔλB changesΔλFSRA − ΔλFSRB. So the smallest change in the refractive index of the analyte
A
B
Here ΔλB = Δneff Δneff = Δnc is the sensitivity of a single ring B. From Eq. (3), the sensitivity of the two cascaded micro-rings resonator is magnified by a factor of M ¼ΔλFSRA = ΔλFSRA −ΔλFSRB as compared with that of a single micro-ring resonator. The FSR of two cascaded micro-rings resonator is given by ΔλFRS = ΔλFRS ΔλFRS = ΔλFRS −ΔλFRS A
B
A
B
ð4Þ
From Eq. (3), the maximal refractive index change is Δnmax = ΔλFSRA / S. 3. Design and fabrication The sensor is designed on SOI substrate with a 220 nm Si layer on a 2 μm SiO2 insulator layer. Since SOI is a high refractive index contrast material, the waveguide should be designed to be very small to keep the waveguide single mode. In our design, the widths of all the ridge waveguides are designed to be 1 μm with a shallow etched ridge height of 20–40 nm in order to maintain the single mode waveguide. We choose directional coupler to couple light into and out of the micro-ring resonators, with the minimal distance between the bus waveguide and ring to be 1 μm as well, so that this optical sensor can be fabricated by using contact photolithography. The interference orders of ring A and ring B at 1550 nm are chosen to be 1000 and 1100 with 10% difference, corresponding to ring diameters of 240 μm and 264 μm, respectively. Because numerical simulation shows the waveguide sensitivity of the waveguide sensitivity of our waveguide structure Δneff / Δnc is higher for TM polarization than for TE polarization [5], about 41.7% and 4.73% respectively, the sensor operated in TM mode is in principle much more sensitive. The FSRs of ring A and ring B's transmission spectrums
Fig. 2. Transmission spectra of ring A (a) and ring B (b); and (c) total transmission spectrum.
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L. Jin et al. / Optics Communications 284 (2011) 156–159
a
b
c
Fig. 3. (a)Optical microscope image of cascaded micro-ring resonator sensor; (b) SEM picture of the direction coupler; and (c) output mode captured by infrared camera.
at 1550 nm are 0.779 nm and 0.709 nm respectively, as shown in Fig. 2. In this case, the minimal refractive index change Δnmin = 2.2 × 10 − 4 can be obtained from Eq. (2) by wavelength interrogation method with the amplification factor M = 11. The ratio between the wavelength shift and one unit of the refractive index change is 3456 nm/RIU. The FSR of two cascaded micro-rings resonator is ΔλFSR = 7.89 nm. Therefore, the maximal refractive index change Δnmax is 2.2 × 10−3. In the same situation for TE mode, the minimal refractive index change and the maximal refractive index change is about one order of magnitude larger. The two cascaded micro-ring resonator sensor was fabricated by using conventional photolithography, a 1.1 μm thick photoresist deposited by spin-coating served as the etch mask for etching of SOI. The SOI was patterned by reactive ion etching (RIE) using CF4 chemistry for 3 min. The height of the ridge waveguide is 40 nm, which is measured by Surface Profiler (Dektak 150). The perimeters of the two rings are about 957 μm and 995 μm, respectively. The light is coupled from the bus waveguide to the rings, and vice versa, by using directional couplers. The whole sensor was covered by Su-8 layer as upper cladding and a sensing window was opened on the top of the ring B by photolithography to form a sample reservoir as shown in Fig. 3.
4. Experimental results and discussions In order to measure the spectral response of the device, we use a tunable laser (Agilent 81600B) with a wavelength resolution of 0.01 nm as the light source and the output is received by a power sensor (Agilent 81635A). The light is coupled into or out of the sensor by lensed fibers. Because the resonant wavelengths of ring A and ring B have slightly different FSRs, an oscillatory envelope function results, with a major peak of the transmission spectrum located at 1546.93 nm, as we can see from Fig. 4. Far away from the major peak, the difference of the resonant wavelengths between the two rings becomes larger, resulting in two separated doublet peaks with low intensity. The water–ethanol mixtures of different volume concentration (10%, 15%, and 20%) were used to demonstrate how the sensor works. The experimental result is shown in Fig. 5. The refractive index of an ethanol solution varies by 3 × 10−3 RIU per 5% concentration variation [16] at 20 °C. Since the resonant wavelength of two micro-rings resonator is determined by the peak of the fixed ring A that coincides with a peak of the sensing ring B, a wavelength shift in the resonant peaks of the sensing ring B with the refractive index variation results
Fig. 4. Measured and calculated transmission spectra of the cascaded micro-rings when ring A and ring B are both covered by Su-8 layer. The two insets show the detail of the transmission peaks far away from the mutual resonant wavelength and close to the mutual resonant wavelength, respectively.
L. Jin et al. / Optics Communications 284 (2011) 156–159
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Fig. 5. Measured transmission spectra of the cascaded micro-rings when ring B is covered by water with different concentrations of ethanol (a) 10%, (b) 15%, and (c) 20%.
in a jump of the fixed ring A in the resonant wavelength of our sensor. Therefore, the resolution can be improved significantly. It can be seen from Fig. 5, the extinction ratio of the central peak reaches 25 dB with a Q factor about 2 × 104 and the 5% change of the concentration results in 4 nm resonant wavelength shift of the sensor. We can deduce the sensitivity S (the ratio between the wavelength shift and the refractive index change) is about 1300 nm/RIU, over an order higher than a single ring sensor [7]. By fitting with experimental results ΔλFSRA and ΔλFSRBare found to be 0.656 nm and 0.629 nm, respectively and FSR of two cascaded micro-rings resonator ΔλFSR is 15 nm. Therefore, we can deduce that the amplification factor M is 24.3. For a single ring in the two cascaded micro-rings resonator, we can also calculate the sensitivity which is about 62 nm/RIU. From Eqs. (1) and (3), the measurement range of the refractive index change is derived to be 1.15 × 10− 2 RIU, and the detection resolution corresponding to a shift of the major peak from one resonant mode of the reference ring to its adjacent mode is 5.05 × 10−4 RIU. It is known from Eq. (3), in order to increase the sensitivity S, one can decrease the difference between ΔλFSRA and ΔλFSRB. However, in this case, the intensity difference between the major peak and its adjacent peaks becomes smaller, which may increase the possibility of detection errors. In Fig. 5, the intensity peaks fluctuate, because of the cleaved facets of the input and output waveguides forming a Fabry–Perot cavity. By eliminating the intensity fluctuation, the intensity ratios among the central peaks can be used to fit the wavelength shift more accurately, allowing the sensitivity to be further increased. The experimental results shown above are measured for TE mode. Due to the resolution limit of our contact photolithographic aligner, shallow etched waveguides were used with 1 μm gap in the directional couplers. This leads to high loss for TM mode due to lateral leakage loss [17]. As a result, we were not able to measure the TM mode sensitivity with this batch of samples. By using deep etched rib waveguide with narrower width and directional couplers with narrower gaps, which can be fabricated by electron beam lithography the propagation loss for the TM mode can be reduced significantly and the sensitivity can be improved by one order of magnitude compared to the TE mode. This is a subject of our future work. 5. Conclusions We have proposed and investigated theoretically a novel highlysensitive EF waveguide biosensor based on silicon-on-insulator with a
two cascaded micro-rings resonator by using Vernier effect. Preliminary experiments show that the Vernier effect improved the sensitivity by 24.3 times as compared with a single micro-ring resonator sensor. A ratio between the wavelength shift and the refractive index change as high as S = 1300 nm/RIU is obtained experimentally. By improving the structure of waveguide for lower loss TM mode, the sensitivity is expected to be further enhanced by one order of magnitude. Because of its compactness, the sensor is promising for multi-channel EF waveguide biosensor applications.
Acknowledgments This project was supported by Zhejiang Provincial Natural Science Foundation of China (Grant No. Y1090239), National Science Foundations of China under grants No. 60807018 and 60788403 and the Fundamental Research Funds for the Central Universities of China.
References [1] Koji Matsubara, Satoshi Kawata, Shigeo Minami, Appl. Opt. Vol. 27 (Issue 6) (1988) 1160. [2] B. Liedberg, I. Lundström, E. Stenberg, Sens. Actuators B 1–3 (1993) 63. [3] A.K. Sharma, R. Jha, B.D. Gupta, IEEE Sens. J. 7 (8) (2007) 1118. [4] Xin Ma, Xu. Xinlong, Zheng Zheng, Kun Wang, Su. Yalin, Jianfeng Fan, Rui Zhang, Lusheng Song, Zhiyou Wang, Jinsong Zhu, Sensors and Actuators A: Physical 1 (2010) 9. [5] A. Densmore, D.-X. Xu, P. Waldron, S. Janz, P. Cheben, J. Lapointe, A. Delâge, B. Lamontagne, J.H. Schmid, E. Post, IEEE Photonics Technol. Lett. VOL. 18 (NO. 23) (2006) 2520. [6] A. Densmore, D.-X. Xu, S. Janz, P. Waldron, T. Mischki, G. Lopinski, A. Delâge, J. Lapointe, P. Cheben, B. Lamontagne, J.H. Schmid, OPTICS LETTERS 33 (6) (2008) 596. [7] Katrien De. Vos, Irene Bartolozzi, Etienne Schacht, Peter Bienstman1, Roel Baets, Opt. Express Vol. 15 (No. 12) (2007) 7610. [8] S.S. Saini, C. Stanford, S.M. Lee, J. Park, P. DeShong, W.E. Bentley, M. Dagenais, IEEE Photonics Technol. Lett. 19 (18) (2007) 1341. [9] P. Cheben, J.H. Schmid, A. Delâge, A. Densmore, S. Janz, B. Lamontagne, J. Lapointe, E. Post, P. Waldron, D.-X. Xu, Opt. Express 15 (5) (2007) 2299. [10] J. Mohr, B. Anderer, W. Ehrfeld, Sens. Actuators A 27 (1–3) (1991) 571. [11] C.K. Madsen, J.H. Zhao, Optical Filter Design and Analysis, Wiley, 1999. [12] Scheilb, Frigyes, Microwave Opt. Technol. Lett. 39 (4) (2003) 257. [13] L.A. Coldren, IEEE J. Sel. Top. Quantum Electron. 6 (6) (2000) 988. [14] J.-J. He, D. Liu, Opt. Express vol. 16 (no. 6) (2008) 3896. [15] L. Jin, M. Li, J.-J. He, Highly-sensitive optical sensor using two cascaded-microring resonators with Vernier effect, Asia Communications and Photonics Conference (ACP2009), Paper TuM4, Shanghai, November, 2009, pp. 2–6. [16] CRC Handbook of Chemistry and Physics, edited by David R. Lide, (90th Edition CD-ROM Version 2010). [17] M.A. Webster, et al., IEEE Photonics Technol. Lett. VOL. 19 (NO. 6) (2007) 429.
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Highly-sensitive silicon-on-insulator sensor based on two cascaded micro-ring resonators with vernier effect Lei Jin, Mingyu Li ⁎, Jian-Jun He Centre for Integrated Optoelectronics, State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, Hangzhou, PR China 310027
a r t i c l e
i n f o
Article history: Received 1 July 2010 Received in revised form 13 August 2010 Accepted 15 August 2010 Keywords: Optical sensor Silicon-on-insulator Cascaded micro-rings resonator
a b s t r a c t A highly-sensitive integrated optical biosensor based on two cascaded micro-rings resonator (MRR) is investigated theoretically and experimentally. The free spectral ranges (FSRs) of two cascaded micro-rings are designed to be slightly different in order to generate Vernier effect. A preliminary investigation of our sensor with a Q factor of 2 × 104 using different ethanol concentrations shows that the Vernier effect can improve the sensitivity to 1300 nm per refractive index unit (RIU), compared to 62 nm/RIU for a single ring sensor. The sensor also has a large measurement range of refractive index change up to 1.15 × 10− 2 RIU. It can be useful for low-cost and highly-sensitive optical biosensor system. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Because of its low-cost, highly-sensitive and ultra-compact features, much research effort has recently been devoted to the development of optical label-free biosensors [1–4]. Surface plasmon resonances in the noble-metal nano-structures and evanescent field (EF) waveguides sensing are the two optical techniques which are most commonly used in label-free biosensing applications such as environmental monitoring, biological recognition, medical diagnostics, food quality and safety analysis [1–8]. Significant effort is put into the development of EF sensors due to their potential for increased sensitivity and suitability for implementing into multi-channel detection. The waveguide EF sensors rely on monitoring the perturbation of the waveguide mode effective refractive index neff due to the various concentrations of an analyte in the solution that is in contact with the waveguide surface. People have developed all kinds of structures and mechanisms such as interferometer-based biosensors [5,6], high-Q optical ring resonator based biosensors [7], and optical fiber based biosensors [8]. Among them, micro-ring resonator can provide a higher sensitivity due to the sharp resonance peak. There are two typical interrogation approaches which have been used in a high-Q optical micro-resonant cavities sensing: intensity interrogation and wavelength interrogation [7]. Intensity interrogation is the simplest way for sensing, which measures the output intensity change at a given wavelength. Although the intensity interrogation has a high limit of detection, the measurement range is very small because of a high-Q resonant cavity. For the wavelength
⁎ Corresponding author. Tel.: +86 571 87953340; fax: +86 571 87953875. E-mail address: [email protected] (M. Li). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.08.035
interrogation, the change in the ambient refractive index can be detected by the magnitude of the wavelength shift in the resonant mode. To detect the small shift of resonant wavelength and improve the sensitivity, the combination of a very narrow line width tunable laser and power meter or a broad band source with a high wavelength resolution optical spectrum analyzer (OSA) can be utilized. But these instruments are very expensive and complex and cannot be integrated with optical sensor on one chip. However, a low wavelength resolution micro-OSA based on array waveguide grating (AWG) or etched diffraction grating (EDG) is low-cost and can be integrated with optical sensor on one chip [9,10]. This requires the sensor to produce a large resonant wavelength shift with a small refractive index change of analyte. In this paper, we investigate an EF waveguide sensor based on two cascaded micro-rings resonator with slightly different diameters to introduce Vernier effect. Although the Vernier effect was used in other applications such as optical filters [11,12] and tunable lasers [13,14], it was only recently proposed for sensor application [15]. By using the Vernier effect we can increase the sensitivity of the effective refractive index measurement without requiring a narrow line width tunable laser or a high-resolution OSA. The experimental results of the cascaded micro-ring resonator sensor based on SOI waveguide will be presented. A wavelength shift sensitivity as high as 1300 nm/RIU (Refractive Index Unit) is obtained. The sensor also has a large measurement range of refractive index change up to 1.15 × 10− 2 RIU. The SOI based biosensor is compatible with standard CMOS processing and potentially allows for cheap mass production and integration with electronics. This technology is readily combined with microfluidic components, and is easily extendable to a multi-array biosensor with thousands of sensing spots for real lab-on-chip applications.
L. Jin et al. / Optics Communications 284 (2011) 156–159
157
Δnminwhich produces a detectable change in the sensor output Δλmin is Δnmin = neff ΔλFSR −ΔλFSR = λB × Δnc = Δneff
ð2Þ
B
A
Here Δneff / Δnc is the ratio of the change in the waveguide effective refractive index to the change in the analyte refractive index, which is determined by the structure of the waveguide. The sensor sensitivity S is the ratio of the change in sensor's output (e.g., resonant wavelength) to the change in the quantity to be measured (e.g., the refractive index of analyte). h i S = Δλ = Δnc = ΔλB = Δneff Δneff = Δnc ΔλFSR = ΔλFSR −ΔλFSR ð3Þ A
Fig. 1. Schematic of the two cascaded micro-rings resonator sensor based on silicon-oninsulator. The reference ring A is covered by SU8 upper cladding layer while the sensing ring B is exposed to analyte in the sensing window.
2. Device structure and operation principle Two micro-rings resonators based on SOI were cascaded by a ridge waveguide to form a high-sensitive optical sensor, as shown in Fig. 1. The whole optical sensor is covered by SU8 upper cladding layer except the sensing ring B, which is exposed to a fluid sample to be analyzed. The free spectral ranges (FSRs) of the two ringsΔλFSRA,ΔλFSRB are designed to be slightly different to employ the Vernier effect by choosing different radii (RA and RB) for ring A and ring B, respectively. To describe the principle of our sensor, the transmission spectra TA and TB for ring A and B are plotted in Fig. 2. In our design, RA b RB results in ΔλFRSA N ΔλFRSB. The total transmission spectrum T equals to TA × TB, as shown in Fig. 2(c). The maximal peak of transmission T is located at the mutual resonant wavelength of ring A and ring B. The variation of the refractive index of the analyte fluid in the sensing window Δnc results in a change in the effective refractive index of ring B Δneff, consequently the resonant wavelength of the ring B changes by ΔλB = λB(Δneff / neff). According to the Vernier effect, the change Δλ in the resonant wavelength of the two cascaded miro-rings resonator is h i Δλ = ΔλB × ΔλFSR = ΔλFSR −ΔλFSR A
A
B
ð1Þ
Since the resonant wavelength of the reference ring A is fixed, the minimal change of Δλmin is ΔλFSRAwhen ΔλB changesΔλFSRA − ΔλFSRB. So the smallest change in the refractive index of the analyte
A
B
Here ΔλB = Δneff Δneff = Δnc is the sensitivity of a single ring B. From Eq. (3), the sensitivity of the two cascaded micro-rings resonator is magnified by a factor of M ¼ΔλFSRA = ΔλFSRA −ΔλFSRB as compared with that of a single micro-ring resonator. The FSR of two cascaded micro-rings resonator is given by ΔλFRS = ΔλFRS ΔλFRS = ΔλFRS −ΔλFRS A
B
A
B
ð4Þ
From Eq. (3), the maximal refractive index change is Δnmax = ΔλFSRA / S. 3. Design and fabrication The sensor is designed on SOI substrate with a 220 nm Si layer on a 2 μm SiO2 insulator layer. Since SOI is a high refractive index contrast material, the waveguide should be designed to be very small to keep the waveguide single mode. In our design, the widths of all the ridge waveguides are designed to be 1 μm with a shallow etched ridge height of 20–40 nm in order to maintain the single mode waveguide. We choose directional coupler to couple light into and out of the micro-ring resonators, with the minimal distance between the bus waveguide and ring to be 1 μm as well, so that this optical sensor can be fabricated by using contact photolithography. The interference orders of ring A and ring B at 1550 nm are chosen to be 1000 and 1100 with 10% difference, corresponding to ring diameters of 240 μm and 264 μm, respectively. Because numerical simulation shows the waveguide sensitivity of the waveguide sensitivity of our waveguide structure Δneff / Δnc is higher for TM polarization than for TE polarization [5], about 41.7% and 4.73% respectively, the sensor operated in TM mode is in principle much more sensitive. The FSRs of ring A and ring B's transmission spectrums
Fig. 2. Transmission spectra of ring A (a) and ring B (b); and (c) total transmission spectrum.
158
L. Jin et al. / Optics Communications 284 (2011) 156–159
a
b
c
Fig. 3. (a)Optical microscope image of cascaded micro-ring resonator sensor; (b) SEM picture of the direction coupler; and (c) output mode captured by infrared camera.
at 1550 nm are 0.779 nm and 0.709 nm respectively, as shown in Fig. 2. In this case, the minimal refractive index change Δnmin = 2.2 × 10 − 4 can be obtained from Eq. (2) by wavelength interrogation method with the amplification factor M = 11. The ratio between the wavelength shift and one unit of the refractive index change is 3456 nm/RIU. The FSR of two cascaded micro-rings resonator is ΔλFSR = 7.89 nm. Therefore, the maximal refractive index change Δnmax is 2.2 × 10−3. In the same situation for TE mode, the minimal refractive index change and the maximal refractive index change is about one order of magnitude larger. The two cascaded micro-ring resonator sensor was fabricated by using conventional photolithography, a 1.1 μm thick photoresist deposited by spin-coating served as the etch mask for etching of SOI. The SOI was patterned by reactive ion etching (RIE) using CF4 chemistry for 3 min. The height of the ridge waveguide is 40 nm, which is measured by Surface Profiler (Dektak 150). The perimeters of the two rings are about 957 μm and 995 μm, respectively. The light is coupled from the bus waveguide to the rings, and vice versa, by using directional couplers. The whole sensor was covered by Su-8 layer as upper cladding and a sensing window was opened on the top of the ring B by photolithography to form a sample reservoir as shown in Fig. 3.
4. Experimental results and discussions In order to measure the spectral response of the device, we use a tunable laser (Agilent 81600B) with a wavelength resolution of 0.01 nm as the light source and the output is received by a power sensor (Agilent 81635A). The light is coupled into or out of the sensor by lensed fibers. Because the resonant wavelengths of ring A and ring B have slightly different FSRs, an oscillatory envelope function results, with a major peak of the transmission spectrum located at 1546.93 nm, as we can see from Fig. 4. Far away from the major peak, the difference of the resonant wavelengths between the two rings becomes larger, resulting in two separated doublet peaks with low intensity. The water–ethanol mixtures of different volume concentration (10%, 15%, and 20%) were used to demonstrate how the sensor works. The experimental result is shown in Fig. 5. The refractive index of an ethanol solution varies by 3 × 10−3 RIU per 5% concentration variation [16] at 20 °C. Since the resonant wavelength of two micro-rings resonator is determined by the peak of the fixed ring A that coincides with a peak of the sensing ring B, a wavelength shift in the resonant peaks of the sensing ring B with the refractive index variation results
Fig. 4. Measured and calculated transmission spectra of the cascaded micro-rings when ring A and ring B are both covered by Su-8 layer. The two insets show the detail of the transmission peaks far away from the mutual resonant wavelength and close to the mutual resonant wavelength, respectively.
L. Jin et al. / Optics Communications 284 (2011) 156–159
159
Fig. 5. Measured transmission spectra of the cascaded micro-rings when ring B is covered by water with different concentrations of ethanol (a) 10%, (b) 15%, and (c) 20%.
in a jump of the fixed ring A in the resonant wavelength of our sensor. Therefore, the resolution can be improved significantly. It can be seen from Fig. 5, the extinction ratio of the central peak reaches 25 dB with a Q factor about 2 × 104 and the 5% change of the concentration results in 4 nm resonant wavelength shift of the sensor. We can deduce the sensitivity S (the ratio between the wavelength shift and the refractive index change) is about 1300 nm/RIU, over an order higher than a single ring sensor [7]. By fitting with experimental results ΔλFSRA and ΔλFSRBare found to be 0.656 nm and 0.629 nm, respectively and FSR of two cascaded micro-rings resonator ΔλFSR is 15 nm. Therefore, we can deduce that the amplification factor M is 24.3. For a single ring in the two cascaded micro-rings resonator, we can also calculate the sensitivity which is about 62 nm/RIU. From Eqs. (1) and (3), the measurement range of the refractive index change is derived to be 1.15 × 10− 2 RIU, and the detection resolution corresponding to a shift of the major peak from one resonant mode of the reference ring to its adjacent mode is 5.05 × 10−4 RIU. It is known from Eq. (3), in order to increase the sensitivity S, one can decrease the difference between ΔλFSRA and ΔλFSRB. However, in this case, the intensity difference between the major peak and its adjacent peaks becomes smaller, which may increase the possibility of detection errors. In Fig. 5, the intensity peaks fluctuate, because of the cleaved facets of the input and output waveguides forming a Fabry–Perot cavity. By eliminating the intensity fluctuation, the intensity ratios among the central peaks can be used to fit the wavelength shift more accurately, allowing the sensitivity to be further increased. The experimental results shown above are measured for TE mode. Due to the resolution limit of our contact photolithographic aligner, shallow etched waveguides were used with 1 μm gap in the directional couplers. This leads to high loss for TM mode due to lateral leakage loss [17]. As a result, we were not able to measure the TM mode sensitivity with this batch of samples. By using deep etched rib waveguide with narrower width and directional couplers with narrower gaps, which can be fabricated by electron beam lithography the propagation loss for the TM mode can be reduced significantly and the sensitivity can be improved by one order of magnitude compared to the TE mode. This is a subject of our future work. 5. Conclusions We have proposed and investigated theoretically a novel highlysensitive EF waveguide biosensor based on silicon-on-insulator with a
two cascaded micro-rings resonator by using Vernier effect. Preliminary experiments show that the Vernier effect improved the sensitivity by 24.3 times as compared with a single micro-ring resonator sensor. A ratio between the wavelength shift and the refractive index change as high as S = 1300 nm/RIU is obtained experimentally. By improving the structure of waveguide for lower loss TM mode, the sensitivity is expected to be further enhanced by one order of magnitude. Because of its compactness, the sensor is promising for multi-channel EF waveguide biosensor applications.
Acknowledgments This project was supported by Zhejiang Provincial Natural Science Foundation of China (Grant No. Y1090239), National Science Foundations of China under grants No. 60807018 and 60788403 and the Fundamental Research Funds for the Central Universities of China.
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