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Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 14 — Jul. 15, 2013
  • pp: 16955–16963
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Integrated interferometric approach to solve microring resonance splitting in biosensor applications

Sam Werquin, Steven Verstuyft, and Peter Bienstman  »View Author Affiliations


Optics Express, Vol. 21, Issue 14, pp. 16955-16963 (2013)
http://dx.doi.org/10.1364/OE.21.016955


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Abstract

Silicon-on-insulator microring resonators have proven to be an excellent platform for label-free nanophotonic biosensors. The high index contrast of silicon-on-insulator allows for fabrication of micrometer-size sensors. However, it also limits the quality of the resonances by introducing an intrinsic mode-splitting. Backscattering of optical power at small waveguide variations lifts the degeneracy of the normal resonator modes. This severely deteriorates the quality of the output signal, which is of utmost importance to determine the performance of the microrings as a biosensor. We suggest an integrated interferometric approach to give access to the unsplit, high-quality normal modes of the microring resonator and experimentally show an improvement of the quality factor by a factor of 3.

© 2013 OSA

1. Introduction

2. Origin of resonance splitting

A perfectly symmetric microring resonator mode in the absense of a bus waveguide is twofold degenerate. Both clockwise (CW) and counterclockwise (CCW) propagation are possible in the microring and both modes are uncoupled. This degeneracy is lifted when the CW-mode and CCW-mode become coupled, e.g. by surface roughness on the waveguide edges and by the proximity of bus waveguides. These deviations from circular symmetry cause forward propagating light to scatter back into the opposite direction, exciting a CCW-mode from a CW-mode and vice versa. Standing-wave modes as a symmetric and antisymmetric superposition of the traveling waves can now be considered as the new eigenmodes of the system. They will however no longer be degenerate as a consequence of the symmetry breaking coupling [8

8. T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Modal coupling in traveling-wave resonators.” Opt. Lett. 27, 1669–71 (2002) [CrossRef] .

]. If the linewidth of the resonance is small enough to distinguish both modes, the resonance splitting will be visible in the output signal. This occurs for high quality resonances, when the backre-flected power exceeds the coupling losses of the resonator. A critical relation between reflected power and microring coupling is derived in [9

9. B. E. Little, J. P. Laine, and S. T. Chu, “Surface-roughness-induced contradirectional coupling in ring and disk resonators.” Opt. Lett. 22, 4–6 (1997) [CrossRef] [PubMed] .

].

Evidence of the importance of backreflected power is provided in Fig. 1, which shows both the pass-port and add-port spectrum of a microring in add-drop configuration. The microring is designed to have a roundtrip length of 36μm and a power coupling ratio of 6 percent. Since only the input port is excited, no power should be present in the CCW-mode and the add port should remain dark. The measurement shows that backscattering in the microring waveguide cannot be neglected, resulting in significant power in the add port and resonance splitting in the pass-signal amounting up to 50 pm. Using the simple coupled harmonic oscillator model described in [8

8. T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Modal coupling in traveling-wave resonators.” Opt. Lett. 27, 1669–71 (2002) [CrossRef] .

], one can easily derive expressions for the CW and CCW-modes of a microring resonator coupled to a waveguide in the all-pass configuration. The coupled mode system is described by by equation 1:
daCWdt=iΔωaCW12τaCW+i2γaCCW+κsdaCCWdt=iΔωaCCW12τaCCW+i2γaCW
(1)

From equation 2, it is clear that the eigenmodes are centered around the new eigenfrequencies ω = ω0 ± 1/2γ. They each have a linewidth of 1/τ, determined by the losses of the coupled microring. The results from equation 2 can be used to calculate both the CW and CCW-mode as well as the transmitted and reflected fields for the microring coupled to an input waveguide by using the relations from equation 3:
t=s+κaCWandr=κaCCW
(3)

When comparing the theoretical transmission to the recorded transmission spectrum of an all-pass microring resonator, we obtain the results from Fig. 2. The resulting fitted parameter values indicate a quality factor of Q = 20850 for the eigenmodes. The mode splitting amounts to Δλ = 79pm and the power coupling ratio of the directional coupler is K2 = 0.0332, which corresponds well to the projected design value for critical coupling. This is summarized in table 1.

Fig. 1 Measured spectra for add-drop microring resonator showing resonance splitting and backscattered power
Fig. 2 Measured spectrum of all-pass microring with resonance splitting. The full line gives the theoretical transmission after fitting the model parameters

Table 1. Model parameter values

table-icon
View This Table

3. Integrated interferometric circuit

As demonstrated in [11

11. J. Knittel, T. G. McRae, K. H. Lee, and W. P. Bowen, “Interferometric detection of mode splitting for whispering gallery mode biosensors,” Appl. Phys. Lett. 97, 123704 (2010) [CrossRef] .

], in the context of a fiber-based system, an interferometric approach can be used to retrieve the unsplit modes of the microring resonator in an output signal. However, such a fiber-based setup is difficult to stabilize and to integrate. Therefore, we have implemented this in an integrated circuit on a single SOI-chip. A layout of the circuit is provided in Fig. 3. Vertical grating couplers are used to couple light from a tunable laser lightsource into the circuit and collect the power at the output. The input light excites the CW-mode in the microring resonator, which in turn excites the CCW-mode as a consequence of mode coupling. The normal modes of the microring resonator are the symmetric and antisymmetric superposition of the CW and CCW-mode, given by equation 2. If the coupling per unit time between the bus waveguide and the microring is represented by κ, the fields transmitted and reflected by the resonator are again given by equation 3. Here, s=ain/2 and |ain|2 is the total input power.

Fig. 3 Schematic representation of the integrated interferometric circuit

The input light is first passed through a 3dB combiner before coupling into the microring. The CW propagating light will couple back to the waveguide to constitute the transmitted field. The CCW propagating light will form the reflected field when coupling back to the waveguide. The input combiner acts as a splitter for the reflected light which is guided through a feedback arm towards a 2×2 multi mode interference (MMI) coupler. The transmitted light is passed to a 3dB MMI-splitter. It is split equally between output 1 and the 2×2 MMI coupler, where it is recombined with the reflected light. The signals from the 2×2 coupler are the result of interference between the transmitted and reflected fields and are collected in output 2 and 3. For the case of a perfect 2×2 MMI coupler, the fields at the output ports for given input fields E1 and E2 are 1/2(E1±iE2). There is a quadrature phase relationship between both output signals. We can summarize the output fields of the interferometric circuit in equation 4:
out1=12tout2=12(12ain+κaCW+eiϕκaCCW)out3=12(12ain+κaCWeiϕκaCCW)
(4)

Here, ϕ is the phase difference between the interfering transmitted and reflected fields. We find that output 1 is always proportional to the pass-signal of the microring resonator in the all-pass configuration. To obtain the required output signals in output 2 and 3, the phase difference ϕ between the transmitted and reflected wave has to be controlled carefully. In this case, this is done by processing a titanium-gold heater on the feedback waveguide. By setting the current through the heater, we can tune the phase difference to the required value. If it is a multiple of π, we see from equation 4 that the signals in output 2 and 3 are given by the sum of a constant and a signal proportional to the complex amplitudes of the normal modes of the resonator, given by equation 2. In other words, the spectral shape of the signals in output 2 and 3 will be identical to this of the complex amplitudes of the eigenmodes. This means we will have access to the unsplit, high-Q normal modes of the cavity. Since the detection limit of a biosensor is limited by the quality factor of the resonance [12

12. J. Hu, X. Sun, A. Agarwal, and L. C. Kimerling, “Design guidelines for optical resonator biochemical sensors,” J. Opt. Soc. Am. B 26, 1032–1041 (2009) [CrossRef] .

] - higher resonator Q-factors give rise to lower detection limits - this provides a tool to improve the detection limit significantly. Even worse, fitting errors introduced by resonance splitting can cause jumps between both modes of a split resonance that lead to false positive or even false negative results when the sensor is implemented in a lab-on-a-chip setting.

4. Fabrication and calibration

The circuit from Fig. 3 is designed and processed in a CMOS pilot line at imec. Using the vertical in- and output couplers on the waveguides [13

13. G. Roelkens, D. Vermeulen, R. Selvaraja, S Halir, W. Bogaerts, and D. V. Thourhout, “Grating-based optical fiber interfaces for silicon-on-insulator photonic integrated circuits,” IEEE J. Sel. Top. Quantum Electron. 17, 571–580 (2011) [CrossRef] .

], the chip can easily be measured in a fiber-to-fiber configuration. The process of depositing tuning heaters on the feedback waveguide is straightforward and requires no high-accuracy allignment steps. To limit optical losses induced by the heater metal, a polymer layer of approximately 1μm is deposited on the chip surface prior to heater definition. For this layer, the photopatternable resin Cyclotene 4022-25 [14

14. The Dow Chemical Company, “Cyclotene advanced electronics resins,” (2013), http://www.dow.com/cyclotene/prod/402235.htm.

] is used to allow the definition of windows over critical device structures. The polymer is derived from B-staged bisbenzocyclobutene (BCB) and is widely used for various post-processing steps on SOI-chips for near-infrared applications [15

15. S. Stankovic, R. Jones, J. Heck, M. Sysak, D. Van Thourhout, and G. Roelkens, “Die-to-die adhesive bonding procedure for evanescently-coupled photonic devices,” Electrochem. Solid-State Lett. 14, H326 (2011) [CrossRef] .

]. After developement, photo-BCB covering the microring, grating couplers and 2×2 MMI is removed.

Figure 4 shows the 2×2 MMI and the edge of the BCB layer. The tuning heater is then processed on the chip with locally opened BCB cover. For that purpose, a lithographic lift-off procedure is used [16

16. K. V. Acoleyen, K. Komorowska, and W. Bogaerts, “One-dimensional off-chip beam steering and shaping using optical phased arrays on silicon-on-insulator,” J. Lightwave Technol. 29, 3500–3505 (2011) [CrossRef] .

]. The heater measures 450 by 2.5μm with large contact pads for easy probing. For characterization of the tuning heater, a Mach-Zehnder-Interferometer (MZI) is used. By comparing the output intensity of the MZI to the theoretical cosine squared interference pattern for different heater currents, we were able to determine the quadratic relationship between heater current and phase change as ϕ = 0.4rad/mA2 × I2. This confirms the phase change in the heated waveguide is proportional to the dissipated power in the metalic heater. The resulting calibration curve is shown in Fig. 5. A phase range of 2π is obtained for heater currents from 0 to 4mA.

Fig. 4 Microscope image of lithographically opened BCB layer. The arrows indicate the edge of the 2×2 MMI that is covered by BCB residue
Fig. 5 Calibration curve for tuning heater. A phase range of 2π is obtained for heater currents from 0 to 4mA

5. Experimental results

The fabricated chip with integrated interferometric circuit is mounted on a vertical optical setup for fiber-to-fiber characterization. To obtain the measured spectra, a TUNICS tunable laser source is used to generate the input signals. Output intensities are measured by a HP-8153 optical power meter. The laser wavelength is swept in 5pm steps while power is recorded. Setting the heater current gives us the ability to change the resonance state in the output signals from one normal mode, over the severely split intermediate state to the other normal mode.

This is demonstrated in Fig. 6 where recorded spectra of the output couplers are shown for different phase differences between the interfering waves. In this figure, the theoretically predicted output signals according to equation 4 are also provided. The values for the model parameters are given by table 1. The correspondence between theory and experiment shows we have a good understanding of the physical effects in the microring resonator. We have to remark the 2×2 MMI is particularly sensitive to processing variations that effect its transmission characteristics. The proximity of the BCB layer to one edge of the MMI, which is seen on Fig. 4, generates an assymetric coupler transmission. This results in a extra phase shift of approximately π/2 in the transmission from input port 2 to output port 3, which was determined by fitting. This phase change has already been accounted for in the theoretical model and heater currents have been adapted accordingly.

Fig. 6 Interferometric output signals for different phase difference between reflected and transmitted field: (top) output 2 (bottom) output 3. The full line gives the theoretically predicted signals and the dotted line gives the experimentally recorded power

The width and corresponding quality factors of the recorded eigenmode spectra compare favourably to those of the resonance signal in output 1, as can be seen from Fig. 7. When determined based on the fitted model of equation 2, we obtain a width of 50pm for the unsplit eigenmodes and one of 130pm for the split all-pass transmission. Since the limit of detection (LOD) scales with the square root of the resonance width, this technique provides a potential LOD improvement with a factor 1.6. One could argue that the total width of the transmission resonance is not a good measure for estimating detection limits. However, when the resonance splitting Δλ is less than or equal to the width of the individual normal modes, splitting will be obscured while still causing significant broadening of the resonance dip. In that case, a fit to the more narrow unsplit mode revealed by this interferometric approach will always be more accurate and provide limit of detection improvements up to a factor 2, while at the same time eliminating the chance of false positive or negative detections due to intrinsic microring effects.

Fig. 7 Split all-pass transmission and interferometric output signals showing unsplit modes for a phase difference Δϕ = 0. Output power of output 1 has been scaled for clarity

6. Conclusion

In this paper, we demonstrate the successful integration of an interferometric circuit to resolve resonance splitting on a SOI-chip. We also show that a simple theoretical model of coupled harmonic oscillators can adequately describe the microring mode splitting. Using a tuning heater, we can sweep the phase difference between the combining reflected and transmitted fields of the microring and change the output signals from a severely split spectrum to the unsplit microring modes with high quality factors. The theoretical predictions are confirmed by the experiment. When comparing the output signals of the interferometric circuit to the split resonance of the all-pass microring, the resonance width is reduced by approximately a factor 3. This can result in a limit of detection improvement by a factor 1.6, while at the same time eliminating the chance of false positive or negative detections due to intrinsic microring splitting.

Acknowledgments

Sam Werquin would like to acknowledge the Special Research Fund of Ghent University (BOF-UGent) for providing a research grand. The research is also supported by the Interuniversity Attraction Poles program of the Belgian Science Policy Office ‘Photonics@be’. Finally, the authors would also like to thank Thomas Van Vaerenbergh for many fruitful discussions.

References and links

1.

K. De Vos, J. Girones, T. Claes, Y. De Koninck, S. Popelka, E. Schacht, R. Baets, and P. Bienstman, “Multiplexed antibody detection with an array of silicon-on-insulator microring resonators,” IEEE Photon. J. 1, 225–235 (2009) [CrossRef] .

2.

M. Iqbal, M. A. Gleeson, B. Spaugh, F. Tybor, W. G. Gunn, M. Hochberg, T. Baehr-Jones, R. C. Bailey, and L. C. Gunn, “Label-free biosensor arrays based on silicon ring resonators and high-speed optical scanning instrumentation,” IEEE J. Sel. Top. Quantum Electron. 16, 654–661 (2010) [CrossRef] .

3.

C. L. Arce, K. D. Vos, T. Claes, K. Komorowska, D. V. Thourhout, and P. Bienstman, “Silicon-on-insulator microring resonator sensor integrated on an optical fiber facet,” IEEE Photon. Technol. Lett. 23, 890–892 (2011) [CrossRef] .

4.

T. Claes, W. Bogaerts, and P. Bienstman, “Experimental characterization of a silicon photonic biosensor consisting of two cascaded ring resonators based on the Vernier-effect and introduction of a curve fitting method for an improved detection limit,” Opt. Express 18, 22747–22761 (2010) [CrossRef] [PubMed] .

5.

J. Wang and D. Dai, “Highly sensitive Si nanowire-based optical sensor using a Mach Zehnder interferometer coupled microring,” Opt. Lett. 35, 4229–4231 (2010) [PubMed] .

6.

W. Bogaerts, P. D. Heyn, T. V. Vaerenbergh, K. D. Vos, S. Kumar, T. Claes, P. Dumon, P. Bienstman, D. V. Thourhout, and R. Baets, “Silicon microring resonators,” Laser Photonics Rev. 6, 47–73 (2011) [CrossRef] .

7.

T. Claes, J. Molera, K. De Vos, E. Schacht, R. Baets, and P. Bienstman, “Label-free biosensing with a slot-waveguide-based ring resonator in silicon on insulator,” IEEE Photon. J. 1, 197–204 (2009) [CrossRef] .

8.

T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Modal coupling in traveling-wave resonators.” Opt. Lett. 27, 1669–71 (2002) [CrossRef] .

9.

B. E. Little, J. P. Laine, and S. T. Chu, “Surface-roughness-induced contradirectional coupling in ring and disk resonators.” Opt. Lett. 22, 4–6 (1997) [CrossRef] [PubMed] .

10.

B. Little, S. Chu, H. Haus, J. Foresi, and J.-P. Laine, “Microring resonator channel dropping filters,” J. Lightw. Technol. 15, 998–1005 (1997) [CrossRef] .

11.

J. Knittel, T. G. McRae, K. H. Lee, and W. P. Bowen, “Interferometric detection of mode splitting for whispering gallery mode biosensors,” Appl. Phys. Lett. 97, 123704 (2010) [CrossRef] .

12.

J. Hu, X. Sun, A. Agarwal, and L. C. Kimerling, “Design guidelines for optical resonator biochemical sensors,” J. Opt. Soc. Am. B 26, 1032–1041 (2009) [CrossRef] .

13.

G. Roelkens, D. Vermeulen, R. Selvaraja, S Halir, W. Bogaerts, and D. V. Thourhout, “Grating-based optical fiber interfaces for silicon-on-insulator photonic integrated circuits,” IEEE J. Sel. Top. Quantum Electron. 17, 571–580 (2011) [CrossRef] .

14.

The Dow Chemical Company, “Cyclotene advanced electronics resins,” (2013), http://www.dow.com/cyclotene/prod/402235.htm.

15.

S. Stankovic, R. Jones, J. Heck, M. Sysak, D. Van Thourhout, and G. Roelkens, “Die-to-die adhesive bonding procedure for evanescently-coupled photonic devices,” Electrochem. Solid-State Lett. 14, H326 (2011) [CrossRef] .

16.

K. V. Acoleyen, K. Komorowska, and W. Bogaerts, “One-dimensional off-chip beam steering and shaping using optical phased arrays on silicon-on-insulator,” J. Lightwave Technol. 29, 3500–3505 (2011) [CrossRef] .

OCIS Codes
(130.3120) Integrated optics : Integrated optics devices
(130.6010) Integrated optics : Sensors
(230.5750) Optical devices : Resonators

ToC Category:
Integrated Optics

History
Original Manuscript: May 28, 2013
Revised Manuscript: June 13, 2013
Manuscript Accepted: June 18, 2013
Published: July 9, 2013

Virtual Issues
Vol. 8, Iss. 8 Virtual Journal for Biomedical Optics

Citation
Sam Werquin, Steven Verstuyft, and Peter Bienstman, "Integrated interferometric approach to solve microring resonance splitting in biosensor applications," Opt. Express 21, 16955-16963 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-14-16955


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References

  1. K. De Vos, J. Girones, T. Claes, Y. De Koninck, S. Popelka, E. Schacht, R. Baets, and P. Bienstman, “Multiplexed antibody detection with an array of silicon-on-insulator microring resonators,” IEEE Photon. J.1, 225–235 (2009). [CrossRef]
  2. M. Iqbal, M. A. Gleeson, B. Spaugh, F. Tybor, W. G. Gunn, M. Hochberg, T. Baehr-Jones, R. C. Bailey, and L. C. Gunn, “Label-free biosensor arrays based on silicon ring resonators and high-speed optical scanning instrumentation,” IEEE J. Sel. Top. Quantum Electron.16, 654–661 (2010). [CrossRef]
  3. C. L. Arce, K. D. Vos, T. Claes, K. Komorowska, D. V. Thourhout, and P. Bienstman, “Silicon-on-insulator microring resonator sensor integrated on an optical fiber facet,” IEEE Photon. Technol. Lett.23, 890–892 (2011). [CrossRef]
  4. T. Claes, W. Bogaerts, and P. Bienstman, “Experimental characterization of a silicon photonic biosensor consisting of two cascaded ring resonators based on the Vernier-effect and introduction of a curve fitting method for an improved detection limit,” Opt. Express18, 22747–22761 (2010). [CrossRef] [PubMed]
  5. J. Wang and D. Dai, “Highly sensitive Si nanowire-based optical sensor using a Mach Zehnder interferometer coupled microring,” Opt. Lett.35, 4229–4231 (2010). [PubMed]
  6. W. Bogaerts, P. D. Heyn, T. V. Vaerenbergh, K. D. Vos, S. Kumar, T. Claes, P. Dumon, P. Bienstman, D. V. Thourhout, and R. Baets, “Silicon microring resonators,” Laser Photonics Rev.6, 47–73 (2011). [CrossRef]
  7. T. Claes, J. Molera, K. De Vos, E. Schacht, R. Baets, and P. Bienstman, “Label-free biosensing with a slot-waveguide-based ring resonator in silicon on insulator,” IEEE Photon. J.1, 197–204 (2009). [CrossRef]
  8. T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Modal coupling in traveling-wave resonators.” Opt. Lett.27, 1669–71 (2002). [CrossRef]
  9. B. E. Little, J. P. Laine, and S. T. Chu, “Surface-roughness-induced contradirectional coupling in ring and disk resonators.” Opt. Lett.22, 4–6 (1997). [CrossRef] [PubMed]
  10. B. Little, S. Chu, H. Haus, J. Foresi, and J.-P. Laine, “Microring resonator channel dropping filters,” J. Lightw. Technol.15, 998–1005 (1997). [CrossRef]
  11. J. Knittel, T. G. McRae, K. H. Lee, and W. P. Bowen, “Interferometric detection of mode splitting for whispering gallery mode biosensors,” Appl. Phys. Lett.97, 123704 (2010). [CrossRef]
  12. J. Hu, X. Sun, A. Agarwal, and L. C. Kimerling, “Design guidelines for optical resonator biochemical sensors,” J. Opt. Soc. Am. B26, 1032–1041 (2009). [CrossRef]
  13. G. Roelkens, D. Vermeulen, R. Selvaraja, S Halir, W. Bogaerts, and D. V. Thourhout, “Grating-based optical fiber interfaces for silicon-on-insulator photonic integrated circuits,” IEEE J. Sel. Top. Quantum Electron.17, 571–580 (2011). [CrossRef]
  14. The Dow Chemical Company, “Cyclotene advanced electronics resins,” (2013), http://www.dow.com/cyclotene/prod/402235.htm .
  15. S. Stankovic, R. Jones, J. Heck, M. Sysak, D. Van Thourhout, and G. Roelkens, “Die-to-die adhesive bonding procedure for evanescently-coupled photonic devices,” Electrochem. Solid-State Lett.14, H326 (2011). [CrossRef]
  16. K. V. Acoleyen, K. Komorowska, and W. Bogaerts, “One-dimensional off-chip beam steering and shaping using optical phased arrays on silicon-on-insulator,” J. Lightwave Technol.29, 3500–3505 (2011). [CrossRef]

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