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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 13 — Jun. 22, 2009
  • pp: 10731–10737
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A reflective microring notch filter and sensor

Haishan Sun, Antao Chen, and Larry R. Dalton  »View Author Affiliations


Optics Express, Vol. 17, Issue 13, pp. 10731-10737 (2009)
http://dx.doi.org/10.1364/OE.17.010731


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Abstract

We present a new design of wavelength selective reflector composed of a Y junction and a singly coupled microring resonator, and demonstrate its biochemical sensing applications with a prototype device. In contrast with other reflectors like distributed Bragg reflectors, this device acts as notch filter at its reflection port. One promising application of the device is for remote sensing of harsh or inaccessible site, where only one optical fiber is required to transmit the input and reflected light signal over a long distance. The design can also be used to make microring cavity lasers.

© 2009 OSA

1. Introduction

Fiber and waveguide Bragg reflectors are critical components of optical communications systems, such as feedback mirrors for distributed feedback lasers and optical add-drop multiplexers for wavelength-division multiplexing. They are also used as sensing elements of, for instance, distributed embedded sensing systems in “smart structures” [1

1. A. Othonos, “Fiber Bragg gratings,” Rev. Sci. Instrum. 68(12), 4309–4341 (1997). [CrossRef]

,2

2. A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putnam, and E. J. Friebele, “Fiber grating sensors,” J. Lightwave Technol. 15(8), 1442–1463 (1997). [CrossRef]

]. Recently, several designs of microring-based wavelength-selective reflectors were proposed to replace grating structures for realizing tunable single-mode lasers [3

3. G. T. Paloczi, J. Scheuer, and A. Yariv, “Compact microring-based wavelength-selective inline optical reflector,” IEEE Photon. Technol. Lett. 17(2), 390–392 (2005). [CrossRef]

6

6. Y. Chung, D.-G. Kim, and N. Dagli, “Widely tunable coupled-ring reflector laser diode,” IEEE Photon. Technol. Lett. 17(9), 1773–1775 (2005). [CrossRef]

]. The advantages of microring structures include easy fabrication, on chip integration with other photonic devices, and wide wavelength tuning range. The reflection spectra of Bragg grating reflectors and these reported microring-based reflectors have isolated narrow band peaks, i.e. they are reflective-type band-pass filters. In this letter, a microring resonator based reflector with narrow notches in the reflection spectrum is presented. In addition to its applications as a reflective-type notch filter in optical communications (e.g. Raman lasers), here we demonstrate its sensing applications, specially as a biochemical sensor.

For the optical sensing applications, interaction of the measurands with the light in the ring waveguide changes the effective index of the guided mode and thus the resonant wavelengths. Detection can be made by monitoring the shift of a resonant wavelength or variation of the reflected light intensity of a wavelength fixed at the largest slope in the transmission spectrum. This sensor configuration combines the advantage of fiber-optic sensors in remote measurement and the advantage of planar sensors in integration and mass production [7

7. G. Robinson, “The commercial development of planar optical biosensors,” Sens. Actuators B Chem. 29(1-3), 31–36 (1995). [CrossRef]

]. Mechanical flexibility and the ability to transmit optical signals over a long distance of the fiber make such sensor attractive for remote measurement in harsh or inaccessible locations. Fabrication, characterization and a biochemical sensing experiment of the sensor are presented in the following sections. We will show that compared with doubly or triply coupled microring reflectors, the design proposed here exhibits higher extinction ratio and narrower linewidth because of less coupling induced loss over the ring path, which can lead to higher sensitivity. Possibility for other sensing applications and methods to further increase the sensitivity are also discussed.

2. Device theory and fabrication

In the device design shown in Fig. 1
Fig. 1 Microscopic images of a fabricated device. (a) Full view. (b) Detail of the coupler region and (c) the Y junction region. The radius of circular ring is 200 μm.
, the Y junction splits equally the input light Ii into two arms (i.e. Ii = 2⋅ Ia). After part of the light Ic coupled into the ring cavity, the other part of the light Ib circulates back to the Y junction and combines to give the total reflected light Ir (i.e. Ir = 2⋅ Ib). Here the Y junction serves as both a power splitter and combiner. Based the universal relations for singly coupled ring resonators [8

8. A. Yariv, “Universal relations for coupling of optical power between microresonators and dielectric waveguides,” Electron. Lett. 36(4), 321–322 (2000). [CrossRef]

], the normalized reflected light intensity are formulated as
IrIi=IbIa=α2+|t|22α|t|cos(θ+ϕt)1+α2|t|22α|t|cos(θ+ϕt)
(1)
where α is the field attenuation factor after one round-trip in the ring resonator, t=|t|eiϕt is phasor field transmission past the coupler region and θ=βL=2πneffL/λ is the total phase shift per ring circumference L for a ring waveguide propagation constant β (with waveguide effective index neff and light wavelength λ). When θ is integer multiples of 2π, i.e. the resonant wavelengthλr=neffL/m, where resonance order m is an integer, resonances are built up and most of the light from the input waveguide is trapped in the ring cavity. If the critical coupling condition α = |t| is achieved, perfect destructive interference happens at the output waveguide of the coupler and no light of that wavelength will be reflected back. Equation (1) remains valid even if the Y junction is asymmetrical and the split ratio is not 50% due to fabrication errors.

The proposed device was fabricated with SU8 polymer (n = 1.565, Microchem Corp.) on a silicon substrate covered by 5 μm thermal oxide (n = 1.445) serving as the lower cladding. An FEI Sirion scanning electron microscopy (SEM) system with an accelerating voltage of 30 kV was used to pattern the 2 μm thick SU8 film. Nanometer Pattern Generation System (NPGS) was used to generate the device designs and to control the writing processes. The waveguide width is 2 μm. The circular ring resonator has a radius of 200 μm and device designs with waveguide to ring resonator separations (coupling gaps) ranging from 0 to 1 μm were fabricated to find out the optimal coupling condition. S-bends are used to separate the two arms of the Y-junction. All the arc bend sections are smoothly connected (i.e. first order derivatives are continuous) to minimize the transition loss and their radius of curvatures are larger than the ring resonator radius to minimize the bending loss.

3. Experiments and results

The setup for measuring the reflection spectra of the sensor is shown in Fig. 2
Fig. 2 Setup for the reflection spectrum and biochemical sensing measurements.
. Individual devices were cleaved from the Si wafers before measurements. The output of an erbium doped fiber amplified spontaneous emission (ASE) broadband source with a wavelength range from 1520 to 1560 nm was polarized through an Agilent 8169A polarization controller, which consists of individually rotatable linear polarizer, half-wave plate, and quarter-wave plate and can synthesize any predetermined state of polarization. Transverse electric (TE) light was fiber coupled to the input port of an optical circulator. Its through port was fiber coupled to the devices. The reflected light from the devices was collected at the drop port of the circulator and directed to an OSA (HP 70951B).

In the measured reflection spectra shown in Fig. 3
Fig. 3 Measured reflection spectra of the devices with different coupling gap sizes. All the ring resonators are circular in shape with radius of 200 μm.
, the device with 300 nm coupling gap shows the highest extinction ratio of more than 11 dB and is close to the critical coupling condition. The free spectral range of the resonant nulls is around 1.15 nm, which agrees with the actual circumference L of the ring resonators. Small ripples in the flat tops of the curves indicate weak higher order guided modes, which is consistent with the mode simulations and experiment results of the similar waveguide structure in [3

3. G. T. Paloczi, J. Scheuer, and A. Yariv, “Compact microring-based wavelength-selective inline optical reflector,” IEEE Photon. Technol. Lett. 17(2), 390–392 (2005). [CrossRef]

]. Curve fitting of the spectra with the theoretical transfer function in Eq. (1) and further calculations suggest that the device quality factor (Q) is approximately 8000. The Q could be increased by reducing the waveguide scattering loss through optimization of the electron beam writing process and post-fabrication annealing of the polymer waveguides [9

9. C.-Y. Chao and L. J. Guo, “Thermal-flow technique for reducing surface roughness and controlling gap size in polymer microring resonators,” Appl. Phys. Lett. 84(14), 2479–2481 (2004). [CrossRef]

]. However, higher Q was not pursued in this proof-of-concept device.

We used a homogeneous biochemical sensing experiments to demonstrate the sensing capability of the device. The air cladding over the SU8 polymer waveguides was replaced by sodium chloride (NaCl) solutions in de-ionized water with mass concentrations of 0~20%. The refractive index of a NaCl aqueous solution changes 0.0018 RIU per 1% mass concentration at 20°C [10

10. D. R. Lide, “Concentrative Properties of Aqueous Solutions,” in CRC Handbook of Chemistry and Physics, 88th Edition (Internet Version 2008) (CRC Press/Taylorand Francis, Boca Raton, FL., 2007), pp. 2640–2640.

]. Variations of the solution refractive index ns disturb the evanescent tail of the guided mode and change the corresponding effective indices neff, which was detected by monitoring the reflection spectrum (or resonant wavelength λr) shift with the same setup described earlier. The measurements (Fig. 4
Fig. 4 Resonant wavelength as functions of the NaCl solution concentration and refractive index.
) show a linear relationship between the resonant wavelength and NaCl solution concentration (and the solution refractive index). If the device sensitivity S is defined as the slope of the relationship between the resonant wavelength and the refractive index of the analyte, we haveS=λr/ns=λr/neffSw=λr/neffSw, where Sw=neff/nsis the waveguide sensitivity and only relevant to the waveguide structure. Line fitting of the measurements indicates our measured device sensitivity to be 63 nm/RIU.

4. Discussion

The waveguide sensitivity was estimated by varying the top cladding refractive index from 1.33 to 1.331 and finding the relevant change of effective index, i.e. Sw=Δneff/Δns, using a full vectorial mode solver (FIMMWAVE, Photon Design). Based on the simulated neff = 1.51, Sw = 0.06, and λr ≈1550 nm, the theoretical device sensitivity is calculated to be 62 nm/RIU and agree quite well with the measurement. It is also noteworthy that our waveguide sensitivity is about two times higher than the simulated one of a polymer slab waveguide (for homogeneous sensing) [11

11. A. Densmore, D. X. Xu, P. Waldron, S. Janz, P. Cheben, J. Lapointe, A. Delage, B. Lamontagne, J. H. Schmid, and E. Post, “A silicon-on-insulator photonic wire based evanescent field sensor,” IEEE Photon. Technol. Lett. 18(23), 2520–2522 (2006). [CrossRef]

]. The wavelength reproducibility and tuning repeatability in 1 min for a current commercial optical spectrum analyzer (OSA, Agilent 86146B) is 2 pm, so the detection limit for homogeneous sensing (defined as minimum detectable refractive index change of the analyte solution) of our device can be as low as 3 × 10−5 RIU. If intensity detection using a wavelength fixed at the largest slope is applied to the same device, the theoretical detection limit would be estimated to be 4 × 10−6 RIU, based on an assumption of actual neff = 1.5 and optical power measurement accuracy of 2.2% (e.g. Agilent 81624B).

The proposed reflector has a singly coupled ring resonator, which contrasts with the microring-based reflectors cited earlier [3

3. G. T. Paloczi, J. Scheuer, and A. Yariv, “Compact microring-based wavelength-selective inline optical reflector,” IEEE Photon. Technol. Lett. 17(2), 390–392 (2005). [CrossRef]

6

6. Y. Chung, D.-G. Kim, and N. Dagli, “Widely tunable coupled-ring reflector laser diode,” IEEE Photon. Technol. Lett. 17(9), 1773–1775 (2005). [CrossRef]

]. They all involve doubly or even triply coupled ring resonators. We also fabricated an alternative design of reflectors which consists of a doubly coupled ring resonator, and compared its performance with the one we described above. Figure 5
Fig. 5 Theoretical transmission spectra of the two designs of microring reflectors. (a) The original design shown in Fig. 1. (b) The design with a doubly coupled ring resonator. α = |t| = 0.8 for both designs.
is the theoretical transmission spectra of the two designs, with α = |t| = 0.8 in both cases. We find that the singly coupled ring design have much higher extinction ratio (infinite in theory) and sharper lineshape, which indicate higher device sensitivity for intensity interrogation. With the similar waveguide index profile and electron beam writing process, the measured spectra of the doubly coupled ring reflector show extinction ratios of 2~5 dB and 1~2 times wider linewidths, which agree with the theoretical analysis. This could be due to the added cavity loss contributed by the additional coupler.

Based on the original design of singly coupled ring reflector, Fano-resonance created by introducing two partially reflecting junctions to the bus waveguide can be used to further increase the device sensitivity [12

12. C.-Y. Chao and L. J. Guo, “Biochemical sensors based on polymer microrings with sharp asymmetrical resonance,” Appl. Phys. Lett. 83(8), 1527–1529 (2003). [CrossRef]

]. Optimizing the waveguide structure and improving the waveguide sensitivity Sw can also lead to higher sensitivity, where silicon-on-insulator (SOI) photonic wire waveguides, anti-resonant reflecting optical waveguides (ARROW) or slot waveguides could be considered [13

13. V. M. N. Passaro, F. Dell’Olio, B. Casamassima, and F. De, “Guided-Wave Optical Biosensors,” Sensors 7(4), 508–536 (2007). [CrossRef]

]. Using thermo-optic or electro-optic waveguide materials, the sensor design can be readily applied to temperature or radio-frequency (RF) electric field detection [14

14. H. Sun, A. Pyajt, J. Luo, Z. Shi, S. Hau, A. K. Y. Jen, L. R. Dalton, and A. Chen, “All-dielectric electrooptic sensor based on a polymer microresonator coupled side-polished optical fiber,” IEEE Sens. J. 7(4), 515–524 (2007). [CrossRef]

]. If fabricated on a flexible substrate, the ring reflector becomes strain or displacement sensors through the photoelasticity of the waveguide material and the ring deformation [15

15. I. Kiyat, C. Kocabas, and A. Aydinli, “Integrated micro ring resonator displacement sensor for scanning probe microscopies,” J. Micromech. Microeng. 14(3), 374–381 (2004). [CrossRef]

,16

16. B. Bhola, H.-C. Song, H. Tazawa, and W. H. Steier, “Polymer microresonator strain sensors,” IEEE Photon. Technol. Lett. 17(4), 867–869 (2005). [CrossRef]

], Finally, by using polymers sensitive to different analytes as waveguide materials and specific bio-receptors immobilized on the waveguide surfaces the reflector can be a multi-functional platform for broad biosensing applications. A sensor array multiplexed with microrings of different sizes serially coupled to the same bus waveguide and functionalized with different molecule recognition capabilities is also possible (Fig. 6
Fig. 6 A multiplexed sensor head with microring elements of different detection capabilities.
).

5. Conclusion

In conclusion, a microring resonator based wavelength selective reflector is proposed and the conception was demonstrated with a device fabricated with electron-beam patterned SU8 polymer waveguides. An extinction ratio greater than 11 dB has been achieved. The homogeneous biochemical sensing experiment was carried out using the device and a sensitivity of 63 nm/RIU was observed, in good agreement with the theoretical model. The minimal detectable ambient refractive index change is estimated to be 3 × 10−5 RIU (with instrument reproducibility 2 pm). The simple design minimizes round-trip loss from additional couplers and preserves a better performance than some other designs of microring reflector sensors. Approaches for further improving the device sensitivity and possibilities for other sensing applications are also discussed. Combined with a single optical fiber for delivering both the input light and reflected signal, the device can be used for remote sensing of an inaccessible spot, for instance, water quality monitoring in bore holes. As a reflective-type notch filter, the device could find applications in optical communications. With the ring waveguide as a laser cavity and the Y-junction as a power combiner, it is still possible to construct a microring cavity laser, either light pumped [17

17. H. Rong, Y.-H. Kuo, S. Xu, A. Liu, R. Jones, M. Paniccia, O. Cohen, and O. Raday, “Monolithic integrated Raman silicon laser,” Opt. Express 14(15), 6705–6712 (2006). [CrossRef] [PubMed]

] or electric pumped [18

18. A. W. Fang, B. R. Koch, K.-G. Gan, H. Park, R. Jones, O. Cohen, M. J. Paniccia, D. J. Blumenthal, and J. E. Bowers, “A racetrack mode-locked silicon evanescent laser,” Opt. Express 16(2), 1393–1398 (2008). [CrossRef] [PubMed]

].

Acknowledgements

This work is supported by NSF Grant Number ECS-0437920, NSF-DMR-0092380, and NSF Center on Materials and Devices for Information Technology Research (CMDITR), Grant Number DMR-0120967. The work was conducted at the Nanotech User Facility at the University of Washington, a member of the National Nanotechnology Infrastructure Network (NNIN) supported by NSF.

References and links

1.

A. Othonos, “Fiber Bragg gratings,” Rev. Sci. Instrum. 68(12), 4309–4341 (1997). [CrossRef]

2.

A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putnam, and E. J. Friebele, “Fiber grating sensors,” J. Lightwave Technol. 15(8), 1442–1463 (1997). [CrossRef]

3.

G. T. Paloczi, J. Scheuer, and A. Yariv, “Compact microring-based wavelength-selective inline optical reflector,” IEEE Photon. Technol. Lett. 17(2), 390–392 (2005). [CrossRef]

4.

J. K. S. Poon, J. Scheuer, and A. Yariv, “Wavelength-selective reflector based on a circular array of coupled microring resonators,” IEEE Photon. Technol. Lett. 16(5), 1331–1333 (2004). [CrossRef]

5.

B. E. Little, S. T. Chu, and H. A. Haus, “Second-order filtering and sensing with partially coupled traveling waves in a single resonator,” Opt. Lett. 23(20), 1570–1572 (1998). [CrossRef]

6.

Y. Chung, D.-G. Kim, and N. Dagli, “Widely tunable coupled-ring reflector laser diode,” IEEE Photon. Technol. Lett. 17(9), 1773–1775 (2005). [CrossRef]

7.

G. Robinson, “The commercial development of planar optical biosensors,” Sens. Actuators B Chem. 29(1-3), 31–36 (1995). [CrossRef]

8.

A. Yariv, “Universal relations for coupling of optical power between microresonators and dielectric waveguides,” Electron. Lett. 36(4), 321–322 (2000). [CrossRef]

9.

C.-Y. Chao and L. J. Guo, “Thermal-flow technique for reducing surface roughness and controlling gap size in polymer microring resonators,” Appl. Phys. Lett. 84(14), 2479–2481 (2004). [CrossRef]

10.

D. R. Lide, “Concentrative Properties of Aqueous Solutions,” in CRC Handbook of Chemistry and Physics, 88th Edition (Internet Version 2008) (CRC Press/Taylorand Francis, Boca Raton, FL., 2007), pp. 2640–2640.

11.

A. Densmore, D. X. Xu, P. Waldron, S. Janz, P. Cheben, J. Lapointe, A. Delage, B. Lamontagne, J. H. Schmid, and E. Post, “A silicon-on-insulator photonic wire based evanescent field sensor,” IEEE Photon. Technol. Lett. 18(23), 2520–2522 (2006). [CrossRef]

12.

C.-Y. Chao and L. J. Guo, “Biochemical sensors based on polymer microrings with sharp asymmetrical resonance,” Appl. Phys. Lett. 83(8), 1527–1529 (2003). [CrossRef]

13.

V. M. N. Passaro, F. Dell’Olio, B. Casamassima, and F. De, “Guided-Wave Optical Biosensors,” Sensors 7(4), 508–536 (2007). [CrossRef]

14.

H. Sun, A. Pyajt, J. Luo, Z. Shi, S. Hau, A. K. Y. Jen, L. R. Dalton, and A. Chen, “All-dielectric electrooptic sensor based on a polymer microresonator coupled side-polished optical fiber,” IEEE Sens. J. 7(4), 515–524 (2007). [CrossRef]

15.

I. Kiyat, C. Kocabas, and A. Aydinli, “Integrated micro ring resonator displacement sensor for scanning probe microscopies,” J. Micromech. Microeng. 14(3), 374–381 (2004). [CrossRef]

16.

B. Bhola, H.-C. Song, H. Tazawa, and W. H. Steier, “Polymer microresonator strain sensors,” IEEE Photon. Technol. Lett. 17(4), 867–869 (2005). [CrossRef]

17.

H. Rong, Y.-H. Kuo, S. Xu, A. Liu, R. Jones, M. Paniccia, O. Cohen, and O. Raday, “Monolithic integrated Raman silicon laser,” Opt. Express 14(15), 6705–6712 (2006). [CrossRef] [PubMed]

18.

A. W. Fang, B. R. Koch, K.-G. Gan, H. Park, R. Jones, O. Cohen, M. J. Paniccia, D. J. Blumenthal, and J. E. Bowers, “A racetrack mode-locked silicon evanescent laser,” Opt. Express 16(2), 1393–1398 (2008). [CrossRef] [PubMed]

OCIS Codes
(130.3120) Integrated optics : Integrated optics devices
(230.4555) Optical devices : Coupled resonators
(280.4788) Remote sensing and sensors : Optical sensing and sensors
(130.7408) Integrated optics : Wavelength filtering devices

ToC Category:
Integrated Optics

History
Original Manuscript: April 17, 2009
Revised Manuscript: May 30, 2009
Manuscript Accepted: May 30, 2009
Published: June 11, 2009

Virtual Issues
Vol. 4, Iss. 8 Virtual Journal for Biomedical Optics

Citation
Haishan Sun, Antao Chen, and Larry R. Dalton, "A reflective microring notch filter and sensor," Opt. Express 17, 10731-10737 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-13-10731


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References

  1. A. Othonos, “Fiber Bragg gratings,” Rev. Sci. Instrum. 68(12), 4309–4341 (1997). [CrossRef]
  2. A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putnam, and E. J. Friebele, “Fiber grating sensors,” J. Lightwave Technol. 15(8), 1442–1463 (1997). [CrossRef]
  3. G. T. Paloczi, J. Scheuer, and A. Yariv, “Compact microring-based wavelength-selective inline optical reflector,” IEEE Photon. Technol. Lett. 17(2), 390–392 (2005). [CrossRef]
  4. J. K. S. Poon, J. Scheuer, and A. Yariv, “Wavelength-selective reflector based on a circular array of coupled microring resonators,” IEEE Photon. Technol. Lett. 16(5), 1331–1333 (2004). [CrossRef]
  5. B. E. Little, S. T. Chu, and H. A. Haus, “Second-order filtering and sensing with partially coupled traveling waves in a single resonator,” Opt. Lett. 23(20), 1570–1572 (1998). [CrossRef]
  6. Y. Chung, D.-G. Kim, and N. Dagli, “Widely tunable coupled-ring reflector laser diode,” IEEE Photon. Technol. Lett. 17(9), 1773–1775 (2005). [CrossRef]
  7. G. Robinson, “The commercial development of planar optical biosensors,” Sens. Actuators B Chem. 29(1-3), 31–36 (1995). [CrossRef]
  8. A. Yariv, “Universal relations for coupling of optical power between microresonators and dielectric waveguides,” Electron. Lett. 36(4), 321–322 (2000). [CrossRef]
  9. C.-Y. Chao and L. J. Guo, “Thermal-flow technique for reducing surface roughness and controlling gap size in polymer microring resonators,” Appl. Phys. Lett. 84(14), 2479–2481 (2004). [CrossRef]
  10. D. R. Lide, “Concentrative Properties of Aqueous Solutions,” in CRC Handbook of Chemistry and Physics, 88th Edition (Internet Version 2008) (CRC Press/Taylorand Francis, Boca Raton, FL., 2007), pp. 2640–2640.
  11. A. Densmore, D. X. Xu, P. Waldron, S. Janz, P. Cheben, J. Lapointe, A. Delage, B. Lamontagne, J. H. Schmid, and E. Post, “A silicon-on-insulator photonic wire based evanescent field sensor,” IEEE Photon. Technol. Lett. 18(23), 2520–2522 (2006). [CrossRef]
  12. C.-Y. Chao and L. J. Guo, “Biochemical sensors based on polymer microrings with sharp asymmetrical resonance,” Appl. Phys. Lett. 83(8), 1527–1529 (2003). [CrossRef]
  13. V. M. N. Passaro, F. Dell’Olio, B. Casamassima, and F. De, “Guided-Wave Optical Biosensors,” Sensors 7(4), 508–536 (2007). [CrossRef]
  14. H. Sun, A. Pyajt, J. Luo, Z. Shi, S. Hau, A. K. Y. Jen, L. R. Dalton, and A. Chen, “All-dielectric electrooptic sensor based on a polymer microresonator coupled side-polished optical fiber,” IEEE Sens. J. 7(4), 515–524 (2007). [CrossRef]
  15. I. Kiyat, C. Kocabas, and A. Aydinli, “Integrated micro ring resonator displacement sensor for scanning probe microscopies,” J. Micromech. Microeng. 14(3), 374–381 (2004). [CrossRef]
  16. B. Bhola, H.-C. Song, H. Tazawa, and W. H. Steier, “Polymer microresonator strain sensors,” IEEE Photon. Technol. Lett. 17(4), 867–869 (2005). [CrossRef]
  17. H. Rong, Y.-H. Kuo, S. Xu, A. Liu, R. Jones, M. Paniccia, O. Cohen, and O. Raday, “Monolithic integrated Raman silicon laser,” Opt. Express 14(15), 6705–6712 (2006). [CrossRef] [PubMed]
  18. A. W. Fang, B. R. Koch, K.-G. Gan, H. Park, R. Jones, O. Cohen, M. J. Paniccia, D. J. Blumenthal, and J. E. Bowers, “A racetrack mode-locked silicon evanescent laser,” Opt. Express 16(2), 1393–1398 (2008). [CrossRef] [PubMed]

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