Strain gauge using Si-based optical microring resonator Longhai Lei,1,2 Jun Tang,1,2 Tianen Zhang,1,2 Hao Guo,1,2 Yanna Li,1,2 Chengfeng Xie,1,2 Chenglong Shang,1,2 Yu Bi,1,2 Wendong Zhang,1,2 Chenyang Xue,1,2 and Jun Liu1,2,* 1
Key Laboratory of Instrumentation Science & Dynamic Measurement, North University of China, Ministry of Education, Taiyuan 030051, China 2
Key Laboratory of Science and Technology on Electronic Test & Measurement, North University of China, Taiyuan 030051, China *Corresponding author: [email protected]
Received 30 May 2014; revised 30 October 2014; accepted 19 November 2014; posted 4 November 2014 (Doc. ID 213095); published 15 December 2014
This paper presents a strain gauge using the mechanical-optical coupling method. The Si-based optical microring resonator was employed as the sensing element, which was embedded on the microcantilevers. The experimental results show that applying external strain triggers a clear redshift of the output resonant spectrum of the structure. The sensitivity of 93.72 pm∕MPa was achieved, which also was verified using theoretical simulations. This paper provides what we believe is a new method to develop micro-opto-electromechanical system (MOEMS) sensors. © 2014 Optical Society of America OCIS codes: (140.4780) Optical resonators; (230.4685) Optical microelectromechanical devices; (220.4880) Optomechanics; (120.1880) Detection. http://dx.doi.org/10.1364/AO.53.008389
1. Introduction
Recently, miniaturization and integration of siliconbased, stress-sensitive structures have received wide attention with advances in microelectromechanical system (MEMS) technology [1,2]. Currently mechanical sensors based on piezoresistive [3], capacitive [4], resonant [5], and optical fiber [6,7] sensing modes were created to convert strain/stress changes to an electrical signal. Such types of sensors are widely used in aerospace, marine exploration, medical diagnostics, and biochemical fields [8–10]. In addition, due to its high sensitivity, a microring resonator based on a silicon-on-insulator (SOI) also has been used in the sensing field. Several research groups have reported sensors based on siliconintegrated optical ring resonators, such as pressure 1559-128X/14/368389-06$15.00/0 © 2014 Optical Society of America
sensors [11,12] and biochemical sensors [13,14]. J. Wouter reported [15] an application of particular interest for strain measurement based on silicon waveguides. The device sensitivity reached 0.5– 0.75 pm∕microstrain. However, there are very few reports on the characteristics of stress sensitivity of this type of optical microring resonator to achieve the detection of embedded mechanical sensor signals in a micro-opto-electromechanical system (MOEMS) structure. This paper proposes a strain gauge using an optical microring resonator. It suggests using a Si-based optical microring resonator for the sensing element. When external strain is put on the structure, the radius of the SOI ring waveguide will be subjected to variation, which causes the optical resonant parameters to change. This ultimately leads to a redshift of resonant spectrum, and shows the excellent characteristics of the structure’s stress sensitivity [11]. The resulting sensing pattern of the SOI 20 December 2014 / Vol. 53, No. 36 / APPLIED OPTICS
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microring resonator via the induced stress is highly sensitive. This result verifies that the optical microring resonator can be employed for embedded high sensitivity stress detection for applications in mechanical sensing, MOEMSs, and single-chip integrated modules. The experimental test, theoretical analysis, and finite element simulation, shows the SOI optical microring resonator’s high level of stress sensitivity. For a pressure range of 0 to 10.399 MPa, the sensitivity obtained was 93.72 pm∕MPa. Simultaneously, a record redshift of the resonant spectrum up to 0.99498 nm was observed. The optical quality factor Q was 3.7 × 104 . 2. Experiments
A 6-in. (150 mm) SOI wafer with 220 nm Si on SiO2 (Shanghai Simgui Technology Co., Ltd.) was used in the experiments. We employed e-beam lithography and an ICP etching process to prepare the optical microring resonator. Two types of samples with ring waveguides with radii of 30 and 60 μm were used for the experiments. Figure 1(a) shows the strain gauge experimental system, which mainly consisted of a laser source (New Focus TLB-6700), compression structure, photoelectric detector (4NIC-K15), and an oscilloscope (Tektronix DPO2024). In the experiment, the output current of the laser source was 70 mA and the scanning wavelength ranged from 1520 to 1570 nm. The SOI structure was fixed on a compression structure, which included a fixed end and a spiral pressurized knob. The compression structure, the input and output optical fibers were fixed on a highprecision, three-dimensional (3D) vibration isolation platform that could adjust the incidence angle of the lensed fiber with the waveguide grating structure. By connecting the output optical fiber to the
photoelectric detector and the oscilloscope, we were able to characterize the “strain-optical” coupling effect. 3. Results and Discussion A. Optical Microring Resonator Fabrication and Characterizations
The SOI cantilever beam optical microring resonator consisted of a bus waveguide, a ring resonator, and a cantilever beam structure, as shown in Fig. 1(b). Employing e-beam lithography, ICP etching, and wet etching process, we created a cantilever beam structure that was 10 mm × 5 mm × 0.678 mm (0.678 mm was the thickness of the substrate material). The bus waveguide (length 2250 μm), which could steadily couple the input and output optical fibers with the grating structure, was located on the fixed end of the structure. The ring resonator was located at the cantilever beam’s headend. It produced deflection deformation via the induced stress and generated radial deformation due to the shear stress. The optical coupling condition also could be changed to further affect the optical resonant spectrum. We were able to study the stress sensitivity of the structure by testing and analyzing the redshift of output resonant spectrum. Figure 1(c) shows the S-4800 scanning electron microscopic (SEM) image of the fabricated SOI microring resonator. It shows that the coupling gap between the microring resonator and the bus waveguide is about 90 nm. The cross-sections of the bus waveguide and the microring resonator were equal in size and rectangular: 453 nm × 220 nm. Compared to the initial designed size (waveguide width and coupling gap were 500 and 100 nm, respectively), the waveguide width and coupling gap appeared to decrease to a certain degree. We believe this change was due to the edge effects in the etching process. This result was considered as reasonable and satisfies the single mode propagation of the light. B. Strain Gauge Test
Fig. 1. Experimental system. (a) Schematic diagram of the strain-optical testing system. (b) Structure diagram of the cantilever beam with SOI waveguide. (c) FESEM image of the microring resonator. 8390
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Using a specially designed compression structure and strain-optical testing system, we subjected the optical resonant spectrum of the SOI structure with ring radii (R) of 30 μm to different deflection deformations via induced stress, as shown in Fig. 2(a). Table 1 shows the strain-optical testing data of the SOI optical microring resonator. Figure 2(b) is the partially enlarged diagram of the normalized resonant spectrum with wavelength range from 1548.0 to 1551.0 nm. In Fig. 2, the redshift amount of the resonant spectrum Δλm decreases after increasing initially with the deflection deformation y of the cantilever beam increasing continuously. Table 1 shows the values obtained. Two physical effects played a role in the shift of the resonant spectrum: the change of the ring circumference and the change of the waveguide’s effective refractive index.
Fig. 2. Resonant spectrum of the optical microring resonator with different deflection deformation. (a) Wavelength range from 1520 to 1570 nm. (b) A partially enlarged diagram.
In a certain range of deflection deformation of the cantilever beam structure (at low stress), the influence of the increasing ring waveguide circumference on the shift of resonant spectrum is about three times of that caused by the changes in the effective refractive index of the waveguide. These waveguide changes are influenced by the variation of the refractive index of the silicon itself due to the photoelastic effect and the shape of the cross-section of the waveguide shrunk due to Poisson’s effect. Note that the two effects are opposite to each other [16]; namely, an increase in the circumference of the ring waveguide causes the redshift and a change in the effective refractive index creates a blueshift of resonant spectrum. In this case, the redshift amount of the resonant spectrum Δλm is mainly affected by the variation ΔR of the ring waveguide radius, and the influence by the slight change in the effective refractive index can be ignored. According to the theory of whispering gallery mode, the relationship between the redshift amount Δλm and the radial deformation ΔR of ring waveguide can be expressed as Δλm
2πneff ΔR: q
(1)
In Eq. (1) R is the radius, neff is the effective refractive index, and q is the different resonant modes of the microring resonator. The parameter q can be any nonzero positive integer and λm is the resonant wavelength. As shown in Eq. (1), when the cantilever beam structure is under certain external stress, the radius of the ring waveguide will produce a radial deformation of ΔR and becomes larger, thus to produce a shift in linear augment of the resonant spectrum, hence the redshift amount of the resonant spectrum increases initially. Table 1.
When the deflection y due to deformation of the structure reaches a certain maximum value and the deformation continues (at high strain), the photoelastic effect and Poisson’s effect on the waveguide structure will be remarkable, and the greater change will occur in the effective refractive index [17] of the waveguide. In this case, the influence on shift of the resonant spectrum by the change of the effective refractive index cannot be ignored, and the two changes together influence the shift of the resonant spectrum. Based on the photoelastic effect and Poisson’s effect, the redshift amount Δλm of resonant spectrum can be expressed as Δλm ΔR Δneff : λm neff R
(2)
In Eq. (2), Δneff is the change in the amount of the effective refractive index. The effect of change on the ring circumference still is dominant [15], and a redshift of 0.20506 nm is obtained experimentally. Only the amount of redshift of the resonant spectrum eventually appears to reduce. C.
Simulations and Discussion
By using finite element simulation and analysis, the stress δ in the ring and radial deformation ΔR of the microring resonator with radii (R) of 30 μm were obtained by the SOI cantilever beam structure deflection deformations of 20, 40, 70, 90, and 110 μm. Table 2 shows these results. Figure 3 is the simulation diagram of deflection deformation of the SOI cantilever beam via the stress induced. X coordinate represents for the direction of the cantilever beam, and Y is perpendicular to the plane of the cantilever beam. There is a deformation distribution y (0 ≤ y ≤ 110 μm) in the vertical direction of the plane of the structure, and that is where the simulation was performed.
Strain-optical Testing Data of the SOI Optical Microring Resonator with Ring Radii of 30 μm.
Pressurization displacement/mm Deflection deformation y∕μm Spectrum redshift amount Δλm ∕nm
0 0 0
0.02 20 0.26781
0.04 40 0.45480
0.07 70 0.77118
0.09 90 0.99498
0.11 110 0.20506
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Fig. 3. Simulation diagram of deflection deformation of the SOI cantilever beam via the stress induced. (a) The amount of deflection deformation y 20 μm. (b) The amount of deflection deformation y 110 μm. (c) The simulation diagram of radial deformation of the ring waveguide structure after the stress induced.
In the process of simulation, via the induced stress, the substrate structure of the cantilever beam produced a deflection deformation y, as shown in Figs. 4(a) and 4(b). The headend of the cantilever beam generated significant strain so that the substrate produced deformation in the direction of the radius of the ring waveguide. Simultaneously, the stress in the substrate was transferred to the ring waveguide, which produced radial deformation ΔR, as shown in Figs. 4(c) and 4(d). From the theory of mechanics of materials, the following relationships are obtained: Dsub
1 Eh3 121 − ν2
(3)
1 Eh3 : 12 b
(4)
and Dwav
In these equations, Dsub and Dwav are the stiffness values of the substrate and ring waveguide materials. Table 2.
Simulation Data of Stress and Radial Deformation of Ring Waveguide with Ring Radii of 30 μm.
Deflection deformation (y∕μm) Stress in the ring (δ∕MPa) Radial deformation (ΔR∕nm)
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E and v are the Young’s modulus and Poisson’s ratio of substrate material. h and hb are the thickness of the substrate material (h 0.678 mm) and the ring waveguide structure (hb 220 nm), respectively. Since h > 100hb and Dsub > 106 Dwav , when stress is applied to the substrate, the radial deformation ΔR of ring waveguide is only caused by the shear stress from the substrate plane and the influence of stress in the ring waveguide on the substrate structure can be completely ignored. Simultaneously, the substrate is in direct contact with the ring waveguide, so that the radial deformation ΔR of the ring waveguide equals the radial displacement of the substrate for a given position. In the simulation data, exact value of the radial displacement was extracted as the radial deformation ΔR. From the simulation results, the linear regression relationship between radial deformation ΔR and the applied stress δ can be observed, as shown in Fig. 5. The radial deformation ΔR is directly proportional to the applied stress when the applied stress δ is in the range of 0 to 10.399 MPa. Therefore, the radial deformation, ΔR of the ring waveguide can be expressed as
0 0 0
20 2.317 47.683
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40 4.651 95.093
70 8.083 165.26
90 10.399 212.62
110 12.716 259.99
Fig. 4. Structure diagram of the cantilever beam. (a) Before stress applied. (b) After stress applied. (c) Deformation diagram of the ring waveguide before stress applied. (d) After stress applied.
ΔR T 1 δ.
(5)
T 1 is the ring waveguide stress sensitivity. It represents the degree of sensitivity of the radial deformation ΔR to the applied stress δ. The parameter T 1 is related to initial radius of the ring waveguide. By curve fitting, the value of T 1 is calculated as 20.446 nm∕MPa. In addition, the relationship between the radial deformation ΔR and the applied stress δ for the waveguide radius R of 60 μm was also obtained and is shown in Fig. 5. For a certain range of stress, the radial deformation ΔR of the ring waveguide is also proportional to the applied stress δ of the ring radii of 60 μm, confirming the rationality of the Eq. (5) and correctness of the simulation analysis. By substituting Eq. (5) in Eq. (1) the following is obtained: Δλm
2πneff T 1 δ T 2 δ; q
(6)
where T 2 is the resonant spectrum sensitivity of stress detection of optical microring resonator. It characterizes the degree of sensitivity of resonant spectrum redshift Δλm responsiveness of the stress δ. T 2 is relevant with the initial radius of ring waveguide and the effective refractive index.
Fig. 6. Linear fitting diagraph of the redshift amount Δλm and the stress δ in the ring with ring radii of 30 μm and 60 μm.
By combining the simulation data and the test results, the relationship between the redshift Δλm of resonant spectrum and stress δ in the ring of radii (R) of 30 μm can be obtained. Figure 6 shows that for stress in a range of 0–10.399 MPa, the resonant spectrum of the microring resonator appears to be linear redshift with stress gradually increasing. This trend is due to the value of redshift Δλm is mainly affected by the variation of the ring circumference. While the stress value reaches to 12.716 MPa (at high strain), the magnitude of redshift of the resonant spectrum decreases. This trend, due to the shift of the resonant spectrum, is influenced by the variation of the ring circumference and the effective refractive index of the structure together. From the test results and linear fitting data, the largest redshift Δλm of resonant spectrum of the SOI microring resonator was observed as high as 0.99498 nm. The quality factor Q of the resonant peak was around 2.7 × 104 . Two values of 93.72 pm∕MPa and 11.06 pm∕microstrain for stress/strain detection sensitivity were obtained experimentally. Compared to the values of references (1.47 pm∕KPa, 0.47–1.3 pm∕microstrain). Zhao and co-workers [11,16,18] reported the resulting value in this paper was slightly lower in resolution, and it was suitable for measurement in a larger stress value range. These results illustrate that the radial deformation of ring waveguide induced by the force exerted on the cantilever beam structure can be monitored via the shift in resonant spectrum of the output from the bus waveguide and can be applied for the measurement of the stress/strain. Simultaneously, the relationship between the redshift amount Δλm of the resonant spectrum and stress δ in the ring of radii (R) of 60 μm also was studied by the strain-optical testing system and finite element simulation. By fitting the data, the stress sensitivity T 2 of the structure was determined to be 44.73 pm∕MPa. The quality factor Q was found to be as high as 3.7 × 104. 4. Conclusion
Fig. 5. Linear fitting diagraph of the radial deformation ΔR and the applied stress δ in the ring with ring radii of 30 and 60 μm.
In summary, this paper has studied the SOI-based optical microring resonator stress/strain sensor 20 December 2014 / Vol. 53, No. 36 / APPLIED OPTICS
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characteristics and sensitivity. The output signal of the sensor was found to be directly proportional to the applied stress in the ring for the stress range of 0 to 10.399 MPa. The stress/strain sensitivity of the structure was determined to be 93.72 pm∕MPa, 11.06 pm∕microstrain. The largest redshift of resonant spectrum was as high as 0.99498 nm and the quality factor Q is 3.7 × 104 . By optimizing the design process, the optical microring resonator can be employed to realize the embedded high sensitivity of stress detection in mechanical sensing and micro-opto-electro-mechanical systems (MOEMSs). This work was supported by the National Science Foundation for Distinguished Young Scholars of China (51225504), the National Natural Science of China (61171056, 91123016, and 51105345) and the Shanxi Provincial Foundation for Leaders of Disciplines in Science, China and the State Key Development Program for Basic Research of China (2012CB723404). References 1. C. Hsin-Nan, C. Tsung-Lin, L. Chun-Te, and C. Kuo-Ning, “Investigation of the hysteresis phenomenon of a silicon-based piezoresistive pressure sensor,” in Microsystems, Packaging, Assembly and Circuits Technology (IEEE, 2007), pp. 165–168. 2. P. Jih-Ping, P. Thayamballi, and G. Chia, “Pressure sensor implementation for head media spacing reduction,” IEEE Trans. Magn. 46, 778–781 (2010). 3. U. Gowrishetty, K. Walsh, S. McNamara, T. Roussel, and J. Aebersold, “Single element three-terminal pressure sensors: a new approach to pressure sensing and its comparison to the half-bridge sensors,” in Proceedings of Solid-State Sensors, Actuators and Microsystems Conference (IEEE, 2009), pp. 1134–1137. 4. A. V. Chavan and K. D. Wise, “A monolithic fully integrated vacuum-sealed CMOS pressure sensor,” IEEE Trans. Electron Devices 49, 164–169 (2002). 5. M. Benetti, D. Cannata, F. DiPietrantonio, C. Marchiori, P. Persichetti, and E. Verona, “Pressure sensor based on surface acoustic wave resonators,” in IEEE Sensors, Lecce, Italy (IEEE, 2008), pp. 1024–1027. 6. L. Mengchao and W. Qiaoyun, “Miniature high- sensitivity diaphragm-based optical fiber Fabry–Perot pressure sensor,”
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in International Conference on Electronics, Communications and Control (ICECC), Zhejiang (IEEE, 2011), pp. 328–330. C. E. Campanella, A. Giorgini, S. Avino, P. Malara, R. Zullo, G. Gagliardi, and P. De Natale, “Localized strain sensing with fiber Bragg-grating ring cavities,” Opt. Express 21, 29435–29441 (2013). K. Harman, B. Hodgins, J. Patchell, and M. Maki, “A rapid deployment guided radar sensor,” in 43rd Annual 2009 International Carnahan Conference on Security Technology, Zurich (IEEE, 2009), pp. 232–238. X. Juncheng, W. Xingwei, K. L. Cooper, G. R. Pickrell, and W. Anbo, “Miniature temperature-insensitive Fabry–Pe/spl acute/rot fiber-optic pressure sensor,” IEEE Photon. Technol. Lett. 18, 1134–1136 (2006). S. Krco, V. Tsiatsis, K. Matusikova, M. Johansson, I. Cubic, and R. Glitho, “Mobile network supported wireless sensor network services,” in IEEE International Conference on Mobile Adhoc and Sensor Systems, Pisa, Italy (IEEE, 2007), pp. 1–3. X. Zhao, J. M. Tsai, H. Cai, X. M. Ji, J. Zhou, M. H. Bao, Y. P. Huang, D. L. Kwong, and A. Q. Liu, “A nano-opto- mechanical pressure sensor via ring resonator,” Opt. Express 20, 8535– 8542 (2012). E. Hallynck and P. Bienstman, “Integrated optical pressure sensors in silicon-on-insulator,” IEEE Photon. J. 4, 443–450 (2012). S. M. Leinders, W. J. Westerveld, J. Pozo, H. P. Urbach, N. de Jong, and M. D. Verweij, “Membrane design of an all-optical ultrasound receiver,” in Proc. IEEE Int. Ultrason. Symp. (IEEE, 2013). C. L. de Korte, H. H. G. Hansen, and A. F. W. van der Steen, “Vascular ultrasound for atherosclerosis imaging,” Interf. Focus. 1, 565–575 (2011). W. J. Westerveld, S. M. Leinders, P. M. Muilwijk, J. Pozo, T. C. van den Dool, M. D. Verweij, M. Yousefi, and H. Paul Urbach, “Characterization of integrated optical strain sensors based on silicon waveguides,” IEEE J. Sel. Top. Quantum Electron. 20, 5900110 (2014). W. J. Westerveld, J. Pozo, P. J. Harmsma, R. Schmits, E. Tabak, T. C. van den Dool, S. M. Leinders, K. W. A. van Dongen, H. Paul Urbach, and M. Yousefi, “Characterization of a photonic strain sensor in silicon-on-insulator technology,” Opt. Lett. 37, 479–481 (2012). J. Cai, Y. Ishikawa, and K. Wada, “Strain-induced bandgap and refractive index variation of silicon,” Opt. Express 21, 7162–7170 (2013). D. Taillaert, W. Van Paepegem, J. Vlekken, and R. Baets, “A thin foil optical strain gauge based on silicon-on-insulator microresonators,” Proc. SPIE 6619, 661914 (2007).
Key Laboratory of Instrumentation Science & Dynamic Measurement, North University of China, Ministry of Education, Taiyuan 030051, China 2
Key Laboratory of Science and Technology on Electronic Test & Measurement, North University of China, Taiyuan 030051, China *Corresponding author: [email protected]
Received 30 May 2014; revised 30 October 2014; accepted 19 November 2014; posted 4 November 2014 (Doc. ID 213095); published 15 December 2014
This paper presents a strain gauge using the mechanical-optical coupling method. The Si-based optical microring resonator was employed as the sensing element, which was embedded on the microcantilevers. The experimental results show that applying external strain triggers a clear redshift of the output resonant spectrum of the structure. The sensitivity of 93.72 pm∕MPa was achieved, which also was verified using theoretical simulations. This paper provides what we believe is a new method to develop micro-opto-electromechanical system (MOEMS) sensors. © 2014 Optical Society of America OCIS codes: (140.4780) Optical resonators; (230.4685) Optical microelectromechanical devices; (220.4880) Optomechanics; (120.1880) Detection. http://dx.doi.org/10.1364/AO.53.008389
1. Introduction
Recently, miniaturization and integration of siliconbased, stress-sensitive structures have received wide attention with advances in microelectromechanical system (MEMS) technology [1,2]. Currently mechanical sensors based on piezoresistive [3], capacitive [4], resonant [5], and optical fiber [6,7] sensing modes were created to convert strain/stress changes to an electrical signal. Such types of sensors are widely used in aerospace, marine exploration, medical diagnostics, and biochemical fields [8–10]. In addition, due to its high sensitivity, a microring resonator based on a silicon-on-insulator (SOI) also has been used in the sensing field. Several research groups have reported sensors based on siliconintegrated optical ring resonators, such as pressure 1559-128X/14/368389-06$15.00/0 © 2014 Optical Society of America
sensors [11,12] and biochemical sensors [13,14]. J. Wouter reported [15] an application of particular interest for strain measurement based on silicon waveguides. The device sensitivity reached 0.5– 0.75 pm∕microstrain. However, there are very few reports on the characteristics of stress sensitivity of this type of optical microring resonator to achieve the detection of embedded mechanical sensor signals in a micro-opto-electromechanical system (MOEMS) structure. This paper proposes a strain gauge using an optical microring resonator. It suggests using a Si-based optical microring resonator for the sensing element. When external strain is put on the structure, the radius of the SOI ring waveguide will be subjected to variation, which causes the optical resonant parameters to change. This ultimately leads to a redshift of resonant spectrum, and shows the excellent characteristics of the structure’s stress sensitivity [11]. The resulting sensing pattern of the SOI 20 December 2014 / Vol. 53, No. 36 / APPLIED OPTICS
8389
microring resonator via the induced stress is highly sensitive. This result verifies that the optical microring resonator can be employed for embedded high sensitivity stress detection for applications in mechanical sensing, MOEMSs, and single-chip integrated modules. The experimental test, theoretical analysis, and finite element simulation, shows the SOI optical microring resonator’s high level of stress sensitivity. For a pressure range of 0 to 10.399 MPa, the sensitivity obtained was 93.72 pm∕MPa. Simultaneously, a record redshift of the resonant spectrum up to 0.99498 nm was observed. The optical quality factor Q was 3.7 × 104 . 2. Experiments
A 6-in. (150 mm) SOI wafer with 220 nm Si on SiO2 (Shanghai Simgui Technology Co., Ltd.) was used in the experiments. We employed e-beam lithography and an ICP etching process to prepare the optical microring resonator. Two types of samples with ring waveguides with radii of 30 and 60 μm were used for the experiments. Figure 1(a) shows the strain gauge experimental system, which mainly consisted of a laser source (New Focus TLB-6700), compression structure, photoelectric detector (4NIC-K15), and an oscilloscope (Tektronix DPO2024). In the experiment, the output current of the laser source was 70 mA and the scanning wavelength ranged from 1520 to 1570 nm. The SOI structure was fixed on a compression structure, which included a fixed end and a spiral pressurized knob. The compression structure, the input and output optical fibers were fixed on a highprecision, three-dimensional (3D) vibration isolation platform that could adjust the incidence angle of the lensed fiber with the waveguide grating structure. By connecting the output optical fiber to the
photoelectric detector and the oscilloscope, we were able to characterize the “strain-optical” coupling effect. 3. Results and Discussion A. Optical Microring Resonator Fabrication and Characterizations
The SOI cantilever beam optical microring resonator consisted of a bus waveguide, a ring resonator, and a cantilever beam structure, as shown in Fig. 1(b). Employing e-beam lithography, ICP etching, and wet etching process, we created a cantilever beam structure that was 10 mm × 5 mm × 0.678 mm (0.678 mm was the thickness of the substrate material). The bus waveguide (length 2250 μm), which could steadily couple the input and output optical fibers with the grating structure, was located on the fixed end of the structure. The ring resonator was located at the cantilever beam’s headend. It produced deflection deformation via the induced stress and generated radial deformation due to the shear stress. The optical coupling condition also could be changed to further affect the optical resonant spectrum. We were able to study the stress sensitivity of the structure by testing and analyzing the redshift of output resonant spectrum. Figure 1(c) shows the S-4800 scanning electron microscopic (SEM) image of the fabricated SOI microring resonator. It shows that the coupling gap between the microring resonator and the bus waveguide is about 90 nm. The cross-sections of the bus waveguide and the microring resonator were equal in size and rectangular: 453 nm × 220 nm. Compared to the initial designed size (waveguide width and coupling gap were 500 and 100 nm, respectively), the waveguide width and coupling gap appeared to decrease to a certain degree. We believe this change was due to the edge effects in the etching process. This result was considered as reasonable and satisfies the single mode propagation of the light. B. Strain Gauge Test
Fig. 1. Experimental system. (a) Schematic diagram of the strain-optical testing system. (b) Structure diagram of the cantilever beam with SOI waveguide. (c) FESEM image of the microring resonator. 8390
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Using a specially designed compression structure and strain-optical testing system, we subjected the optical resonant spectrum of the SOI structure with ring radii (R) of 30 μm to different deflection deformations via induced stress, as shown in Fig. 2(a). Table 1 shows the strain-optical testing data of the SOI optical microring resonator. Figure 2(b) is the partially enlarged diagram of the normalized resonant spectrum with wavelength range from 1548.0 to 1551.0 nm. In Fig. 2, the redshift amount of the resonant spectrum Δλm decreases after increasing initially with the deflection deformation y of the cantilever beam increasing continuously. Table 1 shows the values obtained. Two physical effects played a role in the shift of the resonant spectrum: the change of the ring circumference and the change of the waveguide’s effective refractive index.
Fig. 2. Resonant spectrum of the optical microring resonator with different deflection deformation. (a) Wavelength range from 1520 to 1570 nm. (b) A partially enlarged diagram.
In a certain range of deflection deformation of the cantilever beam structure (at low stress), the influence of the increasing ring waveguide circumference on the shift of resonant spectrum is about three times of that caused by the changes in the effective refractive index of the waveguide. These waveguide changes are influenced by the variation of the refractive index of the silicon itself due to the photoelastic effect and the shape of the cross-section of the waveguide shrunk due to Poisson’s effect. Note that the two effects are opposite to each other [16]; namely, an increase in the circumference of the ring waveguide causes the redshift and a change in the effective refractive index creates a blueshift of resonant spectrum. In this case, the redshift amount of the resonant spectrum Δλm is mainly affected by the variation ΔR of the ring waveguide radius, and the influence by the slight change in the effective refractive index can be ignored. According to the theory of whispering gallery mode, the relationship between the redshift amount Δλm and the radial deformation ΔR of ring waveguide can be expressed as Δλm
2πneff ΔR: q
(1)
In Eq. (1) R is the radius, neff is the effective refractive index, and q is the different resonant modes of the microring resonator. The parameter q can be any nonzero positive integer and λm is the resonant wavelength. As shown in Eq. (1), when the cantilever beam structure is under certain external stress, the radius of the ring waveguide will produce a radial deformation of ΔR and becomes larger, thus to produce a shift in linear augment of the resonant spectrum, hence the redshift amount of the resonant spectrum increases initially. Table 1.
When the deflection y due to deformation of the structure reaches a certain maximum value and the deformation continues (at high strain), the photoelastic effect and Poisson’s effect on the waveguide structure will be remarkable, and the greater change will occur in the effective refractive index [17] of the waveguide. In this case, the influence on shift of the resonant spectrum by the change of the effective refractive index cannot be ignored, and the two changes together influence the shift of the resonant spectrum. Based on the photoelastic effect and Poisson’s effect, the redshift amount Δλm of resonant spectrum can be expressed as Δλm ΔR Δneff : λm neff R
(2)
In Eq. (2), Δneff is the change in the amount of the effective refractive index. The effect of change on the ring circumference still is dominant [15], and a redshift of 0.20506 nm is obtained experimentally. Only the amount of redshift of the resonant spectrum eventually appears to reduce. C.
Simulations and Discussion
By using finite element simulation and analysis, the stress δ in the ring and radial deformation ΔR of the microring resonator with radii (R) of 30 μm were obtained by the SOI cantilever beam structure deflection deformations of 20, 40, 70, 90, and 110 μm. Table 2 shows these results. Figure 3 is the simulation diagram of deflection deformation of the SOI cantilever beam via the stress induced. X coordinate represents for the direction of the cantilever beam, and Y is perpendicular to the plane of the cantilever beam. There is a deformation distribution y (0 ≤ y ≤ 110 μm) in the vertical direction of the plane of the structure, and that is where the simulation was performed.
Strain-optical Testing Data of the SOI Optical Microring Resonator with Ring Radii of 30 μm.
Pressurization displacement/mm Deflection deformation y∕μm Spectrum redshift amount Δλm ∕nm
0 0 0
0.02 20 0.26781
0.04 40 0.45480
0.07 70 0.77118
0.09 90 0.99498
0.11 110 0.20506
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Fig. 3. Simulation diagram of deflection deformation of the SOI cantilever beam via the stress induced. (a) The amount of deflection deformation y 20 μm. (b) The amount of deflection deformation y 110 μm. (c) The simulation diagram of radial deformation of the ring waveguide structure after the stress induced.
In the process of simulation, via the induced stress, the substrate structure of the cantilever beam produced a deflection deformation y, as shown in Figs. 4(a) and 4(b). The headend of the cantilever beam generated significant strain so that the substrate produced deformation in the direction of the radius of the ring waveguide. Simultaneously, the stress in the substrate was transferred to the ring waveguide, which produced radial deformation ΔR, as shown in Figs. 4(c) and 4(d). From the theory of mechanics of materials, the following relationships are obtained: Dsub
1 Eh3 121 − ν2
(3)
1 Eh3 : 12 b
(4)
and Dwav
In these equations, Dsub and Dwav are the stiffness values of the substrate and ring waveguide materials. Table 2.
Simulation Data of Stress and Radial Deformation of Ring Waveguide with Ring Radii of 30 μm.
Deflection deformation (y∕μm) Stress in the ring (δ∕MPa) Radial deformation (ΔR∕nm)
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E and v are the Young’s modulus and Poisson’s ratio of substrate material. h and hb are the thickness of the substrate material (h 0.678 mm) and the ring waveguide structure (hb 220 nm), respectively. Since h > 100hb and Dsub > 106 Dwav , when stress is applied to the substrate, the radial deformation ΔR of ring waveguide is only caused by the shear stress from the substrate plane and the influence of stress in the ring waveguide on the substrate structure can be completely ignored. Simultaneously, the substrate is in direct contact with the ring waveguide, so that the radial deformation ΔR of the ring waveguide equals the radial displacement of the substrate for a given position. In the simulation data, exact value of the radial displacement was extracted as the radial deformation ΔR. From the simulation results, the linear regression relationship between radial deformation ΔR and the applied stress δ can be observed, as shown in Fig. 5. The radial deformation ΔR is directly proportional to the applied stress when the applied stress δ is in the range of 0 to 10.399 MPa. Therefore, the radial deformation, ΔR of the ring waveguide can be expressed as
0 0 0
20 2.317 47.683
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40 4.651 95.093
70 8.083 165.26
90 10.399 212.62
110 12.716 259.99
Fig. 4. Structure diagram of the cantilever beam. (a) Before stress applied. (b) After stress applied. (c) Deformation diagram of the ring waveguide before stress applied. (d) After stress applied.
ΔR T 1 δ.
(5)
T 1 is the ring waveguide stress sensitivity. It represents the degree of sensitivity of the radial deformation ΔR to the applied stress δ. The parameter T 1 is related to initial radius of the ring waveguide. By curve fitting, the value of T 1 is calculated as 20.446 nm∕MPa. In addition, the relationship between the radial deformation ΔR and the applied stress δ for the waveguide radius R of 60 μm was also obtained and is shown in Fig. 5. For a certain range of stress, the radial deformation ΔR of the ring waveguide is also proportional to the applied stress δ of the ring radii of 60 μm, confirming the rationality of the Eq. (5) and correctness of the simulation analysis. By substituting Eq. (5) in Eq. (1) the following is obtained: Δλm
2πneff T 1 δ T 2 δ; q
(6)
where T 2 is the resonant spectrum sensitivity of stress detection of optical microring resonator. It characterizes the degree of sensitivity of resonant spectrum redshift Δλm responsiveness of the stress δ. T 2 is relevant with the initial radius of ring waveguide and the effective refractive index.
Fig. 6. Linear fitting diagraph of the redshift amount Δλm and the stress δ in the ring with ring radii of 30 μm and 60 μm.
By combining the simulation data and the test results, the relationship between the redshift Δλm of resonant spectrum and stress δ in the ring of radii (R) of 30 μm can be obtained. Figure 6 shows that for stress in a range of 0–10.399 MPa, the resonant spectrum of the microring resonator appears to be linear redshift with stress gradually increasing. This trend is due to the value of redshift Δλm is mainly affected by the variation of the ring circumference. While the stress value reaches to 12.716 MPa (at high strain), the magnitude of redshift of the resonant spectrum decreases. This trend, due to the shift of the resonant spectrum, is influenced by the variation of the ring circumference and the effective refractive index of the structure together. From the test results and linear fitting data, the largest redshift Δλm of resonant spectrum of the SOI microring resonator was observed as high as 0.99498 nm. The quality factor Q of the resonant peak was around 2.7 × 104 . Two values of 93.72 pm∕MPa and 11.06 pm∕microstrain for stress/strain detection sensitivity were obtained experimentally. Compared to the values of references (1.47 pm∕KPa, 0.47–1.3 pm∕microstrain). Zhao and co-workers [11,16,18] reported the resulting value in this paper was slightly lower in resolution, and it was suitable for measurement in a larger stress value range. These results illustrate that the radial deformation of ring waveguide induced by the force exerted on the cantilever beam structure can be monitored via the shift in resonant spectrum of the output from the bus waveguide and can be applied for the measurement of the stress/strain. Simultaneously, the relationship between the redshift amount Δλm of the resonant spectrum and stress δ in the ring of radii (R) of 60 μm also was studied by the strain-optical testing system and finite element simulation. By fitting the data, the stress sensitivity T 2 of the structure was determined to be 44.73 pm∕MPa. The quality factor Q was found to be as high as 3.7 × 104. 4. Conclusion
Fig. 5. Linear fitting diagraph of the radial deformation ΔR and the applied stress δ in the ring with ring radii of 30 and 60 μm.
In summary, this paper has studied the SOI-based optical microring resonator stress/strain sensor 20 December 2014 / Vol. 53, No. 36 / APPLIED OPTICS
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characteristics and sensitivity. The output signal of the sensor was found to be directly proportional to the applied stress in the ring for the stress range of 0 to 10.399 MPa. The stress/strain sensitivity of the structure was determined to be 93.72 pm∕MPa, 11.06 pm∕microstrain. The largest redshift of resonant spectrum was as high as 0.99498 nm and the quality factor Q is 3.7 × 104 . By optimizing the design process, the optical microring resonator can be employed to realize the embedded high sensitivity of stress detection in mechanical sensing and micro-opto-electro-mechanical systems (MOEMSs). This work was supported by the National Science Foundation for Distinguished Young Scholars of China (51225504), the National Natural Science of China (61171056, 91123016, and 51105345) and the Shanxi Provincial Foundation for Leaders of Disciplines in Science, China and the State Key Development Program for Basic Research of China (2012CB723404). References 1. C. Hsin-Nan, C. Tsung-Lin, L. Chun-Te, and C. Kuo-Ning, “Investigation of the hysteresis phenomenon of a silicon-based piezoresistive pressure sensor,” in Microsystems, Packaging, Assembly and Circuits Technology (IEEE, 2007), pp. 165–168. 2. P. Jih-Ping, P. Thayamballi, and G. Chia, “Pressure sensor implementation for head media spacing reduction,” IEEE Trans. Magn. 46, 778–781 (2010). 3. U. Gowrishetty, K. Walsh, S. McNamara, T. Roussel, and J. Aebersold, “Single element three-terminal pressure sensors: a new approach to pressure sensing and its comparison to the half-bridge sensors,” in Proceedings of Solid-State Sensors, Actuators and Microsystems Conference (IEEE, 2009), pp. 1134–1137. 4. A. V. Chavan and K. D. Wise, “A monolithic fully integrated vacuum-sealed CMOS pressure sensor,” IEEE Trans. Electron Devices 49, 164–169 (2002). 5. M. Benetti, D. Cannata, F. DiPietrantonio, C. Marchiori, P. Persichetti, and E. Verona, “Pressure sensor based on surface acoustic wave resonators,” in IEEE Sensors, Lecce, Italy (IEEE, 2008), pp. 1024–1027. 6. L. Mengchao and W. Qiaoyun, “Miniature high- sensitivity diaphragm-based optical fiber Fabry–Perot pressure sensor,”
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