IEEE SENSORS JOURNAL, VOL. 11, NO. 4, APRIL 2011 1035 Silicon-Based Stress-Coupled Optical Racetrack Resonators for Seismic Prospecting Wenqin Mo, Zhiping Zhou, Senior Member, IEEE, Huaming Wu, and Dingshan Gao Abstract—A silicon-based stress-coupled optical racetrack res- onator with a crossbeam mass is proposed to detect acceleration for seismic prospecting. Acceleration applied on the crossbeam mass can result in optical phase changes in one racetrack cycle, which leads to a resonant wavelength shift. By systematically op- timizing the resonator structure and mechanical characteristics, a very large wavelength shift of 52 pm under 1 g acceleration is demonstrated by numerical simulation. The maximum frequency of input signal can be up to 200 Hz. This silicon-based compact and high-performance racetrack resonator can have great potential for seismic prospecting. Index Terms—Acceleration measurement, optical resonator, seismic wave. I. INTRODUCTION S EISMIC prospecting applications normally require mea- surement of acceleration with high sensitivity in the frequency range from 10 to 200 Hz [1], [2]. To date, most acceleration sensors are based on capacitive, piezoelectric or magneto-resistive mechanisms [3]. However, these mechanisms suffer from electromagnetic interferences and cannot be used under harsh or explosive atmosphere. Recently, there has been a surge of investigations into optical acceleration sensors because they have advantages such as immunity to electromagnetic interference, light weight and remote sensing capabilities. Fiber Bragg grating (FBG) acceleration sensors have been theoretically proposed and experimentally demonstrated by several groups [1], [2], [4]. Although FBG has achieved the high sensitivity of 77 pm/g [1] and [2], the optic fiber has the necessity of manual positioning [5] and cannot be integrated with other optical functions on a planar-optic chip. Acceleration sensors based on the planar optic waveguide [6] are compact, low loss and easily fabricated by mass implanta- Manuscript received July 20, 2010; revised August 29, 2010; accepted September 05, 2010. Date of publication September 27, 2010; date of current version February 09, 2011. The associate editor coordinating the review of this paper and approving it for publication was Dr. M. Abedin. W. Mo is with the Wuhan National Laboratory for Optoelectronics, College of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China, and also with the Faculty of Mechanical and Electronic Information, China University of Geosciences, Wuhan 430074, China. Z. Zhou is with the State Key Laboratory of Advanced Optical Communi- cation Systems and Networks, Peking University, Beijing, 100871, China and also with the School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0250 USA (e-mail: zjzhou@pku.edu.cn). H. Wu and D. Gao are with the Wuhan National Laboratory for Optoelec- tronics, College of Optoelectronic Science and Engineering, Huazhong Univer- sity of Science and Technology, Wuhan 430074, China. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2010.2079326 tion. Owing to the advantage of the effective long interaction length provided by the resonance effects, microresonators have been shown to achieve a sufficient phase shift [7] and to be ex- tremely sensitive [8], [9]. Therefore, microresonators have been widely studied for biosensing [10], chemical sensing [11], dis- placement [12], and vibration sensing [13], [14]. The resonator- based acceleration sensor was first investigated by varying the gap between the waveguide and microsphere with resolution of 1 mg [13]. The optical MEMS vibration sensor using resonance effect had a wavelength shift of 3.19 pm under 1 g acceleration, and could be used in impact-vibration environments [14]. In ad- dition, the acceleration sensitivity of this configuration is related to both of the dimension of vibration unit and the elasto-optic coefficient of the materials. In this paper, we propose an integrated optical racetrack sensor for seismic detection without elasto-optic effect [15], [16]. The acceleration exerting the stress on the waveguides changes the optical phase per round trip in the racetrack wave- guide and thus causes variation in transmission characteristics of the racetrack resonator. Although the stress on the device will lead to the waveguide length increment and elasto-optic effect, the phase change by the latter can be negated by ap- plying the crossbeam vibration unit. By monitoring the resonant wavelength shift or the optical power variation, the applied acceleration can be detected. To achieve optimal performance of the sensor, the wavelength sensitivity is studied as a function of the crossbeam dimension, the racetrack resonator circumfer- ence and the applied acceleration. Furthermore, the dynamic range of the racetrack resonator is also discussed. II. MODEL AND THEORY A. Structure The proposed seismic sensor is based on a racetrack res- onator, which is integrated with a crossbeam mass sensitive to external seismic wave accelerations. As shown in Fig. 1, the racetrack parameters are the ring radius , the coupling length , and the gap spacing Gap between the resonator and the bus. The acceleration sensing part is formed by the crossbeam mass with the beam dimension of and mass dimension of , which can be realized by the silicon etching process. As we know, when the phase accumulated in a round trip through the racetrack resonator is an integer multiple of , a series of transmission minima occur at separated wavelengths , which refers to the resonant wavelength. When the acceleration is applied on the seismic mass, due to the stress on the crossbeam, the length of racetrack waveguide will be distorted by and effective index will also be changed under elasto-optic effect. Therefore, the 1530-437X/$26.00 © 2010 IEEE