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High-bandwidth, Low-sensitivity Optomechanical MEMS Accelerometers


OBJECTIVE: Develop a chip-integrated optomechanical micro-electromechanical systems (MEMS) accelerometer with 100 ng/Hz^1/2 sensitivity and 10 kHz bandwidth using high finesse optics to readout and dynamically tune sensor parameters. DESCRIPTION: Inertial navigation systems (INS) are a critical asset to the DoD in environments where GPS is either denied or unavailable. At the heart of these systems are precision acceleration and rotation sensors. Recently, MEMS-based accelerometers have found widespread use in INS owing to their small size and ease of fabrication. However they still lack the sensitivity and bandwidth required for accurate long-distance navigation. Typically, MEMS accelerometers use capacitive measurement; their sensitivities are limited by thermal-electronic noise in the readout circuitry [1]. Optical interferometric methods eliminate electronic noise and can approach the thermal-mechanical limit [2], [3]. This thermal-mechanical noise imposes a fundamental trade-off between the sensitivity (ath) and bandwidth (BW) of the accelerometer: ath proportional to (BW/mQ)^1/2, where m is the mechanical resonator mass and Q is its quality factor. Therefore, to achieve a high sensitivity for a given bandwidth, the product mQ needs to be maximized. Furthermore, for high bandwidth devices, a high resolution displacement (x) measurement is required (x proportional to BW^-2), thus imposing requirements on the finesse (F) and input power (P) of the optical readout cavity (x proportional to (F^-1P^-1/2)), which is ultimately limited by laser shot noise. For example, to achieve a sensitivity of a few ng/Hz^1/2 at a bandwidth of 10 kHz, one would require mQ>1 kg and F>1000. Such a sensitivity and bandwidth combination has not been achieved in a commercial device and would reduce the INS error, allowing longer-duration navigation in the absence of GPS. Recently, accelerometers based on optomechanical devices have been developed, which exhibit a sensitivity of a few ng/Hz^1/2 with a bandwidth greater than 10kHz, in a compact form-factor [4], [5]. Optomechanical devices are strongly coupled optical and mechanical systems, in which a high finesse optical cavity is used to both measure and manipulate high-quality MEMs. Such devices have enabled optical radiation-pressure cooling of MEMs to their quantum ground state [6], eliminating thermal noise and enhancing the achievable bandwidth by broadening the mechanical resonance without loss of sensitivity. Furthermore, the cavity-enhanced optical field enables displacement measurement at the standard quantum limit [7], an important fundamental limit for acceleration sensing. Finally, utilizing the high circulating power achievable in a high finesse cavity, one can dynamically control the bandwidth of the MEMS accelerometer via the optical spring effect [8], thus enabling unprecedented in-situ control of accelerometer performance. While optomechanical devices have demonstrated exciting results in the laboratory, significant development is necessary to construct a robust packaged device that incorporates the laser, the optomechanical device, and optical readout circuitry. PHASE I: Design a robust, packaged MEMS accelerometer with highsensitivity optical readout approaching the standard quantum limit for displacement measurement. Such a system should exhibit high opticalmechanical coupling such that a pump laser can manipulate MEMS parameters such as resonance frequency and damping rate. The chosen work should be compatible with an accelerometer with less than100 ng/Hz^1/2 sensitivity and greater than a 10 kHz bandwidth. Exhibit the feasibility of the approach through a laboratory demonstration. Phase I deliverables will include a design review including expected device performance and a report presenting the plans for Phase II. Experimental data demonstrating feasibility of the proposed device is favorable. PHASE II: Fabricate and test a prototype device demonstrating the device performance outlined in Phase I. The Transition Readiness Level to be reached is 5: Component and/or bread-board validation in relevant environment. PHASE III: Once developed, compact, integrated optomechanical accelerometers with high-sensitivity and high-bandwidth would greatly improve military inertial navigation systems, requiring less frequent error correction and updates from GPS. Innovations in Phases I and II will enable such devices to transition out of the laboratory and into fieldable devices. MEMS accelerometers find widespread use in civilian products such as cellphones, seismic detection (geo-physical and oil exploration), automobiles and gravitational wave detection. REFERENCES: 1) G. Krishnan, C. Kshiragar, G. K. Ananthasuresh, and N. Bhat,"Micromachined high resolution accelerometers,"Journal of the Indian Institute of Science, vol. 87, no. 3, Jul. 2007. 2) M. A. Perez and A. M. Shkel,"Design and Demonstration of a Bulk Micromachined FabryPerot mico-g-Resolution Accelerometer,"IEEE Sensors Journal, vol. 7, no. 12, pp. 1653-1662, Dec. 2007. 3) K. Zandi, J. A. Bandlanger, and Y.-A. Peter,"Design and Demonstration of an In-Plane Silicon-on-Insulator Optical MEMS Fabry Perot-Based Accelerometer Integrated With Channel Waveguides,"Journal of Microelectromechanical Systems, vol. PP, no. 99, pp. 1-7, 2012. 4) L. Kumanchik, G. Shaw, J. Pratt, and J. M. Taylor,"An ultra-low noise, wide-bandwidth, micro-optomechanical accelerometer,"arXiV, Sep. 2012. 5) A. G. Krause, M. Winger, T. D. Blasius, Q. Lin, and O. Painter,"A microchip optomechanical accelerometer,"arXiv:1203.5730, Mar. 2012. 6) J. Chan, T. P. M. Alegre, A. H. Safavi-Naeini, J. T. Hill, A. Krause, S. Groblacher, M. Aspelmeyer, and O. Painter,"Laser cooling of a nanomechanical oscillator into its quantum ground state,"Nature, vol. 478, no. 7367, pp. 8992, Oct. 2011. 7) G. Anetsberger, E. Gavartin, O. Arcizet, Q. P. Unterreithmeier, E. M. Weig, M. L. Gorodetsky, J. P. Kotthaus, and T. J. Kippenberg,"Measuring nanomechanical motion with an imprecision below the standard quantum limit,"Phys. Rev. A, vol. 82, no. 6, p. 061804, Dec. 2010. 8) Q. Lin, J. Rosenberg, X. Jiang, K. J. Vahala, and O. Painter,"Mechanical Oscillation and Cooling Actuated by the Optical Gradient Force,"Phys. Rev. Lett., vol. 103, no. 10, p. 103601, 2009.

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