Onur Can Akkaya

Modeling and Demonstration of Thermally Stable High-Sensitivity Reproducible Acoustic Sensors

A thermally stable high-sensitivity compact fiber acoustic sensor with a large bandwidth and a high dynamic range is introduced. The device is based on a photonic crystal fabricated on a compliant single-crystal silicon diaphragm, which is placed near the metalized end of a single-mode fiber to form a Fabry-Perot (FP) cavity. High reproducibility in operating wavelength (±1 nm) is enabled by assembling the sensor in a SiO2 chip using room-temperature sodium silicate oxide bonding. We demonstrate ten FP sensors with measured displacement sensitivities within ±0.6 dB. The response is shown to be polarization independent and thermally stable, with a thermal coefficient of the operating wavelength of 2.9pm/°C over more than 50 °C. An experimental sensor is shown to measure acoustic pressures down to a record low of 5.6 μPa/√Hz at 12.5 kHz with a flatband response greater than 8 kHz and a sensitivity extending down to at least 100 Hz. The dynamic range in pressure is greater than 100 dB. An electromechanical model of the device response is presented and shown to be in good agreement with experimental results.

High-sensitivity thermally stable acoustic fiber sensor

A new design of miniature fiber acoustic sensors with high sensitivity and greatly improved thermal stability and reproducibility of the operating wavelength is presented. It consists of a high-finesse Fabry-Perot (FP) made of a photonic-crystal (PC) reflector fabricated on a compliant Si membrane and placed in close proximity to the reflective end of a fiber. An incident acoustic wave deflects the membrane, which shifts the FP reflection spectrum. This shift is detected as a change in the power reflected by the FP at a fixed wavelength. The reported improvements over an earlier design include (1) the use of photolithography to fabricate the PC diaphragm (higher accuracy and yield), (2) making the sensor chip out of silica instead of Si (higher thermal stability and reproducibility of the FP spectrum), and (3) using silicate bonding to assemble the sensor (higher thermal stability). An experimental fiber acoustic sensor with a finesse of ∼6 is shown to measure pressures in air with a resolution from 180 to 27 µPa/√Hz from ∼700 Hz to ∼8.6 kHz, and as low as 5.6 µPa/√Hz at 12.5 kHz. This last value is ∼4 times better than previously reported. The measured thermal stability is ∼70 times better than that of the earlier Si-based design.

Fabry-Perot fiber sensors with reproducible displacement sensitivities

An all-silica design and silicate bonding are used to demonstrate a Fabry-Perot acoustic fiber sensor for large-scale array applications with a high sensitivity, high thermal stability, and excellent reproducibility in displacement sensitivity (±0.6 dB).

Optomechanical fiber gyroscope

We report a miniature mechanical gyroscope that utilizes optical means to detect rotation-induced displacements in a mechanical structure. It utilizes the Foucault pendulum principle used in some existing MEMS gyroscopes: a rotating reference frame induces a Coriolis force that oscillates the structure about an axis orthogonal to the driving-mode axis. The main difference with similar MEMS gyroscopes is that this rotation-induced oscillation is sensed using a pair of high-finesse fiber Fabry-Perot displacement sensors instead of a capacitive device. The drive axis is also driven by radiation pressure inside a set of auxiliary fiber Fabry-Perot cavities, making this device immune to electromagnetic interference. Calculations predict that a rotation sensitivity on the order of 1°/h/Hz1/2 is achievable. We show that this structure solves several problems associated with MEMS gyroscopes utilizing electrostatic sensing methods.

Haltere-Like Optoelectromechanical Gyroscope

We report an optoelectromechanical vibratory gyroscope inspired by halteres of dipteran flies. The gyroscope utilizes optical displacement sensing to achieve a Brownian-motion-limited displacement sensitivity without mechanical resonant amplification in the sense mode. This yields a threefold difference between the resonance frequencies of the drive and sense modes, corresponding to a theoretical bandwidth of over 200 Hz, without compromising the sensitivity. Our measurements show a noise-equivalent rotation rate of 3°/h/Hz

Optical-fiber-compatible acoustic sensor

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Miniature fiber acoustic sensors using a photonic-crystal membrane

under atmospheric-pressure conditions.