Department of Applied Physics
University PhD Dissertation Defense
Fiber Based Photonic-Crystal Acoustic Sensor
Onur Kilic
Research Advisor: Professor O. Solgaard
11 March 2008 @4:00 p.m.
in
Applied Physics Building, Room 200
(Refreshments at 3:45 p.m.)
Abstract
Photonic crystal slabs can be employed to make various free space optical devices by tailoring the geometrical parameters of the structure such as hole radius, pitch, or hole shape. A standard photonic crystal slab can be used to make efficient optical filters and broadband mirrors. Breaking the symmetry through introducing asymmetric holes also enables polarization sensitive devices such as retarders, polarization beam splitters, and photonic crystals with additional non-degenerate resonances useful for increased sensitivity in sensors. The fabrication of photonic crystal slabs is compatible with microfabrication techniques, making them suitable as key components in micromachined sensor applications.
We report a micromachined acoustic sensor that consists of a Fabry-Perot interferometer made of a photonic-crystal reflector embedded on a compliant silicon diaphragm placed at the tip of a single-mode fiber. The small thickness of the photonic crystal slab makes it ideal as the external reflector that needs to be compliant for high sensitivity. Measurements in air indicate that this sensor has a relatively uniform frequency response up to at least 50 kHz, and detects pressures as low as 18 µPa/Hz1/2. This limit is four orders of magnitude lower than in similar types of acoustic fiber sensors.
We report a micromachined acoustic sensor that consists of a Fabry-Perot interferometer made of a photonic-crystal reflector embedded on a compliant silicon diaphragm placed at the tip of a single-mode fiber. The small thickness of the photonic crystal slab makes it ideal as the external reflector that needs to be compliant for high sensitivity. Measurements in air indicate that this sensor has a relatively uniform frequency response up to at least 50 kHz, and detects pressures as low as 18 µPa/Hz1/2. This limit is four orders of magnitude lower than in similar types of acoustic fiber sensors.
Through a modification in the design, such a sensor can also be used in water. In addition to the high compliance, the advantage for using the photonic crystal slab is that the holes provide a venting channel for pressure equalization so that the hydrophone can be employed in deep-sea applications. Measurements in water over the range of 10 kHz-50 kHz show that the hydrophone has a minimum detectable pressure down to 10 µPa/Hz1/2, close to the ambient noise level. A model was developed to show that after optimization to ocean acoustics, this sensor has a minimum detectable pressure that follows the minimum ambient noise spectrum of the ocean (reaching a minimum of ~10 µPa/Hz1/2 at ~30 kHz) in the bandwidth of 1 Hz-100 kHz. By placing several such sensors with different acoustic power ranges within a single sensor chip, the hydrophone is able of exhibiting a dynamic range in the excess of 200 dB.
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Onur Kilic
Applied Physics
Stanford University
OnurKilic.com
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