Design guidelines for a guided-wave optical pressure sensor based on dependences of sensitivity and resonance frequency on diaphragm dimensions were considered. The guided-wave optical pressure sensor consists of a diaphragm and a single-mode waveguide on the diaphragm. According to our theoretical study, the edge of the diaphragm was found to be the best position to obtain the highest sensitivity. Also, sensitivity is proportional to the cube of the side length of the diaphragm but inversely proportional to the square of the diaphragm thickness. On the other hand, resonance frequency of the diaphragm is proportional to the diaphragm thickness and inversely proportional to the square of the side length. Dependences were experimentally examined for fabricated sensors with different diaphragm dimensions to confirm theoretical predictions. The experimental results agreed well with the theoretical predictions. Moreover, a design assistant chart for the sensor was proposed based on the dependences.
We experimentally investigated the relationship between sensitivity and diaphragm thickness in a glass-based guided-wave optical pressure sensor using intermodal interference between the fundamental TM-like and TE-like modes. The sensor consists of a rectangular diaphragm and a straight single-mode waveguide on the diaphragm. The sensitivity is theoretically known to be inversely proportional to the square of the diaphragm thickness. In this study, to examine this relationship, four sensors with diaphragm thicknesses of 0.30 mm, 0.22 mm, 0.20 mm, and 0.15 mm were fabricated. The area of the diaphragm was 10 mm×10 mm. For the waveguide position nearest to the center of the diaphragm, the measured sensitivities almost agreed with the theoretical ones.
In this study, the signal-to-noise ratio of a glass-based guided-wave optical microphone was successfully improved by
both increasing sensitivity and reducing noise. The optical microphone has a square diaphragm as a pressure-sensitive
structure and a straight single-mode waveguide across the diaphragm. Sensitivity of the microphone and resonance
frequency of the diaphragm are dependent on the area and thickness of the diaphragm. In this study, in order to increase
sensitivity, the diaphragm dimensions were enlarged from 16 mmx16 mmx0.15 mm in the previous study to 20 mmx20
mmx0.15 mm. According to theoretical calculations, the phase sensitivity and resonance frequency were 2.5 mrad/Pa
and 3.4 kHz for a 20 mmx20 mmx0.15 mm diaphragm, respectively. The sensitivity was theoretically expected to be
twice as high as that in the previous study. To reduce noise, a bandpass filter with passband from 300 Hz to 3 kHz was
employed. After fabrication of the optical microphone, sound pressure, ranging from 100 to 122 dB-SPL, was applied to
the microphone with a frequency of 1 kHz. The measured output of the optical microphone was almost proportional to
the sound pressure, and the minimum detectable sound pressure level of the microphone was experimentally evaluated to
be 100 dB-SPL.
In this paper, the feasibility of a glass-based guided-wave optical microphone is described. The optical microphone consists of a rectangular diaphragm and a straight waveguide on the diaphragm. The sensitivity of the microphone and the resonance frequency of the diaphragm are dependent on the diaphragm dimensions. In this study, to confirm operation of the proposed optical microphone, the target values for phase sensitivity and resonance frequency were set at 1.3 mrad/Pa and 5 kHz, respectively. By design considerations, the diaphragm dimensions were determined to be 16 mm × 16 mm × 0.15 mm. After fabrication, a sound wave of 1 kHz and 25 Pa, corresponding to 122 dB-SPL (sound pressure level), was applied to the microphone. In the experiment, the intensity-modulated output with the same frequency as the applied sound wave was obtained, but the observed output was unexpectedly caused by misalignment of the optical components due to mechanical vibration. The estimated output signal by the normal operation of the microphone for a sound pressure of 25 Pa was 1/10 - 1/100 of the noise level, according to the measured output characteristic to static pressure. In order to detect normal speech ranging from 55 to 65 dB-SPL, the S/N ratio should be improved by a factor of more than 104.
In this paper, an optical microphone using a silicon-based guided-wave optical pressure sensor as an opto-mechanical transducer is reported. The pressure sensor consists of a rectangular diaphragm and a straight waveguide on the diaphragm. The sensitivity of the sensor and the resonance frequency of the diaphragm are important factors to determine the characteristics of the microphone, and depend on the diaphragm dimensions. In this study, to examine a feasibility of the proposed optical microphone, the target values of phase sensitivity and resonance frequency were set at 1.6 mrad/Pa and 7 kHz, respectively. By design considerations, the diaphragm dimensions were determined to be 7 mmX7 mmX23 μm. After fabrication of the optical microphone, sound pressure from 5 to 25 Pa, with a frequency of 1 kHz, was applied to the fabricated microphone with a 7 mmX7 mmX27 μm diaphragm. During measurement, a lock-in detection was taken because the fabricated pressure sensor had an unexpected low sensitivity, which resulted in an extremely low S/N ratio. The measured output voltage from the lock-in amplifier was proportional to the sound pressure as expected although the lock-in detection is not practical for the microphone.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.