microphones posses innate
resistance to electro magnetic
interference & harsh environments.
MEMS technology provides a
promising implementation for optical
Here, we discuss the design &
characteristics of an intensity
modulated optical level microphone.
Introduced by Nykolai Bilaniuk in
3 properties of light could be
modulated. They are
Optical Microphone Classification
Based on Transduction Mechanism
Intensity modulating optical
microphone can be sub- divided into
a) Radiated wave intensity modulating
b) Evanescent wave intensity
modulation type devices alter the
polarization of the light when in the presence of an
layer of liquid crystals is subjected to acoustic
field induced shear stresses, which modulate the
polarization of the light passing through.
“a moveable dielectric plate interacts with the
evanescent field of a waveguide excited with both
TE and TM modes,
A mechanism that changes either the
physical length or the refractive index of
an optical test path and recombining the
result with the signal from a reference
The two defined subgroups
optical fibers selected for the optical
microphone are the Thorlabs
AFS105/125Y multimode optical fibers.
for both transmit (Tx) and receive (Rx)
cores of each fiber are color-coded, and
surrounded by a white ring representing the
light source used by this optical
microphone is the HP8168B Tunable Laser
The maximum output power of the laser at
1550 nm is 0.515 mW.
alternate laser source or an LED source
could be used in place of the HP8168B.
There are three schemes for use as
unreferenced output technique.
the referenced output technique.
FABRICATION OF THE OPTICAL
The fabrication of the optical microphone
consists of two parts:
MEMS optical diaphragm chip
Fabricated by MEMS Exchange
MEMS Exchange Process
Both mask and wafers were purchased through the
Wafers Used for Optical Microphone Fabrication
Abeysinghe et al. Packaging Technique.
generation version of the optical microphone
could be implemented with a single, large-core, highNA fiber (instead of a fiber bundle) using an LED as a
light source to improve stability and frequency
A laser can provide 1000 times more power than an
LED source when used as a light source in an
intensity-modulated lever microphone.
Since the performance of a MEMS device is
application specific, multiple packages and an array
packaging technique should be developed to take
advantage of the small size of the MEMS device.
Bandwidth (typically from 1Hz to 10kHz)
Dynamic Range (at least 85dB.)
Signal to Noise Ratio (SNR) in the order of 70dB.
Total Harmonic Distortion (THD) is less than 1%
at 94dBre20μPa over the entire frequency
Sensitivity of the FOM is 100mV/Pa for the
pressure microphones and 1.94 mV/(Pa/m) for the
pressure gradient microphones.
MEMS-based intensity-modulated optical microphone is an
excellent choice for applications with harsh environmental or
Optical MEMS microphones are currently marketed as a
surveillance technology, as an EMI and RFI immune
technology, and as a suitable technology for use in automobile
voice recognition systems
It is also possible to design the optical microphone with a
significantly higher sensitivity and lower MDS by sacrificing
frequency response and reducing the upper limit of the
microphone’s dynamic range.
more sensitive, fiber geometries are required to make an
intensity modulated optical microphone suitable for aeroacoustic measurements.
S. D. Senturia, Microsystems Design. New York: Kluwer
N. Bilaniuk, "Optical Microphone Transduction Techniques,"
Applied Acoustics, vol. 50, pp. 35-63, 1997.
V. P. Klimashin, “Optical Microphone,” Pribory i Tekhnika
Eksperimenta, no. 3, pp. 135-137, May 1979.