17-01-2014, 02:37 PM
OPTICAL MICROPHONE
OPTICAL MICROPHONE.ppt (Size: 1.71 MB / Downloads: 58)
INTRODUCTION
Optical microphones posses innate resistance to electro magnetic interference & harsh environments.
MEMS technology provides a promising implementation for optical microphones.
Here, we discuss the design & characteristics of an intensity modulated optical level microphone.
INTENSITY MODULATION
Intensity modulating optical microphone can be sub- divided into
a) Radiated wave intensity modulating microphone
b) Evanescent wave intensity modulating microphone
POLARIZATION MODULATION
Polarization modulation type devices alter the polarization of the light when in the presence of an acoustic field.
TWO SUBCATEGORIES.
a 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,
MICROPHONE STRUCTURE
The intensity-modulated optical microphone can be divided into four major physical parts.
MEMS chip
Optical fibers
Light source
Detection electronics
Optical Fibers
The optical fibers selected for the optical microphone are the Thorlabs
AFS105/125Y multimode optical fibers.
Used for both transmit (Tx) and receive (Rx) fibers
The cores of each fiber are color-coded, and surrounded by a white ring representing the cladding.
Light source
The light source used by this optical microphone is the HP8168B Tunable Laser Source.
The maximum output power of the laser at 1550 nm is 0.515 mW.
An alternate laser source or an LED source could be used in place of the HP8168B.
FUTURE WORK
Future generation version of the optical microphone could be implemented with a single, large-core, high-NA fiber (instead of a fiber bundle) using an LED as a light source to improve stability and frequency response.
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.
CONCLUSION
MEMS-based intensity-modulated optical microphone is an excellent choice for applications with harsh environmental or size constraints.
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 aero-acoustic measurements.