WELCOME…WELCOME… A SEMINAR ON

OPTICAL MICROPHONE

PRESENTED BY:

JITHIN PRASAD S6, ELX

OUT LINE…OUT LINE…

Introduction Optical microphone Microphone structure Fabrication Future work Implementation Advantages Conclusion

INTRODUCTIONINTRODUCTION

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.

OPTICAL MICROPHONE OPTICAL MICROPHONE TRANSDUSER SCHEMESTRANSDUSER SCHEMES

Introduced by Nykolai Bilaniuk in 1996

3 properties of light could be modulated. They are

Intensity

Polarization

Phase

Optical Microphone ClassificationBased on Transduction Mechanism

INTENSITY MODULATIONINTENSITY MODULATION

Intensity modulating optical microphone can be sub- divided into

a) Radiated wave intensity modulating microphone

b) Evanescent wave intensity modulating microphone

Radiated Wave Intensity-modulating Microphone Types.

Evanescent Wave Intensity-modulating Evanescent Wave Intensity-modulating Microphone Types.Microphone Types.

POLARIZATION MODULATIONPOLARIZATION 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,

Polarization Modulating Microphone Types.Polarization Modulating Microphone Types.

PHASE MODULATIONPHASE MODULATION

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 path.

The two defined subgroups Grating type devices Interferometric devices.

Grating-Type Phase Modulating Microphone Grating-Type Phase Modulating Microphone Types.Types.

Interferometric Phase Modulating Microphone Types

MICROPHONE STRUCTUREMICROPHONE STRUCTURE

The intensity-modulated optical microphone can be divided into four major physical parts.

MEMS chipOptical fibersLight sourceDetection electronics

Block Diagram of the Optical Microphone

MEMS ChipMEMS Chip 2.5mm X 2.5 mm silicon chip with a micro

machined 1 mm diameter silicon nitride diaphragm

Cross Section of the MEMS Chip.

Fiber Bundle in the MEMS Chip - Cross Section Fiber Bundle in the MEMS Chip - Cross Section

Optical FibersOptical 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.

End View of the Optical Fiber BundleEnd View of the Optical Fiber Bundle

Optical Fibers in Steel TubingOptical Fibers in Steel Tubing

Optical Fiber Bundle Drawing.Optical Fiber Bundle Drawing.

Light sourceLight 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.

Detection ElectronicsDetection Electronics

There are three schemes for use as detection electronics.

unreferenced output technique.the referenced output technique.Heterodyne modulation

FABRICATION OF THE OPTICAL FABRICATION OF THE OPTICAL MICROPHONEMICROPHONE

The fabrication of the optical microphone consists of two parts:

The MEMS optical diaphragm chip

Fabricated by MEMS Exchange

The fiber bundle.

MEMS Exchange ProcessMEMS Exchange Process

Wafers Used for Optical Microphone Fabrication

Both mask and wafers were purchased through the MEMS Exchange

Packaging ProcessPackaging Process

Abeysinghe et al. Packaging Technique.

Beggans et al. Packaging Technique.

Kadirval Packaging Technique.Kadirval Packaging Technique.

Proposed PackageProposed Package for the Optical Microphone. for the Optical Microphone.

Proposed Optical Microphone Array Package.

FUTURE WORKFUTURE 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.

IMPLIMENTATION

Microphone Components

PHONE-OR Fibre Optical Microphone

IMPLIMENTATION

ADVANTAGESADVANTAGES Pressure Gradient Accuracy EMI/RF Immunity 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 bandwidth.

Sensitivity of the FOM is 100mV/Pa for the pressure microphones and 1.94 mV/(Pa/m) for the pressure gradient microphones.

CONCLUSIONCONCLUSION

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.

References References S. D. Senturia, Microsystems Design. New York: Kluwer

Academic, 2001. 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.

THANKS….THANKS….