MEMS based Optical microphone

1. WELCOME…
A SEMINAR ON
OPTICAL MICROPHONE
PRESENTED BY:
JITHIN PRASAD
S6, ELX

2. OUT LINE…
 Introduction
 Optical
microphone
 Microphone structure
 Fabrication
 Future work
 Implementation
 Advantages
 Conclusion

3. 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.

4. OPTICAL MICROPHONE
TRANSDUSER SCHEMES
Introduced by Nykolai Bilaniuk in
1996
 3 properties of light could be
modulated. They are
Intensity
Polarization
Phase


5. Optical Microphone Classification
Based on Transduction Mechanism

6. INTENSITY MODULATION
Intensity modulating optical
microphone can be sub- divided into
a) Radiated wave intensity modulating
microphone
b) Evanescent wave intensity
modulating microphone

7. Radiated Wave Intensity-modulating
Microphone Types.

8. Evanescent Wave Intensity-modulating
Microphone Types.

9. 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,

10. Polarization Modulating Microphone Types.

11. PHASE 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.

12. Grating-Type Phase Modulating Microphone
Types.

13. Interferometric Phase Modulating Microphone Types

14. MICROPHONE STRUCTURE
The intensity-modulated optical microphone
can be divided into four major physical
parts.
 MEMS
chip
 Optical fibers
 Light source
 Detection electronics

15. Block Diagram of the Optical Microphone

16. MEMS 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.

17. Fiber Bundle in the MEMS Chip - Cross Section

18. Optical Fibers
 The
optical fibers selected for the optical
microphone are the Thorlabs
AFS105/125Y multimode optical fibers.
 Used
fibers
 The
for both transmit (Tx) and receive (Rx)
cores of each fiber are color-coded, and
surrounded by a white ring representing the
cladding.

19. End View of the Optical Fiber Bundle

20. Optical Fibers in Steel Tubing

21. Optical Fiber Bundle Drawing.

22. 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.

23. Detection Electronics
There are three schemes for use as
detection electronics.
 unreferenced output technique.
 the referenced output technique.
 Heterodyne modulation

24. FABRICATION OF THE OPTICAL
MICROPHONE
The fabrication of the optical microphone
consists of two parts:
 The
MEMS optical diaphragm chip
Fabricated by MEMS Exchange
 The
fiber bundle.

25. MEMS Exchange Process
Both mask and wafers were purchased through the
MEMS Exchange
Wafers Used for Optical Microphone Fabrication

26. Packaging Process
Abeysinghe et al. Packaging Technique.

27. Beggans et al. Packaging Technique.

28. Kadirval Packaging Technique.

29. Proposed Package
for the Optical Microphone.

30. Proposed Optical Microphone Array Package.

31. FUTURE WORK
 Future
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
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.

32. IMPLIMENTATION
Microphone Components

33. IMPLIMENTATION
PHONE-OR Fibre Optical Microphone

34. ADVANTAGES
 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.

35. 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 aeroacoustic measurements.

36. 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.

37. THANKS….