Febry perot sensor

1. ANANT BINEKAR
15MSS0020

2.  Introduction
 Principle
 Operation
 Types
 Application
 Advantages
 Disadvantages
 References

3.  It is a optical sensor.
 Invented by two scienstists Alfred Perot and
Charles Fabry.

4.  For measuring small displacements with high
precision in a harsh environment, the so- called
Fabry–Perot optical cavity can be employed.
 The cavity contains two semireflective mirrors
facing each other and separated by distance L.
 The cavity is injected with light from a known
source (a laser, e.g.) and the photons inside the
cavity bounce back and forth between the two
mirrors, interfering with each other in the process.

6.  In fact, the cavity is a storage tank for light.
At some frequencies of photons, light can
pass out of the cavity.
 The narrow bands of transmitted light are
separated by frequencies that are inversely
proportional to the cavity length:
ν = c/2L
Where, c is the speed of light
v is frequncy
L is length of cavity .

7.  For practical cavities with a mirror separation
on the order of 1 μm, typical values of ν are
between 500 MHz and 1 GHz.
 Thus, by detecting the frequency shift of the
transmitted light with respect to a reference
light source,changes in the cavity dimensions
can be measured with the accuracy
comparable with the wavelength of light.

8.  Whatever may cause changes in the cavity
dimensions (mirror movement) may be the
subject of measurements. These include
strain, force , pressure, and temperature.
 This kind of sensor detects changes in optical
path length induced by either a change in the
refractive index or a change in physical
length of the cavity.

9. Fabry–Perot pressure sensor

10.  Pressure is applied to the upper membrane.
Under pressure, the diaphragm deflects
inwardly,thus reducing the cavity dimension
L.

11.  Small size.
 Accurate.
 Insensitive to environmental effect.
 Immune to many electromagnetic and radio-
frequency interferences.
 can operate reliably in harsh environment.
 Indirect contact to mesurend.

12.  Cost is large.
 Not suitable for high pressure.

13.  Jacob Fraden, “ Hand Book of Modern
Sensors: physics, Designs and Applications”,
3rd ed., Springer, 2003.
 J.M. Lopez-Higuera, 'Introduction to fibre
optic sensing technology',
in Handbook of optical fibre sensing
technology, J.M. Lopez-Higuera Ed, Wiley,
2002.