The document discusses the Fabry-Perot interferometer, an optical sensor used to measure small displacements. It consists of two parallel mirrors separated by a cavity, and works by detecting changes in the frequency of light transmitted through the cavity when the mirror spacing changes due to environmental factors like pressure, strain, or temperature. Measurements with an accuracy comparable to the wavelength of light are possible. Applications include pressure sensing, where pressure deflects a diaphragm and alters the cavity length. Advantages are small size, accuracy, and insensitivity to environmental effects. Disadvantages include high cost and unsuitability for high pressures.

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.