See other catalogues for
MTI Instruments
You may also be interested in
Text version of the page
MTI Instruments, Inc.
325 Washington Avenue Extension
Albany, NY 12205
PH: +1-518-218-2550
OR USA TOLL FREE: 1-800-342-2203
FX: +1-518-218-2506
sales@mtiinstruments.com
www.mtiinstruments.com
APPLICATION NOTE
Fiber Optic Lever Displacement Transducers:
Principles, Improvements, and Applications
Introduction
The recent fl urry of activity in fi ber optic sensors has resulted in a great variety of technically sophisticated devices employing
interference, polarization and wavelength modulation techniques l. While all of these methods offer great promise for certain
specifi c applications and dedicated sensors, the intensity modulated Fiber Optic Lever Displacement Transducer offers a
powerful combination of simplicity, performance, versatility, and low cost, which make it well suited for a wide variety of
laboratory and industrial applications.
The Principle
The basic principle employed in the Fiber Optic Lever Displacement
Transducer comes down to the use of an adjacent pair of fi ber optic
elements, one to carry light from a remote source to an object or target
whose displacement or motion is to be measured and the other to receive
the light refl ected from the object and carry it back to a remote photo
sensitive detector. A complete analysis of the principles and performance
characteristics of this type of transducer was done by R.O. Cook and
C.W. Hamm 2. A fi ber optic element is a fl exible strand of glass or plastic
capable of transmitting light along its length by maintaining near total internal refl ection of the light accepted at its input end,
as shown in Fig-1.
The most commonly used fi bers are called “step index” type and consist of an inner core to carry the light fl ux and an outer
cladding. For total internal refl ection to occur, the index of refraction of the glass in the core (Nl) must be greater than the index
refraction of the glass cladding (N2). The sine of the half angle of the light which will be accepted into the core is defi ned as
the numerical aperature (N.A.) and is given by the formula:
N.A. = Sin 0 = ã(N1
2 - N2
2)
This is the maximum angle at which a light ray incident on the face of the
fi ber can be trapped within the core and refl ected along its length. The light
rays then exiting the other end of the fi ber are also limited to the same angle.
Individual fi bers usually fall in the range of about 0.001” diameter to .010”
diameter although recent advances in the fi ber optic manufacturing technology
has extended the size up to about .060”. Transmission effi ciency is dependent
upon the composition and purity of the glass used in the core and cladding and
on the quality of the optical fi nish on the end surfaces of the fi bers.
Fig. 2 depicts the interaction of adjacent transmit and receive fi bers as the
light is refl ected from a target. It can be seen that at zero gap, the light in
the transmit fi ber would be refl ected directly back into itself and little or no
MTII appnote: fi beroptics.pdf - Page 1 of 3
