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A Fiber-optic based Strain Sensor for Medical Applications
Tarek Elsarnagawy, Ph.D.
Helwan University, Faculty of Engineering
Biomedical Engineering Department
Cairo, Egypt, August 2002
Abstract:
Sensors based on optical fibers use many principles to measure the required
parameter. There are two basic types of fiber-optic sensors (FOS): Extrinsic- and
Intrinsic-FOS. In this thesis an intrinsic FOS is designed to measure strain produced
by a measurand. Alike the well known electrical strain gauges (ESG), a FOS is used
to measure strain. The results are directly compared to the results given by the ESG
and the theoretical calculations. The results are represented and discussed. All
measurements were done at the PTB-Germany (German Calibration Organization).
Introduction:
Optical fiber sensors are devices for measuring strain, temperature,
displacement, pressure, electric currents, magnetic fields and various
other material and environmental properties. Fiber optic sensors provide
several advantages over their electrical counterparts, namely, high
bandwidth, small size, light weight, corrosion resistance, geometrical
flexibility and an inherent immunity to electromagnetic interference
(EMI). Fiber optical sensors, as well as possessing extreme sensitivity,
are electrically passive (which is important for safety in some
applications such as medical applications).
The general principle behind fiber-optic sensors is quite simple. In
communication applications of fiber optics, it is undesirable for the light
waves being sent through an optical fiber to be affected by the
environment, or the message being sent may be corrupted. A fiber optic
sensor detects the effect(s) that a selected environmental property has on
light being sent through the optical fiber. Therefore, by measuring
changes in the some parameters of the light exiting the optical fiber
'sensor', the property of the environment that caused the change in the
light can be measured.
Fiber-optic sensors are used in a variety of applications, including strain,
pressure sensing, gyroscopes, hydrophones, fluid level sensors, 'smart'
materials, temperature sensing, medical devices, 'smart' highways and
civil structures, industrial process monitoring and for a wide range of
aerospace applications.
Experimental Setup
Fiber optical interferometers have been developed into a family of highly
sensitive sensor configurations. It has long been known that optical
interferometry is one of the most sensitive technologies to measure small
displacements. Displacements as small as 10-10 mm are readily resolved
using interferometric techniques. It is, therefore, necessary to convert
whatever is to be measured (pressure, temperature, rotation, etc.) into a
displacement so that the fiber interferometer may be used to sense it. The
common interferometric sensor configurations (a.Michelson, b.MachZehnder, c.Sagnac) that are usually applied in fiber optical sensors are
illustrated in fig.1.
Fig.1
In this work the Michelson fiber optical sensor configuration is used to
sense strain. The used components are as follows:

Optical source:
A pigtail single-mode diode Laser (Seastar, Diode Mitsubishi
ML5415 ( = 826,3 nm) is used to supply the fiber optical sensor
with light (fig.2). The Laser diode module is electronically
temperature and current controlled to ensure a constant wavelength
output, which is important for the interferometric configuration.
Fig.2

Optical fiber:
A single-mode fiber (SM800-FIBERCORE LTD) of an outer
diameter of 125μm, 240μm coating outer diameter, numerical
aperture of 0,11 and a launch spot size of 6μm is used to act as the
sensor and reference of the interferometer. In fig.3 the refractive
index profile of the used fiber is shown.
Fig.3

Fiber coupler
The implemented fiber coupler is a 3x3-coupler (fig.4). One arm is
used as the input from the laser source. The other two arms at the
same side are used as the two outputs to the photodiode. On the
other side only two arms are used as the sensor and reference arms
of the Michelson interferometer. Because the arms of the coupler
are not long enough to achieve a high sensitivity, several meters of
the mentioned fiber type were electrically spliced to the coupler
arms. The third arm of the coupler is made inactive in order to get
only two output signals.
Fig. 4
This type of coupler was chosen because it gives a constant phase
shift between the signals in the coupler arms which is 120 degree.
This leads to the operation at semi quadrature – 90degree – for
means of detection of the induced phase variation due to the
measurand. In accordance, the output signals are two signals with a
phase shift of 120 degree (fig.5).
Fig. 5
The shown photo detectors outputs are then fed into an up-down
counter which counts the zero paths of the signals (in this case 4
pulses for each period, i.e. for each 2π). So, if the measurand
causes a phase shift of 1000 x 2π, the readout of the counter would
be 1000x4=4000 counts. This readout is then a direct measure of
the amount of effect caused by the measurand.

EPO-TEK 353ND
The fixation of the fiber on the deformation body or plate that is
affected by the measurand is achieved using the EPO-TEK 353ND
glue. This type of glue has excellent resistance to many types of
solvents and chemicals and is ideal for bonding fiber optics,
metals, glass, ceramics and most plastics. In the following figure
(fig.6) the embedded fiber in the cured EPO-TEK 353ND is
shown.
Fig. 6

Overall experimental setup
Laser diode
Sensor arm
3X3- Coupler
Input
Mirorred ends
Output
Output
Reference arm
Detectors
Fig.7
Results and Discussion
1. Results with the fiber optical sensor without coating:
In the figure below (fig.8) the result of the strained sensor fiber is
plotted.
2000
Counts
1500
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D em o
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D em o
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D em o
O rigin
D em o
O rigin
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1000
500
0
0.0
0.5
1.0
1.5
2.0
mV/V
Fig. 8
A commercial strain sensor (electrical strain gauges) is used as a
reference (x-axis (mV/V)). A 90 cm fiber is wound and fixed via
the EPOTEK on a mandrel which is strained via a force acting on
its longitudinal axis causing a transversal strain. The curve in fig.7
shows a very good linearity. On the other hand, using an uncoated
optical fiber is very difficult to handle. Moreover, an uncoated
fiber loses its flexibility due to humidity and becomes brittle. As a
result, the usage of uncoated fiber is applicable only if short
lengths are required to act as a strain sensor. It is obvious that using
short lengths of a fiber leads to a low sensitivity according to the
relation:
Φ=β.L
Δ Φ = β L (ε+Δn/n)
Where
Φ: phase shift, β: propagation constant, n: refractive index
of the fiber, L: fiber length, ε: strain
2. Results with the fiber optical sensor without coating:
The same experimental setup was carried on using a coated fiber of
4m length. From the figure below (fig.9) it is obvious that the
sensitivity is increased. According to the relation
G
 4  1


 n 1  n²(1   ) p12  p11
L   2

(where: p12,p11 are the photoeleastic constants, v is the poisons
ratio), the induced phase shift due to the strain can be calculated
(G=1,89x107 rad/m).
The linearity error is found to be 0,015%, which is very good. A
part of the linearity error arises from the fact that the temperature
of the deformation body changes during the strain procedure. And
this in turn causes additional counts to the output of the fiber
optical sensor.
12000
O rigin
D em o
O rigin
D em o
O rigin
D em o
O rigin
D em o
O rigin
D em o
O rigin
D em o
O rigin
D em o
O rigin
D em o
O rigin
D em o
O rigin
D em o
O rigin
D em o
O rigin
D em o
O rigin
D em o
O rigin
D em o
O rigin
D em o
O rigin
D em o
O rigin
D em o
O rigin
D em o
10000
FOS (Counts)
8000
6000
4000
2000
0
0.2
0.4
0.6
0.8
1.0
1.2
Strain gauge (mV/V)
1.4
1.6
1.8
mess71.opj
3x3 coupl., LD, L=4m
Fig. 9
To avoid such a temperature variation problem a sort of
compensation is employed which is fixing the reference arm near
to the sensor arm, or let both arms be affected in an opposite
manner such as it is usually applied with electrical strain gauges.
Hence, in this thesis, a fiber optical strain sensor is illustrated. It showed
an excellent linearity with the force causing the strain. Due to the
advantages of optical fibers and the high sensitivity (resolution of 10 -6) of
the sensor it is optimal for use in medical application to characterize
strains acting on implanted metal plates or any parts where strain is
desired to be measured without affecting the patient with any electrical
power. Furthermore, an interesting research field is the application of the
illustrated fiber optical strain sensor in respiratory monitoring systems
(e.g. breast belts). The results of such experiments will be published in
the near future.
Acknowledgements
This work was supported by the German Aerospace Center and the
Technical University in Brunswick in Germany. I am indebted to Dr. N.
Fuerstenau (German Aerospace Center) for significant contributions in
the realization of this project as well as to Prof. Dr. K. Bethe (TU
Brunswick). Furthermore, I appreciate the cooperation of the team of the
Physikalisch Technische Bundesanstallt (PTB) – Germany for their
contribution in the calibration measurements done at the PTB.
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