An apparatus is provided to allow for quasi distributed sensing of strain within a test object. Strain insensitive fiber is used to deliver a light signal to a strain sensitive fiber in an optical phase locked loop sensor configuration. The use of strain insensitive delivery fiber allows for non-integrated measurements of strain without the use of expensive electronics such as those employed in ODTR techniques. The novelty of the present invention lies in the use of strain insensitive multimode fiber. The inventors had previously developed a similar sensor with strain insensitive fiber, however it was restricted to the use of single or few mode fibers. The use of an optical phase locked loop arrangement allows for the use of multimode strain insensitive fiber

Egalon, Claudio Oliviera

Rogowski, Robert S.

English

NASA Technical Reports Server

y 
NASA CASE NO. LAB_15_
PRINT FIG.
NOTICE
The invention disclosed in this document resulted from research in aeronautical
and space activities performed under programs of the National Aeronautics and Space
Administration. The invention is owned by NASA and is, therefore, available for
licensing in accordance with the patent licensing regulations applicable to U.S.
Government-owned inventions (37 CFR 404.1 et seq.).
To encourage commercial utilization of NASA-owned inventions, it is NASA
policy to grant licenses to commercial concerns.
nonexclusive licensing to promote competition and
Although NASA encourages
achieve the widest possible
utilization, NASA will provide the necessary incentive to the licensee to achieve early
practical application of the invention.
Address informational requests for this invention to the Technology
Applications Team, NASA Langley Research Center, Code 200, Hampton, Virginia
23681-0001.
Address applications for license to the Patent Counsel Office, Mail Stop 212,
3 Langley Boulevard, Hampton, Virginia 23681-0001.
Serial No.: 08/644,655
Filed: 04/23/96 LaRC
https://ntrs.nasa.gov/search.jsp?R=19960034345 2020-06-17T23:51:13+00:00Z
NASA Case No. LAR 1 5 1 59- 1 -SB 
STRAIN INSENSITIVE OPTICAL PHASE LOCKED LOOP 
An apparatus is provided to allow for quasi distributed sensing of strain within a 
test object. Strain insensitive fiber is used to deliver a light signal to a strain 
sensitive fiber in an optical phase locked loop sensor configuration. The use of 
strain insensitive delivery fiber allows for non-integrated measurements of strain 
without the use of expensive electronics such as those employed in ODTR 
techniques. 
The novelty of the present invention lies in the use of strain insensitive multimode 
fiber. The inventors had previously developed a similar sensor with strain 
insensitive fiber, however it was restricted to the use of single or few mode fibers. 
The use of an optical phase locked loop arrangement allows for the use of 
multimode strain insensitive fiber. 
Inventors: Claudio Oliviera Egalon Robert S. Rogowski 
Address: 2658 N. Armistead Ave. B-2 109 Wicomico Turn 
Hampton, VA 23666 Yorktown, VA 23693 
Employer: AS&M NASA LaRC 
Initial Evaluator: none 
Serial No.: 081644,655 
Filed: 4123196 
LAR 15159-1-SB -1- PATENT APPLICATION
STRAIN INSENSITIVE OPTICAL PHASE LOCKED LOOP
10
Oricj_ of the Invention
The invention described herein was jointly made by an employee of
the United States Government and a contract employee during the
performance of work under NASA Contract No. NAS-1-19236. In
accordance with 35 U.S.C. 202, the contractor elected not to retain title.
Background of the lr_vP__.ntion
15
2O
1. Technical Eield_f_the lnvP_.ntJon
The present invention relates generally to distributed monitoring of
strain in a structure through use of embedded or surface mounted optical
fibers and specifically to strain monitoring with an optical phase locked
loop (OPLL).
2. Discussion of the Related Art
25
Optical fibers may be attached to a structure for the purpose of
monitoring strain. As the structure is strained, embedded fibers or fibers
affixed to the surface of the structure will undergo strain as well. As the
fibers are strained, the light transmission characteristics are altered in a
known manner, allowing one to extract information about the strain in
the structure.
510
15
2O
25
LAR 15159-1-SB -2- PATENT APPLICATION
Optical fibers may be generally classified as single mode, few
mode or multimode fibers. These classes refer to the number of modes
propagating through the fibers. The light transmitting cores of single
mode fibers are very small, on the order of a few microns in diameter so
that only one mode may be transmitted. These fibers are more
expensive and more difficult to work with than larger, multimode fibers
which usually have cores on the order of 100 microns. Few mode fibers
fall between these two categories.
As a fiber is strained, the optical path length of light traveling
through the fiber is altered. This produces a phase shift in the light
exiting the receiving end of the fiber. It is by detecting the phase shift
that strain information is extracted.
One simple example of the use of this is a pair of single mode
fibers, one is attached to the test object and one is held as a reference.
They are employed in an interferometer configuration and as the test
object is strained the test fiber will also be strained. The reference fiber
will not be subject to strain and through the use of the interferometer
changes in phase may be compared between the two fibers.
One of the drawbacks of this type of monitoring is that the
information about strain is necessarily integrated over the length of the
detecting fiber. That is, since the phase change depends on the change
in optical path length of the entire fiber, the strain information reflects
strain in the entire fiber. In a test object having widely varying strain
response or in an object in which strain is important at specific points
rather than in the object as a whole, integrated measurements are not
very useful.
Egalon, et. al. (US Pat. No. 5,343,035, hereafter the '035 patent)
disclose an optical fiber strain sensor for use in making measurements
over a small region. The invention according to the '035 patent makes
10
15
2O
25
LAR 15159-1 -SB -3- PATENT APPLICATION
use of a strain insensitive optical lead fiber to carry a light signal to a
strain sensitive optical fiber disposed in the region of interest of the test
object. Thus the sensor is sensitive to strain only in the region occupied
by the strain sensitive fiber and integrated information from the lead fiber
is ignored. The invention according to the '035 patent requires that the
sensing fiber be a two-mode optical fiber.
Lagakos, et. al. (US Pat. No. 4,979,798) make use of lead optical
fibers that are insensitive to strain in the region being monitored. This is
achieved through the leads not being within the test region. This is not
always a viable option when, for example, the test region lies within a
larger region of a test structure which is itself undergoing strain.
Kleinerman (US Pat. No. 5,363,463) makes use of optical time
domain reflectometry (OTDR) to determine temperature measurements in
selected areas of a test object. This method employs a fluorescent dye
that will react in a temperature dependant manner in the presence of an
interrogating light signal. The light given off by the fluorescent dye may
then be monitored and its intensity and time of arrival give both a
temperature reading and an indication of the location at which that
temperature was measured. A drawback of OTDR methods is the
expense of the electronic processing elements that are necessary.
Kleinerman additionally discusses monitoring of microbends within
a fiber. In the case of microbend information it is often practical to
monitor backscattered light. Again, through the use of OTDR it is
possible to find the locations of multiple microbends as revealed by
discontinuities in the backscatter intensity versus a known distance
decay curve.
Summar__ventJon
510
15
LAR 1 51 59-1 -SB -4- PATENT APPLICATION
It is an object of the present invention to provide an optical fiber
arrangement that may be used to sense strain and vibration at specific
locations within a test object.
It is an additional object of the present invention to use multimode
optical fibers to achieve the above object.
It is a further object of the present invention to achieve the
forgoing objects without the use of OTDR methods which require
expensive electronic elements.
To achieve the forgoing objects, strain insensitive optical fiber is
used to carry a light signal to a strain sensitive optical fiber. Information
from the strain sensitive fiber travels through strain insensitive fiber to a
detector. Since the leads are chosen to be strain insensitive, strain in the
region of interest is the only contribution to the signal. The signal from
the sensor is monitored in an optical phase locked loop (OPLL). The use
of the OPLL allows for use of multimode fibers instead of the single or
few mode fibers used in other optical fiber monitoring devices.
BrJ_cript_Jon of the Drawings
2O
25
Fig. 1 is a drawing showing a fiber having a strain sensitive region
and strain insensitive regions of the fiber carrying an optical signal to and
from the strain sensitive region.
Fig. 2 is a drawing showing a system according to the present
invention in an OPLL configuration.
D_scription of Pr_f_,dJEr[_odi_
Referring first to Fig. 1, a multimode optical fiber sensor is shown.
The multimode optical fiber sensor 2 is made up of a strain insensitive
LAR 15159-1-SB -5- PATENT APPLICATION
fiber 4 leading to a strain sensitive fiber 6 located in the region being
monitored and a second strain insensitive fiber 8 leading away from the
strain sensitive fiber. For example the three fibers may be connected by
splicing. Light I enters the sensor and strain 3 is applied.
5 A voltage controlled oscillator 10 is used to directly modulate a
laser 12 and to provide a reference signal to a double balanced mixer 14,
as shown in Figure 2. The laser radiation passes through a multimode
optical fiber sensor 2.
The second strain insensitive fiber 8 leads to a detector 16. The
10 detected signal passes through an amplifier 18, the amplified signal
passes through the mixer 14 and is mixed with the reference signal. The
phases of the two signals are maintained at quadrature by feedback of
the DC error voltage from the mixer 14 to the oscillator 10. A filter 22
removes the radio frequency component coming from the mixer 14.
15 A change in the phase of the modulation is expressed as an error
voltage at the mixer 14 and is compensated by a change in the
modulation frequency. A change in phase length, AL, of the optical fiber
will produce a change in frequency, AF, according to:
AL AF
20 L F (1)
25
Where L is the effective path length of the strain sensitive optical fiber 6
and F is the nominal frequency value. Frequency is monitored by a
counter 24.
An important consideration is the strain insensitive optical fibers 4
and 8. The phase shift due to strain in the OPLL is given by"
(_ _ln- (.OZ S1 ncore (2 2
- Pe_ n core)2c (2)
510
LAR 15159-1-SB -6- PATENT APPLICATION
(P12-vf(P11+ P12 )) (3)
P eff =
2
Where Pll and P12 are
the strain optic coefficients of the fiber, vf is the Poisson's ratio of the
fiber, c0 is the frequency of modulation of the laser, c is the speed of
light, z is the length of the fiber under strain, S 1 is the value of axial
strain and ncore is the refractive index of the fiber core. The fiber is
assumed to be weakly guiding, that is, it meets the inequality:
2 2
n core-nclad
2
2 ncore
_<0.01 (4)
15
From the above, it is evident that a fiber can be made more or less
sensitive by choosing appropriate optical fiber parameters. So, for a
strain insensitive fiber, the core refractive index must be given by"
i 2n core= Peff
(5)
20
25
While for strain sensitive fiber it is best to maximize Eqn. 2.
Using appropriate parameters in Eqn. 5 it is determined that a
strain insensitive fiber is one with a very high core refractive index, e.g.
ncore -_ 4.5. Germanium, for example is an appropriate material for
producing multimode optical fibers with very high refractive index.
Other variations and uses will be apparent to those skilled in the art.
The above embodiments are not exhaustive but rather are given by way of
example. It is understood that the present invention is capable of numerous
modifications within the scope of the following claims.
LAR PATENT APPLICATION
STRAIN INSENSITIVE OPTICAL PHASE LOCKED LOOP
Abstract of_the_DJsclesu[e
5
10
A strain sensor uses optical fibers including strain insensitive
portions and a strain sensitive portion. The optical fibers form a sensitive
arm of an optical phase locked loop (OPLL). The use of the OPLL allows
for multimode optical fiber to be used in a strain insensitive configuration.
Only strain information for the strain sensitive portion is monitored rather
than the integrated strain measurements commonly made with optical
fiber sensors.
31
m
6
_3
i
8_t. = 0 6% = 0
Figure 1.
2.


Strain Insensitive Optical Phase Locked Loop

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server