Published in Symposium and Workshop on Time Domain Reflectometry in
Environmental, Infrastructure, and Mining Applications held at
Northwestern University, Evanston, Illinois, September 17-19, 1994
(Washington, D.C.: U.S. Bureau of Mines, 1994), pp.434-442.
USBM special publication SP 19-94
by Alan D. Kersey
Research Physicist, Naval Research Laboratory
Washington, D.C. 20375
Optical fiber based time domain reflectometry (TDR) techniques provide a powerful enabling technology for sensing in structures and structural components. A particularly interesting approach is based on the use of in-line fiber Bragg gratings which reflect a small portion of the light in the fiber back to the transmitter. Recent work on the use of fiber Bragg grating sensors for distributed strain sensing is described. This technology has the capability of allowing high resolution, multi-point static and dynamic strain sensing at various points along a single fiber cable.
Distributed fiber optic based sensing is a powerful sensing tool for monitoring and profiling a variety of parameters, or 'measurand' fields, such as strain, temperature, force, optical index, and chemical parameters along the length of a fiber cable. Typically, such distributed sensing systems utilize optical time domain reflectometry (OTDR) based techniques to achieve the spatial discrimination required to monitor a parameter at different locations along the fiber, as depicted in FIGURE 1. The basic OTDR technique relies on the intrinsic Rayleigh (or in some cases Brillouin or Raman) backscatter signals to monitor changes in the characteristics of the fiber along it's length (1). Unfortunately, backscatter signals are very weak, and consequently, considerable time averaging is often required to assess and map the spatial changes in loss or scattering coefficients along a fiber. Despite these disadvantages, OTDR-based sensing is a practical technique, and has received considerable research attention (2). An alternative approach to the use of intrinsic fiber scattering for generating return signals is the use of weak in-fiber reflectors to reflect a portion of the light guided in the fiber. Modulation in the intensity (or other characteristic such as wavelength) of the light reflected by the sensing element at a particular location is used as a measure of the measurand at that point. Optical time domain interrogation of a system based on the use of such reflective elements can be highly advantageous in terms of the overall signal to noise ratio (S/N), number of sensors which can be addressed, and span of the sensor system. One device, which is particularly suited for this type of interrogation, is the fiber Bragg grating.
Fiber Bragg gratings (FBG) represent one of the most exciting developments in the area
of fiber optic sensing in recent years, and are currently receiving considerable research
interest. Bragg gratings can be written into Germanium doped optical fiber by exposing the
fiber to an UV interference signal generated either holographically via two beam
interferometry (3) or by using a diffraction mask (
4). The absorption of UV light in the fiber
changes the chemical bonds in the glass (producing defect centers), thus giving rise to a
change in the complex refractive index of the glass. The resulting spatial modulation in the
index of the fiber produces the Bragg grating. This structure is permanent for temperatures
< 350 degrees Celsius, and acts as a very narrow-band in-line wavelength notch filter. In
reflection, the grating reflects strongly at the wavelength,
for which the Bragg resonance
is satisfied, as depicted in FIGURE 2. This Bragg resonance is determined by the
condition:
where 
is the spatial pitch of the grating, and n is the effective index of the fiber. Strain
applied to the grating thus shifts the wavelength at which the resonance condition is
satisfied. Illumination of the grating using a broadband optical source thus produces a
narrowband reflection signal off the grating, the wavelength of which is encoded by the
measurand (5).
This inherent 'wavelength-encoded' nature of the output of FBGs has a number of distinct advantages over other sensing schemes. One of the most important is that, as the sensed information is encoded directly into wavelength which is an absolute parameter, the output does not depend directly on the total light levels, losses in the connecting fibers and couplers or source power. Furthermore, the wavelength encoded nature of the output also facilitates wavelength division multiplexing by assigning each sensor to a different slice of the available source spectrum, as illustrated in FIGURE 3.
The key to a practical sensor system based on FBGs lies in the development of instrumentation capable of determining the relatively small shifts in Bragg wavelength of FBG elements induced by strain or temperature changes in these sensor elements, and in the low cost fabrication of the fiber grating elements (6). This former area has received significant attention lately, with a variety of approaches demonstrated. The range of application areas for FBG sensors could be quite extensive, but currently, most interest is being directed at the development of quasi-distributed, multi-point, strain measurement systems for use as embedded sensors in structural sensing applications.
INSTRUMENTATION FOR FIBER BRAGG GRATING SENSORS
The most straightforward means for the optical interrogation of a FBG sensor element is based on passive broadband illumination of the device, as is depicted in FIGURE 5. Simple filtering techniques based on the use of broadband filters allow the shift in the FBG wavelength of the sensor element to be assessed by comparing the transmittance through the filter compared to a direct 'reference' path (7). A relatively limited sensitivity is obtained using this approach due to problems associated with the use of bulk-optic components and alignment stability. One means to improve on this sensitivity is to use a fiber device with a wavelength dependent transfer function, such as for example, a fiber wavelength-division multiplexing (WDM) coupler (8).
One of the most attractive filter based techniques for interrogating FBG sensors is
based on the use of a tunable passband filter for tracking the FBG signal
(9). Examples of
the type of filter include Fabry Perot filters, acousto-optic filters, and FBG based filters. This
approach can yield sensitivities of 
.
Interferometric techniques have also been used to convert the shift in the wavelength of a returned signal from a grating into a phase shift at the output of an interferometer (10). Of these detection approaches, the interferometric detection technique has demonstrated the highest strain resolution capability. In this case, the wavelength shift of a FBG sensor is detected by utilizing the inherent wavelength dependence of an unbalanced fiber interferometer. The principle and system for a single grating sensor is shown in FIGURE 6. Light from a broadband source is coupled along a fiber to the FBG element, and the wavelength component reflected back along the fiber towards the source is tapped off and fed to a unbalanced Mach-Zehnder interferometer. This light effectively becomes the source light into the interferometer, and wavelength shifts induced by perturbation of the FBG resemble a wavelength (optical frequency) modulated source. The unbalanced interferometer behaves as a spectral filter with a raised cosine transfer function; the wavelength dependence on the interferometer output can be expressed as:
where 
is proportional to the input intensity and system losses, d is the
length imbalance between the fiber arms, n is the effective index of the core,
is the
wavelength of the return light from the grating sensor (sensor signal) and
is a bias phase
offset of the Mach-Zehnder interferometer. This concept has been widely used in the field of
interferometric fiber sensors to introduce a phase carrier signal into an unbalanced
interferometer using direct laser emission frequency (wavelength) modulation for the
purpose of phase demodulation. We employ the converse by using the unbalanced
interferometer as a discriminator to detect the wavelength shifts in the effective source
formed by the strained grating element. For a dynamic-strain-induced modulation in the
reflected wavelength, 
from the grating sensor element, the change
in phase shift (t) is
where 
to is the dynamic strain of the grating, and
is the normalized strain-to-wavelength
shift responsivity of the grating 
. A path imbalance d of 10 mm, an index of
1.46, a strain responsivity of 
and a
wavelength of 
yields a strain
to phase shift conversion responsivity, 
, of 
.
Phase shift detection
capabilities down to 
are achievable with interferometric sensors, which
yields a dynamic strain resolution of 
. Experimentally, this
technique has been shown to have extremely high sensitivity, with a resolution of 0.6
nanostrain/
reported.
MULTIPOINT SENSING CAPABILITIES
Many of these detection schemes are capable of simultaneously monitoring a number
of FBG elements using time domain reflectometry (TDR) based multiplexing. The approach
is depicted schematically in FIGURE 7. Pulsed light from the broadband source is input to the
optical system which comprises a series of fiber Bragg grating elements spaced at regular
intervals along the fiber. Each grating is written at a different nominal Bragg wavelength,
and the returned signal from each grating is directed through to the wavelength
monitoring instrumentation system, which may comprise various options such as filters, or interferometric
elements. The number of elements which may be multiplexed using this approach is determined by the source
bandwidth and the wavelength range required for operation by each sensor (e.g., for a strain range of
, a sensor spacing of at least 5 nm would be required for
gratings). For typical broadband
sources of 50 to 100 nm bandwidth, this provides the capability to monitor 10 to 20 sensor locations.
Using time-domain addressing in conjunction with wavelength-division addressing
allows the capability to increase this by several fold, possibly to more than 100 gratings. l
The concept is shown in
FIGURE 8. Here, the series of gratings each at a different
wavelength are repeated along the fiber length. The gratings are written into the fiber with
a low reflection coefficient (e.g. < 5 %), such that an individual grating reflects only a small
portion of the light at its Bragg wavelength. The remaining optical signal in that
wavelength band passes on to subsequent gratings at the same nominal Bragg wavelength.
At the output of the system, the wavelength 'bands' for each Bragg wavelength are
separated using filtering, and wavelength shift detection based on edge filters or
interferometric elements used. With pulsed source operation, this system produces a series
of output pulses in each wavelength range corresponding to each of the gratings in the
system which falls within that range. Time domain demultiplexing can then be used to
separate the outputs corresponding to each grating nominally operating at
. This
technique offers the capability of supporting a very large number of gratings if the
reflectivity of the gratings are very low (e.g. < 5 %). This will, of course necessitate the use
of high input power from the pulsed broadband source. Fiber based broadband sources,
such as Neodymium, Erbium and Praseodymium doped fiber can deliver several tens of
mW of broadband optical power into the core of a fiber and are ideally suited for this type
of application.
An alternative approach detecting the wavelengths returned by a series of Bragg gratings is to utilize a tunable narrowband source to interrogate an array of hundreds of very weakly reflecting FBGs using direct OTDR analysis. The concept is as shown in FIGURE 9. Here, the array of gratings is written into the fiber with a spatial separation greater than the OTDR resolution capability of the system, and with a reflectivity low enough (< 1% for 100 gratings) to avoid complicating multiple pulse interactions. The tunability of the source can be used to allow the OTDR profile of the fiber to be assessed at different wavelengths. As sections of the fiber experience different strains, the OTDR trace evolves allowing determination of the location and magnitude of the strain. By employing a spatially interleaved series of gratings at different nominal reflection wavelengths, this approach could have the capacity to address a very large number of devices (potentially ~ 1000), thus allowing a long sensor span.
Techniques for the multi-point measurement of strain using a quasi-distributed array of fiber Bragg gratings sensors utilizing both wavelength addressing and optical time domain reflectometry interrogation have been described. This technology can be used to profile strain at many locations along a single fiber path, and potentially will find widespread use in structural monitoring applications.
This work is jointly supported by the Office of Naval Research (ONR) and the Advanced Research Projects Agency (ARPA).
1. Barnoski, M. K., and S. M. Jensen, Fiber waveguides: a novel technique for investigating
attenuation characteristics. Appl. Optics, Vol. 15, 1976, p. 2112.
2. Culshaw, B., Optical Fiber Sensing and Signal Processing. Peter Pereginus, 1984.
3. Meltz, G., W.W. Morey, and W. H. Glenn. Formation of Bragg gratings in optical fiber by
a transverse holographic method. Optics Lett., Vol. 14, 1989, p. 823.
4. Hill, K. O., B. Malo, F. Bilodeau, D. C. Johnson, and J. Albert. Bragg gratings fabricated
in photosensitive monomode optical fiber by UV exposure through a phase mask. Appl. Phy. Lett., Vol. 62, 1993, p. 1035.
5. Morey, W. W., J. R. Dunphy and G. Meltz. Multiplexed fiber Bragg grating sensors. Proc.
Distributed and Multiplexed Fiber Optic Sensors, SPIE, Vol. 1586, 1991, p. 216.
6. Askins, G. C., M. A. Putman, G. M. Williams, and E. J. Friebele. Stepped-wavelength
optical fiber Bragg grating arrays fabricated in line on a draw tower. Optics Lett., Vol.
19, 1994,p.147.
7. Melle, S. M., K Liu and R. M. Measures. A passive wavelength demodulation system for
guided-wave Bragg grating sensors. Photonics Technol. Lett., Vol. 4, 1992, p. 516.
8. Davis, M. A., and A. D. Kersey. All-fiber Bragg grating strain-sensor demodulation
technique using a wavelength division coupler. Electron. Lett., Vol. 30, 1994, p. 75.
9. Kersey, A. D., T. A. Berkoff and W. W. Morey. Fiber Fabry-Perot demodulator for Bragg
grating strain sensors. Optics Letters, Vol. 18, 1993, p. 1370.
10. Kersey, A. D., T. A. Berkoff and W. W. Morey. High resolution fiber grating based
strain sensor with interferometric wavelength shift detection. Electronics Letters, Vol.
28, 1992, p. 236.
