Optical Fiber Technology 9 (2003) 57–79
www.elsevier.com/locate/yofte
Invited paper
Review of the present status of optical fiber sensors
Byoungho Lee
School of Electrical Engineering, Seoul National University, Kwanak-Gu Shinlim-Dong,
Seoul 151-744, South Korea
Received 28 August 2002
Abstract
The current status of optical fiber sensors is reviewed. The optical fiber sensors have certain
advantages that include immunity to electromagnetic interference, lightweight, small size, high
sensitivity, large bandwidth, and ease in implementing multiplexed or distributed sensors. Strain,
temperature and pressure are the most widely studied measurands and the fiber grating sensor
represents the most widely studied technology for optical fiber sensors. Fiber-optic gyroscopes and
fiber-optic current sensors are good examples of rather mature and commercialized optical fiber
sensor technologies. In this paper, among the various fiber-optic sensor technologies, especially,
technologies such as fiber grating sensors, fiber-optic gyroscopes, and fiber-optic current sensors
are discussed with emphasis on the principles and current status. Today, some success has been
found in the commercialization of optical fiber sensors. However, in various fields they still suffer
from competition with other mature sensor technologies. However, new ideas are being continuously
developed and tested not only for the traditional measurands but also for new applications.
 2003 Elsevier Science (USA). All rights reserved.
Keywords: Optical fiber sensor; Fiber grating; Fiber-optic gyroscope; Optical fiber current sensor
1. Introduction
Almost thirty years have passed since the study of optical fiber sensors began. Various ideas have been proposed and various techniques have been developed for various
measurands and applications. To date, some types of optical fiber sensors have been commercialized, but it is also true that, among the various techniques that have been studied,
only a limited number of techniques and applications have been commercially successful.
Optical fiber sensors have advantages such as immunity to electromagnetic interference
E-mail address: byoungho@snu.ac.kr.
1068-5200/03/$ – see front matter  2003 Elsevier Science (USA). All rights reserved.
doi:10.1016/S1068-5200(02)00527-8
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B. Lee / Optical Fiber Technology 9 (2003) 57–79
Fig. 1. Distribution of OFS-15 papers according to measurands. Papers not directly related to measurands, such
as those that discuss fiber grating sensor interrogators, multiplexing, light sources, etc., are not included in the
statistics. If one paper deals with more than one measurand, the counted paper number is equally divided among
the measurands. Special session papers on fiber-optic gyroscopes are included in the statistics.
(EMI), lightweight, small size, high sensitivity, large bandwidth, and ease in signal light
transmission. However, in many fields of application, optical fiber sensors should compete
with other rather mature technologies such as electronic measurements. To appeal to users
already accustomed to other mature technologies, the superiority of optical fiber sensors
over other techniques needs to be clearly demonstrated. Typical users are not interested in
specific techniques involved in measurement. They simply desire sensor systems having
good performances with reasonable price except for very special uses. Hence, optical fiber
sensor systems should be available in the form of complete systems including detecting
and signal-processing electronics. In some cases such as electric protection relaying systems, the sensor systems are simply subsystems of rather larger systems. In some cases
such as optical gyroscopes and optical current sensors, the optical fiber sensors should
compete with other optical bulk sensors as well. Even with these difficulties, considerable
efforts have been made to study of optical fiber sensors, and some of them are now nearing
maturity.
There have been excellent review books and articles on optical fiber sensors such
as Refs. [1–5]. It is impossible to review all of the fiber-optic sensor technologies in
a journal article with a limited length. In this paper, the current status of some of the
actively studied or well-developed optical fiber sensors will be reviewed. Figure 1 shows
the distribution of papers presented at the 15th Optical Fiber Sensors Conference (OFS15: IEEE catalog number 02EX533) according to measurands of interest. The conference
was held in Portland, Oregon, USA in May 2002, and is a major conference in the field of
B. Lee / Optical Fiber Technology 9 (2003) 57–79
59
Fig. 2. Distribution of OFS-15 papers according to technologies. If one paper deals with more than one technology, the counted paper number is equally divided among the technologies. Special session papers on fiber-optic
gyroscopes (FOGs) are included in the statistics.
optical fiber sensors. Hence the statistics of OFS-15 provide information on recent research
trends in the area of optical fiber sensors. The most highly discussed measurands are strain
and temperature, the same as in the conference of five years ago [6]. Figure 2 shows
the technologies involved in the optical fiber sensors presented at OFS-15. Fiber grating
sensors are clearly the most widely studied topic.
In Section 2 fiber grating sensor technology, which is the most popular topic in optical
fiber sensors, is reviewed. In Sections 3 and 4 two rather mature topics—fiber-optic
gyroscopes (FOGs) and fiber-optic current sensors—are reviewed. In Section 5, other
sensors are briefly discussed, which is followed by some concluding remarks.
2. Fiber grating sensors
Although the formation of fiber gratings had been reported in 1978 [7], intensive study
on fiber gratings began after a controllable and effective method for their fabrication was
devised in 1989 [8]. Fiber gratings have been applied to add/drop filters, amplifier gain
flattening filters, dispersion compensators, fiber lasers and so on for optical communications [4]. Extensive studies have also been performed on fiber grating sensors and some of
which have now reached commercialization stages.
Figure 3 shows types of fiber gratings. Under phase matching conditions, a fiber Bragg
grating (FBG) couples the forward propagating core mode to the backward propagating
core mode. A long-period fiber grating (LPG) can couple the forward propagating core
mode to one or a few of the forward propagating cladding modes. A chirped fiber grating
has a wider reflection spectrum and each wavelength component is reflected at different po-
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B. Lee / Optical Fiber Technology 9 (2003) 57–79
Fig. 3. Types of fiber gratings. (a) Fiber Bragg grating, (b) long-period fiber grating, (c) chirped fiber grating,
(d) tilted fiber grating, (e) sampled fiber grating.
sitions, which results in a delay time difference for different reflected wavelengths. A tilted
fiber grating can couple the forward propagating core mode to the backward propagating
core mode and a backward propagating cladding mode. A sampled fiber grating can reflect
several wavelength components with equal wavelength spacing. All these types of gratings have been utilized in various types of fiber grating sensors and wavelength change
interrogators. Among them, however, FBGs are the most widely used as sensor heads.
In FBGs the Bragg wavelength λB , or the wavelength of the light that is reflected, is
given by
λB = 2neff Λ,
(1)
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61
where neff is the effective refractive index of the fiber core and Λ is the grating period. In
Eq. (1) it can be seen that the Bragg wavelength is changed with a change in the grating
period or the effective refractive index. The former is the case for strain and the latter for
temperature variation. The grating period can also be changed with temperature variation,
but around room temperature the effect of temperature on refractive index is about one
order of magnitude larger than that of thermal expansion (or contraction). The typical response of the Bragg wavelength shift to strain is ∼0.64 pm/µε (µε = microstrain) near the
Bragg wavelength of 830 nm, ∼1 pm/µε near 1300 nm, and ∼1.2 pm/µε near 1550 nm [9].
The unit of strain is ε, which is read as “strain”. It is a relative concept, that is, if a 1-mlong fiber is elongated by 1 µm, the strain is 1 µm/1 m = 1 µε. The typical temperature
response is ∼6.8 pm/◦ C near 830 nm, ∼10 pm/◦ C near 1300 nm, and ∼13 pm/◦ C near
1550 nm [9] although the values depend on FBG types [10]. (These FBG types should
not be confused with fiber grating types of Fig. 3. Detailed information can be found in
Ref. [4].) This wavelength-encoded measurand information is a unique characteristic of
FBGs. In addition to the common advantages of fiber sensors, this wavelength-encoded
characteristic provides robustness to noise or power fluctuation and also enables wavelength division multiplexing (WDM). Hence multi-point sensors can be realized using this
technique. Sometimes WDM is combined with spatial division multiplexing (SDM), time
division multiplexing (TDM) and code division multiple access (CDMA) techniques. However, it must be admitted that multi-point FBG sensors are not fully-distributed sensors such
as backscattering sensors or optical time-domain reflectometers (OTDRs) because physical
parameters can be measured only at the FBG positions.
As discussed above, FBGs respond to both strain and temperature. Hence their effects
need to be separated from each other, in order to measure each physical parameter or
to measure both simultaneously. Considerable effort has been focused on this topic and
various solutions have been proposed. A straight forward and very practical approach is
to use a reference grating [11]. Another FBG (the reference grating), which is isolated
from one parameter, e.g., strain, is placed near the sensor FBG. The reference grating
can be on the same fiber as the sensor FBG [12]. Another method is to use two FBGs
with much different Bragg wavelengths (in general, two light sources are needed), which
show different responses to the same measurands [13]. FBGs written on different-diameter
fibers have also been proposed, which give different strain responses, while the temperature
responses are the same [14,15].
Demodulators or interrogators are required for FBG sensors. Their roles are to extract measurand information from the light signals coming from the sensor heads. The
measurand is typically encoded in the form of a Bragg wavelength change, and hence, the
interrogators are typically expected to read the wavelength shift and provide measurand
data. Optical spectrum analyzers are not suitable for real sensor systems because they
are expensive and their wavelength scanning speed is too slow. Various techniques have
been developed for the interrogators [16]. Some are quite simple but are more limited in
measurement resolution, dynamic range or multiplexing, and some are more complicated
and provide better resolution but are more expensive or need stabilization. Not all of them
are appropriate for commercialized systems. Table 1 summarizes interrogator types and
Table 2 summarizes their performances [17–62]. Many of these are laboratory experiment
results. Civic structures such as bridges require the measurement of dynamic strain. When
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B. Lee / Optical Fiber Technology 9 (2003) 57–79
Table 1
Fiber grating sensor interrogator types
Types
Technologies
References
Passive detection schemes
Linearly wavelength-dependent device
CCD spectrometer
Power detection
Identical chirped-grating pair
[17–21]
[22–25]
[26–29]
[30]
Active detection schemes
Fabry–Perot filter
Unbalanced Mach–Zehnder interferometer
Fiber Fourier transform spectrometer
Acousto-optic tunable filter
Matched FBG pair
Michelson interferometer
LPG pair interferometer
[31–33]
[34–43]
[44,45]
[46–48]
[49–52]
[53]
[54]
Other schemes
Wavelength-tunable source
Mode-locked fiber laser with wavelength-time conversion
Optical CDMA correlator
Frequency modulation
Intra-grating sensing
[55–57]
[58]
[59,60]
[61]
[62]
Table 2
Strain measurement resolution of some fiber grating sensor systems/interrogators
Static/quasi-static resolution
±20 µε
∼12 µε
4.12 µε
∼ ±3.5 µε
3.5 µε
∼ ±3 µε
3 µε
±2 µε
1.9 µε
±0.25 µε
40 nε
±8.04 µε
Dynamic resolution
√
1.5 µε/√Hz
6 nε/√Hz
0.5 µε/ Hz
√
5 nε/√Hz
7 pε/ Hz
√
42 nε/√Hz
10 nε/√Hz
10 nε/√Hz
2 nε/√Hz
1.5 nε/√Hz
∼7 fε/ Hz
References (measurement range)
[58] (> 3500 µε)
[44]
[49]
[18] (∼16,000 µε)
[40] (2 kHz)
[20] (>1050 µε)
[50]
[32] (±1300 µε)
[55]
[25]
[54]
[56] (±1000 µε)
[57]
[43]
[51] (> ±100 µε)
[35] (>10 Hz)
[42] (10–2000 Hz)
[36]
the dynamic signal is measured, the minimum detectable strain is determined by the
background noise-level. The magnitude of noise changes with the frequency span because
the noise power in the detector and amplifying circuit depends on the frequency span.
Generally, the amount of noise magnitude in other bandwidths can be approximated by
B. Lee / Optical Fiber Technology 9 (2003) 57–79
63
(a)
(b)
Fig. 4. Some typical interrogators for FBG sensors. (a) Linearly wavelength-dependent device (coupler)
method [20]. (b) CCD spectrometer method [24]. (c) Fabry–Perot filter method [32] (FFP: fiber Fabry–Perot
filter). (d) Unbalanced Mach–Zehnder interferometer (pseudo-heterodyne detection) method [37] (OPD: optical
path length difference, PZT: piezoelectric transducer, BPF: band pass filter).
scaling the power spectral density of the noise by the square root of the bandwidth.
Thus,
√
the normalized minimum detectable dynamic strain is displayed in units of ε/ Hz.
Figure 4 shows some typical types of interrogators. Figure 4a is an example of
interrogators that use linearly wavelength-dependent devices (couplers or filters) [20]. The
Bragg wavelength shift is monitored by the detected power change. This structure is quite
simple and has been commercialized, but it is not suitable for multiplexed sensors. In fact,
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B. Lee / Optical Fiber Technology 9 (2003) 57–79
(c)
(d)
Fig. 4. (Continued.)
this type of power-detection method does not fully use one of the key advantages of FBG
sensors—measurand information is coded as wavelength change and the information is
not affected by fluctuations in light power. In Fig. 4a to remove the light power (or loss)
fluctuation effect, a power-ratio detection technique is used. Figure 4b shows the CCD
spectrometer interrogator [24], which takes advantage of the fact that the diffraction angle
is dependent on the wavelength of the light incident on a diffraction grating. The structure
shown in Fig. 4b adopts both WDM and SDM. Compared with other interrogators such
B. Lee / Optical Fiber Technology 9 (2003) 57–79
65
as those adopting scanning filters and TDM, the advantage of the CCD spectrometer
interrogator is its light-effectiveness, that is, a high signal-to-noise ratio can be obtained
even with low input optical power. A one-dimensional CCD can be used instead of the twodimensional CCD if TDM is used instead of SDM [25]. Figure 4c shows an example of
the scanning fiber Fabry–Perot filter interrogator system. U.S. Naval Research Laboratory
tested this for a 64-FBG sensor system [32]. The basic principle is to locate the WDM
Bragg wavelengths within the free spectral range of the Fabry–Perot filter and monitor their
changes by spectrally-scanning the filter. In real systems, generally, the dithering of the
filter spectrum is adopted for a fine measurement in the case of a single FBG sensor and the
zero-crossing of the time derivative of the received power is monitored in the case of WDM
FBG sensors. Figure 4d shows an unbalanced Mach–Zehnder interferometer interrogator
system adopting a pseudo-heterodyne detection technique [37]. A detailed description
of the principle of operation can be found in Ref. [16]. The unbalanced Mach–Zehnder
interferometer provides very high-resolution interrogation. However, this interferometer
is quite sensitive to its environment such as temperature change or even air fluctuation.
Hence, a reference grating for the interrogator system (this should not be confused with
the reference grating for the sensor head to distinguish the temperature and strain effect) is
required as shown in Fig. 4d and the interrogator should be well-stabilized.
FBG sensors have been tested in various fields such as bridges, dams, mines,
composite laminates, airplanes, generators, ship waterjets, geotechnical fields, and railway
systems [25,43,63–66]. Some FBG sensors have been commercialized for structural
monitoring [67] and oil/gas reservoir monitoring [68]. For example, CiDRA’s FBG
pressure/temperature sensor was reported to show good functionality at 150 ◦ C, 5000 psi
for over two years with output drift within 0.015% full scale/year [68]. Such sensors have
been deployed in oil/gas wells—for example, in a 15,000-feet downhole [68].
3. Fiber-optic gyroscopes
It is generally recognized that the first demonstration of the FOG was achieved 27 years
ago [69], though there had been a few previous studies. The basic concept is based on
the Sagnac interferometer and is quite simple. It is a good example of an application of
special relativity. For a rotating optical fiber coil, as shown in Fig. 5, two lights traveling in
opposite directions in the coil experience different lengths, which results in different travel
times and a phase difference in the two optical waves. Simple analyses [70,71] show that
the phase difference φ is given by
8πN
A · ,
(2)
λc
where N is the number of coil turns, λ the wavelength in vacuum, c the speed of light in
vacuum, A the area vector of the fiber coil (Its magnitude is the area enclosed by the singleturn coil and direction is normal to it.), and the rotating rate (angular frequency) vector.
Like the output of typical interferometers, the output response of the FOG is a raised
cosine function with respect to the phase difference. In the response curve, if the operating
point is at maxima (or minima) such as the point corresponding to φ = 0, the sensitivity
φ=
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Fig. 5. Principle of the fiber-optic gyroscope. In this figure, the fiber coil is rotating in the clockwise (CW)
direction. Hence, for the CW and counter-clockwise (CCW) rotating lights to meet at the same exit point to
interfere with each other, they should have entered the coil at different times at different points in the figure.
Different entering times mean different entering points because the fiber coil is rotating.
is low and the rotational direction of the fiber coil cannot be distinguished due to the
symmetric response. Hence, the operating point is shifted to a position where the response
slope is not zero. This is accomplished by the use of a phase modulator placed at the
end of the fiber coil. There are two types of basic configurations in which a FOG can be
constructed; one is an open-loop configuration and the other is a closed-loop configuration.
As shown in Fig. 6a, a dynamic phase modulation is done at one end of the fiber coil, such
that two lights that start at the coupler at the same time and rotate the fiber coil in opposite
directions experience different optical path lengths within the phase modulator (such as
a fiber wound around a piezoelectric transducer (PZT) tube) due to their different arrival
times at the modulator [72]. This open-loop structure has a limited dynamic range [71].
In the closed-loop configuration a feedback signal is applied to null the output signal and
maintain the operation of the system in the linear range [73,74]. Figure 6b shows such an
example using an integrated optics chip (IOC) to modulate the phase. The IOC is typically
a lithium niobate modulator, which has a larger bandwidth than the PZT and allows a
proper modulation for the feedback control.
When a FOG is at rest, the output signal is a random function that is the sum of a
white noise and a slowly varying function. The photon shot noise is an important source
of white noise and limits the fundamental accuracy of the FOG. The white noise is
expressed in terms of the standard deviation of the equivalent rotation rate per square root
of bandwidth of detection (like the case of the dynamic strain measurement resolution of
fiber grating sensors),
in other words, the equivalent noise power spectral density in the
√
unit of deg /h/ Hz. The white noise causes noise in measured
rotation rate output and
√
results in an angle random walk (ARW) in the unit of deg / h. The slowly varying noise
can be induced by several sources and results in long-term drift or bias instability.
B. Lee / Optical Fiber Technology 9 (2003) 57–79
67
(a)
(b)
Fig. 6. Basic configurations of FOG. (a) Open-loop type (SLD: superluminescent diode. PM: phase modulation).
(b) An example of a closed-loop type (IOC: integrated optics chip modulator).
The performance of a FOG has been continuously improved and is now quite mature
and capable of meeting the most accurate gyroscope requirements. As can be seen in
Fig. 7, the FOG has competing technologies such as mechanical gyroscopes and ring laser
gyroscopes. The cost of the FOG has been constantly falling in recent years thanks to
economies of scale and the dramatic explosion in the optical fiber communications market
(component prices are decreasing) [75]. Currently, FOGs are considered to be the most
cost-efficient solution for various inertial navigation applications. One important advantage
of the FOG is its ruggedness. It contains no moving parts unlike other competitors such
as mechanical gyroscopes and ring laser gyroscopes (dithering is involved in ring laser
gyroscopes). Due to these unique advantages, it seems likely that the FOG will play a
significant role in both military and commercial markets.
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B. Lee / Optical Fiber Technology 9 (2003) 57–79
Fig. 7. Comparison of gyroscope technologies and applications (MEMS: micro-electro-mechanical system technology, DTG: dynamically tuned gyroscope (mechanical type), FOG: fiber-optic gyroscope, RLG: ring-laser
gyroscope, AHRS: attitude heading reference systems).
Some recent reports indicate that Honeywell Space Systems currently manufactures
high-precision
FOGs with a flicker noise below 0.0001 deg/h and an ARW of ∼80
√
µdeg/ h [76] and Ixsea manufactures high-level performance FOGs with a temperature
bias stability
√ better than 0.01 deg/h (0.003 deg/h at ambient) for a noise level of
0.001 deg/ h [75]. FOGs have been developed by many companies and have been sold
or applied in various fields such as commercial aircraft attitude heading reference systems,
military helicopters, missile guidance systems, marine, and submarine navigation systems,
pipe-mapping (gas, power, and communication cables), compasses for tunnel construction,
rockets, and automotive navigation systems [71,75–81].
4. Fiber-optic current sensors
With the growth in the capacity of electric power systems, the role of protection relaying
systems is becoming more important. Such a system can immediately recognize any
sudden failures, such as a surge, and separate the failure parts from the power systems.
These relaying systems require current sensors, referred to as current transformers or
current transducers (CTs). Most CTs currently in use are electromagnetic devices that
suffer from magnetic saturation effects and residual field effects. Moreover, with the superincrease in voltages (several hundred kV) in power distribution systems, the insulation of
the CTs becomes more difficult and expensive. Hence, optical current sensors that do not
suffer from electromagnetic interference are good substitutes for conventional CTs.
In addition to protection relaying systems, the deregulation and growth of independent
power producers and regional transmission companies have created a need for many new
B. Lee / Optical Fiber Technology 9 (2003) 57–79
69
high voltage revenue-metering points [82]. In the transfer of power from a generation
company to a regional transmission company, a 0.5% uncertainty in metering may result
in an uncertainty of millions of dollars per year at a high power metering location [82].
Hence the potential use of optical CTs that provide high accuracy is promising.
The concept of the optical CTs is quite old [83,84] and based on the Faraday rotation
effect, which states that the polarization of light waves is rotated with the propagation of
the light along (or opposite to) a magnetic field inside some material. Optical fiber CTs are
not the only optical CTs. Bulk-optic CTs such as flint glass closed-loop type and crystalbased (Faraday cell) CTs have also been extensively studied and tested. Some examples
of characteristics of bulk devices are as follows—{material, Verdet constant [deg/Oe cm]
at wavelength [µm], temperature coefficient}: {Flint (Pb) glass, 0.84 × 10−3 at 0.85 or
1.1 × 10−3 at 0.63, < ±0.5% (−25–100 ◦ C)}, {BGO, 0.31 × 10−2 at 0.85, < ±1.5%
(−25–85 ◦ C)}, {FR-5 glass, 0.21 × 10−2 at 0.85, < ±15% (−25–85 ◦ C)}, {YIG, 0.27
at 1.30, < ±7.0% (−25–85 ◦ C)}, {RIG, 7.0 at 0.83, < ±0.5% (−20–80 ◦ C)}. Here the
Verdet constant indicates the rotation angle of the polarization per unit magnetic field per
unit propagation length and 1 Oe = 79.6 A/m. Compared with the bulk devices, the Verdet
constant of optical fibers is quite small, 1.2 × 10−4 deg/Oe cm (at 850 nm) to 2.13 × 10−4
deg/Oe cm (at 633 nm), but the optical path length can be increased to compensate for it by
winding the fiber around a current conducting element a large number of turns. Although
the bulk-optic CTs provide better mechanical stability and smaller sizes, optical fiber CTs
provide ease in forming a closed-loop (to make the sensor respond only to the enclosed
current), adjustability of the sensitivity and dynamic range, less insertion loss, and a higher
signal-to-noise ratio.
In optical fiber CTs, linearly polarized optical waves are input to the optical fiber coil.
The linear polarization can be expressed mathematically as a superposition of two circular
polarizations (right-hand and left-hand). The magnetic field induced around a currentcarrying element induces a circular birefringence inside the optical fiber coil. Hence, after
passing through the coil, a relative phase difference between two circular polarization
components is generated, which results in the rotation of the linear polarization angle in
proportion to the enclosed current and the number of fiber turns.
Although the concept is quite simple, in actual implementation, several difficulties that
limit the resolution of the sensor systems are encountered. Optical fibers have some linear
birefringence due to the imperfection of the core shape, which may distort the output
signals of optical fiber CTs. This linear birefringence can be reduced considerably by
annealing [85]. But, it has a difficulty in practice because the annealing is generally done
after the winding process of the fiber coil. An inevitable linear birefringence, induced
by bending of optical fiber around the current-carrying element, exists. Moreover, the
effects of vibration, mechanical stress or strain and temperature variation on the linear
birefringence are also critical. In some applications, high-current-carrying conductors
vibrate. The vibration in such a direction to rotate the fiber coil is more critical because
the fiber coil acts as a FOG in that case [86].
Several techniques have been proposed to cope with these problems. If a birefringence
bias corresponding to 258-degree phase difference is applied, the effect of linear birefringence variation can be minimized [87]. However, the sensitivity is reduced by a factor
of 0.217. Tokyo Electric Power Company has been developing current sensors using flint
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Fig. 8. Reflection-type fiber-optic current sensor.
glass fibers that show a low photoelastic coefficient (4.5×10−10 cm2 /kg) and a high Verdet
constant (∼0.065 min/Oe cm) [88], but they suffer from higher loss, difficulty in splicing
with other fibers and higher cost. A recent report indicates that the system was successful
in hammering and thermal shock tests for a differential current relaying system [86].
The use of twisted optical fiber has also been proposed, in which a high circular
birefringence is induced, thus reducing the effect of linear birefringence [89]. A high
birefringent spun fiber was also proposed for this application, which is pulled from a
rotating silica preform during the fiber draw process [90]. In most of the today’s optical
fiber CTs these types of methods of increasing circular birefringence are in use.
In practice, the linear birefringence and intentionally induced circular birefringence may
vary with time due to vibration and temperature variation. A simple and efficient way to
reduce this effect is to use a reflection type structure as shown in Fig. 8. The intentionally
induced circular birefringence is reciprocal with the reversal of light propagation direction,
while the Faraday rotation is nonreciprocal. Hence if a light traveling along the fiber coil is
reflected by a mirror and then travels back along the fiber coil, the Faraday rotation angle
is doubled, while the effect of the intentionally induced circular birefringence is cancelled.
The structure has another advantage, in that it can be easily mounted around the currentcarrying element because one end of the fiber coil is open. However, if a conventional
mirror is used, the effect of linear birefringence is also doubled. However, a Faraday rotator
mirror can be utilized to minimize the linear birefringence effect, in which case the two
orthogonal linear polarization components are switched at the Faraday rotator mirror and
experience the same amount of phase delay (despite the linear birefringence) after the
round trip [90,91]. Toshiba reported that, for its reflection type optical CT, the influence
of mechanical shock from circuit breaker vibration of 8g (g = gravitational acceleration)
was less than the rms value of the electronic noise [92]. Sagnac interferometer type optical
B. Lee / Optical Fiber Technology 9 (2003) 57–79
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Fig. 9. An example [94] of a high-performance fiber-optic current sensor using technology developed for FOG
(IOC: integrated optics chip modulator, PC: polarization controller).
fiber CTs are also insensitive to reciprocal effects [93]. Several techniques that have been
devised for FOGs can be applied in these optical fiber CTs. With the structure shown
in Fig. 9, EPFL has achieved ±0.2% accuracy over the ±500 kA range for a temperature
range between 0 and 50 ◦ C [94]. The Sagnac structure using a 3 × 3 fiber optic coupler was
also tested. This structure enables the interferometer to be optimally biased at all times [95].
Some other recent reports indicate that Siemens has tested an optical fiber CT with an
annealed fiber coil for a generator with a measurement accuracy error below 0.25% within
the temperature range of 40–90 ◦ C [96] and NxtPhase showed a linearity performance
better than ±0.2% from 0.5 to 120% of the rated current [82]. ABB Corporate Research
has developed a robust fiber-optic current sensor with inherent temperature compensation
of the Faraday rotation effect [97]. Insensitivity of the sensor to temperature within 0.2%
was demonstrated between −35 and 85 ◦ C.
5. Others
Some other types of optical fiber sensors based on very simple concepts have been commercialized. Some examples include the displacement or pressure sensor based on the light
coupling of two fibers, the liquid-level sensor based on frustrated total internal reflection,
the pressure sensor using a wiggled (periodically bent) fiber, and the temperature sensor
based on the detection of radiation from a heated sensor head (blackbody cavity) [98].
One of the most well-developed and commercialized in-line fiber sensors or diagnostic
tools is OTDR, which is based on the monitoring of the backscattering along the fiber.
The concept is also quite old [99] and the OTDR has become a standard technique for
testing optical fiber links [100]. It typically provides sub-meter spatial resolution but
improved techniques can provide mm-order resolution. More complicated methods such as
optical frequency domain reflectometry have been studied to achieve mm or sub-millimeter
(∼10 µm) spatial resolution [101].
While OTDRs are generally aimed at monitoring optical fiber communication links,
active research has also been done on distributed sensors for civil structure monitoring. One
72
B. Lee / Optical Fiber Technology 9 (2003) 57–79
example is a distributed temperature sensor for monitoring concrete setting temperatures of
a large dam [102]. The principle is based on the stimulated Brillouin scattering. An acoustic
wave couples two counter-propagating beams, which are frequency-shifted by an amount
that is dependent on temperature or strain. A commercialized distributed temperature and
strain monitoring system shows the measurement range of up to 20 km with a spatial
resolution of 0.8 m over 1 km (2–5 m at 10 km and 5–15 m at 20 km), a strain resolution
of 20 µε (measurement range of up to 25 mε) and a temperature resolution of 1 ◦ C [103].
Optical fiber acoustic sensors or optical fiber hydrophone systems have also been
intensively studied and tested for marine or submarine applications [104]. Fiber-optic lowcoherence interferometry has also been commercialized for civil applications [105]. The
dynamic range of the commercialized deformation sensor using the method is of the order
of a few tens of mε for elongation (∼5 mε for shortening) [103]. A fiber optic pressure
sensor using the fiber Fabry–Perot interferometer method was also commercialized, which
shows a dynamic range of 0–3000 psi with ±1.0% (of full range) accuracy and ±0.5%
(of full range) linearity [106]. There are other examples of commercialized sensors using
Fabry–Perot interferometers (or thin film Fizeau interferometers) and white-light crosscorrelators: a force and load transducer (load capacity: 4500 kg, resolution: 0.01% of
full scale); strain gauges (range: ±500 to ±5000 µε, resolution: 0.01% of full scale);
a temperature transducer (range: −40 to 350 ◦ C, accuracy: ±0.5 to ±1 ◦ C or 0.5–1% of full
scale); a displacement transducer (linear stroke: 20 mm, resolution of up to 2 µm) [107].
Cryogenic temperature sensing is also an attractive field for fiber-optic sensors [108].
Fiber-optic chemical sensors or biosensors have been continuously studied. The
principles are based on the monitoring of absorbance, reflectance, luminescence, reflective
index change, or light scattering and aimed at the measurement of oxygen, pH, carbon
dioxide, ammonia, detergents, biochemical oxygen demand, pesticides, and humidity
[5,109]. In many cases optical fibers are simply used to guide light to the measurement
point in the specimen. In some cases optical fibers are monitoring the response of a
material deposited on the end of the fiber [110]. In some other cases well-developed sensor
technologies such as FBG temperature sensors have been used for biological tests [111].
A good example of optical fiber chemical sensors in which the fiber itself plays a key role
in the measurement is the use of LPGs [112,113]. As discussed in Section 2, an LPG can
couple the core mode to a cladding mode. If the coating or jacket is removed from the
optical fiber, the evanescent field of the cladding mode that exists outside the cladding
experiences the refractive index change of the outside material. The sensitivity can be
adjusted by fiber etching [114]. An example of a commercialized fiber biosensor is the invivo blood pressure sensor that uses the white light interferometry (pressure range: 500 mm
Hg to 1060 mm Hg, precision: ±1 mm Hg or ±1% of reading) [107].
6. Concluding remarks
In this paper the current status of optical fiber sensors has been briefly reviewed. As soon
as optical fibers were developed, it was concluded that they could also be used for sensors.
Hence, the history of research in optical fiber sensors is almost as old as the history of
optical fiber communication research. Optical fiber sensors have unique advantages such
B. Lee / Optical Fiber Technology 9 (2003) 57–79
73
as high sensitivity, immunity to EMI, small size, lightweight, robustness, and the ability
to provide multiplexed or distributed sensing. Although the optical fiber sensors have not
experienced the dramatic commercial success of optical fiber communications, they have
been continuously and enthusiastically studied.
At present, the economic situation is not good. One of the gloomy forecasts predicts
that the worldwide optical transport market will decline through the year of 2004 before
resuming modest growth in 2005 (by Dell’Oro Group) [115]. It says the 2004 sales will
be only 35% of the peak in 2000. Some other analyst groups predict more rapid economic
recovery. Strategies Unlimited predicts that the optical component market will start the
recovery from 2003. Communications Industry Researchers (CIR) also predicts that the
long haul market will start its recovery from 2003. It also predicts that the market of
integrated optics for multiplexing, switching, and computing will grow to $400 millions by
2006 [116]. These forecasts are on the market related to optical communications. However,
the competitiveness of optical fiber sensors relies on the component cost reduction, which
is greatly dependent on the optical communication market. The sensor forecast of 2006–
2011 published in April of 2002 by the Freedonia Group predicts that U.S. market
for sensors will grow at a healthy pace, fueled by economic recovery in more mature
applications such as process control, industrial machinery, and conventional automotive
sensors. It also predicts that the fastest growth will occur in emerging areas such as microelectro-mechanical systems and telematics, and that nearly all types of opto-electronic
sensors including fiber optic sensors will exhibit healthy gains [116]. Recently Honeywell
Automation and Control Solutions has signed an agreement to acquired Invensys Sensor
Systems [117], and General Electric has employed an aggressive strategy for entering the
sensor’s arena such as temperature, humidity, pressure, gas, and oxygen sensors [118].
Although the sensors involved here are not confined to optical fiber sensors, these trends
are encouraging to the researchers in the field of optical fiber sensors.
Fiber grating sensors have been the most widely studied topic among various optical
fiber sensor technologies. Some fiber grating sensors have been commercialized for civil
health monitoring and oil industries. FBGs have been shipping for WDM devices since
1997 [119]. But for WDM filter applications, FBGs have strong competitors such as
arrayed-waveguide gratings and thin-film filters. Several FBG vendors have recently exited
the market [119]. However, for sensor applications, FBGs provide a unique characteristic
of multi-point EMI-resistant measurement, and they have started to be deployed in civil
and oil industry fields.
There are other rather mature optical fiber sensor technologies such as OTDRs, FOGs,
and optical fiber current sensors. The OTDR has become a standard technique in optical
communication line fault monitoring. Today, FOGs are considered as the most costefficient solution for various inertial navigation applications. Optical current and voltage
sensors became to be installed in real power industry [120].
New ideas are continuously being proposed and tested for not only various traditional
measurands such as strain, temperature, and pressure but also for new applications such
as biosensors. Papers on optical fiber sensors have already begun to appear in many fieldapplication journals such as civil engineering journals, sensor journals, power engineering
journals, chemical engineering journals, and bioengineering journals. In 2002, the number
of papers on optical fiber sensors published in the journals listed in Scientific Citation
74
B. Lee / Optical Fiber Technology 9 (2003) 57–79
Index (SCI) is about 110. Many of the papers are published in the field-application journals.
Hence, the research in the optical fiber sensors is more active than optics people feel from
the journals of their main interest. The U.S. patent applications on optical fiber sensors
during recent two years records several hundreds in number, among which temperature,
strain, displacement, refractive index, damage detection, current, vibration, biological and
chemical sensors are frequent topics [121]. Biosensors for medical applications will be
an important application field of optical fiber sensors. Now, the optical fiber sensors
reached the stage at which optics people should collaborate with specialists in other
fields such as civil engineering, mechanical engineering, material engineering, chemical
engineering, and bioengineering to develop sensor systems useful in real fields and to find
new applications.
With the continuing effort and enthusiasm, the application fields of optical fiber sensors
will be extended and more commercial successes will almost certainly be found.
Acknowledgment
The author thanks Dr. Minho Song for valuable comments on this paper.
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