chapter 1 - Bentham Science

2 Fiber Bragg Grating Sensors Jacques Albert
SHG field helped several advances in FBG science and technology. In fact, the first scientific workshops and
meetings on these two topics were held jointly (more on this later).
FIBER BRAGG GRATINGS BECOME A REALITY
It was the 1989 breakthrough publication by Meltz, Morey and Glenn actually made FBGs practical [7-8]. Since
FBGs were basically permanent holograms written in the core of an optical fiber, they decided to try holographic
techniques to write FBGs from the outside of the fiber. Since it had been determined by then that UV light was the
source of the FBG writing process, two questions had to be addressed: would the cladding of the fiber be
transparent enough to the UV light to reach the core and was there a light source with enough power and spatial
coherence to form interference fringes of sufficient contrast and intensity to write FBGs? Luckily, the cladding of
most optical fibers (even those at the time) is made of very pure undoped synthetic silica, whose absorption edge
lies beyond 180 nm in the UV. Since the most prominent defect band thought responsible for the formation of
FBGs is a germanium oxygen deficient color center with absorption near 242 nm, the cladding would not pose a
problem. For the UV source there was not much to choose from in the late 1980s. They had to use a rather intricate
system made up using a tunable excimer-pumped dye laser, operated at a wavelength in the range of 486-500 nm,
along with a frequency-doubling crystal to provide UV light near 242 nm adequate coherence. The advantage of that
particular system was that they could tune the laser wavelength and thus prove that the FBG writing process was more
efficient near 242 nm. Of course, forming the interference fringe pattern from the outside also allowed the writing of
gratings with almost arbitrary periods, and hence to make wavelength filters and sensors for practical wavelength
bands (near 800, 1300, and 1500 nm for instance). The same group also demonstrated very early on the formation of
tilted gratings that were used then to tap some of the light from the core of the fiber to make it radiate outside [9].
But back to sensors! The FBG is essentially a wavelength filter, bandpass in reflection and band reject in
transmission. Since the period of the grating necessary to reflect core guided light in glass fibers is of the order of
half a micrometer, even millimeter long gratings have Q-factors (number of grating periods / grating length) of at
least 2000, resulting in pass bands that are of the order of 1 nm and less for light at visible and near IR wavelengths.
Furthermore the combined thermo-optic coefficient of the reflection wavelength (hereafter called the “Bragg”
wavelength) was soon calculated and measured to be near 10 pm/°C, arising from a combination of the (weak)
coefficient of thermal expansion of silica glass and the relatively stronger thermo-optic coefficient of the refractive
index (+10 -5 /°C). Similarly for strain, whereas fibers can be stretched without breaking by as much as 1%, with a
corresponding change in the period of the FBG [7, 10]. Therefore with suitable instrumentation to launch light with
at least a few nanometers of bandwidth and to measure the reflected optical spectrum (fused fiber couplers for single
mode fibers and optical spectrum analyzers had been invented and mass produced by the late 1980s!), a fiber with
an embedded FBG could be used to measure temperature or strain in very small spaces, remotely from relatively
large distances, and in areas where electrical devices were not welcomed. Interestingly, the original patent on side
writing, filed in October of 1986, explicitly mentions temperature and strain sensing as claimed applications and
provides the equations to calculate the wavelength shifts of FBGs as a function of these two parameters. Knowing
that Meltz’s group at UTC was concerned with the development of fiber optic (strain and temperature) sensors from
at least the mid 1970s (US patents from 1981), it is fair to say that the need for better fiber sensors was the main
reason FBGs were developed in the first place. The attractive sensing features of FBGs were immediately
recognized by the photonics community and led to an incredible flurry of activity worldwide from the early 1990s
onward to this day. Another aspect of optical sensing where FBGs were recognized to have great potential was in
the development of narrowband, high coherence lasers through the use of FBGs as mirrors for fiber laser cavities
and also as pigtailed output couplers for semiconductor lasers [11-13]. These efforts were made in parallel with
similar research activities to find applications for FBGs in other fields, especially telecommunications. The 1990s
were the golden age of optical telecommunications research as the growth of the demand for long distance
bandwidth was fueled by the birth of the internet and fed by several breakthroughs (low loss single mode fibers and
erbium-doped fiber optical amplifiers). Just to give a flavor of the enthusiasm associated with FBGs in that field, in
2000, near the peak of what has been called the “telecom bubble” the market forecast for FBGs in telecom alone was
predicted by some analysts to reach 1 B$/year within the first half of the decade. At grating prices between 10 $ and
1000 $ depending on the application, that would have been a lot of gratings.
Back to sensors again! It is not so well known as I write this that a large fraction of the early research on FBGs was
for sensing applications. The Optical Fiber Sensors conference had had its first occurrence in 1983, and the field was