Induced attenuation (db/km)
1600 1500 1400 1300 1200 1100
Fiber A; PSC GI MM
Fibers A & B: After exposure to
1. 5 atm of H2 at
300°C for 400 hours
Fiber C: After exposure to
1 atm of H2 at
175°C for 150 hours
Fiber B; PSC GI MM
Fiber C; Std Ge GI MM
1000 900 800
hydrogen-rich environments. One method is coating the fiber
with a hermetic coating, the most common of which is carbon.
Layers of carbon in the form of randomly orientated carbon
platelets can be applied to optical fibers during the fiber draw
5 The carbon layer is much less permeable to hydrogen than other coating materials, so the rate of hydrogen diffusion into the fiber is significantly reduced.
However, there are two disadvantages of this approach: ( 1)
over time, the hydrogen will diffuse through the carbon lay-
er and reach the fiber core; and ( 2) at elevated temperatures
(>170°C), the carbon coating becomes permeable to hydrogen,
losing its hermeticity. Carbon-coated fibers are primarily used
for downhole environments where the temperature is <170°C.
The alternative method involves designing the core of the fiber to be much less sensitive to hydrogen-induced losses. This
approach can significantly reduce the impact of the Si-H and
Si-OH absorption peaks, but interstitial hydrogen-induced losses will still occur. The key is managing the losses at the wavelengths of interest so the DTS system will function normally.
The advantage of this approach is that with the proper fiber
coating, such as polyimide, the fibers can operate up to 300°C
with very little signal degradation at the relevant wavelengths.
The ideal Raman DTS fiber design for high-temperature
(up to 300°C), hydrogen-rich environments is graded-index
50/125 µm multimode fibers with polyimide coating. These
fibers, referred to as pure silica core (PSC) fibers, have fluorine as the only dopant. They are designed with a heavily flu-
orine-doped cladding region for 0.2 NA and a fluorine-doped
silica parabolic graded-index core region.
Controlling the smooth graded-index profile in these fibers
is very challenging but essential since the spatial resolution
of the sensor is defined by the bandwidth. Step index mul-
timode fibers do not have sufficient bandwidth to provide
the necessary spatial resolution. The preferred spatial reso-
lution is 1 m and requires bandwidths of 300 MHz-km at
850 and 1300 nm.
Figure 3 shows the induced attenuation for three different
fibers after exposure to hydrogen. Fiber A is Nufern’s PSC
graded-index multimode fiber (NuSENSOR GR-S50/125-
20P), and Fiber B is another commercially available PSC grad-
ed-index multimode fiber—both are designed for high-tem-
perature, hydrogen-rich downhole environments. Fiber C is
a standard graded-index 50/125 multimode fiber used in the
In Figure 3, the two specifically designed fibers (A and B)
exhibit much lower induced attenuation when exposed to a
high-temperature hydrogen atmosphere, especially in the 1000–
1120nm range used in Raman DTS. The standard 50/125 Ge-doped fiber experiences significantly higher hydrogen-induced
losses and would not work in downhole sensing applications.
In addition to the fiber design, processing conditions can
also determine how well a fiber performs in the presence of
hydrogen. Fibers A and B are commercially available from different suppliers and are made with different processes. They
are both pure silica core, graded-index 50/125 multimode fibers with 0.2 NA and designed for high-temperature downhole Raman DTS. Figure 3 demonstrates the defect sites leading to the formation of Si-OH bonds present in Fiber B are
much greater than in Fiber A.
As mentioned, the induced losses associated with Si-H and
Si-OH formation are permanent. Both fibers will work well
at the standard wavelength range (1000 to 1120 nm) used for
Raman DTS, but clearly Fiber A will perform better at oth-
er wavelengths. Operating at different wavelengths may be-
come important in the future as the oil industry explores and
develops new technologies. For example, the 1310 and 1550
nm wavelengths usually associated with SM fiber operation
may be utilized in applications requiring these specialty multimode fibers.
Critical coating tests
As important as the robustness of the glass in the fiber, the fi-
ber coating must also continue to operate under the same harsh
conditions. The fiber coating of choice for up to 300°C operation is polyimide. Not all polyimides are the same so the actual material selection and the manner in which the coating
is applied must be considered. Testing the fiber coating can
be done in many different ways.
The two critical coating tests are high-temperature perfor-
mance and chemical resistance. Performing a high-tempera-
ture bake demonstrates coating performance at elevated temperatures and can help determine if the coating is sufficiently
cured. If the coating is under-cured, blistering will occur at elevated temperatures. Under-cured coatings do not sufficiently
FIGURE 3. Spectral-induced attenuation of three commercially
available fibers allowed to recover in an inert atmosphere after
exposure to hydrogen. Fibers A and B are two pure silica core, graded-index 50/125 µm multimode fibers exposed to 1. 5 atm of hydrogen
partial pressure. Fiber C is one standard graded-index 50/125 µm
multimode fiber exposed to 1.0 atm of hydrogen partial pressure.