FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

The vertical height of the various cross-hatched regions
represent the loss as a function nf the wavelength
arising from the various sources. Note that the
minimum total attenuation at approximately 0.8 micron
wavelength is approximately 10 dB/km. This is a somewhat
mediocre fiber by today’s standards. The lowest
four regions in Fig. 2.17 correspond to the loss due
to 1 part per million by weight, in silicon oxide (Si02)
glass, of the metallic impurities Cu, Ni, Fe, and Cr,
from bottom to top, respectively. The peak in the
vicinity of the 0.95-micron wavelength is due to the
third harmonic of the hydroxyl (OH-) vibrational mode,
and corresponds to roughly a 20-part-per-million impurity
content. The black dots are predicted values of
losses based on calorimetry-type optical absorption
measurements made on the glass sample from which the
fiber was drawn. The upper cross-hatched region represents
the attenuation due to Rayleigh scattering while
the white region is the remaining difference between
the total loss versus wavelength (the uppermost curve)
and the sum of the previously mentioned losses. The
latter is attributed to regular bending and microbending
losses.
The attenuation versus wavelength curve shown
in Fig. 2.18 is for a currently available very-low-
5.0 . ‘.
3.0 — ‘.> ‘.
2.0 —
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1.0
tion. In the bent region, the ray intersects the corecladding
interface at an angle 13 that is greater than
‘dC and thus it will be partially transmitted out of the
core and into the cladding. This will occur at each
successive reflection from the outer interface and large
losses may occur. Another qualitative explanation of
this type of loss is as follows. In the beam propagating
in the fiber, assuming plane wavefronts, if the
velocity at the center of the core in the bent section
were equal to the c/nl, the “proper” velocity in the
core, then the velocity at the outer edge of the front
would have to be higher than c/nl, which cannot occur.
Radiation in the form of core-to-cladding scattering
results. Finally, from the electromagnetic wave theory
it may be shown that in a waveguide with a constant
bend radius, all of the solutions of the wave equation
represent waves that decay with
along the centerline of the core.
increasing distance
6
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e’ 70,
\
/
/
/
0.5 -
\
Fig. 2.19 Leakage of optical power froman optical
fiber at a constant-radius bend.
Fig. 2.18
0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6
WAVELENGTH (pm)’
Attenuation rate of optical power in a low-
10SS optical fiber as a function of wavelength.
The arrows along the abscissa indicate
the wavelengths of commercially
available lasers.
loss, single mode fiber. The arrows along the lower
abscissa correspond to the wavelengths of currently
available lasers. The peak in the loss curve at the
1.4-micron wavelength is due to the OH- radical, which
has been reduced to a relatively low concentration
level. The minimum at approximately the 1.3 micron
wavelength is of special interest, not only because the
attenuation rate is down to 0.5 dB/km, but also because
this wavelength is very close to the zero-dispersion
wavelength of Si02 which is of special interest in some
applications, as will be discussed shortly. Fig. 2.18
shows that extremely low attenuation rates attainable
with current fiber fabrication techniques at wave -
lengths where high intensity long-life, solid state
laser sources are becoming commercially available. It
is now up to the fiber user to devise techniques and
configurations that can maintain this low loss by not
introducing significant regular (macro) bending and
microbending losses.
Using the latter approach it is possible to
compute the expected loss due to a constant bend radius.
The results of such calculations are shown in Fig. 2.20,
where loss curves versus bend radius are shown for
singlemode fibers at the 0.83-micron wavelength having
different numerical aperture (N.A.). Note the strong
dependence on bend radius and N.A. Referring to Fig.
2.20, consider a fiber with a numerical aperture of
0.1. When a 10-meter length is wound on a l.2-cmradius
mandrel, the attenuation due to bending is approximately
6 dB, i.e., 75 percent of the light energy
injected into the core at the input end is scattered
out of the core while propagating in the ten meters
to the output end. Nhen 10 meters of identical fiber
are wound on a 1.0 cm radius mandrel, the attenuation
due to bending will increase by a factor of about
250,000 to 60 dB, so that only about one millionth of
the original light remains at the end of the fiber.
Just about all of the input light is scattered out of
the core. On the other hand, using the 1.2 cm radius
mandrel and increasing the numerical aperture (N.A.)
to 0.12 reduces these attenuations to 0.16 dB and 1.6
dB, respectively. Therefore, care must be taken in
designing fiberoptic sensors that require bending and
winding of fibers and in specifying fibers for such
applications.
A ray picture of the regular (constant) bend
radius loss mechanfsm is shown in Fig. 2.19. Assume a
ray is traveling to the right at an angle of o less
than the critical angle ec in the straight fiber sec - 2-9