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1. ISSN 1063 7850, Technical Physics Letters, 2010, Vol. 36, No. 9, pp. 789–791. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © A.M. Kurbatov, R.A. Kurbatov, 2010, published in Pis’ma v Zhurnal Tekhnicheskoі Fiziki, 2010, Vol. 36, No. 17, pp. 23–29.
789
Single mode fibers used in fiber optic gyroscope
(FOG) sensing coils should have small losses at least
for one of polarization modes (x mode). If second
polarization mode (y mode) has high level losses, then
we have a polarizing fiber.
At present, FOG coils employ a conventional two
layered fiber with refractive index (RI) of the core
higher than that of cladding RI by 0.015 and more.
This gives one an opportunity to have small enough
fundamental mode field diameter (MFD) and deter
mines extremely high macro and microbending resis
tance of this fiber.
However, this kind of fibers rarely has losses lower
than 1 dB/km (frequently, they reach 2 dB/km). This
can be determined by fundamental mode tunneling
into stress applying rods, where material losses (120–
150 dB/km) take place. High germanium level in the
core also leads to material losses. Further, twisting of
this kind of fiber also leads to losses, because in this
case the boundary of stress applying rods and quartz
cladding features coupling of the fundamental mode
with attenuating higher order modes.
Furthermore, this kind of fiber is absolutely inap
plicable in high level radiation environment. In this
case, germanium silicate core was suggested to be
replaced by nitrogen doped one [1], but high nitrogen
concentration led to a problem of residual material
losses at a wavelength of 1.55 μm due to the nitrogen
absorption peak at 1.52 μm.
As an alternative for FOG sensing coil, a micro
structured fibers with air guiding core were proposed
[2]. This at once removes several problems by many
times reducing the temperature sensitivity [2],
increasing the vibration resistance of the coil, and
almost completely suppressing the Faraday and Kerr
effects. Large birefringence [3] along with the absence
of its temperature fluctuations in these fibers allows
one to radically reduce polarization errors in FOG
fiber ring interferometers (FRIs). Finally, these fibers
are excellently radiation resistant.
However, in practice a lot of basic limitations of
these fibers are still not overcame: losses are large,
polarization characteristics are almost unknown, and
coupling with FRI Y junction channel waveguides is
difficult (technologically and due to mode fields over
lap, because large birefringence fiber air guiding core
is elliptical [3]). To our opinion, this is already enough
to turn to W fibers [4]. Of course, they have much
more limited capabilities than microstructured fibers,
but they can noticeably improve FRI characteristics
right now.
As a new fiber for sensing coil we propose W profile
Panda type fiber [5]. All the above listed kinds of
waveguide losses in this fiber are extremely small.
Material losses in the case of nitrogen doped core
would be small due to low enough core doping by
nitrogen. The polarization characteristics of proposed
fiber are at least no worse than those of conventional
fibers and beside this, there is an opportunity to get
appreciable polarizing effect (dichroism).
Figure 1 shows the RI profile of initial preform for
the proposed fiber. This fiber has, in addition to the
germanium silicate core, a fluorine doped reflecting
cladding (RC) with depressed RI, quartz (outer) clad
ding, and protective coating. Based on the structure
depicted in Fig. 1, we obtained anisotropic PANDA
type fiber, which cross section photograph is presented
in Fig. 2.
Darker regions correspond to lower RIs. RI values
in various regions are listed in table.
New Optical W Fiber Panda
for Fiber Optic Gyroscope Sensitive Coil1
A. M. Kurbatov* and R. A. Kurbatov
Kuznetsov Research Institute for Applied Mechanics, Moscow, 111123 Russia
*e mail: akurbatov54@mail.ru
Received April 22, 2010
Abstract—Based on refractive index W profile, two kinds of single mode fibers are proposed: polarizing
(bend type) and linear polarization maintaining (PM fiber) with low losses (up to 0.35 dB/km). The polar
izing W fiber allows one to combine the dichroism in wide spectral range with opportunity to have almost any
desirable mode field diameter (MFD). The PM fiber low losses are essentially determined by fundamental
mode tight packing in the guiding core. These fibers also can be made radiation resistant by replacing the ger
manium silicate core by nitrogen doped core.
DOI: 10.1134/S106378501009004X
1
The article was translated by the authors.

2. 790
TECHNICAL PHYSICS LETTERS Vol. 36 No. 9 2010
A.M. KURBATOV, R.A. KURBATOV
We have made two fibers, with diameters 80 and
90 μm. Based on the first fiber, we obtained a polariz
ing fiber, and based on the second fiber, we obtained a
polarization maintaining fiber with losses amounting
to 0.35 dB/km.
Polarizing 500 m long fiber due to its winding into
60 mm diameter coil showed x mode losses of
3 dB/km and a dichroism of 30 dB/km. Light source
spectrum width in these measurements was approxi
mately 20 nm. It is a good result considering that a
birefringence value of 3.4 × 10–4
is not very high. Since
winding this fiber with a diameter larger than 60 mm
did not led to dichroism, it is clear that we have got a
bend type polarizer.
W fiber is known due to the fact that even its fun
damental mode may exhibit cutoff at finite wave
length. Cutoff means that mode effective RI becomes
equal to quartz cladding RI:
Calculation of the W fiber fundamental mode cut
off wavelength (threshold) is quite simple. However,
the mode cutoff by no means always means its large
losses. That is why it is necessary to clarify real mech
anisms of fundamental mode losses. Primarily we will
consider the following two of these:
(1) Fundamental mode radiation tunneling into
external quartz cladding and touching the absorbing
coating (in straight fiber).
(2) Bending losses.
In our fiber, the fundamental mode cutoff wave
length is approximately 2.2 μm. First of the loss mech
anisms according to calculations leads to the fact that
fundamental mode losses become significant already
neff n3.=
at wavelength of 1.8 μm. Bending losses due to wind
ing into 40 mm diameter coil shift the beginning of
losses approximately to 1.55 μm. Below we will only
consider the bending losses.
Most important question is the dependence of
bending losses on the parameter χ = τ/ρ, where τ is the
RC radius and ρ is the core radius. According to calcu
lations, RC in our fiber may be arbitrary thin because
the operation wavelength (1.55 μm) is far from funda
mental mode cutoff threshold (2.2 μm).
Figure 3 shows the behavior of wavelength λ0 at
which bending losses are 1 dB/km as a function of
parameter χ (this could be named the fundamental
mode bending cutoff threshold). The same figure pre
sents a plot of the fundamental mode spot diameter
(MFD) evolution as dependent on χ. In order to cal
culate the bending influence, we generalized the
approach that was developed in [6] for conventional
two layer fibers. To confirm the results, we also mod
eled the problem by the method of supermodes [7, 8],
which were calculated by finite difference method
(one of supermodes always significantly resembles the
fundamental mode of straight fiber, so its attenuation
determines bending losses).
From Fig. 3, one can see the following. First of all,
at all χ the bending cutoff shift does not decrease
below 1.55 μm. Second, this curve initially goes down,
then (in the region 1.3 < χ < 1.42) it has approximately
constant level (1.55 μm), and eventually it grows
slowly. This behavior of λ0(χ) curve can be explained
by the model of fundamental mode coupling to higher
order attenuated modes. The coupling coefficients of
these modes monotonically decrease when χ grows.
Synchronism of these modes first increases sharply
and prevails over decrease in the coupling coefficients
(losses grow), then it increases smoothly (losses do not
change), and eventually it stabilizes (losses decrease).
Third, the MFD at χ < 1.6 sharply grows and this
gives the limitation to RC width from below, so one has
to use RC with χ > 1.6.
Calculations using the methods described above
show that, at a birefringence larger than 8 × 10–4
, it is
possible to obtain fiber with large dichroism in a wide
0.005
−0.011
Fig. 1. Initial preform RI profile for W fiber PANDA. Fig. 2. W fiber PANDA cross section photograph.
RI values in various elements of PANDA fiber
Parameter RI Value
Core RI, n1 1.4655
Reflecting cladding RI, n2 1.451
Outer cladding RI, n3 1.46
Stress applying rods RI 1.4515

3. TECHNICAL PHYSICS LETTERS Vol. 36 No. 9 2010
NEW OPTICAL W FIBER PANDA FOR FIBER OPTIC GYROSCOPE SENSITIVE COIL 791
spectral range (100 nm and more) and with low losses.
To increase the dichroism, it is also possible to apply
absorbing/scattering materials located in quartz clad
ding [9, 10].
Generally, the proposed W fiber combines advan
tages of two convenient fibers. First of these is the fiber
with the RI difference between core and quartz clad
ding equal to Δn13 = n1 – n3 (see table); the second
fiber has the RI difference Δn12 = n1 – n2. In the first
fiber, when winding it, one can ensure wide single
polarization spectral window, because birefringence
against Δn13 is significant. But in this case one will not
obtain the desired MFD. In second fiber, there is no
problem with MFD, but there is no chance to obtain
wide single polarization spectral window, because
birefringence against large Δn12 is small. The proposed
W fiber has simultaneously a wide spectral window (as
that in the first fiber) and the opportunity to obtain
desired MFD (as that in the second fiber).
Based on the same W structure, Panda fiber with
diameter 90 μm operates as polarization maintaining
(PM fiber). In this case, dichroism is removed to
longer wavelengths, but due to this x mode losses are
simultaneously sharply reduced. At present, based on
the structure described above, we obtained PM fiber
samples with losses amounting to 0.35 dB/km, which
is not far from the 0.2 dB/km limit.
Small losses in FOG coil could be used in different
ways. For example, in FOG there is a minimal level of
optical signal power reaching the photodetector, for
which photodetector and preamplifier electronic
noise is suppressed. Power reserve which is obtained
due to low loss fiber application could be used to
employ other methods of additional signal phase mod
ulation. This reserve can also be used for coil winding
with several kilometers length, which will improve
FOG sensitivity.
As for h parameter, it appeared to be 2 × 10–5 m–1.
This is good result for birefringence B = 3.4 × 10–4
,
considering that h parameter depends on B approxi
mately as B–2
. Further birefringence growth is purely
technological problem and it is associated with stress
applying rods doping.
Finally, material losses in stress applying rods are
not larger than several hundredths of dB/km (due to
fundamental mode tight confinement in the core). For
the same reason, sensitivity to twisting is absent, and
this also gives to these fibers certain advantages when
using in fiber optic gyroscopes.
Acknowledgments. Authors are grateful to FIRE
RAS 226 Laboratory Head G.A. Ivanov for his help in
fiber fabrication.
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1.0
λ0, μm
χ
1.80
1.8
1.77
1.74
1.71
1.68
1.65
1.62
1.59
1.56
1.53
1.50
10.0
9.8
9.6
9.4
9.2
9.0
8.8
8.6
8.4
8.2
8.0
MFD, μm
1.2 1.4 1.6 2.0 2.2 2.4 2.6 2.8 3.0
Fig. 3. Behavior of wavelength λ0 at which bending losses
are 1 dB/km (solid line) and fundamental mode MFD
(dashed line) depending on the RC width.