This document proposes a new type of optical fiber called a W-fiber Panda that could improve fiber optic gyroscopes. The fiber has a core with nitrogen doping surrounded by stress-applying rods that create birefringence. Testing showed a polarizing version had 3 dB/km x-mode losses and 30 dB/km dichroism, while a polarization-maintaining version had ultra-low losses of 0.35 dB/km. This new fiber combines advantages of existing fibers by allowing a wide single-polarization spectral window as well as a desired mode field diameter. Its low losses could enable longer or more sensitive fiber optic gyroscope coils.

1. 1
New optical W-fiber Panda for fiber optic gyroscope sensitive coil
Kurbatov А.М., Kurbatov R.А.
Kuznetsov Research Institute for Applied mechanics, 111123, Moscow, Aviamotornaya st., 55.
E-mail: akurbatov54@mail.ru
On the base of refractive index W-profile two kinds of single-mode fibers are suggested:
polarizing (bend-type) and polarization maintaining (PM-fiber) with low losses (up to 0.35
dB/km). Polarizing W-fiber allows one to combine the dichroism in wide spectral range with
opportunity to have almost any desirable MFD. PM-fiber low losses are essentially determined
by fundamental mode tight packing in the guiding core. These fibers also could be done to be
radiation resistant by changing of germanosilicate core by nitrogen-doped core.
PACS: 42.81.Qb
Single mode fibers which are 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 polarizing fiber.
At present time in FOG coils a conventional two-layered fiber is used with refractive
index (further RI) of the core higher than cladding RI by amount 0.015 and more. This gives one
an opportunity to have small enough fundamental mode field diameter (MFD) and determines
extremely high macro- and microbending resistance of this fiber.
However this kind of fibers rarely has losses lower than 1 dB/km (frequently they are 2
dB/km). This could 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
on the stress applying rods and quartz cladding boundary a coupling of fundamental mode with
attenuating higher order modes is induced.
Furthermore, this kind of fiber is absolutely inapplicable for exploitation in high level
radiation environment. In this case germanosilicate core is suggested to be exchanged for
nitrogen-doped one [1], but due to nitrogen high concentration a problem of residual material
losses at the wavelength 1.55 μm arises due to nitrogen absorbing peak at 1.52 μm.
As an alternative, for FOG sensing coil a microstructured fibers with air guiding core
were suggested [2]. This at once removes several problems: many times reducing of temperature
sensitivity [2] and vibration resistance growth of the coil, Faraday and Kerr effects are
suppressed almost completely. Large birefringence [3] along with its temperature fluctuations
absence in these fibers lets one to radically reduce polarization errors in FOG fiber ring
interferometers (FRI). 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, coupling with FRI Y-junction channel
waveguides is difficult (technologically and due to mode fields overlap, because large
birefringence fiber air guiding core is elliptical [3]). To our mind this is already enough to turn to
W-fibers [4]. Of course they have much more limited capabilities then microstructured fibers,
but they can noticeably improve FRI characteristics right now.

2. 2
As a new fiber for sensing coil we suggest W-profile Panda-type fiber [5]. Listed earlier
all kinds of waveguide losses are extremely small in this fiber. Material losses in the case of
nitrogen-doped core would be small due to low enough core doping by nitrogen. Polarization
characteristics of suggested fiber at least are not worse than in convenient fiber and beside this
there is an opportunity to get appreciable polarizing effect (dichroism).
So, RI profile of initial preform for suggested fiber is pictured in Fig. 1.
Fig. 1. Initial preform RI profile for W-fiber Panda.
This fiber apart from germanosilicate core has fluorine-doped reflecting cladding (RC) with
depressed RI, quartz (outer) cladding and protective coating. On the basis of performed in Fig. 1
structure we have got anisotropic PANDA-type fiber, which cross section photograph is pictured
in Fig. 2.
Fig. 2. W-fiber PANDA cross section photograph.
Darker regions correspond to lower RI. RI values in different regions are listed in Table.
Table
Parameter Value
Core RI, 1n 1.4655
Reflecting cladding RI, 2n 1.451
Outer cladding RI, 3n 1.46

3. 3
Stress applying rods RI 1.4515
We have got two fibers: with diameters 80 and 90 μm. On the basis of the first one we
have got polarizing fiber, on the basis of the second one we’ve got polarization maintaining fiber
with losses achieving 0.35 dB/km.
Polarizing 500-m length fiber due to it’s winding into 60 mm diameter coil showed x-
mode losses to be 3 dB/km and dichroism to be 30 dB/km. Light source spectrum width in these
measurements is approximately 20 nm. It’s a good result considering that birefringence value
3.4×10-4
is modest enough. Whereas winding this fiber with larger than 60 mm diameter did not
led to dichroism it is clear that we’ve got a bend-type polarizer.
W-fiber is known due to the fact that even its fundamental mode may suffer cutoff at
finite wavelength. Cutoff means that mode effective RI becomes equal to quartz cladding RI:
3nneff  .
W-fiber fundamental mode cutoff wavelength (threshold) calculation is quite simple. However it
is far from situation when mode cutoff means its large losses. That’s why it is necessary to
clarify fundamental mode real losses mechanisms. Primarily we’ll consider the next two of them:
1) Fundamental mode radiation tunneling into external quartz cladding and
touching the absorbing coating (in straight fiber);
2) Bending losses.
In our fiber fundamental mode cutoff wavelength is approximately 2.2 μm. First of the
loss mechanisms according to calculations leads to the fact that fundamental mode losses
become significant already at wavelength 1.8 μm. Bending losses due to winding into 40 mm
diameter coil remove losses beginning approximately to 1.55 μm. Further we’ll consider only
bending losses.
Most important question is bending losses dependence on parameter χ = τ/ρ, where τ is
RC radius and ρ is core radius. According to calculations RC in our fiber may be arbitrary thin,
because operation wavelength (1.55 μm) is far from fundamental mode cutoff threshold (2.2
μm).

4. 4
Fig. 3. Wavelength 0 behavior at which bending losses are 1 dB/km (solid line), and
fundamental mode MFD behavior (dashed line) in dependence on RC width.
In Fig. 3 a behavior of a wavelength λ0 at which bending losses are 1 dB/km is shown in
dependence on parameter χ (this could be named as fundamental mode bending cutoff
threshold). At the same figure a graph of fundamental mode spot diameter (MFD) evolution in
dependence on χ is also presented. To calculate the bending influence we generalized the
approach developed in [6] for convenient two-layer fibers. To confirm the results we also
modeled the problem by supermodes method [7,8], which were calculated by finite difference
method (one of supermodes is always significantly resembles the fundamental mode of straight
fiber, so it’s attenuation determines bending losses).
From graphs one may see the following. First of all, under all χ bending cutoff shift
doesn’t get below 1.55 μm. Second, this curve in the beginning goes down, then in the region
1.3<χ<1.42 it has approximately constant level (1.55 μm), and after that grows slowly. Such
behavior of curve λ0(χ) could be explained by the model of fundamental mode coupling to higher
order attenuated modes. These modes coupling coefficients monotonically decrease when χ
grows. Synchronism of these modes firstly sharply gets better and predominates on couple
coefficients decreasing (losses grow), then it’s get better but smoothly (losses don’t change) and
then it stabilizes (losses decrease).
Third, MFD at χ<1.6 sharply grows and this gives the bottom limitation to RC width, so
one has to use RC with χ>1.6.
Calculation using the described above methods show that at birefringence larger than
4
108 
 it is possible to get fiber with large dichroism in wide spectral range (100 nm and more)
and with low losses. To enlarge the dichroism it is also possible to apply absorbing/scattering
materials located in quartz cladding [9,10].
Generally, suggested W-fiber combines advantages of two convenient fibers. First of
them is the fiber with core and quartz cladding RI difference equal to Δn13 = n1 – n3 (see Table),
the second one has RI difference Δn12 = n1 – n2. In first fiber, when winding it, one may provide
wide single polarization spectral window, because birefringence against Δn13 is significant value.
But in this case one will not get the desired MFD. In second fiber there is no problem with MFD,
but there is no chance to get wide single polarization spectral window, because birefringence
against large Δn12 is small. Suggested W-fiber has simultaneously a wide spectral window, as the
first fiber, and the opportunity to get desired MFD, as in the second one.
Fabricated from the same W-structure Panda fiber with diameter 90 μm operates as
polarization maintaining (РМ-fiber). In this case dichroism is removed to longer wavelengths,
but simultaneously due to this x-mode losses are sharply reduce. At present time on the base of
described above structure we’ve got PM-fiber samples with losses up to 0.35 dB/km, which is
not so 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
optical signal power minimal level reaching the photodetector when photodetector and prior
amplifier electronic noise is suppressed. Power reserve which was got due to low loss fiber
application could be used to apply other ways of signal additional phase modulation. Also this
reserve could be used for coil winding with several kilometers length, which will improve FOG
sensitivity.

5. 5
As for h-parameter it appeared to be 2×10-5
m-1
. This is good result for birefringence
В=3.4×10-4
, considering that h-parameter depends on B approximately as 2
B . 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). Due to the same reason
sensitivity to twisting is absent, and this also gives to these fibers certain advantages when using
in fiber optic gyroscopes.
Authors are grateful to FIRE RAS 226 laboratory head Ivanov G. A. for help in fiber
fabrication.
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