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1. Radiation resistant fibers with depressed claddings for fiber optic gyro
sensing coil
Kurbatov A.M1
., Kurbatov R.A.
Department of Center for terrestrial space infrastructure objects exploiting
Kuznetsov Research Institute for Applied Mechanics,
55 Aviamotornaya St., Moscow 111123, Russia.
ABSTRACT
A two kinds of single mode optical fiber are described: three-layer with depressed fluorine clad (W-fiber) Panda
with Nitrogen core, and isotropic fiber with pure (undoped) quartz core and two fluorine claddings (narrow and wide).
Both of these fibers could be used in fiber optic gyro sensing coil operating in space radiation environment.
Keywords: single-mode optical fibers, radiation resistance.
1. INTRODUCTION
Radiation induced losses in fiber optic gyro (FOG) sensing coil fiber are one of the general problem for FOG.
Solutions are known applying fibers with pure (undoped) silica core1,2
or with Nitrogen core2
.
Earlier we described a Ge-doped core fiber3
based on widely known refractive index (RI) W-profile4
, having a
specifically narrow and deep depressive fluorine cladding and two kinds of fiber for FOG sensing coils basing on this
profile: polarization maintaining (РМ) fiber with low material/bending losses and polarizing (PZ) fiber. One of the basic
reasons of these fibers improved characteristics is a fundamental mode tight packing in the physically uniform core,
when this mode is under much more weak influence of non-uniformities an the core and fluorine clad boundary and at
the boundaries of stress-applying rods. Beside this, a material losses in these rods is eliminated which in convenient
fibers may be significant5
. Finally, according to calculations, these fibers should be effective modal filters.
Further, it is known that Nitrogen-core fiber is radiation-resistant2
. Thus changing the Ge-doped core in earlier
described3
W-fiber to a Nitrogen core should make this fiber also to be radiation-resistant. In present work a first sample
of such Nitrogen Panda fiber is described.
Also basing on earlier described3
W-profile a construction is created with pure silica core and two fluorine
claddings (narrow and wide) which may give one more kind of radiation-resistant fiber for FOG sensing coil. First
sample of such kind of fiber is also described in the present work.
Note that all related to this work calculations were carried out using mathematical models which are
schematically described by us earlier3
. For additional modal filtering estimation we also used one more model for
asimutally symmetric W-fibers with coating having high RI (1.54). Here we also starting from scalar wave equation.
Coating is put to have infinite diameter and the field within it is a cylindrical outgoing wave. In this case a propagation
constant of any mode is a complex number with imaginary part characterizing the propagation losses of this mode.
2. NITROGEN-CORE W-PROFILE PANDA-FIBER.
On Fig. 1 (a) and (b) are shown: three RI profiles in W-fiber preform having a Nitrogen core (beginning, middle
and the end of preform) and cross section photograph of Panda fiber drawn from it. Preform was manufactured by
“Fiberus” company6
specialists (Moscow) according to profile calculated by the present work authors.
1
E-mail: akurbatov54@mail.ru

2. Fig. 1(а). W-profiles of Nitrogen core fiber preform.
Fig. 1(b). Nitrogen core Panda W-fiber cross section photograph.
From Fig. 1(a) it is clear that RI changes in preform cross section are large enough, so they could be reached only by
SPCVD-methid2
. A dip in the profile center reduces a single-mode region comparing with earlier described3
Ge-core
fibers because it reduces fundamental mode effective RI stronger that that of the second-order mode. Thus while having
the same fundamental mode bending losses a second-mode cutoff wavelength in fiber with dip will be higher than that in
the the fiber without dip in the RI profile. However according to calculations modal filtering at the operating wavelength
(1.55 μm) should be effective enough. Dip also increases a fundamental mode field diameter (MFD) due to its field
distortion but still this MFD is lower than the core diameter which means a tight mode packing in it. The drawn fiber
measured standard characteristics are listed in Table.
Further it is possible to noticeably improve waveguide parameters of
described fibers type. Beside this in described fiber a Nitrogen content in the
core is considerably lower than in high-aperture Nitrogen fibers from original
paper2
from which it is also possible to conclude that this is an advantage at
least in the “slow” space radiation environment.
3. PURE-SILICA CORE ISOTROPIC FIBER WITH DOUBLE FLUORINE CLAD.
Another kind of radiation-resistant fibers are the fibers with pure silica core and wide fluorine cladding1,2
. Here
it is possible to combine the low bending losses and effective modal filtering. In this case the core should be single mode
and the fluorine cladding should be seven and more times wider than the core. However in this case fundamental mode is
always not tightly packed in the core (see above). Also according to the calculation here it is at least very difficult to
reach a noticeable dichroism.
On the Fig. 2(a) a three RI profiles are presented in the beginning, middle and in the end of preform for new
fiber which also has a pure silica core but two fluorine claddings (narrow and wide). On Fig. 2(b) a cross section
photograph of drawn from it first isotropic fiber sample is presented having the diameter 200 μm (profile 1 on Fig. 1(a)).
This preform is also manufactured by “Fiberus” company specialists (Moscow) according to profile calculated by the
present work authors.
Table.
Parameter Value
Fiber diameter (μm) 110
Insertion losses (dB/km) 1.8
h-parameter (1/m) 2·10-5
Linear birefringence 3·10-4
Cutoff (μm) 1.42

3. Fig. 2(a). RI profiles in preform for pure silica core fiber at two ends (1 and 3) and in the middle (2).
Fig. 2(b). Isotropic fiber first sample cross section (with diameter 200 μm) drawn from the preform part having profile 1 on
Fig. 2(a).
Drawn fiber diameter (200 μm) could be reduced to 80 μm by removing the part of external quartz cladding without
noticeable influence on fiber optical parameters. This fiber has the core diameter 9 μm, material losses 0.75 dB/km (as it
is shown by spectral losses measurements in this fiber a water concentration (losses peak at 1.39 μm) is large, and bend
losses become noticeable only at loop diameter less than 10 mm. Further, MFD calculated value is 7.6 μm which is
against the 9-μm core means fundamental mode tight packing in it. Generally, basing on such kind of RI profile a fibers
are possible with MFD in the region 6-15 μm which are bend resistant and effective high order modes filters.
Described earlier3
W-fiber is bend resistant because its fundamental mode mathematical cutoff7
is
approximately equal to 2.2 μm. At the same time fundamental mode sharp spectral losses growth (physical cutof) in a
straight 1-m fiber, according to calculations, should begin near 2.1 μm, and in the straight 1000-m fiber near 1.8 μm.
Thus fiber bend resistance is guaranteed by the distance of these values from operating wavelength (1.55 μm). The same
could be said about Nitrogen core W-fiber described above.
In the fiber on Fig. 2(b) role of «fundamental» mode is played by one of the modes of overall fiber cross
section. It has practically the same field form as the fundamental mode of W-fiber3
, but in this case it is always under the
mathematical cutoff (as the rest of the modes). This is what differs this fiber from W-fiber3
and from Nitrogen core W-
fiber. However as for “fundamental” mode physical cutoff its threshold in the fiber from Fig. 2(b), according to
calculations, is approximately equal to 2.2 μm (1-m fiber) and 1.8 μm (1000-m fiber), i.e. there is no difference from W-
fibers3
from this point of view. Due to this fact fiber from Fig. 2(b) is also bend resistant in spite of the fact that its core
RI due to fluorine penetration is even slightly lower than hat of the quartz external cladding.
Generally, according to calculations for fixed core and first fluorine cladding it is possible to apply a lot of
second fluorine cladding kinds some of which are presented at Fig. 3, giving the same “fundamental” mode bending loss.
1 2 3

4. Fig. 3. RI profiles giving the same bending loss.
Here a W-profile3
is shown (profile 1). We carried out a comparative calculations for profiles 1-4 from the point of view
of dichroism and high order modes filtering. For the first case profile 1 is most preferable and profile 4 is least
preferable. In the second case of mode filtering situation is different: moving from 1 to profile 4 mode filtering, starting
from large enough efficiency for profile 1, then grows to even larger values, then reaches maximum (profile 2), and after
that begin to monotonically decrease coming to be very inefficient at the profile 4 (there is no contradiction with above
statement about possible effective mode filtering in the case of single wide fluorine cladding because above we made an
accent that core itself should be single mode and all the profiles of Fig. 3 have few-mode core).
Returning to Fig 2(a) one may see that all its three profiles are close enough to profile 2 on Fig. 3. Further it is
planned to manufacture Panda fibers basing on these profiles.
4. CONCLUSION.
In conclusion, we described two kinds of fiber based on well-known refractive index (RI) W-profile: 1)
Polarization maintaining (PM) Panda fiber with Nitrogen core and fluorine depressed clad; 2) Isotropic fiber with pure
silica core and two fluorine claddings (narrow and wide). The latter should give the good base for polarizing (PZ) pure-
silica core fiber unlike the conventional pure-silica core fiber with single wide fluorine depressed cladding. Both of these
fibers are bend resistant and according to literature data should be radiation resistant, so they may be used in fiber-optic
gyro sensing coil under space radiation environment.
5. AKNOWLEGMENTS.
Authors are grateful to Golant K. M. for preform manufacturing for fibers described in the present work.
REFERENCES.
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[2] Tomashuk A.L., Golant K.M., “Radiation-resistant and radiation-sensitive silica optical fibers,” Proc. SPIE, 4083,
188-201 (2000).
[3] Kurbatov A.M., Kurbatov R.A., “New optical W-fiber Panda for fiber optic gyroscope sensitive coil,” Technical
Physics Letters, 36(9), 789-791 (2010).
[4] Kawakami S., Nishida S., “Characteristics of a doubly clad optical fiber with a low-index inner cladding,” IEEE
Journal of Quantum Electronics, 10(12), 879-887 (1974).
[5] Tajima K., Sasaki Y. J., “Transmission loss of a 125-μm diameter PANDA fiber with circular stress-applying parts,”
Journal of Lightwave Technology, 7(4), 674 (1989).
[6] http://www.fiberus.ru/about.php
[7] M. Monerie, “Propagation in doubly clad single-mode fibers,” IEEE Journal of Quantum Electronics, 18(4), 535-542
(1982).
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