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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.

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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

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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).

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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 4108 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.

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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 2B . 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.

References

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