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 leastfor one of polarization modes (x�mode). If secondpolarization mode (y�mode) has high level losses, thenwe have a polarizing fiber.

At present, FOG coils employ a conventional two�layered fiber with refractive index (RI) of the corehigher than that of cladding RI by 0.015 and more.This gives one an opportunity to have small enoughfundamental 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 lowerthan 1 dB/km (frequently, they reach 2 dB/km). Thiscan be determined by fundamental mode tunnelinginto stress applying rods, where material losses (120–150 dB/km) take place. High germanium level in thecore also leads to material losses. Further, twisting ofthis kind of fiber also leads to losses, because in thiscase the boundary of stress applying rods and quartzcladding features coupling of the fundamental modewith attenuating higher order modes.

Furthermore, this kind of fiber is absolutely inap�plicable in high level radiation environment. In thiscase, germanium silicate core was suggested to bereplaced by nitrogen�doped one [1], but high nitrogenconcentration led to a problem of residual materiallosses at a wavelength of 1.55 μm due to the nitrogenabsorption 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 manytimes reducing the temperature sensitivity [2],increasing the vibration resistance of the coil, and

almost completely suppressing the Faraday and Kerreffects. Large birefringence [3] along with the absenceof its temperature fluctuations in these fibers allowsone to radically reduce polarization errors in FOGfiber ring interferometers (FRIs). Finally, these fibersare excellently radiation resistant.

However, in practice a lot of basic limitations ofthese fibers are still not overcame: losses are large,polarization characteristics are almost unknown, andcoupling with FRI Y�junction channel waveguides isdifficult (technologically and due to mode fields over�lap, because large�birefringence�fiber air guiding coreis elliptical [3]). To our opinion, this is already enoughto turn to W�fibers [4]. Of course, they have muchmore limited capabilities than microstructured fibers,but they can noticeably improve FRI characteristicsright now.

As a new fiber for sensing coil we propose W�profilePanda�type fiber [5]. All the above�listed kinds ofwaveguide losses in this fiber are extremely small.Material losses in the case of nitrogen�doped corewould be small due to low enough core doping bynitrogen. The polarization characteristics of proposedfiber are at least no worse than those of conventionalfibers and beside this, there is an opportunity to getappreciable polarizing effect (dichroism).

Figure 1 shows the RI profile of initial preform forthe proposed fiber. This fiber has, in addition to thegermanium silicate core, a fluorine�doped reflectingcladding (RC) with depressed RI, quartz (outer) clad�ding, and protective coating. Based on the structuredepicted in Fig. 1, we obtained anisotropic PANDA�type fiber, which cross section photograph is presentedin Fig. 2.

Darker regions correspond to lower RIs. RI valuesin various regions are listed in table.

New Optical W�Fiber Pandafor Fiber Optic Gyroscope Sensitive Coil1

A. M. Kurbatov* and R. A. KurbatovKuznetsov Research Institute for Applied Mechanics, Moscow, 111123 Russia

*e�mail: akurbatov54@mail.ruReceived 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 anydesirable mode field diameter (MFD). The PM�fiber low losses are essentially determined by fundamentalmode 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

1The article was translated by the authors.

790

TECHNICAL PHYSICS LETTERS Vol. 36 No. 9 2010

A.M. KURBATOV, R.A. KURBATOV

We have made two fibers, with diameters 80 and90 μm. Based on the first fiber, we obtained a polariz�ing fiber, and based on the second fiber, we obtained apolarization maintaining fiber with losses amountingto 0.35 dB/km.

Polarizing 500�m long fiber due to its winding into60 mm diameter coil showed x�mode losses of3 dB/km and a dichroism of 30 dB/km. Light sourcespectrum width in these measurements was approxi�mately 20 nm. It is a good result considering that abirefringence value of 3.4 × 10–4 is not very high. Sincewinding this fiber with a diameter larger than 60 mmdid not led to dichroism, it is clear that we have got abend�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 becomesequal 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 largelosses. That is why it is necessary to clarify real mech�anisms of fundamental mode losses. Primarily we willconsider the following two of these:

(1) Fundamental mode radiation tunneling intoexternal quartz cladding and touching the absorbingcoating (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 thatfundamental 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 oflosses approximately to 1.55 μm. Below we will onlyconsider the bending losses.

Most important question is the dependence ofbending losses on the parameter χ = τ/ρ, where τ is theRC radius and ρ is the core radius. According to calcu�lations, RC in our fiber may be arbitrary thin becausethe operation wavelength (1.55 μm) is far from funda�mental mode cutoff threshold (2.2 μm).

Figure 3 shows the behavior of wavelength λ0 atwhich bending losses are 1 dB/km as a function ofparameter χ (this could be named the fundamentalmode 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 theapproach that was developed in [6] for conventionaltwo�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 thefundamental mode of straight fiber, so its attenuationdetermines bending losses).

From Fig. 3, one can see the following. First of all,at all χ the bending cutoff shift does not decreasebelow 1.55 μm. Second, this curve initially goes down,then (in the region 1.3 < χ < 1.42) it has approximatelyconstant level (1.55 μm), and eventually it growsslowly. This behavior of λ0(χ) curve can be explainedby the model of fundamental mode coupling to higherorder attenuated modes. The coupling coefficients ofthese modes monotonically decrease when χ grows.Synchronism of these modes first increases sharplyand prevails over decrease in the coupling coefficients(losses grow), then it increases smoothly (losses do notchange), and eventually it stabilizes (losses decrease).

Third, the MFD at χ < 1.6 sharply grows and thisgives the limitation to RC width from below, so one hasto use RC with χ > 1.6.

Calculations using the methods described aboveshow that, at a birefringence larger than 8 × 10–4, it ispossible 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

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 applyabsorbing/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 fiberwith the RI difference between core and quartz clad�ding equal to Δn13 = n1 – n3 (see table); the secondfiber has the RI difference Δn12 = n1 – n2. In the firstfiber, when winding it, one can ensure wide singlepolarization spectral window, because birefringenceagainst Δn13 is significant. But in this case one will notobtain the desired MFD. In second fiber, there is noproblem with MFD, but there is no chance to obtainwide single polarization spectral window, becausebirefringence against large Δn12 is small. The proposedW�fiber has simultaneously a wide spectral window (asthat in the first fiber) and the opportunity to obtaindesired MFD (as that in the second fiber).

Based on the same W�structure, Panda fiber withdiameter 90 μm operates as polarization maintaining(PM�fiber). In this case, dichroism is removed tolonger wavelengths, but due to this x�mode losses aresimultaneously sharply reduced. At present, based onthe structure described above, we obtained PM�fibersamples with losses amounting to 0.35 dB/km, whichis not far from the 0.2 dB/km limit.

Small losses in FOG coil could be used in differentways. For example, in FOG there is a minimal level ofoptical signal power reaching the photodetector, forwhich photodetector and preamplifier electronicnoise is suppressed. Power reserve which is obtaineddue to low loss fiber application could be used toemploy other methods of additional signal phase mod�ulation. This reserve can also be used for coil windingwith several kilometers length, which will improveFOG 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 purelytechnological problem and it is associated with stressapplying rods doping.

Finally, material losses in stress applying rods arenot larger than several hundredths of dB/km (due tofundamental mode tight confinement in the core). Forthe same reason, sensitivity to twisting is absent, andthis also gives to these fibers certain advantages whenusing in fiber optic gyroscopes.

Acknowledgments. Authors are grateful to FIRERAS 226 Laboratory Head G.A. Ivanov for his help infiber fabrication.

REFERENCES

1. A. L. Tomashuk, K. M. Golant, and M. O. Zabezhailov,Fiber Opt. Technol. Mater. Dev., No. 4, 52 (2001).

2. V. Dangui, H. K. Kim, M. J. F. Digonnet, andG. S. Kino, Opt. Express 13, 6669 (2005).

3. S. O. Konorov, L. A. Mel’nikov, A. A. Ivanov,M. V. Alfimov A. V. Shcherbakov, and A. M. Zheltikov,Laser Phys. Lett. 2 (7), 366 (2005).

4. S. Kawakami and S. Nishida, IEEE J. Quant. Electron.QE�10 (12) (1974).

5. A. M. Kurbatov and R. A. Kurbatov, RF Patentno. 2250482 (Priority from 16.09.03; Register20.04.05).

6. C. Vassallo, J. Lightwave Technol. LT�3, 416 (1985).7. P. L. Francois and C. Vassallo, Appl. Opt. 22, 3109

(1983).8. J. A. Besley and J. D. Love, IEEE Proc. Optoelectron.

144, 411 (1997).9. A. M. Kurbatov and R. A. Kurbatov, RF Patent

no. 2250481 (Priority from 19.05.03; Register20.04.05).

10. A. M. Kurbatov and R. A. Kurbatov, RF Patentno. 2269147 (Priority from 26.05.04; Register27.01.06).

1.0

λ0, μm

χ

1.80

1.8

1.771.741.711.681.651.621.591.561.531.50

10.09.89.69.49.29.08.88.68.48.28.0

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 lossesare 1 dB/km (solid line) and fundamental mode MFD(dashed line) depending on the RC width.