Emil Voiculescu 1

Moat Fibers RevisitedLMA Fibers Revisited

Emil VoiculescuTechnical University of Cluj Romania

Emil Voiculescu 2

Previously Reported

1. The LMA fiber having a High-index Ring in the cladding

presented in Naples, and also being reported at the

Photonic West Conference 20081

2. The moat-fiber having a lower refractive index in the

cladding : presented by M Hotoleanu in Naples

Emil Voiculescu 3

Short Recap : 1. the High-Ring type

a. Index profile b. Flat doping of the core

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Power Gain Along the Fiber

Chosen Ytterbium Doped fibers : 20μm- and 25μm-core, double-clad fibers,

code Yb 1200 -25 -250DC, provider Liekki Oy

OK !

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Most powerful higher order

modes are M2 and M6, and their

attenuation is

P1 / P2 = P1 / P6 = 9.2 dB.

The MFD is 14.72 μm for a

20 μm diameter of the core,

meaning that

MFD / 2a = 73.6%.

The normalized effective

area is Aeff / A co = 54.2%

Main results

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2. The Fiber reported in Naples by M Hotoleanu

has been called ‘Moat’ because of the depressed index in the SiO2 ring

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The Preform Index Profile as practically determined at Liekki

However, the tentatively recommended core index differential n1 – n2 = 0.00568, with 0.003 height in the cladding (M Hotoleanu) did not fit well.

We looked for appropriate values in a ‘try and error’ systematical manner, and eventually got the optimal parameters (next).

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Input data to simulate the Moat Fiber

Index profile leading to a quality beam

i.e. to sufficient mode discrimination

Radial doping, as flat doping cancels mode-discrimination

NB : With flat doping mode-power characteristics are overlapping or, even worse, higher-order modes (strongly) prevail

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The setup used for simulation, and the input data

Ytterbium Doped fibers 20μm- and 25μm-core, double-clad fibers,

code Yb 1200 -25 -250DC, provider Liekki Oy

• Other data : λs = 1.064μm, Ps = 300 mW, λP = 976 nm, Pp = 30 W

• Simulator Used : LAD 3.3 of Liekki Oy

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By using the characteristics previously shown, the followingPower distribution among modes results

NB : Playing with the index differential / doping, the combination in slide 8 seems to be optimal : 10logP1 / P8 =9.63dB.

OK

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Slight variation of the index differential and doping, is possible

However, a radial doping encouraging the fundamental mode (right) is necessary.

That means that virtually one use a narrower core.

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Previous data make power in the fundamental mode prevail

10logP1/P8=9.62dB

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Comments

One problem is the effective coverage of the core :

MFD / Dco = 58 % , Aeff / Aco = 33.7% – the numbers are not high enough.

However, the same happens for a plain step-index fiber doped radialy, so, by placing the cladding ring, discrimination took place, and the fundamental mode remained comparatively the same.

The same happens when the fiber is coiled in order to leak out the

higher-order modes. If that is acceptable, the present result is better,

because it does the same without coiling the fiber.

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As flat doping is not working with the moat-fiber,

a doping favoring the fundamental mode might look like that :

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With power / mode distribution still good

10logP1/P8=8.63dB

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Getting closer to the flat doping, the attenuation of the

higher-order modes drops (next) : 10logP0/P8=5.72dB.

Double-step doping characteristic

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Mode-power along the fiber with the previous index / dopant characteristics

MFD/2a = 57.8%

Aeff/Aco = 33.4%

– 5.72dB

0 dB

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In order to preserve a quality beam, one have to depress doping

towards the core-cladding interface

Radial doping, a step- or double-step characteristic, even

triangular doping, basically represent the same : a measure to favor

the fundamental mode against the higher-order modes

By ‘modulating’ the doping profile a virtual thinner core is generated,

so the effective area, and correspondingly the MFD have to be

maximized

Facts regarding moat fiber #2(the core more refringent than the cladding ring)

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Simulation of larger core moat-fibers

Liekki Ytterbium Doped 25μm-core, double-clad fiber, code Yb 1200 -25 -250DC, radialy doped.

a=12.5μm

12.5μm

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As the diameter of the LMA fiber grows, a multimode operation is always more likely to happen

Beam quality being of interest, it would be better that the fundamental mode strongly prevail.

10logP1/P2=4.77dB

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A linear- or triangular-doping would favor the axial modes

and improve the mode power distribution (next).

12.5μm

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To be improved :

10logP1/P2=6.57dB

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30μm-large core

a. Index profile

c. Mode-power distribution

10logP1/P4=5.94dB

Doping profile : linear

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Next : Liekki’s original moat-fiber simulated

a. Index profile recommended by the manufacturer

b. Radial doping

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Simulation result : just three modes, mode M2 being attenuated with 3.67dB

Conclusion : this combination of index / doping is not practical.

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Conclusions to fiber #2By playing with the doping profile concurrently with the imposed moat pattern of the index profile, while maintaining a core more refringent than the ring, a sufficient narrowing of the fiber core has been obtained, associated with substantial attenuation of the higher order modes.

The optimisation done could be really profitable if the core coverage

( MFD, Aeff ) in the fundamental mode would be higher.

However, the core coverage is not worse than that obtained when higher order mode rejection is done by coiling the fiber.

As the diameter of the LMA fiber grows, it is more difficult to reject / to attenuate the higher order modes. It seems that the moat fibers investigated (20 – 30 μm of core diameter) are easier to deal with. However, a new approach / design is possible.

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High Index Cladding Ring

The moat fiber #1 for which the cladding ring is more refringent than the core is

shortly reconsidered here because of its better performances

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Main parameters to deal with

High Index Cladding Ring

This possibility implies a step-index profile, and a flat doping of the core

To be implemented at Liekki

It has been successfully reported at Photonic West 2008

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Best index profile

The strongest rejection of most powerful higher-order modes M6 and M2

gives the necessary index difference in the core n1 – n2 = 0.001765

The optimal ring index difference, obtained for a n1– n2 = 0.001765 step

in the core, is Δh = 0.00317

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For these quantities the following power distribution among modes results:

10logP1/P2=10logP1/P6=9.2dB Modes M2 and M6 overlap

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Top-view giving a qualitative idea

about the core coverage

The MFD is 14.72 μm for a 20 μm diameter of the core, meaning that MFD / 2a = 73.6%.

The normalized effective area is

Aeff / A co = 54.2%

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Transverse cross-section of the power ‘bell’, as provided by the simulator, shows the light distribution in the core

Power density in M1

0

1

2

3

4

5

6

-42 -25.2 -8.4 8.4 25.2 42

The axial power distribution of the fundamental mode M1 shows a peak power density of 5 mW /μm2.

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Eventually, the case when the ring sticks to the core:

A modest result : 5 modes, less than 6dB attenuation of the most powerful mode, MFD = 13.6μm,MFD / 2a = 0.68, Aeff / Aco = 46%.

Technologically not attractive (difficult).

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Results and conclusions to this fiber

A passive ring in the cladding is of great help in rejecting the higher-order modes, and this method can be applied to a large range of LMA fibers. Best results are achieved for core diameters in the range from several microns to 20-25μm

By slightly sliding the ring toward the cladding ( or toward the fiber axis) significant changes take place :

► A ring closer to the core provides a higher effective area

► A more distant ring might increase the higher order modes rejection,

but that comes at the price of lower effective area

Anyway, the coverage of the core area is 1.6 times higher than the one obtained with the lower index ring !

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Perspective / Future work

Result intercomparison with the other participants that have

simulated the moat fibers :

• Dr Jacek Olszewski, Wroclaw University of Technology

• Prof Stefano Selleri, University of Parma

If compatibility / complementarity of the results are of interest, a conference paper would be possible

Simulation of different LMA fibers as those circulated through Liekki round-robin and comparison with experimental results

(Prof Manuel Lopez Amo, Prof Lopez Higuerra, Dr Mathieu Legre)

could be done

Measurements of the moat fibers at Liekki – if these fibers would

be put into fabrication

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References

• References1. Improving the beam quality in LMA fibers.

Emil Voiculescu, Technical University of Cluj-Napoca, Romania et al.

Conference of Integrated Optics, Materials and Technologies (XII).

Paper # 6896-55, SPIE Photonic West 2008.

San Jose Convention Center, CA, USA, Jan 23, 2008.

2. FIDES, European Project COST 299 : Optical Fibers for New Challenges

Facing the Information Society. Memorandum of Understanding,

www.cost299, 2006.

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Acknowledgement

I am grateful to the following co-workers for helping with various

simulations : student Bogdan Ghete, whose graduation project

deals with LMA fibers and assist-prof Csipkes Gabor.

I am grateful to Dr M Hotoleanu and Liekki Oy for providing me with the

fiber data needed, and with the LAD software repeatedly.

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Glossary of Terms

Main parameters of interest

n – the refractive index

n1 – n2 − profile height

Δ = ( n1 – n2 ) / n1 ≤ 1 %

NA = √(n12 – n22) ≈ 0.07

Δ = NA2/2n12

n2 = nSiO2 = 1.4573 – index of

pure silica

n1 = √(NA2+n22) = 1.45898

n1 – n2 = 0.00168

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The mode effective areaThe scalar wave equation

contains – the scalar field function for the fundamental mode, the free-space wave number k = 2π/λ, the propagation constant β and the refraction index profile n(r).

• The spot radius , also called effective modal spot size , is :

and the LAD gives all data to compute it. • The Effective Area is and

• the Mode Field Diameter is .Mode effective area to core area ratio might be called the normalized

effective core ( or normalized coverage) [ %].

01 222

2

2

rnk

dr

d

rdr

d

0

20

22 2

drrdrd

drrw

effw____

w

effwMFD__

2

2__

effeff wA

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Thank you !