Emil Voiculescu 1

Single-mode Operation of LMA Fibers

by using a new profile structure

Emil Voiculescu

Technical University of Cluj, RO

Madeira, 3 – 5 Sep 2008

Emil Voiculescu 2

Previously Reported

Les Houches : ► Index Depression helps with high-order mode

discrimination

Naples : ► The LMA fiber having a High-index Ring in the cladding,

also having been reported at the Photonic West Conference 20081,

► The Moat-fiber having a lower refractive index in the cladding, presented by M Hotoleanu in Naples.

They work up to 20 – 25 m of core diameter.

Berlin : ► Results Intercomparison :

High RI better than Lower RI (Voiculescu), both can be used

(S Selleri3 ), Lower RI is preferable ( J Olszewski6 )

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Madeira, Sep 2008

► Newly developed solution solves the problem of higher

order modes rejection concurrently with magnifying the light

coverage of the core (effective area in the fundamental mode/

MFD)

► The method can be applied to larger-core fibers, up to

40m and more

► The doping can be flat, there is no need to diminish the

peripheral doping to help with higher-order modes attenuation

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Short Recap

in order to explain how the idea

has been developed

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Take the best result reported in Berlin :

Single ring having a higher refractive index than the core

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And the conclusions

as they have been reported there 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 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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For the moment the new arrangement cannot be publicized, however :

The newly-developed model works okay, meaning it virtually operates single-mode in certain conditions, all superior modes being substantially attenuated, and

The effective area can be sufficiently magnified

This way a new structure has been conceived

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The setup used for simulation

• Chosen Ytterbium Doped fibers : 25μm- and 30μm-core, double-clad fibers, code Yb 1200 -25 -250DC, provider Liekki–nLight, experimental

larger-core fibers of 40μm and more, up to 80μm

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

• Simulator Used : Liekki Application Designer LAD 3.3

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Example of the novel Yb LMA Fiber

Index profile for a 30 m-core fiber leading to drastic attenuation of higher-order modes despite the flat doping used ( right-side figure)

Geometry of the profile – cannot be disclosed yet

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Main result showing a 12.9 dB attenuation

of all unwanted modes

NB : Because of the flat doping a strand of only 1.5m of doped fiber is sufficient to reach the maximum gain

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Side view of the fundamental mode

shows the core coverage

MFD/ 2a = 59.2%, Aeff /Acore = 35.4%

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Side view of the M2 power distribution along

the fiber axis shows radial spreading of M2

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Cross-section of M2 power distribution shows M2 peaks outside the core

(evanescence of M2 obvious)

Power density in M2

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

-138,159 -82,8954 -27,6318 27,6318 82,8954 138,159

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Intercomparison :

Similar behavior obtained by Mircea Hotoleanu

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► In order to optimize the index profile, many

parameters had to be varied concurrently.

► Finding the optimal single-mode operation is a

tough job as it regards almost all the

parameters in the index characteristic.

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► Anyway, a systematic approach has been carried out and the previous experience

with LMAs helped to shorten the try-and- error procedure

► The basic (expected) functionality seems to relate the high-order mode rejection mainly with the outer portion / layer, and the core coverage mostly with the inner structure

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► For this class of fibers, and for the following 40m-core fibers, only the flat doping has been considered. This is one of the great advantages that the new topology offers : to emulate a virtual single-mode fiber we don’t have to decrease the peripheral doping. One also can get a larger gain with a shorter strand of fiber this way.

General remarks

Dopant concentration : 1.56 • 1025 / m3

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More than that :

► A significant attenuation of the higher-order modes can be reached without taking special measures. Usually a ≥10dB attenuation of the total power in the higher-order modes can be accepted.

Then, if we can provide more, why not decrease the attenuation, until the Mode-Field Diameter reaches a maximum ? This strategy has been applied for sizing the considered fibers ( 30m- and 40m-core fibers).

► In order to obtain the largest effective area, i.e. maximum MFD, a thin moat resulted.

Because of that, all other geometries are less performant.

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The Etalon

►As the coverage of the core with light should be as high as possible one need a comparison criterion. By taking a reference step-index fiber with the same core-diameter (and same cladding diameter), and assuming the same flat doping, we might say that reaching the same effective area in the fundamental mode can be accepted.

►In fact, that coverage is well surpassed every

time, usually by 1.5 times.

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1. Core Diameter 30 m

Significant Results

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First index profile Power distribution among modes

Attenuation of the most powerful mode ( M9) is : 10log(P1/Pmax) = 10.2dB

MFD/2a = 63.7%

Aeff/Acore = 40.6%

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Second Index Profile Power distribution among modes

10log(P1/Pmax) ≈ 9 dB

10log(P1/Ptotal) ≈ 10log(P1/P9+P6)= 10log(17000/2950) ≈ 8 dB

MFD/2a = 77.9% ≈ 80%

Aeff/Acore = 60.7%

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MFD/2a = 63.7%

Side view of the fundamental mode

demonstrates the improved core coverage

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Also the top view of the fundamental

mode confirms core coverage

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What we got ?

► 10 dB of higher-order modes

attenuation or so, which will do with practical

applications, so we may indulge in trading the

attenuation for a larger effective area

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Top-view picture proves that the most powerful mode leaks out of the core ( in between –15m and +15m → red, i.e. zero Watts )

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2. Core Diameter 40 m

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The reference model (the etalon) first

Step-index fiber Flat doping

Mode power distribution along the fiber shows the usual mode

’scrambling’ as no measure for counteracting that has been taken

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The fundamental mode

MFD / 2a = 51.7 %

Aeff / Acore = 26.7 %

Side view

Cross section

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The proposed fiber

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And the results

Attenuation of the most powerful mode : 10 log(P1/P9) = 10.98 dB

Attenuation of all superior modes : 10 log (P1/PTotal) = 9.33 dB

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Mode field diameter and Effective area

MFD / 2a = 65.4 %

Aeff / Acore = 42.7 %

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Conclusions► The novel fiber structure presented here solves the key problem

associated with LMA fibers : the beam quality.

► This way we can modify a multimode fiber in a quasi-single mode one.

► Compared with the previously developed single-ring fibers, this one offers an extra degree of freedom

► The structure works fine with 40 m cores. It cannot be easily applied to experimental 80m-core LMA fibers or thicker core fibers. Going beyond

25 m of core diameter makes single-mode operation problematic.

► If the experimental results will confirm the simulation results, then such fibers will be manufactured. It is very likely that some samples will be tested soon. A round-robin to confirm performance by measurement could naturally follow the modelling/simulation activity.

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References 1. Improving the beam quality in LMA fibers. E Voiculescu, M Hotoleanu, Csipkes G,

Photonic West Conference 6896, San Jose, CA, Jan 23 2008.

2. Study regarding the Index and Doping profiles in LMA fibers for maximum optical power in the fundamental mode. Graduation Thesis 2008, B Ghete, Technical University of Cluj, RO.

3. Moat fiber finite element method. F Poli, D Passaro, A Cicinotta, S Selleri, University of Parma, COST Presentation, Berlin, Jan 29 2008.

4. Single-mode regime in large mode area moat fiber. D. Passaro, F. Poli, A. Cucinotta, S. Selleri, submitted to ECOC 2008.

5. Large mode area optical fibers for high power amplification. S Selleri, A Cucinotta, F Poli, D Passaro, COST Presentation, Madeira, 2008.

6. Modes in moat fiber. Numerical modeling. J Olszewski, Wroclaw University of

Technology, COST Presentation, Berlin, 2008.

Acknowledgement

I am grateful to student Bogdan Ghete for his work to the topic

through his graduation thesis that I advised, and which got the

maximum score and the ‘Cum Laudae’ apreciation from the

Graduating Board June 2008.

I am grateful to Dr M Hotoleanu and Liekki– nLight for revising

this material, providing me with the fiber data needed, and with

the LAD 4.1 software.

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Thank you !