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LMA Fibers Revisited Emil Voiculescu Technical University of Cluj Romania. Moat Fibers Revisited. 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 2008 1

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moat fibers revisited
LMA Fibers

Revisited

Emil Voiculescu

Technical University of Cluj Romania

Moat Fibers Revisited

previously reported
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

short recap 1 the high ring type
Short Recap : 1. the High-Ring type

a. Index profile b. Flat doping of the core

power gain along the fiber
Power Gain Along the Fiber

OK !

  • Chosen Ytterbium Doped fibers : 20μm- and 25μm-core, double-clad fibers,
  • code Yb 1200 -25 -250DC, provider Liekki Oy
slide5
Main results
  • 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%
2 the fiber reported in naples by m hotoleanu
2. The Fiber reported in Naples by M Hotoleanu

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

the preform index profile as practically determined at liekki
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).
input data to simulate the moat fiber
Input data to simulate the Moat Fiber
  • 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
  • Index profile leading to a quality beam i.e. to sufficient mode discrimination
the setup used for simulation and the input data
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
by using the characteristics previously shown the following power distribution among modes results
By using the characteristics previously shown, the followingPower distribution among modes results

OK

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

slight variation of the index differential and doping is possible
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.

comments
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.

slide14
As flat doping is not working with the moat-fiber, a doping favoring the fundamental mode might look like that :
mode power along the fiber with the previous index dopant characteristics
Mode-power along the fiber with the previous index / dopant characteristics

0 dB

MFD/2a = 57.8%

Aeff/Aco = 33.4%

– 5.72dB

slide18
Facts regarding moat fiber #2

(the core more refringent than the cladding ring)

  • 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

simulation of larger core moat fibers
Simulation of larger core moat-fibers

a=12.5μm

12.5μm

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

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

10logP1/P2=4.77dB

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

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

12.5μm

and improve the mode power distribution (next).

to be improved
To be improved :

10logP1/P2=6.57dB

30 m large core
30μm-large core

10logP1/P4=5.94dB

  • Index profile

Doping profile : linear

c. Mode-power distribution

next liekki s original moat fiber simulated
Next : Liekki’s original moat-fiber simulated
  • Index profile recommended by the manufacturer

b. Radial doping

simulation result just three modes mode m 2 being attenuated with 3 67db
Simulation result : just three modes, mode M2 being attenuated with 3.67dB

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

conclusions to fiber 2
Conclusions to fiber #2
  • By 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.
slide27
High Index Cladding RingThe moat fiber #1 for which the cladding ring is more refringent than the core is shortly reconsidered here because of its better performances
main parameters to deal with
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
best index profile
Best index profile
  • The strongest rejection of most powerful higher-order modes M6 and M2gives 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
for these quantities the following power distribution among modes results
For these quantities the following power distribution among modes results:

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

top view giving a qualitative idea about the core coverage
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%

slide32
Transverse cross-section of the power ‘bell’, as provided by the simulator, shows the light distribution in the core

The axial power distribution of the fundamental mode M1shows a peak power density of 5 mW /μm2.

eventually the case when the ring sticks to the core
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).

results and conclusions to this fiber
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!
perspective future work
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

references
References
  • 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.
  • References
acknowledgement
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.
glossary of terms
n – the refractive indexGlossary of Terms

Main parameters of interest

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

the mode effective area
The mode effective area

The 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) [ %].

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