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Transcript of Emil Voiculescu 1 Moat Fibers Revisited LMA Fibers Revisited Emil Voiculescu Technical University of...
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
Emil Voiculescu 4
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 !
Emil Voiculescu 5
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
Emil Voiculescu 6
2. The Fiber reported in Naples by M Hotoleanu
has been called ‘Moat’ because of the depressed index in the SiO2 ring
Emil Voiculescu 7
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).
Emil Voiculescu 8
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
Emil Voiculescu 9
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
Emil Voiculescu 10
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
Emil Voiculescu 11
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.
Emil Voiculescu 12
Previous data make power in the fundamental mode prevail
10logP1/P8=9.62dB
Emil Voiculescu 13
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.
Emil Voiculescu 14
As flat doping is not working with the moat-fiber,
a doping favoring the fundamental mode might look like that :
Emil Voiculescu 15
With power / mode distribution still good
10logP1/P8=8.63dB
Emil Voiculescu 16
Getting closer to the flat doping, the attenuation of the
higher-order modes drops (next) : 10logP0/P8=5.72dB.
Double-step doping characteristic
Emil Voiculescu 17
Mode-power along the fiber with the previous index / dopant characteristics
MFD/2a = 57.8%
Aeff/Aco = 33.4%
– 5.72dB
0 dB
Emil Voiculescu 18
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)
Emil Voiculescu 19
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
Emil Voiculescu 20
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
Emil Voiculescu 21
A linear- or triangular-doping would favor the axial modes
and improve the mode power distribution (next).
12.5μm
Emil Voiculescu 22
To be improved :
10logP1/P2=6.57dB
Emil Voiculescu 23
30μm-large core
a. Index profile
c. Mode-power distribution
10logP1/P4=5.94dB
Doping profile : linear
Emil Voiculescu 24
Next : Liekki’s original moat-fiber simulated
a. Index profile recommended by the manufacturer
b. Radial doping
Emil Voiculescu 25
Simulation result : just three modes, mode M2 being attenuated with 3.67dB
Conclusion : this combination of index / doping is not practical.
Emil Voiculescu 26
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.
Emil Voiculescu 27
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
Emil Voiculescu 28
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
Emil Voiculescu 29
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
Emil Voiculescu 30
For these quantities the following power distribution among modes results:
10logP1/P2=10logP1/P6=9.2dB Modes M2 and M6 overlap
Emil Voiculescu 31
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%
Emil Voiculescu 32
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.
Emil Voiculescu 33
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).
Emil Voiculescu 34
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 !
Emil Voiculescu 35
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
Emil Voiculescu 36
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.
Emil Voiculescu 37
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.
Emil Voiculescu 38
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
Emil Voiculescu 39
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
Emil Voiculescu 40
Thank you !