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
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
a. Index profile b. Flat doping of the core
has been called ‘Moat’ because of the depressed index in the SiO2 ring
NB :Playing with the index differential / doping, the combination in slide 8 seems to be optimal :10logP1 / P8 =9.63dB.
However, a radial doping encouraging the fundamental mode (right) is necessary. That means that virtually one use a narrower core.
MFD / Dco =58 % , Aeff / Aco = 33.7% –the numbers are
not high enough.
so, by placing the cladding ring, discrimination took place, and the
fundamental mode remained comparatively the same.
higher-order modes. If that is acceptable, the present result is better,
because it does the same without coiling the fiber.
MFD/2a = 57.8%
Aeff/Aco = 33.4%
(the core more refringent than the cladding ring)
towards the core-cladding interface
triangular doping, basically represent the same : a measure to favor
the fundamental mode against the higher-order modes
so the effective area, and correspondingly the MFD have to be
Liekki Ytterbium Doped 25μm-core, double-clad fiber, code Yb 1200 -25 -250DC, radialy doped.
Beam quality being of interest, it would be better that the fundamental mode strongly prevail.
and improve the mode power distribution (next).
Doping profile : linear
c. Mode-power distribution
b. Radial doping
Conclusion : this combination of index / doping is not practical.
( MFD, Aeff ) in the fundamental mode would be higher.
High Index Cladding Ring
10logP1/P2=10logP1/P6=9.2dB Modes M2 and M6 overlap
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%
The axial power distribution of the fundamental mode M1shows a peak power density of 5 mW /μm2.
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).
► 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
simulated the moat fibers :
• Dr Jacek Olszewski, Wroclaw University of Technology
• Prof Stefano Selleri, University of Parma
(Prof Manuel Lopez Amo, Prof Lopez Higuerra, Dr Mathieu Legre)
could be done
be put into fabrication
deals with LMA fibers and assist-prof Csipkes Gabor.
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) [ %].