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Improving the beam quality in LMA fibers by providing a passive ring in the cladding. Emil Voiculescu,Technical University of Cluj, RO, Mircea Hotoleanu, Liekki Oy, FIN, Gabor Csipkes TUCN, RO. The problem.
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Improving the beam quality in LMA fibers by providing a passive ring in the cladding
Emil Voiculescu,Technical University of Cluj, RO,
Mircea Hotoleanu, Liekki Oy, FIN,
Gabor Csipkes TUCN, RO
( n1 – n2 ) / 2.
Example : Depression point position obtained for 30μm-thick core LMA fibers to get sufficient attenuation of the higher order modes
The index profile has to fit the shaded 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) [ %].
First we got that : • Large Index Height in the Ring Does Not Help,• Narrow Ring ( 1 μm or so ) – does not help, as we got :• Many modes scrambled together ( 6 to 8),• Power in modes M2, M3 or M6 comparable with the M1 power, or 2 – 3 times higher. Then : • neither it helps to narrow the groove in the index profile under 1μm, • nor to shrink the ring to the core.
Characteristics : Core diameter 2a = 20 μm, Core index height n1 – n2 = 0.0168
Groove width w = 2μm, Ring index height : Δh = 0.0032, Ring width w = 2 μm
• At the fiber input, all modes have the same power, i.e : 300mW / 6 =50mW;
• At the output end, the most powerful mode rejection ratio is : P1 / Pmax = P1 / P6 = 7.4 dB, to be improved;
• The Mode field diameter over the core diameter is : MFD / Dco = 76.5 % , the effective mode area in M1
over the area of the core Aeff / Aco = 58.5 %.
The dopant concentration was 1.56 x 1026 / m3, as implicit with the simulator
a slightly different regime results →
• This time the rejection of M6 (most powerful higher-order mode) grows to 8.94 dB, which is acceptable;
• The MFD is 13.42 μm for a 20 μm wide core, which gives Aeff / Aco = 45% − to be improved.
► Similar results have been experienced with thicker cores, as for instance with the Yb-1200-25-250DC fiber, having a 25μm diameter core :
▪ Higher order modes have been attenuated to
P1 / Pmax = P1 / P2 = 5.7dB,
▪ MFD / 2a = 15.94μm / 25μm = 63.8%, core coverage
Aeff / Aco = 40.6%.
► By narrowing the ring to 1 μm, M2 raises to a level of (P1− 3dB), which is not acceptable.
► By widening the ring to 4μm, modes M6and M5 take over the
► The same happens by raising up the ring height Δh to 0.004 (instead of the previous value of 0.0032) of the differential index : M6 overcomes M1.
►Great results can be obtained by taking wider cores and placing the ring INSIDE the core. Actually such tries have no practical significance.
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).
The following attempts have been made:
1. The differential index in the core n1 – n2 has been varied among rated 0.00168 (most usual NA = 0.07), and 0.00182 maximum; a flat peak of the higher order modes rejection has resulted.
2. The height of the index difference in the ring Δh, has been varied in accordance with the n1 – n2 adjustments, among 0.003 and 0.0033,
in 10 steps.
NB : Δh = 0.0032
NB : Best index difference in the core n1-n2 = 0.001765
Remarkably, modes M2 and M6 overlap as they change places (positions) exactly on the peak
• This time most powerful higher modes are M2 and
M6, and their attenuation is P1 / Pmax = 9.2 dB.
Empirical criteria suggest that around 10 dB of
attenuation of any mode will do in practice.
• 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%.