1. 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
Emil Voiculescu

2. The problem
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 prevails. A single mode operation would be ideal.
By playing with the index profile concurrently with the doping profile, a sufficient narrowing of the fiber core has been obtained, associated with substantial attenuation of the higher order modes. As reported at Les Houches meeting, in order to attenuate the higher order modes one have to bypass the D-point underneath.
Emil Voiculescu

3. The D-point
The D-point is a deliberate depression in the index profile around a /2 ( a is the core radius), and having an index height of circa
( n1 – n2 ) / 2.
As the fiber core diameter grows, the D-point descends. For really large core fibers ( 2a ≥ 40 μm ), the index profile should be concave to preserve single-mode operation.
Emil Voiculescu

4. 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
Emil Voiculescu

5. Short recap : 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 = – index of pure silica
n1 = √(NA2+n22) =
n1 – n2 =
Emil Voiculescu

6. Does the index profile depression represent the solution to the problem ?
Not quite
By virtually narrowing the core, the effective mode area drops, and that is not always accepted. With high power applications for sure
That is why we looked towards different ways to reject the higher order modes
Emil Voiculescu

7. Main parameters to deal with
Cladding passive ring
Main parameters to deal with
This possibility has been suggested by Dr M Hotoleanu in May, in order to be implemented at Liekki,
and it became our current task.
It implies a step-index profile, and a flat doping of the core.
Emil Voiculescu

8. We got the following hints :
Try large index heights in the ring ( 6x or so)
Vary the ring thickness b1 – b2
Look for the best position of the ring ( vary width w)
Flat doping is more likely to be used, however playing with the dopant is advisable
Numerical Aperture : around 0.07
(later) : Optimize possible results by trimming nco- ncl
Emil Voiculescu

9. 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) [ %].
Emil Voiculescu

10. Effective Area and Mode-Field Diameter
Emil Voiculescu

11. The setup used for simulation
Chosen Ytterbium Doped fibers : 20μm- and 25μm-core, double-clad fibers,
code Yb DC, provider Liekki Oy
Other data : λs = 1.064μm, Ps = 300 mW, λP = 976 nm, Pp = 30 W
Simulator Used : Recently released LAD 3.3
Emil Voiculescu

12. 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.
Emil Voiculescu

13. This index profile represents a result
Characteristics : Core diameter 2a = 20 μm, Core index height n1 – n2 =
Groove width w = 2μm, Ring index height : Δh = , Ring width w = 2 μm
Emil Voiculescu

14. Power distribution among modes
• 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 %.
Emil Voiculescu

15. As specified in the fiber manufacturer task, a flat doping has been considered :
The dopant concentration was 1.56 x 1026 / m3, as implicit with the simulator
Emil Voiculescu

16. By pushing the ring 1 μm outside the fiber axis
a slightly different regime results →
Emil Voiculescu

17. More distant groove / same 2 μm width
• This time the rejection of M6 (most powerful higher-order mode) grows to 8.94 dB, which is acceptable;
• The MFD is μm for a 20 μm wide core, which gives Aeff / Aco = 45% − to be improved.
Emil Voiculescu

18. Similar results
► Similar results have been experienced with thicker cores, as for instance with the Yb DC 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%.
Emil Voiculescu

19. To summarize :
► 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 M6 and M5 take over the
fundamental mode.
► The same happens by raising up the ring height Δh to (instead of the previous value of ) 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.
Emil Voiculescu

20. 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

21. Next : Finding the best index profile
The following attempts have been made:
1. The differential index in the core n1 – n2 has been varied among rated (most usual NA = 0.07), and 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 and ,
in 10 steps.
Emil Voiculescu

22. The strongest rejection of mode M6 (and M2) gives the necessary index difference in the core n1-n2 =
NB : Δh =
Emil Voiculescu

23. The optimal ring index difference, obtained for a n1-n2 = 0
The optimal ring index difference, obtained for a n1-n2 = step in the core, reads Δh =
NB : Best index difference in the core n1-n2 =
Emil Voiculescu

24. For these quantities the following power distribution among modes results:
Remarkably, modes M2 and M6 overlap as they change places (positions) exactly on the peak
Emil Voiculescu

25. Conclusion • 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 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

26. A top-view might give a qualitative idea about the core coverage
Emil Voiculescu

27. Also the axial power distribution is computed as in the following diagram showing a peak power density of 5 mW /μm2
Emil Voiculescu

28. Lower levels of power densities are shown in the strongest higher-order modes
Emil Voiculescu

29. Conclusions
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
The cladding ring procedure is of interest for Liekki, and it is very likely that it would be put into production soon
This material will be reported to the next Photonic West Conference
Emil Voiculescu

30. Thank you !
Emil Voiculescu