Moat Fibers Revisited

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

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

3. Short Recap : 1. the High-Ring type a. Index profile b. Flat doping of the core

4. 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

5. 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%

6. 2. The Fiber reported in Naples by M Hotoleanu has been called ‘Moat’ because of the depressed index in the SiO2 ring

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).

8. 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

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

10. 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.

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.

12. Previous data make power in the fundamental modeprevail 10logP1/P8=9.62dB

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.

14. As flat doping is not working with the moat-fiber, a doping favoring the fundamental mode might look like that :

15. With power / mode distribution still good 10logP1/P8=8.63dB

16. Double-step doping characteristic Getting closer to the flat doping, the attenuation of the higher-order modes drops(next) : 10logP0/P8=5.72dB.

17. Mode-power along the fiber with the previous index / dopant characteristics 0 dB MFD/2a = 57.8% Aeff/Aco = 33.4% – 5.72dB

18. 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

19. 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.

20. 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.

21. A linear- or triangular-doping would favor the axial modes 12.5μm and improve the mode power distribution (next).

22. To be improved : 10logP1/P2=6.57dB

23. 30μm-large core 10logP1/P4=5.94dB • Index profile Doping profile : linear c. Mode-power distribution

24. Next : Liekki’s original moat-fiber simulated • Index profile recommended by the manufacturer b. Radial doping

25. Simulation result : just three modes, mode M2 being attenuated with 3.67dB Conclusion : this combination of index / doping is not practical.

26. 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.

27. 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

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

29. 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

30. For these quantities the following power distribution among modes results: 10logP1/P2=10logP1/P6=9.2dB Modes M2 and M6 overlap

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%

32. 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.

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).

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!

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

36. 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

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.

38. n – the refractive index Glossary 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

39. 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) [ %].

40. Thank you !