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Single-mode Operation of LMA Fibers by using a new profile structure

Single-mode Operation of LMA Fibers by using a new profile structure. Emil Voiculescu Technical University of Cluj, RO Madeira, 3 – 5 Sep 2008. Previously Reported. Les Houches : ► Index Depression helps with high-order mode discrimination

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Single-mode Operation of LMA Fibers by using a new profile structure

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  1. Single-mode Operation of LMA Fibers by using a new profile structure Emil Voiculescu Technical University of Cluj, RO Madeira, 3 – 5 Sep 2008

  2. Previously Reported Les Houches : ►Index Depression helps with high-order mode discrimination Naples : ► The LMA fiber having a High-index Ring in the cladding, also having been reported at the Photonic West Conference 20081, ► The Moat-fiber having a lower refractive index in the cladding, presented by M Hotoleanu in Naples. They work up to 20 – 25 m of core diameter. Berlin :► Results Intercomparison : High RI better than Lower RI (Voiculescu), both can be used (S Selleri3 ), Lower RI is preferable ( J Olszewski6 )

  3. Madeira, Sep 2008 ►Newly developed solution solves the problem of higher order modes rejection concurrently with magnifying the light coverage of the core (effective area in the fundamental mode/ MFD) ► The method can be applied to larger-core fibers, up to 40m and more ► The doping can be flat, there is no need to diminish the peripheral doping to help with higher-order modes attenuation

  4. Short Recapin order to explain how the idea has been developed

  5. Take the best result reported in Berlin : Single ring having a higher refractive index than the core

  6. And the conclusionsas they have been reported there • 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 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.

  7. This way a new structure has been conceived For the moment the new arrangement cannot be publicized, however : • The newly-developed model works okay, meaning it virtually operates single-mode in certain conditions, all superior modes being substantially attenuated, and • The effective area can be sufficiently magnified

  8. The setup used for simulation • Chosen Ytterbium Doped fibers : 25μm- and 30μm-core, double-clad fibers, code Yb 1200 -25 -250DC, provider Liekki–nLight, experimental • larger-core fibers of 40μm and more, up to 80μm • Other data : λs = 1.064μm, Ps = 300 mW, λP = 976 nm, Pp = 30 W • Simulator Used : Liekki Application Designer LAD 3.3

  9. Example of the novel Yb LMA Fiber Geometry of the profile – cannot be disclosed yet Index profile for a 30 m-core fiber leading to drastic attenuation of higher-order modes despite the flat doping used ( right-side figure)

  10. Main result showing a 12.9 dB attenuation of all unwanted modes NB : Because of the flat doping a strand of only 1.5m of doped fiber is sufficient to reach the maximum gain

  11. Side view of the fundamental mode shows the core coverage MFD/ 2a = 59.2%, Aeff /Acore = 35.4%

  12. Side view of the M2 power distribution along the fiber axis shows radial spreading of M2

  13. Cross-section of M2 power distributionshows M2 peaks outside the core(evanescence of M2 obvious)

  14. Intercomparison : Similar behavior obtained by Mircea Hotoleanu

  15. ► In order to optimize the index profile, many parameters had to be varied concurrently. ► Finding the optimal single-mode operation is a tough job as it regards almost all the parameters in the index characteristic.

  16. ► Anyway, a systematic approach has been carried out and the previous experience with LMAs helped to shorten the try-and- error procedure ► The basic (expected) functionality seems to relate the high-order mode rejection mainly with the outer portion / layer, and the core coverage mostly with the inner structure

  17. General remarks ► For this class of fibers, and for the following 40m-core fibers, only the flat doping has been considered. This is one of the great advantages that the new topology offers : to emulate a virtual single-mode fiber we don’t have to decrease the peripheral doping. One also can get a larger gain with a shorter strand of fiber this way. Dopant concentration : 1.56 • 1025 / m3

  18. More than that : ► A significant attenuation of the higher-order modes can be reached without taking special measures. Usually a ≥10dB attenuation of the total power in the higher-order modes can be accepted. Then, if we can provide more, why not decrease the attenuation, until the Mode-Field Diameter reaches a maximum ? This strategy has been applied for sizing the considered fibers ( 30m- and 40m-core fibers). ► In order to obtain the largest effective area, i.e. maximum MFD, a thin moat resulted. Because of that, all other geometries are less performant.

  19. The Etalon • As the coverage of the core with light should be as high as possible one need a comparison criterion. By taking a reference step-index fiber with the same core-diameter (and same cladding diameter), and assuming the same flat doping, we might say that reaching the same effective area in the fundamental mode can be accepted. • In fact, that coverage is well surpassed every time, usually by 1.5 times.

  20. Significant Results 1. Core Diameter 30 m

  21. First index profile Power distribution among modes Attenuation of the most powerful mode ( M9) is : 10log(P1/Pmax) = 10.2dB MFD/2a = 63.7% Aeff/Acore = 40.6%

  22. Second Index Profile Power distribution among modes 10log(P1/Pmax) ≈9 dB 10log(P1/Ptotal) ≈ 10log(P1/P9+P6)= 10log(17000/2950) ≈ 8 dB MFD/2a = 77.9% ≈ 80% Aeff/Acore = 60.7%

  23. Side view of the fundamental mode demonstrates the improved core coverage MFD/2a = 63.7%

  24. Also the top view of the fundamental mode confirms core coverage

  25. What we got ? ► 10 dB of higher-order modes attenuation or so, which will do with practical applications, so we may indulge in trading the attenuation for a larger effective area

  26. Top-view picture proves that the most powerful mode leaks out of the core ( in between –15m and +15m → red, i.e. zero Watts )

  27. 2. Core Diameter 40 m

  28. The reference model (the etalon) first Step-index fiber Flat doping Mode power distribution along the fiber shows the usual mode ’scrambling’ as no measure for counteracting that has been taken

  29. The fundamental mode Cross section MFD / 2a = 51.7 % Aeff / Acore = 26.7 % Side view

  30. The proposed fiber

  31. And the results Attenuation of the most powerful mode : 10 log(P1/P9) = 10.98 dB Attenuation of all superior modes : 10 log (P1/PTotal) = 9.33 dB

  32. Mode field diameter and Effective area MFD / 2a = 65.4 % Aeff / Acore = 42.7 %

  33. Conclusions ► The novel fiber structure presented here solves the key problem associated with LMA fibers : the beam quality. ► This way we can modify a multimode fiber in a quasi-single mode one. ► Compared with the previously developed single-ring fibers, this one offers an extra degree of freedom ► The structure works fine with 40 m cores. It cannot be easily applied to experimental 80m-core LMA fibers or thicker core fibers. Going beyond 25 m of core diameter makes single-mode operation problematic. ► If the experimental results will confirm the simulation results, then such fibers will be manufactured. It is very likely that some samples will be tested soon. A round-robinto confirm performance by measurement could naturally follow the modelling/simulation activity.

  34. References • Improving the beam quality in LMA fibers. E Voiculescu, M Hotoleanu, Csipkes G, Photonic West Conference 6896, San Jose, CA, Jan 23 2008. 2. Study regarding the Index and Doping profiles in LMA fibers for maximum optical power in the fundamental mode. Graduation Thesis 2008, B Ghete, Technical University of Cluj, RO. 3. Moat fiber finite element method. F Poli, D Passaro, A Cicinotta, S Selleri, University of Parma, COST Presentation, Berlin, Jan 29 2008. • Single-mode regime in large mode area moat fiber. D. Passaro, F. Poli, A. Cucinotta, S. Selleri, submitted to ECOC 2008. • Large mode area optical fibers for high power amplification. S Selleri, A Cucinotta, F Poli, D Passaro, COST Presentation, Madeira, 2008. 6. Modes in moat fiber. Numerical modeling.J Olszewski, Wroclaw University of Technology, COST Presentation, Berlin, 2008.

  35. Acknowledgement • I am grateful to student Bogdan Ghete for his work to the topic through his graduation thesis that I advised, and which got the maximum score and the ‘Cum Laudae’ apreciation from the Graduating Board June 2008. • I am grateful to Dr M Hotoleanu and Liekki– nLight for revising this material, providing me with the fiber data needed, and with the LAD 4.1 software.

  36. Thank you !

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