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Air-clad multimode holely fiber coupler for high power transmission Y. Kim, W. Shin, S.C. Bae and K. Oh Department of Informationand Communications,GwangjuInstitute of Science and Technology,Gwangju,500-712, Korea Tel: +82- 62-9 70-2213 Fax: +82-62-9 70-223 7
[email protected]
J. Kobelke, K. Schuster, J. Kirchhof, Institut for PhysikalischeHochtechnologieJena e.V.Albert-Einstein-Strasse 9, 07745 Jena, Germany Abstract: We report fabrication of a 2x2 air-clad multimode coupler. It has an insertion loss of 6.5dB, excess loss of 3.5dB over broadband. Potential application in high power combiner was confirmed. ©2005 Optical Society of America OCIS codes: (060.1810) Couplers, switches, and multiplexers. •( 140.3510)Lasers, fiber
Since the report of periodic air-silica waveguide structure[I,2], intense efforts have been given to single mode holey fibers and photonic band gap fibers for novel optical properties such as high nonlinearity and large chromatic dispersion. The air-silica waveguide is based on inherently large contrast in refractive index and recently air-clad multimode holely fibers have been developed for clad-pumped high power laser application[3]. In this new type of multimode fiber, the core is simply a silica rod and air pockets are surrounding it to provide a high numerical aperture and multimode guidance. The cross section of the fiber is shown in the microscope photograph in Fig.1. Thus far the air clad multimode holely fiber (ACMHF) has been used only as a gain medium in a single strand, yet fused couplers have not been reported. Power combiner based on ACMHF will be very useful in scaling up pump power in clad pumped fiber lasers and high power distributing systems in industrial laser applications. In this paper, we report a novel 2X2 fused taper coupler made of ACMHF, for the first time to the best knowledge of authors. We report fabrication process, splitting ration, spectral response, and insertion loss of the device and discuss potential applications. Air-clad multimode fiber was fabricated using stack-and-drawing of capillaries and silica rods and it has the core diameter of 70gm and an outer diameter was 130gm. Air holes surrounding the core has the diameter about 7gm with pitch period of 6 gm. Two strands of ACMHFs were twisted and mounted on electronically controlled motorized stages. Flame brushing techniques were employed to provide a hot zone, where fibers are fused and elongated. As we further elongated twisted fibers, the outer diameter of air clad fiber become smaller and air hole size gradually decreased. Near the fused region the holes began to completely collapse to form a multimode silica waveguide. The adiabatic transformation of ACMHFs along the fused region is schematically shown in Fig.1 along with side-view photographs. Output
Input
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s
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Air hole .......
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center
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Fig.. 1. Schematicof air-clad multimodeholelyfiber couplerand cross-sectionaland side views of the fiber
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The waist width of air-clad multimode fiber was 20~tm at a optimized pulling length of 8.74mm.
The property of power splitting in multimode coupler is strongly affected by the number of modes that propagate in the fused taper region. Within the tapered multimode fiber, each of modes carries a fraction of its power and then couple to the output arms depending on phase delays, which are primarily affected fused region parameters such as elongation length, cross sectional area. [4]. These parameters, on the other hand, are affected by fiber pulling length in the fused-taper fabrication system. At the output port, the distribution of the power between each corresponding modes in two fibers so that, on average, output ports coming out half power of input power. There is a limit to the degree of tapering that can be applied before the onset of volume loss, which cuts off the highest order mode from cladding-air boundary. Fig 2 (a) shows the output power at 1310nm of main and coupled ports with increasing fiber pulling length. Over the pulling of 4mm, air-clad multimode fiber began the mode coupling cycle between two arms and when the elongation length reached 8.74mm, splitting of the output power by equal half was achieved to result in 50:50 2X2 multimode coupler.
100
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Wavelength [nm]
Fig. 2. (a)Coupled two output p o w e r with pulling length (b)output p o w e r spectra for the m a i n and coupled port.
Fig 2 (b) shows the spectral response of the output ports of the fabricated coupler at 50:50 coupling ratio. The spectra were obtained using a non-polarized white light source. It is noted that output ports have almost the same spectral characteristics over a wide wavelength range from 800nm to 1650nm, which can make the device suitable for a wide band power splitter. Furthermore we noted that the spectral response in the output ports were highly uniform and almost independent of wavelength, which strongly indicates ACMHF coupler was also based on symmetric power transmission as in conventional multimode fiber couplers. In conventional multimode coupler has the spectral uniformity about 0.6dB deviation in output power from 800 to 1600nm.[reference] On the while the proposed device showed power deviation of 0.5dB in the same spectral region, which showed equivalent or improved performances. Compared with conventional multimode fiber couplers, the proposed ACMHF coupler does have important advantages in high power handling due to larger pure silica core area and higher numerical aperture [5].
For the measurement of air-clad multimode coupler performance, we set up a measurement system shown in Fig 3. We used power stabilized Fabry-Perot laser diodes as light sources and the wavelengths were centered at 1310 and 1550 nm. The output and input air-clad multimode fibers of the fabricated coupler were connectorized using a large bore size ferrules. The connectors were then mated to conventional 62.5gm core multimode fiber connector using a FC type adapters. In the measurements all the fiber ends were FC connectorized to secure consistent measurements. Utilizing this set up, we measured the insertion loss, excess loss, and power distribution ratio of air-clad multimode coupler. The measured values are summarized in Table 1.
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2 x 2 Air-clad M u l t i m o d e Coupler MMF . . . . . . . . . . . . . . . . . . .
° :
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Fig. 3. Setup of experiment for transmission quality measurement
Table 1. The measurement of insertion loss, excess loss and split ratio. Wavelength
1310 nm
1550 nm
Input power [dB]
-11.57 dB
-12.75 dB
Insertion loss [dB]
6.9 dB
6.5 dB
Excess loss [dB]
3.9 dB
3.5 dB
Distribution ratio
50 : 50
50 : 50
Here we defined the optical characteristics for the 2X2 splitters as below; Excess loss [dB] =-10xlog ( sum[Poutput]/Pinput ) Insertion loss [dB] = -10xlog (Poutput/Pinput ) Distribution ratio [dB] = Poutput(main) " Poutput(coupled)
(1)
It is noted that the insertion loss and excess loss of ACMHF couplers are slightly higher than those of conventional MMF couplers. The higher loss is attributed to drastic change near the taper where the air clad is collapsed to solid silica and further optimization of process parameter is being pursued by the authors. The output power distribution, however, was perfectly 50:50 making the device an ideal power divider along with a low power deviation of 0.5 dB over 800 to 1650nm. Air-clad multimode fiber in general has a high numerical aperture (N. A.) but N.A. does depend on the silica bridge thickness and the number of bridge. For better performances of the coupler fiber design parameter could be further optimized by taking account of high NA and low insertion loss. In summary, a new 2x2 air-clad multimode fiber coupler was fabricated using fused tapering process. The coupler showed an insertion loss of 6.5 dB and excess loss of 3.5 dB at 1550nm. The device shows an excellent uniformity of 0.5 dB deviation in 50:50 power splitting ratio between the output ports over a wide wavelength range from 800 to 1650 nm. Using high numerical aperture and large pure silica core, the device would be directly applicable to high power laser delivery systems. References [1] J.C. Knight. etal, Opt.Lett. vo121, pp. 1547-1549, 1996 [2] J.K Ranka et al, Opt.Lett. vo125, pp. 796-798, 2000 [3] P. Glas et al, Opt. Express, vol 10, pp286-290, 2002 [4] B. S. Kawasaki et al.,Opt. Lett, vol. 6, pp. 499-501, 1981. [5] Nader A. Issa et al., App. Opt. vo143, pp. 6191, 2004
