We propose and demonstrate bend-insensitive fibers equipped with higher-order mode strippers. The mode stripper is realized by filling a section of air holes with epoxy to attenuate any higher-order modes that are excited at fiber junctions and are confined by the air holes surrounding the core. We found that the higher-order modes are well suppressed with 5 cm-long epoxy columns. An ultralow bending loss of 0.025 dB/turn at a bend diameter of 10 mm, together with single-modeness, is experimentally demonstrated in a bend-insensitive fiber with six air holes 16 μm in diameter.
©2010 Optical Society of America
Fiber-to-the-home (FTTH) applications are growing as the number of high speed internet subscribers increases worldwide. When standard single-mode fibers (SMFs) are used for FTTH, the fiber cable must be installed very carefully to avoid small bends in the fiber path that can cause significant signal loss . For conventional single mode fibers, the smallest bending diameter allowed is around 30 mm, and this restriction causes high cost and large space for the fiber installation. Several designs of bend insensitive fibers (BIFs) have been proposed to reduce the bending loss of single-mode fibers. These fibers are characterized by depressed cladding, low index trench [2–4], hole-assisted cladding [5–7], or a nano-engineered ring in the cladding .
Recently, very low bending loss of less than 0.1 dB/turn at a bending diameter of 10 mm has been reported in hole-assisted fiber , low-index trench [3,4] and nano-engineered ring  designs. Figure 1(a) shows an example of a hole-assisted fiber. The key principle of the fiber is that the hole layer, composed of six or more air holes surrounding a Ge-doped core, provides a low index barrier and prohibits optical leakage from the core in a bent fiber. Usually, large air holes located close to the core build very strong low index barriers, thereby giving lower bending loss. However, such a structure allows inner cladding modes or higher-order modes to be guided by the low index barrier. Such higher-order modes  can be excited by an optical connection with an imperfect mode match between two fibers. If the higher-order mode is not fully attenuated during its propagation along the fiber, it may couple back to the core at a following optical connection to induce modal interference . Therefore, it has been a tricky problem to design low index barriers strong enough for suppression of bending loss for the fundamental mode and, at the same time, leaky enough for the higher-order modes .
In this study, we propose to implement mode strippers at the ends of the fiber cable, instead of carefully designing the hole structures, to suppress the higher-order modes and minimize the modal interference. Any higher-order modes excited at the front end will be removed in the mode stripper section while the core mode is not influenced. This method eliminates the complicated requirement for the hole design, and thus facilitates the development of ultralow bending loss fibers free from modal interference.
2. Fabrication of the higher-order mode stripper
For our experiments, we selected a hole-assisted fiber with six relatively large air holes fabricated by Optomagic, Inc.(Ansan-si, Korea), as shown in Fig. 1. The Ge-doped core has a diameter of d = 9.0 μm. The air holes have a diameter of D = 16 μm, and they are located at the triangular lattice with a lattice constant of Λ = 23 μm. The cutoff wavelength was ~1240 nm, and the mode field diameter was 10.2 μm at 1550 nm. The attenuation was 0.38, 0.342, 0.300, 0.203, and 0.208 dB/km at 1260, 1310, 1385, 1550, and 1620 nm, respectively. This fiber exhibits very low bending loss due to the large hole diameter, but there exist several higher-order modes or inner-cladding modes that are strongly guided by the six air holes.
The mode stripper was realized by filling the air holes with an epoxy for a certain length as shown in Fig. 2 . Since the refractive index of epoxy is higher than that of silica, the inner-cladding modes cannot be confined by total internal reflection. Instead, it is only weakly guided by Fresnel reflections at the silica/epoxy boundary . If the epoxy length in the ends of fiber is long enough, the inner-cladding modes will be completely diffused into the outer cladding to be scattered or absorbed, and only the core mode can propagate in the BIF. The fundamental core mode also can be slightly affected by high index column in terms of optical loss or dispersion. But such effects will be negligible if the epoxy column is only a-few-cm long. In the mode stripping region, the fiber does not have low-index barriers and bend loss occurs as in conventional single mode fibers. Therefore, we assumed that the stripper section is protected from any sharp bends.
The epoxy was injected into the air channels using a dipping method. The BIF was dipped in a long tube filled with epoxy as shown in Fig. 3(a) . Then, the epoxy was injected into air channels automatically by epoxy column pressure and capillary force. This method allows simultaneous treatment of multiple fibers. In our experiment, UV Epoxy (NOA 60, Norland, Inc.) with a refractive index of 1.56 and a low viscosity (300 CPS) was chosen for fast injection and curing. After about 12 hr of dipping, the epoxy was cured under UV radiation, and the filling length (l) was measured from optical microscope images (Fig. 3(b)). In Fig. 3(d), the epoxy length is plotted as a function of the dipping depth, showing linear dependence. Figure 3(c) is a scanning electron microscope image of the fiber cross section in which the six air holes are filled with epoxy.
3. Suppression of modal interference by the mode stripper
To evaluate the mode stripping effect, we observed modal interference  in optical connections for different epoxy lengths. Figure 4(a) shows a schematic of the experimental setup. A 1.5 m-long BIF with a mode stripping section was fusion-spliced to a SMF at the back side. The other end of the BIF was butt-coupled to a SMF. The lowest loss attainable in the butt-coupling was 0.2~0.3 dB. Here, an air gap of 20 μm and transverse offset of 2 μm were intentionally introduced to facilitate excitation of inner cladding modes. The connection loss was found to be about 0.5 dB and 1.5 dB for the fusion splice and the misaligned butt coupling, respectively. The loss of the fusion splice in the SMF-BIF connection was relatively large compared to a typical loss of < 0.05 dB for a SMF-SMF splice. This is explained by the intrinsic offset of the core in BIF that occurred during fiber fabrication . The epoxy-treated end of the fiber could be also fusion-spliced to SMF with the loss of ~0.5 dB, but a low-level arc power had to be used to avoid burning the epoxy. A super-luminescent diode (SLD) with a bandwidth of >50 nm was used as a broadband source, and the output signal was analyzed using an optical spectrum analyzer (OSA).
We measured the transmission spectra in the wavelength ranges of 1250-1350 nm and 1500-1600 nm for several different epoxy lengths l, as shown in Fig. 4(b). The BIF sample with an epoxy length of 50 mm was cut by 10 mm after each measurement. (We do not show the cases of 10 and 30 mm to save space.) When there was no mode stripper in the BIF, modal interference resulted in loss modulation with an amplitude of 0.5~1.4 dB, depending on wavelength. Such a large modulation was due to the intentional misalignment of the fiber connections applied in this experiment. The loss modulation amplitude decreased as the stripper length increased, and it was suppressed below noise level when l = 50 mm. In all cases, the average loss was about 2 dB(0.5 dB from the fusion splice, and 1.5 dB from the butt coupling as previously mentioned). The maximum modulation amplitudes measured in Fig. 4(b) were converted to MPI(Multi-Path Interference) values , which are plotted as a function of stripper length in Fig. 4(c). The fitted lines will be explained below. This graph helps us determine the minimum stripper length required to suppress the modal interference below a target level. We note that the initial MPI value for zero epoxy length is relatively high compared to the conventional cases. When a normal splice or mechanical connectors are applied, the whole MPI data will shift to a lower level. Moreover, fiber bending with a diameter of a few cm applied on any section of BIF also dramatically reduces the modulation amplitude due to the large bending loss of higher-order modes. We suggest using an epoxy length of ~30 mm at both ends of the fiber cables, so that the total length of the mode stripping section becomes 60 mm, and each mode stripping section can fit in the ceramic ferrule of a connector (with a length of 25~30 mm) to be protected from bending loss.
Higher-order mode suppression by the mode stripper was also demonstrated in far-field patterns of the fiber output. The SLD was replaced with a 1550-nm narrow-linewidth laser in the setup shown in Fig. 4(a). The SMF at the back end of the BIF was removed, and the far field of the BIF output was observed using an infrared vidicon camera. Figure 5 shows the far fields for the epoxy length changing from 0 to 50 mm. Many speckles and asymmetric pattern appeared in non-treated BIFs of Fig. 5(a) due to the interference of multiple spatial modes. The field pattern gradually changed to the symmetric Gaussian shape of the fundamental mode as the epoxy length increased. The mode pattern in Fig. 5(f) was found to be very stable under any strain or stress applied to the BIF, whereas such a perturbation produced blinking and reshaping of the pattern in other cases. The results are in good agreement with those of the experiment shown in Fig. 4.
One interesting observation from Fig. 5 is that the attenuation strength of the cladding modes differed significantly for different orders of the cladding modes. The high-order modes generating many speckles quickly disappeared within the first 1 cm of the mode stripper, but the LP11 mode survived much longer. In Fig. 4(c), the linear fitting was performed excluding the data for l = 0 mm to consider the modal interference between LP01 and LP11 modes, only. The MPI slopes of the fitted lines were −0.32 dB/mm and −0.41 dB/mm for 1300 and 1550 nm bands, respectively. The attenuation of the lower-order modes would be further accelerated if proper light-absorbing materials were mixed with the epoxy.
Finally, the bending loss of the BIF was measured using narrow line-width lasers and an optical power meter. Two or three turns of fiber loops were made for a bend diameter in the range of 8 to 14 mm. Each measurement was repeated three times and the average values were recorded. Figure 6 shows the results obtained for the various wavelengths. The bending loss increased with the wavelength as mode confinement becomes weaker. At 1550 nm, the loss was about 0.025 dB/turn for a bend diameter of 10 mm, which is comparable to the lowest value ever reported. We believe that the loss can made even lower simply by increasing the air hole sizes. It is an important advantage of our scheme that there is no specific requirement for the holes sizes or layouts to maintain single modeness of the fiber.
The suggested BIF design with epoxy infiltrated at both ends has the additional benefit of hole sealing. The epoxy will prevent the air holes from being polluted by external gases or liquids. It is known that the bending loss of hole-assisted fibers dramatically increases at low temperatures due to water condensation on the hole surfaces. This problem can be solved effectively if the air holes are filled with dry nitrogen before applying epoxy. Although the required mode stripper length of 5 cm seems to be acceptable when applied to fiber connectors, it may be further reduced by optimization. Proper selection of the coating material surrounding the fiber cladding, as well as optimization of the refractive index and absorption coefficient of the epoxy, are being considered to enhance the attenuation of cladding modes.
We developed a technique to suppress tightly-guided cladding modes in bend-insensitive fibers for high speed FTTH applications. It was found that 5 cm-long epoxy columns could effectively attenuate the higher-order modes and suppress modal interference below 1/30 of the original level before the treatment. An ultralow bending loss of 0.025 dB/turn for a bend diameter of 10 mm at 1550 nm was demonstrated in a fiber with large six air holes, and further improvement is expected. The mode strippers suggested in this work can be applied to various kinds of micro-structured fibers with multiple air holes to suppress the modal interference. The use of mode strippers will allow us to freely optimize the hole layouts to tailor bending loss, dispersion, or other properties without worrying about modal interference.
This work was supported by the Regional Research Center for Photonic Materials and Devices, Chonnam National University, and the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2008-331-C00115).
References and links
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