We demonstrate low-loss splicing between a photonic crystal fiber (PCF) and a single-mode fiber (SMF) with a conventional electric-arc fusion splicer, where nitrogen gas (N2) with a proper pressure is pumped into the air holes of the PCF to control the air-hole collapse ratio so as to optimize the mode-field match at the joint. The method is applicable to both solid-core and hollow-core PCFs. With this method, we achieve a splice loss (measured at 1550 nm) of ~0.40 dB for a solid-core PCF and ~1.05 dB for a hollow-core PCF. The method could find wide applications in the fabrication of PCF-based devices.
©2012 Optical Society of America
Photonic crystal fiber (PCF), which consists of a large number of regular air holes running along the fiber, has gained much attention because it can be designed to yield many useful properties, such as endless single-mode operation, flexible dispersion control, extremely high or low nonlinearity, and high birefringence [1–5]. These properties can be explored for implementing new fiber devices for many applications. To integrate PCFs into fiber communication or sensing systems, robust low-loss splicing of PCFs to single-mode fibers (SMFs) is required. Since the first demonstration of splicing between a PCF and a SMF , many splicing techniques have been reported, including the use of a filament splicer, a CO2 laser, and gradient-index fiber lenses [7–10]. Bourliaguet et al. reported a 0.7 − 1.1 dB splice loss for a solid-core PCF by using an arc of short duration and weak power . Thapa et al. reported a 1.5 − 2.0 dB splice loss by offset electrode arc discharge . Xiao et al. applied repeated arc discharges on the joint to gradually collapse the air holes of a PCF  and achieved splice losses of 0.9 − 1.41 dB and 1.45 − 2.01 dB for solid-core and hollow-core PCFs, respectively. Recently, splicing of a polarization-maintaining PCF to a SMF with an arc fusion splicer was reported, where a low splice loss was achieved by matching the mode-field diameters (MFDs) of the two fibers . Splicing with a CO2 laser to control the air-hole collapse was also demonstrated, which can give an exceptionally low loss for a solid-core PCF . However, the method introduces polarization-dependent loss and cannot be applied to a hollow-core PCF.
The factors that contribute to the splice loss include the transverse offset, the angular misalignment, the mode-field mismatch, and the deformation of the fiber structure . For fusion splicing between a PCF and a SMF, the mode-field mismatch and the air-hole collapse during splicing are the two main factors. Air-hole collapse, in particular, may destroy the light guiding property of the PCF and significantly increase the loss . Under certain conditions, however, a collapse of the air holes can help to reduce the mode-field mismatch and thus lower the loss. For a hollow-core PCF, where light is guided along the central hole of the fiber by the bandgap effect, air-hole collapse must be avoided.
In this paper, we propose a method of splicing between a PCF and a SMF, where nitrogen gas (N2) is pumped into the air holes of the PCF during fusion discharge to help reduce the splice loss. We tried two popular PCFs from NKT Photonics: a solid-core PCF (ESM-12-01), which can provide endless single-mode operation, and a hollow-core PCF (HC-1550-2), which guides light on the principle of the photonic bandgap effect. The SMF used was a telecommunication fiber from Corning (SMF-28e). With our method, the air-hole collapse ratio of the PCF can be efficiently controlled or avoided by adjusting the N2 pressure, which thus allows a more precise control of the MFD of the fiber and hence leads to a lower splice loss.
2. Principle of low-loss splicing
The PCF/SMF splice loss is mainly caused by the MFD mismatch. It is possible to match the MFDs of a SMF and a solid-core PCF by collapsing the air holes of the PCF . In the splicing process, the air holes of the PCF shrink and the relation between the hole diameter d and the distance of the adjacent holes Λ is given by 19]Eq. (1) and Eq. (2), we get20]Fig. 1 (see Table 1 for the fiber parameters).
The MFD of the SMF SMF-28e is about 10 μm. According to Fig. 1, the enlarged mode field of the PCF can match that of the SMF when the hole collapse ratio is 0.5 – 0.6. Figure 2(a) shows schematically how the hole collapse can lead to a change of the MFD of the PCF. As shown in Fig. 2(a), the PCF experiences a smooth transition after arc discharge because of the longitudinally decreasing temperature, which makes the holes collapse gradually along the fiber. As a result, the MFD of the PCF increases gradually from its original value to a value that can match with that of the SMF at the joint. The idea of realizing the MFD match by hole collapse has been achieved by several methods [12,13,21]. In general, both the outer diameter and the pitch shrink longitudinally , which is not desirable. Our method can provide a better control of the MFD of the PCF and yet does not lead to a significant change in the outer diameter.
We simulate the splice between the SMF and the solid-core PCF with a full-vector beam propagation method (OptiBPM, Optiwave inc.). The variation of the splice loss with the transition length at different collapse ratios at the wavelength 1550 nm are shown in Fig. 3(a) . We find that the loss is lowest when the transition length is around 20 μm, which can be achieved with a collapse ratio in the range [0.457, 0.592], in agreement with the results shown in Fig. 1. We also calculate the spectral dependence of the loss. The results are shown in Fig. 3(b), which assumes a transition length of 21.6 μm and a collapse ratio of 0.293. The oscillations shown in Fig. 3(b) suggest that the MFD of the SMF and the PCF have different spectral dependences. Nevertheless, by properly controlling the hole collapse ratio and the transition length, it is possible to achieve a low splice loss over a wide wavelength range.
For a hollow-core PCF, where light is confined in the central hole by the photonic bandgap effect, fusion splicing can easily destroy the bandgap structure of the fiber and introduce a large loss. To achieve a low splice loss, it is necessary to maintain the structure of the fiber as much as possible, which means that no air-hole collapse should be allowed, as shown schematically in Fig. 2(b). To confirm this, we simulate the splice between the SMF and the hollow-core PCF HC-1550-2 with OptiBPM (see Table 1 for the fiber parameters). The variation of the splice loss with the collapse ratio for different transition lengths at the wavelength of 1550 nm are shown in Fig. 4 . The lowest loss occurs at a zero collapse ratio, i.e., when there is no hole collapse.
From the simulation, we know that the key to achieve low-loss splicing between a SMF and a PCF is to control the joint profile. For the solid-core PCF, we should control the collapse of the holes to obtain a suitable transition length, whereas, for the hollow-core PCF, we should avoid the collapse of the holes. However, during arc discharge, the surface tension of the glass-air interface in the microstructure region offers less resistance to deformation than that of the solid glass in the outer cladding of the PCF, so a recess is formed at the end side of the fiber, which leads to deformation of the holes . To solve this problem, we propose pumping N2 into the air holes of the PCF during splicing. The idea is to control the pressure in the holes so that the collapse of the holes can be controlled or avoided.
2. Experiments and discussion
The parameters of the fibers used in our study (SMF-28e, ESM-12-01, and HC-1550-02) are given in Table 1. Figure 5(a) and Fig. 5(b) show the scanning electronic micrographs (SEM) of the cross sections of ESM-12-01 and HC-1550-02 PCFs, respectively.
In our experiments, we sealed the PCF at one end with the SMF and pumped dry N2 into the PCF from the other end with a gas flow controller to introduce pressure in the air holes of the PCF, as shown in Fig. 5(c). We used a fusion splicer (Furukawa Electric Co., S176) to do the splicing. The parameters of the fusion splicer setting we used for the two PCFs are given in Table 2 . To measure the splice loss, we launched continuous-wave laser light into the SMF and measured the output power from the PCF (which was about 1-m long) with an optical power meter (Newport, 1830-C). We did 16 splices for each gas pressure and repeated the experiments at different gas pressures.
The results measured at 1550 nm are shown in Fig. 6(a) and Fig. 6(b) for ESM-12-01 and HC-1550-02, respectively, where the splice loss at each gas pressure is the average of 16 splices. The minimum splice loss for the solid-core PCF ESM-12-01 is 0.40 ± 0.05 dB at a pressure of ~1.3 bar, while the minimum splice loss for the hollow-core PCF HC-1550-2 is 1.05 ± 0.05 dB at a pressure of ~1.7 bar. The side views of several splices are shown in Fig. 7 . As shown in Fig. 7(a), the air holes of the solid-core PCF can be collapsed to a desired ratio at a proper pressure (1.34 bar). On the other hand, as shown in Fig. 7(c), a proper pressure (1.65 bar) can also prevent the air holes of the hollow-core PCF from collapsing. Without applying any pressure to the air holes, however, the air holes in both PCFs are almost completely collapsed, as shown in Fig. 7(b) and (d). By the way, the very low splice loss could be easily achieved from the SMF to the PCFs direction. In our experiment, the splice loss could be 0.1 ± 0.05dB and 0.3 ± 0.05dB for solid core PCF/SMF and hollow core PCF/SMF, respectively.
We calculate the optical mode-field distributions of ESM-12-01 and HC-1550-02 with different air-hole collapse ratios with a mode solver (COMSOL). The results are shown in Fig. 8 . As shown in Figs. 8(a) and 8(b), the MFD of ESM-12-01 is enlarged from ~6 μm to ~11 μm, when the collapse ratio changes from 0% to 60%. On the other hand, as shown in Figs. 8(c) and 8(d), a collapse ratio of just 10% is sufficient to destroy the mode in HC-1550-02, which confirms the need of maintaining the sizes of the holes in a hollow-core PCF during splicing. As shown by our experiments, the application of a proper pressure to the air holes of a hollow-core PCF can prevent the air holes from collapsing.
We have demonstrated an effective method for low-loss splicing between a SMF and a PCF. The method relies on pumping N2 into the air holes of the PCF during fusion splicing to control the air-hole collapse ratio of the PCF. In the case of a solid-core PCF, the pressure can be controlled to achieve the desired collapse ratio and hence the MFD required to best match with the SMF. In the case of a hollow-core PCF, the pressure can be adjusted to prevent the air holes of the fiber from collapsing, which is essential for achieving a low splice loss for such a fiber. Using this method, we have achieved a splice loss of ~0.40 dB for a solid-core PCF (ESM-12-01) and ~1.05 dB for a hollow-core PCF (HC-1550-02), when splicing the PCF to a conventional SMF (SMF-28e). The method should be useful for the development of PCF-based devices for application in optical communication and sensing systems.
This work was supported by Natural Science Foundation of China under Grant No. 61007049 and 60807019, and the Program for NCET (Grant No. NCET-08-0602).
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