We describe the fabrication of elliptical hollow-core photonic bandgap fibers (EC-PBGFs). It was shown that the aspect ratio of the hollow core can be controlled by tuning the negative pressure in the space between the intermediate preform cane and outer jacketing tube, and by placing this preform assembly off-center in the furnace, resulting in lateral tension during the final draw. Modal birefringences of fabricated PBGFs with different aspect ratio were measured using a Sagnac loop interferometer. For the elliptical hollow core PBGF with aspect ratio of 2.34, the modal birefringence was measured to be about 4.6×10-2 at 1,550nm.
©2009 Optical Society of America
Photonic crystal fibers (PCFs) are new kinds of optical fibers that have an arrangement of air holes along the length of the fiber. They are divided into two categories, index guiding fibers and photonic bandgap fibers (PBGFs) [1-2]. Similar to conventional fibers, index guiding fibers guide light in the high index solid core via a modified total internal reflection principle. On the other hand, PBGFs guide light in the low index core region via a photonic bandgap (PBG) effect.
One research area for these fibers that is attracting a great deal of interest is the enhancement of their birefringence, since they have a much higher index contrast than conventional fibers and it is relatively easy to introduce asymmetric geometry around the fiber core. To date, many kinds of highly birefringent (Hi-Bi) PCFs have been proposed, such as a Hi-Bi index guiding PCFs with different air-hole diameters along two orthogonal fiber axes, or asymmetric core shapes, or uniformly oriented elliptical holes [3-9]. It has been theoretically and experimentally demonstrated that their birefringence can be high as on the order of 10-3, which is one order higher than conventional polarization maintaining fibers (PMFs). Also Hi-Bi PBGFs with asymmetric air cores have been proposed [10-14]. Recently, Chen et al. reported a very Hi-Bi PBGF with a group birefringence of 0.025 at the wavelength of 1,550 nm . However, a fabrication method for controlling the aspect ratio, and the affect of the aspect ratio of the air-core on the group birefringence, has not been reported in the literature.
In this paper, we report a means to tune the aspect ratio of the hollow core. It is shown that if the final out jacked cane is placed off-center of the furnace during drawing, which results in asymmetric temperature characteristics near the hollow core, and the amount of evacuated air from the region between the cane and the second out-jacket tube is tuned, it is possible to change the aspect ratio of the hollow core. This is the first experimental demonstration of unrestricted control of the aspect ratio of the hollow core in PBGF, to fabricate a PBGF with the required birefringence. Moreover, we show the dependence of the birefringence on the aspect ratio of the hollow elliptical core.
2. Fabrication of elliptical-core photonic bandgap fibers
The elliptical hollow core photonic bandgap fiber (EC-PBGF) was fabricated via the well-known stack-and-draw technique, as shown Fig. 1. Firstly, a number of 1-mm capillary tubes were stacked, to form the designed hexagonal structure, in which the hollow core was constructed by removing seven central capillaries. Here, we note that all capillary tubes were sealed at one end with a high temperature flame prior to stacking, enabling the structure to be inflated to a high air filling fraction during drawing. And, we inserted two additional 3-mm short capillaries from both ends of the stack, to maintain the hollow core structure during drawing. Subsequently, we inserted these stacked capillaries, with eight rows of air holes surrounding the core set, into a jacketing tube. When this first preform was constructed, it was drawn to the intermediate preform in the shape of a cane with a diameter of about 1 mm. In order to prevent capillaries from over inflating or melting together, the fiber tension must be increased via decreasing the furnace temperature to a greater extent than in a conventional optical fiber drawing process (In our experiment the temperature of the furnace was about 1,850 °C). During this process, we reduced the pressure in the space between the bundle and tube, to eliminate micro defects between the stacked capillaries and out-jacket tube. Also, this reduction of the pressure of the volume enables expansion of the holes. Figure 2 shows a perspective view of the intermediate preform drawn from the first preform assembly comprising the plurality of tubes and jacketing tube.
Next, this intermediate perform with diameter of 1mm was inserted into an out-jacketing tube with an inner diameter of 3.1 mm and an outer diameter of 4.3 mm, prior to drawing into the fiber itself. With lower ends of thereof sealed together, the unsealed ends of the intermediate perform and jacketing tube were connected to an argon gas vessel and a vacuum pump, respectively, and the sealed ends were fed in a vertical orientation at a 5 mm off-center position into the furnace whose dimensions were 35 mm in inner diameter and 185 mm in length. Thus, we could ensure asymmetric temperature characteristics at the hollow core region, i.e., inducing lateral tension, resulting in the elliptical hollow core. Then, the sealed end of the assembly was heated by furnace for about 15 minutes or longer and melted. For the final fiber draw, the feed speed and the fiber draw speed were about 14 mm/min and 15 m/min, respectively. A laser based diameter gauge and coating cup were carefully adjusted by using x-y translation stages to compensate the offset of the assembly. While the final fiber was being drawn, we applied negative pressure to the gap between the intermediate preform and second out-jacket tube, whilst applying positive differential pressure of about 10 mmH2O to the intermediate cane region, to ensure a thinner web structure. By regulating the negative pressure in the gap between the preform cane and jacket, we could change the aspect ratio of the hollow core. As we increased the negative pressure applied to the region between the cane and outer jacket, the aspect ratio of the elliptical hollow core increased.
Figure 3 shows scanning electron microscopy (SEM) images of fabricated EC-PBGFs with different hollow core geometries. In Fig. 3, each hollow core has an elliptical shape, which is directly related to the birefringence of the fiber. As described in the previous paragraph, during fabrication of PBGFs it is possible to control the aspect ratio of the hollow elliptical core by regulating the negative pressure in intermediate preform assembly. In our experiment, the applied negative differential pressure was -5, -10, and -18 mmH2O, respectively. In Fig. 3, it is clear that the distortion of the hole pitch near the core is minimal, as expected. The aspect ratios of the hollow elliptical core were 1.36, 1.52, 2.34, for EC-PBGF1, EC-PBGF2, EC-PBGF3 respectively. The air filling fraction of PBGFs in the cladding exceeded 85 %. Our method worked pretty well and demonstrated that the ellipticity of hollow-core photonic bandgap fiber could be controlled with this technique. However, it must be pointed out that fiber profile becomes very sensitive to unexpected variations in the drawing parameters. Therefore, for fiber of consistently reproductive quality, an automatic procedure for fabricating the perform and drawing the fiber may be needed.
3. Measurement of optical properties of EC-PBGFs
The basic optical properties of EC-PBGFs such as transmission spectrum and group birefringence were measured. The transmission spectrum is shown in Fig. 4, and the inset is the cross section of the measured EC-PBGFs. In order to measure the transmission spectrum of the fabricated EC-PBGFs, we spliced two ends of EC-PBGFs to two single mode fibers (SMFs). When EC-PBGFs were spliced to SMFs, we tuned the splicing parameter of Ericsson FSU-975 splicer to reduce the splice loss between EC-PBGFs and SMF . Here, the length of the fabricated EC-PBGFs for transmission spectrum was about ~3 m.
Figure 4 shows that the photonic bandgap extends from 1,100nm to 1,600nm, and includes small dips. These dips (i.e., loss region) were attributed to the mode coupling between the core and surface mode. At these wavelengths, surface modes couple with the core-guided mode, increasing the loss. Optical losses of EC-PBGFs in the photonic bandgap wavelength range were measured around 1.5 dB/m, 1.7 dB/m, 2.2 dB/m, for EC-PBGF1, EC-PBGF2, EC-PBGF3, respectively. These increases of transmission loss with respect to the aspect ratio of the core were attributed to the fact that the thickness of the core boundary and asymmetric deformation of the holes in the vicinity of the hollow core increase [16-18].
We also evaluated the birefringence of three EC-PBGFs by using a Sagnac loop interferometer at 1,550nm . Initially, in order to measure the birefringence of EC-PBGFs, the Sagnac loop interferometer configuration consisted of a standard 3dB fiber coupler and 1m-long fiber under test (FUT). The two ends of the 3dB fiber coupler were fusion spliced to the FUT, to make the Sagnac loop. The other two ends of the 3dB coupler were directly connected to a tunable laser (Ando AQ4321D) and an optical spectrum analyzer (OSA) (Ando AQ6315B). Measurements were performed by launching linearly polarized light from a tunable laser into FUT and tuning the wavelength of the laser. Because the sample fiber was birefringent, the output of the interferometer was wavelength-dependent, thus, there was modulation in the spectrum measured in OSA. The spacing of the modulation Δλ is related to the birefringence Δn and sample fiber length L by the following equation :
where λ is the operating wavelength and L is the length of EC-PBGFs.
Figure 5 shows the measured interference spectra of EC-PBGFs-based Sagnac loops, which can be used to characterize the modal birefringence. The wavelength spacing between two adjacent peaks at the wavelength of 1,550 nm was 1.88 nm, 0.29nm, 0.052nm, for EC-PBGF1, EC-PBGF2, EC-PBGF3, respectively. Using Eq. (1), the modal birefringences for EC-PBGF1, EC-PBGF2, and EC-PBGF3 were measured as 1.55× 10-3, 8.3× 10-3, and 4.6× 10-2, respectively. This value of the birefringence for the EC-PBGF3 (with the aspect ratio of 2.34) is about twenty times larger than that of a conventional PANDA or bow-tie PM fibers.
We have shown that the elliptical hollow core PBGF can exhibit very high modal birefringence. For practical fabrications, it was shown that the aspect ratio of the hollow core can be controlled by tuning the negative pressure in the space between the intermediate preform cane and outer jacketing tube and by placing this preform assembly off-center in the furnace, resulting in lateral tension during the final draw. To the best of our knowledge, this is the first experimental demonstration of control of the aspect ratio of the hollow core in PBGF via negative vacuum pressure. We postulate that our novel method of control of the shape of the hollow core will be a useful tool for fabricating various kinds of hollow core PBGF.
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