We present a novel technique for fabricating photonic crystal fiber (PCF) that employs a slurry casting method to produce a silica PCF preform. By combining this approach with an OH reduction process, a low-loss PCF (1.1 dB/km at 1.55 μm and 3.1 dB/km at 1.38 μm) was successfully fabricated. Furthermore, by employing air-hole pressure control in the drawing process, a minimum loss of 0.86 dB/km was obtained. This method provides highly flexible air-hole structures and a low fabrication cost.
© 2013 Optical Society of America
Photonic crystal fibers (PCF) have received a lot of attention due to their attractive features, which include endlessly single-mode operation, a low bending loss, high nonlinearity, and widely tunable dispersion . Silica PCF has been realized with various fabrication techniques including a capillary stack and drawing method , a sol-gel casting method [3, 4], an extrusion method , and a preform drilling method . The most widely used of these techniques is the capillary stack and drawing method. By employing this method, PCF can be fabricated with few impurities because the approach utilizes high purity silica capillaries and rods made using VAD technology. PCF obtained using this method has realized the lowest transmission loss of 0.18 dB/km at 1.55 μm as reported by Tajima . However, the method involves complicated processes for manually assembling a number of capillaries with high precision, which is an important issue as regards reducing the fabrication cost. Stacking precision is a key factor in terms of the uniformity of the air hole structure in the fabricated PCF. With the sol-gel casting method, it is difficult to obtain a defect free gel body because voids and bubbles are generated easily in the gel body during the gelation process. Because of these defects, it is very difficult to reduce the fiber loss. Moreover, with preform drilling it is difficult to maintain longitudinal structural uniformity, and the extrusion method cannot be applied to silica PCFs. Recently, a novel PCF fabrication process was also reported with a sand clad process . In this process, silica sand is used to surround capillary assemblies including the interstitial voids between capillaries. The overall assembly can be solidified and collapsed to obtain a microstructure preform. This process can realize a cost effective PCF fabrication process. However, this process has the same problems regarding the capillary assembly process as the capillary stack and drawing method.
In this paper, we propose a novel PCF fabrication process that employs a modified slurry casting method to make a silica PCF preform. This approach results in simple and low cost PCF fabrication and provides improved precision and a highly flexible geometry. The process is used to fabricate a low-loss silica PCF with a reduced OH absorption loss, where the minimum loss is 1.1 dB/km at 1.55 μm and the OH absorption peak is as low as 3.1 dB/km at 1.38 μm. Following our preliminary report on the novel PCF fabrication method using the slurry casting method , here we elaborate on the slurry casting method for PCF preform fabrication, and provide detailed data regarding the PCF transmission properties. We also describe an improvement in the longitudinal structural uniformity and transmission loss that we achieved by employing pressure control in the drawing process, which enabled the minimum loss to be reduced to 0.86 dB/km at 1.55 μm.
2. Fabrication method
Figure 1 shows the PCF fabrication process based on the slurry casting method. The slurry casting technique was originally developed for fabricating silica glass in order to manufacture multi-hole silica capillaries for fiber assemblies . A highly pure SiO2 powder, which is manufactured by the thermal oxidation of silicon chloride, is used as a starting material. The desired amount of SiO2 powder, organic binder, dispersing agent and distilled water are mixed by ball milling. The obtained slurry is poured into a metal mold. Stainless wires with the desired outer diameter are arranged in this mold to fabricate air holes in the preform. It should be noted that various PCF structures can be easily realized depending on the size and geometry of the wires. Once the organic binder has solidified, the wires are removed and the preform is released from the mold. The obtained preform is carefully dried to prevent the generation of cracks. The dried preform is then calcined to remove organic chemicals by oxidation at a high temperature. The calcined preform is passed through a purification step to remove metallic impurities and OH and then sintered at 1400°C. The sintered preform has the desired microstructure as shown in the Fig. 1 inset.
Figure 2 shows photographs of a PCF preform fabricated by the slurry casting method after the drying process. The diameter of the dried preform is 24.6 mm and the length is 480 mm. The dried preform already has a similar structure to the final PCF. By sintering the dried preform, a transparent PCF silica glass preform can be obtained as shown in Fig. 3.The glass preform has a diameter of 20 mm and a length of 400 mm. The diameter and length of a PCF preform decrease by about 81% (the cross section area by about 66.5% and the volume by about 53%) during the sintering process.
This preform is drawn into a fiber while the pressure in the air holes is controlled to obtain a uniform microstructure in the PCF. The PCF drawing process is carried out at 1950°C at a speed of 100 m/min. The PCF is drawn to a diameter of 125 μm. In this work, two types of drawing systems, namely a closed air-hole drawing system and a pressure controlled drawing system, are used to fabricate the PCFs as shown in Fig. 4.In the closed air-hole drawing system, dummy silica rods are connected at both ends of the PCF preform to seal the air holes. In the pressure controlled drawing system, the silica tube is connected with one end of the PCF preform and a silica rod is connected to the other end. When the fiber is drawn, the gas pressure in the air holes of the PCF preform is kept constant to obtain a uniform longitudinal air hole structure. Helium gas is used to control the pressure in the air holes.
Figure 5 shows a cross-sectional view of a PCF fabricated by the slurry casting method. The air-hole diameter is 2.6 μm and the hole pitch is 5.6 μm. It can be seen that a uniform air-hole structure was successfully obtained.
3. Optical properties of fabricated PCF
Figure 6 shows the transmission loss of PCF fabricated by the closed air-hole drawing system without the OH reduction process. The fiber length was 135 m. In this condition, the PCF exhibits high transmission losses of 65.2 dB/km at 1.38 μm and 3.6 dB/km at 1.55 μm. The transmission losses were very high because of the OH absorption loss in the longer wavelength region. In the shorter wavelength region, the transmission loss increases rapidly. This loss increase is caused by metal impurities on the surface of the air holes in the PCF, which can induce the optical absorption of silica glass at a wavelength shorter than 1 μm even if the concentration is as low as 1 ppb .
To reduce the transmission loss, an OH reduction process was employed at 1300 °C in a helium gas atmosphere that included 20% chlorine gas just before the drawing process. The transmission loss of the fabricated PCF is shown in Fig. 7(a).Due to the OH reduction process, the minimum loss was reduced to 1.1 dB/km at 1.55 μm, and the OH absorption peak was as low as 3.1 dB/km at 1.38 μm. These values are excellent compared with those of previously reported PCFs with a similar structure fabricated with a conventional stack and draw method [12, 13]. The measured transmission loss was fitted into the expressionFig. 7(b). From this plot, we estimated the Rayleigh scattering coefficient to be 1.32 dB/km·μm4 and the imperfection loss to be 0.94 dB/km. The transmission loss, Rayleigh scattering coefficient and imperfection loss were successfully reduced with the present OH reduction process.
Figure 8 shows the transmission loss of PCF fabricated by the pressure controlled drawing system with OH reduction. The fiber length was 1.3 km. The transmission loss at 1.55 μm was 0.86 dB/km, which was lower than that of PCF fabricated by the closed air-hole drawing system. However, the OH absorption peak at 1.38 μm was 12.2 dB/km, which was higher than that in Fig. 7(a). This may be attributed to the redissolution of H2O in the helium gas that we used to control pressure, since the helium gas is contaminated with H2O, which is adsorbed on the inner surface of the polyvinyl chloride tube used for the gas flow. The imperfection loss of this PCF was estimated to be 0.5 dB/km, which was improved by 0.44 dB/km compared with that fabricated by the closed air-hole drawing system. Using our slurry casting method, the diameter of each hole in the PCF preform can be controlled to within ± 1 μm along its length, and we can obtain a PCF preform with a uniform air hole structure. Therefore, the pressure control drawing system is very effective for achieving more precise control of the air hole structure along the fiber length and for reducing the imperfection loss of PCF.
Figure 9 shows an optical time-domain reflectometer (OTDR) waveform of the 1.3 km PCF measured at 1550 nm. As shown in Fig. 9, the fabricated PCF was uniform along its length and no discontinuities were observed. These results suggest that the pressure controlled drawing system makes it possible to obtain PCF with a uniform longitudinal microstructure.
Table 1 compares the transmission properties with those of previous PCFs fabricated by the capillary stack and drawing method [12, 13] and the sol-gel method . It can be seen that the transmission losses of our PCFs are lower than those reported in  and . Furthermore, the slurry casting method allows us to realize both a lower transmission loss and a lower Rayleigh scattering coefficient than with the sol-gel method. The difference between the imperfection losses with the sol-gel method and the slurry casting method possibly depends on the surface roughness of the air holes or micro-defects such voids and bubbles generated in the silica glass.
On the other hand, the transmission loss of PCF reported by K.Tajima  is still lower than with the slurry casting method. In , K.Tajima reports that the surface roughness of air holes is responsible for both the imperfection loss and the Rayleigh scattering. It is also demonstrated that the intrinsic loss of PCF could be reduced below that of a conventional pure silica core fiber by removing the surface imperfections of the air holes. The estimated imperfection losses of our PCFs are 0.5~0.94 dB/km, which still leaves the possibility of further improvement. By removing the surface roughness in the air holes using the approach for VAD glass capillaries reported in , we expect to reduce the intrinsic loss to a level comparable to that obtained with standard fibers.
The slurry casting method has been successfully used to fabricate a low-loss silica PCF. The fabricated PCF had a minimum loss of 0.86 dB/km at 1.55 μm, an imperfection loss of 0.5 dB/km and a Rayleigh scattering coefficient of 1.7 dB/km·μm4. The slurry casting method is a very convenient and attractive method for fabricating PCF preforms with a highly flexible geometry, and has good potential for allowing PCF to be fabricated with a simple and inexpensive process.
References and links
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