A simple method to prevent reflection at optical fiber interfaces
Optical fibers are used in optical communication devices and photometric analyzers. Antireflection (AR) layers at the tips of optical fibers are indispensable for reducing optical noise and propagation loss by Fresnel reflection at interfaces, which depends on the relative refractive indices of the two media. However, conventional thin-film AR layers are costly to prepare, and many thin-film layers are required for broadband AR.
AR structures consisting of subwavelength gratings (SWGs), which have periodic structures with the periods smaller than operating wavelengths, have been extensively investigated1, 2 and achieve the desired refractive indices for ideal AR conditions. SWGs can be fabricated at low cost by nanoimprint lithography (NIL).3, 4 However, conventional NIL, in which molds and substrates are pressed by two parallel plates, is challenging at the tips of flexible and long optical fibers. We have developed a dedicated UV-NIL system for this purpose.
We use the optical fiber core itself to guide light from a mercury lamp to cure a UV-curable polymer and form a 2D binary SWG at the tip of an optical fiber: see Figure 1(a).5 Figure 1(b) shows the refractive index distribution of the SWG and its surrounding media. The effective refractive index of the SWG can be controlled by its filling ratio, and thus we can obtain an optimum refractive index for a single-layer AR coating. Figure 2 shows how we prepare the SWG. First, a mold and the tip of an optical fiber are coated with a UV-curable polymer. Next, the optical fiber is pressed to the mold and the UV-curable polymer is polymerized using a mercury lamp in which the light is propagated into the core from the other end of the optical fiber. During this step, only the polymer near the core region is polymerized. Finally, the optical fiber is released from the mold.
Figure 3 shows scanning electron microscopy images of the fabricated SWG at the tip of the optical fiber. It is clear that the SWG is selectively fabricated at the center of the core. The measured period, diameter, and height of the SWG are 700, 560, and 250nm, respectively. Figure 4 shows measured reflectance spectra at the tip of the optical fiber. In the case of the tip without the SWG, measured reflectance is about 3.8%, but for the tip with the SWG, measured reflectance is less than 0.27% over wavelengths between 1460 and 1580nm. For example, reflectance is only 0.2% at 1550nm.
In conclusion, we have successfully used a dedicated UV-NIL system to fabricate an SWG with a period of 700nm, a width of 560nm, and a height of 250nm at the tip of a single-mode optical fiber for an optical communications system. Measured reflectance decreased to less than 0.27% at measured wavelengths between 1460 and 1580nm. For example, reflectance decreased to 0.2% at a wavelength of 1550nm. Theory shows that lower reflectance is possible, and as a next step in this direction we plan to improve the fabrication accuracy.
Part of this work was supported by the Creation of Innovation Centers for Advanced Interdisciplinary Research Areas Program of the Japanese Ministry of Education, Culture, Sports, Science and Technology.
Yoshiaki Kanamori received master's and doctoral degrees from Tohoku University in 1998 and 2001, respectively. He has been an associate professor there since 2007, working on the research and development of nanophotonics and optical microelectromechanical systems.