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Antireflection (AR) layers at the tips of optical fibers are indispensable in high efficiency and low noise applications. We realized the AR structures with two-dimensional binary subwavelength gratings (SWGs) at the tips of optical fibers by using a dedicated UV nanoimprint machine. Using this technique, ideal AR structures with desired refractive indices can be realized at low cost in principle. The SWG with the period of 700 nm was fabricated at the tip of a single-mode optical fiber for optical communications system. The reflectance was decreased to less than 0.27% at measured wavelengths between 1460 nm and 1580 nm.
©2013 Optical Society of America
1. Introduction
Optical fibers have been used as components of optical communication devices and photometric analyzers. At the tips of optical fibers, a part of light is reflected back because of the Fresnel reflection which causes optical loss and noise. In order to reduce the Fresnel reflection, antireflection (AR) films have been fabricated at the tips of optical fibers. However, the cost issue remains because conventional AR films have been fabricated using vacuum devices such as sputtering devices and evaporation systems. In addition, it is difficult to fabricate ideal AR films due to the limitation of the refractive index of materials.
Recently, AR structures using subwavelength gratings (SWGs) have been extensively investigated [1–18]. Because the SWGs have periods smaller than the incident wavelength, they behave as uniform thin films with effective refractive indices based on effective medium theory. Since the effective refractive index can be defined using a filling ratio of a grating material, a desired refractive index between the refractive indices of the grating and the surrounding medium can be artificially realized by designing the SWG structure. Two-dimensional SWGs do not depend on the polarization direction. Therefore, they show promise for use in many optical components. The SWG with a tapered grating [1–11] suppresses reflection over a wide spectral bandwidth and a large field of view, although the higher grating height is needed to obtain lower reflectivity. On the other hand, the one with a binary grating [12–18] suppresses reflection completely at a design wavelength with lower grating height corresponding to a quarter-wavelength in the optical length. David et al. fabricated a two-dimensional binary SWG with a period of 200 nm on a polycarbonate substrate for AR structures in the visible range [12]. Hadobás et al. fabricated 300 nm periodically structured silicon SWGs [13]. Motamedi et al. reported on AR structures using silicon binary SWGs for infrared optical components [14].
Nanoimprint lithography is one kind of cost-effective nano-patterning methods which can make submicron structures easily at low cost because of process in the air, low machine cost, and short time fabrication [19–24]. There are also several reports on fabrication of the SWGs using nanoimprint lithography [10–12]. However, it is difficult to fabricate the SWGs at the tips of flexible and long optical fibers by using conventional nanoimprint machines in which molds and substrates are pressed by two parallel plates.
In this paper, we fabricate AR-SWGs with two-dimensional binary gratings at the tips of optical fibers by using a dedicated ultra violet (UV) nanoimprint machine we developed. Optical characteristics of the fabricated SWGs are measured.
2. Design
Figure 1 shows a schematic view of an SWG at the tip of an optical fiber. On a core region of the optical fiber, the SWG with a two-dimensional binary grating, which consists of a UV-curable polymer, is formed. The optical fiber consists of SiO2 having a single-mode property at a wavelength of 1550 nm.
Generally, optical fibers can accept or emit light in the range of angles which are characterized by numerical aperture (NA). The period of the SWG should be designed to be enough small not to generate high-order diffraction lights at the angles characterized by the NA of 0.22 in the used optical fiber. When the light is incident from an incident medium with a refractive index of ni to a substrate medium with a refractive index of ns, the condition of the SWG which generates only the zero-order diffraction light is expressed as follows:
where Λ and λ are the period and the incident wavelength, respectively. The refractive indices of the air and the core are 1.0 and 1.458, respectively, at a wavelength of 1550 nm. By taking into account the Eq. (1), the SWG period is designed to be 700 nm. Next, the SWG height h is designed with AR condition for a single-layer AR coating at a normal incident angle. The AR condition can be expressed as follows: where N is an optimum refractive index of the AR layer, which is calculated to be 1.207 from Eq. (2). Using Eq. (3), the optimum SWG height at the wavelength of 1550 nm is calculated to be 321 nm. With the period and the height described above, the optimum SWG diameter to obtain the minimum reflectance was calculated to be 521 nm using rigorous coupled-wave analysis (RCWA) [25].3. Fabrication
Figure 2 shows the fabrication steps. Firstly, a mold and the tip of an optical fiber are coated with a UV-curable polymer (PAK-01, Toyo Gosei Co.) as shown in Fig. 2(a). The optical fiber (OKOP-SM28, Okano Cable Co.) with a single-mode property and a physical contact polishing grade was used. Next, the optical fiber is pressed with the pressure of 0.18 MPa to the mold (Fig. 2(b)). The mold is heated to 50 ◦C in order to reduce the required pressure since viscosity of the UV-curable polymer is reduced with increasing the temperature. Next, 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 (Fig. 2(c)). At this step, only the UV-curable polymer near the core region is selectively polymerized. In order to prevent oxygen inhibition, the polymerization is carried out in nitrogen environment. The light is irradiated to the UV-curable polymer at the intensity of 1.1 μW for 5 min. Finally, the optical fiber is released from the mold (Fig. 2(d)).
A schematic of a dedicated UV nanoimprint machine for the tips of optical fibers is shown in Fig. 3 . The machine consists of a fixing jig, stages, goniometers, a load cell, a heater, a nitrogen gas line, a CCD camera, a thermocouple, a temperature indicator, a pressure indicator, and the mercury lamp. An optical fiber is fixed with the fixing jig. UV light from the mercury lamp is introduced from an opposite side of the optical fiber. The tip of the optical fiber is pressed to the mold when the stage is moved down. The pressure is measured with the load cell and indicated with the pressure indicator. The temperature is measured with the thermocouple and indicated with the temperature indicator. The CCD camera is used for alignment. The nitrogen gas is flowed into between the optical fiber and the mold.
Figure 4 shows a scanning electron microscope (SEM) image of the fabricated mold consisting of silicon. The period, the hole diameter, and the height are 700 nm, 520 nm, and 270 nm, respectively. Although the height became approximately 50 nm lower in comparison with the designed one, the period and the hole diameter were fabricated as designed. The height can be improved by increasing the etching time. The mold was fabricated as follows. Firstly, a single-crystal silicon wafer with a 400 μm thickness was coated with an electron-beam resist (ZEP520A, Zeon Co.) with a 390 nm thickness, followed by an electron-beam patterning (JBX-5000LS, JEOL Ltd.) at the dose of 80 μC/cm2, the beam current of 100 pA, and the acceleration voltage of 50 keV. After that, using the resist as a mask, silicon was etched by using a fast atom beam (FAB-60LM, Ebara Co.) of an SF6 gas. Next, a residual resist was removed with H2SO4 and H2O2 solutions. Finally, the mold was coated with a release agent (0.1 wt% Optool DSX, Daikin Industries, Ltd.) to prevent adhesion of the UV-curable polymer to the mold.
Figure 5 shows an SEM image of the fabricated SWG at the tip of the optical fiber. It is clear that the SWG is fabricated selectively centering on the core region. The period, the diameter, and the height of the SWG are 700 nm, 560 nm, and 250 nm, respectively. When compared with the mold, the period is exactly same value. However, the diameter and the height of the SWG became 40 nm wide and 20 nm low, respectively. They seem to be caused by the structural deformation because the tip of the optical fiber came in contact with a fixing jig and the SWG was compressed in a vertical direction, at a surface measurement after the releasing of the optical fiber from the mold. As the cause of the reduction of the height, additionally, insufficient filling of the UV-curable polymer to the mold are considered. As shown in Fig. 5, the diameter of the cured area is 420 μm which is larger than the core diameter of 9.5 μm and the clad diameter of 125 μm. Because the UV-light is diffracted by the SWG and propagated into the UV-curable polymer in a horizontal direction, the cured area becomes wider than the core diameter.
4. Optical characteristics
The optical characteristics were measured by using a micro-spectroscope system in which a tunable laser (8164A, Agilent Technologies, Inc.) was used as a light source. The measured values were compared with the calculated values which were obtained using the RCWA. A calculation model, as shown the sectional view in Fig. 6 , was constructed from the fabricated SWG structure which was observed by an SEM. As shown in Fig. 6, existence of a residual UV-curable polymer at the nanoimprint process is taken into account in the calculation. The period Λ of 700 nm, the diameter a of 560 nm, the height h of 250 nm, and the residual polymer thickness d of 4500 nm were used in the calculation. Also, refractive indices of 1.458 and 1.504 were respectively used for the core (SiO2) and the UV-curable polymer. The refractive index of the UV-curable polymer was a measured value obtained by means of prism coupling technology using a laser with the wavelength of 1550 nm.
Fig. 6 A schematic of the sectional view of a calculation model which was constructed from the fabricated SWG structure observed by an SEM. Λ, a, h, and d show the period, the diameter, the height, and the residual thickness of the UV-curable polymer, respectively. n means a refractive index.
Figure 7 shows the reflectance spectra at the tip of the optical fiber. In the case of the tip without the SWG, the measured reflectance is about 3.8% which almost agrees with a calculated value of 3.5%. In the case of the tip with the SWG, on the other hand, the measured reflectance is decreased to less than 0.27% at measured wavelengths between 1460 nm and 1580 nm. For example, the reflectance is decreased to be 0.2% at the wavelength of 1550 nm. In comparison with the calculated values, the measured reflectance almost agrees with the calculated values. Although the calculated spectrum fluctuated like a sinusoidal wave, it is caused by interference within a layer of the residual UV-curable polymer. On the other hand, interference characteristics do not appear in the measured spectrum because coherency of the light is less and the flatness of the interference surfaces are not perfectly flat compared with the calculated model.
5. Conclusion
AR-SWGs with two-dimensional binary gratings were fabricated at the tips of optical fibers by using a dedicated UV nanoimprint machine. The period, the diameter, and the height of the SWG were 700 nm, 560 nm, and 250 nm, respectively. The SWG was formed selectively centering on the core region. The optical characteristics were measured by using a micro-spectroscope system. The reflectance was decreased to less than 0.27% at measured wavelengths between 1460 nm and 1580 nm. For example, the reflectance was decreased to be 0.2% at the wavelength of 1550 nm. By taking into account of a residual UV-curable polymer underneath of the SWG layer, the measured results almost agreed with the calculated values.
Acknowledgments
A part of this work was supported by Creation of innovation centers for advanced interdisciplinary research areas Program.
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