A polarization insensitive technique for highly efficient coupling between SOI waveguides and high mode field diameter single-mode fibers is reported. The proposed coupling structure is based on an inverted taper structure coupled to a fiber-adapted waveguide. The fiber-adapted waveguide is made by using the SiO2 layer under the Si waveguiding layer of the SOI wafer thus avoiding the use of extra materials such as polymers. The proposed coupling structure is aimed for being integrated with V-groove auto-alignment techniques. Coupling losses of 3.5dB and 3.7dB to 8μm mode field diameter single-mode fibers have been estimated by means of 3D-BPM simulations for TE and TM polarizations respectively and a 1550nm input signal wavelength.
© 2007 Optical Society of America
Efficient coupling between standard single-mode fibers and single-mode waveguides is a key point in silicon photonics. An optical integrated circuit is useless without an interface to the outside world. The small size of single-mode silicon-on-insulator (SOI) waveguides (typically 500nm width and around 200nm thickness) compared with the high 8μm diameter of a single-mode fiber makes coupling inefficient. A direct end-fire coupling between a SOI single-mode waveguide and a standard single-mode optical fiber means much more than 20dB of coupling losses for TE polarization and 1550nm input signal wavelength. Three-dimensional (3D) tapers have been reported to achieve 3D spot-size conversion between the spot-sizes of the waveguide and the fiber . More complex structures such as two different tapers formed at different levels have also been proposed . However, the complexity of the fabrication significantly increases in 3D approaches and the coupling length is more than 1mm. A more elegant and compact solution that is compatible with planar processing techniques is the use of two-dimensional (2D) inverted tapers to achieve 3D spot-size conversion [3–9]. In this case, the width of the taper is gradually decreased thus delocalizing the mode profile out of the waveguide core.
The tip of the inverted taper can be directly attached to the optical fiber [4,5]. However, in this case, very precise control on the chip facets position is required and the coupling structure is polarization sensitive. Therefore, the mode out of the inverted taper is usually coupled into another fiber-adapted waveguide. This fiber-adapted waveguide is made of low index contrast polymer materials and it is placed on top of the inverted taper [6–9]. However, the inverted taper and the low-index contrast waveguide are optimized for low coupling losses into small core optical fibers with typically 3-4 μm mode field diameters. In this paper, a new coupling structure based on the inverted taper is proposed for low coupling losses between SOI waveguides and high mode field diameter single-mode fibers. The proposed approach is polarization insensitive and it takes advantage of the SiO2 layer under the Si waveguiding layer of the SOI wafer to obtain the fiber-adapted low-index contrast waveguide. Furthermore, the structure is designed by means of simulations with the aim of future integration with V-groove structures thus allowing passive alignment and easier packaging .
2. Proposed structure
The proposed inverted taper-based structure is shown in Fig. 1(a). The single-mode SOI waveguide is tapered down by the inverted taper. SOI wafers of 205nm/3μm Si/SiO2 layer thicknesses have been taken into account, so the Si waveguide width has to be around 500nm to achieve single-mode propagation. The main inverted taper design parameters are the inverted taper length (Lt) and the inverted taper tip width (Wt) both illustrated in Fig. 1(a). If the substrate of the SOI wafer is removed, it is possible to use the SiO2 layer as the fiberadapted waveguide, as shown in Fig. 1(a). Due to the SiO2 SOI wafer layer thickness, the height of the SiO2 waveguide is fixed to 3μm. The objective is to find the optimum SiO2 waveguide width and the optimum inverted taper parameters to achieve the lowest coupling losses between the proposed structure and the single-mode fiber. Furthermore, the design is carried out to obtain a polarization insensitive structure. The most important application of the proposed structure is its direct integration with V-groove auto-alignment structures as illustrated in Fig. 1(b). A step by step analysis of the proposed inverted taper-based structure is presented on the following sections. The analysis starts with the SiO2 waveguide design to achieve the lowest coupling losses to high mode field diameter single-mode fibers. Then, the optimum inverted taper tip width and length are designed.
3. SiO2 waveguide design
The objective is to find the optimum SiO2 waveguide width to get the lowest coupling losses between the SiO2 waveguide and the fiber. Coupling losses have been theoretically calculated by means of the following overlap integral :
where F 1 and F 2 are the fiber and SiO2 waveguide fundamental mode profiles, respectively. The fundamental mode profile of the SiO2 waveguide has been obtained by means of a 3D mode solver based on the Beam Propagation Method (BPM). The fundamental mode profile of the fiber has been approximated by a Gaussian beam. Figure 2 shows the estimated coupling losses between the SiO2 waveguide and the fiber as a function of the fiber mode field diameter (MFD) for different SiO2 waveguide widths and a 1550nm input signal wavelength. A polarization insensitive behaviour of the coupling efficiency in the SiO2 waveguide-fiber interface is achieved so results shown in Fig. 2 are valid for both TE and TM polarizations. As it is shown in Fig. 2, the estimated coupling losses for an 8μm wide SiO2 waveguide and a single-mode fiber with MFD=8μm are around 3dB. If the SiO2 waveguide gets wider (e.g. 10μm width), the estimated coupling losses are only 0.3dB less than for the 8μm wide SiO2 waveguide. Therefore, in order to minimize as much as possible the SiO2 waveguide dimensions, the 8μm wide SiO2 waveguide case has been chosen.
4. Inverted taper design
4.1 Inverted taper tip width design
The interface depicted in Fig. 3 has been considered to find the optimum tip width (Wt) of the inverted taper. The design process is similar to the one described in the previous section. According to Fig. 3(a), it is possible to obtain the fundamental mode profiles of both the fiber-adapted SiO2 waveguide with and without the inverted taper on top, as it is shown in Fig. 3(b). Coupling losses are then calculated by means of the overlap integral expressed in Eq. (1).
Figure 4 shows the coupling losses between the SiO2 waveguide with and without the inverted taper on top as a function of the tip width for TE and TM polarizations and for a 1550nm input signal wavelength. As it can be seen in Fig. 4, almost negligible coupling losses for both TE and TM polarizations are achieved when the inverted taper tip width is lower than 200nm. On the other hand, coupling losses also do not significantly change as the inverted taper tip width is higher than 400 nm. This occurs because the mode is mainly located in the high index contrast silicon waveguide and therefore only a small part of the mode profile overlaps with the mode profile of the SiO2 waveguide. In our case, inverted taper tip widths lower than 200nm are desirable to achieve a polarization insensitive coupling structure, as the SiO2 waveguide-optical fiber interface is also polarization insensitive. However, wider taper tips are more suitable to reduce the complexity of the fabrication. Therefore, the optimum value will be 200 nm.
4.2 Inverted taper length design
The optimum inverted taper length has been designed by means of three-dimensional (3D) BPM simulations using the inverted taper tip width (Wt=200nm) and the SiO2 waveguide width (Wg=8μm) that were found to be the optimum ones in the previous sections. Figure 5 shows the field distribution in a 400μm long inverted taper obtained by means of a 3D-BPM simulation. The structure is excited by the SiO2 waveguide fundamental mode at a 1550nm wavelength for each polarization. The power coupled at the 500nm wide Si waveguide is measured to evaluate coupling losses.
Figure 6 shows the coupling losses as a function of the inverted taper length for both TE and TM polarizations and a 1550nm input signal wavelength. Looking at Fig. 6, it can be seen that coupling losses decrease as the inverted taper gets longer due to the lower mode mismatching. Furthermore, it is interesting to notice that this improvement is similar for both TE and TM polarization. On the other hand, it can be seen that the highest coupling losses for the case of a zero length taper are in agreement with the results shown in Fig. 4 for the case of 500nm width. An inverted taper lengh of 400μm has been chosen as the most optimum one taking into account the trade off between minimum coupling losses and short lengths for both polarizations. For this length value, 0.5dB coupling losses are achieved for TE polarization while 0.7dB coupling losses are achieved for TM polarization.
It is important to remind that as the fiber-SiO2 waveguide interface has not been considered in the 3D BPM simulation on Fig. 5 the total coupling losses of the structure will be the sum of the coupling losses in the fiber-SiO2 waveguide interface, which were estimated in 3dB for both polarizations, and the 0.5dB and 0.7dB coupling losses estimated in Fig. 6 for a 400μm long inverted taper and TE and TM polarizations respectively. Therefore, the total coupling losses for the considered wavelength of 1550 nm will be 3.5dB for TE polarization and 3.7dB for TM polarization.
The influence on the coupling performance when a part of the 400μm long inverted taper is located on top of the SiO2/Si layers instead on being completely located on top of the SiO2 waveguide has also been investigated in order to reduce the length of the SiO2 waveguide that is hanging on air and thus increasing the mechanical robustness of the structure. Figure 7 shows a description of the simulated structure where the goal is to find the minimum value of the d parameter without degrading coupling losses.
Coupling losses as a function of the d parameter for both TE and TM polarizations are shown in Fig. 8. It can be seen that coupling losses increase for high d values due to leakage losses into the silicon substrate. However, coupling losses are almost flat for d values smaller than 325μm indicating that light quickly couples from the SiO2 waveguide to the higher index of the inverted taper, as it can also be observed in Fig. 5. Therefore, it can be concluded that only the first 75μm of the inverted taper length must be necessary located on top of the SiO2 waveguide.
5. Spectral response
The spectral response of the proposed coupling structure has also been obtained taking into account the design parameters calculated previously at a 1550 nm input signal wavelength. Figure 9 shows the spectral response of the coupling losses of a 500nm wide single-mode SOI waveguide coupled to an 8μm MFD single-mode fiber by means of the proposed coupling structure. It can be seen an almost flat spectral response in the considered wavelengths range for both TE and TM polarizations. This flat spectral response is achieved because the coupling structure does not rely on any resonant effect. Finally, it is important to point out that coupling losses could be reduced by using fibers with a lower MFD, as it can be seen in Fig. 2.
In this paper, we report a polarization insensitive fiber-to-SOI waveguide efficient coupling technique using an inverted taper-based structure. The SiO2 layer of the SOI wafer is considered to make the fiber-adapted waveguide thus avoiding the use of extra materials such as polymers. The proposed coupling structure is aimed for being integrated with V-groove auto-alignment techniques. Parameters have been theoretically designed to minimize the coupling losses to a single-mode fiber with MFD=8μm. Coupling losses of 3.5dB and 3.7dB for both TE and TM polarizations respectively and a 1550 nm input signal wavelength have been obtained using a 400μm long inverted taper. Lower coupling losses may be achieved by using fibers with a lower MFD. Furthermore, a flat spectral response is achieved.
Financial support by EC under project 017158-PHOLOGIC and Spanish MEC and EUFEDER under contract TEC2005-07830 SILPHONICS is acknowledged. Authors also acknowledge financial support by Generalitat Valencia.
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