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In this paper, an efficient and novel optical fiber-to-waveguide mode size converter for Si Photonics devices with sub-micron waveguides is developed on the SOI platform. This optical converter is composed of a suspended SiO2 waveguide and overlapped Si nano-tapers located in the center of suspended SiO2 waveguide. Laterally connected SiO2 beams provide structural support for the suspended SiO2 waveguide. The optical input signal from the optical fiber is launched into the suspended SiO2 waveguide, and then coupled into the Si nano-taper. The measured coupling loss using a lensed fiber with 5 µm spot diameter is 1.7 ~2.0 dB/facet for TE mode and 2.0 ~2.4 dB/facet for TM mode in the wavelength range of 1520 ~1600 nm. When a cleaved fiber with 9.2µm spot diameter is used, the coupling losses for both TE and TM modes are less than 4.0 dB/facet at 1550 nm. For the case of lensed fiber, the alignment tolerances for both TE and TM modes are about ± 1.7 µm for 1 dB excess loss in both X and Y axes. The alignment tolerances for both modes of TE and TM are relaxed, exceeding ± 2.5 µm for 1 dB excess loss in both X and Y axes when a cleaved fiber is used.
©2010 Optical Society of America
1. Introduction
Silicon photonics devices based on Silicon-on-Insulation (SOI) have become an active research and development field, due to the high refractive index contrast between the silicon core and the oxide cladding. The high refractive index contrast enables high-density integrated optical circuits, which can be fabricated by standard complementary metal-oxide-semiconductor (CMOS) technology. It is possible to realize mass production and reduce the cost effectively. In the past decade, silicon photonic has been attracting to many research groups in the world and many optical SOI components have been demonstrated, including compact and low loss SOI passive waveguide devices [1–3], high speed silicon modulator [4–8], high speed GeSi modulator [9–11], silicon Raman lasers [12–14] and other thermo-optic devices [15,16]. Although silicon photonics devices on SOI platform have many advantages and have been widely developed, there is a disadvantage for silicon photonics devices’ application. The waveguide dimension of silicon photonics device is very small, only several hundreds of nanometers. Thus, there is a mode size mismatch between the nano-scale Si waveguide and the single mode fiber (SMF). The coupling loss is very high if the nano-scale silicon waveguide directly couples with the single mode fiber. At the same time, the alignment tolerance is small; and it is a challenge to package silicon photonics devices with single mode fiber. So, the optical mode size converter between the single mode fiber and nano-scale Si waveguide is a vital component for all of silicon photonics devices.
Some important research works have contributed to the development of low loss optical mode size converter for silicon photonic devices. The simplest structure is Si nano-tip covered with a low refractive index SiO2 layer [17,18]. This converter has 2.0 ~3.0 dB/facet coupling loss with a tapered fiber. This kind of converter is an excellent characterization solution with the tapered fiber for silicon photonic devices; however, it is difficult to be packaged because of the tight alignment tolerance. A low loss polymer converter has been demonstrated by NTT [19]. The polymer covers the nano-scale Si tip and the coupling loss to optical fiber is about 0.8 dB/facet. However, the process is not compatible with CMOS technology. Later, NTT fabricated the same converter using SiON material as the cladding waveguide and the coupling loss is about 2.5 dB/facet [20]. Another important converter is the vertical grating coupler [21–23]. The vertical grating coupler has a flex alignment tolerance; however, it is a wavelength dependent device.
In this work, we designed and fabricated an efficient and novel optical fiber-to-waveguide mode size converter for Si Photonics devices based on nano-scale waveguides on the SOI platform. It is almost wavelength independent. It is composed of a suspended SiO2 waveguide and an overlapped Si nano-taper located in the center of the suspended SiO2 waveguide. In order to strengthen the suspended SiO2 waveguide, narrow SiO2 beams were fabricated to connect the suspended SiO2 waveguide to the deposited SiO2 layer. If compared to previous converters, the process is very simple and easy to control. At the same time, this converter provides a good solution to package with the optical fiber.
2. Design and fabrication
As we know, the optical single mode fiber is made of the SiO2 material. Compared to other materials, the major advantage of SiO2 converter is the improvement in transmission and reduction in return loss from optical fiber to converter. In order to confine the optical mode within the SiO2 waveguide, it is necessary to etch and isolate the SiO2 waveguide from the Si substrate, so as to form a suspended SiO2 waveguide.
Figure 1 shows the schematic diagram of our converter. It is composed of a suspended SiO2 waveguide and a Si nano-taper. In order to compress the mode size in the vertical direction (Y axis), a partial etch of SiO2 is used to create a ridge on top of the SiO2 waveguide which tapers laterally down to a minimum width of 300 nm at its termination, forming a terminated taper. The suspended SiO2 waveguide is also laterally tapered such that the dimensions are larger at the input facet and smaller at the Si taper. This lateral taper serves to compress the input optical mode size to match the Si nano-taper mode size in the horizontal direction (X axis).
A lateral Si nano-taper is used to match the optical mode of the SiO2 waveguide in the horizontal direction. At the same time, in order to enlarge the vertical optical mode size of the Si nano-taper to match the mode of the SiO2 waveguide, an overlapped Si nano-taper is employed. The optical signal from the optical fiber is first coupled into the suspended SiO2 waveguide. Then, the optical signal propagates in the suspended SiO2 waveguide and is laterally and vertically compressed by the SiO2 lateral taper and the SiO2 terminated taper, respectively. Then, the optical signal is coupled into the overlapped Si nano-taper, and finally into the Si waveguide.
The suspended SiO2 waveguide is 6 µm × 6 µm in cross-section at the input end. Laterally, it tapers down from 6 µm to 2 µm in width along a 60 µm-long section. The overlapped Si nano-tapers are located in the narrow SiO2 waveguide and have a 5 µm distance from the end of the above-mentioned lateral taper. The gap between the SiO2 waveguide and the adjacent SiO2 layer of 2 µm is to avoid the optical signal leakage into the adjacent SiO2 layer. Also, in order to avoid the optical signal leakage into the Si substrate, the gap between the SiO2 waveguide and Si substrate is more than 2 µm. Another SiO2 terminated taper with a 300 nm-wide tip is designed on the lateral taper. It is 1 µm thick and 50 µm long. SOI wafers with 220 nm top Si layer and 2 μm buried oxide (BOX) was used. As such, the overlapped Si nano-taper is near the center of suspended SiO2 waveguide, allowing for optimal coupling efficiency between the suspended SiO2 waveguide and the Si nano-taper. The overlapped tapers are shown in Fig. 1 (inset). The two Si tapers have the same lengths and widths; but have different thicknesses. The designed top Si taper is 150 nm wide and 140 nm thick. The suspended SiO2 waveguide is about 150 um long. In order to provide the support for the suspended SiO2 waveguide, 2 pairs of SiO2 beams are designed, as shown in Fig. 1. The length of the beam is only 1 µm along the propagation direction to minimize the optical crossing loss. The designed dimension of Si waveguide connect with the overlapped Si nano-taper is 220 nm (thickness) × 500 nm (width). The tip width of the Si nano-taper is critical for the coupling loss. In fact, the optical loss caused by the beam can be negligible. So, in the simulation, the structure is without the narrow beams. The suspended structure is simulated using the beam propagation method (BPM) of RSOFT software, shown in Fig. 2 . The input fiber mode field diameter is 6 µm. Figure 2(a) is the mode field distribution with the 30 nm wide Si tip and it shows coupling efficiency is very good. Figure 2(b) is the coupling loss per facet vs. tip width of Si nano-taper and it shows that a coupling efficiency of up to 82% (~0.9 dB/facet) can be obtained for TE mode at a wavelength of 1.55 um when the tip width is 30 nm. Due to the limitation of our process, we fabricated the Si taper with a 110nm-wide tip, which corresponds to a simulated coupling loss of about 1.4 dB/facet.
Fig. 2 Simulation results of suspended converter at 1550 nm for TE mode: (a) Mode field distribution (Z axis is the propagation direction); (b) coupling loss per facet vs. tip width of Si nano-taper
In our experiment, a 200 mm-diameter Si wafer with 220 nm top Si and 2 µm BOX was used. The process flow chart is shown in Fig. 3. After 75 nm PECVD SiO2 deposition as the hard mask (HM), the waveguide and top taper structures are patterned using 248 nm deep UV lithography technology, as shown in Fig. 3(a). The photo-resist (PR) pattern size is closed to the design value. Then, the PR pattern is trimmed using O2 gas before etching the SiO2 hard mask. After trimming, the tip width of the taper and the waveguide width become 110 nm and 450 nm, respectively. The SiO2 hard mask is then opened using carbon tetrafluoride (CF4) and 140 nm Si is partially etched using chlorine (Cl2) and oxygen, as shown in Fig. 3(b). The formation of top Si nano-taper is completed by the first partially Si etching. In order to form the bottom Si nano-taper, the wafer is patterned and trimmed again; and then, the remained 80 nm Si is etched using Cl2, shown as in Fig. 3(c), 3(d). After this process, the overlapped Si nano-taper and Si waveguide is formed. In Fig. 4(a) , 4(b), the scanning electron microscope (SEM) images of overlapped Si nano-taper are shown. The left SME image is the bottom Si nano-taper on the BOX; and the right SME image is the top Si nano-taper on a slab waveguide which is the same layer with the bottom Si nano-taper. After formation of overlapped Si nano-tapers, a 4 µm PECVD SiO2 layer is deposited. So, the SiO2 surrounds the Si waveguide. Then, the 1 µm-thick SiO2 terminated taper is patterned and etched, as shown in Fig. 3(e), 3(f). Its SEM image is shown in Fig. 4(c). Later, the SiO2 waveguide and deep trench are patterned, as shown in Fig. 3(g). After patterning, the SiO2 layer is etched by octofluorocyclobutane (C4F8); and Si layer is etched by sulfur fluoride (SF6) to form the suspended SiO2 waveguide and Si deep trench, as shown in Fig. 3(h). The processed suspended converter is shown in Fig. 4(d). Figure 4(e) is the input port of the suspended converter. The deep trench process eliminates the need for mechanical polishing prior to optical measurements. Finally, the processed wafer is diced for measurement. In order to get the coupling loss of the suspended converter, we design the cut-back structures, including 5 Si waveguides (WG1~WG5) with the same suspended converters at the input/output ports. All of cut-back structures have the same dimension and the same bend waveguides. The diameter of bend waveguides is 20 µm, so the bend loss can be negligible. The lengths of WG1~WG5 are 4.06 mm, 8.06 mm, 12.06 mm, 16.06 mm and 20.06 mm, respectively.
Fig. 4 SEM images of the suspended converter: (a) Bottom Si taper; (b) Top Si taper; (c) SiO2 terminated taper; (d) Suspended converter; (e) Input port of suspended converter.
3. Results and analysis
The coupling loss of the converter can be characterized by the cut-back method using a high-precision tunable light source with broad band wavelengths, a polarization controller, a polarizer and an optical high-precision power meter. First, we used two lensed optical polarization-maintaining (PM) fibers (mode spot diameter ~5 µm) to couple the Si cut-back waveguides with input/output mode size converters. In order to measure the insertion loss of TE/TM mode, we adjust the polarization controller and rotate the PMF holder before testing the waveguides.
The measured insertion loss of TE/TM mode is shown in Fig. 5 . According to these spectra of insertion loss, we can calculate the coupling loss and propagation loss at different wavelengths, as shown in Fig. 6 . At the wavelength range of 1520 ~1600 nm, the coupling loss and propagation loss is 1.7 ~2.0 dB/facet and 2.5 ~2.6dB/cm for TE mode, respectively; and, 2.0 ~2.4 dB/facet and 2.9 ~3.0 dB/cm for TM mode, respectively. We also measured the coupling loss using the cleaved fiber with a 9.2 µm mode spot diameter at 1550 nm. The coupling loss at 1550nm is 3.8 dB/facet for TE mode and 4.0 dB/facet for TM mode, as shown in Fig. 7 .
Fig. 5 Insertion loss of cut-back waveguides for TE/TM mode measured with 5 µm spot diameter lensed fibers
The alignment tolerance of the optical converter is a very critical parameter for the fiber assembly. Hence, the tolerances of our converter in both X and Y axes at 1550 nm are characterized. From the above results of cut-back structures, the propagation loss of Si waveguide is 2.5 dB/cm for TE mode and 2.9 dB/cm for TM mode. The total propagation loss of Si waveguide is decoupled from the measured insertion loss; and the measured alignment tolerances are shown in Fig. 7. For the lensed fiber with a 5 µm mode spot diameter, the alignment tolerances in both X and Y axes are ± 1.7 µm for 1dB excess loss. For the case of cleaved fiber with 9.2 µm mode spot diameter, the alignment tolerances in both X and Y axes exceed ± 2.5 µm for 1dB excess loss. We also used the lensed fibers and cleaved fibers to couple with the Si nano-tip only covered by the SiO2 cladding, similar to the structure reported in reference [17]. Without the suspended structure, the coupling loss of Si nano-taper is 2.5 ~3.0 dB/facet using the lensed PM fiber for both TE and TM modes, with an alignment tolerance of less than ± 1 µm for 1 dB excess loss. It also shows that the coupling loss of the nano-taper coupled with cleaved fibers is very high (~8 dB/facet) for both TE and TM modes. So, this suspended converter can improve the alignment tolerance and reduce the coupling loss.
4. Conclusion
An efficient and novel optical fiber-to-waveguide mode size converter for Si Photonics devices has been presented in this paper. This converter is composed of the suspended SiO2 waveguide and the overlapped Si nano-taper. The measured results show coupling loss of the converter coupled with the lensed fibers is 1.7 ~2.0 dB/facet for TE mode and 2.0 ~2.4 dB/facet for TM mode in the broad wavelength range of 1520 ~1600 nm. The alignment tolerance is ± 1.7 µm for 1dB excess loss in both X and Y axes at 1550 nm. For the case of the cleaved fiber with 9.2 µm mode spot diameter, the coupling loss is less than 4.0 dB/facet for TE/TM mode; and the alignment tolerance exceeds ± 2.5 µm for 1dB excess loss. The converter can be further optimized to reduce the coupling loss with the cleaved fiber by increasing the dimension of the input SiO2 waveguide in X and Y axes. It would be a useful component to improve the coupling efficiency in the fiber attachment process.
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