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Using silicon photonic wire waveguides, we constructed compact 1 × 1, 1 × 2, and 1 × 4 Mach-Zehnder interferometer type optical switches on a silicon-on-insulator substrate and demonstrated their switching operations through the thermo-optic effect. These switches were smaller than 140 × 65, 85 × 30, and 190 × 75 μm, respectively. At a 1550-nm wavelength, we obtained an extinction ratio larger than 30 dB, a switching power as low as 90 mW, and a switching response time of less than 100 μs. Furthermore, switching operations were successfully demonstrated for the 1 × 4 switch.
©2005 Optical Society of America
1. Introduction
Silicon photonics is attracting much attention [1–5] due to the integration leverage from well-developed silicon processing technologies and the monolithic integration of both photonic and electronic devices. Moreover, there is a growing demand for the fabrication of even more compact optical devices on silicon-on-insulator (SOI) substrates. This is because the silicon-channel waveguides that are formed on SOI substrates can confine the optical field to a small core area, one that is more than 100 times smaller than the modal area of a standard single-mode fiber [2]. In particular, silicon photonic wire waveguides with a nanometer-order cross-sectional-size silicon core are considered to be a promising platform for various types of optical devices [6–10]. This is because devices based on such waveguides can have a bending radius of less than several micrometers due to the large difference in the refractive index between the silicon core (n = 3.5) and silica cladding materials (n = 1.5) or air (n = 1).
There is also a growing demand for compact optical switches for use in photonic integrated circuits in future photonic networks. Since the large silicon thermo-optical coefficient, dn/dT, can strongly affect the lightwaves that propagate in a silicon waveguide through the thermo-optic effect, a thermo-optically controlled optical switch on a silicon waveguide would therefore be compact and easy to control. A lot of research has been devoted to thermo-optic switches fabricated on SOI substrates [11–15]. Included in this is a new 1.5-mm-long optical switch based on silicon photonic wire waveguides with a 600 × 260-nm cross-section [12]. This switch showed a 32-dB insertion loss and a 10-dB extinction ratio at 50-mW switching power. However, these switching characteristics are still insufficient for actual photonic network applications. Additionally, 1.5 mm is still too large for future monolithic integration.
Therefore, we propose more compact 1 × N optical switches based on silicon photonic wire waveguides that are controlled through the thermo-optic effect. To our knowledge, the 1 × 1 and 1 × 2 switches we fabricated are the most compact Mach-Zehnder interferometer (MZI) type optical switches ever reported, and the 1 × 4 switch is believed to be the most compact 1 × 4 switch ever developed. Here, we report in detail the device structures, the fabrication processes, and the switching characteristics.
2. Structures and fabrication
Microscopic views of the optical switches we fabricated are shown in Fig. 1. The 1 × 1 and 1 × 2 switches are basic switch units, and the 1 × 4 switch is composed of three 1 × 2 optical switches. The elemental components of the 1 × 1 and 1 × 2 switches are a Y-splitter for dividing the light beam, a thermo-optically controlled MZI for tuning the phase of the propagating light beam, and a 3-dB coupler with a Y-splitter for the 1 × 1 switch or with a directional coupler (DC) for the 1 × 2 switch. These components are based on silicon photonic wire waveguides on an SOI substrate. Silicon photonic wire waveguides are also used to connect the devices externally and the elements internally in the 1 × 4 switch. The silicon photonic wire waveguide has a silicon core with a 300-nm square cross-section; the core is surrounded by silica cladding. With this structure, the waveguide acts as a single-mode waveguide. Due to the strong optical confinement of silicon photonic wire waveguides, the bending loss for a 90° circular bend with a 5- or 10-μm radius is less than 0.1 dB. The radii of the bends in our switches are either 5 or 10 μm, and these small bends are the primary reason for the reduction in device size. The 3-dB coupler with the DC is only about 5 μm long because the effective coupling length of the DC is about 10 μm when the distance between the DC cores is 300 nm [16]. The Y-splitter used in the 1 × 2 switch is only about 7 μm long because a large splitting angle (> 4.8°) is possible for the waveguide. The length of the 3-dB coupler is similar to that of a 3-dB MMI coupler [17]. The length of the MZI branches is 40 μm. Metal thin-film heaters are arranged over the silica cladding layer and on the MZI branches for thermo-optic control. It should be noted that only one of the two heaters on the two branches is active and connected to the electrode pads, while the other one is only a dummy heater used to provide structural balance between the two branches. The electric resistance of the active heater is 30 Ω for the 1 × 1 switch and 100 Ω for the 1 × 2 and 1 × 4 switches. Excluding the electrode pads, the net device footprints are 140 × 65 μm for the 1 × 1 switch, 85 × 30 μm for the 1 × 2 switch, and 190 × 75 μm for the 1 × 4 switch. The 1 × 2 switch is more compact than the 1 × 1 switch because the design of the Y-splitter used in the 1 × 2 switch was optimized, compared with that used in the 1 × 1 switch. The silicon photonic-wire-based optical switches we fabricated are thus quite compact, and the 1 × 4 switch is believed to be the most compact 1 × 4 switch ever fabricated.
For the fabrication, we used SOI substrates with a 300-nm-thick silicon top layer and a 1-μm-thick buried oxide (BOX) layer. The substrates were first patterned using electron beam lithography. Then, an inductively coupled plasma dry etcher was used to etch the silicon top layer down to the BOX layer to form the waveguide core pattern. Next, the waveguide core was covered by a 1-μm-thick silica layer, and a vertical symmetric cladding waveguide structure was formed using the BOX layer. Finally, over the silica layer, metal thin-film heaters and their electrode pads were sputtered onto the MZI branches.
3. Experiments
The optical switches were characterized using a wavelength tunable laser and a spectrum analyzer. One tapered optical fiber was used for coupling light into the silicon photonic wire waveguide, and another tapered fiber was used to collect the light output from the waveguide for analysis. To measure the response speed of the switch, we used a pulse power supply.
First, we characterized the 1 × 1 optical switch. At 1550 nm, we measured device transmittance at various heating powers, as plotted in Fig. 2. We measured it for both polarizations of the incident light beam: the electric field vector perpendicular to the substrate (described here as “TM polarization”) and the electric field vector parallel to the substrate (“TE polarization”). As shown in Fig. 2, the maximum and minimum transmissions did not appear at the same heating power for the two polarizations due to the polarization-dependent propagation constant of the waveguide. It might be possible to reduce this polarization dependence by using properly adjusted stress-induced birefringence with silica cladding, as done for rib-type silicon waveguides [18]. At maximum transmittance, the device insertion loss was 15 dB for the TM polarization and 22 dB for the TE polarization. These losses were mainly caused by propagation losses of about 2 dB/mm for the TM polarization and about 2.5 dB/mm for the TE polarization and coupling losses between the fiber and silicon photonic wire waveguides of about 6 dB/facet for the TM polarization and about 8 dB/facet for the TE polarization, since the total loss of the bends in the devices was negligible (less than 0.1 dB/bend for both polarizations). The propagation losses mainly arose from the surface roughness of the sidewall of the waveguide core; it might be possible to reduce them to about 3 dB/cm by optimizing the lithography process [8]. On the other hand, the coupling loss can be reduced to less than 0.5 dB/facet by using a mode spot-size converter [10]. In short, device insertion loss can be reduced to less than 2 dB by using a spot-size converter and an optimized process.
The maximum extinction ratio for the 1 × 1 optical switch was 27 dB for TM polarization and 19 dB for TE polarization. The heating power needed for complete switching was approximately 90 mW for both polarizations. Although this is higher than that suggested by Espinola et al. [12], it could be reduced to the 10–30 mW level by optimizing the structures of the waveguide and micro heater [13, 14]. Although the width of the micro heaters should be minimized to reduce the switching power, it presently depends on the heater power rating.
The wavelength-detuning characteristics of the 1 × 1 optical switch were investigated by tuning the wavelength of the incident light beam while fixing the heating power at the maximum extinction point for the TE and TM polarizations at a 1550-nm wavelength. As shown in Fig. 3, the wavelength can be detuned by over 15 nm for both polarizations while maintaining an extinction ratio of more than 20 dB for the TM polarization and 15 dB for the TE polarization. This means the switching bandwidth is more than 15 nm under the above conditions.
The switching response time was measured at a 1550-nm wavelength. As shown in Fig. 4, the ON/OFF switching time was typically around 100 μs. However, it might be possible to further reduce the time to less than ten μs by reducing the thickness of the lower silica cladding layer [13, 14], although a cladding layer that is too thin might increase the propagation loss.
Next, we measured the dependence of the transmittance of the 1 × 2 optical switch on the heating power at 1550 nm. Due to the polarization dependence of the device, the measurement was carried out only for TM polarization. As can be seen in Fig. 5, as the heating power was increased, the light outputs from both ports (1 and 2) alternately switched ON and OFF. The light output from port 2 was minimal at 80 mW, while that from port 1 was minimal at 170 mW. Therefore, the switching power needed to change output ports was 90 mW. The device insertion loss was about 15 dB, and the maximum extinction ratio was more than 30 dB. The switching response time and wavelength-detuning characteristics of the 1 × 2 switch should be similar to those of the 1 × 1 switch since the fundamental structures of the devices are similar.
Finally, we tested the switching operation of the 1 × 4 optical switch. Figure 6 shows top-view infrared images of the device taken during switching. A light beam input from the left side of the device was output in order from ports 1 to 4 on the right side when the heating power of the individual 1 × 2 optical switch elements was appropriately adjusted.
4. Conclusion
We have designed 1 × 1, 1 × 2, and 1 × 4 optical switches that are controlled through the thermo-optic effect and are based on silicon photonic wire waveguides, which strongly confine light in their 300 × 300-nm cross-sectional cores. These switches are very compact: their footprints are 140 × 65, 85 × 30, and 190 × 75 μm, respectively. For the 1 × 1 switch, the extinction ratios were 27 dB for TM polarization and 19 dB for TE polarization. The switching power was 90 mW, while the switching response time was about 100 μs. The switch bandwidth was over 15 nm, while the extinction ratio remained above 20 dB for TM polarization and 15 dB for TE polarization. For the 1 × 2 switch, we obtained an extinction ratio of more than 30 dB for TM polarization. The switching power needed to change between output ports 1 and 2 was 90 mW. We successfully demonstrated switching operation for the 1 × 4 switch, which we believe is the most compact 1 × 4 optical switch ever fabricated. These switches offer a significant opportunity for future monolithic integration of silicon-based photonic circuits.
Acknowledgments
We sincerely thank Junichi Fujikata for his valuable input and advice. This study was carried out as part of the “Photonic Network Project” under contract to the New Energy and Industrial Technology Development Organization (NEDO) and the “Focused Research and Development Project for the Realization of the World's Most Advanced IT Nation,” IT Program, MEXT.
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