SOI-Based Thermo-Optic Waveguide Switch Matrix with Spot Size Converters

A SOI-based thermo-optic waveguide switch matrix worked at 1.55μm, integrated with spot size converters is designed and fabricated for the first time. The insertion loss and polarization dependent loss are less than 13dB and 2dB, respectively. The extinction ratio is larger than 19dB. The response time is less than 5μs and the power consumption of the switch cell is about 200mW. Optical switch or switch matrix is one of the key devices for optical communication systems, including optical cross connection (OXC) and optical add-drop multiplexing (OADM) systems. Planar waveguides technologies, based on silica[1], polymer[2] and SOI[3]-[6]are very frequently employed for optical switching applications. SOI technology is recently attracting more and more attention due to its mature production technology. Thermo-optic switch matrix in Ref.[4] is made on SOI wafer with 8μm-thick device layer. The switch power is about 340mW and switching time is 13μs. The insertion loss is larger than 20dB due to the dominating contribution of energy radiation in S-bend waveguides and transmission loss of such a device as long as 4.6cm. 2×2 switch [3] on SOI with 5μm-thick device layer has lower switching power and faster speed, but the insertion loss is still 14dB, to which 4.5dB coupling loss between waveguide and fiber contributes. Switch [5] based on SOI waveguide with 0.6μm×0.26μm cross-section presents much lower switching power of 50mW and faster speed of less than 3.5μs. Due to the large mode mismatch between waveguide and fiber, however, the insertion loss is 32dB, which is unacceptable for practical application. One way to overcome the coupling difficulty is with a tapered waveguide. Results using a pseudo-vertical taper, tapering from a 12μm by 12μm input down to waveguide on order of 4-5μm, have given coupling losses as low as 0.5dB/facet[7]. In this letter, an SOI thermo-optic waveguide switch matrix at 1.55μm is designed and fabricated. By combination of spot size converter (SSC) and miniaturized device, smaller insertion loss and faster switching speed are achieved simultaneously, compared with our previous work [3],[4]. As to our knowledge, it is the first demonstration of the integration of spot size converters with a switch matrix based on SOI. A bonding and back-etching SOI (BE-SOI) wafer with an 8μm top silicon layer and a 0.5μm buried silicon oxide is used here. Fig.1 shows a schematic diagram of the spot size converter. It has a uniform height but varying width and is formed by inductively coupled plasma (ICP) etching technology. In order to increase the alignment tolerance in photolithography, the width of the inner rib is designed to be 8μm, which is smaller than that of the middle rib 12μm. Fig.2 shows the calculated tolerance of the spot size converter to fiber misalignments. For perfect alignment the calculated coupling loss of the spot size converter is approximately 0.44dB. Fig.3 shows the thermo-optic switch matrix architecture. The device consists of twelve 2×2 switch cells, and the S-bend connectors are used to enlarge the distance of neighboring input/output ports to127μm for convenience of device package. It can be considered either a blocking 8×8 switch matrix, or a strictly non-blocking 4×4 one, if only four input ports and output ports are utilized. Switch cell, as shown in Fig.4, is based on the Mach-Zehnder interferometer (MZI) structure. Isolating groove is introduced between two neighboring arms in order to avoid mode coupling and reduce switching power consumption. Multimode interferometers (MMIs) are used as power splitters and combiners in the MZI structure due to their large fabrication tolerances and compact sizes. The general interference MMI, which is 1920μm long while 20μm wide, is used here to increase the fabrication tolerance. Considering single mode condition for SOI rib waveguide, the height and the width of the single mode waveguide are 4μm and 3.5μm, respectively. The etching depth is designed to be 2μm. The device is fabricated on SOI wafer. ICP etching technology is used twice to form the spot size converters and the other switch matrix waveguides. Cr-Au heaters are then sputtered and patterned on the MZ arms. At last, the fabricated device is diced and polished for test purposes. The device is measured by a fiber-chip-fiber system. Fig.5 shows the fiber-chip-fiber insertion losses and polarization dependent losses for the four channels of the switch matrix. The insertion loss is smaller than 13dB, and the polarization dependent loss is less than 2dB. Compared to the insertion loss 7dB of the straight waveguide on the same wafer (the insertion loss including the reflection loss 1.6dB per facet, the coupling loss 0.44dB for both input and output coupling with fibers, and the transmission loss 1dB/cm), an excess loss of less than 7dB is derived from the loss of S-bends and MMIs. The maximum crosstalk is less than -18dB. Figure 6 shows the output power of 8’ and 6’ output ports versus heating power when switch cell 6 is modulated. From this plot and plots for all the other output ports (not given here), it can be seen that the extinction ratios of eight channels are all larger than 19dB. The switching power of different switch cell is less than 200mW. Because of the high heat conductivity of silicon, SOI thermo-optic switch has a much faster response speed, in comparison with silica or polymer waveguide switches [1, 2]. Fig.8 shows the response time of the switch matrix. The 10%-90% response time at the rising and falling edges are 4.6 and 1.9μs, respectively. The speeds are faster than our previously published results [3, 4], owing to the adoption of the thinner device layer of 4μm. IN Conclusion, we have designed and fabricated a thermo-optic SOI waveguide switch matrix at 1.55μm with spot size converters for the first time. The device has better performance than formerly reported. The insertion loss and polarization dependent loss are less than 13dB and 2dB, respectively. The extinction ratios are larger than 19dB. The switching power of single cell is less than 200mW. The response time is less than 5μs. Fig.1 Spot size converter structure −4 −3 −2 −1 0 1 2 3 4 0 1 2 3 4 5 6 7 Misalignment,μm Lo ss ,d B fibre to waveguide y fibre to waveguide x fibre to fibre Fig.2 Calculated coupling loss of a SSC-fiber interface from misalignment Fig.3 Thermo-optic switch matrix architecture Fig.4 Schematic structure of the MMI-MZI 2×2 switch cell 152

Optical switch or switch matrix is one of the key devices for optical communication systems， including optical cross connection (OXC) and optical add-drop multiplexing (OADM) systems.Planar waveguides technologies, based on silica [1], polymer [2] and SOI [3]- [6]are very frequently employed for optical switching applications.SOI technology is recently attracting more and more attention due to its mature production technology.Thermo-optic switch matrix in Ref. [4] is made on SOI wafer with 8µm-thick device layer.The switch power is about 340mW and switching time is 13µs.The insertion loss is larger than 20dB due to the dominating contribution of energy radiation in S-bend waveguides and transmission loss of such a device as long as 4.6cm.2×2 switch [3] on SOI with 5µm-thick device layer has lower switching power and faster speed, but the insertion loss is still 14dB, to which 4.5dB coupling loss between waveguide and fiber contributes.Switch [5] based on SOI waveguide with 0.6µm×0.26µmcross-section presents much lower switching power of 50mW and faster speed of less than 3.5µs.Due to the large mode mismatch between waveguide and fiber, however, the insertion loss is 32dB, which is unacceptable for practical application.One way to overcome the coupling difficulty is with a tapered waveguide.Results using a pseudo-vertical taper, tapering from a 12µm by 12µm input down to waveguide on order of 4-5µm, have given coupling losses as low as 0.5dB/facet [7].
In this letter, an SOI thermo-optic waveguide switch matrix at 1.55µm is designed and fabricated.By combination of spot size converter (SSC) and miniaturized device, smaller insertion loss and faster switching speed are achieved simultaneously, compared with our previous work [3], [4].As to our knowledge, it is the first demonstration of the integration of spot size converters with a switch matrix based on SOI.
A bonding and back-etching SOI (BE-SOI) wafer with an 8µm top silicon layer and a 0.5µm buried silicon oxide is used here.Fig. 1 shows a schematic diagram of the spot size converter.It has a uniform height but varying width and is formed by inductively coupled plasma (ICP) etching technology.In order to increase the alignment tolerance in photolithography, the width of the inner rib is designed to be 8µm, which is smaller than that of the middle rib 12µm.Fig. 2 shows the calculated tolerance of the spot size converter to fiber misalignments.For perfect alignment the calculated coupling loss of the spot size converter is approximately 0.44dB.Fig. 3 shows the thermo-optic switch matrix architecture.The device consists of twelve 2×2 switch cells, and the S-bend connectors are used to enlarge the distance of neighboring input/output ports to127µm for convenience of device package.It can be considered either a blocking 8×8 switch matrix, or a strictly non-blocking 4×4 one, if only four input ports and output ports are utilized.Switch cell, as shown in Fig. 4, is based on the Mach-Zehnder interferometer (MZI) structure.Isolating groove is introduced between two neighboring arms in order to avoid mode coupling and reduce switching power consumption.Multimode interferometers (MMIs) are used as power splitters and combiners in the MZI structure due to their large fabrication tolerances and compact sizes.The general interference MMI, which is 1920µm long while 20µm wide, is used here to increase the fabrication tolerance.Considering single mode condition for SOI rib waveguide, the height and the width of the single mode waveguide are 4µm and 3.5µm, respectively.The etching depth is designed to be 2µm.
The device is fabricated on SOI wafer.ICP etching technology is used twice to form the spot size converters and the other switch matrix waveguides.Cr-Au heaters are then sputtered and patterned on the MZ arms.At last, the fabricated device is diced and polished for test purposes.
The device is measured by a fiber-chip-fiber system.Fig. 5 shows the fiber-chip-fiber insertion losses and polarization dependent losses for the four channels of the switch matrix.The insertion loss is smaller than 13dB, and the polarization dependent loss is less than 2dB.Compared to the insertion loss 7dB of the straight waveguide on the same wafer (the insertion loss including the reflection loss 1.6dB per facet, the coupling loss 0.44dB for both input and output coupling with fibers, and the transmission loss 1dB/cm), an excess loss of less than 7dB is derived from the loss of S-bends and MMIs.The maximum crosstalk is less than -18dB.
Figure 6 shows the output power of 8' and 6' output ports versus heating power when switch cell 6 is modulated.From this plot and plots for all the other output ports (not given here), it can be seen that the extinction ratios of eight channels are all larger than 19dB.The switching power of different switch cell is less than 200mW.Because of the high heat conductivity of silicon, SOI thermo-optic switch has a much faster response speed, in comparison with silica or polymer waveguide switches [1,2].Fig. 8 shows the response time of the switch matrix.The 10%-90% response time at the rising and falling edges are 4.6 and 1.9µs, respectively.The speeds are faster than our previously published results [3,4], owing to the adoption of the thinner device layer of 4µm.
IN Conclusion, we have designed and fabricated a thermo-optic SOI waveguide switch matrix at 1.55µm with spot size converters for the first time.The device has better performance than formerly reported.The insertion loss and polarization dependent loss are less than 13dB and 2dB, respectively.The extinction ratios are larger than 19dB.The switching power of single cell is less than 200mW.The response time is less than 5µs.