Wafer-s cale s ilicon p hotonic s witches beyond d ie s ize l imit: supplementary material

Wafer-scale silicon photonic switches


Section I. Wafer Arrangement Information
The wafer-scale silicon photonic switches are realized by stitching three types of the basic blocks (the switch block, the input coupler block, and the output coupler block) as described in the main text of the paper.By arranging the desired number of switch blocks and the corresponding input/output coupler blocks, switch fabrics with different port counts can be integrated on the same wafer without any change of photomask reticles.
In this work, we have designed 2x2, 3x3, and 4x4 switch blocks on a 6-inch wafer, corresponding to 160x160, 240x240, and 320x320 switch fabrics, respectively.The input coupler blocks are located at the western and eastern sides of the switch arrays, and the output coupler blocks at the northern and southern sides.The arrangement of the basic blocks on the wafer is shown in Fig. S1(a).The outer areas of the wafer are filled with dummy switch blocks to achieve consistent pattern density for uniform chemicalmechanical polishing (CMP) across the wafer.Figures S2(b) shows the fabricated 6-inch wafer, containing five 160x160 switches, two 240x240 switches, and one 320x320 switch.
Although we demonstrated multiple silicon photonic switches with various switch sizes on the wafer, it is also possible to realize a single ultra-large switch.For example, a 6-inch wafer can potentially accommodate up to 10x10 switch blocks to realize a 800x800 switch fabric.Advanced silicon photonics foundries are typically based on 8-inch or 12-inch wafer process facilities, thus even larger scale silicon photonic switches are achievable by this methodology.

Section II. Basic Block Design
In this section, we describe the detailed design of the three basic building blocks.Figure S2 shows their simplified layouts.Each block occupies an area of 1x1 cm 2 .

Switch Block
The switch block consists of 80x80 switch array with 80 input, 80 through, 80 add, and 80 drop ports at the western, eastern, northern, and southern sides of the block, respectively, as shown in Fig. S2(a).
The unit cell of the switch array is based on the silicon photonic MEMS switch we previously reported in ref [7] of the main text and it has a footprint of 110x110 μm 2 .We employ the same designs of the adiabatic couplers and the MMI (multimode interference) crossings though we increase the bus waveguide width to 800 nm to achieve lower propagation loss.The width of the waveguide ports at each side is tapered from 800 nm to 10 μm over a length of 500 μm.The pitch of waveguide ports is 110 μm as they are straight extension of the bus waveguides of the 110x110 μm 2 switch unit cell array.

Input Coupler Block
Figure S2(b) shows the layout of the input coupler block.The 10μm-wide waveguides at the eastern edge are tapered to 600-nm wide waveguides for single-mode propagation.These routing waveguides are connected to the two interleaved grating coupler arrays with 127-μm pitch which is matched to the pitch of the fiber array.
At the western side, we include the mirror-imaged routing waveguides and grating couplers.These ports provide interface to the through ports of the switch when this block is attached to the eastern side of a switch array.Figure 1(a) shows that the input coupler blocks are attached to both of the western and eastern sides of the switch arrays.
Passive test structures are included in the middle of the block.The test structures are intended for characterizing the waveguide crossing loss and the propagation loss, the two main sources of the switch loss.The results of the test structures will be discussed in the following section.

Output Coupler Block
The output coupler block has two interleaved grating coupler arrays connected to the tapered waveguides at the northern edge of the block.Another set of routing waveguides and grating coupler arrays provide interface to the southern edge, which can be used as add ports when the block is attached to the northern side of a switch array.
Instead of test structures, the output coupler block has long routing waveguides, which allows the grating couplers to be farther away the switch arrays.The extra space will be useful for simultaneously aligning input and output fiber arrays.

Section III. Measurement of Passive Optical Test Structure
The propagation loss and the crossing loss are the two major contributions to the switch loss.In order to measure these losses, we used the passive test structures in the input coupler block.The propagation loss of the 800-nm wide rib waveguide was measured with two methods.First, the propagation loss was measured using the cut-back method with waveguide spirals of various lengths as shown in Fig. S3(a).From the linear fitting of the measurement results, the propagation loss was estimated to be 0.43 dB/cm.The deviations from the fitted line are probably due to the nonuniformity of the grating coupler efficiencies.Second, optical frequency domain reflectometry was used to measure an 8cm long spiral waveguide.Figure S3(b) shows the plot of the return loss amplitude versus length.The spikes occur at the fiber and grating coupler interfaces.The measured loss is 0.45 dB/cm, which agrees well with the cut-back method result.The MMI crossing loss was measured from the test structure with various number (80, 160, 240, 320, and 400)

Section IV. Estimation of Power Consumption
The MEMS switch is an electrostatic device, and its energy consumption can be estimated from its capacitive energy.The total MEMS actuator area in a switching cell including a pair of adiabatic couplers is 4 x 35 μm x 3 μm, and the electrode gap in the ON state is 185 nm.From the capacitance of the MEMS electrodes and the assumption of the 65-V switching voltage, the switching energy is estimated to be about 42 pJ.Assuming the maximum speed of switching operation of 1 MHz, the peak power consumption is calculated to be 42 μW.
Ideally, there are no current flow and power consumption in the steady state.However, we observed a small amount of leakage current as shown in Fig. S5.The measured leakage current is less than 1 nA around the switching voltage (65V).The additional power consumption caused by the leakage current is smaller than 0.065 μW, which is negligible compared with the switching power consumption calculated above.The maximum power consumption of the 240x240 switch occurs when the switch is fully reconfigured at 1 MHz rate.In a matrix switch, two cells will change states in each row.Therefore, the maximum power consumption is 42 μW x 240 x 2 = 20.2mW.

Fig. S2 .
Fig. S2.Simplified layouts of (a) the switch block, (b) the input coupler block, and (c) the output coupler block.Each block occupies an area of 1x1 cm 2 .

Fig. S3 .
Fig. S3.Waveguide loss measurements from (a) cut-back method and (b) reflectometry method.The waveguide loss is measured to be 0.45 dB/cm.

Fig. S4 .
Fig. S4.Crossing loss measurements from various numbers of MMI crossings.The cross loss is measured to be 0.016 dB/crossing.

Fig
Fig. S5.Measurement of the leakage current.