We present a 4x4 spatially non-blocking Mach-Zehnder based silicon optical switch fabricated using processes fully compatible with standard CMOS. We successfully demonstrate operation in all 9 unique switch states and 12 possible I/O routing configurations, with worst-case cross-talk levels lower than −9 dB, and common spectral bandwidth of 7 nm. High-speed 40 Gbps data transmission experiments verify optical data integrity for all input-output channels.
©2010 Optical Society of America
Photonic networks-on-chip (NoC) are an attractive option for on- and off-chip communications in high performance chip multiprocessors (CMPs) for increasing the bandwidth, lowering the latency and reducing the power [1,2]. Significant advancements in silicon photonics technology have provided a viable path toward the realization of photonic NoCs because of its compatibility with CMOS and the capability for dense integration [3,4].
A broadband, low-power and temperature-insensitive silicon optical switch is a key device for these networks, providing high-throughput and reconfigurability. Discrete 1x2 or 2x2 silicon photonic switches have previously demonstrated a throughput bandwidth of hundreds of Gb/s and nanosecond-scale switching speeds [5–9].
However systems-level investigations of such switches have been limited due to design, fabrication, packaging and test complexity. An optical 4x4 switch using thermo-optically tuned silicon microring resonators was recently reported [10–12]. In this work, we demonstrate experimentally the full functionality of a 4x4 spatially non-blocking silicon photonic switch with all designed router paths and states successfully verified. The device is based on 2x2 Mach-Zehnder (MZ) electro-optic switching elements which are capable of routing high-speed data streams with nanosecond switching time , in contrast with microsecond speeds in thermo-optically tuned resonator based switches. Using non-resonant MZ based switching elements also permits broadband operation over multiple wavelength division multiplexed (WDM) optical signals, which is difficult to implement using resonance-based switches. Our 4x4 switch has low cross-talk (< −9 dB), an optical bandwidth of 7 nm, low power consumption (< 20.4 mW), and a demonstrated bandwidth-per-port of 40 Gbps.
2. Design of 4x4 non-blocking silicon photonic switch
2.1 Non-blocking 4x4 photonic switch architecture
The present 4x4 photonic switch was designed using a bidirectional non-blocking configuration, similar to those previously reported [10–12]. As shown in Fig. 1 , the four ports are labeled north (N), south (S), east (E), and west (W), corresponding to a two-dimensional routing scheme. In order to facilitate a more convenient testing arrangement, all input waveguides are directed to one chip edge and all output waveguides are directed to the other chip edge. The non-blocking topology guarantees an internal path from any input to any output port as long as no two packets are destined for the same output (output port contention), and as long as packets are not allowed to ingress and egress from the same port (U-turn). Such a non-blocking switch architecture will simplify the network-level routing algorithms necessary to minimize (or entirely eliminate) packet contentions in a NoC made up of such devices .
The 4x4 non-blocking switch consists of six Mach-Zehnder-based electro-optic switching elements and six low-loss waveguide crossings. Each MZ switching element is electrically wired for independent control with an external power supply. Table 1 shows the 12 possible physical paths (3 for each input port) for the switch. At the intersecting cell between a given input port column and output port row, the MZ switching elements encountered in the optical pathway are listed. The table illustrates that optical signals must pass through either one or three MZ switching elements along each possible input-output path. Note that the diagonal entries of the table represent U-turn states, and therefore are not allowed.
Signals from each input port can traverse to three possible output ports, since U-turns are not allowed (denoted by crossed-out diagonal elements). Input signals must pass through either one or three Mach-Zehnder switching elements on their way from input to output.
2.2 Subcomponent 2x2 Mach-Zehnder switching elements
Each switching element includes a balanced 2x2 Mach-Zehnder interferometer (MZI). Its schematic is shown in Fig. 2a and the microscope image of a fabricated MZI is shown in Fig. 2b. One arm of the MZI contains a 200-μm-long active p-i-n diode, where the π phase shift required for switching action is implemented via carrier injection [8,13]. The spectral bandwidth of the 2x2 switch is determined by the 50% directional couplers within the MZI. The design of the directional couplers has been discussed previously . Figure 2c shows the spectral switching response of a typical 2x2 MZ switch. The optical power levels in each output port were normalized against the sum of the transmitted power in both ports, measured at zero applied voltage. In the “off” (or cross) state, the switch has −15 dB cross-talk with an optical bandwidth of 50 nm (indicated with dotted lines in Fig. 2c) and is presently centered for optimal operation at a wavelength of 1530 nm. At 1.06 V forward bias, the 2x2 MZ switch reaches the “on” (or bar) state with cross-talk lower than −15 dB over the entire wavelength range between 1470 nm to 1590 nm. Due to free-carrier absorption, an insertion loss of 1.0 dB is observed for the “on” state. The MZI element is capable of switching between the “on” and “off” states with nanosecond switching times, as reported previously .
2.3 Low-loss broadband waveguide crossings
The waveguide crossings were designed using a pair of 1x1 multimode interference (MMI) couplers, overlapping each other at 90 degrees [14,15]. Six waveguide crossings are used in the 4x4 switch design to enable all I/O ports measurement on a single edge. The low-loss broadband waveguide crossing consists of a 500 nm wide single-mode waveguide, center-feeding a 1x1 MMI coupler. The dimensions of the 1x1 MMI coupler are 3.5 μm long and 1 μm wide. A second identical MMI coupler (and its single-mode access waveguides) intersects at 90 degrees through the middle of the first MMI coupler, thus producing a waveguide crossing in which the optical signals experience low diffraction, low loss, and low cross-talk. This design was measured to have an insertion loss of 0.3-0.5 dB/crossing, within the wavelength range of interest from 1520 to 1540 nm.
3. Fabrication of 4x4 non-blocking silicon photonic switch
The 4x4 switch was fabricated on a 200 mm silicon-on-insulator (SOI) substrate with 2 μm thick buried oxide and 220 nm thick top silicon layer, in standard complementary metal-oxide-semiconductor (CMOS) fabrication line. After shallow trench isolation, rib waveguides were then patterned, leaving a 50 nm thick silicon slab. To control line-width uniformity, 193-nm deep-UV lithography was used in these patterning processes. Subsequently highly doped n+ and p+ contact regions were formed 500 nm away from the waveguide core by ion implantation into the silicon slab. Afterwards conventional CMOS process was resumed with dopant activation, Ni silicide ohmic contact formation, W via plugs, and Cu metal interconnects. Finally optical couplers were formed as spot-size converters between tapered-lensed fibers and inversely tapered SOI waveguides, using a SiOxNy layer deposited and patterned after Cu metallization. Additional details on the fabrication process have been published elsewhere .
The 4x4 switch has a footprint of 300 x 1600 μm2, which can be significantly reduced by simply shrinking the distance between the MZ switching elements. All waveguides have cross-sectional dimensions of 500 nm by 220 nm. The microscope image of the fabricated 4x4 switch is shown in Fig. 3 .
4. Measurement results
The routing functionality of the 4x4 switch was characterized using TE-polarized input light from a broadband LED source, and recording the output using an Optical Spectrum Analyzer. The transmittance spectra for a given input port were obtained by in-coupling light and subsequently measuring the spectrum of transmitted light at each of the four output ports. The transmission at each output port was then normalized against the sum of the power spectra from all four output ports. The analysis was then repeated for all remaining input ports.
Due to process variations, some MZ switching elements required a small forward bias to correct for phase errors in order to obtain a sufficient extinction ratio. The electrical power applied to each MZ switching element to reach the “off” (or cross) state was tuned individually near 1530 nm wavelength, and is listed in Table 2 .
The transmittance spectra for the 4x4 switch, when all MZ switching elements are in the “off” state, are shown in Fig. 4 . The steady state power consumption is 11.3 mW, which is the sum of the “off” state power applied to the six MZs as listed in Table 2. As pointed previously, this “off” state power consumption was induced solely because of the phase errors induced by process variation. Consistent with our design, the light signals from Ei, Si, Wi and Ni input ports are routed to So, Eo, No and Wo output ports, respectively. In Fig. 4, the horizontal dotted lines show the location of −10 dB cross-talk, while the vertical dotted lines show the optical bandwidth at −10 dB cross-talk. It can be seen that the off-state cross-talk levels are lower than −10 dB over a wavelength range of more than 24 nm when considering all input ports individually. However the range of collective wavelengths where all possible paths exhibit less than −10 dB cross-talk is reduced to 10 nm because it is limited to shorter than 1534 nm by the path Ei to No and longer than 1524 nm by the path Wi to So.
Subsequently a different steady-state electrical forward bias signal was applied to the p-i-n diode of each MZ switch, and fine-tuned to obtain maximum extinction for each MZ in its “on” (bar) state. The electrical power applied to reach the “on” state of each MZ switching element is also listed in Table 2.
Using these optimized bias settings for the individual MZ switching elements, all designed states of the 4x4 switch were characterized. Table 3 shows the total of 9 unique switch states, with the associated operation status of each MZ element identified as either “0” (off) or “1” (on). These nine unique router states define all permutations in which the four input ports can simultaneously transfer data to the four output ports in parallel.
The 4x4 switch can operate in one of the above nine states. Mach-Zehnder switching elements are operated in either the “0” (off or cross) state or the “1” (on or bar) state to generate all unique switching combinations.
Figure 5 shows the transmittance spectra of the 4x4 non-blocking silicon photonic switch in each of the states numbered #1 to #8 in Table 3. State #9, the “off” state of the 4x4 switch, is depicted separately in Fig. 4. The red, black, green, and blue traces represent transmission to the South, West, North, and East output ports, respectively. The four columns respectively represent transmittance spectra from the East, South, West and North input ports. It can be seen that in each of the eight states all of the four input signals are switched to the correct output port as designed. For example, in switch state #5, according to Table 3 the signals from Ei, Si, Wi, and Ni input ports are routed to Wo, No, Eo and So output port respectively, while experimentally we observed dominant optical power at these output ports where the transmittance are denoted with black, green, blue and red traces respectively in the fifth row in Fig. 5.
Listed on the right side of each row in Fig. 5 are the steady-state power consumption, collective optical bandwidth and cross-talk for each switch state. The steady-state power is calculated in the same way as that used for state #9 previously. First the required “on” or “off” status for the six MZ switching elements is found in Table 3 for the specific switch state. Then the power used for each MZ switching element to reach its “on” or “off” condition (shown in Table 2) are added together. The steady-state switching power varies from 7.4 mW to 20.4 mW.
The collective optical bandwidth for each switch state is obtained similarly to that for state #9. In this work, the collective optical bandwidth for each individual switch state ranges from 7 nm to >120 nm. A common optical bandwidth for the 4x4 switch is then selected to satisfy all possible switch states. In almost all of the nine switch states, the cross-talk between channels is better than −10 dB within the spectral range from 1524 nm to 1531 nm, namely a common wavelength range as large as 7 nm for the 4x4 optical switch. The only exception is for state #4 where the worst-case cross-talk of −9 dB occurs for the Ei input. Many states in fact exhibit extinction ratio better than −25 dB. The insertion loss of the 4x4 optical switch varies from 0.6 dB to 5.8 dB, depending on the switching state and I/O port.
Sensitivity to dimensional variation in photonic devices fabricated on the high-index contrast SOI platform has been investigated by Selvaraja et al. . The performance of the 4x4 non-blocking optical switch in the present work is mainly limited by the waveguide width variation caused by process non-uniformity. In the future, tighter process control on critical waveguide dimensions can be implemented using more advanced patterning techniques in CMOS technology, such as phase-shifted masks and immersion lithography. Moreover, the insertion loss may be reduced by integrating efficient silicide thermo-optic heaters to compensate any phase offsets, as opposed to electrically biasing the MZI p-i-n diodes [17,18]. Finally, to further reduce power consumption and improve the cross-talk and spectral bandwidth of the 4x4 optical switch, novel switching element designs including digital [19, 20] and/or ultra-broadband  MZ switches reported by Van Campenhout et al., can be adopted.
The performance of the 4x4 non-blocking switch was also evaluated using an optical data pattern consisting of a 40 Gbps 27-1 pseudo-random bit sequence (PRBS). The experimental setup comprises a single tunable laser source, which was externally modulated using a LiNbO3 MZ modulator. The optical signal passed through a fiber polarization controller, selecting the fundamental TE waveguide mode, and then was coupled to and from the silicon chip using tapered-lensed fibers. An erbium-doped fiber amplifier (EDFA), tunable grating filter, and variable optical attenuator were used at the switch output, prior to detection of the signal using a high-speed oscilloscope fitted with a 65 GHz bandwidth optical detector module.
The eye diagrams shown in Fig. 6 were recorded at an input wavelength of 1530 nm for all 12 possible designed input-output routings. All eyes appear open, indicating distortion-free transmission of the 40 Gbps optical data through the 4x4 non-blocking silicon photonic switch. The increased noise level present in some of the eyes (i.e. East Input to South Output, North Input to West Output, etc.) occurs due to the additional insertion loss (and accordingly larger EDFA noise figure) incurred by passing through three MZ switching elements in these specific configurations versus only one MZ switching element in the lower-noise cases.
In summary, we have fabricated and demonstrated full functionality of a 4x4 non-blocking silicon photonic switch. Through all possible designed routes the cross-talk is less than −9 dB, with the common optical bandwidth being as large as 7 nm. Successful transmission is demonstrated for a 40 Gbps 27-1 PRBS optical data pattern. The demonstrated 4x4 switch performance can in principle enable routing of at least a 200 Gbps aggregate WDM signal (i.e. consisting of 5 discrete 40 Gbps optical channels with 1 nm spacing), resulting in signal switching with low energy consumption of 0.037-0.1 pJ/bit.
This work was supported in part by the DARPA APS Program, under contract HR0011-08- C-0102. The views, opinions, and/or findings contained in this article are those of the authors and should not be interpreted as representing the official views or policies, either expressed or implied, of DARPA or the Department of Defense. The authors also gratefully acknowledge the efforts of the staff of the Microelectronics Research Laboratory (MRL) at the IBM T. J. Watson Research Center, where the devices were fabricated.
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