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We design and fabricate a four-port optical router, which is composed of eight microring-resonator-based switching elements, four optical waveguides and six waveguide crossings. The extinction ratio is about 13 dB for the through port and larger than 30 dB for the drop port. The crosstalk of the measured optical links is less than −13 dB. The average tuning power consumption is about 10.37 mW and the tuning efficiency is 5.398 mW/nm. The routing functionality and optical signal integrity are verified by transmitting a 12.5 Gb/s PRBS optical signal.
©2011 Optical Society of America
1. Introduction
Next-generation chip multiprocessors (CMPs) demand more on- and off-chip communications as the number of cores integrated on a single die and the clock speed continue to increase. Global metallic interconnect delays increase exponentially while shrinking feature sizes are pushed by new technology. Moreover, higher data rate and longer electrical links induce larger signal loss and distortions and therefore signal integrity issue becomes a tough challenge for networks-on-chip (NoC) design. NoC based on traditional electrical interconnects are becoming a bottleneck for improving the performance of CMPs due to their limited bandwidth, long delay and high power consumption [1–5]. Optical interconnects have broader bandwidth and lower latency compared with electrical interconnects [6,7]. Studies have demonstrated that optical interconnect could be an option to resolve these issues when it can supply the sufficient interconnection density and is monolithically integrated with CMOS devices. Recent advances in silicon photonics make it viable to develop photonic NoC using the standard CMOS processes [8–13]. Many compact and functional passive/active photonic components have been made, such as high-speed modulators [9], reconfigurable filters [10], detectors [11], and switches [12], and they can act as blocks to construct photonic NoC.
As one of the key elements in photonic NoC, on-chip optical routers have received much attention in recent years. On-chip optical router has single-stage and multi-stage routing topologies. A single-stage optical router based on microring resonators (MRRs) has been reported [14]. Subsequent experiments have verified the routing functionality of such MRR-based router employing single-stage routing topology [15]. A non-blocking router based on Mach-Zehnder interferometer (MZI) has also been reported, with broad bandwidth and low crosstalk [16]. The latter scheme adopts multi-stage switching topology to reduce the number of switch elements and crossings. However, the MZI-based router occupies larger area and has higher power consumption compared with the MRR-based router, which limits its applications in photonic NoC.
In this paper, we present the design, fabrication and characterization of a non-blocking four-port optical router based on MRRs. To the best of our knowledge, the optical router comprises minimum waveguides, MRRs and waveguide crossings. Notably, power consumption, loss, crosstalk and device size are important figures of merit for the optical router. Reduction of MRRs can reduce the power consumption and area occupation of the optical router. Reduction of crossings can offer advantages in terms of insertion loss and crosstalk. MRR-based switching elements are analyzed using scattering matrix method. The optical router is fabricated with standard CMOS process and characterized fully by transmission spectrum tests. The optical router has the average power consumption of 10.37 mW and the worst crosstalk of less than −13 dB. Clear eye diagrams are obtained by high-speed 12.5 Gbps optical signal transmission experiments, which verifies the signal integrity for input-output channels of the optical router.
2. Router architecture and MRR-based switching elements
2.1 Topology of the optical router
Figure 1(a) shows a bidirectional non-blocking four-port optical router, which comprises four waveguides, eight MRRs and six waveguide crossings. Single ring coupled with two waveguides, acting as the elemental building block, is employed for the proof of concept of the optical router. In this paper, ON and OFF states of the MRR are defined as follow. If the MRR is in OFF state, an incident light with the wavelength of λ0 propagates from the input port to the through port. If the MRR is in ON state, the incident light is coupled into the MRR and transferred to the drop port. All MRRs in the optical router have the same radius. Thus, the optical router can operate a single wavelength channel as well as a WDM signal with channel spacing equal to the free spectral range (FSR) of the MRR.
Fig. 1 (a) Schematic of a bidirectional four-port non-blocking optical router based on MRRs and (b) one waveguide coupled with 2(N-2) MMRs from the input port to the output port in N-port non-blocking router.
The router is non-blocking to relieve the contention issues in photonic NoC made up of such devices [14,16]. As shown in Table 1 , the optical router has 12 point-to-point optical links, each of them is either directly connected by a waveguide (denoted as T-Link) or established by a resonant MRR (denoted as D-Link). Any four optical links can work simultaneously for the proposed router as long as not two optical signals are injected from the same input port or transferred to the same output port. To construct such a non-blocking N-port router, some rules should be followed: 1) N waveguides are needed, and each of them connects one input port and one output port; 2) each waveguide is coupled with 2(N-2) MRRs from its input port to the corresponding output port, in which the former (N-2) MRRs drop the target channel from this input port to other output ports respectively and the latter (N-2) MRRs add the channels from other (N-2) input ports to this waveguide’s output port, seeing Fig. 1(b). In the other words, N(N-2) MRRs are needed for N-port bidirectional non-blocking optical router. Note that the number of crossings increases with increasing the number of ports.
Table 1. Twelve unidirectional optical links in the four-port optical router
The optical router adopts a single-stage switching architecture, which means that any input-output link of the optical router is established by no more than one resonant MRR. Due to the narrow bandwidth of MRRs, single-stage routing topology is preferred. On the one hand, single-stage topology can avoid precise control of the resonant wavelengths of the switching elements in different stages. On the other hand, single-stage topology can simplify the verification of the optical router because each optical link has an independent physical channel. Moreover, the optical router can offer bidirectional communication between two cores connected by any optical link, which is completed by using two unidirectional waveguides for each port because of the absence of on-chip circulator; see Fig. 1(a).
Different from the previously reported optical router [14], four MRRs coupled with two parallel waveguides are used in the proposed structure to minimize the number of waveguide crossings. Waveguide crossing leads to not only additional transmission loss and return loss but also crosstalk in the arms of the waveguide crossing. Minimizing the number of waveguide crossings reduces the loss and crosstalk of the optical router, which can bring a significant benefit for the optical router’s application in the large-scale photonic NoC [17–19]. Statistical analysis shows that each optical link has an average 2 waveguide crossings with the variance of 0.67 for the proposed structure while each optical link has an average 3.33 waveguide crossings with the variance of 2.89 for the previously reported structure (see Table 2 ). This indicates that the proposed structure has not only lower transmission and return loss and crosstalk but also better channel uniformity than the previously reported structure. As shown in Fig. 1(a), the input of each port is next to its output to make it easy that one router connects to its neighboring routers in photonic NoC, such as mesh [2] and torus networks [5].
Table 2. Statistical analysis of the crossings used in the proposed structure and the previously demonstrated structure
2.2 MRR-based switching elements
For a switching element, low loss, low crosstalk and broad bandwidth are required to guarantee the error-free transmission of a high-rate optical signal. In the proposed router, add-drop filter based on single ring is the elemental building block to construct optical router. In order to investigate the behavior of the MRR with different structural parameters, scattering matrix method [20] is used to modeling both the MRR coupled with two parallel waveguides and the MRR coupled with two crossed waveguides. The responses of the drop and through ports (Pd and Pt) of the add-drop filter are given by Eqs. (1) and (2), where θ = 4π2Rneff/λ, t and κ is the self- and cross-coupling coefficients respectively (Here, we assume that the MRR has two identical lossless couplers (t2 + κ2 = 1)), α and R is the loss coefficient and radius of the ring, neff is the effective refractive index, and λ is the wavelength of light in vacuum, the parameter x in Eq. (1) is equal to 1 or 0.5 (x = 1 is for the MRR coupled with parallel waveguides, while x = 0.5 is for the MRR coupled with crossing waveguides).
When the MRR is in ON state, the input signal is directed to the drop port with a loss ILd, while the MRR is in OFF state, the input signal is directed to the through port with a loss ILt. ERd and ERt are respectively extinction ratios of the drop-port and through-port transmission spectra. From Eqs. (1) and (2), the transmission spectra of the drop and through ports of the MRR are determined by α and κ. For the MRR with a certain radius, its loss coefficient is mainly determined by the fabrication process and its coupling coefficient κ is controlled by the gap between the bus waveguide and the ring waveguide. Figure 2 shows the variations of ILd, ILt, 3-dB bandwidth, ERd and ERt with the loss of the ring and the coupling coefficient. Both ILd and ILt decrease with decreasing the loss of the ring, while ERd and ERt increase with decreasing loss of the ring. Therefore, lower loss of the ring is preferred. As shown in Figs. 2(a), 2(b), 2(d), and 2(f), larger κ should be selected to get a lower ILd, a larger 3-dB bandwidth and a larger ERt, but smaller κ is preferred for a lower ILt and a larger ERd (seeing Figs. 2(c) and 2(e)). Thus, to design the MRR-based switching element is a tradeoff among its figures of merit.
Fig. 2 The contours of drop-losses ILd for MRR coupled with parallel waveguides (a) and MRR coupled with crossing waveguides (b), through-loss ILt (c), 3-dB bandwidth (d), extinction ratio ERd (e) and ERt (f) with the loss of the ring and coupling coefficient. In the simulation, the radius of the MRR is 10 μm.
2.3 Fabrication
The device is fabricated on an 8-inch silicon-on-insulator (SOI) wafer with 220-nm-thick top Si layer and 2-μm-thick buried dioxide layer. The process flow schematics are shown in Fig. 3 . 248-nm deep ultraviolet photolithography and inductively coupled plasma etching are used to form Si photonic waveguides. In order to enhance the coupling efficiency between the waveguides and the lensed fibers, spot size converters (SSCs) are integrated on the input and output terminals of the waveguides (Fig. 4(a) ). The SSC is a 200-μm-long linearly inversed taper with 180-nm-wide tip. After the waveguide is etched, a 1500-nm-thick silica layer is deposited on the Si core layer as the separate layer by plasma enhanced chemical vapor deposition (PECVD). Then a 200-nm-thick TiN is sputtered on the separate layer and Ω-shaped TiN heaters are fabricated using ultraviolet photolithography and dry etching. Via holes are etched after depositing a 300-nm-thick silica layer by PECVD. Then aluminum wires and pads are fabricated. Finally, the end-face of the SSC is exposed by a 110-μm-deep etching process as the world-to-chip interface.
Fig. 4 Scanning electronic micrograph (SEM) of (a) the SSC, (b) the parallel MRR, (c) the crossing connected MRR, and (d) the MMI-based waveguide crossing. (e) Micrograph of the four-port non-blocking optical router.
The micrograph of the device is shown in Fig. 4(e). The optical router is formed with submicron rib waveguide since it is the implementation of high-speed switch based on MRRs through plasma dispersion effect [21]. The single mode condition and polarization property of submicron rib waveguide have been investigated using computer simulation [22]. For the optical router, the bus waveguide and MRR are formed with a submicron rib waveguide with width of 400 nm, height of 220 nm and slab thickness of 70 nm. FEM calculation shows that the waveguide only supports TE-like fundamental mode, which agrees with the experimental data in our previously published work [23]. All MRRs in the router have the same radii of 10 μm, and symmetric gaps are selected to be 400 nm to get a tradeoff among the figures of merit. Input/output waveguides are specially aligned to guarantee the characterization of all three paths from not less than one input port to other three output ports. There are two groups of such paths that are measurable, including paths from North input port to East/South/West output ports and ones from West input port to East/South/North output ports. This issue can be resolved by developing the multi-channel coupler between fiber array and silicon waveguide array [24].
3. Results and Discussion
3.1 Operation wavelength and power consumption
A broadband source and an optical spectrum analyzer are used to characterize the routing functionality of the device. The response spectra are obtained at West output port and North output port with light launched from North input port and West input port respectively. The transmission spectra in the wavelength range from 1546.1 to 1548.5 nm are shown in Fig. 5 . Results show that the resonances have the 3-dB bandwidth of 0.12 nm (about 15 GHz) and the ER of 13 dB for the through output. Initial resonances of MRRs are different due to process non-uniformity. The operation wavelength is selected to be at 1548.1 nm, which is larger than the initial resonances. Resonance of each MRR is distinguished by tuning each respectively through thermo-optic effect. Suppose all MRRs are in OFF state when none of MRRs is powered on, initial resonance and ON-state power consumption for each MRR are given in Table 3 .
Fig. 5 Transmission spectra at (a) West output port and (b) North output port with light injected from North input port and West input port respectively.
Table 3. Initial resonance, resistance of heater and on-state power consumption for each MRR comprising the optical router
As shown in Table 4 , the optical router has nine routing states, each of which comprises four independent paths. Statistical analysis shows that the optical router has the minimum tuning power consumption of 0 mW, the maximum power consumption of 20.079 mW and the average power consumption of 10.365 mW. There are two methods to reduce the tuning power consumption of the optical router. On the one hand, improving process uniformity can reduce the resonance difference of the MRRs in order to cut aggregate wavelength shift required. On the other hand, optimizing the thermo-optic modulating structure of the MRR can improve heating power efficiency, such as employing thermal isolation trenches [24]. In the optical router, an Ω shape heater is employed and the thermo-optic tuning efficiency is about 5.398 mW/nm (as shown in Fig. 6 ).
Table 4. Nine routing states of the optical router and corresponding power consumption (Ei/o, Si/o, Wi/o and Ni/o indicate East, South, West and North input/output ports respectively)
Fig. 6 (a) Resonance shift of the MRR with the applied power, as well as (b) the tuning efficiency of the heater.
3.2 Extinction ratio and crosstalk
Another important figure of merit for optical router is ER, which is also measured for each of six measurable links. Figures 7(a) -7(c) show the response spectra at East/South/West output ports while injecting light at North input port and applying the corresponding on-state voltages to the heaters of R6/R2/R6 respectively. Figures 7(d)-7(f) show the response spectra at East/South/North output ports while injecting light at West input port and applying the corresponding on-state voltages to the heaters of R1/R5/R1 respectively. Results show that D-link has an ER of larger than 13.5 dB and T-link has an ER of larger than 13.4 dB at the 1548.1 nm operation wavelength. The maximum ER of the T-Link is determined by ER of the through-port spectrum of the MRR coupled with the corresponding waveguide, while the maximum the ER of the D-Link is determined by the ER of the drop-port spectrum of the corresponding MRR. Results show that the ER is more than 13 dB for the through-port spectra, and more than 30 dB for the drop-port spectra. Notably, there is a tradeoff between the ER and the power consumption.
Fig. 7 Response spectra at (a)-(c) East/South/West output ports with light injected from North input port and at (d)-(f) East/South/North output ports with light injected from West input port. Black curve/red curve shows spectrum before/after the corresponding path is set up.
Crosstalk is also an important figure of merit of the optical router, which significantly limits the scalability of the photonic NoC employing such optical routers. Crosstalk of the optical router stems from the MRRs and the waveguide crossings. MMI-based crossings, shown in Fig. 4(d), are employed to reduce the loss and crosstalk [25]. The scattering loss is 0.4 dB/crossing and the crosstalk is less than −30 dB. Figure 8 shows the transmission spectra at all output ports when each of the six links is independently set up. We can clearly see that the leakage from the main link to other three unexpected output ports is less than −13 dB for all six measurable links. Therefore, the crosstalk of the device is estimated to be less than −13 dB and can be further reduced by adopting MRRs satisfying the critical coupling condition.
Fig. 8 Transmission spectra of the optical router when North-East, North–south, North-West, West-East, West-South and West-North links are set up respectively. The two rows respectively represent transmission spectra from the North and West input ports. The black, red, blue, and cyan curves represent transmission to the East, South, West and North output ports, respectively.
3.3 High-rate signal transmission
The performance of the four-port optical router is also characterized by eye diagrams using a 12.5 Gb/s non-return-to-zero signal. An MZ intensity modulator is used to externally modulate the continuous wave, which is driven by a 12.5 Gb/s pseudo-random bit sequence (PRBS) generated by a pulse-pattern generator. The optical signal is amplified by a high-power erbium-doped fiber amplifier (EDFA) before being injected into the input waveguide of the optical router. At the output port, a tunable filter is used to reduce the noise induced by EDFA. Finally, the optical signal is sent to a digital communication analyzer to observe the waveforms and record the eye diagrams.
Figure 9 shows the back-to-back eye diagram as well as the eye diagrams of signals at the output ports of the optical router. Clear and open eye diagrams verify signal integrity of input-output paths and the routing functionality of the optical router. In principle, the optical router can simultaneously operate multi-wavelength signals with channel spacing equal to the FSR of the MRR. Notably, the power consumption is transparent to the transmitted signal speed and the number of wavelength channels multiplexed. Thus the optical router can make full use of the power consumption by operating more wavelength channels. Compact and low power consumption comb switches with small FSR are preferred.
Fig. 9 Back-to-back eye diagram and eye diagrams at corresponding output ports for 6 measurable paths, using 12.5 Gb/s 27-1 PRBS optical signal at the wavelength of 1548.1 nm
4. Conclusion
We have designed and fabricated a bidirectional four-port non-blocking optical router based on MRRs in SOI platform with standard CMOS process. The optimization of the topology minimizes the number of waveguide intersections in the optical router, which reduces the crosstalk and the transmission and return loss stemming from the crossings and improves the loss uniformity of different paths of the optical router. Results show that the MRR has the extinction ratio of about 13 dB for the through port and more than 30 dB for the drop port. The heater has the tuning efficiency of 5.398 mW/nm, and the average power consumption of the optical router is about 10.37 mW while the operation wavelength is selected to be 1548.1 nm. At the same time, the balanced crosstalk and extinction ratio for the T-link and D-link are obtained. The ERs of T-link and D-link are both larger than 13 dB, and the crosstalk is less than −13 dB. Routing functionality and optical signal integrity are verified by transmitting a 12.5 Gb/s PRBS NRZ optical signal through the optical router.
Acknowledgments
This work has been supported by the National Natural Science Foundation of China (NSFC) under grant 60977037, and by the National High Technology Research and Development Program of China under grant 2009AA03Z416.
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