Thermal switch

A 4x4 thermal switch based on multiple rings is simulated. The temperature profile of a ring under DC bias is calculated using the HEAT solver. The effective index and the group index of the bent waveguide (ring) as a function of temperature are calculated using MODE. The HEAT and MODE results are then used in the INTERCONNECT circuit solver to analyze the switching behaviour of the multi-ring switch by controlling the temperature of one of the rings.

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Overview

Understand the simulation workflow and key results

Network systems are used for routing, which is the ability to define the path of an input source on a network with a high extinction ratio. Photonics Networks-on-Chip is becoming increasingly popular for their ability in routing optical light with high bandwidth and reduced power in chip multiprocessors. Here we show a 4×4 photonic router that operates in a single wavelength and its outputs are controlled by thermally active ring resonators. By turning the switch on and off, an extinction ratio of ~20 dB is observed [1].

Step 1: Unit cell thermal simulations (HEAT)

Run the thermal simulations to calculate the temperature profiles of two adjacent rings (unit cell) for varying applied bias voltages.

Step 2: Mode optical simulations (FDE)

The effective and group indices of a bent waveguide is calculated at room temperature (300K). An “Index perturbation” material model with dn/dT= 1.86e ^-4 is used for silicon. This will enable a sweep over simulation temperature to calculate the variation in effective index of bent waveguide mode as a function of temperature.

Step 3: Thermal switch circuit simulation (INTERCONNECT)

A 4x4 thermally tuned switch is constructed based on multiple ring resonators. Using the results obtained from the previous steps, the temperature, effective index, and group index of active rings are updated. Transmission is monitored in the output channels when the switch is turned on and off and shows how the light path can be controlled in the router.

Run and results

Instructions for running the model and discussion of key results

Step 1: Unit cell thermal simulations

The first step in the simulation of the thermal switch is the thermal simulation of the unit cell. The thermal_switch_DEVICE.ldev file contains the unit cell of the router shown above which includes ring R5 and R6.  Using the symmetry of the unit cell, we can turn only one ring (R6) on. The simulation setup thus contains the heater and external contacts for R6 only.  The thermal boundary conditions are applied at the bottom of the substrate and at the external metal contacts setting them at room temperature. Voltage boundary conditions are applied at the external contacts of the heater for R6 to drive the heater.  A schematic of the simulation setup can be seen below.

The heat transport and electrical conductive transport equations are solved self-consistently to extract the temperature profile of the switching circuit for a given applied voltage. A 3D simulation accurately determines the temperature profile within a volume containing a small number of components; components that are sufficiently far away are assumed to be thermally isolated. In each simulation, we record the temperature profile in a 2D cross-section located at the middle of the waveguide plane. The average temperature of each segment of the ring is then determined by averaging over the area occupied by that segment.

The Thermal_switch_DEVICE.ldev file sweeps the voltage on the heater from 0 to about 2.5 V. Once the simulation is run, the temperature profile of the rings can be visualized from the 2D temperature monitor (left). The log plot on the right shows that the heat couples to part of the inactive ring and can lead to a thermal crosstalk. This effect is only noticeable in the segment nearest to the active ring.

Next, we can use the data from the temperature monitor to calculate the average temperature on four segments of the ring.  The script file thermal_switch_ring_temp.lsf can be used to identify the triangular elements that fall inside each quarter ring as shown below and then calculate the average temperature of them. The results are saved into Tref.mat file and will be imported for simulation in INTERCONNECT.

Step 2: Mode optical simulations

The thermal_switch_MODE.lms simulation file contains a single ring.  The MODE solver region calculates the optical mode on a cross-section of the ring at 300 K for the bend radius in the middle of the ring (9.775 um) to model the optical mode accurately.

The image below shows the E-field intensity of the fundamental TE mode, with effective and group indices of 1.4528 and 4.3216, respectively. Here the effective and group indices are calculated for the central wavelength of ~1.55. In the first order of approximation, INTERCONNECT can calculate the effective index of the modes at other wavelengths using group index values.

neff = getsweepdata('T_sweep','neff');
neff = real(neff);
T = getsweepdata('T_sweep','T');
?T_coeff = (neff(end)-neff(1)) / (T(end)-T(1)); 
plot(T,neff,'Temperature (K)','n_eff');

The effective index increases linearly as the temperature increases, with a slope of 2e ^-4 (1/K).

Step 3: Thermal switch circuit simulation

The circuit is a 4×4 optical router and is composed of 8 rings as shown in the Overview. The ON/OFF state of each ring is controlled by an externally applied voltage. The single optical ring switch compact model is built by two couplers and four pieces of waveguides and an optical crossing. The whole ring structure is divided into four parts to give flexibility to temperature assigning. The schematic design for a single ring structure is shown below:

The middle part of the router with two ring structures coupled together has the compact model design as below:

The 4x4 router can then be built by using the two compact models shown above. In the schematic design diagram shown below, R1, R2, R7 and R8 are based on the first compact model (single ring compact model); R3_R4 and R5_R6 are based on the second compact model (coupled ring compact model).

The waveguide element properties including effective and group indices as well as thermal properties such as effective index temperature sensitivity are set based on the results from step 1 and step 2.

In the Thermal_switch_circuit.icp simulation file we have provided four additional circuits: two of them to analyze the spectrum of a single ring in the ON/OFF states, and two of them that show the spectrum of the coupled rings with and without thermal crosstalk.

To analyze the thermal switch behavior, we first compare the spectrum of the ON and OFF rings. When the heater is turned on, the resonance frequency shifts from 1552.3 to 1555.5:

The Thermal_switch_circuit.lsf script file runs the simulations for the cases where R6 is switched from OFF to ON state. The plot below shows that the transmission at ~1555.5 nm in the east output port is reduced by ~30 dB when the R6 is turned ON:

Based on the following routing table, by using different combinations of the rings, signal inputs from each input port can go to each output port:

Important model settings

Description of important objects and settings used in this model

Unit cell thermal simulations

Mesh override region : Two mesh overrides over the R6 ring (ring and pads sections) are used to properly resolve the heater elements.

Average Ring Temperature : A ring is divided into four segments and the average temperature of each segment is calculated by integrating the temperature profile over each segment and dividing it with its area.

Mode optical simulations

Index perturbed material : An index perturbed material model is used for silicon with dn/dT = 1.86e ^-4 to capture the shift in the effective index as a function of temperature. In this example, we assume that the temperature of the ring is uniform along its cross-section. A parameter sweep has been set up to sweep the temperature of the ring by setting the temperature of the simulation region (FDE) from 300 K to 500 K. Alternatively, one can import the heat results for different voltage values from step 1 as a temperature grid attribute into MODE to capture the variation in temperature in the waveguide cross-section. In the first order of approximation the temperature variation in the effective index as a function of wavelength is ignored (i.e. dn/dT is not a function of wavelength).

Temperature sweep : We assume that the shift in silicon material index is linear within the temperature of interest (300 K-500 K) and thus use only two simulations. If the shift is nonlinear, the “table of values” in the material library should be used for index perturbed silicon material.

Thermal switch circuit simulation

Voltage range : The INTERCONNECT circuit is based on component-level simulations using HEAT and MODE. The circuit may not function properly for parameter values outside the range of what was characterized in the component simulations. For example, if the modulator voltage is outside the range simulated in HEAT, linear interpolation will be used to extrapolate the data.  This can introduce errors.  It is recommended to run the component simulations to cover the full parameter space of interest.

Temperature-dependent spectrum of ring resonator : The resonance frequency of the ring is red shifted with temperature. Four additional circuits in the Thermal_switch_circuit.icp simulation file are provided to study the spectrum of the single and coupled rings in the On and Off states. We select a resonant wavelength near 1555 nm for the ring in ON-state for routing.

Thermal cross talk : We have provided the thermal cross talk results. A resonance frequency shift of ~0.5nm is observed between the results with thermal crosstalk included and excluded, and its effect is studied in ref. [2-5].

Updating the model with your parameters

Instructions for updating the model based on your device parameters

When updating the model to match your component parameters, it is important to remember that multiple solvers and simulation files are involved. Changes must be made consistently in all the associated files. For example, changes to the ring radius must be made in both the HEAT, FDE, and INTERCONNECT simulation files. Updated results from the component simulations must then be carried over into INTERCONNECT.

Key parameters that might require frequent changes include:

• Ring radius (HEAT, FDE, INTERCONNECT)
  • When updating the ring radius in HEAT, simulation objects such as mesh override position, waveguide cross position, and heater_R6 structure group must be updated accordingly. A change in ring radius will affect the effective and group indices and their values has to be updated in INTERCONNECT.
• Coupling gap distance which will affect the coupling coefficient in INTERCONNECT.
• Material properties (FDE, HEAT).
• Bias voltage (HEAT, INTERCONNECT).

Taking the model further

Information and tips for users that want to further customize the model

Router states : In this example we studied the router response when the R6 ring switches from Off to On state. The router response can be calculated for all 9 states of the above table.

Different ring/bus configurations : In this example we have divided the ring into four segments based on the distinctive segmentation of the temperature profile. However, if you are considering different configurations for the rings and buses, you might need to consider different ways of segmenting it. This would affect the average temperature calculation from HEAT simulation and the construction of the ring and waveguide elements in the INTERCONNECT.

Additional resources

Additional documentation, examples and training material

Related publications

1. N. Sherwood-Droz et al, “Optical 4x4 hitless silicon router for optical Networks-on-Chip (NoC),” Opt. Express, 16 (20), 15915-15922 (2008).
2. L. Pavesi, D. J. Lockwood, “Silicon Photonics I”, Springer (2004).
3. C. DeRose et al, “Thermal crosstalk limits for silicon photonic DWDM interconnects,” Optical Interconnect Conference (IEEE, 2014).
4. P. Dong et al, “GHz-bandwidth optical filters based on high-order silicon ring resonators,” Opt. Express, 18 (23) 23784-23789 (2010).
5. A. Alam et al, "Modeling Thermal Crosstalk in Silicon Photonics," Accepted for presentation at Integrated Photonics Research, Silicon, and Nano-Photonics, July 2016.

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