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Tunable Cascaded Lattice Filter for Detector Array
The S-Matrix/Custom PDK Generation Utility creates custom PDK models to augment existing PDKs, create new PDKs, and generate IP. Larger devices can be split into smaller, passive, linear elements for more efficient circuit-level simulation of the entire structure where each element is represented by a custom PDK. We've previously used the example of a simple ring resonator to demonstrate the simulation procedures and capability of the utility.
Figure 1 illustrates an Si-wire-based lattice filter that was proposed to increase the FSR of a channel dropping filter when compared to a single ring filter for a WDM system. This approach allows the channel bandwidth to be controlled by the number of periods (stages) included. Furthermore, the filter channel position can be tuned by a delay line, the straight waveguide section, connecting the half ring sections.
Fig. 1: Schematic of Si-wire-based cascaded ring coupler [1]
We use the S-Matrix/Custom PDK Generation Utility to simulate and design one period of the lattice filter cascaded ring coupler, and then use OptSim Circuit to simulate several cascaded periods. The layout of the full structure can be input into Synopsys’ OptoDesigner for mask layout. Figure 2 shows the one unit ring setup in the RSoft CAD Environment. The Si wire in the structure has a 400nm x 200nm cross section. The cladding material is silicon dioxide. The radius of the ring is 2.5 µm and the gap between the ring and straight waveguide is 175nm at the closest point. Five delay line lengths of 0, 50nm, 100nm, 150nm, 200nm, are chosen to form different filter groups.
Fig .2: One unit ring in RSoft CAD [1]
First, the S-Matrix/PDK Generation Utility is used to calculate the spectrum at each port of the one-unit structure shown in Figure 2. The S-Matrix Utility also creates an OptSim Circuit model, a GDSII mask, and a model for OptoDesigner. Figure 3 shows the spectra when launched from Ports 1 & 2, respectively, for one delay line length. Note that these simulations were done using 2.5D (EIM) FDTD.
Fig. 3: The spectra at each port from S-Matrix calculation
Figure 4 shows a schematic in OptSim Circuit using eight periods, each modeled by a custom PDK.
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Fig. 4: a) An OptSim Circuit schematic using eight-unit ring coupler PDKs to create a cascaded ring coupler circuit, and b) the OptSim Circuit schematic.
A wideband source launches light to the circuit as shown in Figure 4b. Analyzers at all four ports are used to measure power spectra. The through and drop spectra are shown in Figure 5a, and the results match very well with published results [1]. Figure 5b shows the spectra with several different delay line lengths.
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Fig. 5: (a) Power spectra at the through and drop ports of 8 ring coupler circuit in Fig.4 without delay line; (b) Spectra with different delay line length (nm)
Next, we build a 4-channel detector array using cascaded designs with different delay line lengths, each tuned to a particular wavelength. The rest of the components in the design are taken from the AIM PDK. The spectra of the six ring coupler are shown in Figure 6. When compared to eight periods, the FSR of six periods is narrower, but it has balanced ER and bandwidth requirements at 1.5 µm to 1.55 µm wavelength range. The number of cascaded periods can be adjusted at the circuit level to optimize trade-off between ER and filter bandwidth. This significantly reduces computational overhead, which would otherwise be required by device-level simulation.
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Fig. 6: (a) Power spectra at the through and drop ports of 6 ring coupler circuit in Fig. 4 without delay line; (b) Spectra of 6 ring coupler with different delay line length (nm)
Figure 7 illustrates this 4-channel detector array with custom and official PDK. 50, 100, 150, 200 nm delay line-length were picked to accommodate center channels of 1504.8, 1519.2, 1533.6, and 1548.0 nm, respectively. The AIM Photonics PDK elements provide input coupling, signal splitting, and photodetection.
Fig. 7: The schematic setups in OptSim Circuit for 4-channel detector
We put this detector array in an OptSim Circuit schematic and operate it with a 15Gbps NRZ signal. With approximately -11dBm launch power per channel, we obtained very open eye at all four wavelength channels, as shown in Figure 8.
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Fig. 8: (a) Four-channel detector-array PIC in OptSim Circuit; (b) The eye diagram for each channel
OptSim Circuit provides a netlist to the schematic of Figure 7 to generate corresponding layout in PhoeniX Software OptoDesigner, as shown in Figure 9. It uses the same interface as standard PDK components.
Fig. 9: Mask in OptoDesigner for the OptSim Circuit photonic integrated circuit (PIC) of Fig. 7
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References:
- Koji Yamada, et al., “Silicon-wire-based ultrasmall lattice filters with wide free spectra ranges,” Optics Lett., vol. 28, No. 18, pp. 1663-1664, Sept. 2003.