We designed and demonstrated a tri-layer Si3N4/SiO2 photonic integrated circuit capable of vertical interlayer coupling with arbitrary splitting ratios. Based on this multilayer photonic integrated circuit platform with each layer thicknesses of 150 nm, 50 nm, and 150 nm, we designed and simulated the vertical Y-junctions and 3D couplers with arbitrary power splitting ratios between 1:10 and 10:1 and with negligible(< −50 dB) reflection. Based on the design, we fabricated and demonstrated tri-layer vertical Y-junctions with the splitting ratios of 1:1 and 3:2 with excess optical losses of 0.230 dB. Further, we fabricated and demonstrated the 1 × 3 3D couplers with the splitting ratio of 1:1:4 for symmetric structures and variable splitting ratio for asymmetric structures.
© 2017 Optical Society of America
Si3N4(core) and SiO2(cladding) based optical waveguides are attractive because of its relatively low propagation loss  than silicon waveguides and small footprints  compared to silica waveguides. Further, Si3N4/SiO2 waveguide fabrication techniques support a low-loss multilayer waveguide platform for 3D photonic integrated circuits (PIC) . The interlayer coupling in multilayer platform utilized various methods including directional couplers , grating couplers , reflector mirrors , and inverse tapers couplers [7–9]. The inverse taper vertical couplers demonstrate ultra-low coupling loss  and have been widely studied on the coupling of silicon-to-silicon  and silicon-to-Si3N4 [8, 9]. However, the reported works focus on the coupling limited to single-output structures on bi-layer platforms. Based on the inverse taper coupling method, we propose multi-layer multi-output couplers with various coupling ratio.
Recently, we demonstrated a bi-layer Si3N4/SiO2 waveguide platform optimized by varying the interlayer gap between the two waveguide cores of an identical thickness . By repeating the optimized Si3N4 deposition and waveguide fabrication  while maintaining reasonable wafer bowing, we introduce additional Si3N4 layers to further reduce crosstalk and losses when multiple waveguides need to cross and intersect inevitably in single layer photonic integrated circuits. In this work, we present a tri-layer platform with non-identical layer thicknesses. By adding a new layer, we design waveguide crossing between non-neighboring layers to enlarge the gap between overlaid waveguides and reduce layer-to-layer crosstalk. Additionally, we design non-identical layer thicknesses to utilize the variable confinement factor and mode sizes of Si3N4 for functional purposes, such as large mode size for optical gyroscope with ultra-low propagation loss  and small mode size for compact arrayed waveguide grating with tight bending .
In the tri-layer platform, we exploit the vertical dimension as a new degree of freedom in photonic device design. Using the tri-layer platform, we propose vertical Y-junctions, 1 × 3 3D couplers, and 1 × 4 3D couplers with arbitrary power splitting ratios with wavelength range of 1530-1565 nm. The proposed tri-layer Si3N4 3D couplers are more attractive because of its simplified design for low loss couplers compared to the traditional silicon Y-junctions  and 2D asymmetric Y-junctions , as well as the good fabrication tolerance compared to the photonic crystal Y-junctions .
2. Platform design
Si3N4 waveguides with SiO2 cladding have a broad range of confinement factor values depending on the core thickness and width values . For waveguides with thinner cores, the ultra-low propagation loss is reported in . Also, the confinement becomes smaller, resulting in a larger mode size, which matches the mode of the single mode fibers better, leading to a lower facet coupling loss. Figures 1(a)-1(d) simulate power coupling between Si3N4 waveguides and single mode silica fibers with varying the fiber tip spot size and the Si3N4 core tip width for the core thicknesses of 50 nm and 150 nm. The typical tip spot size of the lensed fiber or cleaved single mode fiber is in the range of 2-10 μm. The fiber modes with small beam sizes at the tip of lensed fiber are not the guiding Linearly Polarized (LP) modes in the single mode fiber at 1550 nm wavelength. As an assumption, we used Gaussian beams with varying beam diameter for simulating optical modes at the fiber side . Within this range, the 150 nm thick Si3N4 waveguide requires an edge coupler with the tip width below 500 nm to match the mode size of fiber with 5 μm beam diameter, while the 50 nm thick Si3N4 waveguide provides better reliability on low loss facet coupling with waveguide width between 2 μm and 5 μm.
With a thicker waveguide core, confinement factor becomes larger, and device footprint becomes smaller because of tight bending. Therefore, to combine both advantages between thin and thick Si3N4 waveguides, we designed a platform including non-identical waveguide thickness on each layer and asymmetric inverse taper vertical coupler for inter-layer coupling. Figure 2 shows the schematic of the tri-layer platform. There are three Si3N4 layers with the thicknesses of 150 nm, 50 nm, and 150 nm, respectively. We designed the inter-layer coupling between neighbor layers, the middle 50 nm thick Si3N4 layer coupling to the top or bottom 150 nm thick Si3N4 layer. Figure 3(a) illustrates the simulated vertical coupling between 50 nm and 150 nm thick Si3N4. As a result, the simulated asymmetric vertical coupling introduces excess loss less than 0.1 dB with neighbor-layer gap smaller than 1.4 μm.
Another consideration of multilayer platform is that devices on different layers are overlaid to reduce footprint. On the tri-layer platform, we design waveguide crossing between non-neighboring layers, where the gap for inter-layer crossing is twice of that for interlayer vertical coupling. Figures 3(b)-3(d) illustrate the simulation of transmission, reflection, and crosstalk for inter-layer crossing with varying non-neighboring gap. As a result, with the non-neighboring gap larger than 1.2 μm, the simulated inter-layer crossing loss, reflection and crosstalk are less than 0.1 dB, −33.5 dB, and −34.5 dB, respectively. We used Lumerical MODE solutionsTM and FDTD solutionsTM for simulation.
3. Simulation of vertical Y-junctions and 3D couplers
Based on the tri-layer platform, we designed vertical Y-junctions with the input port on the middle layer of 50 nm thick Si3N4 and two output ports on the 150 nm thick Si3N4 layers. Figure 4(a) shows the schematic, where three inverse taper structures overlap with the tip width of 250 nm. Figures 4(b) and 4(c) are the simulation of the Poynting Vector for TE polarization in a vertically symmetric structure and power splitting of each port with varying interlayer gap in a vertically asymmetric structure . The design of input port utilizes a larger mode size on the 50 nm thick Si3N4 layer for strong coupling to other layers, while the output power equally splits into each port with identical gaps. The simulated Poynting Vector values increase beyond 1.0 due to narrower waveguide width for single mode condition on the top and bottom layers. When the upper gap is larger than the lower gap, the power relocates from the top port to the bottom port, and the power splitting ratio drops from 1:1 to less than 1:9.
Furthermore, we designed 1 × 3 vertical couplers by adding one output port to the vertical Y-junction. Figure 5(a) shows the schematic of two paralleled inverse tapers with 4 μm separation on the top layer, and one port on the bottom layer. For a symmetric structure, the two paralleled ports on the top layer are laterally 2 μm away from the input port and the transmission on each port is equal. To achieve arbitrary power splitting, the paralleled ports are laterally shifting along y-axis and the power distribution varies along with shifting for all ports. Figures 5(b)-5(e) are the simulation of mode distribution of the output ports with 0, 0.5, 1.0, and 2.0 μm lateral-offset, respectively. Figure 5(f) illustrates the corresponding transmission from 1:1:6 to 0:5:5 with varying the top layer lateral-offset up to 2 μm.
Figures 6(a)-6(d) show another 1 × 3 vertical coupler with power splitting of 1:1:1 by tuning the interlayer gap. The designed values of the upper and lower gaps are 600 nm and 1.8 μm with 2 μm lateral distance from the center to the top ports. The simulated mode overlapping integral between the input and each output port are equal within ± 3% of 1.5 μm to 1.6 μm wavelength range. Usually, 1 × 3 couplers based on self-imaging method introduce a phase difference between the output ports . In our simulation, there is no imagery part difference of the transmission coefficients between the three output ports of the 1 × 3 vertical couplers.
Similarly, we designed a 1 × 4 vertical coupler by adding one port on the bottom layer to the 1 × 3 vertical coupler. Without lateral-offset, the symmetric structure split the input power into 1:1:1:1 with small wavelength dependence. Figures 7(a) and 7(b) illustrate the schematic of the 1 × 4 vertical coupler and the output mode distribution with 600 nm inter-layer gap.
Figure 8(a) shows the polarization dependency and the wavelength dependency of the proposed Y-junction, 1 × 3 coupler, and 1 × 4 coupler. Figures 4, 5, and 7 illustrate the structures for simulation. Due to symmetry, the outputs have equal transmission for 1 × 2 and 1 × 4 couplers, as well as the two ports on the top layer of the 1 × 3 coupler. As a result, all the outputs indicate low wavelength dependency. The transmission variation is below 0.1%, 3% and 0.1% for 1 × 2, 1 × 3, and 1 × 4 couplers from 1530 nm to 1565 nm wavelength. Besides, the transmission of 1 × 2 and 1 × 4 couplers indicate low polarization dependency between TE and TM modes with the difference below 1.4% and 0.3%, respectively. However, the 1 × 3 coupler, which breaks the spatial symmetry, shows a transmission difference of 17% between TE and TM input modes. The simulated excess losses are 0.18 dB, 0.3 dB, 0.03 dB, 0.08 dB, 0.16 dB, and 0.21 dB for 1 × 2 TE, 1 × 2 TM, 1 × 3 TE, 1 × 3 TM, 1 × 4 TE, and 1 × 4 TM couplers at 1550 nm wavelength.
Figure 8(b) shows the influence of tapering length of the proposed 3D couplers with TE or TM input mode at 1550 nm wavelength. As a result, as tapering length decreases from 100 μm to 25 μm, the total loss of the 1 × 2 coupler increases from 0.18 dB to 0.38 dB for TE input, and from 0.3 dB to 0.47 dB for TM input. For 1 × 3 and 1 × 4 couplers, the total loss variation is less than 0.1 dB within this tapering length range. The symmetric structures including 1 × 2 and 1 × 4 couplers indicate a constant power splitting with varying tapering length, while the asymmetric structure of the 1 × 3 coupler shows the tapering length dependency on the power splitting ratio.
4. Device fabrication and characterization
We deposited waveguide core and cladding by Low-Pressure Chemical Vapor Deposition (LPCVD) on 6-inch silicon wafers. We fabricated the devices layer-by-layer using ASMLTM PAS 5500 300 deep-UV lithography stepper and Inductively Coupled Plasma (ICP) dry etching. The layer-to-layer misalignment is within 50 nm. Between the layers, we used Chemical Mechanical Planarization (CMP) to achieve Root Mean Square (RMS) roughness less than 1 nm, layer thickness accuracy ± 10 nm, and non-uniformity of 5% . We diced the wafer into chips of 1 cm × 1 cm.
The essential element of the tri-layer platform for vertical coupling is the asymmetric vertical coupler between 50 nm and 150 nm thick Si3N4 layers. Figure 9(a) is the photo image of an asymmetric vertical coupler, where the Si3N4 taper tip on the 150 nm thick layer is lying above the Si3N4 inverse taper on the 50 nm thick layer. For the measurement, we used an Optical Vector Network Analyzer (OVNA) system to characterize the device under test and single mode polarization maintaining lensed fibers as input and output with TE polarization. We measured the transmission of straight waveguides on each layer as the reference. The repeatability accuracy and noise level of the measurement system are within 0.1 dB and below −50 dB after calibration. We measured the propagation loss of 0.4-0.6 dB/cm for the 150 nm thick Si3N4 waveguides by using cutback method, and 3.5 dB/m for the 50 nm thick Si3N4 waveguides by using the optical frequency domain reflectometry (OFDR) method. Figures 9(b) and 9(c) illustrate the measured transmission of straight waveguides and the fitting curve for the cascaded asymmetric vertical couplers. As a result, the asymmetric vertical coupling loss is 0.185 dB per coupling. All fabricated devices have 100 μm long taper couplers.
Furthermore, we fabricated the vertical Y-junctions with a fixed lower gap value of 600 nm and varying the upper gap values of 600 nm, 780 nm, and 860 nm, respectively. Figure 10(a) is the photo image of the fabricated vertical Y-junctions. We characterized the devices with TE input at 1550 nm wavelength. Figure 10(b) is the measured transmissions of each output port normalized to the asymmetric vertical coupler insertion loss, which includes the fiber to input port coupling loss, the vertical coupling loss, and the output port to fiber coupling loss. As a result, the power splitting is 1:1 for identical gaps and 3:2 for non-identical gaps.
Adjusting inter-layer gap is an effective approach for arbitrary power splitting. However, a multi-project wafer run requires that the devices integrated on one single platform share the common inter-layer gap. Under this condition, adjusting the lateral-offset of inverse tapers allows achieving arbitrary power splitting rations for a given inter-layer gap. For a symmetric vertical Y-junction with identical inter-layer gaps, the lateral-offset method reduces optical transmission of the shifted port. In the case of unlimited shifting, the input port and un-shifted port become a vertical coupler and most of the optical power transmission switches to the un-shifted port. Figure 10(c) illustrates the transmission of the vertical Y-junction with equal gap values of 600 nm and lateral-offset on the top port. The power split ratio of 3:2 can be achieved as the lateral-offset values increased to 500 nm.
We also fabricated and characterized 1 × 3 couplers with Port#1, Port#3 on the bottom layer, and Port#2 on the top layer. Figures 11(a) and 11(b) are the device photos of 1 × 3 couplers and the zoomed-in view of input port on the middle layer surrounded by three inverse taper tips on top and bottom layers. The separation of paralleled inverse tapers on the bottom layer is 4 μm. The lower gap and upper gap values are 700 nm and 500 nm, respectively. Normalized to straight waveguide insertion loss, Fig. 10(c) illustrates the transmission spectrum with 2 μm lateral-offset, where Port#1 on the bottom layer is underneath the input port, and Port#3 is laterally 4 μm away from the input port. In this case, the transmission value of Port#3 is low due to the large distance from the input port, while the transmission of Port#1 and Port#2 are −2.7 dB and −3.7 dB due to non-equal gaps. With reduced lateral-offset at the bottom layer, the transmission of Port#1 and Port#3 varies along with the distance from the input port. For a laterally symmetric structure, the transmission of Port#1 and Port#3 is −7.7 ± 0.2 dB and −7.4 ± 0.2 dB, while the transmission of Port#2 increases to −1.9 ± 0.2 dB.
We designed and demonstrated a tri-layer Si3N4 platform integrated with multiple waveguide thicknesses. Compared to the bi-layer platform, tri-layer combines advantages from variable layer thickness and realizes simplified ultra-low crossing loss by using a non-neighboring cross. Based on the tri-layer platform, we designed vertical Y-junctions, 1 × 3 couplers, and 1 × 4 couplers with arbitrary power splitting ratio by tuning inter-layer gaps and lateral-offset. We fabricated and characterized an asymmetric vertical coupler with excess coupling loss of 0.185 dB, the vertical Y-junctions with power splitting from 1:1 to 3:2, and the 1 × 3 coupler with power splitting from 1:1:4 to 1:22:27. The simulated total loss of the vertical Y-junctions and 1 × 3 couplers are 0.18 dB and 0.03 dB for TE mode. The fabricated devices have less than 0.25 dB excess losses compared to the simulation values. For the effect of fabrication misalignment, the lithography tool we used provides less than 50 nm misalignment. With simple calculation from the experiment data of 1 × 2 coupler with lateral offsets, 50 nm offset results in power variation of about 0.5%, which is below 0.1 dB. Overall, by introducing multiple Si3N4 layers for integration, we demonstrate photonic devices converted from 2D to 3D. The 3D coupler with variable power splitting ability is one candidate for various demands of the future photonic 3D integration.
Lockheed Martin Corporation (subaward from W31P4Q-15-C-0003) Defense Advanced Research Projects Agency (DARPA) (W31P4Q-15-C-0003).
Fabrication of the devices utilized the facilities at the Marvell Nanofabrication Laboratory (Berkeley, CA) and at the Center for Nano-MicroManufacturing (Davis, CA).
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