We propose and demonstrate broadband, entirely mode-evolution-based, polarization splitter-rotators (PSR) using sub-wavelength grating (SWG) assisted adiabatic waveguides for two SOI platforms. Our PSRs are more compact than previously demonstrated entirely mode-evolution-based designs. The devices were fabricated using two fabrication processes and, in both cases, the measured spectra show close matches to the simulation results. One of the processes uses standard optical lithography and, hence, this is the first time that an SWG-based PSR has been experimentally implemented using such a process. Finally, measurements for arbitrary input polarizations on an active, automated polarization receiver, that uses one of our PSRs, are also presented.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
CorrectionsMinglei Ma, Anthony H. K. Park, Yun Wang, Hossam Shoman, Fan Zhang, Nicolas A. F. Jaeger, and Lukas Chrostowski, "Sub-wavelength grating-assisted polarization splitter-rotators for silicon-on-insulator platforms: erratum," Opt. Express 28, 17122-17123 (2020)
Silicon-on-insulator (SOI) is a promising platform for the photonics industry as it allows mass production at a low cost by leveraging existing complementary metal-oxide semiconductor (CMOS) fabrication technology. However, due to the aspect ratio of SOI waveguides (typically ∼2:1) and the high-index contrast between the silicon and the surrounding media, integrated photonic devices suffer from sizeable modal birefringences and, therefore, typically require control of the input polarization if one desires that only transverse-electric (TE) or transverse-magnetic (TM) modes are launched. This can be achieved using properly oriented polarization-maintaining (PM) fiber, however, this is a costly solution for real-world optical communication systems. Therefore, in order to allow standard (non-PM) fiber to be used at the input, a polarization-transparent integrated photonic device, i.e., a photonic polarization receiver, is needed [1–3]. A polarization splitter-rotator (PSR) is an essential element in such a polarization receiver. We can categorize the PSRs that have been recently experimentally demonstrated into three types: those that are entirely mode-coupling based devices [4–8], those that are hybrid mode-evolution-based/mode-coupling-based devices [9–11], and those that are entirely mode-evolution-based devices [12, 13]. The hybrid devices use mode evolution for the rotation function and mode coupling for the splitting function, whereas the other two types typically use either mode coupling or mode evolution for both functions. All of the devices that use mode coupling have lower tolerances to fabrication variations, have limited bandwidths, but are relatively compact in their sizes, compared to the entirely mode-evolution-based devices which have greater tolerances to fabrication variations, wider bandwidths, but are less compact. Nevertheless, it has been theoretically shown that more compact entirely mode-evolution-based PSRs are possible [14,15]. Sub-wavelength grating (SWG) waveguides have been used in various photonic devices due to the ability to engineer the index and dispersion properties of metamaterial-based waveguides . SWG waveguides also lead to more compact and broadband devices, compared to regular SOI waveguide-based devices [6,17–19] and, thus, they provide a pathway to improve the designs of mode-evolution-based PSRs.
In this paper, we propose and demonstrate SWG-assisted, mode-evolution-based PSRs using two SOI processes: an air cladding, single etch-step, electron-beam (E-beam) lithography process and a SiO2 cladding, multiple etch-step, CMOS-compatible, optical lithography process. Our air-clad PSR has been reported earlier in  and our SiO2-clad PSR is the first implementation of an SWG-based PSR in a standard optical lithography process. The test devices were fabricated in the two SOI processes and their spectra responses were measured. In addition, we designed and implemented a polarization receiver, based on our SWG-assisted PSR, for use in the front-ends of photonic integrated circuits (PICs) intended for high-speed optical data processing systems.
2. SWG-assisted adiabatic polarization splitter-rotator using a single etch-step E-beam lithography process
2.1. Operating principle and design
The schematic of the proposed PSR is shown in Fig. 1. An adiabatic nano-taper, an SWG-assisted adiabatic coupler, and an adiabatic mode splitting section are the three main parts of the proposed PSR. Air cladding is used to break the vertical symmetry so that efficient polarization conversion can be achieved. As shown in Fig. 1(a), light injected into the first-order TE mode (the TE00 mode) at the left-hand side (LHS) of the adiabatic nano-taper is expanded into the TE00 mode at the right-hand side (RHS) of the nano-taper, whereas light injected into the first-order TM mode (the TM00 mode) at the LHS of the nano-taper evolves into the second-order TE mode (the TE01 mode) at the RHS of the nano-taper. Figure 1(b) shows the three segments of the nano-taper. The widths and lengths are optimized to maximize the mode-conversion efficiency and minimize losses in the nano-taper, see for example in . To understand the mode expansion/evolution in the adiabatic nano-taper, we calculated the effective indices of the eigenmodes in an air-clad, SOI strip waveguide with a thickness of 220 nm. Figure 2 shows the effective indices of the first three eigenmodes in the waveguide as a function of the waveguide’s width. It can be seen that there is a hybrid mode region between the TM00 mode and the TE01 mode when the waveguide width increases from 0.62 μm to 0.72 μm. Thus, this leads to mode evolution from the TM00 mode to the TE01 mode when a TM00 mode propagates along the adiabatic nano-taper. In contrast, the effective index of the TE00 mode in the strip waveguide increases but shows no mode conversion as the strip waveguide widens.
An SWG-assisted, adiabatic mode evolution section, which we will refer to as an SWG-assisted adiabatic coupler (SWG-AC), is designed such that the TE00 and TE01 modes on the RHS of the nano-taper are well matched to the TE00 and TE01 modes on the LHS of the two-waveguide system, respectively. This two-waveguide system consists of the upper, regular strip waveguide and the lower, SWG-assisted strip waveguide, and, eventually, at the outputs of the SWG-AC, the TE00 and TE01 modes of the two-waveguide system are predominately located in the upper waveguide and in the lower waveguide, respectively. The two waveguides are tapered to minimize the losses in the two system modes, as their mode distributions evolve from the LHS of the SWG-AC to the RHS of the SWG-AC. Specifically, to achieve the optimal field distributions of the two TE system modes in the SWG-AC, we use the finite-difference time-domain (FDTD) bandstructure calculations  to determine an appropriate period, Λ = 200 nm, and fill factor, ff = 0.6. In addition, as regards the mode splitting section, the TE00 mode at the RHS of the SWG-AC is directed into the TE00 mode of an isolated, 450 nm wide, strip waveguide using a low loss S-bend, and, a high-efficiency SWG taper is used to evolve the TE01 mode at RHS of the SWG-AC into the TE00 mode of the second isolated, 450 nm wide, strip waveguide. Overall, the injected TE00 mode is received at the through-port of the device, whereas the injected TM00 mode is converted to the TE00 mode at the cross-port of the device. Compared with the reported adiabatic PSR in , our SWG-based PSR is also adiabatic but requires a shorter coupler length (L4 = 100 μm here versus L = 300 μm in ). Further details will be discussed in Section 3.1.
2.2. Simulation and measurement results
To fully understand our proposed PSR, we investigated an efficient simulation approach based on FDTD S-parameter matrix calculations and a circuit modelling method. Initially, using Lumerical FDTD Solutions, we obtained the S-parameter matrices for each section of the device, i.e., the nano-taper, the SWG-AC, and the splitting section, and created compact model representations in Lumerical INTERCONNECT. Then, we calculated the transmission response of the entire PSR using the compact models in Lumerical INTERCONNECT. This approach is more computationally efficient than running a full 3D FDTD simulation for the entire structure. The simulated transmission spectra are shown in Fig. 3. In Fig. 3(a), we can see that for an injected TM00 mode, the output light was mainly received at the cross-port with less than 0.5 dB insertion loss. At the through-port, the TE00 mode crosstalk was less than −18 dB over a 120 nm bandwidth, from 1500 nm to 1620 nm, and the TM00 mode crosstalk stayed less than −19 dB in the wavelength range from 1546 nm to 1620 nm. However, the TM00 mode crosstalk rose to nearly −15 dB below 1530 nm, which can be reduced by replacing the S-bend with a compact broadband polarization beam splitter [21, 22]. For an injected TE00 mode, on the other hand, the input light propagated to the through-port with less than 0.4 dB insertion loss, and less than −25 dB crosstalk was received at the cross-port over a broad wavelength range from 1500 nm to 1620 nm, as shown in Fig. 3(b).
Our air-clad, single etch-step PSRs were fabricated by Applied Nanotools Inc. using electron-beam lithography (EBL) with a 5 nm grid spacing. Figs. 4(a)–4(d) show scanning electron microscope (SEM) images of a fabricated PSR. It can be seen that the designed rectangular corrugations on the SWG-AC and the splitting section have been accurately fabricated, as shown in Figs. 4(b)–4(d). The fabricated devices were experimentally characterized using a custom-built test setup . An Agilent 81600B tunable laser was used as the light source and Agilent 81635A optical power sensors were used as the output detectors. Broadband grating couplers (GCs)  were used to couple light into and out of the devices. Figs. 5(a) and 5(b) show the measurement results for a fabricated PSR when the TM00 and the TE00 modes were launched, respectively (here, the transmission spectra have been calibrated using the responses of the GC calibration pairs). As shown in Fig. 5(a), when the TM00 mode was launched, the output light was mainly collected at the cross-port and the measured crosstalk at the through-port, for both the TM00 mode and the TE00 mode, had wavelength dependent responses, that were similar to the simulated spectra. As shown in Fig. 5(b), the launched TE00 mode mainly propagated to the through-port, and the crosstalk was less than −20 dB over a 120 nm wavelength range (1500 nm to 1620 nm), which is close to the simulation results. It should be noted that the measured optical powers for the transmission of both the TE00 mode at the cross-port (in Fig. 5(a)) and the TE00 mode at the through-port (in Fig. 5(b)) are reasonably well matched to the simulation results. The insertion loss was less than 1.4 dB from 1530 nm to 1620 nm. The errors in the measured transmissions were approximately ±0.5 dB for the TE00 mode measurements and ±1.0 dB for TM00 mode measurements, respectively. We attribute this due to the measurement alignment inaccuracies and calibration errors in the GC port measurements.
3. SWG-assisted adiabatic polarization splitter-rotator using a standard optical lithography process
3.1. Operating principle and design
For the first time, we demonstrate a broadband PSR using SWG-assisted adiabatic waveguides on a standard optical lithography platform. Instead of an air-clad nano-taper, an adiabatic bi-level taper (see Fig. 6(a)) was designed as the first section of our proposed PSR. A shallow-etched silicon slab waveguide, with a 90 nm thickness, was used to break the vertical symmetry of the waveguide for efficient polarization conversion. This enabled us to use a symmetric SiO2 cladding so that our device would be fully compatible with standard foundry processes and, hence, would allow us to integrate our device in active PICs. Following the bi-level taper were an SiO2-clad SWG-AC and a splitting section, similar to the air-clad PSR shown in Figs. 1(a) and 1(c). However, due to both our use of the SiO2 cladding layer and the fabrication process rules, the design parameters of the SiO2-clad SWG-AC were optimized and found to be W3 = 850 nm, W4 = 150 nm, W5 = 450 nm, W6 = 550 nm, L4 = 100 μm, g = 180 nm, Λ = 300 nm, and ff = 0.5. Similar to the analysis of the nano-taper, the effective indices of the first three eigenmodes along the first half of the bi-level taper were calculated and are presented in Fig. 6(b). Again, there is a hybrid mode region, in which the mode conversion between the TM00 mode and the TE01 mode occurs. Accordingly, in the first half of the bi-level taper, the rib and slab waveguides widen from 0.45 μm and 0.45 μm to 0.55 μm and 1.55 μm, respectively. On the other hand, we can see that the TE00 mode simply propagates along the bi-level taper without any mode hybridization. In the second half of the bi-level taper, which is designed to ensure efficient TM00 to TE01 mode conversion and to provide a transition to the following SWG-AC in strip waveguides, the width of the rib waveguide increases linearly to 0.85 μm and the width of the slab waveguide decreases linearly to the same value. Also, to optimize the TM00 to TE01 mode conversion in the bi-level taper, in our simulations we swept the lengths of the two sections of the taper, LA and LB, as shown in Fig. 7(a). Here, LA = 35 μm and LB = 30 μm was found to result in a 99.8% mode conversion efficiency at a center wavelength of 1550 nm. Finally, using the 3D FDTD solver, we simulated the mode evolutions in the bi-level taper for both TM00 mode and TE00 mode inputs, as shown in Fig. 7(b) and 7(c), respectively.
Additionally, we swept the length of the adiabatic coupler section(L4 in our SWG-AC) for our SWG-based design and the reported design in . Based on the simulation results shown in Fig. 7(d), we can see that, for the TE00 mode, both of the designs have similar transmission responses at various coupler lengths. This is because the mode evolution is predominantly confined to the upper strip waveguide for the TE00 mode. However, for the TE01 mode, the mode evolution happens in both waveguides of the adiabatic coupler section. Due to the smaller effective index of TE01 mode in our SWG-AC, the field of the TE01 mode, in the two-waveguide system, is less confined to the silicon region, i.e., stronger mode overlap occurs. Therefore, compared with the adiabatic coupler section with two regular strip waveguides in , the mode evolution along our SWG-AC will occur more rapidly for the TE01 mode, i.e., our SWG-AC requires a shorter coupler length (∼90 μm) to achieve a full mode evolution.
3.2. Simulation and experimental results
Using the simulation method previously introduced in Section 2.2, we also simulated the transmission spectra of the SiO2-clad PSR. As shown in Fig. 8(a), where a TM00 mode is injected at the input-port, the output light, in the TE00 mode at the cross-port, has less than 0.5 dB insertion loss. The crosstalk at the through-port is obtained separately for the TE00 mode and the TM00 mode. As shown in Fig. 8(b), where a TE00 mode is injected at the input-port, the output, in the TE00 mode at the through-port, has a less than 0.3 dB insertion loss, and the crosstalk at the cross-port was also obtained for the TE00 mode across a wavelength range from 1500 nm to 1620 nm.
Our PSRs were fabricated on an SOI platform using 193 nm optical lithography at the Institute of Microelectronics (IME), Singapore. To experimentally characterize the fabricated devices, we used an edge coupler setup with a fiber array having a mode-reducing transposer (made by PLC Connections) to couple light into and out of our devices. Figure 9 shows the measured transmission spectra of the test device for both TM00 mode and TE00 mode inputs. The transmission spectra have been calibrated using the responses of edge coupler calibration pairs. We can see that the measurement results, i.e., the optical power measured at the target output-ports and the crosstalk-ports for both TM00 mode and TE00 mode inputs, are close to the simulation results shown in Fig. 8. The insertion loss was less than 1.3 dB over a wavelength range from 1500 nm to 1620 nm. However, we should also note the slight optical power discrepancies between the simulations and measurements, which are mainly attributed to measurement alignment errors in edge coupling and to fabrication imperfections. In addition, instead of the flat responses seen in the simulations, oscillations can be observed in the measured spectra, which are mainly due to the Fabry-Perot effect created by reflections between the chip facets at the input and outputs ports of the test device . Finally, the figure of merits of the fabricated mode-evolution-based PSRs were summarized as shown in Table. 1.
3.3. Polarization receiver
To demonstrate the functionality of our PSR in an optical communication system, we implemented a silicon photonic polarization receiver that is able to automatically adapt to an arbitrary polarization state for an incoming data stream from a single-mode fiber . The arbitrary polarization state of the input light, as shown in Fig. 10(a), was generated using a HP11896A polarization controller (PC) in the input path. The input light was coupled into the polarization receiver using a tapered edge coupler. As the first section of the polarization receiver, the PSR passively coupled the TE and TM portions of the captured input light into the TE00 modes of its two outputs and then passed them into a balanced Mach-Zehnder Interferometer (MZI). Using two thermal phase shifters (H1 and H2 shown in Fig. 10(a)), the optical power at the output port was optimized by minimizing the optical power in the feedback port. The thermal phase shifter (H1) was used to obtain the equal optical powers at the two outputs of the first 3dB coupler. Then, to obtain an optimized output after the second 3dB coupler, the thermal phase shifter (H2) was used to adjust the phase difference between the two inputs of the second 3dB coupler, i.e., the two outputs of the first 3dB coupler. Detailed system modelling and analysis of the MZI has been discussed in [2,25]. Figure 10(b) shows an optical micrograph of a fabricated polarization receiver.
For the experiment, firstly, we generated a 12.5 Gbps 231 − 1 pseudorandom binary sequence (PRBS) optical data stream using a Mach-Zehnder modulator, which was then passed through the PC. Next, the automated control algorithm, demonstrated in , was implemented and the optical power at both the output port and the feedback port were recorded before, during, and after the automated optimization process, as shown in Fig. 11(a). The optical light collected at the output port was detected by an external high-speed photodetector (PD) and finally sent to an Agilent 86100A sampling oscilloscope with a 50 GHz 83484A sampling head. The displayed eye diagrams of the data stream before, during, and after the optimization process were also recorded and are shown in Fig. 11(b): before the automated optimization, the polarization receiver’s MZI was not functional and the arbitrary polarization state led to a small eye amplitude (58 mV in Fig. 11(b)(i)); during the optimization process, the eye started changing (Fig. 11(b)(ii)) indicating that the optimization process was affecting the eye; after the optimization process, a large eye amplitude (124.9 mV in Fig. 11(b)(iii)) was obtained and stabilized using our automated tuning system.
In conclusion, we have demonstrated broadband polarization splitter-rotators using sub-wavelength grating-assisted adiabatic waveguides in two silicon-on-insulator fabrication processes: an air-clad, single etch-step process and a SiO2-clad, multiple etch-step, CMOS compatible process. The SiO2-clad device is the first demonstration of a fabricated polarization splitter-rotator based on sub-wavelength grating waveguides using a standard optical lithography process. We measured the spectra of both fabricated sub-wavelength grating-assisted adiabatic polarization splitter-rotators and compared them with the simulation results. In addition, we used one of the proposed polarization splitter-rotators in an active polarization receiver intended for use in high-speed optical communication systems. The measured responses of the circuit indicate that our polarization splitter-rotators have good compatibility with active photonic integrated circuits, fabricated using a commercial silicon-on-insulator platform.
Natural Sciences and Engineering Research Council of Canada (NSERC) Silicon Electronic Photonic Integrated Circuit (SiEPIC) program; China Scholarship Council (CSC).
We would like to thank CMC Microsystems and Lumerical Solutions Inc. for the design and simulation software. We also acknowledge Han Yun for helpful discussions.
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