We demonstrate novel polarization management devices in a custom-designed silicon nitride (Si3N4) on silicon-on-insulator (SOI) integrated photonics platform. In the platform, Si3N4 waveguides are defined atop silicon waveguides. A broadband polarization rotator-splitter using a TM0-TE1 mode converter in a composite Si3N4-silicon waveguide is demonstrated. The polarization crosstalk, insertion loss, and polarization dependent loss are less than −19 dB, 1.5 dB, and 1.0 dB, respectively, over a bandwidth of 80 nm. A polarization controller composed of polarization rotator-splitters, multimode interference couplers, and thin film heaters is also demonstrated.
© 2014 Optical Society of America
Silicon-on-insulator (SOI) has attracted significant attention as an integrated optics platform owing to its high index contrast, capability for integration with electro-optic modulators and detectors, and compatibility with CMOS fabrication processes . However, silicon (Si) is not the most ideal CMOS-compatible material for certain optical functions. For passive components, silicon nitride (Si3N4) waveguides with silica (SiO2) claddings can be superior to Si waveguides since Si3N4 has greatly reduced thermo-optic effects and optical nonlinearities compared to Si, and it has high transparency into the visible wavelength range. The lower index contrast of a Si3N4 waveguide also reduces the waveguide losses due to sidewall roughness scattering [2–7], sensitivity to variations in waveguide dimensions, and dispersion. Several recent demonstrations have shown the integration of Si3N4 waveguides onto SOI photonic platforms [7–9] or the integration of SOI waveguides onto a Si3N4 waveguide platform  to combine active and passive functionalities for CMOS-compatible integrated photonics.
In this work, we present a polarization rotator-splitter (PRS) and a polarization controller in a custom Si3N4-on-SOI integrated photonics platform. These devices are important for controlling the polarization characteristics of photonic integrated circuits, and they are used in polarization diversity schemes as well as polarization (de)multiplexers [3, 11]. In contrast to demonstrations of hybrid integrated devices that separate functionalities between the constituent layers [7–10], our work here emphasizes that new device designs can be enabled by composite waveguides formed from the multiple layers. Preliminary results of this work were presented in . Our PRS design is based on TM0-TE1 mode conversion in a composite Si3N4-Si waveguide. The design is entirely adiabatic and is inherently more broadband and fabrication tolerant than the first demonstration of a polarization splitter-rotator in a Si3N4-on-SOI platform in , which used a directional coupler for polarization splitting and a Si3N4-on-SOI adiabatic polarization rotator. Compared to our PRS demonstration based on SOI ridge waveguides in [14, 15], our Si3N4-on-SOI design has improved fabrication yield since it lacks the inherently difficult to control partial-etch depth of SOI ridge waveguides.
The paper is organized as follows: we provide an overview of the Si3N4-on-SOI platform and fabrication in Section 2; we present design and measurement results for the PRS in Section 3; finally, we apply the PRS to an active polarization controller in Section 4.
2. Overview of the Si3N4-on-SOI platform and fabrication
The integrated Si3N4-on-SOI platform used in this work is illustrated in Fig. 1(a). This platform is similar to that presented in [8, 9, 16], but with different thicknesses. In contrast to , the waveguides in both the Si3N4 and SOI have moderate confinement. Our platform consists of a 400 nm thick Si3N4 layer on top of the 150 nm thick top Si layer of a SOI wafer. The Si3N4 and Si layers are separated by a 50 nm thick planar layer of SiO2. The layer thicknesses are chosen to enable efficient coupling between the Si3N4 and Si using adiabatic tapers without greatly reducing the electro-optic capabilities of the Si layer. The separation between the Si3N4 layer and the TiN thin film heaters is 1.5 μm. Four types of waveguides can be formed in this platform as shown in Fig. 1(a); Si3N4 strip, Si strip, Si rib, and composite Si3N4-Si. Since the thermo-optic coefficient of Si is significantly larger than Si3N4, we use TiN above Si strip waveguides for thermal tuners. PN junctions and germanium photodiodes for modulators and detectors (not shown in this work) can be formed on the Si level.
This platform was fabricated using a custom fabrication process at IME, A*STAR. The process started with a 200 mm SOI wafer with a 220 nm thick top Si and a 2 μm thick buried oxide (BOX). The top Si layer was first thinned down to 150 nm. Si rib and channel waveguides were then formed by deep ultraviolet (DUV) lithography (248 nm exposure) and anisotropic reactive ion etching. Next, SiO2 was deposited followed by planarization using chemical-mechanical polishing to obtain a 50 nm SiO2 thickness above the Si waveguides. Si3N4 was deposited through low-pressure chemical vapor deposition (LPCVD) and Si3N4 waveguides were formed using dry etching. Lastly, a SiO2 top cladding was deposited, followed by steps to define TiN thin film heaters and their contact metals. Cross-sectional transmission electron micrographs (XTEMs) of a fabricated Si strip waveguide, Si3N4 strip waveguide, and Si3N4-Si waveguide are shown in Figs. 1(b)–1(d). The SiO2 thickness between the Si3N4 and Si in Fig. 1(d) is about 10 nm, which is thinner than the nominal value of 50 nm and can be attributed to the wafer-scale variation of the chemical-mechanical polishing.
3. Adiabatic polarization rotator-splitter
Our PRS design is shown in Fig. 2(a). It uses the vertical asymmetry of a composite waveguide formed out of the Si3N4 and Si layers to adiabatically transform the fundamental transverse magnetic (TM0) mode to the first order transverse electric (TE1) mode before separating the TE1 and TE0 modes into two output waveguides using an adiabatic coupler. The TM0-TE1 mode conversion in vertically asymmetric structures was proposed in , and we have recently used this concept to demonstrate a SOI PRS and a polarization controller circuit in . Similar to , the PRS design implemented here is entirely adiabatic to improve the fabrication tolerance and operation bandwidth.
3.1. Principle of operation and design
Figure 2(b) illustrates the PRS operation, which is based on mode evolution. The two modes with the highest effective indices at various positions along the PRS are indicated (i.e., “mode 1” and “mode 2”). At the input, a single-mode 900 nm wide Si3N4 input waveguide is widened to 1.4 μm. Then, a Si waveguide begins with a 180 nm wide tip underneath the Si3N4. At this point, modes 1 and 2 are TE0 and TM0, respectively, and are mostly confined in the Si3N4. The Si waveguide is then widened while the Si3N4 width is unchanged, enabling the conversion from TM0 to TE1. In this section of the PRS, the composite Si3N4-Si waveguide is vertically asymmetric, leading to a large difference in the effective indices of modes 2 and 3 throughout the structure [Fig. 2(c)]. As a result, a TM0 input remains as mode 2 through the TM0-TE1 mode converter. It first evolves into a “hybridized” mode, which has TM0 and TE1 characteristics, and then the TE1 mode. A TE0 input remains in mode 1, the TE0 mode. After the TM0-TE1 mode converter, the Si3N4 is tapered down and terminated with a blunt 200 nm wide tip while the Si width is constant at 930 nm. Since the TE0 and TE1 modes are confined primarily in the Si, the Si3N4 termination is low-loss and has the benefit of restoring vertical symmetry, which prevents any further interaction between the TM0 and TE1 modes.
An adiabatic coupler follows the TM0-TE1 mode converter. The design is similar to that in , and it uses only fully-etched Si waveguides. The TE0 and TE1 modes are now the supermodes of the adiabatic coupler. A narrow waveguide starts with a 180 nm wide tip next to a broad 930 nm wide waveguide, in which both the TE0 and TE1 modes are almost entirely confined. Then, the narrow waveguide widens to a width of 500 nm and the broad waveguide width decreases to 630 nm, while the gap remains at 200 nm. This transition causes the TE1 mode to become mostly confined in the narrow waveguide while the TE0 mode is confined in the broad waveguide. The narrow waveguide is then bent away using an arc with a radius of 500 μm, and the TE0 and TE1 modes evolve into the TE0 modes of the isolated top and bottom waveguides, respectively. Finally, adiabatic transitions couple light from the Si waveguides to 900 nm wide Si3N4 output waveguides.
3.2. Experimental results
Figure 3 shows the microscope images of the fabricated PRS. The total device length is 576 μm. The device is connected to Si3N4 inverse-taper edge couplers at both ends for efficient coupling to tapered fibers with spot sizes of about 2 μm in diameter. To eliminate the effects of fiber polarization rotation on the device measurements, we used free-space coupling and free-space polarizers. Specifically, we measured the PRS by coupling light from a tunable laser onto/off the chip using objective lenses, and we placed manually-adjustable, free-space, linear polarizers at the input and output of the chip to set the input polarization and analyze the output polarization.
The measured transmission spectra of the PRS are shown in Fig. 4. The PRS transmission spectra have been normalized to the edge coupler transmission spectra to remove contributions from the measurement apparatus and edge couplers. The legend indicates the input polarization and the setting of the output polarizer, e.g., “TE → TM” is the measurement of the TM component of the output with a TE-polarized input. The polarization crosstalk at both outputs was less than −19 dB, the insertion loss was less than 1.5 dB, and the polarization-dependent loss (PDL) was less than 1.0 dB over a wavelength range from 1500 nm to 1580 nm. These values have an error of about ±0.5 dB due to errors in aligning the coupling lenses to the chip. The oscillations in the spectra of the crosstalk components were due to small errors in the alignment of the input and output free-space polarizers. Compared to , our PRS has a similar insertion loss and a broader bandwidth. The polarization crosstalk of our PRS can be further improved by using polarization clean-up filters at both outputs, such as directional couplers, waveguide bends, or additional PRSs .
4. Polarization controller
To extend the PRS concept and to demonstrate the tunability of the platform, we implemented the polarization controller shown in Fig. 5(a). The polarization controller consists of a PRS followed by a series of 3-dB multimode interference couplers (MMIs) and phase shifters, and finally, a second PRS to combine the two branches. The design is based on the proposal in , and allows any input polarization state to be transformed into any output polarization state. The phase shifters were implemented using TiN thin film heaters above Si waveguides, and adiabatic transitions were used to transfer light between the Si3N4 routing waveguides and the Si waveguides in the phase shifters.
Figure 5(b) shows an optical microscope image of the fabricated polarization controller. We measured the polarization controller using free-space coupling with a free-space linear polarizer at the input of the chip to set the input polarization. The output polarization state was analyzed with a polarimeter (Agilent N7788B). The input wavelength was fixed at 1550 nm. The polarization controller insertion loss was < 4.5 dB. Only the top heater of each pair was driven; the unused heaters balanced the loss of each pair of arms in the polarization controller. The heater numbering is indicated in Fig. 5(b).
Figure 6 shows measurement data for the polarization controller using a TE-polarized input and a 45° linearly polarized input. The output polarization state was plotted on the Poincaré sphere as two of the heaters were separately tuned. For a TE-polarized input [Fig. 6(a)], Heater 2 was powered at various steps corresponding to phase shifts between 0 and about π radians. For each Heater 2 power, the phase shift at Heater 3 was swept from 0 to 2π radians. The Heater 3 sweeps traced parallel circular orbits on the Poincaré sphere, and the spacing between the orbits was set by the Heater 2 power step size. When the Heater 2 power was 0 mW, the Heater 3 sweep should have ideally produced no change in the output polarization, but the crosstalk of the PRSs and non-ideal 3-dB MMIs led to a small distorted path traced on the Poincaré sphere . Similar parallel circular orbits were seen for a 45° linearly polarized input when Heater 1 was stepped and Heater 2 was swept [Fig. 6(b)]. Overall, controlling two heaters allows us to reach any point on the Poincaré sphere for any input polarization, and the choice of these two heaters depends on the input and desired output polarization.
In practice, where the polarization controller is used to stabilize a fluctuating input polarization (e.g., in a coherent receiver or polarization-division multiplexed link), applying a control algorithm for the five heaters eliminates the impact of non-ideal PRSs and 3-dB MMIs. Only one or two thermal tuners will cause distorted paths to be traced on the Poincaré sphere at a given bias and input polarization, and these heaters can be avoided. In addition, the heaters can be “reset” when they reach their maximum power dissipation, i.e., we can gradually reduce the power to a heater while modifying the remaining heater powers to maintain the output polarization state [18, 19]. In , a similar polarization controller geometry was used in a silicon-photonic polarization-division multiplexed receiver, and the simultaneous control of all heaters enabled stabilization (with resets) of a fluctuating input polarization from a standard single-mode fiber despite imperfect 3-dB MMIs, separation of polarization, and polarization rotation. In our polarization controller, Heaters 4 and 5 were functional, but a demonstration of polarization stabilization and reset procedures using Heaters 4 and 5 is beyond the scope of this work.
In summary, we have demonstrated a polarization rotator-splitter and a polarization controller in a Si3N4-on-SOI integrated optics platform. The polarization rotator-splitter is based on TM0-TE1 mode conversion in a composite Si3N4-Si waveguide, and is entirely adiabatic for improved tolerance to fabrication error and large operating bandwidths. This device design can be applied to fully active Si3N4-on-SOI platforms and other SOI photonics platforms where Si3N4 can be deposited near the Si waveguides. This demonstration shows that not only can a hybrid photonic platform support optical functionalities in the constituent layers separately, it can also enable new device designs that rely on composite waveguides formed from the layers and the close interaction of light between the layers.
W.D.S. and J.K.S.P. thank Brian Withnell of Agilent Canada for the loan of the N7788B optical component analyzer and CMC Microsystems for the loan of the Keithley 2400 and 2602 sourcemeters. The financial support of the Natural Sciences and Engineering Research Council of Canada and the Canada Research Chairs program is gratefully acknowledged.
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