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Abstract
A compact, ultra-broadband and high-performance silicon TE-pass polarizer is proposed and demonstrated experimentally. It is based on partially-etched (ridge) waveguide adiabatic S-bends, input/output tapers and side gratings on a silicon-on-insulator (SOI) platform. A compact footprint and weak back reflections are obtained due to the bent waveguide and the tapers, respectively. An extremely high extinction ratio is achieved by scattering the undesired light in the slab section using the side gratings. The 3D FDTD simulations show a TE loss less than 0.3 dB and an extinction ratio greater than 30 dB over a 500 nm wavelength range (1200 nm to 1700 nm). Measured results show a high TM loss (> 35 dB) and a low TE insertion loss (< 1.5 dB), over a 200 nm wavelength range (1450 nm to 1650 nm). The measured TE loss is < 0.6 dB at a communication wavelength of 1550 nm. The footprint of the optimized design is 65 µm × 20 µm.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Controlling the light on chip-scale is a subject of perennial interest in integrated photonics [1,2]. Silicon-on-insulator (SOI) photonics waveguides have, attracted great potential as they allow sharp bends and compact dense components on a photonics integrated chip. Most devices in silicon photonics circuits are sensitive to the polarization state of the input light. Therefore, a precise control of the transverse electric (TE) and transverse magnetic (TM) modes is required, demanding on-chip polarization handling devices [3,4].
Several integrated polarization control devices have been developed in SOI platforms, such as polarization splitters and rotators [5–13]. Polarization splitters have been used to split two orthogonal polarizations, and polarization rotators have been utilized to rotate one polarization state. A polarization splitter can also be used as a polarizer if one of its outputs is used [14]. A simpler approach is to block one undesired mode and has been previously reported as an on-chip polarizer. Polarizers based on plasmonic structures [15–21] have compact footprints, but they introduce a high insertion loss. Additionally, they are complex and the deposition of metal layers requires non-conventional fabrication steps. Subwavelength grating (SWG) based polarizers are also compact but they usually have a drawback of back reflections of the undesired mode [22–25]. These back reflections can be reduced by engineering the period of the grating structure such that the undesired mode leaks from the waveguide with low reflections as reported in [26,27]. On-chip polarizers based on a tapered structure and a bent waveguide have also been investigated in [28] and [29], respectively.
Polarizers based on partially-etched waveguides have also been demonstrated in previous studies [30,31], yielding an attenuation of the undesired TM mode ($\approx$ 25 dB) on a SOI platform [30]. Such waveguides on lithium niobate-on-insulator (LNOI) platforms lead to the lateral leakage loss for both TE and TM fundamental modes and consequently, TE-/TM- pass polarizers have been demonstrated on a LNOI platform in [31]. These polarizers on both platforms are based on straight partially-etched waveguides and show an adequate performance with a significantly long device (1 mm). In order to achieve a compact partially-etched waveguide polarizer, Azzam et al. [32] proposed both TE- and TM-pass polarizers on a SOI platform. However, the short length of the polarizer was achieved at the expense of high back reflections of the unwanted mode. The strong back reflections are obtained because the input/output waveguides were directly connected to the polarizer section. These reflections were then greatly reduced by inserting tapers between the input/output waveguides and the polarizer section, but then the extinction ratios were also dropped significantly. Consequently, a long polarizer section is required to achieve good extinction ratios. Recently, Liu et al. proposed a TE pass polarizer with two double layer tapers and one shallow-etched waveguide, covering an ultrabroad operation band (1260 nm to 1675 nm) [33]. However, they use air as a top cladding, which makes their design incompatible with most metal BEOL (back end of the line) processes. The majority of standard available silicon photonic processes indeed offer selective oxide open regions (i.e. for sensing applications). But oxide as a top cladding is a better approach for non-sensing devices for the following reasons: (1) to make the mode symmetric, (2) to protect the waveguide structure from the outside environment ( dust, humidity, chip scratch, etc.). Additionally, the fabrication requires more complex steps to do selective openings by oxide removal.
In this work, we propose and demonstrate a partially-etched (ridge) waveguide TE-pass polarizer on a SOI platform with oxide cladding, based on cascaded adiabatic S-bends assisted with side gratings. A comparison of this work with other experimentally demonstrated polarizers is presented in Table 1. This proposed design expands on our work presented in [34,35], with great improvement in the design and the performance. It does not show any back reflections and mode conversion in the bent ridge waveguides. The footprint of the polarizer has been significantly reduced due to the bent structures. consequently, the ridge waveguide and the tight adiabatic bends exhibit ultra high loss of the TM mode, while the TE mode is effectively lossless. The TM mode leakes away from the ridge waveguide and side gratings help in scattering this undesired mode, leading to an ultra high TM extinction.
Table 1. Summary of experimentally demonstrated polarizers. The values of IL and ER are for 1550nm wavelength.
2. Polarizer design and simulation results
The polarizer studied in this work is based on the idea that the leakage and radiation losses in the ridge waveguide bends can differ significantly for the TE and TM modes. The proposed structure is shown in Fig. 1 on a SOI platform with a 2 µm-thick buried oxide (BOX) layer. Figure 1(a) shows the top view of the polarizer which is surrounded by silicon dioxide. The polarizer section of the proposed design is composed of 3 partially-etched waveguide adiabatic S bends with side gratings. The input and output waveguides are connected with the polarizer section through 5 µm-long adiabatic tapers. These tapers are inserted between the strip (input/output fully-etched) and ridge (partially-etched) waveguides to minimize mode mismatch and reduce back reflections. The silicon layer is 220 nm-thick and is defined as (H), while the ridge waveguide is obtained by partially etching away 130 nm of the silicon, leaving a slab thickness (h) of 90 nm, as shown in the cross section of Fig. 1(b). The width (W) of the ridge waveguide is 500 nm, and the device is covered with silicon dioxide from the top. Such waveguides with the selected waveguide dimensions, etched slab (h) and waveguide width (W), exhibit high lateral leakage loss for the TM fundamental mode as TE-slab modes [36–40]. In [40], Tummidi et. al. calculated the TM loss as function of waveguide width and ridge height, indicating an ultra-high TM loss for a waveguide width of 500 nm and a slab height of 90 nm. Both fundamental TE and TM modes are launched into the polarizer from the left side where an adiabatic taper is used to achieve a smooth transition of the refractive index. The ridge waveguide is implemented as 3 adiabatic S bends with the curvature changing linearly with its length, as reported in [29]. Such adiabatic S bends ensure unaffected TE mode propagation. Additionally, the TE mode in the ridge waveguides does not see any lateral leakage and is a perfectly guided mode due to its large refractive index. On the other hand, as the TM mode is much less confined than the TE mode in the bent waveguide, it suffers from strong radiation loss. Another loss mechanism for the TM mode in this designed ridge waveguide is TM lateral leakage loss which is due to the TM-to-TE mode conversion at the ridge walls. With the designed waveguide geometries, the phase matching condition between the TM ridge mode and the TE slab modes is satisfied. Consequently, the TM mode leaks into the radiating TE slab mode and propagates away from the waveguide. Based on this phenomena, a TE pass polarizer was proposed by using a straight ridge waveguide [30], showing moderate performance with an extremely long (1 mm) footprint. The footprint was made ultra-short in [32] by directly connecting the strip waveguide with the ridge waveguide. This abrupt change results in strong back reflections. In our design, we have achieved weak back reflections by inserting adiabatic tapers. At the same time, a greatly reduced footprint is realized by using a bent ridge waveguide. As a result, both lateral leakage and TM radiation at the bends help in achieving a high performance and a micron-scale compact polarizer with an added advantage of ultra-low back reflections.
Fig. 1. Schematic illustration of the proposed TE-pass polarizer with 3 adiabatic S bends and side gratings. The design parameters are the bend angle ($\theta$), and minimum bend radius ($R_{min}$): (a) top view, (b) 3D view and cross-sectional view.
The polarizer performance was studied with the 3D finite difference time-domain method (FDTD) using the lumerical package in our simulations [41], with refractive indices of $3.45$ and $1.45$ for the silicon and SiO2, respectively (at a wavelength of 1550 nm). The polarizer performance can be explained using Fig. 2 which shows the electric field profiles (amplitude of electric fields) of the fundamental TE and TM modes in the cross-section of the studied ridge waveguide at a 1550 nm wavelength with a constant radius of 6 µm. Figure 2(a) shows that the TE mode is very well confined inside the waveguide. However, the fundamental TM mode has a significant radiated field due to the weak modal confinement. This can be illustrated from Figs. 2(b) and 2(c), depicting $E_x$ and $E_y$ components of the fundamental TM mode, respectively. Figure 2(d) and Fig. 2(e) depict the top view of the fundamental TM and TE modes propagation through the polarizer, respectively, at a 1550 nm wavelength. The TM mode entering the input sees an extremely high leakage and radiation loss, leading to a negligible fraction of the TM light at the output (Fig. 2(d)). On the other hand, the launched TE mode passes through the bends unaffected as shown in Fig. 2(e). It continues to propagate through the adiabatic bends with no loss, and a high TE intensity can be seen at the output. As the TM mode propagates through the bends, it leaks and radiates out of the ridge waveguide, leading to a small residual fraction of TM inside the waveguide and a strong field in the slab sections. The light in the slab propagates along the bends and needs a long propagation distance to disappear. In order to reduce the amount of light in the slab section, gratings can be placed along the bent waveguide to scatter the unwanted light in the slab and reduce the amount of light reaching the output (see Fig. 2(d)). The distance between the ridge waveguide and the side gratings is 1.3 µm which exhibits no affect on the TE mode propagation inside the ridge waveguide.
Fig. 2. Electric field profiles for the fundamental: (a) TE mode, (b) x-component of TM mode and (c) y-component of TM mode in the cross-section of a bent ridge waveguide with 6 µm radius, W $=$ 500 nm, H $=$ 220 nm and h $=$ 90 nm, (d) Top view of TM mode propagation depicting high lateral leakage and radiation loss, (e) TE mode propagation through the polarizer is unaffected.
The design parameters of the polarizer are the bend angle ($\theta$) and the minimum bend radius ($R_{min}$). A high-performance on-chip polarizer was evaluated based on (1) high extinction ratio (in this case, high loss for the TM mode relative to TE); (2) a low insertion loss (i.e. low loss for TE polarization); (3) low back reflections; (4) a negligible mode conversion in the bent waveguides and (5) a small on-chip footprint. The TE insertion loss and the extinction ratio (ER) were first studied as a function of minimum radius of curvature $R_{min}$ with a fixed bend angle $\theta$ $=$ 60$^\circ$, as shown in Fig. 3(a) and 3( b). Interestingly, an improvement in the ER is observed at certain wavelengths with an increase in the bend radius which is opposite to the case of the fully-etched waveguide bends [29] as shown in Fig. 3(b). This opposite trend is explained based on the fact that light sees a larger propagation distance with the increase in radius. As it propagates more, it loses more power (lateral leakage loss) along the path, leading to a high TM loss. As far as radiation loss at the bends is concerned, it decreases with the increase of bend radius. However, in the present design, lateral leakage loss increases with a larger radius and the combined effect of the leakage, and the bend loss shows high TM loss at the larger radii. Consequently, to achieve low IL in Ref. [29], a larger radius is needed but it leads to a low ER. However, in the present design, both an ultra-low IL and an extremely high ER are obtained at a larger radius. Figure 3(b) depicts that the ER for $R_{min}$ $=$ 6 µm is above 25 dB over the 500 nm wavelength range (1200 nm to 1700 nm), covering all O, E, S, C, L & U bands. Furthermore, it shows an ultra-high ER of 40 dB and 53 dB at the communication wavelengths of 1310 nm and 1550 nm, respectively. On the other hand, as the bend radius increases, the loss of the TE mode decreases and this is desirable for a high-performance TE-pass polarizer. Figure 3(a) shows great improvement in the TE transmission with a radius of 6 µm, showing a loss of only 0.1 dB at the communication wavelengths of 1310 nm and 1550 nm. The desired and broadband performance of this polarizer is successfully achieved with a radius of 6 µm. Further increase in the bend radius is not considered as it leads to an undesired increase in the footprint of the polarizer. Further investigation of the polarizer was done for different bend angles $\theta$ $=$ 30$^\circ$, 45$^\circ$ and 60$^\circ$ with a fixed $R_{min}$ $=$ 6 µm, as shown in Fig. 3(c) and 3(d). It is observed from Fig. 3(c) that the TE transmission curves are overlapping as the bend radius is fixed. Conversely, a high ER is noted with the increase of the bend angle. This improvement is because of the increased propagation distance at the larger angles, leading to a high lateral leakage loss.
Fig. 3. Calculated TE transmissions and extinction ratios through the polarizer for different bend radii $R_{min}$ $=$ 3, 4, 5 and 6 µm, with fixed bend angle $\theta$ $=$ 60$^\circ$, over a 500 nm wavelength range: (a) TE transmission (insertion loss), (b) Extinction Ratio (dB). Calculated spectral responses for different bend angles $\theta$ $=$ 30$^\circ$, 45$^\circ$ and 60$^\circ$, with fixed $R_{min}$ $=$ 6 µm: (a) TE transmission (insertion loss), (b) Extinction Ratio (dB).
The TE spectral transmissions and ERs with and without side gratings are plotted in Fig. 4(a) and 4(b), respectively, with a fixed $\theta$ $=$ 60$^\circ$ and a $R_{min}$ $=$ 6 µm. It confirms that the desired scattering of the undesired TM mode cannot be achieved without side gratings as can be seen by the black curve of Fig. 4(b). For the current design, a grating period of $p$ $=$ 600 nm with a $50\%$ fill factor was used to efficiently scatter the TM mode as depicted by the red curve of Fig. 4(b). A substantial increase in the TM extinction is achieved as the chosen dimensions of the side gratings effectivey scatter the undesired TM mode. This is because the grating period is not in the subwavelength regime. Additionally, a minimum feature size of 300 nm makes the fabrication quite easy. On the other hand, side gratings have no impact on the transmission of the TE mode (both curves are overlapping in Fig. 4(a)). As discussed earlier, input/ output tapers are used in this design to reduce back reflections. The calculated TE and TM reflections are shown in Fig. 4(c). The obtained results show that the back reflections are below −30 dB and −25 dB for the TE and TM modes, respectively, covering full wavelength range. Additionally, the TM-TE mode conversion is an important mechanism in such partially-etched bends as reported in [42]. The TE-to-TM and the TM-to-TE mode conversions were studied for the optimized polarizer with $\theta$ $=$ 60$^\circ$ and a $R_{min}$ $=$ 6 µm as shown in Fig. 4(d). It exhibits an extremely weak (below −30 dB) mode conversions, leading to a no cross-talk in the polarizer.
Fig. 4. Calculated TE transmissions and extinction ratios through the polarizer for minimum bend radius $R_{min}$ $=$ 6 µm and $\theta$ $=$ 60$^\circ$, over a 500 nm wavelength range: (a) TE transmission (insertion loss) with and without side gratings, (b) ERs with and without side gratings, (c) TE and TM reflections, (d) mode conversion.
In order to exponentially improve the system performance, the reduction of the lateral and vertical dimensions of the integrated circuits is of immense need for the current CMOS technology. Our proposed TE pass polarizer works efficiently on both 220 nm and 160 nm thick substrates over a wide wavelength range from 1200 nm to 1700 nm. Figure 5 shows the polarizer performance on 160 nm-thick substrates with silicon layer thicknesses of H $=$ 160 nm and h $=$ 60 nm. The TE transmission and ER are shown in Fig. 5(a) and 5(b), respectively. The obtained TE transmission is $\approx$ 0.1 dB at both communication wavelengths. A high ER is also calculated and is above 20 dB and 30 dB at the 1310 nm and 1550 nm wavelengths, respectively. Moreover, weak back reflections and mode conversions are also achieved for both fundamental modes as shown in Fig. 5(c) and 5(d), respectively. Consequently, the proposed polarizer performs greatly on both 220 nm and 160 nm thick substrates over a wide wavelength range, covering all O, E, S, C, L & U bands.
Fig. 5. Calculated TE transmission and extinction ratio through the polarizer for minimum bend radius $R_{min}$ $=$ 6 µm and $\theta$ $=$ 60$^\circ$, with silicon layer thickness H $=$ 160 nm and h $=$ 60 nm: (a) TE transmission (insertion loss), (b) Extinction Ratio, (c) TE and TM reflections, (d) mode conversion.
3. Fabrication and measurement results
The optimized version of the TE-pass polarizer ($R_{min}$ $=$ 6 µm and $\theta$ $=$ 60$^\circ$) was fabricated at King Abdullah University of Science and Technology (KAUST), on a silicon-on-insulator platform with a 220 nm-thick silicon layer and a 2 µm-thick buried oxide layer. The structures were patterned using electron beam lithography, and covered with a 2 µm-thick oxide layer from the top. SEM images of the fabricated design are shown in Fig. 6(c-e). Figures 6(d) depicts the top view of the polarizer. Whereas, Fig. 6(c) and Fig. 6(e) are the cross-sectional views at two different positions, obtained using the focused ion beam method.
Fig. 6. Transmission spectra of TE and TM polarized light for $R_{min}$ $=$ 6 µm, and angle $\theta$ of $60^{\circ }$: (a) Measured, (b) Simulated. SEM images: (c) cross-sectional view at position 1, (d) Top view, (e) cross-sectional view at position 2.
The light was edge coupled into the silicon chip from a lensed fiber. A tunable laser (1.45 µm to 1.65 µm) was utilized as a light source, and a polarization controller was used to switch between the TE and TM modes. The transmitted light was collected at the output with a second lensed fiber. The insertion loss was measured with $15$ cascaded polarizers that substantially reduced the impact of fiber misalignment and measurement setup errors. The insertion loss of a single polarizer was extracted from the total loss through the concatenated polarizers. The measured TE and TM transmissions were compared to the transmission through a reference straight waveguide, in order to subtract the input/output coupling losses. The obtained transmission spectra through a polarizer designed with $R_{min}$ $=$ 6 µm, and angle $\theta$ of $60^{\circ }$ are shown in Fig. 6(a), over a 200 nm wavelength range. It shows TM loss >35 dB and TE loss <1.5 dB, over the whole wavelength range. The obtained TE curve indicates a low transmission of <0.6 dB at a communication wavelength of 1550 nm. The measured average TE spectral transmission (Fig. 6(a) (red curve)) is close to the calculated value (see Fig. 4(a) (red curve)). For TM spectral transmission, the measured blue curve in Fig. 6(a) and the calculated red curve in Fig. 4(b) are in good agreement. The simulated values shown in Fig. 4(a) and 4(b) (red curves) are re-plotted in Fig. 6(b), over a 200 nm wavelength range. The excellent agreement demonstrates the predictive power of our simulation scheme that led to this proof of concept. The feasibility of the proposed polarizer is further confirmed by measuring the TM back reflections, using return loss module (Agilent 81610A). The obtained result is shown in the inset of Fig. 6(a), indicating a return loss of $\approx$25 dB at 1550 nm wavelength. We attribute the relatively small differences between simulations and experiments to deviations in the fabrication leading to slightly different widths and resulting dielectric constants. The experimentally obtained high TM loss (>35 dB) and a low TE loss (<1.5 dB), over a 200 nm wavelength range, confirm that a high-performance, low-loss TE-pass polarizer has been successfully implemented.
4. Conclusion
This work presents a novel polarizer integrated on a SOI platform, and is based on a series of ridge waveguide adiabatic S-bends with side gratings. Adiabatic bends lead to a lossless TE transmission. The increase in bend radius and bend angle lead to high TM loss resulting in a substantial increase in extinction ratio. Furthermore, a significant improvement in TM loss is obtained with the side gratings. Measurements of the fabricated polarizer prove the feasibility of the proposed approach.
Funding
Khalifa University of Science, Technology and Research (FSU-2020-04, FSU-2021-023).
Acknowledgments
The authors acknowledge Khalifa University for partially funding this project under faculty startup award (FSU-2020-04). This project also received partial funding from Khalifa University under Award No. FSU-2021-023 and by the European Union’s Horizon 2020 research and innovation program under grant agreement Nos 101021857, (Odysseus) and 832876 (aqua3S).
Disclosures
US Patent 11,061,186
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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