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Abstract
A high performance compact silicon photonics polarization splitter is proposed and demonstrated. The splitter is based on an asymmetric directional coupler. High extinction ratios at the through and drop ports of the polarization splitter are achieved by using an on-chip TE-pass polarizer and a TM-pass polarizer, respectively. The splitter, implemented on a silicon-on-insulator platform with a 220 nm-thick silicon device layer, has a measured insertion loss lower than 1 dB (for both TE and TM modes) and extinction ratio greater than 25 dB (for TM mode) and greater than 36 dB (for TE mode), in the wavelength range from 1.5 µm to 1.6 µm. The footprint of the device is 12 µm × 15 µm.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Silicon photonics technology, driven by its compatibility with complementary metal-oxide semiconductor (CMOS) processes, has promised high-performance computing and data communication [1]. These devices are highly sensitive to the polarization state of the input light and require precise control of the transverse electric (TE) and transverse magnetic (TM) modes for optimal performance. On-chip polarization control can be achieved using polarizers, polarization splitters and rotators [2]. A polarization splitter (PS) is a key component of polarization diversity circuits [3], which can split and couple the TE or TM mode to the adjacent waveguide, at an appropriate coupling length. PSs based on directional couplers (DCs) have been extensively studied in the literature, with various configurations [4–15]. The DCs are based on an evanescent mode coupling technique, which usually couples the TM mode to the adjacent waveguide. On the other hand, the TE mode propagates with a negligible coupling. In reality, a small fraction of the TM light always remains in the input waveguide, due to incomplete coupling to the adjacent waveguide. Moreover, a small fraction TE polarized light still couples to the adjacent waveguide, leading to small polarization extinction ratios (ERs) in the through and drop ports of the PS. In order to achieve high extinction ratios, the undesired polarizations in both ports must be filtered out.
Various configurations of polarization splitters have been reported in the literature, showing moderate ERs [4–7]. In another approach, MMI-based polarization splitters have been reported in [16,17], exhibiting moderate ERs. In order to increase the ER, a polarization splitter was fabricated using a double stage directional coupler [7], but the improvement in the ER was only from 10 dB to 15 dB. This approach also introduces an additional TE loss in the second stage. Recently, a double stage polarization splitter using triple bent waveguide DC has also been reported in [14] with significant ER. The polarization extinction ratio was greatly improved in [8], by cascading three bent directional couplers. The first DC splits both orthogonal polarizations, while the other two directional couplers have been introduced in the cross and through ports, in order to filter out the undesired polarizations. In [10], the polarization crosstalk was improved from 13 dB to 22 dB by inserting a DC-polarization filter with a rotator-splitter. Another approach to achieve high ER was used in [11] where the DC was defined as a slot waveguide and a partial-etched waveguide. Polarization splitters based on triple waveguide directional couplers have also been investigated in [9], employing three silicon waveguides, and in [12], using two silicon waveguides and one sub-wavelength waveguide, leading to an increased ER.
In [13], a high extinction ratio of the TE mode was achieved due to the appearance of a TM bandgap in the periodic waveguide, but the extinction ratio of the TM mode in the input waveguide was only 18 dB at 1550 nm wavelength. Similarly, in [15], a sub-wavelength grating TM-pass polarizer was used to filter out undesired TE mode and achieve high extinction ratio in the drop port. Polarizers [18–21] are important building blocks of the polarization diversity circuits and can be used to remove any unwanted polarization states in the through and drop ports of the PS, eliminating polarization crosstalk.
In this work, a polarization splitter based on an asymmetric directional coupler (ADC) is utilized, consisting of a strip waveguide and a 1D (periodic) photonic crystal waveguide. The TM mode is coupled from the input strip waveguide to the drop port, while the TE mode propagates through the input waveguide, unaltered. The periodic waveguide in the drop port also works as a TM-pass polarizer, filtering out the undesired TE light. On the other hand, adiabatic bends are introduced in the through port, working as a TE-pass polarizer and filtering out the undesired TM light. Consequently, this approach results in a PS with ultra-high extinction ratios in both ports. The extinction rations at both ports can be further enhanced by using more adiabatic bends in the through port and number of periods in the drop port, leading to extremely enhanced ERs as compared to the values reported in the literature [4–7].
2. Splitter design optimization
A polarization splitter with a high extinction ratio and a short coupling length is essential for efficient, compact and nanoscale silicon photonics integrated circuits. Although DC-based polarization splitters can be designed with compact device dimensions, their performance is limited due to the small extinction ratio. In this work, an ADC-based polarization splitter is designed, with on-chip TE-pass and TM-pass polarizers in the through and drop ports, respectively. Figure 1(a) shows a schematic diagram of the proposed design, based on a 220 nm-thick silicon layer. A 3D view of the proposed PS is shown in Fig. 1(b). The device is surrounded by silicon dioxide and the center wavelength of operation is 1550 nm.
Fig. 1. Schematic illustration of the proposed polarization splitter: (a) top view with all design parameters shown, $R_{min}$ $=$ 1.3 µm, (b) 3D view, (c) three different designs of the asymmetric directional coupler, (d) calculated TM mode power transfer to the drop port for all designs.
The width of the input waveguide is $w_c$ $=$ 350 nm, and the coupling gap between the strip waveguide and the periodic waveguide is $G_c$. The coupling length is defined as $L_c$. A slot is introduced in section 1 of the periodic waveguide, with its width defined as $w_s$ (see Fig. 1(a)). The width $w_1$ in section 1 is 220 nm, and the width $w_2$ in section 2 is 300 nm, as shown in the inset of Fig. 1(a). The period of the 1D photonic crystal waveguide is $\Lambda$ $=$ 700 nm. The magnified view of the asymmetric directional coupler is also shown in the inset of Fig. 1(a).
The mixed TE and TM modes are launched in the input waveguide. After propagating through a coupling length, $L_c$, the TM light is transferred to the drop port, while the TE mode propagates with no significant coupling. The ADC is studied with different configurations of the drop port as shown in Fig. 1(c). The drop port of ADC is defined as one slot section (Design A), two slots and one strip waveguide section (Design B), and periodic waveguide which acts as a TM-pass polarizer (Design C). Following the ADC, there lies a periodic waveguide in the drop port (Design A and B) which acts as a TM-pass polarizer. The purpose of this study is to show that an efficient TM mode coupling to the drop port can be achieved with all the three configurations, as shown in Fig. 1(d). The TM power transfer to the drop port is slightly lower in Design C, which is due to the numerous alternating sections. This coupling can be slightly improved by using only one slot section as in Design A.
The drop port of PS, consisting of a periodic waveguide, allows the TM mode to propagate and blocks the TE mode, yielding the function of a TM-pass polarizer. It blocks the undesired TE light, leading to a high ER in the drop port. Moreover, the periodic waveguide works in the radiation regime of the dispersion diagram, which radiates out the TE mode away from the waveguide, instead of reflecting it back. In order to achieve an extinction ratio above 38 dB over a 100 nm wavelength range, only 20 periods are required. On the other hand, the periodic waveguide introduces very low TM loss. The through port of the PS is implemented with 2 adiabatic bend turns with $R_{min}$ $=$ 1.3 µm and two different angles, as shown in Fig. 1(a). The adiabatic bends radiate out the undesired fraction of the TM polarization, leading to a high extinction ratio in the through port. Additionally, an adiabatic bent taper (see Fig. 1(a)) is used to increase the width of the input waveguide from 350 nm to 500 nm. It helps eliminating the radiation loss of the TE mode through the bends, yielding a low insertion loss (IL) in the through port.
In order to obtain an efficient coupling to the drop port, the phase matching condition for the TM mode must be satisfied. Figure 2(a) and Fig. 2(b) show the effective indices ($n_{\mathit {eff}}$) of both fundamental modes, as a function of the width of the strip waveguide ($w_c$) and width of the slot ($w_s$) in the slot waveguide, respectively. The horizontal dotted line in Fig. 2 shows that for $w_c$ (between 330 nm and 370 nm) and $w_s$ (between 60 nm and 100 nm), the phase matching condition for the TM mode is satisfied. The vertical dotted lines show the central values of the core and slot widths where phase matching occurs. On the other hand, the phase-matching condition for the TE mode is not satisfied, which helps in suppressing evanescent coupling. Also, the TE mode being well confined inside the waveguide, has negligible interaction with the adjacent periodic waveguide (if the coupling gap $G_c$ is chosen appropriately), leading to negligible coupling. Hence, the TM mode can be coupled from a 350 nm wide input strip waveguide to the slot waveguide with a 80 nm wide slot. Another interesting finding from Fig. 2 is that the $n_{\mathit {eff}}$ does not change much as slot width varies between 60 nm and 100 nm, which makes the performance highly insensitive to fabrication errors. Moreover, Fig. 2 shows that the TM mode effective index is similar in both sections of the periodic waveguide. Similarly, the TM mode effective index in the input waveguide and section 2 of the periodic waveguide are close. The reason for choosing a slightly smaller width of section 2 is to get enough extended part, 110 nm, of section 1 above and below section 2. The chosen dimensions result in a similar TM mode effective index in the input waveguide and both sections of the periodic waveguide, leading to an efficient TM mode power coupling in all three designs of A, B and C.
Fig. 2. Calculated TE and TM mode effective indices as a function of: (a) core width $w_c$ of strip waveguide, (b) slot width $w_s$ of slot waveguide.
Additionally, it was observed from the rigorous simulations that if we change the number of periods in the coupling section of the drop port, it does not require a change in coupling length as the TM mode effective index is similar in both section 1 and section 2 of the elementary waveguide sections used in Designs A, B and C. Further optimization in each case requires a change in coupling length of $\approx$ 0.1 µm. This slight change in the coupling length usually occurs during the fabrication process. This performance also makes our polarization splitter fabrication-friendly.
From Fig. 1(d), Design A has a better TM power transfer to the drop port, but it does not block the TE transferred power to the drop port, leading to a significant cross-talk in the device (see Fig. 5 and [22]). This is a significantly detracting factor, leading to the selection of Design B for fabrication. Moreover, placing of the periodic waveguide (TE mode filter) after the coupling section, as in Design A, requires additional footprint to obtain the desired ER. Introducing the periodic sections in the coupling region of the ADC (Design B and C) allows a more compact solution.
An efficient coupling of the TM mode and no coupling of the TE mode to the periodic waveguide can be further explained and confirmed by the theory of supermodes in the coupling region of Design B. Figures 3(a)–3(d) show the amplitude of the |$E_x$| component of the electric field of the even and odd TM supermodes, indicating a large overlap of the mode of the input waveguide with both cross-sections of the ADC (outlined), for a coupling gap of $G_c$ = 240 nm. When the TM mode is launched, both supermodes are excited equally and consequently the optical energy can be transferred completely between the two waveguides. The amplitude of the |$E_y$| components of the electric field of the fundamental TE modes are depicted in Figs. 3(e)–3(h). For the two supermodes in Figs. 3(e) and 3(h), the overlap is small for both cross-sections. For the other two supermodes, Figs. 3(f) and 3(g), although the overlap is larger, these two supermodes do not support propagation in the drop waveguide. As a result, no significant coupling occurs to the drop port for the TE mode.
Fig. 3. (a-b) Amplitude of |$E_x$| components of even and odd TM super modes in the cross-section 1 of asymmetric directional coupler, (c-d) |$E_x$| component of even and odd TM super modes in the cross-section 2 of asymmetric directional coupler; (e-h) Amplitude of |$E_y$| components of TE modes in both cross sections of asymmetric directional coupler, indicating that the power is confined in one individual waveguide.
An important design parameter is the coupling length ($L_c$), which depends on coupling gap ($G_c$) between the strip and the periodic waveguide. The coupling length can be estimated from the effective indices of the TM supermodes, which are written in Figs. 3(a)–3(d). The $L_c$ can be calculated using the following formula:
where $\lambda$ is the vacuum wavelength, and $n_{eff-even}$ and $n_{eff-odd}$ are the effective indices of the even and odd super modes, respectively. Most importantly, the calculated $L_c$ in both cross-sections of the coupling region are in close agreement, which further confirms that the periodic waveguide works as a normal waveguide for the TM mode. For a coupling gap of $G_c$ $=$ 240 nm, the calculated coupling length $L_c$ is $\approx$ 5 µm, at 1550 nm wavelength.The Design B of the proposed polarization splitter was simulated using the 3D-FDTD method, and the transmitted powers at the output of the through and drop ports were calculated after launching each mode at the input. Figure 4(a) and Fig. 4(b) depict the top view of the fundamental TE and TM modes propagation through the splitter, respectively, at 1550 nm wavelength. As the coupling condition for the TE mode are not satisfied, only a negligible fraction of the light can couple to the drop port. This undesired fraction of the coupled light is blocked by the periodic waveguide (Fig. 4(a)), which acts as a TM-pass polarizer, and a negligible light fraction is shown at the output of the drop port. Moreover, the TE light continues to propagate through the adiabatic bends in the through port, with no significant loss, and a high intensity can be seen in the output of the through port in Fig. 4(a). On the other hand, the TM mode couples efficiently to the drop port, as shown in Fig. 4(b). As the periodic waveguide acts as a TM-pass polarizer, the light will continue to propagate through it with negligible loss. Furthermore, any undesired TM light in the through port is lost at the adiabatic bends, and a negligible fraction is left in the output of the through port. Consequently, the periodic waveguide and the adiabatic bends show high TE and TM extinction ratios in the drop and through ports, respectively. On the other hand, without TE and TM-pass polarizers, the ERs at both ports are very low with slight change in insertion losses, as shown in Fig. 5. The schematic diagram with no filters is shown in Fig. 5(a). The TE and TM ERs at the drop port and thru port are shown in Fig. 5(b) and Fig. 5(e), respectively. They depict that the ERs are less than 20 dB at both ports at 1550 nm wavelength. A similar performance has already been reported in [22]. In order to improve the ERs, the polarization splitter with the TE- and TM-pass filters in both ports is then studied.
Fig. 4. Top view of light propagation at 1550 nm wavelength through the polarization splitter, (a) TE mode, (b) TM mode.
Fig. 5. Calculated spectral responses of the polarization splitter with no filters at both ports: (a) Schematic with no filters, (b) TE transmittance, drop port, (c) TE transmittance, thru port, (d) TM transmittance, drop port, (e) TM transmittance, thru port.
The calculated spectral power transmission and reflection through both ports (with filters) and for each modes are shown in Fig. 6. The curves of different colours in each window of Fig. 6 correspond to different coupling gaps. The TM power in the through and drop port is shown in Fig. 6(a) and Fig. 6(b), respectively. An efficient coupling to the drop port is obtained as shown by the TM drop port transmission curve (Fig. 6(b)), with loss less than 0.7 dB over a 100 nm wavelength range for $G_c$ $=$ 240 nm. It is also noted that the change in coupling gap between 210 nm and 240 nm introduces a slight change in the performance. Moreover, an ultra-high extinction of the undesired TM mode at the through port is calculated (Fig. 6(a)), which is greater than 30 dB over a 100 nm wavelength range for $G_c$ $=$ 210 nm. The power transmission of the TE fundamental mode is shown in Fig. 6(d). A high TE transmission at the through port is achieved, with loss less than 0.7 dB over a 100 nm wavelength range. Additionally, it shows a loss of only 0.25 dB at 1550 nm wavelength. On the other hand, a high TE extinction (greater than 38 dB over a 100 nm wavelength range), in the drop port is achieved as shown in Fig. 6(e). The extinction ratios can further be enhanced by increasing the number of periods of the periodic waveguide and the number of adiabatic bends in the drop and through ports, respectively. Additionally, reflected power for TE and TM are also calculated as shown in Fig. 6(f) and Fig. 6(c), respectively. The obtained power reflection for both modes are less than 30 dB over a 100 nm wavelength range. The footprint of the proposed polarization splitter is only 12 µm$\times$15 µm, with 20 periods in the drop port and two adiabatic bends in the through port.
Fig. 6. Calculated spectral responses of the proposed polarization splitter: (a) TM transmittance, thru port, (b) TM transmittance, drop port, (c) TM reflectance, input port, (d) TE transmittance, thru port, (e) TE transmittance, drop port, (f) TE reflectance, input port.
3. Experimental results
The polarization splitter with Design B of the ADC was fabricated on an silicon-on-insulator (SOI) substrate with 220 nm-thick silicon device layer over a 3 µm-thick buried oxide layer on a 675 µm-thick silicon handle layer. The device layout was patterned by electron beam lithography followed by the pattern transfer to the silicon layer through reactive ion etch, using a fluorine based chemistry ($\textrm{SF}_{6}$ and $\textrm{C}_{4}\textrm{F}_{8}$). The chip was subsequently covered with 1.1 µm-thick $\textrm{SiO}_{2}$ through plasma enhanced chemical vapor deposition.
SEM images of the fabricated polarization splitter are shown in Figs. 7(a)–7(d). The perspective view and top view are shown in Fig. 7(a) and Fig. 7(c), respectively. The critical dimensions of ADC and one period of TM-pass polarizer in the drop port are shown in Fig. 7(b) and Fig. 7(d), respectively.
Fig. 7. Experimental results: SEM images of the polarization splitter fabricated with Design B of ADC, (a) perspective view, (b) critical dimensions of ADC, (c) top view, (d) critical dimension of the one period of the TM-pass polarizer in the drop port, (e) measured TM power at both ports, (f) measured TE power at both ports.
The fabricated polarization splitter was tested on a standard integrated optics setup with a tunable laser source (1.5 µm to 1.6 µm). Light was edge coupled from a standard tapered lensed fiber, and the input light polarization state (TE or TM) was controlled by a polarization controller. In order to improve light coupling to the waveguides, inverse taper-based spot size converters were used at the input and output of the silicon chip. The output power was measured with an InGaAs photodetector. Test structures with $10$ cascaded devices in series were used in order to get a reliable measurement of the insertion loss. The average transmission through the $10$ cascaded polarization splitters was compared to the transmission through a reference straight waveguide, in order to subtract the coupling loss.
Figure 7(e) and Fig. 7(f) illustrate the measured transmission spectra at the through and drop ports of the fabricated polarization splitter for the TM and TE modes, respectively. The TM and TE insertion losses are shown by the red curves in Fig. 7(e) and Fig. 7(f), respectively. The obtained transmissions show that the IL of both fundamental modes is less than 1 dB over a 100 nm wavelength range (see the magnified view in the insets of Fig. 7(e) and Fig. 7(f)). On the other hand, the TM and TE extinction ratios are shown by the black curves in Fig. 7(e) and Fig. 7(f), respectively. The measured transmissions confirm that the TE extinction ratio is above 36 dB over a 100 nm wavelength range and that the TM extinction ratio is above 30 dB over a 50 nm wavelength range. Although the TM extinction ration is lower, it can be easily further enhanced by using more adiabatic bends in the through port.
4. Conclusion
In conclusion, we have experimentally demonstrated a compact silicon polarization splitter based on a strip waveguide and a periodic waveguide. The measured extinction ratios and insertion losses of the fabricated polarization splitter are greater than 30 dB and less than 1 dB, respectively, for both polarized modes at 1550 nm wavelength. The splitter is broadband, yielding an IL less than 1 dB (both TE and TM modes) and ER greater than 25 dB (TM mode) and greater than 36 dB (TE mode), over a 100 nm wavelength range. The proposed polarization splitter is realized by a simple single etch fabrication process. Ultra-high extinction ratios are achieved by integrating the splitter with on-chip polarizers at both ports. The ER can be further enhanced greatly by using more periods and bends in the drop port and through port, respectively. The increased number of periods and bends does not considerably affect the IL at both ports.
Funding
Khalifa University of Science and Technology (CIRA-2018-110); Semiconductor Research Corporation (GRC 2713.001).
Acknowledgments
The authors acknowledge Dr. Sergio Sanchez Martinez from the Khalifa University Research Computing team for his support with the use of the high-performance computing facilities.
Disclosures
The authors declare no conflicts of interest.
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