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Abstract
In this work, we propose an ultra-broadband and ultra-compact polarization beam splitter (PBS) on a standard silicon-on-isolator platform. Assisted by a tapered subwavelength-grating waveguide and a slot waveguide, the working bandwidth of the directional-coupler-based PBS covers the entire O-, E-, S-, C-, L- and U-bands and the coupling length is only 4.6 µm. The insertion losses (ILs) of the device are simulated to be less than 0.8 dB and the extinction ratios (ERs) are larger than 10.9 dB at the wavelength range of 1260-1680 nm for both TE and TM polarizations. The experimental results show the average ILs are less than 1 dB for both polarizations at our measured wavelength ranges, which are consistent with the simulation results. It has the largest 1-dB bandwidth among all the reported broadband PBSs to the best of our knowledge.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Silicon photonics is a promising solution for future data communication and computation for its low power consumption and high bandwidth. To increase the capacity of integrated photonic circuits, multiplexing technologies are in great needs. Polarization beam splitter (PBS) as a key component for polarization-division multiplexing technology has attracted intensive research attention. Many studies about PBSs attempt to achieve very low insertion losses (ILs) as well as high extinction ratios (ERs) [1–4]. In addition to these merits, increasing the operation bandwidth for a PBS is also desirable for applications such as hybrid multiplexing of wavelength and polarization simultaneously [5] or dual-band operating circuits for passive optical network with two polarizations involved [6,7]. Previous attempts to increase the bandwidth of PBS include adopting structures of bent directional couplers (DCs) [8–10], slot DCs [2,11,12], subwavelength-grating (SWG) DCs [13,14] as well as multimode interference (MMI) [15,16]. Even with these new structures, their bandwidths are still less than 200 nm. To further increase the bandwidth of PBS, Zhao et al. cascaded bend DCs to realize 320-nm bandwidth [17] and Lin et al. cascaded MZIs to achieve 350-nm bandwidth [18]. However, these PBSs based on cascaded structures have coupling length of over 30-µm and 100-µm respectively, which is not desirable for large-scale photonic integrated circuits. Recently, ultra-compact PBSs with footprint of less than 10 microns were demonstrated by optimizing QR-code structures [19] or topology structures [20]. However, PBS based on QR-code structure with hundreds of design degree-of-freedom (DOF) takes thousands of simulations until an acceptable solution is found. Topology-optimized PBS optimizing the device pattern via gradient decent algorithm needs less simulations, but it is prone to fall into local optimal and exhibits small features that are hard to process. What’s more, these inverse-designed devices with very high DOF are more likely to be influenced by fabrication derivations [21].
In this article, we propose a PBS on a standard 220-nm silicon-on-isolator (SOI) platform. The PBS is a DC consisting of a through waveguide and a cross waveguide. The transverse electric (TE) polarization propagates along the through waveguide, while a slot is introduced to the cross waveguide to prevent its coupling. The transverse magnetic (TM) polarization can be coupled to the cross waveguide in an ultra-broad wavelength range by adding tapered SWG to the through waveguide. These SWG patterns which manipulate the refractive indices in the DC can also shrink the footprint of proposed PBS. The simulated ILs of the proposed PBS are less than 0.8 dB and the ERs are larger than 10.9 dB in the wavelength range of 1260-1680 nm for both TE and TM polarizations. To the best of our knowledge, this is the largest bandwidth for silicon-based PBS demonstrated so far. The 4.6-µm coupling length is also much shorter than other demonstrated ultra-broadband PBSs. For the fabricated PBS, the performance at the wavelength range of 1260-1360 nm and 1480-1640 nm are measured. The average measured ILs for TE polarization are 0.4 dB at 1260-1360 nm and 0.5 dB at 1480-1640 nm. For TM polarization, the average ILs are 1.0 dB and 0.5 dB for the two ranges respectively. The ERs are larger than 10 dB at all the measured wavelength ranges. The experimental results are consistent with theoretical analyses.
2. Design and simulation
The proposed PBS is based on a DC assisted by a SWG waveguide and a slot waveguide, as illustrated in Fig. 1(a). It is designed on a 220-nm SOI platform with silica cladding, which is widely used for silicon photonics. It requires only one single etch step for fabrication. This PBS is a three-port device consisting of two parallel waveguides as through waveguide and cross waveguide. In the direction of x-axis, this PBS consists of three regions as multiplexing region, coupling region and de-multiplexing region. The magnified detail of coupling region is provided in Fig. 1(b). Two parallel waveguides with the same width wc = wt = 600 nm are separated by a gap with width of wg = 100 nm. The cross waveguide is etched with a slot with width of ws. The through waveguide is etched with linearly tapered SWG, where the initial and end widths are w1 and w2 respectively. The period and length of the SWG are $\mathrm{\Lambda }$ and a respectively. The total length of the coupling region is Lc. Following the coupling region, the through waveguide is connected with an S-band to separate the two waveguides. The offsets of the S-band are 4 µm along x-direction and 2.5 µm along y-direction. In the cross waveguide, the slot also extends for 1 µm after the coupling region to avoid coupling of TE polarization during separation.
Fig. 1. (a) 3-D schematic of proposed PBS. (b) 2-D structures of the coupling region with detailed parameters description.
2.1 Design principle
DC is the most common and the simplest method for splitting and combing light in silicon photonics. When two closely placed waveguides share the same effective refractive index, i.e., phase matching condition is satisfied, light will couple from one waveguide to another periodically. The minimum propagation length for the maximum coupling is called the cross-over length. Due to the birefringence of SOI waveguide, the cross-over lengths for TE (LTE) and TM (LTM) polarizations are different. To separate the two polarizations, the minimum coupling length should be an odd multiple of the cross-over length for one polarization and an even multiple of the cover-over length for the other polarization. As a result, the coupling length is usually hundreds to thousands of microns. Besides, the cross-over length and coupling strength vary with different wavelengths. For a DC with fixed coupling length, the maximum coupling only occurs at some certain wavelengths, which limits its working bandwidth.
Slot waveguide has been proved to be polarization-selective [11,12]. Considering the fabrication process as well as the supporting ability of fundamental modes, we set the slot width to be wg=100 nm. The polarization profiles and the effective refractive indices are calculated using the finite-element method (FEM, Lumerical MODE Solutions). As shown in Fig. 2(a-b), the electric field for TE polarization in slot waveguide is confined in the slot, while for TM polarization it is identical to a strip waveguide. The refractive indices at the wavelength range of 1260-1680 nm are shown in Fig. 2(c), where solid lines represent TE polarization and dashed lines represent TM polarization. From strip waveguide to slot waveguide, the average refractive indices for TE and TM polarization decrease by 0.71 and 0.19, respectively. Therefore, introducing a slot will create a large refractive index reduction for TE polarization, while it has relatively small influence to TM polarization.
Fig. 2. Electric fields of TE and TM polarizations at 1550-nm wavelength in (a) a 600×220 nm2 strip waveguide and (b) a strip waveguide with a 100-nm wide slot. (c) Effective refractive indices of TE and TM polarizations in strip and slot waveguides at the wavelength range of 1260-1680 nm.
To enable ultra-broadband coupling, the through waveguide is etched with tapered SWG structure. A waveguide periodically repeating silicon and silica is equivalent to a homogenous and anisotropic waveguide if the grating period is much smaller than the wavelength of light so that it operates at the subwavelength regime [22]. It has been widely used for manipulating the effective refractive indices for different polarizations. The effective refractive index of equivalent material of length-wise SWG waveguide can be calculated by effective-medium theory [23] as
where $f = {a / \Lambda }$ is the duty cycle of the SWG waveguide. For an SWG waveguide as shown in Fig. 3(a), it can be regarded as an equivalent slot waveguide. The refractive index of slot n0 can be calculated by Eq. (1). As a proof of concept, the grating period of the SWG is set to be $\Lambda $ = 156 nm to ensure its operation in the subwavelength regime for the entire communication wavelength range of 1260-1680 nm. The teeth length of SWG is set to be a = 86 nm for the consideration of fabrication resolution. The widths of the SWG and the equivalent slot are set to be 200 nm. Then we simulate the propagation of TE and TM polarizations in the SWG waveguide and in its equivalent slot waveguide by 3D finite-difference time domain method (FDTD, Lumerical FDTD Solutions). The electric field distributions of TE and TM polarizations at 1550 nm are shown in Fig. 3(b-c). It is clear that the electric fields of the SWG waveguide and the equivalent slot waveguide are almost the same. We also calculate the accurate effective refractive indices of both the SWG waveguide and equivalent slot waveguide. The values are close to each other as shown in Fig. 3(b-c). Therefore, it is reasonable to replace a SWG waveguide with an equivalent slot waveguide.Fig. 3. (a) Schematic of SWG and equivalent slot waveguides. Electric fields distributions of two kinds of waveguides with (b) TE polarization and (c) TM polarization at 1550-nm wavelength input.
To find the suitable width of SWG teeth for ultra-broadband coupling, the width of SWG is varied from 0 to 600 nm. Replacing the SWG waveguide with its equivalent slot waveguide, the effective refractive indices for TE and TM polarizations at different wavelengths (1260 nm, 1310 nm, 1550 nm and 1680 nm) are calculated by FEM as shown in Fig. 4(a-b). The solid lines indicate the SWG waveguide, while the dashed lines suggest the cross waveguide with a silica slot. The circles show the mode matching points where the effective refractive indices at through waveguide are equal to the values at cross waveguide. For TE polarization as shown in Fig. 4(a), the mode matching conditions for different wavelength are satisfied at the orange region where the SWG teeth widths vary from 300 nm to 440 nm. For TM polarization, the mode matching conditions are satisfied at the blue region where the SWG teeth widths vary from 160 nm to 190 nm. By setting the SWG width to be varying near 175 nm, the TM polarization can be efficiently coupled from 1260 nm to 1680 nm, while for TE polarization the mode matching condition cannot be satisfied. To reduce reflection at the transition between strip waveguide and SWG waveguide [22], we taper the SWG width from w1 = 106 nm to w2 = 231 nm in the coupling region.
Fig. 4. The effective refractive indices at wavelength of 1260 nm, 1310 nm, 1550 nm and 1680 nm for (a) TE polarization and (b) TM polarization at SWG waveguide and slot waveguide when the width of SWG teeth is varied from 0 to 600 nm.
2.2 Simulation results
Specifying the PBS with the above parameters, the coupling length is swept for the best figure-of-merit (FOM) using 3D FDTD. During sweep, the transmission for 420 wavelength points evenly sampled from 1260 to 1680 nm are summed up for the calculation of FOM. At each wavelength point, the average value of TE and TM polarization transmission efficiencies at through and cross ports are recorded as $T_{\textrm{thru}}^{\textrm{TE}}$ and $T_{\textrm{cross}}^{\textrm{TM}}$. The optimum coupling length is found to be 4.6 µm. We summarize all the key parameters of the optimized PBS in Table 1.
Table 1. Key parameters of the proposed PBS
The PBS specified with the key parameters in Table 1 is simulated by 3D FDTD. The electric field propagation profiles at 1310 nm and 1550 nm for TE and TM polarizations are shown in Fig. 5(a). These figures suggest that TE polarization propagates along the through waveguide, while TM polarization is coupled to the cross waveguide at the coupling region. The transmission spectra at through port and cross port are collected as$T_{\textrm{thru}}^{\textrm{TE}}$, $T_{\textrm{cross}}^{\textrm{TE}}$, $T_{\textrm{thru}}^{\textrm{TM}}$ and$T_{\textrm{cross}}^{\textrm{TM}}$, which are shown in Fig. 5(b). The performance of PBS can be evaluated in the terms of IL and ER, defined as
Fig. 5. (a) The simulated electric filed propagation profiles at 1310-nm and 1550-nm wavelength and (b) the transmission spectra at wavelength range of 1260-1680 nm with TE and TM polarizations input.
2.3 Tolerance analysis
The fabrication tolerance of the proposed PBS is simulated by 3D-FDTD method. Feature size derivation $\beta$ is mainly caused by over-etching ($\beta$> 0) or under-etching ($\beta$< 0) as shown in Fig. 6.
We consider a small fabrication error to be $\beta ={\pm} 10\,\textrm{nm}$ and a large fabrication error to be $\beta ={\pm} 30\,\textrm{nm}$. The proposed PBS under the condition of over-etching and under-etching are calculated by 3D FDTD. The transmission efficiencies for TE and TM polarizations at through port and cross port are shown in Fig. 7(a-d), where the solid lines represent TE polarization input and the dashed lines represent TM polarization input. From Fig. 7(a-b), it is clear that the ILs are below 0.8 dB and ERs are larger than 10 dB when then fabrication error is within ${\pm} 10\,\textrm{nm}$. When the fabrication error is as larger as ${\pm} 30\,\textrm{nm}$, the worst IL is 1 dB and the minimal ER is 9.1 dB. What’s more, we can notice that under the same derivation degree the over-etched PBS has better ERs but worse ILs than under-etched PBS. From the fabrication error analysis in Fig. 7, we can conclude that our proposed PBS has good tolerance to fabrication error.
Fig. 7. Fabrication tolerance analysis with (a) +10 nm derivation, (b) -10 nm derivation, (c) + 30 nm derivation and (d) -30 nm derivation.
3. Experimental results and discussions
3.1 Device fabrication and measurement setup
Our PBS is fabricated on an SOI wafer with 220-nm top silicon layer and 3-µm thick buried oxide layer. The device is patterned by electron beam lithography (JBX-9500FS, JEOL) and inductively-coupled plasma (PlasmaPro System100 ICP180, Oxford Instrument) etching process. The scanning electron microscope (SEM) image of fabricated PBS are shown in Fig. 8. After etching, a 1-µm thick silica cladding is deposited on the top for protection. The SEM image shows that the fabricated PBS has a derivation of around $\beta ={+} 30\,\textrm{nm}$from our optimized design.
Two groups of structures are fabricated to measure the performance of the proposed ultra-broadband PBS. The Group 1 is for measuring the performance of PBS at wavelength range of 1260-1360 nm, while the Group 2 is for measuring the performance of PBS at wavelength range of 1480-1640 nm. Four grating couplers (GCs) coupling light from single-mode fiber into the chip are utilized for the measurement. These GCs utilizes two different etch depth of 150 nm and 220 nm. For TE mode coupling, a dual-etch grating design from literature [24] is adopted for S/C/L-band (1480-1640 nm) and a shallow-etched uniform grating (period: 630 nm; etch width: 430 nm; etch depth: 150 nm) is adopted for O-band (1260-1360 nm) coupling. For TM mode coupling, two subwavelength GC [25] both with 220-nm full etch depth are design for S/C/L-band (period: 1010 nm; subwavelength grating period: 350 nm; etch dimension: 404 nm × 250 nm) and O-band (period: 750 nm; subwavelength grating period: 300 nm; etch dimension: 483 nm × 170 nm). In order to cover the measurement wavelength ranges, different fiber tilt angles are tuned.
The measurement setups are shown in Fig. 9. Two tunable lasers, one with wavelength tuning range of 1260-1360 nm (TSL-550, Santec) and another with wavelength tuning range of 1480-1640 nm (TSL-710, Santec) are used as the light sources for the two groups separately. The input polarization is controlled by a fiber polarization controller (PC). After transmitting through the devices under test (DUT), an optical power meter (MPM-210, Santec) is used to collect the light from the output GCs. Since the ILs for our PBS are relatively low, we cascade identical PBSs as shown in Fig. 9(b) to increase accuracy for measurement. The ILs for single PBS are obtained by fitting the losses of different number of cascaded PBSs. For measurement of ER, two branches at the output of PBS are connected with two identical GCs as shown in Fig. 9(c). The output power difference between the crosstalk (CT) ports and IL ports are calculated as the ERs of a single PBS.
Fig. 9. (a) Setups for measuring the performance of PBS at wavelength range of 1260-1360 nm and 1480-1640 nm. (b) Microscope images of ILs measurement layout with cascaded PBSs. (c) Microscope images of ERs measurement layout with single PBS.
3.2 PBS measurement results and analysis
The transmission spectra of the two groups with TE and TM polarizations input are shown in Fig. 10(a-d). Within the wavelength range of 1260-1360 nm, the average IL for TE polarization is 0.4 dB. For TM polarization, the average IL is 1.0 dB. For both polarizations, the worst ERs are around 10 dB. For another wavelength range of 1480-1640 nm, the average ILs for TE and TM polarizations are both around 0.5 dB. The ERs for TE polarizations are larger than 10 dB, while for TM polarization they are larger than 15 dB.
Fig. 10. Measured transmission efficiencies with TE polarization input (a) at wavelength range of 1260-1360 nm and (b) 1480-1640 nm. Measured transmission efficiencies with TM polarization input (c) at wavelength range of 1260-1360 nm and (d) 1480-1640 nm.
Apart from some dips at certain wavelength of ILs at TE polarization, the experimental results are consistent with simulation results. These dips are caused by the back reflection between cascaded PBSs due to Fabry-Perot effect. The free spectral range (FSR) for the ripples can be predicted by
where ${\lambda _0}$ is the central wavelength and ${n_g} \approx 4$ is the group effective refractive index of silicon waveguide. L = 25 µm is the length between two SWG through waveguides. The calculated FSR is 8.6 nm at ${\lambda _0}$ = 1310 nm and 12.0 nm at ${\lambda _0}$ = 1550 nm, which are very close to the experimentally observed periods of about 9.0 nm for O-band and 12.0 nm for C-band.3.3 Discussion
The measurement results show that even with 30-nm fabrication derivation, the proposed PBS still has average ILs of 0.4 dB and 0.5 dB for TE polarization within two measured wavelength range of 1260-1360 nm and 1480-1640 nm, respectively. For TM polarization, the averages ILs are 1.0 dB and 0.5 dB for the two ranges respectively. In addition, the ERs for both TE and TM polarizations are larger than 10 dB. We make a comparison of our PBS with other reported broadband PBSs as shown in Table 2. Our proposed PBS has the largest simulated bandwidth (420 nm) and the measured experimental results are consistent with simulated results. The measurement is limited by the tuning range of our tunable lasers. Apart from the bandwidth, it is still worth notice that the coupling length of our PBS is only 4.6 µm, which is very compact compared to other broadband PBSs. We have also noticed that our proposed PBS does not exhibit very high ER and the worst ER is around 10 dB. It can be further improved by introducing TE- or TM-polarizers after the PBS [26,27].
Table 2. Comparison of Proposed Broadband PBSs on SOI Platform.a
4. Conclusions
In summary, we theoretically propose and experimentally demonstrate an ultra-broadband and ultra-compact PBS on SOI platform which covers O-, E-, S-, C-, L- and U-bands. The coupling length of our proposed PBS is 4.6 µm and only one etch step is required for fabrication. The simulation results show that the ILs of our proposed PBS are less than 0.5 dB for TE polarization and less than 0.8 dB for TM polarization with large bandwidth of 420 nm (1260-1680 nm). The ERs for both TE and TM polarizations are larger than 10.9 dB. The fabricated device is measured at two separated wavelength ranges. The measured average ILs for TE polarization are 0.4 dB and 0.5 dB at wavelength ranges of 1260-1360 nm and 1480-1640 nm, respectively. For TM polarization, the measured average ILs are 1 dB and 0.5 dB. The ERs for both polarizations are larger than 10 dB at the entire measured wavelength range. These measurement results are consistent with our simulation results. From the measured low ILs and high ERs at two separated wavelength bands, we can assume that the proposed PBS is able to work well at the unmeasured wavelength ranges of 1360-1480 nm and 1640-1680 nm. Even with some fabrication error and limited by the bandwidth of tunable light source, the experimental demonstrated PBS still shows potential 1-dB bandwidth to be 420 nm, which is the largest experimental demonstrated 1-dB bandwidth reported so far to the best of our knowledge.
Funding
Science, Technology and Innovation Commission of Shenzhen Municipality (JCYJ20180507183815699, WDZC20200820160650001); Guangdong Basic and Applied Basic Research Foundation (2021A1515011450).
Acknowledgments
The authors would like to thank H-chip Technology Group for device fabrication.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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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