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Abstract
We propose an all-silicon design of a multi-band transverse-magnetic-pass (TM-pass) polarizer. The device is based on one-dimensional gratings that work under different regimes that depend on the polarization. With a tapered structure, it is revealed that the operation bandwidth can be extended by multiplexing the diffraction in O-band and the reflection in S-, C-, and L-bands for the transverse-electric (TE) mode. By simulation, we achieve a 343 nm device bandwidth with insertion loss (IL) < 0.4 dB and polarization extinction ratio (PER) > 20 dB. The operation wavelength range covers commonly-used optical telecommunication bands including the O-, S-, C-, and L- bands. Experimental results also show IL < 1.6 dB and PER > 20 dB from 1265 nm to 1360 nm corresponding to the O-band, and from 1500 nm to 1617 nm that corresponds to the C-band. The device is a single-etched design on the standard 220 nm silicon-on-insulator (SOI) with silicon oxide cladding. Such a simple and compatible design paves the way for developing practical multi-band silicon photonic integrated circuits.
© 2021 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In the past decade, we have witnessed the rapid development of photonic integrated circuits (PICs) on the silicon-on-insulator (SOI) platform. Due to the polarization-sensitive nature of the SOI, polarization-handling devices have played important roles in circuit design, including polarization beam splitters (PBSs) [1–13], polarization splitter-rotators (PSRs) [14–22], and polarizers. Among them, the polarizers are used to block the unwanted polarization and minimize the polarization crosstalk, thus are basic components for PICs that work with a single polarization.
Various types of polarizers have been reported using different principles. Metal-silicon hybrid TE-pass [23] and TM-pass [24–27] polarizers exhibit relatively high bandwidth with compact structures, but they tend to be lossy due to the absorption of metal. Chen et al. [28] demonstrated a low-loss hybrid plasmonic TM-pass polarizer by polarization-dependent mode conversion but it has a narrow operating bandwidth. Nevertheless, such hybrid designs require additional post processing and consequently add complexity to fabrication. Therefore, there is an increased interest in all-silicon polarizers with high performances. TM-pass polarizers based on doped silicon waveguide have been proposed [29,30]. These designs only involve silicon and silicon dioxide and are compatible with the complementary metal oxide semiconductor (CMOS) process. However, they share similar working principles with metal-silicon polarizers, therefore having relatively high insertion losses (ILs). Periodic structures such as subwavelength gratings (SWGs) [31–33] and photonics crystals [34–37] demonstrate polarization-sensitive band structures as explained in [38], hence can function as polarizers as well. However, those devices are usually designed for only one or two optical bands (~100 nm bandwidth). Consequently, they are not suitable for high-capacity multi-band silicon PICs that work with most or all optical wavelength bands (O-, E-, S-, C-, L-, and U-bands).
Efforts have been put to realize the broadband operation in all-silicon polarizers. Xu et al. [39] proposed a TE-pass polarizer with a 415 nm bandwidth enabled by the anisotropic nature of SWG, whilst Liu et al. [40] demonstrated a TE-pass polarizer that covers the same bands based on different mode properties of TE and TM in a shallow-etched silicon waveguide. Both devices maintain low ILs and high polarization extinction ratios (PERs) over all optical bands, but they both use air claddings that are not practical for packaging. As for the TM mode, Wang et al. [41] showed a high-performance Bragg-grating-based silicon TM-pass polarizer that covers a 264 nm bandwidth, but the device is designed on a sandwich-structured waveguide with two silicon layers and an oxide layer in the middle, vertically. Thus the device is not compatible with most device designs on the single-layer SOI platform, and it is only verified by simulation. A hyperuniform disordered silicon photonic (HUDSiP) TM-pass polarizer was demonstrated [42]. Such structures exhibit wide and isotropic bandgaps, enabling a 240 nm bandwidth (1440 - 1680 nm). However the IL threshold over the operation bands is relatively high (~3 dB). A recent work [43] reported a high-performance all-silicon TM-pass polarizer that covers the 415 nm bandwidth by a double slot Euler waveguide bending. In spite of the ultra-broadband operation, the device depends on the index profile asymmetry realized by the air cladding, which introduces the same packaging issue as [39,40].
In this work, we propose an all-silicon multi-band TM-pass polarizer based on one-dimensional gratings. The device is designed on 220 nm SOI with With a careful selection of parameters, the grating operates under the subwavelength regime over all the optical telecommunication bands for TM mode, while it works as a Bragg reflector for TE mode over S-, C- and L- bands. Furthermore, we discover that the O-band TE-polarized light will diffract if taper gratings are inserted between the strip waveguide and the center grating. Hence, a tapered structure is employed to multiplex the various regimes and consequently extend the operation wavelength range. As a result, our proposed device achieves IL < 0.4 dB and PER > 20 dB over a 343 nm band in total in simulation. Measured spectra of the fabricated device also exhibit IL < 1.6 dB and PER > 20 dB over the ranges [1265 nm, 1360 nm] and [1500 nm, 1617 nm], which cover most measurable wavelengths.
2. Design and analysis
2.1 Theory
We design the TM-pass polarizer on a standard SOI platform with a 220-nm-thick silicon layer on a 2 µm buried oxide (BOX) layer, with a silicon dioxide cladding on top. The schematic of our proposed device is displayed in Fig. 1. Basically, the gratings-based TM-pass polarizer consists of tapered strip waveguides (red) and grating waveguides (blue). Starting at the end of the access waveguide with a width of $W_\textrm {a}$, the waveguide width is linearly tapered from $W_\textrm {a}$ to $W_\textrm {e}$ with a length of $L_\textrm {t}$. Then the width is symmetrically increased back to $W_\textrm {a}$ at the other end of the device. Since the device is designed for all the optical telecommunication bands, the access width $W_\textrm {a}$ is preset as 350 nm so that the single-mode condition is satisfied for all targeted wavelengths. The grating waveguides with the pitch $\Lambda$ and the fill factor $f$ are placed on where the strip waveguide is tapered. The gratings can be divided into three sections as indicated in Fig. 1. The center section is a grating waveguide containing $N$ bars with a width of $W$. On both sides, we introduce a taper grating with $N_\textrm {t}$ bars. The widths of the grating bars evolve linearly from 0 to $W(N_\textrm {t}-1)/N_\textrm {t}$ so that the adjacent bars maintain a constant width difference, $W/N_\textrm {t}$. Such taper designs ensure the adiabaticity of the grating waveguide and the confinement of the mode, therefore minimizing the coupling and propagating loss of the TM mode. Under such definitions, the taper length $L_\textrm {t}$ becomes a function of the grating parameters rather than an arbitrary value. The device length $L$, which equals to $2L_\textrm {t}$, can then be expressed as:
Fig. 1. (a) Schematic of the proposed grating-based TM-pass polarizer. (b) Cross-section of the SOI waveguide. The device is designed on such a standard platform with a 220 nm silicon layer and a silicon oxide top cladding.
It is revealed in [38,44] that 1-dimensional gratings can work under 3 types of regimes, namely diffraction, reflection and subwavelength. The operation regime of a certain grating depends on the ratio between the pitch ($\Lambda$) and the free space wavelength ($\lambda _0$) of the incident light. In order to make the device functional, the TE-polarized light has to be reflected by the grating as a Bragg reflector, while the grating is working under the subwavelength regime for TM mode. Specifically, the following two conditions should be satisfied simultaneously:
2.2 Optimization of the geometries
Since the TE mode effective index in a strip SOI waveguide is more sensitive to width change than that of the TM mode, we firstly study the dependence of operation bandwidth on $W$. $W$ is swept from 700 nm to 1500 nm with a step of 200 nm. The other parameters are determined with rough estimations. $\Lambda = 320\;\textrm {nm}$, $f = 0.6$, $W_\textrm {e} = 110\;\textrm {nm}$ are chosen so that the center wavelength of the Bragg reflector is near 1500 nm. $N_\textrm {t}$ and $N$ are both temporarily set as 20. The performance is evaluated by 3-D finite difference time domain (FDTD) simulation, where we extract the transmission and reflection spectra of both TE and TM modes covering a 500 nm wavelength range from 1200 nm to 1700 nm.
According to the TE mode reflection spectra shown in Fig. 2(b), the upper cutoff frequency (3 dB) of the Bragg reflection regime moves from 1494 nm to 1637 nm when $W$ changes from 700 nm to 1500 nm. Meanwhile for the TM mode, the range of Bragg reflection is rather insensitive to the width variation. It can also be observed that the range of the bandgap (Bragg reflection regime) under the TE mode cannot be effectively expanded by the increase of $W$. If one only utilizes this bandgap, the polarizer will not achieve an ultra broadband operation.
Fig. 2. Calculated (a) transmission and (b) reflection spectra of both TE and TM modes with different grating widths.
However, it is interesting to see that the TE mode transmission on the left-hand side (shorter wavelengths) of the bandgap is also suppressed when $W$ is increased. For example, when $W = 1300\;\textrm {nm}$, the TE mode transmission is below −10 dB within the wavelength range [1200 nm, 1635 nm]. Judging by the TE mode reflection spectrum in Fig. 2(b), the Bragg reflection regime only covers [1371 nm, 1620 nm] (3 dB bandwidth). While in Fig. 2(a), the TM mode transmission with $W = 1300\;\textrm {nm}$ is higher than −1 dB within the band [1279 nm, 1700 nm]. Therefore, the device still functions as a TM-pass polarizer between $\lambda _0 = 1279\;\textrm {nm}$ and $\lambda _0 = 1371\;\textrm {nm}$. Such bandwidth extension is the fundamental cause of the broader operation band compared with other reported TM-pass polarizers based on periodic structures. This will be explained with details later.
A problem with wide gratings is that the grating waveguide supports multi modes rather than a single mode. If we compare $W = 1300\;\textrm {nm}$ and $W = 1500\;\textrm {nm}$, we will notice that the compression of TE mode transmission on the left-hand side of the reflection regime is slightly worse with $W = 1500\;\textrm {nm}$. Since the length of the taper is the same, it is inferred that high order modes (TE1, TE2,…) tend to be excited with a wider grating. Moreover, high order modes have smaller effective indices than the fundamental mode (TE0). Thus they can still go through the grating at shorter wavelengths according to Eq. (2). Consequently, we determine $W = 1300\;\textrm {nm}$.
Following the analysis of the potential excitation of high order modes in the proposed device, one can see that the grating taper length, equivalently $N_\textrm {t}$, is an important parameter to optimize. Therefore the device performances are evaluated with 5 different $N_\textrm {t}$ ranging from 10 to 30. Before the sweep, the geometries of the grating are marginally adjusted to achieve the desired operation bands. Specifically, the TM mode is required to work under the subwavelength regime with $\lambda _0 > 1260\;\textrm {nm}$ (the start of O-band), and the TE mode should be blocked with $\lambda _0 < 1625\;\textrm {nm}$ (the end of L-band). As a result, $\Lambda$ is set to 318 nm and $f$ is set to 0.675. Note that a larger fill factor $f$ results in a redshift of the TE mode spectrum which extends the bandwidth. However, $f$ cannot be too close to 1 as the gap size will be reduced, and it becomes harder to fabricate such small features. Thus, $f = 0.675$ is the result of this trade-off so that the minimum feature is still larger than 100 nm.
Figure 3 shows the calculated device performance with different taper lengths. Here we use IL and PER as the performance indicators, which are defined as follows:
where $T^\textrm {TM}$ and $T^\textrm {TE}$ are the transmission of TM and TE mode through the device, respectively.Fig. 3. Calculated (a) IL and (b) PER spectra with various $N_\textrm {t}$ at $W = 1300\;\textrm {nm}$. The PER in O-band is increased by extending the taper.
Simulation results shown in Fig. 3 indicate that an increase in grating taper length effectively reduce the transmission in O-band without affecting the overall bandwidth. Accordingly, $N_\textrm {t} = 25$ is determined since it is large enough to keep the PER above 20 dB in the whole O-band, and it makes a compact device with a footprint of only 21.942 µm. Under the finalized parameters, the device achieves < 0.4 dB IL and > 7.5 dB PER over a 390 nm band from 1245 nm to 1635 nm. Benefiting from the aforementioned taper designs, the IL is very low over such a wide band. However, the minimum PER is relatively low, which originates from the notch around $\lambda _0 = 1400\;\textrm {nm}$. Nonetheless, we manage to locate the notch in E-band (not commonly used) from 1372 nm to 1410 nm by choosing $W$ and $f$ carefully, so that PER is higher than 20 dB outside this wavelength range.
2.3 Further exploration of the device principle
To better understand the principle of the device, we calculate and exhibit in Fig. 4(a-e) the electric field distributions of light propagating through the device at 5 wavelengths: 1300 nm, 1375 nm, 1450 nm, 1525 nm and 1600 nm. For 1450 nm - 1600 nm, the light is reflected by the grating according to the field distributions, while at $\lambda _0 = 1300\;\textrm {nm}$ one can observe that the the light is diffracted. However, the diffraction emerges at the taper gratings rather than the center grating. To figure out the function of the taper gratings, three different device configurations are compared. In the first configuration, $N$ is set to 1 so that the structure only contains taper gratings with $2N_\textrm {t} + 1 = 51$ bars. For the second test, $N_\textrm {t}$ is chosen as 1 while $N$ is set back to 20. The original set of parameters are used as a benchmark, becoming the third setting. The calculated TE mode transmission and reflection spectra of the three configurations are demonstrated in Fig. 5.
Fig. 4. Normalized electric field distributions of both TE and TM modes at $\lambda _0 =$ (a) 1300 nm, (b) 1375 nm, (c) 1450 nm, (d) 1525 nm, (e) 1600 nm.
Fig. 5. Calculated TE mode (a) transmission and (b) reflection spectra of 1) the proposed device, 2) the device without taper gratings and 3) the device without the center grating. The results show that the taper gratings actually contribute to the bandwidth extension in O-band.
By the O-band transmission shown in Fig. 5(a), if no taper grating is used, the center grating with uniform widths cannot effectively block the TE mode light with $\lambda _0 < 1330\;\textrm {nm}$. On the contrary, the device that only contains the taper gratings show similar performance as the benchmark device around O-band. Since both transmission and reflection are limited, it can be determined that the proposed device works under a diffraction regime, and this regime actually arises from the taper gratings. On the other hand, the center grating is also crucial because it extends the bandgap and improves the performance where the TE mode is reflected. In short, the smooth combination of the taper and center gratings successfully extends the operation bandwidth by multiplexing the diffraction regime with the Bragg reflection for the TE mode, while maintaining low loss for the TM mode.
To conclude, we have designed an all-silicon TM-pass polarizer with simulation that covers [1245 nm, 1372 nm] $\cup$ [1410 nm, 1626 nm], with IL < 0.4 dB and PER > 20 dB. The 343 nm operation band includes O-, S-, C-, L- bands, and part of E- band. The broadband feature is achieved by multiplexing different regimes with different sections of the structure, and details can be discovered in Table 1. One should be aware that the performance can be further improved if we increase $N_t$ or $W$, at a cost of larger size and longer delay. In the next section, we report the experimental results for the current set of parameters as a proof of concept.
Table 1. A summary of the grating-regimes multiplexing as the primary device principle
3. Fabrication and characterization
The device is fabricated by the NanoSOI fabrication process provided by Applied Nanotools Inc. The wafer is a standard 220 nm SOI platform with a 2.2-µm-thick top cladding oxide. The scanning electron microscopy (SEM) image of the device is displayed in Fig. 6. For test purposes, we apply broadband vertical grating couplers [45–47] to connect the device-under-test (DUT) with our measurement system. The test system includes the Yenista TUNICS T100S-HP O-band and C-band tunable lasers as the sources, and the Yenista CT400 passive optical component tester as the power meter. A polarization controller (PC) is inserted between the laser and the device to minimize the coupling loss, and an isolator is placed before the PC to eliminate the reflected light. Since the device works from O- to C-, L-bands, and the grating couplers are designed only for a single band and a single polarization, four measurements need to be performed for one device (TE/O, TM/O, TE/C, TM/C). Limited by the wavelength range of the tunable lasers, we obtain the transmission spectra of both TE and TM modes within the ranges [1260 nm, 1360 nm] and [1500 nm, 1630 nm]. In each measurement set, two back-to-back connected grating couplers are fabricated near the DUT for normalization.
Because the simulated IL for the TM mode is relatively low, 20 copies of the device are cascaded as the DUT in the TM mode characterization, then the IL is calculated as the one-twentieth of the overall loss in dB.
The measured IL and PER spectra are shown in Fig. 7, along with the calculated data as a reference. The IL is below 0.6 dB from 1500 nm to 1630 nm, while in O-band one can observe the edge of the Bragg reflection regime at $\lambda _0 = 1265\;\textrm {nm}$, which is a slight redshift (~20 nm) from the simulation. For $1265\;\textrm {nm} < \lambda _0 < 1360\;\textrm {nm}$ the maximum measured IL is 1.6 dB. Note that there are several notches in the O-band IL spectrum. As reported in Fig. 2(b), the TM mode reflection becomes intense in O-band since it is close to the reflection regime. It is then inferred that the cascade of devices forms a series of Fabry-Pérot cavities that result in the notches. Therefore, one can estimate the actual single-device ILs more optimistically around those maxima. Regarding the TE mode performance, PER > 20 dB is achieved over all the measurable wavelengths except for $\lambda _0 > 1617\;\textrm {nm}$. In simulations, the cutoff wavelength with the same PER threshold is 1626 nm. The 9 nm gap may be caused by over-etching in the fabrication process. Still, the measured PERs are in accordance with simulated values.
Fig. 7. Measured IL and PER spectra from (a) 1260 nm to 1360 nm, and from (b) 1500 nm to 1630 nm. Calculated IL and PER spectra are displayed in dotted lines as references.
4. Conclusion
In summary, we have demonstrated an all-silicon TM-pass polarizer that operates in multiple optical telecommunication bands. The grating-based device utilizes polarization-dependent grating regimes to block the TE mode while remaining transparent to the TM mode. Firstly, a wide grating at the center is applied to enhance the birefringence so that the TM mode is under subwavelength regime for all optical bands (> 1.26 µm) while the grating works as a Bragg reflector with TE mode for S-, C- and L-bands. Moreover, it is revealed that taper gratings with increasing widths can cause the O-band TE-polarized light to diffract as well. Therefore, the proposed tapered structure is able to extend the operation bandwidth by multiplexing the diffraction and reflection regimes for TE mode. With 3-D FDTD simulation, the device achieves IL < 0.4 dB and PER > 20 dB over [1245 nm, 1372 nm] $\cup$ [1410 nm, 1626 nm] (343 nm band in total) using a ~22-µm-long structure. The operation band covers the whole O-, S-, C- and L- bands, which are mostly employed in practical optical communication systems. The proposed device has been fabricated and characterized. Experimental results indicate IL < 1.6 dB and PER > 20 dB from 1265 nm to 1360 nm, and IL < 0.6 dB and PER > 20 dB from 1500 nm to 1617 nm. The measured spectra prove that the device functions well over most targeted wavelengths. Last but not least, the device is designed on a standard 220 nm SOI with oxide cladding, so that it is endowed with compatibility to numerous active/passive silicon photonics components. To the best of our knowledge, this is one of the largest bandwidth achieved with such a standard platform for all-silicon TM-pass polarizers. Plus the single-etch structure and a minimum feature size larger than 100 nm, our proposed device contributes towards the development of practical multi-band PICs.
Acknowledgments
The authors would like to acknowledge Lumerical Inc. and CMC Microsystems for providing design tools, and Applied Nanotools Inc. for the device fabrication.
Disclosures
The authors declare no conflicts of interest.
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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