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We present a compact silicon polarization rotator (PR) based on mode-hybridization by breaking the cross-sectional symmetry of a double-stair waveguide. The device fabrication is fully compatible with the commonly used silicon photonics processes with no extra masks required. The dependence of device performance on the double-stair waveguide dimensions is investigated using FDTD simulations. Characterizations of the fabricated devices reveal that the 23-μm-long PR exhibits a polarization extinction ratio (PER) of >17 dB in the wavelength range of 1500-1540 nm. The maximum PER exceeds 30 dB at 1518 nm.
© 2015 Optical Society of America
1. Introduction
Silicon-on-insulator (SOI) platform is being considered as one of the most promising material platforms to develop photonic devices and on-chip integration for telecom and datacom applications [1]. Photonic devices made on the SOI platform are compatible with complementary metal oxide semiconductor (CMOS) electrical circuits to combine the merits of photons and electrons. SOI strip waveguides have a very small cross-sectional area of ~0.1 μm2 and an ultra-small bending radius of 1-2 μm [2]. To date, various compact silicon photonic devices have been realized, such as modulators [3, 4], photodetectors [5, 6], delay lines [7, 8], and switches [9, 10]. However, photonic integrated devices based on SOI sub-micrometer waveguides also suffer severe polarization-sensitivity issues because of the large structural birefringence [11], which may cause polarization mode dispersion (PMD), polarization dependent loss (PDL), and polarization dependent wavelength characteristics (PDλ) [12]. These drawbacks greatly limit their application range.
To make a photonic circuit polarization independent, the simplest way is to use a square-core waveguide with symmetric cladding. However, as silicon waveguides have sub-micrometer dimensions, fabrication errors as small as a few nanometers can result in large birefringence. The accuracy of waveguide geometry needs to be well controlled to nanometer size, which poses a great challenge to fabrication. Another way to circumvent this issue is to adopt a polarization diversity scheme in which polarization beam splitters (PBS) and polarization rotators (PR) are the two key components [12]. Input light with arbitrary polarization is firstly split into two beams with orthogonal polarization states by the PBS, and then one of the components is rotated and converted to the other polarization by the PR. The two beams are then processed in parallel with two identical structures, and finally the polarization-converted beam is rotated back and combined with the other polarization into one output optical beam. A convenient way to realize both polarization splitting and rotation is to use a polarization splitter-rotator (PSR) [13], which works essentially as cascaded PBS and PR.
There exist roughly three mechanisms that a PR can be built on, including mode evolution, mode coupling, and mode hybridization. In mode evolution, silicon waveguide is top-clad with a-Si or SiN material [14, 15] with specially designed tapers to enable gradual mode conversion between orthogonal polarization states. Such kind of top-cladding increases the complexity of fabrication and sharp tips at the end of tapers necessary for low conversion loss are also difficult to make. A pure silicon solution is proposed in [16, 17], but in their structure the input and output silicon waveguides have different thicknesses, uncommon in most photonic integrated chips. The structure in [18] solves this problem yet at the cost of a longer device length of 230 μm. Polarization conversion can also be realized based on mode coupling [19–22]. When two modes with orthogonal polarizations have equal effective refractive indices, strong mode coupling occurs in the waveguide, and with proper taper designs, one mode can be effectively converted to the other. Due to the large birefringence of silicon waveguides, the conversion usually occurs between fundamental transverse magnetic (TM) and high order transverse electric (TE) modes and subsequently the high order TE mode is converted to the fundamental TE mode. There are also several reports on mode-hybridization based PRs [23–26]. By breaking the symmetry of the silicon waveguide cross section, the propagation modes are hybridized, allowing optical power to be transferred periodically between the two desired polarization states. The rotation is essentially enabled through interference of the two hybridized modes. Asymmetric waveguide cross section can be obtained by etching subwavelength trenches [23, 24] or using asymmetric cladding [25, 26]. However, the narrow trenches (~10 nm wide) are difficult to pattern and etch with controllable profiles; asymmetric cladding yet requires extra material (e.g., SiN). Recently, a PR is realized on a simple strip waveguide by cutting one upper corner of the waveguide in a two-step etch process [27], following the original idea in [28]. The pure silicon solution without the need of extra materials is quite attractive, but the measured PER is relatively low around 6 dB within a 30 nm bandwidth.
In this paper, we report a PR based on a double-stair silicon waveguide fabricated with three etch steps. Compared to the two-etch-step structure with single-stair cross section [27], it has a higher polarization extinction ratio (PER) and a broader optical bandwidth. Besides, mode coupling loss at the interfaces is reduced due to smaller effective refractive index difference between the input/output and double-stair waveguides. Our design is fully compatible with the silicon photonic devices commonly adopted by the research community, where three etch steps are originally used to form grating couplers, rib (with slab) and strip (without slab) waveguides [29]. The measured PER is over 17 dB over a bandwidth of 40 nm.
2. Device design and simulations
Figure 1(a) shows the structure of our PR consisting of a double-stair waveguide connected with regular strip waveguides at the input and output ends. The double-stair waveguide supports two orthogonal hybrid modes S1 and S2 as the lowest order modes, rotating 45° with respect to the x and y axes by designing an appropriate waveguide cross-section. Each of these guided modes possesses an equal magnitude of Ex and Ey components so that they are almost fully hybridized. Figure 1(b) shows the polarization rotation process along the waveguide. As the TE (or TM) polarized input light is launched into the waveguide, the two hybrid guided modes are nearly equally excited. The polarization conversion occurs as the two modes propagate along the waveguide for a particular length
where Lπ is the half-beat length of the two guided modes, which is defined aswhere β1 and β2 are the propagation constants of the two hybrid guided modes, respectively. In our design, we choose k = 0 to minimize the conversion length Lc so that Lc = Lπ. Due to the strong confinement and asymmetry of the specific double-stair profile, the difference of propagation constants between these two modes is very large, favoring a small Lc.
Fig. 1 (a) Schematic structure of the double-stair polarization rotator. Insets show the cross-sections of (i) input/output section, and (ii) polarization rotation section. (b) Polarization rotation process along the waveguide. The arrows indicate the direction of mode electric field. z = 0 is the starting position of the polarization rotation section.
The key parameters to characterize a PR are the PER and polarization conversion efficiency (PCE). The PER is defined as
where PTE and PTM represent the output powers for TE and TM modes, respectively. The PCE can be expressed asWe use the commercial software MODE Solutions from Lumerical to perform modal analysis and FDTD Solutions to simulate light propagation in the device. We assume a quasi-TE polarized input mode (Ex component dominant). Note that because the PR is reciprocal, similar results can be expected for a quasi-TM polarized input mode. The wavelength is set at λ = 1550nm, and the refractive indices for the SiO2 insulator layer and Si guiding layer are n1 = 1.444 and n2 = 3.48, respectively. To be consistent with device fabrication, we choose the waveguide height H = 220 nm, the first step Ha = 70 nm representing the etch depth for grating couplers, the second step Hb = 130 nm representing the etch depth for rib waveguides, and the last step Hc = 20 nm representing the etch depth for the remained silicon slab. In our simulation, we study the effect of stair widths Wa, Wb, and Wc on device performances.
We first set Wc = 0 to simplify the structure so that there are only two independent variables. In this case, the double-stair waveguide degrades into a single-stair waveguide. Figure 2 shows the relationship of Wb and Wa in order to realize 45° polarization angle. The corresponding half-beat length Lπ is calculated from Eq. (2). From the diagram, it is learned that with the increase of Wa, a smaller Wb is needed. The half-beat length Lπ reaches the minimum when Wa = 190 nm. Therefore, we choose three sets of Wa (200, 190, and 180 nm) in our following simulations when Wc is taken into consideration for further optimization.
Fig. 2 Required Wb to achieve 100% hybridness and corresponding half-beat length Lπ when Wa varies. Wc is set to 0 and the conversion section is a single-stair waveguide.
Figures 3(a)-3(c) show the dependence of Wb on Wc for a fixed Wa. With the increase of Wc, Wb needs to decrease correspondingly to ensure 100% hybridness. The loss decreases with the increasing Wc because of less mode mismatch between the input/output waveguide and the double-stair waveguide. However, to guarantee a low waveguide propagation loss, the waveguide width W = Wa + Wb should not be too small (otherwise the sidewall roughness induced scattering loss will increase significantly). Therefore, we choose Wb to be 120, 160 and 170 nm for the three sets of Wa (devices denoted as PR-1, PR-2, and PR-3) as a good compromise for real device fabrication. Table 1 summarizes the device design parameters. We also study the influence of dimension deviation on PER of the device. Figure 4 shows that the PER deteriorates when Wa, Wb, and Wc deviate from the optimal values for PR-2. One can see that the device still exhibits a PER of >10dB within ± 20 nm of ΔWa, ± 50 nm of ΔWb, and ± 5 nm of ΔWc.
Fig. 3 Dependence of Wb, half-beat length Lπ and loss on Wc for (a) Wa = 200 nm, (b) Wa = 190 nm, and (c) Wa = 180 nm.
Table 1. Design parameters for three PRs
Fig. 4 Tolerance simulation of PER when the critical dimensions (a) Wa, (b) Wb and (c) Wc deviate from the optimal values for PR-2.
We simulate the modal profiles of the two hybridized modes in PR-2 as shown in Fig. 5(a). The arrows in each diagram represent the modal electric field. The near 45° rotation of the arrows suggests nearly 100% hybridness of the two modes. The large overlap between the two modes guarantees efficient polarization conversion. Figure 5(b) depicts the evolution of Ex and Ey components along the light propagation direction when the TE mode is launched into the input port. The Ex component gradually vanishes while the Ey component becomes stronger during propagation, indicating efficient conversion occurs from input TE to output TM mode.
Fig. 5 (a) Electric field distribution of the hybridized modes in the double-stair waveguide for PR-2. (b) Evolution of Ex and Ey components along the PR with TE mode input.
3. Fabrication and experiments
The PR devices were fabricated using CMOS fabrication processes on a silicon-on-insulator (SOI) wafer. Figure 6 shows the device fabrication process flow. The device patterns are defined by 248-nm DUV photolithography with three masks used for different layers. The first mask defines the grating couplers with an etched depth of 70 nm. The second mask is used to define the silicon waveguide with an etched depth of 130 nm. The third mask defines the slab region outside which silicon is etched through down to the buried oxide layer. It should be noted that a silicon dioxide hard mask is used in waveguide etching, which guarantees a vertical sidewall at the waveguide left edge during the following slab etch. Finally, a 1.5-μm-thick silicon dioxide layer is deposited using plasma-enhanced chemical vapor deposition as the top cladding. Figure 7(a) shows the optical microscope image of a typical PR cascaded with a PRS and TE and TM grating couplers (GCs) to facilitate testing.
Fig. 7 (a) Optical microscope image of the entire test structure. (b) Scanning electron microscope (SEM) image of the PBS.
We characterize the components by measuring their transmission spectra. Light from a tunable laser first passes a fiber-based polarization controller and then couples into the device through the on-chip input TE-GC. The grating period of the TE-GC is 0.59 μm with a duty cycle of 50%. The coupled light in the silicon waveguide then goes through the PR followed by a PBS which is used to separate the TE and TM components after polarization rotation. The PBS is made up of a 35.1-μm-long three-waveguide directional coupler [30] as shown in Fig. 7(b). Once the two polarizations are separated, they couple out via the output TE-GC and TM-GC. The TM-GC has a period of 0.92 μm with a duty cycle of 50%. Both TE-GC and TM-GC were designed to have high polarization selectivity, i.e., low loss for one specific polarization but high isolation (>30 dB) for the other. The out-coupled light is finally received by a power meter. By scanning the laser wavelength and recording the output power, we obtain the device transmission spectrum.
To extract the optical responses of the PR, we first characterize the TE-GC, TM-GC, and PBS. The test structures for GCs are composed of two identical TE-GCs or TM-GCs connected by a 320-μm-long silicon waveguide. Figures 8(a) and 8(b) show the measured transmission spectra of the GCs. The coupling losses for both TE and TM polarizations are around 5 dB/facet at the grating central wavelengths. The test structures for the PBS are composed of a PBS connected with GCs at input and output ends. Figures 8(c) and 8(d) show the spectra of PBS normalized with the GCs under TE and TM inputs, respectively. The label TE-PBS-TE in Fig. 8(c) indicates the transmission at the TE output port with TE input (similar for others). It can be observed that the PBS has a low insertion loss of less than 1 dB (TE) and 2 dB (TM), and a low polarization crosstalk of around −30 dB for both polarizations.
Fig. 8 Measured transmission spectra for (a) TE-GC, (b) TM-GC, (c) PBS under TE input, and (d) PBS under TM input.
Finally, we measure the transmission spectra of the entire structure shown in Fig. 7(a). By normalizing with the GCs and PBS, we obtain the response solely due to the PR. Figure 9 shows the normalized TE-TE and TE-TM spectra at the two outputs for the three PRs in Table 1. The noise in the curves is due to Fabry-Perot oscillations from the chip facets and waveguide junctions, as well as the experimental setup which is not fully removed by normalization. The PR-2 has the best performance in terms of TE-TE residual transmission (<-17 dB) over a 40 nm wavelength range from 1500 to 1540 nm. Figure 10 shows the PER as a function of wavelength for the three PRs. The PER of PR-2 is >17 dB over a 40 nm bandwidth and >10 dB over an 80 nm bandwidth. The maximum PER is >30 dB reached around 1518 nm. Given the fact that the GCs and PBS have high polarization isolation (>30 dB) and the input polarization state is carefully controlled, we believe that the measured high PER is originated from the PR.
Fig. 9 Measured normalized TE-TE and TE-TM transmission spectra for (a) PR-1, (b) PR-2, and (c) PR-3.
Table 2 compares our device with various state-of-the-art pure silicon PRs. It can be seen our double-stair PR has a relatively compact size and the PER is very high. The performance of our PR is much better than the single-stair waveguide based PR demonstrated in [27], where 6 dB PER over a 30 nm bandwidth is achieved in a 25-μm-long device with the best PER <10 dB.
Table 2. Comparison of Various Pure Silicon Polarization Rotators
4. Conclusion
We have demonstrated compact and broadband PRs utilizing mode hybridization in double-stair silicon waveguides. The fabrication of the PRs is fully compatible with common silicon photonic devices without need for extra mask layers. The measurement of various designs reveals that the best performance is achieved in a 23-μm-long device featuring a high PER of >17 dB over a wavelength range of 40 nm. The maximum PER exceeds 30 dB around 1518 nm, which, to the best of our knowledge, is the highest PER achieved in pure silicon PRs.
Acknowledgments
This work was supported in part by the 973 program (ID2011CB301700), the 863 program (2013AA014402), the National Natural Science Foundation of China (NSFC) (61127016, 61107041, 61422508), STCSM Project (14QA1402600). We also acknowledge IME Singapore for device fabrication.
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