Silicon photonic devices based on binary blazed gratings

Silicon photonic devices based on binary blazed gratings Zhiping Zhou Li Yu

Optical Engineering 52(9), 091708 (September 2013) Silicon photonic devices based on binary blazed gratings Zhiping Zhou Li Yu Peking University State Key Laboratory of Advanced Optical Communication Systems and Networks School of Electronics Engineering and Computer Science Beijing, 100871, China E-mail: zjzhou@pku.edu.cn Abstract. Optical technology is poised to revolutionize short-reach communication systems, and the leading technology is silicon photonics. Silicon photonic devices have attracted more and more attention and have been increasingly studied in recent years. Grating, which functions as a building block for many passive and active devices, is widely used in silicon photonics. This review presents some silicon photonic devices based on binary blazed gratings, such as grating couplers, beam splitters, polarization beam splitters, broadband reflectors, and narrow filters, that demonstrate much better performance than those based on uniform gratings, owing to the novel characteristics of binary blazed gratings. 2013 Society of Photo-Optical Instrumentation Engineers (SPIE) [DOI: 10.1117/1.OE.52.9.091708] Subject terms: binary blazed grating; coupler; splitter; reflector; filter. Paper 121672SS received Nov. 16, 2012; revised manuscript received Jan. 30, 2013; accepted for publication Feb. 6, 2013; published online Mar. 8, 2013. 1 Introduction In recent years, silicon photonic devices have attracted more and more attention and have been gaining a great deal of applications due to their unique combination of low fabrication costs, low power consumption, easy integration with other functions, performance enhancements resulting from electronic photonic integration, and compatibility with the world s most successful technology for producing electronics, complementary metal oxide semiconductor (CMOS). 1 As integrated circuit feature sizes continue to shrink and chip sizes continue to expand, new devices, new interconnects, and new integration schemes must be developed to meet the demand for high-density data communication and high-speed data processing. Diffractive elements, such as binary blazed gratings, are extensively used in optical systems. They can be used for many applications in optical circuits: for example, coupling, splitting, and so on. Binary surface-relief diffractive elements, composed of subwavelength microstructures carefully arranged and etched in a transparent material, may exhibit high diffraction efficiencies. Compared with conventional uniform gratings, binary blazed gratings (BBGs) may exhibit higher diffraction efficiencies. For the pure uniform binary grating coupler, coupling efficiency is <50% in the case of vertical coupling. On the other hand, a blazed grating can blaze all diffracted light into a single diffraction order, which suppresses the second- and higher-order diffractions and enhances the efficiency and directionality of the gratings. Other types of blazed gratings, such as triangular- grating and parallelogram-shaped gratings, cannot be fabricated with standard etch processes. The BBG is composed of variable subwavelength pillars with uniform height, which is a kind of binary version of the triangular tooth shape of the blazed grating and can be easily fabricated in only one etching step. It is compatible with mature CMOS technology, which makes mass production possible. In the world of nanophotonics, because the feature size of devices is in the region of subwavelengths, diffractive 0091-3286/2013/$25.00 2013 SPIE elements exhibit many new characteristics and behave differently. In this paper, we give an overview of our work in silicon photonic devices based on BBGs. First, we propose a binary blazed grating coupler that has a high coupling efficiency between single-mode fiber and Si waveguide. Second, we design two different kinds of beam splitters to couple the input light from fiber and split it into two waveguides: a broadband beam splitter, which is designed for the wavelength of 1.55 μm under TE polarization with vertical coupling scheme, and a polarization-independent beam splitter. Polarization independence can be realized because both transverse electric (TE) and transverse magnetic (TM) models are effectively coupled and split simultaneously. Third, we design two different kinds of polarization beam splitters (PBSs) by using the strong polarization dependence of the BBG coupler: single-layer and double-layer, both with high extinction ratios. For the double-layer PBS, the coupling length is merely 14 μm, the extinction ratio is better than 20 db for both polarizations over a 40-nm wavelength range, and the coupling efficiencies for two polarizations are 59% (TE) and 50% (TM). Then, we propose a highperformance broadband polarization-insensitive reflector based on the diffraction character of the BBG. Up to 96% reflectivity over a wavelength range of 1.2 to 1.7 μm was achieved both theoretically and experimentally. Finally, we design a narrow-band polarization-independent filter with high angular tolerance and tunable resonant wavelength and bandwidth. 2 Compact Vertical Grating Coupler Although blazed grating can be designed for high-efficient vertical coupling, the conventional saw-tooth profile is difficult to fabricate. We have proposed a compact binary blazed grating for vertical coupling between fiber and silicon-on-insulator (SOI) waveguide, which shows high efficiency and easy fabrication processes with conventional CMOS technology. 2 Compared with uniform grating, BBG can realize vertical coupling and exhibit much higher coupling efficiency (>70%), and it has multisubpart profiles, which can be flexibly adjusted in the design process. Optical Engineering 091708-1 September 2013/Vol. 52(9)

A high-performance and compact fiber-to-waveguide binary blazed subwavelength grating coupler was designed based on SOI. 3 The binary blazed grating is composed of an asymmetric subgrating structure in which a period consists of two subgratings with identical etching height and different widths, shown in Fig. 1. Each ridge s width is modulated to obtain the blaze effect. 4 Several key parameters influence the coupling efficiency of binary blazed grating coupler, such as thickness, period, height, and fill factor. To optimize the mode matching, a relatively high coupling efficiency was obtained for the fiber and waveguide interface. Figure 1 shows the schematic diagram of the binary blazed grating coupler. Figure 2 shows the SEM picture of a fabricated binary blazed grating coupler. BBG has only one efficient diffraction order. Blazing effect is achieved by using multisubpart profiles. The second-order reflection is suppressed; thus the directionality of the grating can be enhanced and high coupling efficiency can be achieved. Figure 3 shows the simulation result obtained using OptiFDTD software. The plotted field is the real part of Ey and Poynting vector. For a 1.55-μm wavelength, the coupling efficiency is about 65% when we consider the TE mode and normal incidence. In addition, the fabrication errors, including the etching depth and width of binary grating, are also taken into account, as shown in Figs. 4 and 5. This indicates that the coupling efficiency is >50% when the period changes from 0.588 to 0.618 μm. Thus, the tolerance error of width is about 30 nm. Simultaneously, it can be seen that the tolerance error of etching depth beyond 30 nm is also obtained, since the coupling efficiency is >50% when the etching depth changes from 0.105 to 0.137 μm. Fortunately, this is enough to control conveniently in practical fabricating processes. The coupling efficiency can reach up to 80% if Bragg reflector layers are added. Finally, the device layout is simple, feasible, etched in one step, and compatible with standard CMOS technology processing. 3 Beam Splitter 3.1 Broadband Beam Splitter for TE Mode The beam splitter (BS) is designed for the wavelength of 1.55 μm under TE polarization with vertical coupling scheme as shown in Fig. 6. Rigorous coupled-wave analysis Fig. 1 Structure of binary blazed grating coupler. Fig. 4 Relationship of coupling efficiency and period. Fig. 2 Scanning electron microscope (SEM) picture of a fabricated binary blazed grating coupler. Fig. 3 Distribution of the optical field in the waveguide by software OptiFDTD. The plotted field is the real part of Ey (a) and the Poynting vector (b). Fig. 5 Relationship of coupling efficiency and etching depth. Optical Engineering 091708-2 September 2013/Vol. 52(9)

Fig. 6 Scheme of a binary blazed grating beam splitter. is used for optimization and numerical characterization of the device. The grating length is only 9 μm, and the grating height is comparable with waveguide thickness. The finite difference time-domain (FDTD) method, a powerful and accurate method for finite size structure, was chosen to simulate and design the binary blazed grating splitter. Figure 7 shows the relevant Poynting vectors. Obviously, the simulation shows that the beam splitters for TE light are designed to split the incident light beam into two beams of equal power (nearly 50% split), which travel in opposite directions in the waveguide. The coupling efficiency for the right and left branches of the waveguide are 47% and 52%, respectively. Their difference of power value is only about 5%, which is small enough to be suitable for a beam splitter. As seen in Fig. 8, the difference is equal to zero at the wavelength of 1.575 μm, but corresponding to the coupling efficiency of 0.27. In this case, the power difference of two output ports is <15% over a 75-nm (1.50 1.575 μm) wavelength bandwidth range. Fig. 8 Coupling efficiency as a function of wavelength. 3.2 Polarization-Independent Beam Splitter A novel polarization-independent beam splitter at a wavelength of 1.55 μm is proposed as shown in Fig. 9. We optimize the architecture design of waveguide grating by doublestructure pattern, which consists of the grating discussed above and its symmetrical counterpart. This type of design pattern can remodulate the distribution of fill factor, affect the coupling efficiency of grating, and result in equal power coupling and splitting in waveguide. By using additional reflection layers (i.e., Bragg reflector) under substrate, higher coupling efficiency can be achieved. With reflection layers, the up/down ratio and coupling efficiency can both be improved. An optimum design of grating beam splitter is given in Fig. 10, which adds three reflect layers under substrate. For TE polarization and normal incidence, the distribution of Poynting vector (Sz) and power is given in Fig. 11. Itis obvious that the power coupling into two branches of waveguide is nearly identical. As seen in Fig. 12, from 1.53 to Fig. 9 Symmetrical structure of binary blazed grating splitter. Fig. 7 Calculated Poynting vector component in silicon-on-insulator (SOI) platform. (a) Distribution of Poynting vector in waveguide. (b) Wave profile in waveguide. Fig. 10 Binary blazed grating splitter with three reflect layers. Optical Engineering 091708-3 September 2013/Vol. 52(9)

Fig. 11 Distribution of Poynting vector (a) and coupling power (b) for transverse electric (TE) mode. 1.57 μm, the coupling efficiencies of two ports of the waveguide are relatively high, and power differences as low as 6% also are achieved. The beam splitters for TE light are designed to split incident light beam into two beams of nearly equal power (41% and 43%), and the power difference of two output ports is <3% over a 20-nm wavelength bandwidth range. Simultaneously, simulation results for TM polarized wave are presented in Fig. 13. The coupling efficiency of TM mode for the right and left branches of waveguide are 33% and 40%, respectively. As seen in Fig. 14, from 1.51 to 1.59 μm, the power differences of two ports of the waveguide are <10% with 80-nm wavelength bandwidth. The polarization independence that can be realized means both TE and TM mode are effectively coupled and split simultaneously. 4 Polarization Beam Splitter Fig. 12 Coupling efficiency as a function of wavelength for TE. 4.1 Single-Layer Polarization Beam Splitter Polarizing beam splitters (PBSs) are basic functional elements in photonic integrated circuits 5 for separating two orthogonally polarized light beams, which play important Fig. 13 Distribution of Poynting vector (Sz) (a) and power (b) for transverse magnetic (TM) mode. Optical Engineering 091708-4 September 2013/Vol. 52(9)

As seen in Fig. 17, TE and TM polarized waves can achieve high diffraction efficiencies (>95%) in the range of 4 deg to 4 deg. The grating PBSs can be potentially used in routing or switching for computer communications or telecommunications. Fig. 14 Coupling efficiency as a function of wavelength for TM. roles in numerous optical information processing applications. A schematic of the proposed single-layer PBS is shown in Fig. 15. Figure 16 shows a flat TE stopband from 1.53 to 1.62 μm with reflectance R>99%, and the TM pass band is nearly flat in the wavelength range of interest with transmittance T>97%. The resonance wavelength is 1.51 μm. High reflectance and large bandwidths are associated with the characteristics of the large refractive index difference among materials and the properly configured top grating profile. Moreover, the asymmetric profile of the top grating layer can work to remove the leaky mode degeneracy of the grating PBS, which opens up the possibility of a flat reflection band for the TE polarization. 4.2 Double-Layer Polarization Beam Splitter In this section, a polarization beam splitter using a two-layer grating coupler is proposed (see Fig. 18). It can directly couple the normally incident light from fiber into two separate waveguides according to their polarization states while splitting them. The upper layer is designed for TE-mode coupling, while the lower layer is for the TM mode. Normally incident light with both polarizations can be separately coupled into two waveguides. It realizes high coupling efficiency and a good extinction ratio by using binary blazed grating couplers. We can control the width of each pillar (or f) to obtain the desired refractive index distribution. The effective refractive index (ERI) distributions are very different for the TE and TM modes (shown in Fig. 19). When the binary blazed grating is designed for the TE mode, the ERI distribution of the TM mode will be far from its optimized distribution, hence little TM light will be coupled into the waveguide through the binary blazed grating for the TE mode, and vice versa. Fig. 15 A schematic view of a single-layer polarization beam splitter. Fig. 17 Angular spectrum of the PBS grating at a wavelength of 1.57 μm. Fig. 16 Reflectivity spectra (a) and transmissivity spectra (b) response of a broadband polarization beam splitter (PBS) structure. Optical Engineering 091708-5 September 2013/Vol. 52(9)

Fig. 18 Schematic of a two-layer grating coupler as a PBS. (a) Upper grating coupler for TE mode. (b) Spacer between two couplers. (c) Lower grating coupler for TM mode. (d) SiO 2 spacer. (e) Si substrate. Fig. 19 Effective refractive index (ERI) of TE mode (solid curve) and TM mode (dashed curve) as a function of fill factor. Figure 20 shows the simulation results obtained with Opti-FDTD software. The coupling efficiencies are 58% and 50% for the TE and TM mode, respectively. As seen in Fig. 21, the extinction ratio is better than 20 db for both polarizations in the wavelength range of 1530 to 1570 nm. The upper layer has a smaller extinction ratio due to the scattering of the lower grating surface. Some light can be reflected upward by the lower layer surface and coupled into the waveguide, which makes the extinction ratio worse. Fig. 21 Extinction ratio for each layer as a function of wavelength. This PBS can be used as a polarimeter to detect the polarization state of incident light from the fiber by comparing the energies in the two waveguides. Moreover, it can be placed anywhere on a chip, because it allows planar coupling, which makes the system design more flexible. 5 Broadband Reflector Here we propose and fabricate a multilayer-based highperformance subwavelength multisubpart profile grating reflector (MPGR), which experimentally demonstrates combined merits of high reflectivity over a ultrabroadband spectrum and low sensitivity to incident angle. Rigorous coupled-wave analysis for multilayered grating structures is adopted to design and optimize the structure. The proposed structure has a multilayer configuration with a six-subpart surface-relief grating etched onto the top silicon layer, as shown in Fig. 22. The optimized results are: x1 x2 x3 x4 x5 ¼ 0.37 0.5 0.58 0.8 0.9 μm, T ¼ 1 μm, θ ¼ 0, grating thickness ðtgþ ¼ 0.64 μm, middle thickness ðtmþ ¼0.04 μm, and buffer layer thickness ðtbþ ¼1 μm. Figure 23 shows a scanning electron microscope (SEM) picture of a fabricated MPGR. The optical measurement setup is shown in Fig. 24. Figure 25 shows the theoretical and experimental reflectivity spectra of the MPGR for TE polarization. From the figure, we can see that R>0.99 for a broad wavelength range from 1.54 to 1.84 μm is obtained theoretically, while R>0.97 from 1.56 to 1.8 μm is achieved experimentally. As shown in Fig. 26, there are two transmittance dips inside the high-reflectance band, each of which corresponds to a guided mode resonance reflector (GMR). This shows that the broad reflection band results from coexistence Fig. 20 Simulation results of TE input (a) and TM input (b). Fig. 22 Schematic of a multilayer-based subwavelength multisubpart profile grating reflector (MPGR). Optical Engineering 091708-6 September 2013/Vol. 52(9)

Fig. 23 SEM image of a fabricated MPGR. Fig. 26 Transmittance of the MPGR on a log scale. Fig. 24 Schematic of the measurement setup. Fig. 27 Theoretical and experimental angular response of the MPGR at the wavelength of 1.68 μm. Fig. 25 Theoretical and experimental reflectivity spectra of the MPGR normally illuminated by TE polarized wave. This proposed structure also acts as a broadband polarization-insensitive reflector. As we can see, Fig. 28 shows the reflectivity spectra of the reflector for both TE and TM polarizations, and Fig. 29 shows the transmittance on log scales. As displayed in Fig. 28, the width of the reflection band with R>0.99 is about 140 nm over the 1.62 to 1.76-μm range. For TE polarization in Fig. 29, the resonance wavelengths are 1.55 and 1.8 μm, respectively. For TM polarization, the corresponding values are 1.63 and 1.71 μm, and interaction of the TE guided modes. Furthermore, the high-index-contrast grating layer can expand resonances and eventually lead to the formation of broadband reflectance spectra. Figure 27 displays the theoretical and experimental angular response of the MPGR at the wavelength of 1.68 μm. As presented, a>0.99 reflectivity for incident angle at the range of 16 deg to 16 deg is obtained theoretically, while R>0.97 for incident angle at the range of 13.8 deg to 14 deg is obtained experimentally. These remarkable large angular tolerances are mainly due to the coexistence and interaction of guided modes resulting from the high-index-contrast materials and modulation profile of grating. Fig. 28 Reflectance spectra of the reflector normally illuminated by both TE and TM polarized waves. Optical Engineering 091708-7 September 2013/Vol. 52(9)

Fig. 29 Transmittance spectra of the reflector normally illuminated by both TE and TM polarized waves. respectively. This shows that, inside the wavelength range of interest (1.62 to 1.76 μm), the simultaneous coexistence and interaction of leaky modes of both TE and TM polarized waves result in the polarization-insensitive properties of the reflector. Figure 30 presents the angular response of the reflector at a wavelength of 1.68 μm. For TE polarization, the result reveals a reflectivity of >0.99 for incident angles in the range of 16 deg to 16 deg. For TM polarization, it can be seen in the figure that the structure can achieve high reflectivity (R >99%) in the range of 14.8 deg to 14.8 deg. These remarkable large angular tolerances are mainly due to the coexistence and interaction of leaky modes. As seen in Fig. 31, the insertion loss (IL) is <0.04 db for both polarizations across the wavelength range of interest (1.62 1.76 μm). The polarization-dependent loss (PDL) is <0.02 db over the 140-nm bandwidth. It is shown that the proposed reflector can yield high reflectivity (low IL) and low PDL over a broadband spectrum, making the reflector highly suitable for applications where polarization state is uncertain. Fig. 31 TE and TM insertion loss (IL) and polarization-dependent loss (PDL). polarization-independent filter under full conical incidence is presented in Fig. 32; full conical incidence (α ¼ 90 deg) is chosen instead of classic incidence (α ¼ 0 deg). The properties of the grating filter are investigated by rigorous coupled-wave analysis. 6.1 Polarization Independence For full conical incidence, the incident wave vector is parallel to the grooves of the grating. A pair of waveguide modes, which have mirror symmetry with respect to the incidence plane, can be excited. By optimizing grating parameters, the resonance conditions corresponding to both polarization states can coincide. Thus it is possible to realize a polarization-independent grating filter. 7 This characteristic enables this device to be applied in a field where the polarization state is unknown or unstable. 8 As seen in Fig. 33, the filter 6 Filter Guided-mode resonance grating filters have attracted attention due to their unique advantages over classic multilayer structures and fiber Bragg grating filters in a system of dense wavelength demultiplexing. 6 A BBG-based Fig. 32 Scheme of a binary blazed grating in a conical mounting. Fig. 30 Angular behavior of the reflector at a wavelength of 1.68 μm. Fig. 33 Spectral reflectivity of the grating filter for two different polarization states. Optical Engineering 091708-8 September 2013/Vol. 52(9)

demonstrates high reflectivity (R >99%) at its resonant wavelength, which stays the same under two different polarization states. It indicates that this grating filter is polarization independent. Fig. 34 Full conical incidence for resonant excitation of two identical guided modes. 6.2 Tunable Resonant Wavelength As shown in Fig. 34, for full conical incidence, both polarization states can couple into only TE or TM mode in the waveguide. Two TE modes are to be excited, and then we have to satisfy α 2 0 þ n T λ 2 ¼ β 2 TE : (1) If we want to excite two TM modes, we have the same relation, replacing β TE by β TM. For classic incidence, only a single polarization-independent solution exists, meaning a tunable polarization-independent device is not possible. For full conical incidence, the two input polarization states both excite a pair of waveguide modes having mirror symmetry with respect to the incidence plane, fulfilling the conditions for tunable resonant wavelength. From Fig. 35, we can see that the resonant wavelength of this grating filter shifts from 1558.5 to 1531.5 nm when the angle of incidence ranges from 42 deg to 48 deg. Plus, the curve shape stays almost constant over the whole tuning range with different bandwidths. Fig. 35 Spectral reflectivity of the grating filter for two different polarization states and angles of incidence. 6.3 Tunable Bandwidth Figure 36 shows that the bandwidth of this filter can be tuned from 0.6 to 90 nm, while the property of polarizationindependence can be maintained. Fig. 36 Spectral reflectivity of the grating filter for different polarization states when θ ¼ 45 deg (a), 22 deg (b), and 10 deg (c). Optical Engineering 091708-9 September 2013/Vol. 52(9)

fully etched BBG can be used to realize equal power splitting operations under the condition of TE polarization incidence. Fig. 37 Transmittance spectra of the grating filter illuminated by both TE and TM polarized waves when θ ¼ 45 deg (a) and 10 deg (b). As seen in Fig. 37, by adjusting the angle of incidence, the position where the guided-mode resonance occurs will change, which results in the change of the bandwidth. 7 Conclusions In conclusion, we proposed five types of silicon photonic devices based on binary blazed gratings. The devices realize the functions of coupling, beam splitting, polarization beam splitting, reflection, and filtration. All of these devices are based on an SOI platform with BBG and are compatible with conventional CMOS technology, which makes them possible for mass production. There are some methods to improve the properties of the BBG-based devices. For example, the coupling efficiency can be further increased by antireflection layer coated structure. A high-efficiency BBG nearly vertical coupler with fully etched slots can be designed by optimizing the filling factor of the first period based on Bloch mode theory to reduce the back reflection. A novel symmetrical chirped grating beam splitter based on Acknowledgments This work was partially supported by National Natural Science Foundation of China (Grant No. 61177058), the Major International (Regional) Cooperation and Exchange Program of the National Natural Science Foundation of China (Grant No. 61120106012), and National High Technology Research and Development Program of China (Grant No. 2011AA010302). References 1. W. Bogaerts et al., Nanophotonic waveguides in silicon-on-insulator fabricated with CMOS technology, J. Lightwave Technol. 23(1), 401 412 (2005). 2. J. Feng and Z. Zhou, High efficiency compact grating coupler for integrated optical circuits, Proc. SPIE 6351, 63511H (2006). 3. J. Yang et al., High-performance and compact binary blazed grating coupler based on an asymmetric subgrating structure and vertical coupling, Opt. Lett. 36(14), 2614 2617 (2011). 4. Z. Zhou and T. J. Drabik, Optimized binary, phase-only, diffractive optical element with subwavelength features for 1.55 mm, J. Opt. Soc. Am. A 12(5), 1104 1112 (1995). 5. T. K. Liang and H. K. Tsang, Integrated polarization beam splitter in high index contrast silicon-on-insulator waveguides, IEEE Photon. Technol. Lett. 17(2), 393 395 (2005). 6. F. Lemarchand et al., Study of the resonant behavior of waveguide gratings: increasing the angular tolerance of guided-mode filters, J. Opt. A Pure Appl. Opt. 1(4), 545 551 (1999). 7. D. Lacour et al., Polarization independence of a one-dimensional grating in conical mounting, J. Opt. Soc. Am. A Opt. Image Sci. Vis. 20(8), 1546 1552 (2003). 8. G. Niederer et al., Design and characterization of a tunable polarizationindependent resonant grating filter, Opt. Express 13(6), 2196 2200 (2005). Biographies and photographs of the authors are not available. Optical Engineering 091708-10 September 2013/Vol. 52(9)