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Abstract
We report here on one-dimensional (1D) grating couplers based on hybrid silicon/LNOI platform for polarization-independent and high-efficient single-polarization coupling efficiencies. A low index oxide buffer layer was introduced in between the top silicon high index grating coupler and bottom LNOI waveguide. With optimal design of the buffer layer thicknesses, modal and index matches can be tuned for either single polarization or both TE/TM polarization coupling applications. Over 70% coupling efficiency can be achieved for single polarization based on the basic uniform 1D grating coupler design without any bottom reflectors incorporated. Polarization independent coupling efficiency of 51% was also achieved. The spectral bandwidth is over 50 nm with polarization dependent loss of 0.1 dB. The proposed structure is simple to fabricate. Detailed modal and loss analysis suggest different dominant loss mechanisms in the proposed hybrid structure, where the introduction of the bottom mirror may not result in significant improvement in coupling efficiency, as the dominant loss mechanism arises from the top reflection loss.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Lithium niobate on insulator (LNOI) is an emerging platform [1] in integrated optics [2–4], nonlinear optics [5,6] and quantum optics [7,8] because of its large low-loss transparency window (0.35-5 µm), excellent electro-optical coefficient, large nonlinearity coefficient and a range of other unique properties such as piezoelectric [9], thermo-optical [10], photorefractive [11] and elasto-optical effects [12,13]. Compatibility between LN photonics and optical fibers remains a challenge due to their large modal size mismatch [14]. Grating coupler characterized by relaxed positioning tolerance and convenient multipoint wafer characterization is widely used to bridge the gap between optical fibers and photonic integrated chips [15–17]. End-facet polishing can be avoided, which is challenging for LNOI because thin film easily falls off with grinding and polishing stress. Most of the work reported on LN grating coupler so far are mostly based on the direct etching of LN material [18–21]. However, it is well known that high quality etching of LN remains to be of great challenge, which limits the overall demonstrated device performance on LNOI platform [22,23] and account for relatively low coupling efficiency of less than 20% in measurement while around 40% in simulation on the X-cut LNOI platform [18–20] and about 45% on the Z-cut LNOI platform with bottom reflector [21]. Additionally, the relatively smaller index contrast between LN and SiO2 buffer layer also presents a theoretical limit on device performances [24].
On the other hand, SOI platform has enjoyed great successes due to the higher index contrast, and mature micro- and nano-fabrication processes [25]. High performance passive devices can now be realized both theoretically and experimentally [26,27].
Nevertheless, intrinsically polarization and wavelength sensitive characteristics limit the use of grating coupler in some cases where photonic circuits do not preserve polarization of light [28–30]. Polarization-diversity grating couplers proposed so far are 2D grating couplers by superposition of two 1D TE-polarization grating couplers [28,29,31–33], output coupler with different coupling angles for TE and TM polarizations [34], subwavelength 2D polarization-independent grating coupler [30] and multi-layer Si3N4 grating on Si [35]. All structures mentioned above need stringent fabrication procedures.
We report here a hybrid coupling scheme for high efficiency grating couplers on LNOI, by leveraging high index SOI grating designs [24,36,37]. A thin low index oxide buffer layer is incorporated between top silicon grating coupler and bottom LNOI waveguide [24]. By tuning the oxide buffer thickness, modal overlap and coupling strength, as well as the effective index for both TE and TM modes can be tuned differently, which offers a powerful option to optimize the coupler design for either single polarization or both polarizations. Based on the compact footprint uniform one-dimensional (1D) grating design, we are able to achieve over 70% coupling efficiency for single polarization, and over 50% coupling efficiency for both polarizations, with less than 0.1dB polarization dependent loss over a wide spectral range of 50 nm. Further design optimization can result in performances similar to those reported from SOI platform, by incorporating a non-uniform grating design and/or other structures to suppress the losses associated with reflection/transmission [38,39].
2. Device design
Shown in Fig. 1 are the proposed schematics of the hybrid Si/LNOI grating couplers, where the key design parameters are defined as grating period (Λ), duty cycle (f), thicknesses for silicon grating layer (tSi), LN waveguide layer (tLN), silicon oxide buffer layer (tox) between Si and LN waveguide layers, and buried SiO2 layer (tBOX) below the LN waveguide. Angle θ is the tilt angle of the single mode fiber. The top Si layer can be added by amorphous silicon grown with PECVD process [24]. However, considering instability of amorphous silicon and its limitations in high optical power applications such as nonlinear optics, crystalline silicon (c-Si) nanomembranes can be considered here. Based on the micro transfer printing process [40–42], dissimilar crystalline nanomembrane materials can be heterogeneously integrated together. For the proposed structure here, a small piece of unpattern c-Si can be transfer printed from released SOI wafer onto LNOI waveguide with a thin oxide buffer in between. After that, grating structures can be formed on transfer printed c-Si. This process is highly compatible to the CMOS process and scalable without complicated alignment requirements. Finally, the structure is coated with a SiO2 cladding for index matching between the hybrid coupler and optical fiber.
Fig. 1. Schematics of grating coupler on the hybrid LNOI platform with silicon on top: (a) A 3D sketch and (b) Cross-sectional view with key parameters defined for the Si grating and LNOI waveguide layers.
In order to establish a baseline, we first designed a basic grating coupler on LNOI platform with center wavelength of 1,550 nm, by forming etched gratings directly onto LN waveguide layer. The designs reported here are all based on 2-D finite-difference time-domain (FDTD) method. Simulations are first carried out to reproduce results presented in an earlier published paper on 2D polarization-independent grating coupler on SOI to validate the simulation model we use here [30]. Material dispersion is considered here for design accuracy. The refractive index of LN, Si and SiO2 are 2.22, 3.48 and 1.44 at 1,550 nm wavelength, respectively. The background material is air for simplicity. An optimized design is shown in Fig. 2(a). For a simple uniform grating design without a bottom reflector, the coupling efficiency for TE mode is 47.7% at λ = 1.55 µm for a 300 nm LNOI platform (Case B1 as listed in Table 1), where the grating is formed by completely etching through the whole 300 nm LN layer. By partially etching (200 nm etching depth) a 400 nm thick LN waveguide, the peak coupling efficiency increases slightly to 53% (Case B2). The coupling efficiency for TM mode (red curves) is also plotted in Fig. 2(a) as comparison. Because of the lower effective index of TM mode in this case, the phase matching condition is not satisfied so that the coupling doesn’t happen. The 1-dB and 3-dB bandwidth are 90 nm and 150 nm in Case B1 and 70 and 110 nm in Case B2, respectively, which are much wider than 1D uniform gratings on silicon and a little bit wider than silicon nitride grating coupler. The designed parameters based on basic LNOI platform are listed in Table 1, in which te is etching depth and other parameters have the same definitions as those in Fig. 1(b). Note the fiber position is defined as the distance between the center of the fiber core and the front end of the grating in the x axis. It influences the overlap between the power out of the grating and the fiber, which influences the coupling efficiency.
Fig. 2. Simulated coupling efficiency for (a) basic LNOI grating structure and (b) hybrid Si/LNOI grating structure without buried oxide layer (tox = 0), where blue curves represent TE mode, red curves represent TM mode, respectively. Case B1 represent grating on the 300 nm full-etched LNOI platform, while Case B2 represent grating on the 400 nm LNOI platform with 200 nm partially etched depth. The design parameters are summarized in Table 1 for Case B1 and B2 and Table 2 for Case H1, respectively.
Table 1. Key design parameters and performances for basic LNOI grating couplers.
Table 2. Key design parameters and performances for hybrid Si/LNOI grating couplers
Please note that when we select the material parameters for our design, we are based on the following considerations: (a) The layer thicknesses should be optimal for waveguide design and fabrication. We reference common values based on the reported structures and the commercially available wafer structures. (b) The coupling angle and index matching conditions are also considered based on the SOI and other reported platforms for practical implementation and experimental realization.
We then consider the hybrid Si/LNOI coupler design as shown in Fig. 1. We started our design by simply placing a thin Si layer on top of LN layer, i.e., the low index oxide buffer tox=0. An optimized design is shown in Fig. 2(b). A coupling efficiency of 52.6% can be achieved, which shows similar performance as compared to the basic LNOI grating structure shown in Fig. 2(a). The 3-dB bandwidth is 86 nm in this structure, lower than that in directly etching into LNOI waveguide [43]. The corresponding key design parameters are listed in Table 2 as Case H1 listed in Table 2. The thickness of top Si layer is kept constant at 340 nm, which is a relatively common commercial thickness for SOI wafer. As shown in the red curve in Fig. 2(b) which represents TM mode coupling in the structure, it achieves about 20% coupling efficiency as well. Polarization dependent loss in hybrid Si/LNOI structure is smaller than that in basic LNOI grating coupler.
Based on these findings, we explore further the impact of the oxide buffer. A thin layer of low index buffer material such as silicon dioxide can be added between top Si grating layer and LN waveguide layer to further enhance the coupling efficiency. It gives an additional degree of freedom to adjust the coupling efficiencies of TE and TM modes. By changing the buffer oxide thickness, the peak coupling efficiencies at the target wavelength of 1.55 µm for TE and TM modes are plotted in Fig. 3(a). Shown in Fig. 3(b), coupling efficiency of 70% can be achieved with tox = 150 nm. This is an optimized high-efficiency design for TE mode without any bottom reflector such as metal and DBR stacks based on the hybrid Si/LNOI structure. The 3-dB bandwidth decreases to 68 nm in this design. The key design parameters of this optimized structure for TE mode are listed in Table 2 Case H2.
Fig. 3. (a) Oxide buffer impact on the coupling efficiencies for TE and TM modes in the hybrid Si/LNOI structure; (b) Coupling efficiencies for TE and TM modes for a design with optimized high-efficiency design for TE mode in Case H2.
The buffer oxide layer thickness tox gives an additional degree of freedom to achieve polarization-independent grating coupler and can also act as an etching-stop layer in the fabrication process. The thickness of the buried oxide layer affects the coupling strength between the input light and the waveguide modes. It is optimized to find an equilibrium condition which achieves equal coupling efficiency for TE and TM modes. Under the design parameters given in Table 2 Case H3, polarization-independent grating coupler is obtained. As shown in Fig. 4(a), coupling efficiency from waveguide to a SMF fiber for both TE and TM modes are 51%. The polarization dependent loss (PDL), defined as $PDL({\textrm{dB}} )\textrm{ = }10 \times |\lg ({\textrm{TE}/\textrm{TM}} )|$, is within 0.1 dB in the wavelength range of 1537-1587 nm and only 1.5 × 10−3 dB at λ = 1.55 µm, which is shown in Fig. 4(b). Fiber position also plays a role in finding an equilibrium condition between TE and TM modes.
Fig. 4. (a) Coupling efficiencies for TE and TM modes for a polarization-independent coupler design; and (b) Polarization dependent loss (PDL) for the design shown in (a). The design parameters are summarized in Table 2 for Case H3. PDL is within 0.1 dB in the wavelength range of 1537-1587 nm and only 1.5 × 10−3 dB at 1550 nm.
3. Field distribution and transmissivity analysis
To understand the underlying mechanism, we investigated the field distributions in these coupler designs for both TE and TM modes at λ = 1.55 µm. The results are shown in Fig. 5(a-d) for both the basic LNOI grating structure and the hybrid Si/LNOI grating structure, respectively. Figure 5(a) shows the Ez field distribution of TE mode in an infinite periodic grating structure in Case B2 on the basic LNOI platform with trenches etched onto LN waveguide. The simulation source is a simple planewave with a 7° oblique angle. The supported mode for TE polarization in the grating is a fundamental mode. The diffracted power from the incident wave is coupled into the guide waveguide mode because of the phase matching. Corresponding Hz field distribution for TM mode is shown in Fig. 5(b). TM polarized light is diffracted and transmitted through the grating without any confined mode. As for the field distribution in the polarization-independent hybrid Si/LNOI grating structure in Case H3, as shown in Fig. 5(c,d), both TE and TM modes satisfy the phase matching condition and realize the fiber-chip coupling. The simulation source is a planewave with a 12° oblique angle. The supported modes in the grating and waveguide stacks are hybrid modes. It should be noted here that the proposed hybrid coupler scheme can have very different modal overlap properties as compared to the basic coupler design. This is very important in achieving independent control of both TE and TM modes, with engineered PDL values. It should be noted that the TE mode is confined mostly in LN thin film, while the TM mode is distributed in both silicon grating layer and LN layer. The difference is due to the different boundary conditions of TE and TM modes. The thickness of the oxide buffer layer tunes the effective index and the coupling strength. In optimized design for TE mode (Case H2), a thicker tox helps to confine the TE mode better in LN waveguide. An optimized design for TM mode is also included in Table 2 Case H4, in which a thinner tox helps to enhance the coupling strength between silicon grating and LN layer. The coupling efficiency in this case is 60% at 1550 nm, with 1 and 3 dB bandwidths are 51 and 133 nm, respectively. Since the hybrid Si/LNOI structure is more asymmetrical, the difference between the effective indexes for TE and TM mode is smaller. The PDL is intrinsically less than that loss in a basic LN structure.
Fig. 5. Field distribution at λ = 1.55 µm in an infinite 1D periodic grating structure on the (a,b) basic LNOI grating structure in Case B2 and (c,d) polarization-independent hybrid Si/LNOI grating structure in Case H3. (a,b) Ez field of TE mode; (c,d) Hz field of TM mode.
Finally, we investigated loss mechanism by evaluating the transmission and reflection losses. The transmissivity from all directions of the grating for basic partial-etched LNOI grating structure in Case B2 and hybrid Si/LNOI optimized high-efficiency grating for the TE mode in Case H2 are shown in Fig. 6(a) and (b) respectively. Solid lines denote TE mode and dashed lines denote TM mode. Power is coupled from the SMF fiber to the waveguide. Right represents the expected power coupled into the LN waveguide. Transmission represents the power leaked into substrate. Reflection represents power reflected upwards. Left represents the power coupled into the opposite direction from the light propagation direction in the waveguide. The power transmitted to substrate is reduced obviously in hybrid Si/LNOI grating for TE mode than that in the grating directly etching into LN. It shows that an added bottom reflector to this hybrid structure will not provide much improving performance. The coupling efficiency will not be limited by the thickness of oxide buffer layer between substrate and thin film as well. The power leaked into the substrate in TM mode (dashed blue curve in Fig. 6(b)) is also reduced to some extent, which accounts for the higher coupling efficiency than the case in Fig. 6(a). The dominant loss mechanism for the hybrid Si/LNOI platform is the reflection loss, which is different from the high transmission loss in basic LN grating design. Non-uniform grating design can be designed as those reported from SOI platform to improve impedance matching between waveguide and the nearest part of the grating to reduce scattered field, as well as to reduce high order modes in the part of grating which interacts strongly with light [37,38]. The advantages of this hybrid Si/LNOI design compared with grating coupler with DBR [44] include higher flexibility and possibility in material integration, as well as easier and more stable fabrication processes on LNOI platform.
Fig. 6. Transmissivity from all directions of the grating for (a) Basic partial-etched LNOI grating structure in Case B2 and (b) Hybrid Si/LNOI optimized high-efficiency grating for TE mode in Case H2. Solid lines denote TE mode and dashed lines denote TM mode. Power is coupled from the SMF fiber to the waveguide
4. Conclusion
We propose here a hybrid Si/LNOI grating coupling design with a low index oxide buffer layer. The paper concludes different grating coupler designs on the LNOI platform to achieve polarization-independent and high-efficiency single-polarization grating couplers. Basic uniform LNOI grating structure fabricated by directly etching into LN and hybrid Si/LNOI grating structure can both achieve the coupling efficiency of around 50%. Buried oxide buffer layer between the Si grating and LNOI waveguide helps to achieve 1-D polarization-independent grating coupler with the coupling efficiency of 51% for TE and TM modes at λ = 1.55 µm. The polarization dependent loss is within 0.1 dB in the wavelength range of 1537-1587 nm. High-efficiency of 70% for TE mode can be obtained based on hybrid Si/LNOI structure without any bottom reflector. This work provides preconditions for future integrated photonics including active and passive integration as well as sensors based on Mach-Zehnder or common-path interference.
Funding
National Natural Science Foundation of China (51921005); National Key Research and Development Program of China (2017YFB0902701); China Scholarship Council (201906210359).
Disclosures
The authors declare no conflicts of interest.
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