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Abstract
A two-dimensional grating coupler for coupling light between a standard single-mode fiber and ridge waveguides on an X-cut lithium niobate thin-film is designed and demonstrated. Using circular holes for grating cells, simulated coupling losses reach −3.88 dB at 1550 nm and −5.78 dB at 1563 nm with 1-dB bandwidths of 49 nm and 45 nm for P-polarized and S-polarized light inputs, respectively. Experimentally, peak coupling losses of −5.13 dB at 1561 nm and −7.6 dB at 1568 nm are obtained for P-polarized and S-polarized light inputs, respectively, and corresponding 1 dB bandwidths are about 30 nm. An approach to improve the coupling performance of the grating coupler is also proposed using two crossing ellipses as grating cells as well as a bottom metal reflector. The coupling loss and the polarization dependent loss are decreased to around −3.4 dB and 0.44 dB, respectively.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Lithium niobate (LN) is a versatile material for optical integration due to its outstanding properties including wide transparency window, large refractive index, and excellent electro-optic, acousto-optic, nonlinear coefficients [1]. Conventionally, an LN waveguide is fabricated using titanium diffusion or proton exchange. These types of waveguides have weak optical confinement and thus are unsuitable for densely integrated circuits. Recently, a new type of wafer structure, i.e., LN on insulator (LNOI), emerges and draws great interests. By bonding an LN thin-film of a sub-micron thickness on a silica buffer layer, the LNOI structure exhibits a high refractive index contrast and thus can accommodate waveguides with strong optical confinement, as well as a low loss [2]. So far, many photonic devices with excellent performances have been realized, including microring/microdisk resonators [3,4], photonic crystal cavities [5], electro-optic modulators [6,7], frequency comb devices [8], etc.
Although the on-chip functionalities based on LNOI are developing, a low loss optical interface from a fiber to an LNOI waveguide is still challenging. The size of a typical ridge waveguide on LNOI is below one micron, which is much less than the mode field size of a standard single-mode fiber (SMF). Therefore, a high coupling loss may occur due to the large mode mismatch [1,2]. Inverse taper based edge couplers or grating couplers can be employed to solve this problem. Inverse taper couplers facilitate a high coupling efficiency and a wide working wavelength range [9–11]. The best reported coupling loss of such couplers so far reached −1.32 dB [11]. However, the tiny tip end needed in this type of coupler is still challenging to fabricate on LNOI due to non-vertical side-walls resulted from the LN etching [9]. Additionally, inverse taper couplers can be only used at chip edges. Grating couplers, on the other hand, are more versatile considering their advantages of flexible layout, wafer scale testing, and large alignment tolerance. On the LNOI platform discussed here, one dimensional grating couplers (1D GCs) have been extensively studied [12–20], among which the coupling loss of −3.06 dB with a 1-dB bandwidth of 55 nm has been demonstrated so far [15]. It is worth noting that 1D GCs normally work for only one polarization. Yet, two-dimensional grating couplers (2D GCs), considered as two 1D GCs orthogonally positioned, can couple the input light of an arbitrary polarization to its two arms with the same polarization. It is also one key element for building polarization diversity circuits [21]. During recent years, high performance 2D GCs have been developed on the Silicon-on-Insulator (SOI) platform with high coupling efficiencies and low polarization dependent losses (PDLs) [22–27]. Despite its success on SOI, a 2D GC on LNOI is still missing so far.
In this paper, we demonstrate a 2D GC with circular grating patterns on LNOI. Theoretically, optimized peak coupling losses are −3.88 dB at 1550 nm for the P-polarized light input and −5.78 dB at 1563 nm for the S-polarized light input. Their 1-dB coupling bandwidths are 49 nm and 45 nm, respectively. Experimentally, −5.13 dB and −7.6 dB peak coupling losses are demonstrated for the two polarizations, respectively. Measured 1-dB coupling bandwidths are ∼30 nm. Also, we propose an approach to improve the coupling efficiency and PDL performance by using two crossing ellipses as grating cells and a bottom metal reflector on the LNOI wafer.
2. Design
The schematic structure of the proposed 2D GC and its cross-sectional view are presented in Fig. 1. The LNOI wafer structure consists of an LN slab, a buried oxide (BOX) layer, and a silicon substrate. Waveguides and patterns of grating scatters, which are arranged in square, are etched in the LN slab. Two linear tapers are used to connect the respective ports of the 2D GC to a single mode LN ridge waveguide. The symmetric plane of the present structure is also indicated in Fig. 1(a). A fiber is placed on top of the GC within the symmetry plane. It is tilted for −10 degrees with respect to the normal direction of the chip surface. Two polarization states of the input light, i.e., P-polarization (electric field parallel to the symmetry plane) and S-polarization (electric field perpendicular to the symmetry plane), are used to evaluate the coupling efficiency and the PDL of the GC. Here, the PDL is defined as the absolute value of the difference between the coupling spectra of P-polarized and S-polarized light inputs. As shown in Fig. 1(b), the LNOI wafer that we adopted here has a t = 400 nm thick LN layer on top of a b = 3 µm thick BOX layer. Circular grating scatters have a period of p and a diameter of d, measured at the central depth of the hole. Their etching depth e are the same as that of the LN ridge waveguides. The side wall angle (SWA) of all etched LN patterns are assumed 60°, which is a typical value from the current fabrication technology.
Fig. 1. (a) Schematic structure of the proposed 2D GC. (b) Cross-sectional view of the grating and the waveguide on the LNOI structure.
A three-dimensional finite-difference time-domain algorithm is employed to optimize the proposed 2D GC. In order to simulate the SMF, a Gaussian beam with a waist diameter of 10.4 µm is input from top of the grating with an incident angle of −10 degrees as discussed above. The coupling efficiency is calculated as the total transmission to the fundamental transverse electrical (TE) mode in the two LN ridge waveguides. The etching depth e is 200 nm, which is one half of the total LN layer thickness. Then, the period p and the diameter d of the grating scatters are the remaining parameters needed to optimize. For convenience, we use duty cycle dc, instead of d, for optimization. Here, dc is defined as the area percentage of the etched hole within one grating period, i.e., dc = $\pi {({d/2} )^2}/{p^2}$. We sweep p and dc while monitoring the coupling efficiency with a P-polarized light input. The results are presented in Fig. 2(a). The optimal position, i.e., p = 849 nm and dc = 0.5 (d = 338 nm), where the transmission reaches the peak, is marked with an asterisk. Figure 2(b) shows the coupling spectra of the optimal structure. Here, the best coupling loss reaches −3.88 dB at 1550 nm for the P-polarized light input and −5.78 dB at 1563 nm for the S-polarized light input. The 1-dB coupling bandwidths are 49 nm and 45 nm, respectively. The PDL is relatively large, i.e., 2.17dB at 1550nm.
Fig. 2. (a) Contour plot of the coupling efficiency at 1550 nm wavelength for the P-polarized light input for various structural parameters. (b) Simulated coupling spectra, as well as PDLs, of the optimized 2D GC for P-polarized and S-polarized light inputs. The values of period p and duty cycle dc are taken from the asterisk position in (a).
The fabrication tolerance of the 2D GC was further investigated. Since the period p of the grating can be defined rather accurately using modern fabrication technologies, we have then varied the hole diameter d, the etching depth e, and the SWA separately on the optimized structure. Simulated coupling spectra are shown in Fig. 3. For sweeping SWA, the volume of each etched grating hole is kept constant. One can find that the peak coupling wavelength are relatively insensitive to the SWA for both polarizations. For the hole diameter, ${\pm} $60 nm variations in d would result in ${\mp} $13 nm shifts of the peak coupling wavelength. The peak coupling efficiency is however insensitive to Δd. The etching depth, on the other hand, poses a greater impact on both coupling efficiency and wavelength. The peak coupling loss varies for 0.62 dB for the P-polarized input and 0.7 dB for the S-polarized input, when e changes for 60 nm. The peak coupling efficiency tends to increase when the grating holes are etched more deeply. This implies that 200 nm is not the optimal etching depth for the present grating coupler. Yet, in order to be compatible to other devices on LNOI, such as modulators which are normally etched by one half of the total LN thickness [7], e = t/2 = 200 nm is still adopted throughout the paper.
Fig. 3. Impacts of the variations in (a) hole diameter d, (b) etching depth e, and (c) SWA on the coupling loss.
In the previous design, the coupling spectra of the two polarizations are different in both peak coupling losses and wavelengths, which leads to large PDLs. In our earlier work for a 2D GC on SOI [27], we have proven that central wavelengths of the two coupling spectra can be made coincide by using two crossing ellipses as grating cells. This approach is also applied in the present structure as shown in Fig. 4(a). Through a series of simulations, optimized parameters in this type of structure are found to be ${r_L}$ = 972 nm, ${r_S}$ = 476 nm, and p = 845 nm. Here, we further use a gold mirror just above the silicon substrate to improve the coupling efficiency. The BOX layer thickness b was thereby swept, and the resulting coupling efficiencies for the two polarizations at 1550 nm wavelength are plotted in Fig. 4(b). As expected, periodic oscillations due to interference are presented. However, the maximal values and the corresponding BOX layer thicknesses are different for P-polarized and S-polarized light inputs. Nevertheless, we can choose a suitable b for a better PDL without largely compromising the coupling efficiency. Here, b = 2.58 µm is adopted, and the full coupling spectra, as well as PDLs, in this case is shown in Fig. 4(c). As compared to the structure in Fig. 1, the PDLs in the present structure are greatly improved to < 0.44 dB within the entire simulated wavelength band. At the same time, peak coupling losses are also improved to −3.36 dB and −3.42 dB for P-polarized and S-polarized light inputs, respectively.
Fig. 4. (a) Schematic structure of the 2D GC with two crossing ellipses as grating cells. (b) Simulated coupling efficiencies of P-polarized and S-polarized light inputs as a function of the BOX layer thickness b at 1550 nm wavelength, and maximal PDLs within the 1-dB coupling bandwidth. (c) Coupling losses of the two polarizations and the corresponding PDLs at b = 2.58 µm.
3. Fabrication and measurement
Due to the wafer available at hand, the first 2D GC with circular grating patterns without the gold reflection mirror was fabricated. the LNOI wafer with t = 400 nm and b = 3 µm was purchased from Jinan Jingzheng Electronics Co., Ltd. The grating patterns, as well as the ridge waveguides, were defined in an ma-N 2403 resist layer on top of the LNOI chip using electron beam lithography. Inductively-coupled-plasma reactive-ion etching (RIE) process was used to transfer the resist pattern to the LN layer. Here, the grating and waveguide structures were fabricated using one etching step. Optical and scan-electron microscope pictures of one finished sample are displayed in Fig. 5. Two linear tapers of 250 µm long are attached to the 2D GC for adiabatic coupling to two single-mode LN ridge waveguides of 1.5 µm width. At the other ends, two 1D GCs for TE mode were prepared for light out-coupling.
Fig. 5. (a) Optical microscope picture of the fabricated structure. Scan-electron microscope pictures of (b) 2D GC and (c) zoom-in view of grating scatters.
A broad-band super luminescent diode and an optical spectrum analyzer were used to test the spectral responses of fabricated devices. The response of the out-coupling 1D GC was first measured using a structure with two such 1D GCs connected back to back. For measuring the 2D GC, the polarization state of the input light was adjusted to either P or S polarization. In order to count the final coupling efficiency of a 2D GC, the light powers measured from the two out-coupling 1D GCs were summed after normalizing out the coupling loss of the 1D GC. The LN ridge waveguides connecting the grating couplers were considered lossless as the measured propagation losses are small, i.e., ∼1 dB/cm [6]. Figure 6 shows measured coupling spectra of a fabricated 2D GC. The best coupling loss of −5.13 dB is obtained for the P-polarized light input at 1561 nm wavelength, and −7.6 dB for the S-polarized light input at 1568 nm. Their 1-dB bandwidths are 32 nm and 29 nm, respectively. Compared to simulation results shown in the same figure, the peak coupling wavelengths of the measured spectra exhibit red shifts. The peak coupling losses are also higher than the simulated ones by 1.25 dB and 1.82 dB for P-polarized and S-polarized light inputs, respectively. According to Fig. 3(b), these deviations probably resulted from the inadequate etching depth of the grating holes as compared to that of the waveguides due to, e.g., the RIE lag effect.
Fig. 6. (a) Measured and simulated coupling spectra of the 2D GC for P-polarized and S-polarized light inputs and (b) corresponding PDLs.
4. Conclusion
We have designed and demonstrated a 2D GC on the LNOI platform by directly etching the LN layer to form the grating and the waveguides. Peak coupling losses of −3.88 dB at 1550 nm wavelength for the P-polarized light input and −5.78 dB at 1563 nm for the S-polarized light input are obtained theoretically for an optimized structure using circular grating scatters. Corresponding 1-dB bandwidths are 49 nm and 45 nm, respectively. Experimentally, the fabricated sample exhibits peak coupling losses of −5.13 dB and −7.6 dB for the two polarization, respectively, with slightly red-shifted peak coupling wavelengths. The measured 1-dB bandwidths are ∼30 nm. We have also proposed an approach to improve the coupling efficiency as well as PDLs by using two crossing ellipses as grating cells and a bottom metal reflector on an LNOI structure with an optimized BOX layer thickness. This kind of LNOI wafer is also commercially available. With this approach, the coupling loss and the PDL have been decreased to around −3.4 dB and 0.44 dB, respectively.
Funding
National Major Research and Development Program (2019YFB1803902); National Natural Science Foundation of China (61675069); Science and Technology Planning Project of Guangdong Province (2019A050510039).
Disclosures
The authors declare no conflicts of interest.
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