1. Introduction

Lithium niobate (LN) material, due to its excellent electro-optic, acousto-optic, and nonlinear properties, has been widely used in photonic industry for building high performance active and nonlinear devices [1]. Traditionally, bulk LN material and low-optical-confinement waveguides are employed for those devices, which hinders large-scale integration of them. Recently, lithium niobate on insulate (LNOI) wafer structure has drawn great interests [2]. By making a sub-micron thick LN thin film on top of a low refractive index buffer layer, typically SiO₂, the refractive index contrast is largely improved. Together with the high quality patterning processes of the LN material, the propagation loss of a ridge LN waveguide has been pushed down to dB/m scale [3]. Highly-compact, high-performance electro-optic modulators [4,5], nonlinear optical devices [6], etc., have also been demonstrated on the LNOI platform.

Similar to other high-refractive-index-contrast platforms, such as silicon-on-insulator (SOI), the coupling of a sub-micron sized LN waveguide to an ordinary single-mode fiber remains as one challenging issue. To solve this problem, inverse taper based edge couplers [7,8] or vertical grating couplers [9–17] can be employed. Among them, grating couplers have shown advantages of easy fabrication and wafer scale testing. They have been successfully applied on the SOI platform [18]. On the LNOI platform discussed here, mainly two types of grating couplers have been introduced. One is to directly pattern the LN waveguide to form the grating [9–14], just as that on SOI. However, the refractive index of LN is much lower than that of silicon. In order to obtain a suitable grating strength, high-aspect-ratio deep etching of LN is required, which is still technically challenging. The best coupling efficiency demonstrated so far reaches 20.4% on an ordinary LNOI wafer [13] and 44.6% with a bottom metal reflector [11]. Another approach relies on deposited silicon gratings on top of the LN waveguide [15–17]. Theoretically, <−2dB coupling loss can be obtained using this kind of grating coupler [17]. However, the silicon material prepared using conventional physical deposition processes, such as sputtering or evaporation, gives high optical losses [15]. High-quality amorphous silicon (α-Si) by low temperature plasma enhanced chemical vapor deposition (PECVD) is a better choice for making gratings over the LNOI structure [16], but the stability of α-Si is still questioning. Additionally, the etching of the silicon layer after deposition in this case needs to be carefully tuned in order not to attack the LN structure beneath [16,17].

In this paper, we demonstrate a novel type of grating couplers for thin film LN waveguide, where the grating consists of thin Au stripes fabricated with a standard lift-off process. Such a metal based grating coupler has been proved feasible on the SOI platform [19]. The optimized grating coupler exhibits a coupling efficiency of 50.4% theoretically on an LNOI structure of a 400 nm thick LN layer. Experimentally, the coupling efficiencies reach 43.8% and 33.7% for 400 nm and 600 nm thick LN layers, respectively.

2. Design

The schematic structure of the proposed metal based grating coupler is presented in Fig. 1. The LNOI wafer consists of a thin LN layer with a thickness of t and a buried oxide layer (BOX) with a thickness of b. The silicon substrate is assumed infinitely thick. A uniform Au grating with a period of p, a line width of w, and a thickness of m is made directly on top of the LN waveguide. A SiO₂ over-cladding layer with a thickness of o is also considered. A cleaved ordinary single-mode fiber with a mode field diameter of 10.4 µm is mounted above the grating coupler. The angle between the fiber and the vertical direction to the chip is fixed at 8°. The Au stripe here, acting as a cladding material for the LN waveguide, would render a local change in the effective refractive index for the propagating mode. This forms effectively a grating for the optical mode. Since the thickness of the Au layer is thin, the optical losses due to metal absorption is low. An efficient grating coupler can be obtained using this type of structure.

 

Fig. 1. (a) Three-dimensional schematic structure of the proposed metal based grating coupler on the LNOI platform. (b) Two-dimensional structure used for simulations showing all the parameters of the grating and the LNOI wafer structure.

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The coupling efficiency from the fundamental transverse-electrical (TE) mode in the LN slab waveguide to the single-mode fiber is analyzed using a two-dimensional finite-difference time-domain algorithm as shown in Fig. 1(b). The structural parameters discussed above are optimized for the highest coupling efficiency at 1.55 µm wavelength using a particle swarm optimization algorithm. The optimal structural parameters and the coupling performances are listed in Table 1 for some common LN layer thicknesses t. The detailed coupling and back reflection spectra of the structures in Table 1 are presented in Fig. 2. As expected, the coupling spectra exhibit typical Gaussian-like profiles centered at 1.55 µm. One can also find that the thickness t of the LN layer affects the coupling efficiency. This is due to the fact that the grating strength, and hence the collection efficiency of the diffracted light by the fiber, is highly related to t. The best coupling is achieved when t = 400 nm with a peak coupling efficiency of 50.4%. The 1-dB coupling bandwidths for all cases shown in Fig. 2(a) are about 50 nm. The back reflections in the LN waveguide are all small, i.e., less than 1%, within the 1-dB coupling bandwidths as also shown in Fig. 2(a).

 

Fig. 2. (a) Simulated coupling efficiency and back reflection spectra of the optimized grating structures. The parameters are adopted from Table 1. (b) Peak coupling wavelengths with respect to the changes Δp of the grating period from its optimal value. The extracted shift rates are also marked using the corresponding colors.

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Table 1. Performances and structural parameters of proposed grating couplers optimized for 1.55µm.

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Grating performances related to variations of the structural parameters are further studied in details. Peak coupling wavelengths with respect to changes Δp of the grating period from its optimal value in Table 1 are presented in Fig. 2(b) for different LN layer thicknesses. Linear fits to the calculated data indicate that the shift rates are about 0.8–1.0 nm/nm. The influence of the metal layer thickness m and the line width w is also investigated. Results for t = 400 nm and t = 600 nm are shown in Fig. 3. For a relatively large variation of 100 nm in the line width w, peak coupling wavelengths shift within ±5 nm range, and peak coupling efficiencies only drop within 5% for both two cases. As for the metal thickness t, nearly 100% increase would only result in a decrease of the coupling efficiency for about 10%, and peak coupling wavelengths are almost unaffected. Since the grating period p can be rather accurately defined using any modern fabrication technologies, and the variation in w and t discussed in Fig. 3 can also be easily met, we can conclude that the proposed grating coupler shows an excellent fabrication tolerance.

 

Fig. 3. Influence of the grating line width w and thickness m on the coupling spectra for (a) 400 nm thick LN layer and (b) 600 nm thick LN layer. Parameters for each curve are listed with the corresponding color. The rest of parameters is adopted from Table 1.

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The thicknesses of the BOX layer b and the over-cladding layer o would also affect the coupling efficiency [17]. When properly designed, a constructive interference can be obtained for light diffracted upward towards the fiber. Theoretically, when scanning these thicknesses, periodic oscillations in the coupling efficiency would be obtained. Numerical analyses shown in Fig. 4 confirms that multiple optimal values indeed exist for these two parameters. The figures listed in Table 1 is just one of those optima and can be adapted according to, e.g., Fig. 4, without affecting the grating performances.

 

Fig. 4. Influence of (a) over-cladding thickness o and (b) BOX layer thickness b on the coupling for 400 nm and 600 nm thick LN layers. Parameters, except listed, are adopted from Table 1.

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3. Fabrication and measurement

The designed grating structures, as well as thin-film LN ridge waveguides, were fabricated. Two types of LNOI wafer structures with LN layer thicknesses t of 400 nm and 600 nm were used, which are commercially available from Jinan Jingzheng Electronics Co., Ltd. The BOX layer thicknesses b for these two structures are 3 µm and 1.5 µm, respectively, which are not exactly the optimal values according to Table 1 and Fig. 4, but not far away from those. Ultra-violate contact photo-lithography and dry etching processes were employed to fabricate the ridge waveguide in the thin-film LN. The etching depth for both LNOI structures is t/2. The testing structure consists of a short straight waveguide of 1.5 µm width and two linear tapers expanding the waveguide to a width of 13 µm, which matches to the size of the fiber mode. The linear taper is designed so that the mode transition loss is negligible. The grating pattern was then defined directly on top of the 13 µm wide waveguide sections using electron beam lithography. Metal films consisting of Ti/Au/Ti were deposited on the chip using electron beam evaporation. The Ti layers here has a thickness of 2 nm, and are used for increasing the adhesion of Au. The grating structure was subsequently formed with a lift-off process. Finally, An SiO₂ over-cladding was prepared using PECVD. Some pictures of fabricated samples are displayed in Fig. 5.

 

Fig. 5. Optical and scan-electron microscope pictures of a fabricated grating testing structure just before the SiO₂ over-cladding deposition.

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Finished samples were then characterized. The transmission spectrum of the testing structure was measured using a broad-band super luminescent diode (SLD) and an optical spectrum analyzer. The light was led to the grating couplers with two cleaved single-mode fibers. Before the input grating, a polarization controller was used to align the input polarization to TE. No polarizer was included after the output grating. Due to the moderate refractive index contrast of the LNOI structure, the polarization dependence of the designed grating coupler structure is not as obvious as those on, e.g., SOI [17]. However, in the fabricated testing structure, linear tapers and a narrow straight waveguide section are incorporated. During the linear tapers, the coupled transverse magnetic mode, which can be considered as stray light, will be converted to high-order TE modes due to mode hybridization [20], and then dissipated in the narrow straight waveguide. Therefore, the alignment of the input polarization can be achieved by maximizing the transmission when adjusting the polarization controller.

Figure 6 shows the measured coupling efficiency spectra of fabricated grating couplers on two LNOI structures of t = 600 nm, b = 1.5 µm and t = 400 nm, b = 3 µm. Structural parameters, except ones listed in the figure, are adopted from Table 1. The wavelength response of the SLD, as well as the measurement setup, was first measured as shown in Fig. 6(a). The coupling efficiency of a single grating coupler was then deducted as half of the normalized spectrum in dB scale. Here, the insertion losses of the tapers and the straight waveguide were considered zero. The coupling efficiency of the grating coupler is therefore slightly under estimated. Measured peak coupler efficiencies (coupling losses) for the fabricated grating couplers reach 33.7% (−4.72 dB) and 43.8% (−3.58 dB) for the two LNOI structures, respectively, centered at 1.55 µm wavelength. The 1-dB coupling bandwidth is larger than 50 nm for both cases. In Fig. 6, simulated coupling spectra are also plotted, which are matched well to the measurement results. In Fig. 6(c), the measured and simulated spectra for three gratings with slightly different grating periods p are also presented. The extracted shift rate of the peak coupling wavelength with respect to Δp is 1.0 nm/nm, which is consistent with simulation results.

 

Fig. 6. Measured and simulated coupling spectra of grating couplers on two LNOI structures, i.e., (a) & (b) t = 600 nm, b = 1.5 µm and (c) t = 400 nm, b = 3 µm. Parameters, except listed, are adopted from Table 1. The raw data for the fiber-to-fiber transmission used for normalization and the transmission of the grating testing structure is shown in (a).

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4. Conclusion

We have demonstrated an efficiency grating coupler on the LNOI platform, which is made of Au stripes and can be fabricated easily using a standard lift-off process. Theoretically, the proposed grating coupler has shown a coupling efficiency of 50.4% at 1.55 µm wavelength. An 1-dB coupling efficiency of 58 nm has also been obtained. Simulations have also proven that the proposed grating coupler exhibits good fabrication tolerances. Experimentally, peak coupler efficiencies for optimized grating couplers reach 43.8% and 33.7% for two LNOI structures with t = 400 nm and t = 600 nm, respectively. The present grating coupler can be readily used for light access for quick and on-wafer characterization of thin-film LN devices. The coupling efficiency could be improved by using a bottom reflective mirror and a proper apodization of the grating lines [13].