Ultra-broadband and low-loss edge coupler for highly efficient second harmonic generation in thin-film lithium niobate

Abstract. Thin-film lithium niobate is a promising material platform for integrated nonlinear photonics, due to its high refractive index contrast with the excellent optical properties. However, the high refractive index contrast and correspondingly small mode field diameter limit the attainable coupling between the waveguide and fiber. In second harmonic generation processes, lack of efficient fiber-chip coupling schemes covering both the fundamental and second harmonic wavelengths has greatly limited the overall efficiency. We design and fabricate an ultra-broadband tri-layer edge coupler with a high coupling efficiency. The coupler allows efficient coupling of 1  dB  /  facet at 1550 nm and 3  dB  /  facet at 775 nm. This enables us to achieve an ultrahigh overall second harmonic generation normalized efficiency (fiber-to-fiber) of 1027  %    W  −  1 cm  −  2 (on-chip second harmonic efficiency ∼3256  %    W  −  1 cm  −  2) in a 5-mm-long periodically-poled lithium niobate waveguide, which is two to three orders of magnitude higher than that in state-of-the-art devices.


Introduction
Lithium niobate is an ideal material for nonlinear photonics due to its exceptional nonlinear-optic properties, wide transparency range, and ferroelectric properties. 1,2 Periodically poled lithium niobate (PPLN) waveguides, where the periodic domain inversion allows for a quasi-phase-matched (QPM) second-order nonlinear [χ ð2Þ ] process, have been widely used in wavelength conversion, 3,4 optical parametric oscillation, [5][6][7][8] photon pair generation, [9][10][11][12] and supercontinuum generation. [13][14][15] As the strength of the nonlinear interaction is proportional to the optical intensity, the employment of high-contrast waveguides with strong optical confinement can greatly improve nonlinear optical efficiency. Unfortunately, the legacy PPLN waveguides are based on weakly confined waveguides with index contrasts of ∼0.02, leading to low nonlinear interaction strengths. Therefore, the traditional PPLN device requires a long interaction length for high conversion efficiency, making it difficult for large-scale photonic integrated circuits. In recent years, thin-film lithium niobate (TFLN) has emerged as an attractive platform for compact and high-performance optical modulators [16][17][18] and χ ð2Þ nonlinear optical devices [19][20][21][22][23][24] due to the high refractive index contrast (Δn ∼ 0.7) offered by TFLN waveguides. TFLN-based PPLN devices have been demonstrated that outperform their legacy counterparts in both nonlinear optical efficiency and device footprint. [25][26][27][28] For instance, the efficiency of second harmonic generation (SHG) has been improved over 20 times in TFLN-based PPLN devices. 26 However, TFLN-based PPLN devices face a major challenge of how to achieve efficient and broadband off-chip coupling. For example, in Ref. 29 an onchip SHG conversion efficiency as high as 3757% W −1 cm −2 was achieved in a 5-mm PPLN waveguide, but the collected second harmonic (∼780 nm) power is only several μW when the input pump (∼1560 nm) power is nearly about 100 mW due to the lack of a well-designed coupling mechanism. For SHG, an ideal device requires an efficient coupling scheme for both the near-infrared (∼1550 nm) and near-visible (∼775 nm) bands. Recently, several off-chip coupling schemes based on an edge coupler have been demonstrated for efficient coupling at 1550 nm. [30][31][32] Nonetheless, an efficient edge coupler that can cover both near-infrared (∼1550 nm) and near-visible (∼775 nm) has not been reported yet.
In this paper, we propose and demonstrate an efficient and ultra-broadband edge coupler in an x-cut TFLN platform for the fundamental transverse electric (TE 00 ) mode by employing a low-index SiO 2 waveguide and a tri-layer TFLN waveguide taper structure. The measured off-chip coupling loss for 1550 and 775 nm is 1 and 3 dB∕facet, respectively. In addition, we demonstrate a highly efficient SHG in a TFLN-based PPLN waveguide with this ultra-broadband edge coupler, exhibiting an ultra-high overall SHG normalized efficiency of 1027% W −1 cm −2 and a corresponding on-chip efficiency of 3256% W −1 cm −2 , respectively.

Design and Simulation
The proposed edge coupler is based on an x-cut TFLN platform (from NANOLN) with 360-nm lithium niobate and 2-μm SiO 2 on silicon (Si) substrate. As illustrated schematically in Fig. 1(a), the coupler consists of a suspended SiO 2 waveguide, and a tri-layer spot size converter (SSC) including top-, middle-, and bottom-layer tapers. The light from lensed fiber is first coupled into SiO 2 waveguide, and then transferred to the TFLN rib waveguides through the SSC. The TFLN rib waveguides have a top width of 1.2 μm, a slab thickness of 180 nm, and a rib height of 180 nm. A cross sectional view of the coupler is depicted in Fig. 1(b), where H 1 , H 2 , and H 3 represent the thickness of the bottom-, middle-, and top-layer of the SSC, respectively. Figure 1(c) shows the top-view of the SSC, in which L i , w i (i ¼ 1, 2, 3), and w tip denote the lengths, widths, and tip width of tapers. We note here that the w tip is usually larger than 80 nm, which is limited by the fabrication process.
We select 1550 and 775 nm as the target wavelengths for designing the SiO 2 waveguide and the TFLN SSC. First, we calculate the mode overlap between the SiO 2 waveguide and lensed fiber to optimize the size of the SiO 2 waveguide. The mode field diameters of lensed single-mode optical fibers are set to be 3.5 μm at 1550 nm and 2.5 μm at 775 nm, respectively.
As shown in Fig. 2(a), the size of the SiO 2 waveguide is set to be 4 μm × 4 μm to ensure a mode overlap efficiency of >95% for both wavelengths. Secondly, we carefully designed the SSC, especially the thickness and the tip-width of the bottom layer, to achieve adiabatic mode transition from SiO 2 waveguide to the TFLN rib waveguide. We calculated the coupling loss between the SiO 2 waveguide and LN bottom layer as functions of the tip-width at both 775 nm [ Fig. 2 Fig. 2(c)]. These results are obtained by eigenmode expansion (EME) method by setting a sufficiently long length of LN bottom taper. In Fig. 2(b), for a thickness H 1 > 120 nm, the coupling loss between the SiO 2 waveguide and LN bottom layer is more than 2 dB at 775 nm if the tip-width is larger than 80 nm. This reveals that a double layer SSC with a bottom layer thicknesses of 180 nm cannot achieve an efficient coupling for 775-nm light. Thus, we adopt a TFLN SSC with tri-layer structure. The tip width of the tri-layer structure is 100 nm, and the thicknesses of bottom-, middle-, and top-layer are set as 90, 90, and 180 nm, respectively. With this bottom tip size, the minimum coupling loss from the SiO 2 waveguide to the LN bottom layer can be <0.5 dB at both wavelengths. Additionally, we simulated the lengths of the SSC by EME solver. As shown in Figs. 2(d)-2(f), the taper lengths were designed to be L 1 ∼ 200 μm, L 2 ∼ 100 μm, and L 3 ∼ 100 μm to allow a sufficient mode transfer. All the other parameters of the SSC are given in Table 1. Figure 3(a) shows the transverse electric (TE 00 ) mode field distribution of 1550 and 775 nm at different cross-sections of the designed edge coupler. We can see that the mode fields of both wavelengths are first well-confined in the SiO 2 waveguide [labeled I in Fig. 3(a)]. Subsequently, the 775-and 1550-nm wavelength light is coupled to the TFLN rib waveguide via the bottom-, middle-, and top-layer of SSC. Figures 3(b) and 3(c) show the snapshot of the mode evolution process at both wavelengths. A simulated coupling loss of 0.5 dB∕facet at 1550 nm and 1 dB∕facet at 775 nm is obtained for TE 00 mode after optimization, while that for the transverse magnetic (TM 00 ) mode is more than 3 and 7 dB∕facet at the two wavelengths, showing a polarization dependence of the designed coupler. This is advantageous for nonlinear frequency conversion based on x-cut TFLN platform, where the TE 00 mode is usually used to exploit the maximum nonlinear coefficient d 33 of lithium niobate. For comparison, we calculated the coupling efficiencies of the proposed edge coupler with tri-layer SSC and the traditional coupler with double layer SSC, respectively. As illustrated in Figs. 4(a) and 4(b), these simulated results clearly show that the present edge coupler with tri-layer SSC (the solid curves) features ultra-broadband operation covering both 1550-and 775-nm bands, while the edge coupler with double layer SSC (the dotted curves) only works well in the 1550-nm band.

Fabrication
The device fabrication process is as follows. First, the rib waveguides and the top layer of SSC are defined with HSQ resist by electron-beam lithography (EBL), and transferred to TFLN by using Ar ion in inductively coupled plasma (ICP) system.
The etching depth of waveguide is 180 nm. Then, the middleand bottom-layers of the SCC are prepared by EBL, photolithography, and ICP etching. Figures 5(a), 5(b), and 5(c) show the scanning electron microscope (SEM) pictures of the tips of the bottom-, middle-, and top-layer, respectively. After the fabrication of the SSC, a 2-μm-thick SiO 2 is deposited by plasmaenhanced chemical vapor deposition. The SiO 2 waveguides with supporting arms are patterned by photolithography and etched by CHF 3 ∕Ar chemistry. Finally, the underlying silicon substrate is removed by two-step etching processes. An isotropic etching with SF 6 is first utilized to suspend the SiO 2 waveguide, while a Bosch deep silicon anisotropic dry etch process with SF 6 ∕C 4 F 8 is used to expose the coupling interface between optical fiber and chip. A smooth sidewall and coupling interface are observed in the SEM pictures in Figs. 5(d) and 5(e).

Coupling Efficiency Characterization
The coupling efficiency of the edge coupler is characterized by several 5-mm-long reference waveguides. The propagation losses of the waveguides are 0.3 dB∕cm at 1550 nm and 0.5 dB∕cm at 775 nm. Two lensed fibers and a fiber polarization controller (FPC) are applied for TE 00 mode coupling. We measured the coupling transmission spectra at near-infrared band (1535 to 1565 nm) and near-visible band (765 to 780 nm). Figures 6(a) and 6(b) show the mean values of coupling losses for the TE 00 mode and its standard deviation over six devices on the same wafer. The mean coupling losses are 1 dB∕facet at 1550 nm and ∼3 dB∕facet at 775 nm, respectively. The measured results show lower coupling efficiency than the simulated one, which is predominately originated from the scattering losses of the multi-layer structure and propagation loss of SiO 2 waveguide with supporting arm which are estimated to less than 0.5 dB at 1550 nm and 0.25 dB at 775 nm according to 3D FDTD simulation.

Second Harmonic Generation
We also fabricate 5-mm-long PPLN waveguides along with designed edge coupler for SHG. The width and thickness of the PPLN waveguide is designed as 1.2 μm and 180 nm. According to the QPM condition Λ ¼ λ pump 2ðn SH −n pump Þ , we obtained the poling period for type-0 scheme by simulating the TE 00 mode effective indexes at second harmonic (∼775 nm) and pump (∼1550 nm) wavelength. Taking fabrication error into consideration, several PPLN waveguides with poling periods ranging from 3.10 to 3.18 μm in 10-nm steps are prepared in experiment. The  periodic poling process can be divided into two steps. 21 First, the comb-shaped electrodes consisting of 30-nm NiCr and 40-nm Au were deposited on the x-cut TFLN before the fabrication of waveguide. Then several high-voltage pulses about 300 V were applied, and the period domains were checked via SHG imaging technique using confocal microscope to optimize the pulse duration time and the number of pulses. Figure 7(a) shows the period domain observed by SHG-confocal microscope imaging, which shows the domain-inverted region with duty cycle close to 50:50 (red frame).
The experimental setup for SHG is shown in Fig. 7(b). The pump light from a C-band tunable laser is adjusted to TE 00 via an FPC, and subsequently coupled from the lensed fiber into the PPLN waveguide utilizing the edge coupler. The generated second harmonic signal is coupled out from the chip by a second lensed fiber and finally monitored with a Si photodiode power sensor or an optical spectrum analyzer (OSA). We first investigate the SHG spectral profile of the PPLN waveguide by scanning the pump wavelength. As depicted in Fig. 7(c), the main QPM peak is centered at 1563.1 nm with a full width at halfmaximum bandwidth of 1.5 nm. By fixing the pump wavelength at 1563.1 nm and gradually increasing the pump power, we obtain the quadratic power dependence of the second harmonic wave on the pump light [see Fig. 7(d)] and a slope of 814% W −1 is extracted by linearly fitting the experimental data. The normalized efficiency is calculated to be 3256% W −1 cm −2 , which is 70% of the theoretical efficiency (∼4600% W −1 cm −2 ). 33 The lower maximum efficiency is mainly caused by the inhomogeneity of the thin-film thickness over the 5-mm waveguide length. 34, 35 We define an overall normalized efficiency  by dividing the collected second harmonic power by the square of the input pump light and the square of the poled waveguide length. By taking account of the off-chip coupling efficiency k pump ∼ 80% (1 dB per facet) at 1563.1 nm and k SH ∼ 50% (3 dB per facet) at 781.55 nm, the overall normalized efficiency is counted to be 1027% W −1 cm −2 according to formula of η on-chip ¼ η overall k SH ·k 2 pump .

Conclusions
We implement an edge coupler featuring low coupling loss and ultra-broad bandwidth. The off-chip efficiency at 755 and 1550 nm is 3 and 1 dB∕facet, respectively. The experimental coupling efficiency of the coupler in the near-visible light band is 3 dB higher than the simulated efficiency of the traditional two-layer structure. Based on this designed coupling scheme, we acquire an ultrahigh overall SHG normalized efficiency of 1027% W −1 cm −2 in a 5-mm-long PPLN waveguide, corresponding to a high on-chip SHG normalized efficiency of 3256% W −1 cm −2 . Compared with the state-of-the-art (Table 2), the overall conversion efficiency of the present PPLN devices is increased by two to three orders of magnitude, benefitting from the improvement of coupling efficiency at the pump light and second harmonic wave. This high-efficiency and ultra-broadband edge coupler greatly reduces the power consumption in the process of nonlinear frequency conversion, showcasing obvious advantages in practical nonlinear applications.