We demonstrate hybrid amorphous silicon uniform grating couplers for efficient coupling between the standard single-mode fiber and sub-micron lithium niobate waveguides. The grating couplers exhibit coupling efficiency of −3.06 dB and 1-dB bandwidth of 55 nm. The amorphous silicon grating couplers can also provide a universal building block applicable to other photonic platforms such as silicon nitride waveguides, whose moderate refractive index values prevent high efficiency grating couplers to be fabricated in the native waveguide.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Lithium niobate (LN) has excellent electro-optic (r33) and optical nonlinear (d33) properties, with very low optical absorption in a wide wavelength range. Its refractive index (~2.2) allows to form relatively strongly confined waveguides with conventional cladding material such as silica. Novel technologies including ion slicing and wafer bonding enable production of LN films on insulator (LNOI) substrates for fabricating waveguide devices with loss value of less than 0.03 dB/cm , and high quality factor microcavities of with Q factor exceeding 106 [1–3]. The sub-micron sized waveguide is an order of magnitude smaller than the conventional diffusion-type LN waveguides, enabling high performance Mach-Zehnder modulators (MZM) with low drive voltage  and ultra-board bandwidth up to 100 GHz . Optical fields are highly confined in these sub-micron waveguides and structures, facilitating enhanced optical field intensity and low threshold for nonlinear optics applications such as second-harmonic generations [6–8].
Conventional edge coupling between standard single mode fiber (SMF) and LN waveguides has relatively high coupling loss , mainly due to the optical mode mismatch between the fiber and waveguide. The best edge-coupling efficiency demonstrated by lensed fiber to LN ridge waveguide is about −5 dB per facet [4,6]. Coupling sub-micron sized LN waveguides with flat-end SMF posts even higher challenges, and no such edge-coupling is reported yet.
Grating couplers, without dicing, cleaving and polishing processes , are favorable couplers between SMF and sub-micron-sized waveguides. They have achieved extensive successes in silicon photonics on silicon on insulator (SOI) substrates [10–17]. The highest coupling efficiency of silicon-only apodized grating coupler (AGC) is ~−1 dΒ [16,17]. In the case of sub-micron sized LN waveguides, uniform grating couplers directly etched on waveguide [18–20] were recently demonstrated with −10-dB coupling efficiencies. The coupling efficiency can be improved to −6.9 dB by employing back reflectors [21,22]. The coupling efficiencies are much lower than those in SOI (~−3 dB), mainly due to the LN has lower refractive index (~2.2) than Si (~3.5), and etching LN grating teeth with vertical profile is very difficult.
Here, we demonstrate a hybrid grating coupler on LN thin film using amorphous silicon (a-Si) as grating teeth. The a-Si grating has two-fold benefits in achieving high coupling efficiency: firstly, a-Si has higher refractive index [23–26]; secondly the etch profile of the a-Si is sharper than that of the LN etch. Similar grating structures has been demonstrated using focused ion beam etching method, realizing SMF to Si/LN rib-loaded waveguide coupling with efficiency of −18 dB .
2. Design and simulation
Figure 1 shows the schematic of our device structure. We use an x-cut lithium niobate on insulator on silicon substrate (LNOIXSi) chip as the substrate. The orientation of + z axis is out of the paper, and the light travels along the y axis. We define T and Λ as the teeth width and period of the a-Si grating, da-Si as the height of the a-Si grating and as the thickness of the silica buffer layer between a-Si and LN thin film. The light coming out of the grating will be coupled into a SMF with an angle of θ with respect to the normal direction of the chip surface. The grating coupler is coated with silica cladding.
The governing equation for the phase-matching condition of a grating coupler is :28], was applied to optimize the coupling efficiency and 1-dB bandwidth. The teeth width T, period Λ, and height of a-Si grating da-Si, and silica buffer layer were set as variables to obtain a uniform grating for transverse-electric (TE) mode with optimized efficiency-bandwidth product. A while loop is employed in the iterative optimization, and in each iteration, all parameters are tuned by 5 nm. The iteration stops when the optimal results are reached. The achieved optimized parameters are listed in Table 1. The thickness of silica cladding dclad is set to be 1.5 μm.
Figure 2 shows the distribution of Ez of the TE polarization. The maximum simulated coupling efficiency is − 2.7dB at 1553.5 nm, taking into account the overlap integral between upward electric filed and the Gaussian profile that approximates the fiber mode.
3. Device fabrication and measurement
Our devices were fabricated on 14 mm × 12 mm x-cut LNOIXSi chips diced from a 3-inch wafer from NANOLN, with LN film of 600 nm thickness bonded on top of a 2-μm silica buffer layer on silicon substrate. A brief description of the grating coupler process flow is shown in Fig. 3(a). The LN waveguide patterns were defined by E-beam lithography (EBL) and subsequently etched with an optimized argon plasma in an inductively coupled plasma (ICP) etching system. The details of the process (from the step1 to 4 in Fig. 3(a)) was described in our previous work . The etch depth of LN is ~240 nm. After LN sidewall cleaning and mask removal, a 90-nm-thick silica layer were deposited by inductively coupled plasma chemical vapor deposition (ICP-CVD) to improve the adhesion of a-Si growth. Afterwards, the 220-nm-thick a-Si layer was grown with SiH4/Ar at 300°C using a plasma enhanced CVD process. The refractive index of a-Si is 3.467 at 1550 nm measured by ellipsometer. Hydrogen silsesquioxane (HSQ, FOX-16 by Dow Corning) is spin-coated on the a-Si for EBL. The grating patterns are transferred into the a-Si layer using ICP etching with an HBr chemistry. Figure 3(b) shows the scanning electron microscope (SEM) image of the hybrid a-Si grating coupler, which is sitting on the end of the LN adiabatic taper. The zoom-in image Fig. 3(c) reveals feature of the a-Si grating coupler with vertical and smooth sidewalls but a relatively rough top surface. Finally, the devices are coated with a 1.5-μm thick silica top cladding.
4. Measurement results and discussions
Figures 4(a) and 4(b) show the microscope images taken from downward and horizontal cameras when we measure the device using conventional single-mode fiber as input and output. The length of the adiabatic taper is 500 μm. Figure 4(c) shows the spectrum measurement setup. We used a high-precision external cavity tunable laser (Agilent 81600B) as the light source in 1550-nm range. The input polarization was adjusted by a fiber polarization controller. The position and angle of the two coupling fibers were aligned by two six-axis stages and monitored by the downward and horizontal microscopes. The estimated fiber angle to the chip normal is about 12° in Fig. 4(b). The output power was detected by an optical power meter. The tunable laser source and the power meter were connected by a computer to enable wavelength scanning and power data recording. The measurement setup is calibrated via Agilent lightwave measurement system (Agilent 8164B) and its supporting plug-in (insertion loss measurement). The output power of tunable laser is 0 dB in the range of 1500 nm to 1600 nm. The calibration began with scanning the insertion loss of SMF 1 and polarization controller. The insertion loss of the SMF 1 and polarization controller was in the range of −0.2 dB to −1.8 dB, and set as the back-ground reference. Then we obtained a flatten spectrum with a fluctuation of ± 0.015dB in the whole scanning wavelength. The insertion loss of each SMFs 2 and 3 is ~−0.3 dB, which is directly measured by power meter. Therefore, all measured losses have been calibrated to exclude the insertion loss of the fibers and polarization controller.
Figure 5(a) shows the measured spectrum of samples with four different waveguide length. And Fig. 5(b) shows the measured fiber-to-fiber loss at 1552.5 nm. The fitted line intercept at approximately −6.13 dB, representing the total coupling loss of both input and output grating couplers. The single grating coupling efficiency is thus −3.06 dB. The slope of the fitted line indicates that the waveguide propagation loss is 4.3 dB/cm.
Figure 6 shows the wavelength dependency of the grating coupler. The measured transmission spectrum with waveguide widths of 1 μm (solid line) is compared with the simulation result (dashed line). Table 2 summarizes key values between the experiment and the simulation. The measured coupling loss of −3.06 dB is approximately 0.3 dB lower than the simulated one. The measured 1-dB bandwidth is 55 nm and comparable to the simulated bandwidth of 59 nm. The incident angle to achieve best coupling in the experiment is about 12 degree, slightly deviates from the simulated 13.5 degree. Material loss, fabrication deviations and the coupling stage deviations could account for the discrepancies between the measurement and the simulation. It is worth mentioning that the optical loss of the adiabatic taper attributes to the loss term of grating coupler. Eigenmode expansion (EME) method proved that the optical loss of the adiabatic taper is −0.066 dB. That means the coupling efficiency of our grating coupler may be higher than −3 dB.
Table 3 compares our work and demonstrated grating couplers on LN thin film in previous literatures. Our hybrid coupler sets a record of the demonstrated grating coupler on thin-film LN waveguide with an improvement of >3 dB over the nearest reported data and also wider bandwidth. The coupling efficiency of −3.06 dB is also quite comparable to demonstrated uniform silicon grating couplers in silicon photonic waveguide platforms .
We designed and demonstrated hybrid a-Si uniform grating couplers for TE mode on LN thin film ridge waveguides. Coupling efficiency up to −3.06 dB and 1-dB bandwidth of 55 nm have been experimentally obtained, significantly improving the state-of-art in fiber-waveguide coupling of LN thin film devices. The grating performance can be further improved by optimizing the thickness of the silica cladding and by employing nonuniform AGC structures that can also minimize the Fabry-Perot reflections [15–17,30]. The demonstrated a-Si grating couplers pave the way to LN photonic chips with low fiber-to-fiber insertion loss. Entirely compatible to standard fabrication flow with no high temperature (400 °C or above) processing, such a-Si grating coupler has the potential to become a universal building block applicable to other photonic platforms such as silicon nitride waveguides where the modest waveguide refractive index prevents the fabrication of high coupling efficiency grating coupler directly in the waveguide material.
National Natural Science Foundation of China (NSFC) (U1701661, 61490715); Science and Technology Program of Guangzhou (201707020017); Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01X121).
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