Highly-efficient thin film LiNbO3 surface couplers connected by ridge-waveguide subwavelength gratings

The ridge waveguide integrated grating couplers (GCs) in lithium niobate on insulator (LiNbO3, LNOI) were designed, fabricated and characterized. Two ends of the tapered GCs were connected by the subwavelength gratings (SWG) waveguide of a sub-micrometric-diameter, the photonic-wire SWG structure was featured with the profile of side-walls corrugations, and the effect of geometrical dimensions on the output optical response was investigated. All the devices structure patterns for the integrated LNOI GCs could be simultaneously defined by one step of electron-beam lithography, and then easily fabricated by the optimized dry-etching processes, followed by samples surface cleaning. After the fabrication, a low coupling loss of − 5.1 dB/coupler at the telecommunication wavelength of 1561 nm was measured in the best thin-film LiNbO3 (TFLN) surface grating coupler for quasi-transverse-electric (quasi-TE) polarized signals, and a broad 3-dB optical bandwidth of wider than 95 nm was also obtained. The compact components exhibited magnificent performance, and might show the potential functionalities for the TFLN-based integrated optical waveguide devices.


Introduction
Single crystal thin film lithium niobate (TF-LiNbO 3 , TFLN) and the resulting LN-on-insulator (LNOI) wafer, have been emerging as an attractive platform for the applications of high-performance photonic integrated components (PICs), due to LN excellent properties of electro-optic, non-linear optics, high refractive index contrast between photonic waveguide core and its cladding, as well as the available wide optical transparency ranges from 0.4 to 5.2 lm [1][2][3][4]. Among all the light-coupling approaches, unlike the edge couplers, surface grating couplers (GCs) were considered as the fundamental passive components, and had been widely explored for high efficiently interfacing optical fiber to photonic waveguide, especially for those increasing applications in the optical waveguide devices and circuits [5][6][7][8]. For the existing researches focused on the sub-wavelength structured waveguide such as the common types of long-period diffraction gratings, Bragg gratings and short-period subwavelength gratings (SWG), and the dispersion relation between frequency (f) and wavenumber (b) for respective gratings waveguide in the optical integrated devices had been investigated [9][10][11]. It was reported that waveguide SWG can be considered as a low-loss, homogeneous medium as long as the effective transmission wavelength is many times larger than the period of waveguide SWG [12,13]. SWG waveguide used as the Bragg gratings was simulated, in order to effectively suppress the side-lobes of transmission spectrum [14]. The short-period SWG structured in the middle of SiO 2 microfiber was fabricated, and used to reduce the out-of-plane scattering and radiation losses [15]. The light propagation length in the waveguide SWG structure was properly controlled, and then used to manipulate the light mode [16]. Also, the SWG structures fabricated on LNOI had been enormously investigated. For example, the tapered LNOI GCs connected via a few hundreds of micro-meters-length SWG waveguide was theoretically reported, and it revealed that the integrated GCs with sidewalls corrugated SWG geometry could be regarded as the band-rejection filter [17]. In addition, a non-linear LNOI optical element was fabricated by periodically altering the structure dimensions of short-period photonic-wire waveguide [18]. Besides, the long-length LNOI resonator structure integrated with both uniform and chirped SWG waveguide was fabricated to realize the nonlinear light-matter interactions in the fields of modulators [19].
However, the SWG structures embedded in the center of photonic-wire rib waveguide, which could be used as a low-loss optical waveguide, have not been systematically investigated for its applications of light-coupling and propagation on LNOI platform. As we know, the traditional TFLN GCs usually have a high insertion loss (IL). For instance, a minimum loss of -12 dB/coupler for quasi-transverse-electric (quasi-TE) polarization was measured at 1510 nm in the tapered GCs structure with both silica cladding and bend bridged photonic-wire waveguide [20]. Multiple integrated GCs with different lengths of straight waveguide were implemented and studied, and they exhibited a minimum loss of around -9.95 dB/coupler at 1542 nm [21]. A maximum coupling efficiency (CE) of around -7 dB/coupler was experimentally obtained at the wavelength around 1550 nm for vertical coupling integrated GCs, and its hard-etching z-cut TFLN layer was set to be perfectly etched and the device structure and geometrical parameters were ultimately optimized [22]. Noticed that there ever reported a relatively low loss in LNOI GCs, but these GCs based on traditional structure were purposely improved and thus realized the low loss by conducting sufficient optimizations, including adding the oxide upper cladding or refractive index matching materials [20,22], setting the highly-cost and complicated bottom reflector [23,24], employing the curved focused and chirped gratings with fully optimized geometrical parameters [25], requiring more complex structure design and fabrication processes [26], and simultaneously adopting multiple optimizations [27,28]. Meanwhile, the output performance of the existing integrated LNOI GCs were even sensitive to the alignment-error and manufacturing process imperfections, as well as with a narrow 3-dB optical bandwidth, BW [25,26,28]. Therefore, it is still interesting to investigate the lowloss LNOI GCs with easy fabrication process for the purpose of facilitating the large-scale production of PICs.
In our work, the highly-efficient LNOI GCs integrated with waveguide SWG have been fabricated. The integrated GCs were still designed with simple device structure and were of ease manufacturing. All the structures for the integrated TFLN device were monolithically defined in the chip, which could be patterned by one step electron-beam lithography (EBL), followed by dry-etching processes. The light coupling and transmission characteristics of the LNOI GCs for quasi-TE mode have been systematically investigated, and the performance in terms of device structural and optical properties for the fabricated GCs embedded with waveguide SWG have been characterized and analyzed, respectively.

Device design and simulations
The schematic top-view of the whole structure of ridge-waveguide integrated LNOI GCs is plotted, as shown in Fig. 1a. Seen from Fig. 1a, two ends of tapered GCs are bridged by the SWG waveguide structure, as located in the middle zone, L gc is the total length of a single end of straight ridge-waveguide gratings with a nominal width of w gc . L taper is the length of ridge-waveguide taper. K swg is the periodicity of short-length SWG, w swg is the width of a single SWG stripe, then w swg /K swg is denoted as the filling factor of SWG (FF swg ). L swg is the total length of SWG stripes, the bridging structure of SWG ridge-waveguide is designed with periodic width perturbation in z direction, and thus Dw is the variation of width wavy amplitude for SWG waveguide. The enlarged cross-section of a part of ridgewaveguide GCs transmission model is shown in Fig. 1b. K is the periodicity of the diffraction grating stripes, w is the width of a single grating stripe, then w/K is the filling factor of gratings (FF). The total thickness of TFLN device layer is of H = 600 nm, h is the etching depth of ridge waveguide. Seen from Fig. 1b, the injected light source is on the top left, and it is located above the TFLN layer, the optical fiber core is regarded as the source and set to be tilted with an incident angle of h.
For the GCs, the relation of light energy distributions was analyzed. In our case, M1 is the top power monitor used to record the original light diffracted downwards the GC, M2 is the power monitor used to record the light transmission through the left side, M3 is the power monitor used to record the light diffracted downwards and leaked into the bottom substrate, and M4 is the power monitor used to record the light transmission through the right side, as schematically plotted in Fig. 1b. To achieve the single mode light propagation in LN photonic waveguide, the ridge waveguide with a rectangular cross-section profile was firstly considered under the ideal condition, and the resulting electric fields (E y ) distribution was simulated when operated at 1550 nm for quasi-TE mode, as shown in Fig. 1c. Seen from Fig. 1c, it is clear found that the quasi-TE field is effectively confined in the single mode ridgewaveguide with a top width of w wg-top , and it revealed that the light mode also shows a dependence on the waveguide geometry such as the profiles of photonic waveguide sidewalls and surfaces, respectively.
It was reported that the GCs performance experience a dependence on the devices structure design and parameters selections as well as the actual fabrication [27,29], to obtain a maximum CE product for the ridge-waveguide integrated LNOI GCs. The geometry and structure parameters of the proposed devices were fully considered such as the set of deeper etching depth of h for a better light mode confinement, large diffraction gratings period of K, Fig. 1 a Schematic top-view of the proposed ridge waveguide LNOI GCs integrated with the middle waveguide SWG structure, b the cross-section view of the uniform gratings structure interfacing with optical fiber for the light-coupling, and the transmission spectrum recorded by the monitors is also shown, and c the image of simulated quasi-TE fields (E y ) distribution of light mode at the wavelength of 1550 nm for the photonic ridgewaveguide with an ideal rectangular profile and the corresponding duty cycle of FF. As for the effective media theory and gratings diffraction conditions [30][31][32][33], the light-coupling and transmission processes for the whole GCs structure integrated with the waveguide SWG were simulated by a commercial software (from Lumerical, Ansys), as processed by the built-in particle swarm algorithm (PSA), and consequently the geometrical parameters were fully optimized. Based on the existing models as reported in reference [13], our designed waveguide SWG was also firstly equivalent to the ideal structure with a homogeneous dielectric, and thus a uniform waveguide with a rectangular cross-section was built. Then, the relationship between the equivalent effective index of SWG waveguide (n eff ) and the propagation wavelength (N eff ) was expressed below: k eff ¼ k=N eff , where k is the transmitted light wavelength in vacuum. Meanwhile, since the fabricated photonic waveguide was always with trapezoidal cross-section profile, owing to the actual fabrication imperfections, especially for the fine device structures such as the deeply etched waveguide gratings and short-period SWG. For the fabricated waveguide with a practical trapezoidal cross-section, the optical transmission of square wavy shaped sidewalls in the waveguide SWG structure was also characterized, as shown in Fig. 2. Seen from Fig. 2, it is clear found that the simulated light fields propagated along the photonic waveguide in y direction. Besides, we even note that there also existed varied effects of light-matter interactions for different SWG waveguide geometries and dimension sizes, namely, the light mode could be periodically converted in the square wavy SWG region, and consequent used for highly-efficient mode conversion and light transmission.
Based on the existing researches [24,31], we noted that the best optical characteristics of GCs experience a dependence on the structural parameters and the fabrication tolerances, such as the selections of K, h, FF and w, as well as the optical fiber position. Referred to the first-order diffraction of Bragg gratings condition as shown below: N eff À N topÀcladding Á sin h ¼ k=K, where N topÀcladding denoted to the top-cladding refractive index, and it was set of 1 owing to the lack of top cladding and thus to keep consistent with our following experiments. Then, the effect of geometrical dimensions on the optical response was analyzed. To gain an insight into the output performance of the integrated GCs, the self-specified design-intent process of sweeping parameters was also conducted for the separate portion of GCs model, as shown in Fig. 3. Seen from Fig. 3a, it is clear find that there is an additional coupling loss of these GCs when operated at 1550 nm, and an average discrepancy is of around Fig. 2 The zoomed-in view of sidewalls corrugated SWG waveguide embedded in the middle zone of GCs structure (not to scale), and the simulated transverse-electric fields (E y ) distributions at 1550 nm when the light-modes gradually propagated in y direction and changed through the wavy SWG structure 0.36 dB with a 0.011 deviation in FF of the GCs. Also, the simulated CE characteristics show a dependence on the etching depth of h for the case of a 10 nm deviation. Besides, the effect of SiO 2 lower cladding thickness on the CE was also studied, and the scanned result is shown in Fig. 3b. Seen from Fig. 3b, it is obvious to find that the CE curve oscillates between the minimum and maximum value with respect to the buried oxide of SiO 2 thickness (it denoted as the lower cladding). And then, the optimized SiO 2 lower-cladding thickness corresponding to one of the maximum light-coupling was optionally selected in our case, since there also existed the constructive interference of light modes among the part of light power radiated upward at a particular etching-depth of h, which could be partly attributed to the existence of partial light power scattered to the bottom Si substrate and further slightly reflected back by the SiO 2 /Si interface. Besides, as far as we known, there also might exist a part of light power diffracted upward at the top surface of partially etched LN grating-stripes, as referred to the existing references [24,28].
The optical transmission properties for the fiber-to-GC process were systematically investigated, and the result of cross-sectional field distributions (E z ) for TEpolarized optical wave in the simulation region was analyzed, as presented in Fig. 4a. Seen from Fig. 4a, it is easy to find that there is a part of light modes diffracted downwards and further reflected upwards due to the SiO 2 /Si interface, the small input fiber angle of h inc was positive and slant, in order to reduce the backward reflection and realize the effective optical fiber-to-waveguide light coupling [34,35]. It is noted that a part of light modes injected from optical fiber was effectively coupled into the GCs, and then transmitted through the chip and coupled out from the other end of ridge-waveguide GCs. Meanwhile, there also existed the influence of partial light mode radiated downwards and further leaked into the bottom high-refractive index of Si substrate (n Si =3.45), which can be owed to the fact that there is one of the light mode loss sources in the GCs [28]. According to the proposed model structure as shown in Fig. 1, the spectra for the GCs when operated at the telecom wavelength ranges was simulated. The optical response was recorded by these four monitors, and consequently the optical transmissivity were extracted, as plotted in Fig. 4b, respectively. Seen from Fig. 4b, it is clear to see that there is a peak value at 1550 nm for the M4 transmission curve, and correspondingly a minimum value of light leaky for the M3 curve is also obtained in our fully optimized GCs.
After the optimizations, the resulting optimum device geometry and parameters for the proposed GCs integrated with SWG waveguide were obtained, and partially listed below in Table 1. In detail, the design tolerance of the geometrical parameters was set with a 10 nm deviation, and then, K swg was optimized to be of 500 nm, L gc was of 20.8-lm-long on the optical response, and b the CE characteristic as a function of the SiO 2 lower cladding thickness with 10.9-lm-width (w gc ), Dw was of 0.2 lm. Moreover, during the simulations, the linewidth corrections process was also implemented in combination with the consideration of hard-etching nature of LN and thus the multiple transferring errors of device patterns during actual fabrications. The waveguide taper was also fully optimized and used for effective mode adiabatic transition, and then L taper was selected with a fixed lateral length of 200 lm. Besides, in our case, a 500-lm-thick Si substrate was selected and used to support the bonded TFLN wafer. The lower cladding layer of silicon dioxide (SiO 2 ) used as the bonding material in the LNOI wafer, was optionally selected to be of 4.7 lm, as referred to the simulations, since it was thick enough and subsequent the light mode directly leaked into the bottom substrate of high-index Si was negligible.

Experiments
In our experiments, all the ridge-waveguide optical components were simultaneously fabricated on the customized x-cut LNOI wafer, as supplied by Nanoln Company using the mature crystal-ions slicing techniques [1,4]. The brief fabrication process flow of the deeply-etched LNOI integrated GCs is plotted in Fig. 5. Seen from Fig. 5, the LN crystallographic axes (x, y, z) is plotted with the solid blue arrows, the optical axis is in z direction, and the light mode propagates in ? y direction. Firstly, the purchased LNOI wafer was sliced into 1.0 9 1.2 cm 2 segments, and the sample surface was fully cleaned by wet solutions of acetone, isopropanol and distilled water, respectively. Then, the hard mask of amorphous silicon (a-Si) film with around 800-nm-thick was deposited on the TFLN layer by plasma enhanced  chemical vapor deposition (PECVD) tools, followed by the positive electron-beam resist of ZEP-520A spin-coating with around 400-nm-thick. Afterwards, all the device structures patterns were monolithically defined by one step of electron-beam lithography (EBL) process (from Vistec EBPG 5200). Then, the standard dry-etching process by using the inductively coupled plasma-reactive ion etching (ICP-RIE) tools was employed (by SPTS DRIE-I), to etch the a-Si hard mask by the SF 6 and C 4 F 8 mixed gases, and the residual resist was removed by wet etching with the acetone solutions, and then the sample was treated with the oxygen plasma, followed by the improved schemes of multi-steps of Argon-ions (Ar ? ) plasma etching process in ICP setups (by Sentech SI 500), to transfer the device patterns into the underlying single crystal LN film with an expected etching depth of h. Accordingly, once the target depth of LN ridgewaveguide structures was defined, then the residual a-Si hard mask was removed by the wet etching with the diluted potassium hydroxide solutions, followed by the improved surface cleaning process as developed in our group. Moreover, the detailed procedures of fluorinebased gases mixture were analyzed when reacted with a-Si mask in the ICP-RIE etching setup. The chamber temperature was cooled down of 5°C, and the pressure was of 10 mTorr. For the parameters of SF 6 and C 4 F 8 mixed gases and the resulting proportions, namely, the SF 6 gas rate was set of 20 standard cubic centimeter per minute (sccm), and the C 4 F 8 gas rate was of 45 sccm. To effectively etch the a-Si film, the conditions of applied bias voltage with a RF power of 13 W and ICP-RIE power of 400 W were selected, respectively. More importantly, during the etching of a-Si hard mask, the a-Si film was intentionally optimized to be not fully etched through. There still existed a few tens of nanometer un-etched a-Si film, to prevent the chemical reaction between the fluorine-based gases and LN in case of overetching a-Si film, and subsequent avoiding the hardremoved byproduct of LiF. After that, the next step for etching LN film was conducted, and the improved Ar ? plasma dry-etching process was implemented with multiple etching sub-periods. In detail, the residual a-Si was firstly etched by Ar ? plasma, thus the underlying LN was etched. For the Ar ? etching parameters in the ICP setup, the chamber temperature was set of 0°C, the pressure was of 0.7 Pa, the Ar ? rate was 80 sccm. Besides, the energy condition of a relatively high RF power of 400 W and ICP power of 600 W was selected to enable a direct current (DC) ion-beam bias voltage, in which it was sufficient for accelerating the Ar ? vertically towards the LN sample, in combination with the highly-efficient ion-bombardment and physical etching of TFLN device layer, and subsequently resulted in a much smoother profile and more precipitous waveguide side-walls. After the fabrications, the standard APM-based process using water-bath heating of NH 4 OH: H 2 O 2 : H 2 O mixed solutions, and multi-cycles cleaning of the samples, were also developed and used to improve the profile of the fabricated device structures, and the resulting temperature was constantly controlled to be of around 80°C for each cleaning period of 10 min.

Measurements and discussions
For the device structure characterizations of the fabricated LNOI integrated GCs, the measurements of high resolution optical confocal laser scanning microcopy (LSM) were performed, and scanning electron microscope (SEM) characterizations were conducted at a low operating voltage mode due to the electrical insulative property of LN. The optical micrograph of a partial of tapered GCs structure is presented in Fig. 6a, it is obvious to find that the deeply etched grating-stripes are uniform, and the device surface is relatively clean by using the surface cleaning process. And a clear side-view of enlarged SEM image of the grating-stripes structure is also captured, as exhibited in Fig. 6b. Seen from Fig. 6b, we note that the fabricated gratings are of trapezoid cross-section. Also, it can be found that the sidewalls of the gratings structure is not smooth after fabrication, which could be mainly attributed to the effects of etched LN re-deposition, the sputtered byproducts and particles, as well as the contaminations existed in the chamber during the fabrication processes, as similarly referenced to the existing literatures [29,36]. In order to reduce the sidewalls roughness of originally fabricated devices, and obtain much steeper waveguide sidewalls, on the one hand, the improved Ar ? dry-etching was conducted by adopting with multiple etching sub-periods, and the fine milling step by step was also combined, as described before in our experiment details. Moreover, after the fabrication, the optimized RCA cleaning process was also implemented for ultimately cleaning of the samples surface. It is clear to find that the sidewall of the etched ridge-waveguides structure is slant after the Fig. 6 The structure characterizations for the fabricated LNOI integrated GCs devices. a Optical image of the uniform GCs and a part of tapered waveguide structure, b side-view of enlarged SEM graph of the grating-stripes layout after the fabrication, and c topview of SEM image of the partial device structure embedded with waveguide SWG, and the scale bar is also inserted fabrication, and the total size of deeply-etched ridgewaveguide gratings was measured to be of 11-lmwide. The width of a single ridge-waveguide grating stripe was measured to be of about 595 nm, and then the area of the measured GCs region was about 20.9 9 11 lm 2 . The SEM image for the whole SWG structure is exhibited in Fig. 6c, the measured width of a single ridge-waveguide SWG stripe was about 290 nm. All the measured results were basically in good agreement with the originally designed parameters, since the geometrical parameters could be affected by the fabrication imperfections, especially for defining the fine grating and photonic-wire waveguide structures.
For the measurements of optical transmissivity, the input signal injected into the on-chip devices was originated from a laser source of continuous-wave (by Agilent TLS 81960 A), and the optical power sensor (by Keysight 81636B) were employed to measure the optical transmission response for the fabricated integrated LNOI GCs. The schematic of the testing systems is plotted in Fig. 7. Seen from Fig. 7, we can see that the TE-polarized mode was emitted from the tunable laser, the input laser power was fed into the devices by the optical fiber, and then the output transmission signals were collected by the optical spectrum analyzer for interfacing the standard single mode fiber (SMF) to GCs device after multiple manual adjustments for reaching its maximum throughput. The concrete parameters in the measurement setups are presented as follows: the laser output power (P out ) used for injecting into the input fiber was set to be of 10 dBm, the operating wavelength was ranged from 1.504 to 1.62 lm, and the step of sweeping wavelength was 2 pm. The mode of input light was adjusted by the polarization controller. Considering that the optical fibers were fixed on the holder, and the position can be moved and precisely adjusted by the manipulator stage, but the relative incident angle of h with respect to the sample surface is fixed to be of 10°both in simulations and experiments, as for the case of actual availability in our measurement systems.
The transmission spectra for the integrated GCs were characterized. For a comparison, multiple LNOI devices designed with the same geometry were also simultaneously fabricated in the same substrate, and thus they were used as the reference samples, and the resulting transmittance curves were measured, as shown in Fig. 8. For a comparison, the simulated optical transmissivity for the conventional GCs was also plotted, as marked with the solid black line. Seen from Fig. 8, it is clear find that there is an obvious transmission difference between the simulations and measurements, which might be mainly attributed to the fabrication errors and alignment tolerance for the integrated GCs. Besides, since the simulations were conducted with ideal GCs model, in our case, the sidewalls scattering loss was negligible due to the designed rectangular cross-section waveguide. While, considering the hard-etching nature of LN, we note that the sidewall of waveguide is always slant after the devices fabrication, which could be mainly attributed to the re-deposition of etched LN and consequent the re-sputtered effect. Moreover, considering that the multiple processes of transferring device patterns into the TFLN device layer, thus the originally designed etching depth of h was not effectively reached finally, and (or) the expected value of h was obtained but always with a sacrifice of the waveguide top width of w wg-top . In addition, the measurement of optical response also experienced large dependence on the fiber locations and the device geometry, such as the parameters of w, K and K swg , as discussed before. Besides, the characteristics of central operation wavelength were also analyzed, when the devices operated at around 1550 nm. It is found that the peak wavelength is red-shifted a few nanometers for our fabricated GCs, which might be closely related to a lack of the best selection of h inc and due to the actual fabrication tolerance.
Seen from the obtained transmission characteristics of the LNOI GCs in Fig. 8, it is clear to find that a low IL of around -5.1 dB/coupler when operating at 1561 nm was achieved in the best device, where the total losses include the GCs coupling loss, the waveguide propagation loss and any losses in the whole device, as analyzed before. Considering that the measured device #1 to #4 in Fig. 8 was originally designed with the same structure, and there might exist the local non-uniformity effect of the ions-sliced and bonded LNOI chip, while we noted that there is only a slightly difference between these fabricated GCs. Seen from Fig. 8, it is even obvious to find that the output performance of all these fabricated devices (with identical structure) were repeatable and stable, and the measured results revealed that the sample is homogenous. The average loss was calculated to be of -5.35 dB/coupler at the wavelength of 1550 nm, and the minor discrepancy between these devices might be attributed to the imperfect fabrication processes. In addition, since there was no set of cladding layer in our case, thus the efficiency could be further improved by depositing with a proper upper cladding layer. It is easy to find that the curve of CE slightly oscillates, which was partly attributed to the effect of the Fabry-Perot-like resonant structure formed in the waveguide GCs, as previously reported in literatures [14,37]. In addition, the property of 3-dB optical BW was also characterized with the dashed blue line, as shown in Fig. 8, and a broad 3-dB BW was extracted to be of more than 95 nm, which was limited by the intrinsic property in surface GCs structure and the capabilities of our experimental setups.
Compared with the existing researches of integrated LNOI GCs [22,23,28], our easily fabricated GCs integrated with waveguide SWG structure were highly-efficient, which could be mainly attributed to the contributions of self-designed LNOI-on-Si wafer, optimal geometrical parameters and improved manufacturing process. Since the preferable schemes of multi-steps etching within each single cycle was developed in our fabrication process, in combination with the improved cleaning process, due to the fact that experimental results highly depend on the fabrication tolerance and actual manufacturing processes, especially for the structure parameters of h, and w. Besides, the obtained high performance devices structure, which might be partly owing to the designed GCs with short-period SWG structure, thus the index of sidewalls-wavy waveguide core could be effectively tuned, the out-plane scattering and radiate losses in LNOI ridge-waveguide were reduced, as revealed in the literatures [15,30]. And it could not be denied that the scheme of GCs integrated with lowloss SWG waveguide is a good solution to obtain a high CE for effectively interfacing optical fiber and waveguide. In our following work, we will make great efforts to improve the output performance in a wide margin, and further optimize the integrated GCs design and structure parameters with the full consideration of actual fabrication process, as well as properly solving the problems as mentioned above, then to yield the output performance close to the ideal model.

Conclusions
In conclusion, the ridge-waveguide LNOI integrated GCs with a high efficiency have been demonstrated. The fabricated integrated GCs were of simple structure, and embedded with the low-loss SWG waveguide in the middle region. The measured optical response for the best GC exhibited a low IL of -5.1 dB/coupler, and with a wide 3-dB bandwidth of around 95 nm. The fabrication process for our devices was easy, which could be completely patterned by one step of EBL, followed by dry-etching with complementary metal oxide semiconductor (CMOS) compatible processes. The results may greatly boost the large scale production of surface light-coupling for photonic waveguide devices.