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Abstract
The hybridization of mono-crystalline silicon and lithium niobate thin films (Si-LNOI) combines the excellent electrical properties and mature micro-nano processing technology of Si, as well as the remarkable optical properties of LN. The Si-LNOI platform will drive new and promising integrated photonics devices. High-efficiency fiber-waveguide optical coupling is necessary to realize the full potential of devices and practical applications. In this study, a spot-size converter (SSC) was designed and demonstrated for efficient edge coupling between a Si-LNOI waveguide and lens fiber. The SSC was fabricated by standard semiconductor process, which consisted of an inverted-tapered Si and a silicon-rich nitride (SRN) waveguide overlying the inverted-tapered Si. At a wavelength of 1550 nm, the TE and TM light achieved coupling losses of 1.9 and 2.1 dB/facet, respectively. The coupling efficiency was stable in the wavelength range of 1500–1600 nm. The tolerance of alignment was also evaluated.
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1. Introduction
As an excellent material platform for integrated optics, LNOI (lithium niobate on insulator) not only retains the versatile properties of LN crystal, but also has a large refractive-index contrast [1]. High-performance devices, such as electro-optic modulators, optical frequency combs, and nonlinear-optical devices, have been achieved on LNOI [2,3]. However, LN is an insulator with insignificant electrical properties. Si has been the most widely used semiconductor material with important electronics and micromachining advantages, and SOI (silicon on insulator) integrated photonic devices have been well developed. SOI has high refractive index contrast, giving it a strong ability to confine light and a small bending radius of a waveguide [4]. Many integrated optical devices, such as directional couplers, splitters, distributed waveguide Bragg gratings, and arrayed waveguide gratings, have been successfully demonstrated [5,6]. However, Si is a centrosymmetric crystal and lacks electro-optic and nonlinear-optical effects, which restricts its application in integrated optics.
The hybridization of mono-crystalline silicon and lithium niobate thin films (Si-LNOI) has been fabricated by ion implantation and wafer bonding technology [7], which combines the excellent electrical properties and mature micro-nano processing technology of Si, as well as the remarkable electro-optic, acousto-optic, and nonlinear-optical properties of LN. Compared with bonding LN on SOI (LN-SOI) [8,9], Si-LNOI is more convenient for device design and fabrication [10]. In addition, compared with amorphous Si (a-Si) deposited on LNOI [10], mono-crystalline Si thin films have lower transmission loss [7,11]. Based on Si-LNOI, several high-performance photonic devices have been proposed and fabricated, including wavelength conversion devices [12] and electro-optic modulators [11,13]. To realize the full potential of these devices and practical applications, high-efficiency fiber-waveguide optical coupling is necessary. However, the mode field diameter (MFD) of single mode fiber (SMF) is approximately 10 µm, while the mode spot size of an optical waveguide based on Si-LNOI is smaller than 1 µm. Mode mismatch between waveguides and SMF make the coupling challenging. Grating coupling and edge coupling (butt coupling) are the most common ways to realize fiber-to-waveguide coupling [14]. Grating couplers allow on-wafer device evaluation. However, they operate in a narrow bandwidth range and are sensitive to polarization. Edge coupling has a wide operational bandwidth and is insensitive to polarization [15–18]. Usually, the coupling efficiency of edge coupling has been improved by matching the mode of SMF and the waveguide (for example, broadening the size of the waveguide mode and reducing the size of the spot mode in the SMF).
In this study, an inverted-tapered spot-size converter (SSC) was designed and demonstrated on the Si-LNOI platform for efficient edge coupling. The SSC was fabricated using semiconductor process, which consisted of an inverted-tapered Si and a silicon-rich nitride (SRN) waveguide overlying the inverted-tapered Si. The tip width and length of the inverted-tapered Si were 30 nm and 100 µm, respectively. At 1550 nm, the measured coupling losses of TE and TM modes in experiments we conducted were 1.9 and 2.1 dB/facet, respectively. The coupling efficiency was stable in the wavelength range of 1500–1600 nm. The tolerance of alignment was also evaluated.
2. Design and simulation
The Si-LNOI was fabricated by ion implantation and wafer bonding technology [7]. First, hydrogen ions were implanted into the polished side of a Si wafer. Then, the implanted surface of the Si wafer was directly bonded to a LNOI wafer (commercially available) at room temperature. Next, the bonded pair was annealed at high temperature. During the annealing process, gas bubbles were formed in the implanted layer due to the accumulation of H+ to form H2. With the increase of bubbles, the ion implanted layer was peeled off, leaving a peeled Si thin film on the LNOI. Subsequently, further annealing was used to repair the lattice damage caused by the ion implantation. Finally, chemical mechanical polishing (CMP) was used to remove the surface damage layer of the Si thin film and reduce the surface roughness. The materials from top to bottom were: Si thin film, LN thin film, SiO2 layer and Si substrate. The LN thin film was X-cut with a thickness of 600 nm. The thickness of the SiO2 layer and Si substrate were 2 and 500 µm, respectively. Lens fiber was used to reduce the MFD of the SMF from 10 µm to approximately 2.5 µm. An inverted-tapered SSC to convert the mode field from the lens fiber to a Si-LNOI waveguide was designed to improve the coupling efficiency. The schematic structure of the SSC is shown in Fig. 1. The width and height of the Si waveguide were designed to be 500 and 300 nm, respectively, to operate in single mode condition. The SSC consisted of two parts: an inverted-tapered Si waveguide and an SRN waveguide overlaid on the inverted-tapered Si waveguide. The mode size of the inverted-tapered Si waveguide decreased with decreasing waveguide dimension, and then increased with decreasing waveguide dimension due to less optical confinement. The SRN had a wide light transparent window, and the refractive index could be adjusted by modifying the Si/N ratio. Here, the refractive index of SRN was 2.200 measured by ellipsometry at 1550 nm. The height and width of the SRN waveguide were 2.0 and 2.5 µm, respectively.
The top view of the SSC is shown in Fig. 2(a). The direction of optical transmission was parallel to the Y axis of LN. The coupling loss mainly consisted of two parts. The first was the mode-conversion loss from the lens fiber to the cross-section 1 (CS1), which was determined by mode matching of the lens fiber and CS1, and the second was mode-conversion loss toward the Si waveguide (CS1 to CS4). Figure 2(b) shows the cross-section of the SSC at different positions and the corresponding mode field distribution simulated by the full-vectorial finite-difference method using MODE within Lumerical's DEVICE Multiphysics Simulation Suite (Ansys Canada Ltd., Canada). First, the optical power in the lens fiber was coupled into the SSC (CS1). Then, the mode gradually came into the inverted-tapered Si (from CS2 to CS3). Finally, the mode was converted to a single-mode Si waveguide (from CS3 to CS4).
Fig. 2. (a) Top view of SSC and (b) cross-section of SSC at different positions and corresponding mode field distribution (λ=1550 nm).
To reduce the mode-conversion loss from CS1 to CS3, the tip width of the inverted-tapered Si must be small enough and the length of the inverted-tapered Si long enough for adiabatic conversion of the optical mode in the waveguide. Figure 3(a) shows the power coupling of CS1's and CS2's modes as a function of tip width. The power coupling of both the TE0 and TM0 modes decreases with increasing tip width. Compared with the TE0 mode, the power coupling of the TM0 mode was more sensitive to the change of tip width. When the power coupling was 90%, the TE0 mode had a tip width of approximately 100 nm, while the TM0 mode had a tip width of approximately 60 nm. When the tip width was less than 30 nm, the power coupling of both TE0 and TM0 modes could reach more than 98%. Figure 3(b) shows the mode-conversion losses as a function of taper length from CS2 to CS3, which was simulated by the finite-difference time-domain method (FDTD) using FDTD within the aforementioned simulator. The mode-conversion losses of both the TE0 and TM0 modes decreased with increasing taper length. When the taper length was greater than 100 µm, the mode-conversion loss of both TE0 and TM0 modes was approximately 0. Considering the fabrication tolerance, the tip width and taper length were chosen to be 30 nm and 100 µm, respectively. Figure 4 shows the power coupling of CS1's and CS2's modes as a function of wavelength (1500–1600 nm) when the tip width was 30 nm. Since the optical confinement ability of Si waveguides decreased with increasing wavelength, the power coupling of both the TE0 and TM0 modes increased with increasing wavelength. However, the power coupling of both the TE0 and TM0 modes varied very little (<0.01) in the 1500–1600 nm range. This means that when the tip width of the inverted-tapered Si was 30 nm, its effect on the operational bandwidth of SSC was negligible.
Fig. 3. (a) Power coupling of CS1's and CS2's modes as function of tip width of inverted-tapered Si and (b) mode-conversion losses from CS2 to CS3 as a function of taper length.
Fig. 4. Power coupling of CS1's and CS2's modes as function of wavelength when tip width of inverted-tapered Si was 30 nm.
3. Fabrication and measurement
We fabricated the SSC on a Si-LNOI wafer. The fabrication process is shown in Fig. 5. First, a layer of photoresist (ZEP) was spin-coated onto the Si-LNOI surface, and the Si waveguide/inverted-tapered Si was patterned using electron beam lithography (EBL) and inductively coupled plasma (ICP) etching. Then, SRN was deposited on the sample surface by plasma-enhanced chemical vapor deposition (PECVD). Subsequently, photoresist (ZEP) was spin-coated on the SRN surface and patterned using EBL. Next, chromium (Cr) was deposited as a hard mask and the SRN waveguide was patterned by ICP etching. Finally, the chrome mask was removed. A scanning electron microscopy (SEM) image of the fabricated SSC is presented in Fig. 6(a); Fig. 6(b) shows the cross-section of the SRN waveguide. The width and height of the SRN waveguide were 2.5 and 2.2 µm, respectively. The roughness of the SRN waveguide surface originates from two sources: one was the deposited gold to enhance the conductivity of the sample, and the other was the redeposition during the focused ion beam (FIB) etching process. Figure 6(c) shows a SEM image of the inverted-tapered Si. Figure 6(d) shows the image of the tip of the inverted-tapered Si, which had a tip width of approximately 30 nm.
Fig. 6. SEM images of (a) SSC, (b) cross-section of SRN waveguide, (c) inverted-tapered Si, and (d) tip of inverted-tapered Si.
The prepared sample was cut into two parts, one including the SSC and the straight Si waveguide (sample 1), and the other including the straight Si waveguide only (sample 2). Figure 7 shows the coupled measurement system. First, sample 1 was placed on the sample stage. A linearly polarized beam was emitted by a tunable semiconductor laser (Santec TSL-210VF) and transmitted through a single mode polarization-maintaining (PM) fiber. Then, the light was coupled into the SSC via a lens fiber. A positioner was used to adjust the direction of the polarized light. The output light from the Si waveguide was coupled into another lensed fiber and then detected by a Ge detector. The MFD of the two lensed fibers was 2.4 µm, which was measured by the beam scanning method. The coupling loss could be estimated by subtracting the propagation loss of the Si waveguide and the coupling loss between the Si waveguide and lensed fiber from the total insertion loss [19,20]. The coupling loss between the Si waveguide and lensed fiber could be measured from sample 2. The propagation loss of the Si waveguide was measured using the Fabry-Pérot method [10,21]. Figure 8 shows the normalized transmission of TE and TM mode in Si waveguide as a function of wavelength. At 1550 nm, the transmission losses of the TE and TM modes were 7.3 and 7.7 dB/cm, respectively. The transmission loss might be due to the scattering caused by the roughness of the side wall of the waveguide.
Fig. 8. Normalized transmission of (a) TE and (b) TM mode in Si waveguide as a function of wavelength.
Figure 9 shows the measured coupling losses of the TE and TM modes as a function of input wavelength. At 1500–1600 nm, the TE mode has a coupling loss of 1.8–1.9 dB/facet (1.9 dB/facet at 1550 nm), while the TM mode has a coupling loss of 2.1–2.2 dB/facet (2.1 dB/facet at 1550 nm). The main sources of coupling loss were reflection at CS1 and fiber-to-CS1 mode mismatch. According to the simulation, the light reflected at the CS1 was approximately 13% due to the high refractive index of SRN. Higher coupling efficiency could be achieved by applying an optical coating to the CS1 to reduce reflections. Growing thicker SRN allows for better mode matching of the CS1 and lensed fiber. However, there is trade-off between SRN thickness and the stress caused by thick SRN. We also evaluated the alignment tolerances of the SSC and lens fiber, which were measured by moving the lens fiber in the X, Y, and Z direction from the optimal coupling position. The X, Y and Z directions were consistent with the X, Y and Z axes of LN crystal, as shown in Fig. 6(b). In the X and Z directions, the measured alignment tolerance for 1 dB additional loss was approximately ±0.75 µm for both TE and TM modes, as shown in Fig. 10(a) and (c), respectively. In the Y direction, a misalignment of 1 µm would generate an additional loss of about 0.8 dB for both TE and TM modes, as shown in Fig. 10(b).
Fig. 9. Coupling loss of TE (black squares) and TM (red circles) modes as function of input wavelength.
Fig. 10. Additional loss of SSC as function of fiber-chip misalignment in (a) X, (b) Y, and (c) Z directions.
4. Conclusions
In summary, the SSC was designed, fabricated, and characterized in Si-LNOI platform for efficient edge coupling. At 1550 nm, the measured coupling losses of TE and TM modes were 1.9 and 2.1 dB/facet, respectively. Experimental results showed that coupling efficiency was stable in the wavelength range of 1500–1600 nm. The tolerance of alignment was also evaluated. Our study provided useful reference value for the further development of more efficient edge-coupling devices on this platform. In addition, the coupling loss was one of the key parameters of photonic devices, and our study would promote the wide application of Si-LNOI platform in photonic integrated circuits.
Funding
National Key Research and Development Program of China (2018YFB2201700, 2019YFA0705000); Natural Science Foundation of Shandong Province (ZR2020LLZ007).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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