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Abstract
Spot size converter (SSC) plays a role of paramount importance in the silicon photonics integrated circuit. In this article, we report the design of a reconfigurable spot size converter used in the hybrid integration of a DFB laser diode with a silicon photonic waveguide. Our SSC consists of subwavelength gratings and thermal phase shifters. Four subwavelength grating tips are used to improve horizontal misalignment tolerance. Meanwhile, the phase mismatch between two input waveguides is compensated by phase shifters to minimize insertion losses. Our simulated result has yielded a minimum insertion loss of 0.63 dB and an improvement of the horizontal misalignment from ±0.65 µm to ±1.69 µm for 1 dB excess insertion loss at the wavelength of 1310 nm. The phase shifters are designed to compensate any phase error in both the fabrication and bonding processes, which provides a completely new edge-coupling strategy for the silicon photonics integrated circuit.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Silicon photonics has been proven to be one of the most promising technologies in large-scale photonic integrated circuits (PICs) industry. Its fabrication process compatibility with complementary metal-oxide-semiconductor (CMOS) fabrication lines enables the large scalability of PICs at low cost [1–5]. Over the past several decades, most optical devices have successfully been monolithically integrated onto silicon photonic chip, including on-chip modulators and photodetectors [6]. However, on-chip integration of high-performance light sources is still a challenging task due to the indirect bandgap nature of silicon-based material.
So far, significant efforts have been made to achieve silicon-based monolithic light sources including Si Raman lasers [7] and strained Ge lasers [8]. However, their limited wall-plug efficiency and complex fabrication processes make them unsuitable to current need of high power and low cost in PICs industry. Another hybrid approach that combines III-V actives on silicon passive waveguides presently offers significant advantages in laser performances by utilizing both strong electrically-pumped optical gain coefficients in III-V materials and low optical loss of silicon waveguide. Heterogeneous integrated laser structures have been demonstrated by using bonding [9–13] or micro-transfer-printed [14] technologies. In these approaches, the laser light is coupled to passive waveguides via butt coupling or evanescent coupling method. Compared to evanescent coupling, which requires complex post-bonding processes for the coupler, the butt coupling method directly bonds laser devices onto the pre-fabricated passive waveguide coupler chips, enhancing fabrication yield.
However, butt-coupling method usually requires either active alignment technology where the laser is powered on during assembly [15] or extreme high-accuracy (< 0.5 µm) passive alignment by precise flip-chip bonder [16,17]. Both alignment processes suffer from low throughput and high cost. Therefore, it is essential to develop a butt coupler for passive bonding process with high alignment tolerance.
In addition, to enhance the coupling efficiency, the butt-coupling method also requires an SSC to compensate the mode mismatch between laser source and silicon waveguide. Several types of SSC have been proposed including trident SSC [16], multi-tip edge couplers [18,19], multimode edge-couplers [20,21]. However, none of them can simultaneously meet all the requirements of lower insertion loss, relaxed alignment and fabrication tolerance as well as broad-band operation.
In this work, to address the abovementioned challenges, we propose a reconfigurable SSC design operating at 1310 nm wavelength. The paper is organized as follows: Section 2 describes the overall structures and concept for III-IV based laser source coupling to silicon photonics platform. Section 3 describes the detailed design of operation principle and design procedure, in which tolerance analysis for the optimum structures of two case is presented, validating the superiority of the proposed design.
2. Device concept and simulation conditions
A schematic drawing of our proposed reconfigurable spot size converter (rSSC) with 3D view and top views is shown in Fig. 1. The device consists of a 220-nm-thick silicon device layer, a metal heater on the SOI platform layer, and a 3-µm-thick BOX layer as well as 3-µm-thick top cladding silicon dioxide layer. It is composed of three sections as shown in Fig. 1(a). Section A consists two identical meta-trident spot size converters (mtSSCs), which is used to couple the light emitted from DFB laser into two individual silicon waveguides. The mtSSCs are known to yield higher fabrication tolerance than conventional trident spot size converters (tSSCs) [22,23]. Figure 1(b) shows the detail structure of mtSSC. It consists of two tapered subwavelength gratings (SWGs) whose width linearly increases from Wtip to Ws with a length of L1, and an evanescent coupled region with a length of L2. To reduce the mode mismatch scattering, a short section of SWG buffer linearly increased from Wc to Wcm with a length of L3 is added to the center waveguide [24]. In section B, the typical silicon strip waveguide width (Wg) is 380 nm. The phase of light in these two single mode silicon waveguides is tuned by two independent thermal phase shifters, which are used to compensate the phase differences caused by horizontal misalignment (y direction) of the laser and silicon waveguide. In order to avoid thermal crosstalk [25], four curved waveguides with bending radius of 5 µm are used, resulting in a distance of 42.2 µm between these two silicon waveguides. Section C is a tSSC as shown in Fig. 1(c). By using this structure, the light from two individual silicon waveguides can be combined into one output waveguide and then connect with other silicon devices, such as splitter, modulators and detectors. The overall device footprint is 52 µm × 205 µm.
Fig. 1. Sketch of the proposed reconfigurable spot size converter on SOI wafer. (a) the 3D view, (b) meta-trident spot size converter, (c) trident spot size converter
The 3D finite-difference time-domain (FDTD) and finite difference eigenmode (FDE) method of the MODE Solutions from Lumerical commercial software are utilized to simulate the performances of the proposed rSSC. Thermal phase shifter designs are simulated with the combination of a thermal solver and an optical mode solver in Lumerical DEVICE. The mode field diameters (MFD) of the DFB laser are set to be 2.8 µm in the horizontal direction and 2 µm in the vertical direction. The device performances as well as the following discussions are under the context of TE mode.
3. Design and performance
3.1 Subwavelength grating
SWGs in SOI are periodic structures formed by interleaved silicon and silica materials at subwavelength scale [26], as shown in Fig. 2(a). According to the homogenization theory or effective-medium theory, a composite medium comprising different materials combined at subwavelength scale (Λ < λ) can be approximated as a homogeneous media with an equivalent refractive index neq as shown in Fig. 2(b), where Λ is the grating period (pitch) and λ is the wavelength of light. 3D-FDTD algorithm was used to model the dispersion relation of SWGs and equivalent waveguide. The simulation window was set up around a single unit-cell of one period length Λ with perfectly matched layer (PML) around the waveguide transverse cross-section and Bloch boundary conditions along the propagation axis. Figure 2(c) shows the fundamental quasi-TE mode dispersion relation for an SWG with width of 200 nm, period of 340 nm and duty-cycle of 50%. It also plots the dispersion relation of an equivalent homogeneous waveguide with a core refractive index of 2.53 obtained by simulation. The dispersion curve of SWG away from the bandgap resonance matches that of an equivalent waveguide, while it becomes flat as the Bragg condition is approached. A ∼20 THz photonic bandgap is observed in this SWG structure.
Fig. 2. (a) SWG illustration defining its geometric parameters. (b) Artificial homogeneous waveguide with an equivalent refractive index neq which represents the SWG in (a). (c) Dispersion relation (frequency f versus wavenumber β) for the SWG and the WG. (d) SWG Bloch mode index (black line) and Mode overlap (red line) between an equivalent waveguide representing the SWG at the tip and the DFB laser as a function tip width Wtip. The insert shows DFB laser modal distribution.
The effective indices of the SWG were obtained from this dispersion relation shown in Fig. 2(c) by using the equation of nB = cβ/(2πf), where c is the speed of light in vacuum and β is the propagation constant (β = 2πkx/Λ). By calculating the Bloch mode index nB of SWGs with various tip widths, homogeneous waveguides with an equivalent refractive index (neq) of widths ranging from 150 nm to 300 nm were also acquired. Then, their corresponding modal field distributions in the homogeneous waveguides were simulated to calculate the mode overlaps of DFB laser and SWG. The DFB laser modal distribution is estimated from its far field divergence angle and is shown in Fig. 2(d) insert. Finally, the results of Bloch mode index nB and mode overlaps of DFB laser and SWGs with various tip widths are shown in Fig. 2(d). The coupling efficiency ηm was calculated by performing the integrals of mode field overlap between the coupler tip mode and laser mode [27]:
3.2 Meta-trident spot size converter (mtSSC)
For mtSSC structure shown in Fig. 1(b), the input beam is initially captured by Bloch supermodes of dual-SWG arms, which is subsequently transformed into the middle Si waveguide by evanescent coupling. We choose the tip width Wtip = 200 nm and taper lengths L1 = L2 = 20 µm and perform FDTD simulations to obtain their corresponding coupling loss regarding to other structure parameters, including the distance between two SWG arms, width of end SWG arm Ws as well as the ratio between L3 and L2. The simulation results are shown in Fig. 3(a)-(c). the minimum coupling loss of 0.38 dB can be obtained when d = 1.1 µm, Ws = 240 nm and L3/L2 = 0.15. Figure 3(d)-(h) show the |E| field distribution of light coupled from laser source to mtSSC, indicating smooth mode evolution from dual-SWG arms to center silicon waveguide. Because the DFB laser is characterized to have strong optical confinement in the vertical direction, the near field pattern becomes a quite flattened elliptic shape. Since the pitch of the subwavelength gratings is much smaller compared with the wavelength, mode can transmit at a very low propagation loss.
Fig. 3. Coupling loss of mtSSC vs. (a) d, (b) Ws and (c) L3/L2 as shown in Fig. 1(b). (d) Field distributions along the mtSSC. (e) Mode profile of the DFB laser. Mode profile along the mtSSC at (f) x = 0 µm, (g) x = 20 µm and (h) x = 40 µm.
3.3 Reconfigurable spot size converter (rSSC)
In order to increase the horizontal misalignment tolerance (y direction), the rSSC as shown in Fig. 1(a) is composed of two mtSSCs. The input light firstly enters the four tips of SWG and then splits into two silicon waveguides with different intensity as well as phase regarding to horizontal deviation Δy. The simulation results are shown in Fig. 4(a)-(f), For Δy = 0 µm, light will propagate in two individual silicon waveguides with identical intensity and phase. However, for Δy = +1 µm (Δy = −1 µm), the power of light in upper waveguide is weaker (stronger), while carrying a phase advance (delay) compared to the lower waveguide. Therefore, we conducted FDTD simulation of the total transmittance and the phase differences Δφ as a function of the horizontal deviation Δy. The results are shown in Fig. 4(f) and Fig. 4(e), respectively. For 1 µm alignment deviation, the power ratio and phase difference between these two waveguides are 0.72 and 1.22 rad, while these are 0.97 and 1.92 rad for 2 µm alignment deviation.
Fig. 4. (a) Mode profile of the beam along section A. Phase vs. propagating axis at the output waveguide of section A for three different alignment deviation Δy, (b) Δy = 0 µm, (c) Δy = +1 µm and (d) Δy = −1 µm. (e) Phase difference between two output waveguides vs. alignment deviation. (f) Power transmission of two output waveguides vs. alignment deviation. For Fig. 4(b)-(d) and (f), the black line (red dot line) represents upper (lower) waveguide as shown in Fig. 4(a).
The coupling efficiency of section C shown in Fig. 1(c) can be affected by the power ratio and phase difference between these two waveguides. As is shown in Fig. 5(a), the coupling efficiency changes periodically with the value of phase difference when considering equal power in these two waveguides. The maximum coupling transmittance is reached when the two arms are phase matched. Figure 5(b) shows the coupling efficiency as a function of power ratio (PR) between two waveguides when phase is perfectly matched. It is interesting to note that the coupling efficiency of the total power into the middle waveguide increases monotonically as the power differences between the two input waveguides decrease. This phenomenon is primarily due to the higher-order mode excitation when the input powers are not perfectly balanced.
Fig. 5. Section C, (a) Transmission vs. phase differences between two arms. (b) Transmission vs. power ratio between two arms
To compensate the phase difference between these two waveguides, two thermal phase shifters are implemented as shown in Fig. 1(a). The metal heater is titanium nitride (TiN), and is located 1 µm above the silicon waveguide with a length of 100 µm. The phase shifter’s performance is simulated from a combination of a thermal solver and an optical mode solver in Lumerical Device. The thermal and optical FEEM engines solve the Heat Diffusion equation in a wide area (40 µm wide x 18 µm high) around the heater, and Maxwell’s equations at 1310 nm in the optical waveguide area (4 µm wide x 3.5 µm high), respectively. The temperature at the bottom of the thermal simulation region (in the Si substrate) is fixed to be the ambient temperature of 300 K, and a fixed convection of 10 W/(m2·K) is defined between the oxide layer and air above it. Figure 6(a) shows the cross-section view and simulated temperature distribution of the phase shifter. A π phase shift is obtained when 18.1 mW power is applied on the TiN resistor to reach a maximum temperature of 355 K within TiN and 332 K within Si waveguide. Figure 6(b) shows the optical phase versus heater power of this phase shifter. The optical phase is linearly increased with the heater power and the extracted Pπ is 18.1 mW. This power consumption can be further reduced by introducing thermal isolation trench alongside the TiN resistor.
Fig. 6. Thermal and optical simulation of phase shifter with the Finite-Element Eigenmode (FEEM). (a) Temperature distribution at Pπ, 18.1 mW is applied on the TiN resistor. (b) Optical phase vs. heater power for a 100 µm long TiN resistor.
With all sections combined, the overall coupling efficiency η of rSSC is defined as:
where ${\eta _A}$ is determined by the modes mismatch between the laser and the multi SWG tips and the mode evolution efficiency in section A, ${\eta _C}$ is the mode evolution efficiency in section C. The propagating loss of thermal phase shifter in section B is not considered. The insertion loss of the overall spot size converter is defined as IL = −10log(η), which is shown in Fig. 7(a). The simulated insertion loss of perfectly aligned mtSSC and rSSC is 0.38 dB and 0.69 dB, respectively. The misalignment tolerance for 1 dB loss penalty are ±0.65 µm (mtSSC) / ±0.77 µm (rSSC without phase shifter) / ±1.69 µm (rSSC) in the horizontal direction and ±0.49 µm (mtSSC) / ±0.47 µm (rSSC) in the vertical direction as well as 3.8 µm (mtSSC) / 3.6 µm (rSSC) in the propagating direction. Apparently, there is a trade-off between minimum insertion loss and alignment tolerance. Although the alignment tolerance of our proposed rSSC is slightly smaller than that of mtSSC in both vertical and propagating direction, this value is comparably improved in horizontal direction, which is much more important for a flip-chip bonded laser on silicon photonics chips [17,28]. It is worth noting that the insertion loss remains nearly flat within ±1 µm in horizontal direction, which significantly reduces the requirement for the alignment accuracy of the bonding equipment. This is primarily due to the implementation of multiple coupling tips that allows for better misalignment tolerance. In addition, the insertion loss can also be minimized by phase shifters in section B after bonding process. For a flip-chip assembly, the melting and solidification process of the solder will cause alignment deterioration due to coefficient of thermal expansion (CTE) mismatch between the laser diode chip and the waveguides. A standard deviation of σ = 0.37 µm with an average misalignment of 0.1 µm was reported by Tanaka for 17 test samples bonded under standard conditions using highly precise flip-chip bonding technology [28]. Compared to mtSSC, our rSSC device exhibits an extra merit of post-fabrication tuning that minimizes any phase difference induced insertion loss.Fig. 7. Insertion loss vs. (a) Horizontal misalignment deviation and (b) laser wavelengths 1210−1410 nm. Tip dimension deviation of (c) mtSSC and (d) rSSC.
Device dispersion characteristic is also evaluated. The insertion loss of rSSC is evaluated over entire O band, which remains less than 1 dB loss in the wavelength range of 1210 nm to 1410 nm, as shown in Fig. 7(b). By deploying adiabatic couplers, our device enables super broadband operation. The fabrication tolerance of mtSSC and our rSSC is also compared. Figure 7(c) and (d) provides the contour plot of simulated insertion loss of both devices when the SWG duty cycle and the tip width deviate from the designed value. As indicated from the plots, rSSC shares similar fabrication tolerance to that of mtSSC and can maintain an insertion loss smaller than 1 dB even when deviated from design by 20%. Table 1 shows the comparisons of the performances of the proposed device with various state-of-the-art SSCs on SOI platform. It indicates that, the rSSC owns greater superiority in horizontal misalignment tolerance and bandwidth while maintaining low insertion loss.
Table 1. Comparison of various types of SSCs
4. Conclusions
In this paper, we propose a novel type of spot size converter for edge-coupled laser source on silicon photonic chip. The minimum insertion loss of this reconfigurable spot size converter is 0.63 dB and the horizontal misalignment tolerance up to 1 dB loss penalty is ± 1.69 µm for a DFB laser with mode field diameters of 2.8 µm in the horizontal direction and 2 µm in the vertical direction. Our design improves the alignment tolerance over two times comparing to other reported SSCs [16,18,30] due to expanded input ports as well as the application of phase tuning section. It also shows an ultra-broadband performance with less than 1 dB insertion loss over the entire O band. Our device lowers the requirement of alignment accuracy of the bonding equipment, enables higher yield and lower costs for large scale manufacturing, provides a potential solution of passive-aligned edge-coupled light source for PICs.
Funding
Chongqing Science and Technology Commission (cstc2020jscx-cylhX0006).
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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