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Abstract
We propose and demonstrate a 64-channel SiN-Si dual-layer optical phased array (OPA). By taking advantages of both SiN and Si materials, high-power handling and efficient modulation could be achieved simultaneously. In addition, steering range and emission loss are improved by introducing the non-uniform dual-layer antenna. Thinned array efficiently utilized in microwave phased array is first introduced to the OPA. Design details and the corresponding simulation results are presented, and the proposed OPA is successfully fabricated and experimentally characterized. 2D scanning with a steering range of 120°×13.9° and with a resolution of 0.052°×2.72° is demonstrated and a total loss of 12.66 dB is also measured, making it promising for high-resolution long-distance light detection and ranging (Lidar) applications.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the developments of autonomous driving, remote sensing and 3D imaging [1–4], the light detection and ranging (Lidar) has been investigated extensively [5,6] in the past decade. Among all kinds of Lidar technologies, the optical phased array (OPA) is commonly regarded as the most prominent candidate due to its non-mechanical beam steering characteristic. On the other hand, silicon (Si) chip-level integrated OPAs are demonstrated with low-cost, high reliability and small-footprint [7–11]. However, due to the strong non-linear absorption effects, silicon based OPAs are suffering from limited emitting optical power. Thus, long-distance detection is severely constrained. Alternatively, silicon nitride (SiN) based OPAs [5,12–16] are widely investigated in recent years due to their excellent high-power handling performance. Furthermore, SiN-Si dual-layer OPAs [6,17–21], in which SiN is commonly used for splitter and Si is for phase shifter, are proposed by taking advantages of both high-power handling capability from SiN and excellent modulation performance from Si.
Aside from the splitter and phase shifter, optical antenna is another essential component in OPA, determining the performance on steering range, resolution and emitting power. Optical antenna of SiN-Si OPA can be designed either on SiN or Si layer though dilemma exists for either choice. Antenna on Si layer can make compact unit pitches thus large steering range is easy to achieve [18]. However, the high mode confinement by silicon waveguide results in short emitting length, which brings out low longitude resolution. Alternatively, emitting length of SiN antenna could be improved to 3.16 mm [5] due to its relatively low refractive index.
Typically, the unit pitches of SiN antenna are larger than half wavelength [21]. In this case, grating lobes exist inevitably and severely limits the steering range. To enlarge the steering range, non-uniform design for SiN antenna can be introduced to suppress grating lobes and expand antenna aperture. Thus, the steering range and detection resolution could be improved simultaneously. More recently, non-uniform antennas with sparse array [22–26] are proposed to improve the far-field beam quality. However, the unit pitches of sparse array are required to be integer multiples of half-wavelength, which reduces its effect on both side lobe suppression and aperture expansion. Novel non-uniform antenna with improved performance is still highly desired.
Meanwhile, detection distance range is also an essential merit of OPA. High emitted power can be beneficial to high signal-noise ratio (SNR), enhancing the robustness against the noise and then increasing the maximum ranging distance. Compared with Si based OPA, SiN can handle high transmission power [20]. Thus, watt-level optical power could be injected. However, the emitted power also depends on loss of the circuit, especially the antenna induced one. Therefore, low-loss design for optical antenna is in concern. To reduce the emission loss, reported SiN antenna is either based the tri-layer structure [19,21] or special designs such as chain and fish-bone structures [17], which makes the fabrication process complex and difficult. Table 1 summaries the performance of current SiN-Si OPAs. To be noted, the relatively low longitudinal resolution in this work is due to the length of antenna is not optimized specifically. It could be further improved by introducing the weakly perturbed structure into proposed grating design [27].
Table 1. Performance comparison of reported SiN-Si OPAs
In this paper, we propose and demonstrate a 64-channel SiN-Si dual-layer OPA. By utilizing non-uniform antenna with optimized thinned array, large steering range and high resolution could be achieved. Additionally, we introduce the Si assisted grating reflector into the antenna to reduce the emission loss. Compared with the existed low-loss antenna designs, fabrication process of the proposed structure is efficiently simplified. Design details and the corresponding simulation results are presented. The proposed OPA is successfully fabricated and the measured results show steering range of 120°×13.9° with resolution of 0.052°×2.72°. The total loss of the fabricated OPA is measured to be 12.66 dB, and it could be used for high-resolution long-distance Lidar applications.
2. Design and simulation
Schematic of the proposed SiN-Si dual-layer OPA is shown in Fig. 1. The OPA is composed of a SiN fiber-chip coupler, SiN cascaded power splitters, SiN-Si dual-layer transition couplers, Si phase shifters and SiN-Si non-uniform antenna.
Light from a tunable laser is coupled into the circuit via the SiN fiber-chip coupler, and divided into 64 channels by 6-level cascaded 1 × 2 multimode interference (MMI) based splitters. The lights in all channels are then coupled into the Si phased shifters after transition though SiN-Si couplers. To be noted, the optical power into silicon waveguide is weakened greatly after splitters. Thus, the non-linear effects in silicon waveguides could be ignored. By adjusting the electrical powers on those thermal-optic (TO) phased shifters, the phase of each channel could be controlled independently. The lights with adjusted phase from all the channels are then emitted into the free space through the grating based optical antenna.
Cross-section of the grating is shown in the inset of Fig. 1. By utilizing non-uniform dual-layer antenna, grating lobes and emission loss are suppressed simultaneously. Finally, the lights are interference constructively and forming a far-field pattern with certain steering angle. Scanning on θ and $\psi $ axis dimensions could be achieved by adjusting the electrical power on phase shifters and input wavelength, respectively. Design details and corresponding simulation results of the key components are presented in the following sections.
2.1 SiN-Si dual-layer coupler
In order to couple the light from SiN splitters to Si phase shifters efficiently, SiN-Si dual-layer coupler is needed. Structure of the adopted coupler is shown in Fig. 2 (a). Adiabatic mode transition could be achieved by longitudinal evanescent coupling between SiN and Si tapers. The gap between the two layers is 250 nm while the coupling length is optimized to be 100µm. Width of the SiN taper is varied from 1µm to 300 nm while Si varies from 140 nm to 500 nm. The coupling efficiency is simulated to be > 96.5% within the wavelength range of 1500-1600 nm, as shown in Fig. 2 (b).
Fig. 2. (a) Structure of the proposed SiN-Si dual-layer coupler; (b) Simulated coupling efficiency within wavelength range of 1500-1600 nm.
2.2 Non-uniform dual-layer antenna
The most important component of OPA is the optical antenna, which determines the performance on steering range, resolution and detection distance range. Schematic of the proposed non-uniform dual-layer antenna is shown in Fig. 3 (a).
Fig. 3. (a) Schematic of the proposed optical antenna; (b) Distribution of proposed antenna; (c) Schematic of the thinned array.
In order to achieve both large steering range and high resolution, non-uniform antenna with thinned array is proposed. Figure 3 (b) represents the corresponding distribution. The thinned array is an architecture utilized in microwave phased array [28], which has been proven to be efficient on offering large emission aperture and great side-lobe suppression. Inspired by this concept, the thinned array is introduced into the proposed antenna. Figure 3 (c) illustrates the operating process of the thinned array, and green circles represent units. Compared with the sparse array [22–24,26], total width of the array is firstly fixed to guarantee sufficiently large aperture, while the unit pitches d1-dn of the thinned array could be randomly selected rather than set as integer multiples of half-wavelength d, offering more degree of freedom on pitch optimization. To be noted, the minimum unit pitch needs to be set appropriately to avoid optical crosstalk between adjacent antenna units. Detailed design is elaborated as follows.
The far-field intensity distribution of proposed non-uniform antenna could be expressed as:
A genetic algorithm [29] is utilized to optimize the pitch distribution. Flowchart of the optimization is shown in Fig. 4 (a). A group of randomly generated pitches ${d_n}$ is set as the initial population and sent to calculate the figure of merit (FOM). Side mode suppression ratio (SMSR) is set as the FOM, which is used to evaluate the quality of beam forming. Iteration process is included with selection, crossover and mutation, for updating the population. This iteration will continue until the FOM meets the set value. And then the best population will be outputted and set as the antenna pitch distribution.
Fig. 4. (a) Flowchart of the optimization; (b) Optimized pitch distribution of the proposed thinned array; (c) Corresponding normalized far-field intensity distribution.
In our design, operating wavelength is set to be 1550 nm. To balance the performance and fabrication difficulty, number of units is set to be 64, total width of the antenna is set as 2 mm, and the minimum unit pitch is set to be 10µm. Figure 4 (b) shows the pitch distribution after optimization. Corresponding far-field intensity distribution is simulated and shown in Fig. 4 (c), indicating the beam resolution of 0.043° and SMSR of 11.4 dB.
Fig. 5. (a) Cross-section of proposed dual-layer antenna assisted Si grating reflector; (b) Simulated emission loss within wavelength range of 1500-1600 nm.
Low-loss design for single antenna is also introduced and Fig. 5 (a) illustrates the cross-section of the Si assisted grating reflector. By utilizing silicon waveguides with periodic etching as the reflector, downward emission will be reflected back and transformed into the upward emission. Thus, 3 dB improvement of the emission efficiency could be achieved theoretically. The period, dual-layer offset and duty cycle of the silicon reflector are optimized to be 1.2µm, 0.83µm and 0.5, respectively. Simulation result is shown in Fig. 5 (b), indicating the emission loss is 1.487 dB at 1550 nm.
2.3 2D scanning performance
Based on the simulation results in last section, 2D scanning performance of the proposed OPA is further demonstrated. In the θ axis dimension, the horizontal scanning is controlled by adjusting the phase difference between the gratings. Assuming the unit pitch between gratings is d, operating wavelength is λ, the optical phase difference between adjacent gratings is α, and the phase controlled steering angle θ is defined by:
Meanwhile, the $\psi $ axis scanning is achieved by operating wavelength tuning. The longitudinal scanning angle $\psi $ is defined by:
where neffstands for the effective refractive index of the gratings, while $\Lambda $ stands for the period of the grating.The θ axis far-field steering simulation results of the proposed OPA are shown in Fig. 6 (a), indicating an aliasing-free beam-steering range of ∼ 120°. Meanwhile, $\psi $ axis far-field steering simulation results of the proposed OPA are shown in Fig. 6 (b). The longitudinal steering range of proposed OPA is 14.2° by adjusting the wavelength from 1540 nm to 1630 nm. Thus, 2D scanning with steering range of 120°×14.2° could be achieved.
Fig. 6. Simulated far-field beam intensity (a) in θ axis and (b) in ψ axis dimension, different colors represent different steering angles.
3. Fabrication and measurements
3.1 Fabrication of OPA
Figure 7 (a) shows the microscope image of the fabricated 64-channel OPA, while Fig. 7 (c-e) show the images of the SiN-Si thinned antenna, phase shifters and dual-layer couplers, respectively. The chip is fabricated on Silicon-On-Insulator (SOI) wafer with 220 nm silicon layer and 3 µm silica layer. 400 nm SiN layer is deposited by plasma enhanced chemical vapor deposition (PECVD). 248 nm deep ultraviolet photolithography and inductively coupled plasma (ICP) etching are used to form the waveguides. Length and tip width of the SiN fiber-chip coupler is 200µm and 300 nm, respectively. Footprint of the 1 × 2 MMI splitter is 28.8µm × 7µm. Length of the dual-layer coupler is set to be 100µm. 250µm-long TiN heaters are deposited on top of the Si waveguides for phase modulation. By applying different electrical power on the heaters, heat transferred to the silicon waveguide will change, thus the phase of the light in the waveguide could be adjusted. The power consumption for π phase shift is measured to be 18.79 mW. To avoid crosstalk between different channels, silica cladding around the heaters are etched.
Fig. 7. (a) Microscope image of the fabricated SiN-Si dual-layer OPA chip; (b) Photograph of the packaged OPA; Microscope image of (c) the SiN-Si thinned antenna, (d) the phase shifter array and (e) the SiN-Si layer coupler.
Total loss of fabricated OPA is measured as 12.66 dB, including 2.5 dB of the fiber-chip coupler, 0.36 dB of the splitters, 0.2 dB of the dual-layer couplers, and 9.6 dB emission and side-lobe loss of the antenna. Photograph of the packaged OPA is shown in Fig. 7 (b). OPA chip is wire bonded to a printed circuit board (PCB), for easy conjunction with a programmable voltage source. Phase in each channel could be accurately and independently controlled by computer, ensuring the fast and efficient beam-steering. In addition, fiber array (FA) and thermoelectric cooler (TEC) are also packaged for optical coupling and thermo control.
3.2 OPA beam steering performance
Figure 8 shows the experimental setup of the OPA beam steering characterization. The light from a tunable laser is launched into the OPA via the polarization adjustment by a polarization controller. By adjusting the electrical power of the voltage source, the phases of 64 channels could be precisely controlled. The lights with adjusted phase from all the channels are then emitted through the optical antennas, forming a pattern at the screen. An infrared charge coupled device (CCD) camera is then utilized to capture the far-field beams on the screen.
By adjusting electrical power on the phase shifters, beam steering in horizontal direction can be achieved. Beam spots and corresponding far-field intensity distributions at steering angles of -60°, -30°, 0°, 30° and 60° are shown in Fig. 9 (a-b), respectively. The distorted beam spots at higher angles are caused by the divergent characteristic of grating structure [18]. To reduce the distortion, weakly perturbed structure could be introduced into the grating, which is proven to expand the emitting length and shrink the distortion effectively [27]. Resolution at steering angle of 0° is measured to be 0.052°× 2.72°. Besides, SMSR of the spot is measured to be 9.96 dB.
Fig. 9. (a) Far-field beam spots at steering angles of -60°, -30°, 0°, 30° and 60° in horizontal direction ($\theta $); (b) Corresponding far-field intensity distributions.
By tuning the input wavelength, the longitudinal scanning of proposed OPA is further performed. Figure 10 (a) shows the captured beam spots at wavelength of 1540 nm, 1550 nm and 1630 nm, while the steering angle in horizontal direction remains to be 0°. Corresponding far-field intensity distributions are shown in Fig. 10 (b). Thus, the longitudinal steering range is 13.9° by tuning the wavelength from 1540-1630 nm. To be noted, the scanning range can be improved by expanding the wavelength tuning range. To verify the 2D scanning performance of the fabricated OPA, the ‘CUG’ and ‘HUST’ logos are further illustrated, as shown in Figs. 10 (c-d).
Fig. 10. (a) Far-field beam spots at wavelength of 1540 nm, 1550 nm and 1630 nm in longitudinal direction ($\psi $); (b) Corresponding far-field intensity distributions; (c) 2D scanning demonstration of ‘CUG’ logo and (d) ‘HUST’ logo.
In addition, an integrating sphere (S146C from Thorlabs) is set at appropriate location to capture the main lobe of pattern. The total loss of OPA could be calculated by deduction between input power and displayed value on the power meter. When the input optical power is 2.5 mW, the output main lobe power of OPA is measured to be 136.15 µW. Thus, total loss of the OPA is calculated to be 12.66 dB. The proposed SiN/Si OPA is proven to have the capability of handling high power. When the input power coupled by SiN fiber-coupler is under 3.2W, the power transmitted into the Si waveguide will be <0.05W after 1:64 splitting. In this circumstance, nonlinear effect in Si waveguide can be ignored, as it starts to appear when the transmitted power is over 0.05W [18]. After deduction of the emission loss, the maximum output power from the proposed OPA could be as high as ∼ 1.714W, which is sufficient for most Lidar applications. There are also various ways to mitigate the nonlinear effect in Si waveguide, such as utilizing p-i-n junction to sweep out free-carriers [30] and optimizing waveguide structure [31]. We believe the output power can be further scaled when introducing those techniques into the OPA design.
4. Conclusion
In summary, combining the high-power handling of SiN and efficient steering ability of Si, we propose and demonstrate a 64-channel SiN-Si dual-layer OPA. To realize both large steering range and low loss, antenna with optimized non-uniform design and silicon assisted grating reflector are utilized. Design details and the corresponding simulation results are presented. The proposed OPA is successfully fabricated, and 2D scanning is performed with steering range of 120°${\times} $13.9° experimentally. The proposed OPA could be used for high-resolution long-distance Lidar applications.
Funding
National Natural Science Foundation of China (61911530161, 62135004, 62175220, 62205312); Key Research and Development Program of Hubei Province (2021BAA005), Fundamental Research Funds for the Central Universities (G1323523065).
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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