With the development of autonomous driving, three-dimensional imaging, remote sensing, and mapping, light detection and ranging (LiDAR) has received great attention. Although there are various LiDAR products on the market [
Photonics Research, Volume. 8, Issue 6, 912(2020)
Design and fabrication of a SiN-Si dual-layer optical phased array chip
A SiN-Si dual-layer optical phased array (OPA) chip is designed and fabricated. It combines the low loss of SiN with the excellent modulation performance of Si, which improves the performance of Si single-layer OPA. A novel optical antenna and an improved phase modulation method are also proposed, and a two-dimensional scanning range of
1. INTRODUCTION
With the development of autonomous driving, three-dimensional imaging, remote sensing, and mapping, light detection and ranging (LiDAR) has received great attention. Although there are various LiDAR products on the market [
However, silicon-based OPA also has many issues that need to be solved, especially the loss of beam on the chip. Although the processing technology of the silicon-optical platform is relatively stable, there is still a loss of about 3 dB/cm for the optical waveguide, and there are some additional losses, such as coupling loss and transmitting and receiving loss. It is very unfavorable. Moreover, silicon has a strong nonlinear absorption effect, such as two-photon absorption, free carrier absorption, and large lowest order nonlinear effects, which makes the Si single-layer OPA chip unable to handle higher energy light [
In this paper, we propose a SiN-Si dual-layer OPA chip, which is fabricated on a SiN-on-SOI foundry platform. The SiN is specifically , and the refractive index at the wavelength of 1550 nm is 1.996. The SiN layer is located above the SOI substrate with a spacing of 150 nm silicon dioxide. The silicon devices and the SiN devices are located on two layers and do not interfere with each other. The front-end devices of the OPA chip are some SiN devices, mainly including an input coupler and a cascaded beam splitter, and each device is connected through SiN waveguides. The back-end phase modulators and optical antenna are both silicon devices and are connected through silicon waveguides. The proposed SiN-Si dual-layer OPA chip has excellent low-loss characteristics. Because the front-end devices are made of SiN, the chip can handle very large optical power, providing a basis for long-range detection.
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2. STRUCTURE AND CHARACTERISTICS
We have proposed a SiN-Si dual-layer OPA chip, as shown in Fig.
Figure 1.Optical micrograph of the proposed SiN-Si dual-layer optical phased array.
The high-power beam was coupled into the chip through the input coupler, and was divided into several beams by the cascaded MMI. Then the splitted beams were coupled into the silicon waveguides through dual-layer transitions. In this way, the beam arriving in the silicon waveguide has been divided into many parts, and the energy will not be very large, so that it will not suffer from the nonlinear effects of silicon. The beams in the silicon waveguides were then modulated by Si phase modulators, and finally emitted into space through an optical antenna. When the chip is used for receiving, the working principle is similar to that of emission. The signal light is received through the optical antenna, and then modulated by phase modulators. After the cascaded MMI, the beam is detected by the off-chip InGaAs detector or on-chip Ge-Si detector.
The specific structures are shown in Fig.
Figure 2.Micrographs of the separate devices: (a) SiN edge coupler, (b) SiN grating coupler, (c) SiN MMI, (d) SiN-Si dual-layer transitions, (e) phase modulators, and (f) optical antenna.
In the following, we have simulated each device of the proposed OPA chip.
A. Input Coupler
Because the grating coupler and the edge coupler have their own advantages, we designed these two devices and provided them for the OPA chip together. Among them, the grating coupler has large alignment tolerance, and the package with the fiber is more mature. And the edge coupler has a larger bandwidth, which is very advantageous for the coupling of a multi-wavelength laser.
We have designed a SiN-Si double grating coupler with a structure similar to that in the literature [
As shown in Fig.
Figure 3.(a) Schematic of the proposed SiN-Si double grating coupler. (b) Sectional view of the proposed SiN-Si double grating coupler. (c) Simulated far-field spot of the proposed SiN-Si double grating coupler. (d) Simulated coupling efficiency of the proposed SiN-Si double grating coupler.
We adopted a double-layered grating structure to reduce light leakage to the substrate, and optimize the grating period and duty cycle to make the light intensity distribution match the mode in the fiber as much as possible. As shown in Fig.
In addition, we also designed the SiN edge coupler with spot size converter (SSC) structure, as shown in Fig.
Figure 4.(a) Schematic of the proposed SiN edge coupler. (b) Simulated coupling efficiency of the proposed SiN edge coupler.
B. SiN MMI
The beam splitters described in the chip are a number of cascaded SiN MMIs, so the performance of each SiN MMI determines the performance of the overall OPA chip. It mainly includes loss and uniformity of light splitting. As shown in Fig.
Figure 5.(a) Schematic of the proposed SiN MMI. (b) Field distribution in the proposed SiN MMI. (c) Simulated transmission efficiency of the proposed SiN MMI.
Figure
C. SiN-Si Dual-Layer Transition
Efficiently coupling the beam in the SiN waveguide into the Si waveguide is a prerequisite for the smooth operation of the proposed OPA chip. We designed a SiN-Si dual-layer transition to achieve this function. The structure of it is shown in Fig.
Figure 6.(a) Schematic of the proposed SiN-Si dual-layer transition. (b) Light intensity transfer between two layers. (c) Optical mode change process at the proposed SiN-Si dual-layer transition. (d) Light transfer efficiency between two layers.
As shown in Figs.
D. Phase Modulator
We adopted thermo-optic phase modulators to achieve phase adjustment. The specific structure is shown in Fig.
Figure 7.(a) Schematic diagram of the thermo-optic phase modulator. (b) Power consumption of phase modulators with different structures.
We calculated the power consumption of phase modulators with different structures. As shown in Fig.
E. Optical Antenna
Optical antennas are adopted to emit beam into space. The usual structure is an arrayed waveguide grating, where the grating period, duty cycle, and etch depth are all related to wavelength. The maximum scanning range of the OPA is related to the pitch of the gratings. In order to avoid crosstalk between waveguides in general optical antennas, the antenna pitch is set to more than one wavelength. For example, if the working wavelength is 1.5 μm to 1.6 μm, the antenna spacing is set to 1.65 μm. However, such a large antenna pitch will severely limit the scanning range of the OPA. We proposed a new optical antenna for this issue. In our previous work, we proposed a new type of optical antenna with a high contrast grating (HCG) structure, which can increase the scanning efficiency while increasing the scanning range [
The proposed optical antenna is shown in Fig.
Figure 8.(a) Schematic diagram of the proposed antenna. (b) Scanning far-field spots of the proposed optical antenna.
We analyze the reflection of the antenna and the vector at the beginning of the antenna, and the results are shown in Figs.
Figure 9.(a) Reflection of the antenna. (b) Vector at the beginning of the antenna. (c) Upward and downward emission of the antenna.
We have analyzed the near and far fields of the input light of a single waveguide in different channels, and the results are shown in Fig.
Figure 10.(a) Near field and far field with channel1 input light. (b) Near field and far field with channel16 input light. (c) Near field and far field with channel32 input light.
3. TEST RESULTS
A. Device Performance
We performed loss tests on some unit components of the OPA chip, and the results are shown in Fig.
Figure 11.Test results of the separate devices: (a) loss of the grating coupler, (b) loss of the waveguide, (c) loss of MMI, and (d) loss of SiN-Si dual-layer transition.
The waveguide loss is very important for the OPA chip. We measured the Si waveguide and SiN waveguide, and the results are shown in Fig.
Then we tested the beam splitter. We compared the performance of the designed SiN MMI with the Si MMI. As shown in Fig.
Then, we characterize the loss of the SiN-Si dual-layer transition, as shown in Fig.
Although the absolute value of the test results of the above devices cannot reach the international best level, introducing SiN devices into OPA chips is a very good solution for the improvement of optical loss and on-chip light intensity.
In addition, we also compared the characteristics of the Si thermo-optic phase modulator and SiN thermo-optic phase modulator, as shown in Figs.
Figure 12.(a) Modulation characteristics of Si thermo-optic phase modulator. (b) Modulation characteristics of SiN thermo-optic phase modulator.
Regarding the speed of the phase modulator, we tested the rising and falling edges of the MZM. As shown in the Fig.
Figure 13.Speed test results of phase modulator.
B. Scanning Performance
We tested the scanning characteristics of the proposed OPA chip. To this end, we built two test systems, as shown in Fig.
Figure 14.(a) Photo of far-field test system and the schematic diagram. (b) Photo of scanning test system and the schematic diagram.
Through the far-field test system shown in Fig.
Figure 15.(a) Beam steering in
We tested the full scan range of our OPA chip through the scanning test system shown in Fig.
Figure 16.(a) Scanning range in
The spot size we measured at 1550 nm is , and the spot size obtained by simulation is , as shown in Fig.
Figure 17.Simulation result of far-field spot size.
We tested the output spot power of SiN-Si OPA and Si OPA as a function of input power, and the results are shown in Fig.
Figure 18.Output spot power of Si OPA and SiN-Si OPA as a function of input power.
4. IMPROVEMENTS
We adopted point-by-point optimization of the light spot to characterize the scanning performance of the OPA, which is very inefficient. Although a good optimization algorithm can speed up the optimization rate, if too many points need to be optimized, the workload is still large. We explored a better phase modulator solution that only needs to be optimized once. We use a thermo-optic modulator array of equal proportions; that is, the heating length of each waveguide is proportional. And the premise of adopting this solution is that the antenna spacing is equal.
As shown in Fig.
Figure 19.Schematic diagram of proportional heating length phase modulators.
The specific derivation process is shown below. The phase difference between the ()th channel and the th channel is expressed as where is the coefficient of change of the refractive index with temperature, is the length difference of the heater, is the temperature difference provided by the two heaters, and is the working wavelength. Among them, where is the voltage value changed by each channel, and is the resistance value of the heater. The width and thickness of different heaters are the same—only the length is different—so Eq. (
5. CONCLUSION
We designed and fabricated a SiN-Si dual-layer OPA chip, which has better low-loss characteristics. The SiN-Si OPA chip we tested has a loss equivalent to that of the Si OPA chip at low input power. But the losses of the SiN waveguide and SiN MMI are smaller than those of silicon; the advantage of low loss will be greater in the case of a larger array. Moreover, SiN-Si OPA chips are not affected by non-linear effects of high-power light. Compared to Si OPA chips, the performance improvement is significant. This work is groundbreaking, which is a boost for the implementation of the OPA-based LiDAR. In addition, we have adopted a brand new optical antenna and tested the scanning characteristics of the antenna. The antenna can achieve a large field of view of more than , and this result is also very leading. In addition, we provided an improved phase modulation method—namely, proportional heaters—to make the OPA work more easily.
Acknowledgment
Acknowledgment. We are very grateful to the Institute of Microelectronics (IME) [30] for completing the chip manufacturing and guaranteeing that the chip can work normally. And we give special thanks to VanJee Technology Co., Ltd. [31] for providing the driver circuit supporting the OPA chip.
[9] C. V. Poulton, A. Yaacobi, Z. Su, M. J. Byrd, M. R. Watts. Optical phased array with small spot size, high steering range and grouped cascaded phase shifters. Advanced Photonics 2016, IW1B.2(2016).
[14] T. Kim. Realization of Integrated Coherent LiDAR(2019).
[15] M. Gehl, G. Hoffman, P. Davids, A. Starbuck, C. Dallo, Z. Barber, E. Kadlec, R. K. Mohan, S. Crouch, C. Long. Phase optimization of a silicon photonic two-dimensional electro-optic phased array. CLEO: Science and Innovations, JTh2A.39(2019).
[16] C. V. Poulton, P. Russo, B. Moss, M. Khandaker, M. J. Byrd, J. Tran, E. Timurdogan, D. Vermeulen, M. R. Watts. Small-form-factor optical phased array module for technology adoption in custom applications. CLEO: Applications and Technology, JTh5B.6(2019).
[17] J. Notaros, M. J. Byrd, M. Raval, M. R. Watts. Integrated optical phased array butterfly architecture for independent amplitude and phase control. Integrated Photonics Research, Silicon and Nanophotonics, IM4A.4(2019).
[19] H. Hashemi. Large-scale monolithic optical phased arrays. Optical Fiber Communication Conference, Tu3E.5(2019).
[20] B. Zhang, N. Dostart, A. Khilo, M. Brand, K. Al Qubaisi, D. Onural, D. Feldkhun, M. A. Popović, K. Wagner. Serpentine optical phased array silicon photonic aperture tile with two-dimensional wavelength beam steering. Optical Fiber Communication Conference, M4E.5(2019).
[25] W. Xie, J. Huang, T. Komljenovic, L. Coldren, J. Bowers. Diffraction limited centimeter scale radiator: metasurface grating antenna for phased array LiDAR(2018).
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Pengfei Wang, Guangzhen Luo, Yang Xu, Yajie Li, Yanmei Su, Jianbin Ma, Ruiting Wang, Zhengxia Yang, Xuliang Zhou, Yejin Zhang, Jiaoqing Pan, "Design and fabrication of a SiN-Si dual-layer optical phased array chip," Photonics Res. 8, 912 (2020)
Category: Silicon Photonics
Received: Jan. 15, 2020
Accepted: Apr. 2, 2020
Published Online: May. 19, 2020
The Author Email: Jiaoqing Pan (jqpan@semi.ac.cn)