Open Access
Add to BibSonomy
Abstract
Optical phased array (OPA) is a promising beam steering component for light detection and ranging (LiDAR) systems. For most LiDAR applications, two-dimensional (2D, lateral and longitudinal) beam steering with large field of view is required. To achieve large lateral and longitudinal field of view, waveguide with nonuniform spacing and broadband tunable laser source is commonly utilized, resulting in complex structure and high cost. Here, a 2D OPA with large field of view is proposed and demonstrated on the silicon-on-insulator platform. Assisted by an improved optical antenna and polarization switch, lateral and longitudinal steering range could be both significantly improved. The experimental results show the steering ranges are 99.24° × 15.62° and 96.48° × 16.08° for transverse electric mode and transverse magnetic mode, respectively. The proposed scheme provides a promising approach to realize the integrated OPA with large field of view.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
As the rapid development of automatic driving and remote sensing [1,2], Light Detection and Ranging (LiDAR) technology has attracted more and more attention. The optical beam steering components utilized in conventional LiDAR systems are usually based on mechanically rotating reflective mirrors, liquid-crystal spatial light modulators and micro-electro-mechanical-systems (MEMSs) [3,4]. However, these schemes suffer from low response speed, bulky structure and small field of view (FOV), which limit the practical applications. Alternatively, the integrated optical phased array (OPA) that provides a non-mechanical optical beam steering method has been intensively developed recently [5–7]. Integrated OPAs have been investigated on a variety of material platforms, such as indium phosphide [8], silicon nitride [9–11] and silicon-on-insulator (SOI) [12–15]. Among those platforms, SOI is widely regarded as a promising candidate [16] with the merits of compactness, scalability, low power consumption and compatibility with complementary metal-oxide-semiconductor (CMOS) fabrication process.
For most LiDAR applications, two-dimensional (2D, including lateral and longitudinal directions) beam steering with large FOV is needed. However, the steering range of the integrated OPA is restricted by the pitch between adjacent radiators [17]. Although it can be improved greatly by reducing the pitch to half-wavelength, the crosstalk between adjacent radiators would become a new issue correspondingly. This factor restricts the further expansion of the lateral steering range. A specially designed grating is proposed to solve the problem, at the cost of more critical fabrication process [18]. In traditional 2D steering OPA schemes where gratings are utilized as radiators, a large lateral steering range is hard to realize because the pitch between adjacent gratings is much larger than half-wavelength [5,19–21]. On the other hand, the wavelength tuning method is commonly used to realize longitudinal steering, and the steering range is restricted by the tuning range of the laser and the bandwidth of the devices. Although several methods have been explored to increase the longitudinal steering range, such as thermo-optic controlling grating, compound period grating and grating sub-array [11,22,23], several disadvantages still exist, including high power consumption, interdependent dual beams, and increased system complexity.
Here, we propose and demonstrate a silicon 2D OPA with large FOV. The steering range could be significantly enlarged by using specifically designed optical antenna assisting by the polarization switching. The improved optical antenna is based on the principle of radiation after interference, and the beam radiates from a single grating. By utilizing this structure, the crosstalk between channels is eliminated. Hence, the spacing of waveguide array (WA) could be narrowed and a large lateral steering range could be realized. On the other hand, for longitudinal steering, polarization switching is introduced to alleviate the dependence on the tunable laser source, thanks to the different effective refractive indexes and thus diffraction angles induced by different polarization states. Compared with single polarization case, the longitudinal steering range could be doubled. Although the similar concept has been proposed recently, the polarization states are controlled by off-chip devices [24] and only simulation results are presented [24,25]. Our experimental results indicate the steering ranges of transverse electric (TE) and transverse magnetic (TM) polarizations are 99.24° × 15.62° and 96.48° × 16.08°, respectively. The proposed OPA provides an alternative way to realize a larger 2D beam steering range.
2. Structure and principle
Figure 1(a) illustrates the schematic of the proposed silicon OPA. The device mainly includes five parts, namely edge coupler, power splitter, polarization converters, phase shifters and optical antenna. To split the fundamental TE (TE0) mode into four coherent channels, two stages of the multimode interference (MMI) coupler are configured. The polarization converter is introduced in each channel to realize the optional polarization switching, and it is realized by cascading a Mach-Zehnder interferometer (MZI) and a bi-level mode converter [26,27]. If a π phase shift is introduced by heating the upper branch of each MZI, the TE0 mode would be converted to fundamental TM (TM0) mode. Additional four independent TiN micro-heaters are applied to manipulate the channel phase difference, and thus the lateral steering angle could be tuned [6,7,14,19–22]. After phase controlling, the beam would transmit to the antenna. Being different from traditional antenna based on grating array, an interference region (IR) is introduced to separate the waveguide array and the single grating, as illustrated in Fig. 1(b). The working principle of the antenna is explained as follows: the TE0 or TM0 mode with specific phase is fed into the IR to form the interference pattern. Then, the whole pattern transmits to the grating and finally radiates to free space [15]. To be noted, the waveguides of the WA with different widths are also utilized to reduce the crosstalk [28–30], as presented in Fig. 1(b). The pitch of the WA d is set to be 775 nm, and LWA, LIR, and W represent the WA length, the IR length and the grating width, respectively.
Fig. 1. (a) Schematic of the proposed silicon 2D beam steering OPA. α represents the phase difference between adjacent channels. (b) Schematic of the improved optical antenna and its steering angle. Ψ represents the lateral steering angle, while θ represents the longitudinal one.
In the proposed scheme, the lateral steering range is enlarged by adopting the improved antenna. The lateral steering angle can be formulated as:
where d and $\mathrm{\alpha }$ are the center distance and phase difference of adjacent channels, respectively. $\mathrm{\lambda }$ represents the wavelength. The angle can be enlarged by narrowing the channel space in conventional antenna with grating arrays, but narrower spacing results in greater crosstalk. Compared to the conventional structure, the beam here radiates from the single grating rather than grating arrays, and the crosstalk can be eliminated and the lateral steering range can be enlarged.For the longitudinal steering angle, it is determined by the effective refractive index and wavelength. The longitudinal steering angle θ is expressed by the following equation:
where ${n_{eff}}$ represents the effective refractive index in the grating and $\mathrm{\Lambda }$ represents the duty cycle of the grating.As shown in Eq. (2), when the wavelength or the effective refractive index changes, the steering angle of the antenna will also change. Therefore, if the polarization is switchable, the longitudinal steering range would be greatly improved by utilizing the effective refractive index difference between TE and TM modes. Thus, the 2D steering with a large longitudinal steering range can be realized.
3. Simulated results
The lateral steering is accomplished by tuning the phase difference. Figures 2(a)–2(d) qualitatively depict the TE0 mode far-field distributions under four different lateral steering states, while Figs. 2(e)–2(h) depict the cases of TM0 mode. When the phase differences increase, the main lobe intensity decreases while the grating lobe intensity increases. On the other hand, the beam steering angle also increases. Once the phase difference reaches 180°, the main lobe and the grating lobe will have equal intensities, resulting in the steering angle reaches the maximum.
Fig. 2. TE0 mode far-field distributions at (a) 0°, (b) 60°, (c) 120°, (d) 180° phase difference between adjacent waveguides. TM0 mode far-field distributions at (e) 0°, (f) 60°, (g) 120°, (h) 180° phase difference between adjacent waveguides. The wavelength is set to be 1550 nm. α represents the phase difference between adjacent channels.
One should notice that the longitudinal steering angle slightly moves with the variation of lateral steering angle and the trajectory is arc shaped. It is caused by the following reason: the beam transmitting to the grating is divergent, and it has different incidence directions to the gratings under different lateral steering states, leading to the formation of an arc trajectory [15].
In our scheme, although 180° lateral steering can be realized by using a half-wavelength pitch WA according to the Eq. (1), the lateral far-field distribution is modulated by the waveguide diffraction envelope. Thus, the lateral angle of the maximum light intensity in the far-field is smaller than 180°. Defining the angle of maximum intensity as the steering angle, we quantitatively extract the lateral far-field distribution for TE0 and TM0 modes under six different phase differences, as illustrated in Figs. 3(a) and 3(b). At 1550 nm, the lateral steering range for TE0 and TM0 modes are 134.4° and 105.6°, while the maximum full width at half maxima (FWHM) are 24° and 23.4°, respectively. The reason for the large beam width is the low number of channels, and this does not degrade the steering characteristics. It could be noticed that FWHM decreases as the beam steers, and the reason is as follow: the interference pattern is formed on the chip rather than in the free space. More specifically, the beams interfere in the IR and then radiated through the grating. As a result, the far-field pattern illustrated in Figs. 3(a) and 3(b) is the result of Fourier transform of the interference pattern.
Fig. 3. Lateral steering angles under six different phase differences for (a) the TE0 and (b) the TM0 mode. (c)The TE0 and the TM0 modes lateral far-field intensity distributions when the relative phase between adjacent channels equals 0°. The wavelength is set to be 1550 nm.
Figure 3(c) presents the lateral far-field intensity distributions for TE0 and TM0 mode when the relative phase between adjacent channels equals 0°. With the polarization changing from TE0 to TM0, the maximum light intensity in the far-field decreases 13.8%, arising from the diffraction efficiency for TE0 mode is higher than that for TM0 mode.
Figures 4(a)–4(f) qualitatively demonstrate the longitudinal far-field beam distributions for TE0 and TM0 modes. The longitudinal steering angle changes with the wavelength and polarization for a given phase difference. Compared to the traditional OPA, the proposed scheme has two independent FOVs corresponding to the two polarization, and this broadens the steering range under the same wavelength tuning range. From Fig. 4, one could find that the steering angle decreases with the wavelength increasement, for both TE0 and TM0 modes according to Eq. (2). Figures 5(a) and 5(b) quantitatively demonstrate the steering angles with different wavelength and polarization. The results show that the steering range for TE0 mode is 16.48° (from 38.84° to 22.36°), while it is 16.17° (from -32.14° to -15.97°) for TM0 mode, with a wavelength tuning range from 1510-1630 nm. The 3 dB beam width at longitudinal direction for TE0 and TM0 modes are 3.84° and 3.4° at 1550 nm, respectively. It is worth noting that there is a blind area between the two FOVs, which is due to the significant birefringence effect caused by the different propagation constants of TE0 and TM0 polarization in the waveguide. The blind area could be eliminated by birefringence-engineering [31] and utilizing narrower grating period.
Fig. 4. TE0 modes longitudinal far-field distributions at (a) 1510, (b) 1550 and (c) 1630 nm. TM0 modes longitudinal far-field distributions at (d) 1510, (e) 1550 and (f) 1630 nm. The relative phase α is set to be 0°.
Fig. 5. Longitudinal steering angles at 1510, 1550 and 1630 nm for (a) the TE0 and (b) the TM0 mode. The relative phase between adjacent channels is set to be 0°.
4. Fabrication and characterization
Figure 6(a) depicts the microscope image of the fabricated device. The device was fabricated on a 220 nm thick SOI wafer with 2 µm buried oxide layer. TiN micro-heaters were deposited on top of the channel waveguides for phase controlling. The entire device is covered by a SiO2 cladding, forming a buffer layer between the TiN and the waveguides. As presented in Fig. 6(b), the packaged chip was wire-bonded to a print circuit board (PCB) for applying voltage.
The fabricated device is demonstrated using the experimental platform illustrated in Fig. 7. The whole system consists of a power supply (DC voltage), a tunable laser, the chip under test, an observation screen, a prime lens and an infrared camera (CCD). The PCB is located 2.5 cm away from the screen. To capture the far-field beam of the 2D OPA, we placed the chip vertically so that the far-field beam is directly radiated on the observation screen. The tunable range of the laser source is 1510-1630 nm. The beam radiated by the OPA was first imaged on the screen, and then captured by the infrared camera assisting by the lens.
Figure 8 illustrates the infrared spots and the steering range of TE0 and TM0 modes, respectively. Figures 8(a) and 8(b) show the lateral steering characteristics. The arc shaped trajectory is consistent with the simulation. The lateral steering ranges of TE0 and TM0 achieve 99.24° and 96.46°, respectively. Figures 8(c) shows the longitudinal steering ranges of TE0 and TM0 are 15.62° (from 22.71° to 38.33°) and 16.08° (from -16.04° to -32.12°), respectively, by changing the wavelength of the tunable laser from 1510 to 1630 nm. Compared to traditional schemes, the polarization switching OPA has two fields of view and the longitudinal steering range is almost doubled, without sacrificing the lateral steering angle. For a proof-of-concept demonstration, the device has only four channels, and a finer light spot can be expected by adopting more channels. In addition to the FOVs, we also measured side-mode suppression ratio (SMSR), which are 12.08 and 13.54 dB for TE0 and TM0 modes at 1550 nm, respectively. The main beam power of the two modes are -8.4 and -8.8 dBm when the optical power injected into the chip is 0 dBm, respectively. The loss includes the ∼7 dB antenna transmission loss.
Fig. 8. Lateral steering range light spot of (a) the TE0 and (b) the TM0 mode by changing the difference between adjacent channels at 1550nm, respectively. (c) Longitudinal steering range of TE0 and TM0 mode by tuning the wavelength with the phase difference 0°. The horizontal plane is flush with the chip at 0°.
The experimental results show that the losses of the polarization converter are about 0.35 and 0.31 dB for TE0 and TM0 modes, respectively. The power consumption in each channel for converting polarization is 14.8 mW, while that for manipulating phase is 26.8 mW. Compared with doubling the FOV by using multiple OPAs, the proposed scheme reduces power consumption with more compact size. The footprint of the demonstrated chip is only 3.1mm×1.5mm.
5. Conclusions
We propose and demonstrate a polarization switching 2D silicon OPA to alleviate the dependence on the tunable laser source, and the longitudinal steering range is nearly doubled compared with conventional single polarization scheme. The improved antenna reduces the crosstalk between channels occurring in traditional 2D steering OPA schemes based on arrayed gratings, and 2D steering with a large lateral steering range is achieved. The measured steering ranges are 99.24° × 15.62° and 96.48° × 16.08° for TE0 and TM0 mode, respectively. The proposed scheme provides a promising approach to realize a larger FOV.
Funding
National Natural Science Foundation of China (61922034, 62135004); Key Research and Development Program of Hubei Province (2021BAA005); Program for HUST Academic Frontier Youth Team (2018QYTD08); Innovation Project of Optics Valley Laboratory (OVL2021BG005).
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.
References
1. X. Chen, H. Ma, J. Wan, B. Li, and T. Xia, “Multi-view 3d object detection network for autonomous driving,” Proceedings of IEEE Conference on Computer Vision and Pattern Recognition (IEEE, 2017), pp. 1907–1915.
2. M. Shangguan, H. Xia, C. Wang, J. Qiu, S. Lin, X. Dou, Q. Zhang, and J. W. Pan, “Dual-frequency Doppler lidar for wind detection with a superconducting nanowire single-photon detector,” Opt. Lett. 42(18), 3541–3544 (2017). [CrossRef]
3. Y. Wang, G. Zhou, X. Zhang, K. Kwon, P.-A. Blanche, N. Triesault, K.-S. Yu, and M. C. Wu, “2D broadband beamsteering with large-scale MEMS optical phased array,” Optica 6(5), 557–562 (2019). [CrossRef]
4. J. Lindle, A. Watnik, and V. Cassella, “Efficient multibeam large-angle nonmechanical laser beam steering from computer-generated holograms rendered on a liquid crystal spatial light modulator,” Appl. Opt. 55(16), 4336–4341 (2016). [CrossRef]
5. J. Sun, E. Timurdogan, A. Yaacobi, E. Shah Hosseini, and M. R. Watts, “Large-scale nanophotonic phased array,” Nature 431(7431), 195–199 (2013). [CrossRef]
6. J. K. Doylend, M. J. Heck, J. T. Bovington, J. D. Peters, L. A. Coldren, and J. E. Bowers, “Two-dimensional free-space beam steering with an optical phased array on silicon-on-insulator,” Opt. Express 19(22), 21595–21604 (2011). [CrossRef]
7. K. Van Acoleyen, W. Bogaerts, J. Jágerská, N. Le Thomas, R. Houdré, and R. Baets, “Off-chip beam steering with a one-dimensional optical phased array on silicon-on-insulator,” Opt. Lett. 34(9), 1477–1479 (2009). [CrossRef]
8. J. K. Doylend, M. J. R. Heck, J. T. Bovington, J. D. Peters, M. L. Davenport, L. A. Coldren, and J. E. Bowers, “Hybrid III/V silicon photonic source with integrated 1D free-space beam steering,” Opt. Lett. 37(20), 4257–4259 (2012). [CrossRef]
9. C. S. Im, B. Bhandari, K. P. Lee, S. M. Kim, M. C. Oh, and S. S. Lee, “Silicon nitride optical phased array based on a grating antenna enabling wavelength-tuned beam steering,” Opt. Express 28(3), 3270–3279 (2020). [CrossRef]
10. M. C. Shin, A. Mohanty, K. Watson, G. R. Bhatt, C. T. Phare, S. A. Miller, M. Zadka, B. S. Lee, X. Ji, I. Datta, and M. Lipson, “Chip-scale blue light phased array,” Opt. Lett. 45(7), 1934–1937 (2020). [CrossRef]
11. N. A. Tyler, D. Fowler, S. Malhouitre, S. Garcia, P. Grosse, W. Rabaud, and B. Szelag, “SiN integrated optical phased arrays for two-dimensional beam steering at a single near-infrared wavelength,” Opt. Express 27(4), 5851–5858 (2019). [CrossRef]
12. K. V. Acoleyen, W. Bogaerts, and R. Baets, “Two-dimensional dispersive off-chip beam scanner fabricated on silicon-on-insulator,” IEEE Photonics Technol. Lett. 23(17), 1270–1272 (2011). [CrossRef]
13. K. V. Acoleyen, K. Komorowska, W. Bogaerts, and R. Baets, “One-Dimensional Off-Chip Beam Steering and Shaping Using Optical Phased Arrays on Silicon-on-Insulator,” J. Lightwave Technol. 29(23), 3500–3505 (2011). [CrossRef]
14. P. F. Wang, G. Z. Luo, H. Y. Yu, Y. J. Li, M. Q. Wang, X. L. Zhou, W. X. Chen, Y. J. Zhang, and J. Q. Pan, “Improving the performance of optical antenna for optical phased arrays through high contrast grating structure on SOI substrate,” Opt. Express 27(3), 2703–2712 (2019). [CrossRef]
15. P. F. Wang, G. Z. Luo, Y. Xu, Y. J. Li, Y. M. Su, J. B. Ma, R. T. Wang, Z. G. Yang, X. L. Zhou, Y. J. Zhang, and J. Q. Pan, “Design and fabrication of a SiN-Si dual-layer optical phased array chip,” Photonics Res. 8(6), 912–919 (2020). [CrossRef]
16. W. Bogaerts and L. Chrostowski, “Silicon Photonics Circuit Design: Methods, Tools and Challenges,” Laser Photonics Rev. 12(4), 1700237 (2018). [CrossRef]
17. M. J. R. Heck, “Highly integrated optical phased arrays: photonics integrated circuits for optical beam shaping and beam steering,” Nanophotonics 6(1), 93–107 (2017). [CrossRef]
18. Y. J. Lee Kong, A. A. Noman, Y. Tang, G. Chang, R. Chen, and M. Qi, “Aliasing-free Beam Steering from an Optical Array Emitter with Half-wavelength Pitch,” in OSA Advanced Photonics Congress (AP)2020, paper ITh2H.6.
19. K. V. Acoleyen, H. Rogier, and R. Baets, “Two-dimensional optical phased array antenna on silicon-on insulator,” Opt. Express 18(13), 13655–13660 (2010). [CrossRef]
20. D. Kwong, A. Hosseini, J. Covey, X. Xu, Y. Zhang, S. Chakravarty, and R. T. Chen, “Corrugated waveguide based optical phased array with crosstalk suppression,” IEEE Photonics Technol. Lett. 26(10), 991–994 (2014). [CrossRef]
21. S. A. Miller, Y. C. Chang, C. T. Phare, M. C. Shin, M. Zadka, S. P. Roberts, B. Stern, X. Ji, A. Mohanty, O. A. J. Gordillo, U. D. Dave, and M. Lipson, “Large-scale optical phased array using a low-power multi-pass silicon photonic platform,” Optica 7(1), 3–6 (2020). [CrossRef]
22. S. Kim, J. You, Y. Ha, G. Kang, D. Lee, H. Yoon, D. Yoo, D. Lee, K. Yu, C. Youn, and H. Park, “Thermo-optic control of the longitudinal radiation angle in a silicon-based optical phased array,” Opt. Lett. 44(2), 411–414 (2019). [CrossRef]
23. D. Wu, W. Guo, and Y. Yi, “Compound period grating coupler for double beam generation and steering,” Appl. Opt. 58(2), 361–367 (2019). [CrossRef]
24. Zhujun Shi, Wei Ting Chen, and Federico Capasso, “Wide field-of-view waveguide displays enabled by polarization-dependent metagratings,” Proc. SPIE 10676, 1067615 (2018). [CrossRef]
25. J. Chen, X. Yan, D. Dai, and Y. Shi, “Polarization Multiplexing Silicon-Photonic Optical Phased Array for 2D Wide-Angle Optical Beam Steering,” IEEE Photonics J. 13(2), 1–6 (2021). [CrossRef]
26. C. Sun, W. Wu, Y. Yu, G. Chen, X. Zhang, X. Chen, D. J. Thomson, and G. T. Reed, “De-multiplexing free on-chip low-loss multimode switch enabling reconfigurable inter-mode and inter-path routing,” Nanophotonics 7(9), 1571–1580 (2018). [CrossRef]
27. Y. Qin, Y. Yu, M. Ye, and X. Zhang, “High-order mode polarization rotator for all optical SDM-PDM signals processing,” In Asia Communications and Photonics Coference (ACP), ATh3A.50, (2014).
28. M. R. Kossey, C. Rizk, and A. C. Foster, “End-fire silicon optical phased array with half-wavelength spacing,” APL Photonics 3(1), 011301 (2018). [CrossRef]
29. C. T. Phare, M. C. Shin, S. A. Miller, B. Stern, and M. Lipson, “Silicon optical phased array with high-efficiency beam formation over 180 degree field of view,” https://arxiv.org/abs/1802.04624.
30. W. Xu, L. Zhou, L Liang, and J. Chen, “Aliasing-free optical phased array beam steering with a plateau envelope,” Opt. Express 27(3), 3354–3368 (2019). [CrossRef]
31. Winnie N. Ye, Dan-Xia Xu, Siegfried Janz, Pavel Cheben, Marie-Josée Picard, Boris Lamontagne, and N. Garry Tarr, “Birefringence Control Using Stress Engineering in Silicon-on-Insulator (SOI) Waveguides,” J. Lightwave Technol. 23(3), 1308–1318 (2005). [CrossRef]
