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Abstract
Two-dimensional (2-D) integrated optical phased arrays (OPA) have many applications from optical imaging to LiDAR. Conventionally, 2-D beam-steering in an N × N OPA requires N2 phase shifters placed within the phased array aperture, resulting in a high per-element power consumption while limiting the minimum achievable element-to-element spacing. In this paper, we report an OPA architecture, where for 2-D beam-steering in an N × N OPA, only 2N phase shifters outside of the array aperture are used, which significantly reduces the total OPA power consumption and eliminates electrical routing within the aperture. As a proof of concept, an 8 × 8 OPA is implemented that uses 16 phase shifters to perform 2-D beam-steering without tuning the wavelength. Using the aperture size of 77 µm × 77 µm for the implemented OPA transmitter, far-field beam-steering over a range of about 7° is demonstrated.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Corrections
31 August 2022: A typographical correction was made to Fig. 2. A typographical correction was made to paragraph 2 of page 27184. A typographical correction was made to all paragraphs on page 27185.
1. Introduction
An optical phased array (OPA) is an array of optical emitting or receiving antennas (elements) that enables beam-forming and steering and is widely used in various applications including optical communications [1], LiDAR systems [2–4], projection systems [5], and 3-D imaging [6]. In such systems, the far-field beam is formed and steered by controlling the relative phases between optical elements within the array aperture. Benefiting from integrated photonic platforms, a number of OPAs have been demonstrated [2–15]. In the conventional implementation of a 2-D N × N element OPA, N2 phase shifters are used to perform per-element optical phase adjustment [2–5,7,8].
To form a narrow beam with a large side-lobe suppression ratio, which is steerable over a large field-of-view, a large number of closely placed elements (ideally with element spacing approaching half of the wavelength) within a large aperture is required. This results in a large power consumption in a conventional 2-D OPA with per-element phase shifters and makes the optical and electrical routing within the array aperture rather challenging.
In 2-D OPAs with grating based elements (e.g. grating couplers), this challenge can be addressed by steering the beam in one dimension through adjusting the relative phases between the elements, while using wavelength tuning for beam-steering in the other dimension [12–15]. Despite excellent beam-steering results, this method requires a highly tunable laser (over a large wavelength range) and typically achieves a smaller steering range through wavelength tuning compared to the relative phases adjustment between the OPA elements.
In this paper, we introduce a novel OPA architecture, where for 2-D beam-steering in an N × N OPA, only 2N phase shifters outside of the array aperture are used to adjust the relative phases between elements and no laser wavelength tuning is performed. In the proposed scheme, the number of phase shifters for an N × N array is reduced from N2 (for a conventional 2-D OPA) to 2N, significantly reducing the power consumption of an OPA with a large number of elements. Furthermore, placing the phase shifters outside of the array aperture enables realization of OPAs with compact element spacing and eliminates the electrical routing within the aperture and the corresponding metallic-induced optical loss.
As a proof of concept, we have implemented an 8 × 8 OPA, which uses a single wavelength laser source and 16 phase shifters outside of the aperture to perform 2-D beam-steering. Using the aperture size of 77 µm × 77 µm (element spacing of 11 µm) for the implemented OPA transmitter, the far-field beam-steering range of approximately 7° is achieved.
2. Theory of operation
In an OPA, the beam forming and steering is performed by controlling the relative phases between the elements. Figure 1(a) shows the structure of the proposed N × N OPA, where N nanophotonic waveguides serving as rows cross N nanophotonic waveguides serving as columns. Consider the case that the electric field of optical wave entering the mth row is written as ${E_m} = {a_m}{e^{j{\phi _m}}}$, and the electric field of optical wave entering the nth column is written as ${E_n} = {b_n}{e^{j{\theta _n}}}$ where ${a_m}$, ${b_n}$, ${\phi _m}$, and ${\theta _n}$ are the amplitude of the optical field in the mth row, the amplitude of the optical field in the nth column, the phase of the optical signal in the mth row, and the phase of the optical signal in the nth column, respectively. Note that the term ${e^{j{\omega _0}t}}$, representing the optical frequency of the signal, is dropped for simplicity. Before the crossing point of the mth row and the nth column, two directional couplers with varying coupling ratio followed by a Y-junction are used to combine a fraction of the light from the mth row with a fraction of the light from the nth column. The Y-junction output is connected to a grating coupler, GCmn, acting as the OPA element. The length of each directional coupler as well as the etch level in the coupling region is adjusted to ensure all grating couplers receive the same power. The electric field at the input of GCmn is written as
Fig. 1. Conceptual schematic of the proposed N × N OPA, where optical signals traveling in the row and column waveguides have (a) different phases and amplitudes and (b) different phases but the same amplitude.
In this case, the relative phase between GCmn and GC(m+1)n can be written as
As the wave propagates from one element to the next on a row or column, it experiences an additional static phase introduced by devices placed on each waveguide between elements (such as waveguide crossings and directional couplers with varying etch levels and lengths). This is shown in Fig. 2(f). This non-uniform static phase between elements can be optimized to increase the power within the desired 2-D steerable beam. For example, the power of the main lobe in Fig. 2(c) can be almost uniformly distributed among the main and side lobes if a linearly increasing static phase is created between elements on each column. In this case, while the beam can be steered in the X direction by setting $\Delta \theta_{n}$, the diffraction pattern in the Y direction is set by the varying static phase between adjacent elements on each column. Assuming the static phase between the $(i+1)^{\text {th }}$ and $i^{\text {th }}$ elements on each column (in radians) to have the form of $\theta_{s, i}=1.86 i-0.16$, the pattern in Fig. 2(c) will be changed to the one in Fig. 2(g). Similarly, same relative static phase condition on each row (i.e. $\phi_{s, i}=1.86 i-0.16$) changes the pattern in Fig. 2(d) to the one shown in Fig. 2(h). For the case that optical waves propagate in rows and columns, the superposition of the diffraction patterns in Fig. 2(g) and Fig. 2(h) results in the formation of the main beam in Fig. 2(i) that can be steered in 2-D. Note that the power of different side-lobes, the number of side-lobes, and the side-lobe spacing, could be set using relative static phase and/or relative static amplitude optimization. Figure 2(j) to 2(o) show the beams-steering to different angles (for $\phi_{s, i}=\theta_{s, i}=1.86 i-0.16$), which is performed by setting $\Delta \theta_{n}$ and $\Delta \phi_{m}$. Compared to the main beam in Fig. 2(a), the formed beam in Fig. 2(i) has about 3.5 dB lower intensity.
For the case that equal power for main and side lobes is desired while the power is uniformly distributed across all antenna emitters, directional couplers with varying length and etch levels can be used to create a linearly increasing relative phase between adjacent elements while coupling the same power to all elements, which is achieved in this work using an optimization process resulting in coupling length and etch levels shown in the table in Fig. 4(c).
In practice, the fabrication process variations may affect uniform power distribution across the main and side lobes. Note that in measurements, the side-lobes within the diffraction pattern may not clearly appear in the captured far-field pattern due to the non-linearity and the contrast enhancement feature of an IR camera. Also, note that the proposed architecture here can be used to form and steer a single beam and is not capable of continuous multi-beam steering.
Fig. 2. (a) The pattern (array factor) of an 8x8 OPA with 16 phase shifters, and, (b) with the scaled color map. (c) Pattern with no optical signals in the rows, for $\Delta \theta_{n}=110^{\circ}$ (d) Pattern with no optical signals in the columns, for $\Delta \phi_{n}=110^{\circ}$. (e) Superposition of the beams in (c) and (d). (f) OPA with static relative phases between elements. (g) The pattern in (c) after static phase adjustment. (h) The pattern in (d) after static phase adjustment. (i) Superposition of the beams in (g) and (h). Beam-steering to different angles are shown in (j) to (o).
3. System design
Figure 3(a) shows the structure of the implemented OPA transmitter. The input light is coupled into the chip using an on-chip grating coupler and is guided to a network of Y-junction splitters. The splitter network uniformly splits the coupled light into 16 branches. The phase of optical wave in the nanophotonic waveguide in each branch is adjusted through a thermal phase shifter. After phase adjustment, 8 nanophotonic waveguides serving as rows and 8 nanophotonic waveguides serving as columns are used to guide the optical waves to an array of 8 × 8 grating couplers serving as the OPA elements. Each grating coupler is placed near the crossing point of a row and a column waveguide and is fed by two directional couplers followed by a Y-junction combiner used to combine a fraction of the light from the corresponding row and column waveguides. The directional couplers have varying lengths to ensure that all grating couplers receive the same optical power. The spacing between the grating couplers within the array aperture is 11 µm and they are placed symmetrically with respect to the row and column waveguides as shown in Fig. 3(b). The microphotograph of the photonic chip is shown in Fig. 3(c). The chip has an area of 1000 µm × 950 µm, while the emitter aperture area is 77 µm × 77 µm. The chip is fabricated in the IME 180 nm silicon-on-insulator (SOI) photonic process.
Fig. 3. (a) The structure of the implemented 8 × 8 OPA with 16 phase shifters placed outside of the array aperture. (b) The structures of the grating couplers (as OPA elements) and directional couplers with varying lengths. (c) The microphotograph of the implemented 8 × 8 OPA chip fabricated in the IME 180 nm SOI process.
Figures 4(a)–4(c) show the structure of different photonic devices used in the implementation of the proposed OPA. Figure 4(a) shows the 200 µm long thermal phase shifter, which consists of a metal heater on top of a silicon waveguide and two thermally isolating deep trenches on the sides to enhance the thermal efficiency. The phase shifter has a measured resistance of 350 Ω and Iπ of 4.5 mA.
Fig. 4. (a) The structure and the microphotograph of the implemented thermal phase shifter. (b) The structures of the nanophotonic waveguides, the waveguide crossing, directional couplers with varying length, the Y-junction [18], and the grating coupler serving as the OPA element. (c) Finite-Difference Time-Domain (FDTD) simulation results for the directional couplers with varying length and different silicon etch levels within the coupling region.
Figure 4(b) shows the structure of a grating coupler (serving as the OPA element) along with the corresponding row and column waveguides and the feed network. The IME 180 nm SOI process offers three silicon etch levels that are used in the design of the compact directional couplers, grating couplers, and the compact waveguide crossings. Directional couplers with varying lengths are used in order to equally distribute the optical power in each row and column. To make the element spacing as small as possible, the coupling lengths should be minimized, which is achieved by partially etching the gap in the coupling region to increase the coupling coefficient for a given length. Considering the transmission and the coupling coefficients of the directional couplers, as well as the loss of the waveguide crossings, the required coupling lengths and the etch depth for the 7 directional couplers in each row/column are simulated, which are shown in Fig. 4(c). Note that the 500 nm wide 220 nm thick single-mode nanophotonic waveguides have a measured loss of under 2 dB/cm.
The design of the grating coupler serving as the OPA element follows the one presented in [17], however, the device dimensions and the number of gratings are modified to optimize the coupling efficiency for a compact footprint. The simulated efficiency of the grating coupler is about 30% at 1500 nm. To further reduce the element spacing, a compact waveguide crossing structure is designed that has a simulated insertion loss of under 0.3 dB with an isolation of more than 30 dB. To achieve symmetric routing, the grating couplers are placed at a 45 degree angle with respect to the column waveguides.
4. Characterization and measurement results
The measurement setup for characterizing the proposed OPA is shown in Fig. 5(a), where a laser emitting 1 mW at 1502 nm is coupled into the chip input grating coupler using a single-mode optical fiber. Two digitally controlled 8-channel thermal phase modulator drivers were used to control the row and column phase shifters. A FJW FIND-R-Scope 85700A infrared camera with adjustable optics were used to monitor the far-field interference pattern of the OPA.
Fig. 5. (a) Measurement setup used to characterize the OPA chip. (b) Measured far-field interference pattern showing four grating lobes. (c) Demonstration of the 2-D far-field beam-steering. (d) Two consecutive resolvable spots while diagonal beam-steering is performed. The bottom figure shows the beam profiles.
In order to demonstrate 2-D beam-steering, the thermal phase shifters were used to control the phase of the optical signals in each row and column.
To account for the different waveguide routing lengths and process variations, the OPA was first calibrated (using the procedure in [5]), where the far-field interference pattern is compared with the simulation result for the case that both row and column signals have zero phase offset. Then, by adjusting the currents of the thermal phase shifters around the corresponding calibration point, the 2-D beam-forming was performed. Figure 5(b) shows four grating lobes of the far-field interference pattern of the OPA. Figure 5(c) shows the 2-D beam-steering results (only one grating lobe is shown). By controlling the phase of the thermal phase modulators in the rows and columns, beam-steering range of about 7° is demonstrated, which is in agreement with the simulation results in Fig. 2(a). Figure 5(d) shows two consecutive resolvable spots as the main lobe is steered diagonally. The corresponding beam profiles are shown in the bottom section of Fig. 5(d).
The estimated output power for each grating coupler (emitter) is about 38 dB lower than the power coupled into the input grating coupler. Also, for the case that the optical power coupled into the chip is set to -2 dBm, the measured radiated power in the main lobe is about -45 dBm.
5. Conclusion
A novel architecture to perform 2-D beam-steering in an N × N OPA with only 2N phase shifters placed outside of the array aperture is presented. In this case, the number of phase shifters is reduced by a factor of N/2, which results in a significant power consumption reduction for a 2-D OPA with a large number of elements. As a proof of concept, an 8 × 8 OPA with 16 phase shifters outside of the aperture has been implemented and used to demonstrate robust 2-D beam-steering over a range of about 7°.
Funding
Defense Advanced Research Projects Agency (FA8650-18-1-7828).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
References
1. S. J. Spector, B. F. Lane, M. R. Watts, L. D. Benney, J. G. Delva, A. E. Hare, A. F. Kelsey, J. M. Mlynarczyk, E. S. Hosseini, C. V. Poulton, and J. P. Laine, “Broadband imaging and wireless communication with an optical phased array,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (online) (Optical Society of America, 2018), paper SM3I.7
2. W. Xie, T. Komljenovic, J. Huang, M. Tran, M. Davenport, A. Torres, P. Pintus, and J. Bowers, “Heterogeneous silicon photonics sensing for autonomous cars [Invited],” Opt. Express 27(3), 3642–3663 (2019). [CrossRef]
3. J. Sun, E. Timordugan, A. Yaacobi, E. Shah Hosseini, and M. R. Watts, “Large-scale nanophotonic phased array,” Nature 493(7431), 195–199 (2013). [CrossRef]
4. C. V. Poulton, M. J. Byrd, P. Russo, E. Timordugan, M. Khandaker, D. Vermeulen, and M. R. Watts, “Long-range LiDAR and free-space data communication with high-performance optical phased arrays,” IEEE J. Sel. Top. Quantum Electron. 25(5), 1–8 (2019). [CrossRef]
5. F. Aflatouni, B. Abiri, A. Rekhi, and A. Hajimiri, “Nanophotonic projection system,” Opt. Express 23(16), 21012–21022 (2015). [CrossRef]
6. F. Aflatouni, B. Abiri, A. Rekhi, and A. Hajimiri, “Nanophotonic coherent imager,” Opt. Express 23(4), 5117–5125 (2015). [CrossRef]
7. B. Abiri, F. Aflatouni, A. Rekhi, and A. Hajimiri, “Electronic two-dimensional beam steering for integrated optical phased srrays,” in Optical Fiber Communication Conference, OSA Technical Digest (online) (Optical Society of America, 2014), paper M2K.7.
8. R. Fatemi, B. Abiri, and A. Hajimiri, “An 8×8 heterodyne lens-less OPA camera,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (online) (Optical Society of America, 2017), paper JW2A.9.
9. S. Chung, H. Abediasl, and H. Hashemi, “15.4 A 1024-element scalable optical phased array in 0.18µm SOI CMOS,” in 2017IEEE International Solid-State Circuits Conference (ISSCC), San Francisco.
10. C. V. Poulton, P. Russo, E. Timurdogan, M. Whitson, M. J. Byrd, E. Hosseini, B. Moss, Z. Su, D. Vermeulen, and M. R. Watts, “High-performance integrated optical phased arrays for chip-scale beam steering and LiDAR,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (online) (Optical Society of America, 2018), paper ATu3R.2.
11. K. Van Acoleyen, H. Rogier, and R. Baets, “Two-dimensional optical phased array antenna on silicon-on-insulator,” Opt. Express 18(13), 13655–13660 (2010). [CrossRef]
12. J. K. Doylend, M. J. R. 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]
13. D. N. Hutchison, J. Sun, J. K. Doylend, R. Kumar, J. Heck, W. Kim, C. T. Phare, A. Feshali, and H. Rong, “High-resolution aliasing-free optical beam steering,” Optica 3(8), 887–890 (2016). [CrossRef]
14. J. C. Hulme, J. K. Doylend, M. J. R. Heck, J. D. Peters, M. L. Davenport, J. T. Bovington, L. A. Coldren, and J. E. Bowers, “Fully integrated hybrid silicon two dimensional beam scanner,” Opt. Express 23(5), 5861–5874 (2015). [CrossRef]
15. T. Komljenovic, R. Helkey, L. Coldren, and J. Bowers, “Sparse aperiodic arrays for optical beam forming and LIDAR,” Opt. Express 25(3), 2511–2528 (2017). [CrossRef]
16. J. Goodman, Introduction to Fourier Optics (McGraw-Hill, 1996).
17. A. Mekis, S. Gloeckner, G. Masini, A. Narasimha, T. Pinguet, S. Sahni, and P. D. Dobbelaere, “A grating-coupler-enabled CMOS photonics platform,” IEEE J. Sel. Top. Quantum Electron. 17(3), 597–608 (2011). [CrossRef]
18. Y. Zhang, S. Yang, A. E. Lim, G. Q. Lo, C. Galland, T. Baeher-Jones, and M. Hochberg, “A compact and low loss Y-junction for submicron silicon waveguide,” Opt. Express 21(1), 1310–1316 (2013). [CrossRef]
