Hybrid 3D Photonic Integrated Circuit for Optical ... - Semantic Scholar
Hybrid 3D Photonic Integrated Circuit for Optical Phased Array Beam Steering Binbin Guan1, Chuan Qin1, Ryan P. Scott1, Burcu Ercan1, Nicolas K. Fontaine2, Tiehui Su1 and S. J. B. Yoo1 1
Department of Electrical and Computer Engineering, University of California, Davis, One Shields Ave, Davis, CA, 95616, USA 2 Bell Laboratories, Alcatel-Lucent, 791 Holmdel Rd, Holmdel, NJ 07733, USA [email protected]
Abstract: We demonstrate a hybrid integrated optical phased array (OPA) based on a 2D photonic integrated circuit and 3D waveguides. The 4×4 OPA supports 4.93° horizontal and vertical beam steering near 1550 nm with 7.1-dB loss. OCIS codes: (050.6875) Three-dimensional fabrication; (250.5300) Photonic integrated circuits; (110.5100) Phased-array imaging systems
Nonmechanical, electronically controlled optical beam steering and beam forming by use of optical phased arrays (OPAs) [1, 2] is of great interest for applications such as light detection and ranging (LIDAR), free-space optical communications, and optical switches. Of particular interest are low-loss integrated OPA implementations that provide monochromatic 2-D beam steering (i.e., simultaneous elevation and azimuth angles) for higher-power applications in high-vibration environments (e.g., unmanned aerial vehicles). Previous 2-D integrated photonic OPA demonstrations either required a wavelength tunable source [1] or they utilized large-scale silicon photonic integrated circuits (PICs) [2] that accompanied limitations due to relatively large coupling losses, large waveguide losses, and strong optical nonlinearities. We demonstrate a hybrid-integrated device for 2-D OPA beam steering that is low loss and works with a simple monochromatic laser (i.e., no wavelength tuning). The hybrid device uses a silica planar lightwave circuit (PLC) coupled to a 3D waveguide structure fabricated by ultrafast laser inscription (ULI) [3]. Fig. 1(a) presents a 3D illustration of the hybrid device. After entering the center waveguide, a star coupler evenly distributes light to 16 waveguides that have thermo-optic phase shifters. The phase shifters provide active phase tuning and phase-error correction (PEC) before the silica PLC outputs couple their light to the 3D waveguides. The 3D waveguides transform the 1×16 waveguide array to a 4×4 waveguide array at the output facet [Fig. 1(b)]. As a result, the hybrid device can steer the beam in two dimensions arbitrarily when proper phase values are applied to the phase shifters. Since the hybrid device’s waveguides are path-length matched to < 1 µm, only the wavelength range of the star coupler and phase shifter limits the operating spectral bandwidth of the beam steering to cover the entire C-band. Fig. 1(c) shows a representation of the relative phases needed for each output waveguide for different beam steering (details in caption). The multiple inputs of the silica PLC provide coarse beam steering by exploiting the phase tilt imposed by the star coupler for off-center inputs. Simultaneously illuminating inputs, forms multiple beams.
Fig. 1. (a) Hybrid-integrated OPA device for 2-D beam steering. (b) Output facet detail. (c) Output waveguide phases for steering various directions: (i) on axis, (ii) horizontal, (iii) vertical, and (iv) both horizontal and vertical. (d) Photo of output facet showing 4×4 waveguide array.
We previously characterized the silica PLC performance [4] and it has an excess loss of 3.5 dB (i.e., loss beyond the designed splitting loss) and output power non-uniformity of < 1.5 dB. The 3D waveguides were fabricated in Corning Eagle2000 glass at UC Davis by the ULI technique [3], which utilizes the nonlinear process of multi-
photon absorption to provide a permanent refractive index increase in the glass at the focus of the femtosecond writing beam. The refractive index contrast in the fabricated device was ~0.5%, and the waveguide core size was optimized for a minimum mode field diameter (MFD) of 6.5 µm. Based on this MFD, the waveguide center-tocenter spacing was set to 18 µm, which is the minimum separation for < −20 dB coupling between waveguides. In general, a higher index contrast allows a smaller MFD and closer waveguide spacing. From sin /( 2d ) , we expect to achieve a 4.93° steering angle θ in both the horizontal and vertical dimensions. Here d is the waveguide center-to-center spacing, and δθ is phase difference between waveguides. The maximum steering angle is defined when δθ is equal to ±π. The 3D waveguide loss was ~3.6 dB, and the total hybrid device loss was ~7.1 dB. Fig. 2(a) shows the near field amplitude and phase measurement arrangement that is based on shearing interferometry [5]. The optical lens arrangement created a magnified image of device output on the infrared camera where it interfered with a tilted reference beam. Based on the near field phase measurements, we applied phase-error correction (PEC) to remove the phase errors that arise from deviations during device fabrication. Fig. 2(b) shows a comparison of measured near field phase of the output waveguides before and after PEC. Fig. 2(c) shows that the flat phase after PEC creates a well-formed on-axis (centered) beam in the far field (blue lines). The measured far field pattern in Fig. 2(c) shows beam steering in the horizontal and the vertical dimensions. The maximum horizontal steering was from +2.47° (green line) to −2.47° (not shown) for a total horizontal steering angle of 4.93°. The vertical steering showed similar results. Fig. 2(d) shows a 3D view of the measured far field beam profile for two cases: the on-axis (centered) beam with flat near field phase, and simultaneous horizontal and vertical beam steering [near-field phases as shown in Fig. 1(c)(iv)]. In summary, we have demonstrated an innovative, low loss integrated hybrid optical phased array device for 2-D beam steering. The proposed device can steer beams without wavelength tuning and has the potential for high-power and dual-polarization operation [4]. We achieved a 4.93° beam steering in both vertical and horizontal dimensions. Future work includes, decreasing total device loss through mode matching and 3D waveguide optimizations, and increasing the 3D waveguide index contrast through optimization of the ULI process so that the waveguide array spacing can be decreased and the beam steering angle correspondingly increased.
Fig. 2. (a) OPA amplitude and phase characterization arrangement. (b) Near field output phase without and with PEC. (c) Far field beam steering in the horizontal and vertical direction. (d) 3D view of far field beam steering for center position and simultaneous horizontal/vertical steering.
References [1] W. Guo, et al., “Two-dimensional optical beam steering with InP-based photonic integrated circuits,” IEEE J. Sel. Top. Quantum Electron., 19, 6100212-6100212 (2013). [2] J. Sun, et al., “Large-scale nanophotonic phased array,” Nature, 493, 195-199 (2013). [3] Y. Nasu, et al., “Low-loss waveguides written with a femtosecond laser for flexible interconnection in a planar light-wave circuit,” Opt. Lett., 30, 723-725 (2005). [4] B. Guan, et al., “Free-space coherent optical communication with orbital angular, momentum multiplexing/demultiplexing using a hybrid 3D photonic integrated circuit,” Opt. Express, 22, 145-156 (2014). [5] W. Steinchen, et al., Digital shearography: theory and application of digital speckle pattern shearing interferometry vol. PM100 (SPIE Press, 2003).
This work was supported in part by the ONR award N00014-13-1-0158.
Department of Electrical and Computer Engineering, University of California, Davis, One Shields Ave, Davis, CA, 95616, USA 2 Bell Laboratories, Alcatel-Lucent, 791 Holmdel Rd, Holmdel, NJ 07733, USA [email protected]
Abstract: We demonstrate a hybrid integrated optical phased array (OPA) based on a 2D photonic integrated circuit and 3D waveguides. The 4×4 OPA supports 4.93° horizontal and vertical beam steering near 1550 nm with 7.1-dB loss. OCIS codes: (050.6875) Three-dimensional fabrication; (250.5300) Photonic integrated circuits; (110.5100) Phased-array imaging systems
Nonmechanical, electronically controlled optical beam steering and beam forming by use of optical phased arrays (OPAs) [1, 2] is of great interest for applications such as light detection and ranging (LIDAR), free-space optical communications, and optical switches. Of particular interest are low-loss integrated OPA implementations that provide monochromatic 2-D beam steering (i.e., simultaneous elevation and azimuth angles) for higher-power applications in high-vibration environments (e.g., unmanned aerial vehicles). Previous 2-D integrated photonic OPA demonstrations either required a wavelength tunable source [1] or they utilized large-scale silicon photonic integrated circuits (PICs) [2] that accompanied limitations due to relatively large coupling losses, large waveguide losses, and strong optical nonlinearities. We demonstrate a hybrid-integrated device for 2-D OPA beam steering that is low loss and works with a simple monochromatic laser (i.e., no wavelength tuning). The hybrid device uses a silica planar lightwave circuit (PLC) coupled to a 3D waveguide structure fabricated by ultrafast laser inscription (ULI) [3]. Fig. 1(a) presents a 3D illustration of the hybrid device. After entering the center waveguide, a star coupler evenly distributes light to 16 waveguides that have thermo-optic phase shifters. The phase shifters provide active phase tuning and phase-error correction (PEC) before the silica PLC outputs couple their light to the 3D waveguides. The 3D waveguides transform the 1×16 waveguide array to a 4×4 waveguide array at the output facet [Fig. 1(b)]. As a result, the hybrid device can steer the beam in two dimensions arbitrarily when proper phase values are applied to the phase shifters. Since the hybrid device’s waveguides are path-length matched to < 1 µm, only the wavelength range of the star coupler and phase shifter limits the operating spectral bandwidth of the beam steering to cover the entire C-band. Fig. 1(c) shows a representation of the relative phases needed for each output waveguide for different beam steering (details in caption). The multiple inputs of the silica PLC provide coarse beam steering by exploiting the phase tilt imposed by the star coupler for off-center inputs. Simultaneously illuminating inputs, forms multiple beams.
Fig. 1. (a) Hybrid-integrated OPA device for 2-D beam steering. (b) Output facet detail. (c) Output waveguide phases for steering various directions: (i) on axis, (ii) horizontal, (iii) vertical, and (iv) both horizontal and vertical. (d) Photo of output facet showing 4×4 waveguide array.
We previously characterized the silica PLC performance [4] and it has an excess loss of 3.5 dB (i.e., loss beyond the designed splitting loss) and output power non-uniformity of < 1.5 dB. The 3D waveguides were fabricated in Corning Eagle2000 glass at UC Davis by the ULI technique [3], which utilizes the nonlinear process of multi-
photon absorption to provide a permanent refractive index increase in the glass at the focus of the femtosecond writing beam. The refractive index contrast in the fabricated device was ~0.5%, and the waveguide core size was optimized for a minimum mode field diameter (MFD) of 6.5 µm. Based on this MFD, the waveguide center-tocenter spacing was set to 18 µm, which is the minimum separation for < −20 dB coupling between waveguides. In general, a higher index contrast allows a smaller MFD and closer waveguide spacing. From sin /( 2d ) , we expect to achieve a 4.93° steering angle θ in both the horizontal and vertical dimensions. Here d is the waveguide center-to-center spacing, and δθ is phase difference between waveguides. The maximum steering angle is defined when δθ is equal to ±π. The 3D waveguide loss was ~3.6 dB, and the total hybrid device loss was ~7.1 dB. Fig. 2(a) shows the near field amplitude and phase measurement arrangement that is based on shearing interferometry [5]. The optical lens arrangement created a magnified image of device output on the infrared camera where it interfered with a tilted reference beam. Based on the near field phase measurements, we applied phase-error correction (PEC) to remove the phase errors that arise from deviations during device fabrication. Fig. 2(b) shows a comparison of measured near field phase of the output waveguides before and after PEC. Fig. 2(c) shows that the flat phase after PEC creates a well-formed on-axis (centered) beam in the far field (blue lines). The measured far field pattern in Fig. 2(c) shows beam steering in the horizontal and the vertical dimensions. The maximum horizontal steering was from +2.47° (green line) to −2.47° (not shown) for a total horizontal steering angle of 4.93°. The vertical steering showed similar results. Fig. 2(d) shows a 3D view of the measured far field beam profile for two cases: the on-axis (centered) beam with flat near field phase, and simultaneous horizontal and vertical beam steering [near-field phases as shown in Fig. 1(c)(iv)]. In summary, we have demonstrated an innovative, low loss integrated hybrid optical phased array device for 2-D beam steering. The proposed device can steer beams without wavelength tuning and has the potential for high-power and dual-polarization operation [4]. We achieved a 4.93° beam steering in both vertical and horizontal dimensions. Future work includes, decreasing total device loss through mode matching and 3D waveguide optimizations, and increasing the 3D waveguide index contrast through optimization of the ULI process so that the waveguide array spacing can be decreased and the beam steering angle correspondingly increased.
Fig. 2. (a) OPA amplitude and phase characterization arrangement. (b) Near field output phase without and with PEC. (c) Far field beam steering in the horizontal and vertical direction. (d) 3D view of far field beam steering for center position and simultaneous horizontal/vertical steering.
References [1] W. Guo, et al., “Two-dimensional optical beam steering with InP-based photonic integrated circuits,” IEEE J. Sel. Top. Quantum Electron., 19, 6100212-6100212 (2013). [2] J. Sun, et al., “Large-scale nanophotonic phased array,” Nature, 493, 195-199 (2013). [3] Y. Nasu, et al., “Low-loss waveguides written with a femtosecond laser for flexible interconnection in a planar light-wave circuit,” Opt. Lett., 30, 723-725 (2005). [4] B. Guan, et al., “Free-space coherent optical communication with orbital angular, momentum multiplexing/demultiplexing using a hybrid 3D photonic integrated circuit,” Opt. Express, 22, 145-156 (2014). [5] W. Steinchen, et al., Digital shearography: theory and application of digital speckle pattern shearing interferometry vol. PM100 (SPIE Press, 2003).
This work was supported in part by the ONR award N00014-13-1-0158.
