Abstract

Optical phased arrays are a promising beam-steering technology for ultra-small solid-state lidar and free-space communication systems. Long-range, high-performance arrays require a large beam emission area densely packed with thousands of actively phase-controlled, power-hungry light emitting elements. To date, such large-scale phased arrays have been impossible to realize since current demonstrated technologies would operate at untenable electrical power levels. Here we show a multi-pass photonic platform integrated into a large-scale phased array that lowers phase shifter power consumption by nearly 9 times. The multi-pass structure decreases the power consumption of a thermo-optic phase shifter to a ${{\rm P}_\pi }$ of ${1.7}\;{\rm mW/}\pi $ without sacrificing speed or optical bandwidth. Using this platform, we demonstrate a silicon photonic phased array containing 512 actively controlled elements, consuming only 1.9 W of power while performing 2D beam steering over a ${70}^\circ \times {6}^\circ $ field of view. Our results demonstrate a path forward to building scalable phased arrays containing thousands of active elements.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Optical phased arrays [115] provide a non-mechanical, robust approach to beam steering; however, their scalability has been limited by the high power consumption of their embedded large number of phase shifters. In order to achieve a widely steerable, low-divergence output beam that can enable free-space communication systems [16,17] and ranging of objects over 100 m away for applications like autonomous vehicle lidar [1820], a large emission area (or aperture) containing thousands of densely packed phase shifters is needed. These phase shifters typically consist of power-hungry thermo-optic phase shifters that consume tens of milliwatts of power, leading to an overall power consumption of hundreds of watts across the array. The largest active phased array to date, reported by Chung et al. [12], contains 1024 elements and consumes 55 W of power from the thermo-optic phase shifters. Hutchison et al. [7] demonstrated a 128-element independently controllable phased array, which consumes $ \sim 10 \; {\rm W}$ via traditional thermo-optic phase shifters. There has been a recent demonstration of a phased array containing 512 elements by Poulton et al. [19,20] using electro-optic phase shifters consuming less power. However, the device length and drive voltage [21,22] need to be reduced to be compatible with the state-of-the-art CMOS electronics. In general, the overwhelming power consumption and high voltages needed to drive large arrays of phase shifters has prohibited larger element-count demonstrations of active phased arrays thus far. Therefore, a low-power, low-loss phase shifter that does not sacrifice switching speed or operating optical bandwidth is crucial for the scalability of these systems.

Here we show a multi-pass photonic platform that reduces the power consumption of an optical phase shifter while maintaining both its operation speed and broadband low loss for enabling scalable optical systems. Typical interference-based structures, like resonant cavities, increase the efficiency of phase shifters by making light pass through them multiple times, but they do so at the expense of narrowing the optical bandwidth. Here, in contrast, our multi-pass platform relies on spatial mode multiplexing to circulate light multiple times through a phase shifter. A conversion to a different orthogonal spatial mode occurs each time light is recirculated, eliminating unwanted interference and maintaining broadband operation. By embedding a thermo-optic phase shifter in this multi-pass structure, light accumulates phase shift from all passes. In Fig. 1(a) we show the multi-pass platform consisting of a multimode waveguide where the phase shifter is embedded, surrounded by multiple passive mode converters as well as single-mode input and output waveguide ports. In the multi-pass structure shown here, light circulates back and forth a total of 7 times. Figure 1(b) shows a schematic of the light path. Light is input into the multi-pass structure in the ${{\rm TE}_0}$ mode, and upon exiting the multimode waveguide it is converted to the ${{\rm TE}_1}$ mode via a mode converter and sent back to the multimode waveguide in the opposite direction. Upon exiting the multimode waveguide, light is then converted to the ${{\rm TE}_2}$ mode and sent back to the multimode waveguide in the forward direction, and so on. To ensure that the output of the device is in the fundamental mode, light propagating in the highest order mode is converted back to the ${{\rm TE}_0}$ mode when exiting the device. We also design the structure to ensure that only one mode in the multimode waveguide is selectively dropped into the mode converter, converted to the next higher-order mode, and then added back, while all other modes remain unperturbed. In Fig. 1(c), we show as an example the ${{\rm TE}_2}$-to-${{\rm TE}_3}$ converter. One can see that the ${{\rm TE}_2}$ mode of the multimode waveguide is dropped to the mode converter by coupling to the ${{\rm TE}_0}$ mode of the narrow access waveguide via a directional coupler [2328], and then selectively coupled back into the phase shifter by converting to the ${{\rm TE}_3}$ mode through another directional coupler. We use adiabatic directional couplers [2932] in order to achieve a high tolerance to fabrication variations, which thus enables low insertion loss over a broad bandwidth (see Supplement 1 Section 1).

 figure: Fig. 1.

Fig. 1. Multi-pass photonic structure based on mode multiplexing. (a) Schematic (not to scale) of a seven-pass structure that utilizes seven spatial modes. (b) Schematic description of the light path, illustrating mode conversion at each pass. (c) Schematic (not to scale) of a structure that converts the ${{\rm TE}_2}$ mode to the ${{\rm TE}_3}$ mode and reverses the propagation direction, while transmitting all other lower-order modes. (d) Optical microscope image of a seven-pass multi-pass structure embedded in a MZI. A resistive heater is fabricated on top of the ${{\rm SiO}_2}$ cladding to induce phase shifting via the thermo-optic effect. MC, mode converter; DC, directional coupler; WG, waveguide.

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 figure: Fig. 2.

Fig. 2. Measured phase shift and bandwidth performance of multi-pass phase shifters. (a) Accumulated phase shift induced by the three-pass, five-pass, and seven-pass multi-pass thermo-optic phase shifters, extracted by measuring the transmission spectra of the MZIs. As a reference we also show a standard single-pass phase shifter (denoted by one-pass). The dashed lines are the linear fits to the data. The three-pass, five-pass, and seven-pass structures decrease power consumption by 3.3, 5.9, and 8.9 times, respectively. (b) Insertion loss for three-, five-, and seven-pass devices as a function of wavelength, extracted from the MZI transmission spectra. The dashed lines are cubic spline smoothing functions that exclude the artifacts due to the interference from facet reflections. One can see that the bandwidth in which the insertion losses remain less than 3 dB above the minimum loss is at least 100 nm for all structures. The inset shows the measured transmission spectra of the MZIs for the different multi-pass structures. The free spectral range of the interference fringes decreases as the number of passes increases, confirming an increase of the optical path length.

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Using the multi-pass platform, we show that the power consumption of a thermo-optic phase shifter is reduced by nearly 9 times to 1.7 mW per $\pi $ phase shift. In order to measure the accumulated phase shift, we embed the multi-pass structure in a Mach–Zehnder interferometer (MZI) as shown by Fig. 1(d). We measure the phase shift of the MZI interference fringes at different applied powers for a standard single-pass device as well as for three-pass, five-pass, and seven-pass multi-pass structures. All devices have the same multimode waveguide width of 2.44 µm where the thermo-optic phase shifter is embedded. We observe a clear decrease in power consumption as the number of passes increases as shown in Fig. 2(a). In order to achieve a full $\pi $ phase shift, the consumed power was 15.4, 4.6, 2.6, and 1.7 mW for the one-pass, three-pass, five-pass, and seven-pass phase shifters, respectively. This corresponds to a decrease in power consumption by 3.3, 5.9, and 8.9 times in the three-pass, five-pass, and seven-pass phase shifter, respectively. Note that power is decreased by a factor slightly higher than the number of passes because the effective refractive indices of the higher-order modes are more sensitive to temperature change due to their higher group index (see Supplement 1 Section 3). We observe only 3.7% variation in phase shift over a broad wavelength range from 1525 to 1600 nm wavelength (see Supplement 1 Section 6 and Fig. S11). We measure a phase tuning time $\tau $ of 6.5 µs, independent of the number of passes (see Supplement 1 Fig. S9). This is in contrast to other methods of decreasing the phase shifter power consumption, such as isolation trenches or undercuts [33], which greatly increase the tuning time. The power-time product ${{\rm P}_\pi } \cdot \tau $ figure of merit of our seven-pass multi-pass phase shifter is ${11.1}\;{\rm mW} \cdot {\unicode{x00B5}\rm s}$, which is lower than that of other state-of-the-art thermo-optic phase shifters, including heaters on thermally isolated free-standing waveguides [33] (${{\rm P}_\pi } \cdot \tau = {56.1}\;{\rm mW} \cdot {\unicode{x00B5}\rm s}$) and doped-silicon heaters with adiabatic bends [34] (${{\rm P}_\pi } \cdot \tau = {30.5}\;{\rm mW} \cdot {\unicode{x00B5} \rm s}$). Other standard low-power thermo-optic phase shifters [39,40] have lower insertion loss but still have higher electrical power consumption. Since the phase shifter power scales linearly with the number of elements while the laser power suffers only from a one-time phase shifter loss, our platform is advantageous when the system is large scale.

We demonstrate efficient phase shifting over a bandwidth exceeding 100 nm and a low insertion loss of 0.44 dB per pass for the first five passes. In Fig. 2(b) we show the measured insertion losses of the multi-pass structures extracted from the visibility of the MZI interference fringes. One can see that the minimum insertion losses are 1.2 dB (at the wavelength of 1570 nm), 2.2 dB (at 1594 nm), and 4.6 dB (at 1601 nm) for the three-pass, five-pass, and seven-pass structure, respectively. The 3-dB bandwidths of all multi-pass structures exceed 100 nm. From these measured results, we estimate that the first five passes have an insertion loss of 0.44 dB per mode-conversion event. The sixth and seventh passes have higher insertion losses of 1.2 dB per mode-conversion event, due to their closer mode effective indices leading to higher fabrication sensitivity. Note that these insertion losses are not fundamental and can be mitigated using more robust adiabatic coupler designs [30,35,41]. By taking advantage of the lowered dispersion of subwavelength waveguides with an optimized adiabatic transition, we can achieve wideband and efficient higher-order mode conversion [41]. By using the optimized converters, we can achieve the total insertion loss of 2–3 dB across 150 nm bandwidth for the seven-pass phase shifter (see Supplement 1 Fig. S2).

 figure: Fig. 3.

Fig. 3. Optical phased array containing 512 multi-pass phase shifters. (a) Schematic (not to scale) of optical phased array, showing out-of-plane beam emission (red arrows) and 2D steering ($\varphi $ and $\theta $). (b) Optical microscope image of the silicon waveguide layer of the fabricated chip, showing several stages of the binary splitter tree and a portion of the array of multi-pass phase shifter structures. (c) Packaged device, consisting of the ${8}\;{\rm mm} \times {15}\;{\rm mm}$ phased array chip wire-bonded to a silicon interposer, along with an optical fiber input. The grating emission area is highlighted in green.

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Using the multi-pass phase shifter platform, we demonstrate low-power 2D beam steering using a silicon optical phased array with 512 actively controlled channels. The optical phased array consists of a single input waveguide feeding into a nine-stage binary splitter tree, followed by a parallel array of 512 independently controlled multi-pass phase shifters, which is followed by a one-dimensional array of long grating emitters [Fig. 3(a)]. Two-dimensional beam steering is achieved in the horizontal ($\varphi $) direction via controlling the phase front across the array aperture and in the vertical ($\theta $) direction via wavelength tuning through the wavelength-sensitive grating emitters. The splitter tree contains ${1} \times {2}$ multimode interferometer (MMI) 50/50 splitters, each exhibiting less than 0.03 dB loss per splitter. In order to reduce the overall insertion loss while still gaining the multi-pass power efficiency, we use five-pass multi-pass phase shifters in this implementation. These phase shifters are separated by 28 µm to minimize thermal crosstalk between channels, and they are subsequently fanned in to a 1.3 µm emitter pitch. This waveguide fan-in design ensures that each channel has an equal propagation length. Finally, we use a weak single-etch step side-wall grating with a grating pitch of 520 nm and an 8 nm corrugation width to create a long emission over a 2 mm length. The long grating emission length produces a low divergence beam in the vertical ($\theta $) dimension. It is designed for higher wavelength sensitivity by tilting the nominal emission angle to 45° from the normal (see Supplement 1 Section 7). We fabricate this array on a 220 nm thick silicon on insulator (SOI) wafer in a university cleanroom (see Supplement 1 Section 11). An optical micrograph of the device is shown in Fig. 3(b). We use a 2.5D integration approach to route the electrical control signals to the dense array through an electrical silicon interposer to a printed circuit board (PCB) and external control circuitry (see Supplement 1 Section 12). The packaged device is shown in Fig. 3(c).

 figure: Fig. 4.

Fig. 4. Measured 2D beam steering and system power consumption. (a) Characterization of the output beam in the far field. Line cuts of the $\varphi $ and $\theta $ directions show near-diffraction-limited beam divergence of ${0.15}^\circ \times {0.08}^\circ $ (diffraction limit of ${0.133}^\circ \times {0.08}^\circ $). Inset shows full 2D far-field image of the beam. (b) The measured far-field emission pattern above the chip showing a ${70}^\circ \times {6}^\circ $ field of view, demonstrating 7.5 dB peak-to-sidelobe ratio for the beam pointing toward ($ + {35}^\circ $, 45°). (c) Electrical power consumption of the multi-pass phase shifters for converged beams at different steering angles (shaded region represents 1 standard deviation from the mean). One can see that only ${1.9} \pm {0.2}\;{\rm W}$ total power is consumed for any angle in the reported field of view.

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We demonstrate beam steering over a ${70}^\circ \times {6}^\circ $ field of view with a total power consumption of 1.9 W. In order to create a flat phase front and form a single converged output beam, we run an initial calibration on the phased array using a global optimization algorithm [36]. The algorithm maximizes the power on a single pixel photodiode placed in the far field approximately 20 cm from the chip output. We characterize the far-field beam emission using a two-axis motorized scanning mirror (see Supplement 1 Section 13). We measure a beam divergence of ${0.15}^\circ \times {0.08}^\circ $ seen in the far-field beam scan as well as in the line cuts in both $\varphi $ and $\theta $ directions [Fig. 4(a)]. The horizontal ($\varphi $) beam divergences are close to the theoretically predicted values of 0.133°. The vertical beam divergence ($\theta $ direction) indicates an effective aperture size of 1.1 mm projected at 45° from a physical emission aperture length of 1.6 mm. Figure 4(b) shows single isolated beams at the limits of our steering range, illustrating that there are no visible aliasing lobes. The peak-to-sidelobe ratio of 7.5 dB is observed across the entire field of view, measured when the beam points toward (35°, 45°). The peak-to-sidelobe ratio is less than the theoretical value due to the strong background noise from the input fiber and some defective phase shifters from fabrication imperfections. To achieve the 6° vertical steering, the laser wavelength is tuned between 1552 nm and 1575 nm, within the typical tuning range of chip-scale integrated lasers [37], corresponding to a sensitivity of 0.265°/nm. In order to quantify the power consumption of the array, we converge 14 separate beams in different directions across our field of view and measure the power consumption of the active phase shifters for each beam. As shown in Fig. 4(c), the total power required for steering in any direction is ${1.9} \pm {0.2}\;{\rm W}$.

We have demonstrated a low-power, large-scale optical phased array enabled by a novel multi-pass phase shifter structure. The multi-pass photonic platform decreases the power consumption by nearly $ 9\times $ while maintaining low loss across at least 100 nm of continuous optical bandwidth. By improving the design of mode conversion structures, the insertion loss of these structures can be reduced even lower to enable even higher enhancements in phase shifting efficiency [30,38,41]. Other phase tuning mechanisms could be embedded into our multi-pass platform to decrease power consumption and drive voltages [21,22]. By reducing the power without sacrificing bandwidth or speed, these low-power phase shifters form the core of a scalable approach to large phased arrays for wide-angle, long-range lidar and free-space communications systems.

Funding

Defense Advanced Research Projects Agency (HR0011-16-C-0107).

Acknowledgment

This work was performed in part at the Advanced Science Research Center at the City College of New York and at Columbia Nano Initiative Nano Fabrication Facilities at Columbia University.

Disclosures

S. A. M., C. T. P.: Voyant Photonics (I,E); M. L.: Voyant Photonics (I); Y. C. C., S. P. R., B. S., M. L.: Columbia University (P).

 

See Supplement 1 for supporting content.

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References

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Supplementary Material (1)

NameDescription
» Supplement 1       Device design, simulation, measurements and fabrication

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Figures (4)

Fig. 1.
Fig. 1. Multi-pass photonic structure based on mode multiplexing. (a) Schematic (not to scale) of a seven-pass structure that utilizes seven spatial modes. (b) Schematic description of the light path, illustrating mode conversion at each pass. (c) Schematic (not to scale) of a structure that converts the ${{\rm TE}_2}$ mode to the ${{\rm TE}_3}$ mode and reverses the propagation direction, while transmitting all other lower-order modes. (d) Optical microscope image of a seven-pass multi-pass structure embedded in a MZI. A resistive heater is fabricated on top of the ${{\rm SiO}_2}$ cladding to induce phase shifting via the thermo-optic effect. MC, mode converter; DC, directional coupler; WG, waveguide.
Fig. 2.
Fig. 2. Measured phase shift and bandwidth performance of multi-pass phase shifters. (a) Accumulated phase shift induced by the three-pass, five-pass, and seven-pass multi-pass thermo-optic phase shifters, extracted by measuring the transmission spectra of the MZIs. As a reference we also show a standard single-pass phase shifter (denoted by one-pass). The dashed lines are the linear fits to the data. The three-pass, five-pass, and seven-pass structures decrease power consumption by 3.3, 5.9, and 8.9 times, respectively. (b) Insertion loss for three-, five-, and seven-pass devices as a function of wavelength, extracted from the MZI transmission spectra. The dashed lines are cubic spline smoothing functions that exclude the artifacts due to the interference from facet reflections. One can see that the bandwidth in which the insertion losses remain less than 3 dB above the minimum loss is at least 100 nm for all structures. The inset shows the measured transmission spectra of the MZIs for the different multi-pass structures. The free spectral range of the interference fringes decreases as the number of passes increases, confirming an increase of the optical path length.
Fig. 3.
Fig. 3. Optical phased array containing 512 multi-pass phase shifters. (a) Schematic (not to scale) of optical phased array, showing out-of-plane beam emission (red arrows) and 2D steering ($\varphi $ and $\theta $). (b) Optical microscope image of the silicon waveguide layer of the fabricated chip, showing several stages of the binary splitter tree and a portion of the array of multi-pass phase shifter structures. (c) Packaged device, consisting of the ${8}\;{\rm mm} \times {15}\;{\rm mm}$ phased array chip wire-bonded to a silicon interposer, along with an optical fiber input. The grating emission area is highlighted in green.
Fig. 4.
Fig. 4. Measured 2D beam steering and system power consumption. (a) Characterization of the output beam in the far field. Line cuts of the $\varphi $ and $\theta $ directions show near-diffraction-limited beam divergence of ${0.15}^\circ \times {0.08}^\circ $ (diffraction limit of ${0.133}^\circ \times {0.08}^\circ $). Inset shows full 2D far-field image of the beam. (b) The measured far-field emission pattern above the chip showing a ${70}^\circ \times {6}^\circ $ field of view, demonstrating 7.5 dB peak-to-sidelobe ratio for the beam pointing toward ($ + {35}^\circ $, 45°). (c) Electrical power consumption of the multi-pass phase shifters for converged beams at different steering angles (shaded region represents 1 standard deviation from the mean). One can see that only ${1.9} \pm {0.2}\;{\rm W}$ total power is consumed for any angle in the reported field of view.

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