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Monolithic microwave phased arrays are turning mainstream in automotive radars and high-speed wireless communications fulfilling Gordon Moores 1965 prophecy to this effect. Optical phased arrays enable imaging, lidar, display, sensing, and holography. Advancements in fabrication technology has led to monolithic nanophotonic phased arrays, albeit without independent phase and amplitude control ability, integration with electronic circuitry, or including receive and transmit functions. We report the first monolithic optical phased array transceiver with independent control of amplitude and phase for each element using electronic circuitry that is tightly integrated with the nanophotonic components on one substrate using a commercial foundry CMOS SOI process. The 8 × 8 phased array chip includes thermo-optical tunable phase shifters and attenuators, nano-photonic antennas, and dedicated control electronics realized using CMOS transistors. The complex chip includes over 300 distinct optical components and over 74,000 distinct electrical components achieving the highest level of integration for any electronic-photonic system.
© 2015 Optical Society of America
1. Introduction
In 1965, Gordon Moore predicted that advancement in integrated circuit technology will enable monolithic microwave phased arrays that can revolutionize radar [1]. Fulfilling his prophecy, monolithic radio frequency, microwave, millimeter wave, and tera-Hertz phased arrays are turning mainstream in automotive radars [2–4], high-speed wireless communications [5–7], imaging, and other commercial and noncommercial systems. The ability to independently control the amplitude and phase of each phased-array element, essential in creation of arbitrary radiation patterns, is at the centerpiece of monolithic microwave phased arrays. Electronically-controlled optical phased arrays enable imaging, lidar, display, chemical-bio sensing, surveillance, and holography among other applications. Advancements in fabrication technology has enabled realizations of monolithic nanophotonic phased arrays [8–14], albeit without the independent phase and amplitude control ability, integration with the necessary electronic circuitry, and accommodation for receive and transmit functions.
Here, we report the first monolithic optical phased array transceiver with independent control of amplitude and phase for each element using electronic circuitry that is tightly integrated with the nanophotonic components on the same substrate using a commercial foundry CMOS Silicon on Insulator (SOI) process. The monolithic phased arrays chip includes 8 × 8 elements where each element consists of a thermo-optical tunable phase shifter, a thermo-optical tunable attenuator, a radiating grating coupler acting as a nanophotonic antenna, and dedicated control electronics realized using CMOS transistors with 180 nm minimum channel length. Optical switches enable receiving and transmitting functions as well as built-in array calibration modes. The complex chip includes over 300 distinct optical components and over 74,000 distinct electrical components achieving the highest level of integration for any complex electronic-photonic integrated system [15–17]. In addition to the aforementioned conventional applications, monolithic electronically-controlled optical phased array transceivers may enable low-cost quantum-secured wireless information exchange in future hand-held devices.
2. OPA transceiver
The Optical Phased Array (OPA) includes 64 radiating elements in a uniform two-dimensional configuration with independent amplitude and phase control of the optical field at each element. The small wavelength of optical frequencies, 1550 nm in our demonstration, dictates small size for each unit element to reduce the unwanted grating lobes in the far-field radiation pattern. The tunable optical phase shifters and attenuators are local to each unit element in a compact layout to ease the light distribution (Fig. 1). Control electronics for all the tunable components are integrated on the same chip at a distant location from the optical array, and the control electrical signals are routed to the array elements using the available metal lines in the CMOS process. Independent control of phase shift and amplitude of optical field at each element enables creation of arbitrary radiation patterns and removes the necessity for precision matching between elements in a large array. The realized OPA is a transceiver in that it can radiate the light with desired patterns in the transmitting mode and receive the light incident to the array at the desired directions.
Fig. 1 (a) Schematic of the 8 × 8 monolithic optical phased array transceiver with independent amplitude and phase control at each element. In the transmit mode, the laser light is coupled in to the input Grating Coupler (GC) and delivered to each of the 64 antennas using silicon waveguides. In the receive mode, the impinged light on the optical phased array, after being collected and processed by each element, is combined into one field and is sent out through the output GC; (b) Chip microphotograph of the fabricated optical phased array transceiver in a commercial 180 nm Silicon-On-Insulator (SOI) CMOS process featuring over 300 distinct optical components and over 74,000 distinct electrical components; (c) Close-up view of the Optical Phased Array core consisting of 64 optical variable phase shifters, 64 optical variable attenuators, 64 optical nano-antennas, and 128 metal lines routing the control signals from the on-chip electronic circuitry to the optical core; (d) Close-up view of a unit optical element consisting of a variable phase shifter, a variable attenuator, and a nano-antenna.
The OPA transceiver is fabricated in the IBM 7RF-SOI commercial CMOS process without any additional post processing. This production process features CMOS transistors with 180 nm minimum channel length and is widely used to realize RF switches for the front-end of cellular phones [18]. Available materials and processing steps such as etching, doping, implantation, and deposition in a commercial CMOS foundry, optimized solely for electronic devices, are used to realize optical components and functions. All the photonic and electronic devices are realized in the active layers above the 1 μm silicon oxide layer of the SOI wafer. A 145 nm thick crystalline silicon typically intended for the source, drain, and channel of the MOSFETs, is used to make the passive photonic structures such as waveguides [19]. The measured loss of an implemented single-mode waveguide is 1.27 dB/mm corresponding to an estimated sidewall roughness variance and coherence length of 7.5 nm and 10 nm, respectively. The measured loss of waveguide bends are around 0.71 dB per U-turn for sharp bends and 0.2 dB per U-turn for bend radius above 5 μm. To create the grating couplers and nanophotonic antennas, poly-silicon, crystalline silicon, and their combination are used to enable efficient coupling of the light in and out of the chip [20]. In the transmitting mode, laser signal is coupled into the chip through a larger grating coupler, routed to all 64 elements using silicon waveguides and directional couplers, and consequently reradiated following a desired pattern as a function of elements’ phase shift and amplitude settings. Directional couplers are judiciously designed to deliver almost equal optical powers to all 64 elements. After fabrication, slight mismatches in the optical powers delivered to array elements are observed due to common fabrication tolerances. This small nonuniformity in optical power delivery to array elements is well with- in the range of optical amplitude control that is independent at each element. In the receiving mode, another large grating coupler is used to transfer the optical signal that impinges on the array, after being collected and processed by each element and then combined into one field, off the chip. The measured coupling efficiencies of these larger grating couplers are 30%. Mach Zehnder Interferometers (MZI), realized with two 50-50 directional couplers (DC) as power splitter/combiner with one active heater on top arm, are used as optical switches to route the light into the desired waveguides. The heater changes the phase shift of the optical field due to the thermo-optical effect, and results in constructive (switch ON) or destructive (switch OFF) combining of the optical field in two arms by providing 0 or π phase shift difference in the two arms. The heater is implemented using the poly-silicon (PC) layer that is typically used as the gate of MOSFETs in the commercial CMOS process. We used p-doped PC as heater and placed it very close to optical silicon waveguides with 0.5 μm edge-to-edge margin to avoid unwanted doping leakage into the main waveguides that would have in turn increased the optical propagation loss. The measured electrical resistance of the heater is 814 Ω. The heater requires 14.2 mW electrical power to provide π phase shift in the optical field. The electrical current used to heat-up the PC heater is controlled by a programmable electrical 7-bit Digital-to-Analog Converter (DAC) that is implemented on the same chip using MOSFETS with 180 nm and 250 nm channel lengths. One such optical switch, with a measured isolation of 30 dB, separates the array from input and output grating couplers in transmit and receive modes respectively (T/R Switch). Same optical switch is used to send light into the individual array elements in a calibration mode.
3. Array elements and calibration
Each array element consists of a tunable thermo-optical attenuator followed by a tunable thermo-optical phase shifter that is coupled into a nanophotonic antenna. The tunable attenuator is made of an MZI with compact 1 × 2 Multi-Mode Interferometric (MMI) power splitter/combiner in a total footprint of 29 μm × 7 μm. The longer arm is heated up by p-doped PC heater with a measured electrical resistance of 1.18 kΩ that provides a measured 16 dB amplitude tunability with burning 14.9 mW DC power. The tunable phase shifter is realized as a 94.4 μm-long meandered silicon waveguide with three polysilicon heaters connected in parallel electrically to increase the thermal efficiency and also reduce the required voltage swing (Fig. 2). A 2π optical phase shift is achieved by consuming 27.2 mW power in the heater with 1 kΩ electrical resistance. The nanophotonic antenna is realized as a 3.55 μm × 1 μm compact grating coupler with 50% radiation efficiency. The overall passive optical loss inside each array element that is 3 dB before radiation is mainly due to the sharp bends and waveguide sidewall roughness. To avoid thermal cross-talk between heaters, an edge-to-edge distance of 5 μm is considered in the design. The center-to-center spacing of array elements is 33 μm in both dimensions.
Fig. 2 (a) Optical variable attenuator is constructed as an interferometer with polysilicon heater in between. The relative phase change between the two paths, due to thermo-optical effect that is more prominent in the longer branch, results in amplitude tunability of over 16 dB; (b) Thermo-optical variable phase shifter is a meandered waveguide with poly-silicon heaters in between to increase the power efficiency and decrease the voltage swing that is needed to cover 2π optical phase shift. Optical antenna is designed with 50% radiation efficiency and compact footprint of 1 μm × 3.55 μm. Combination of polysilicon and crystalline silicon are used to modulate the silicon height and hence create grating couplers to radiate light; (c) Measured phase and amplitude response of a typical optical variable attenuator as a function of heater power consumption at 1550 nm wavelength; (d) Finite difference time domain simulations of far field (left) and near field (right) radiation patterns of each individual nano-antenna.
Built-in performance monitoring and calibration is essential in large-scale arrays. This is particularly true in optical arrays operating at short wavelengths where small fabrication mismatches lead to significant changes in the radiation pattern. In this demonstration, the residues of optical power at the end of the main bus, 11% of the coupled input, and each row, 1.2% of the coupled input, are fed into on-chip schottky photodiode power monitors. The photodetectors are designed based on p-doped silicon and Tungsten schottky interface with measured responsivity of 1 mA/W at −1 V reverse bias and minimum detectable power equal to 1.5 μW. This low responsivity is sufficient for monitoring the powers needed for array calibration. Since silicon is transparent at wavelength 1550 nm, schottky junction is used to lower down the band-gap energy (Schottky barrier) [21] to around 0.45 eV which is below the photon energy 0.8 eV. This increases the probability of photon absorption. Germanium, available in some commercial CMOS processes (not available in our process) as the additional doping for the source/drain regions of PFETs for strain engineering, can be used to enhance the responsivity [22, 23].
4. Electrical routing
The monolithic optical phased array transceiver has a total of 129 independent heaters, 64 for phase shifters and 64 for attenuators, and one for the front-end optical switch, that must be controlled independently. One terminal of all the heaters are connected electrically to a common terminal using the three bottommost available metal layers (bottommost copper and other two aluminum), and the other terminals are routed independently to the designated driver circuitry using the topmost available metal layer (aluminum). This topmost metal layer is 4 μm thick and 2 μm wide, and requires a minimum line-to-line spacing of 2.8 μm due to the lithography limitations. Commercial CMOS processes require a near uniform distribution of metal layers for consistent Chemical Mechanical Polishing (CMP). All the routing is done judiciously to minimize the effect of metal lines on the optical devices while satisfying the foundry metal density requirements. The digitally controlled integrated electronic consists of a programmable 7-bit Digital to Analog Converter (DAC) followed by the current drivers that drive the polysilicon heaters via the aforementioned metal interconnects (Fig. 3). The driver stage is designed to handle up to maximum 10 V swing without breaking the transistors resulting in 10 mA of current across the 1 kΩ heater electrical resistance. The 16 dB range for the tunable optical attenuators and the 2π range for the tunable optical phase shifters require a maximum voltage swing of 6 V that is well within the designed value. The chip is programmed serially through a computer using clock, reset, and load signals.
Fig. 3 (a) Schematic of the control electronic circuitry for each thermo-optical heater. The current of each heater, Ih, is controlled by an independent 7-bit Digital to Analog Converter (DAC). Programmed bits, b0 to b6, are set using digital shift registers. DAC generates controllable currents by copying and multiplication of reference current, Iref, that is generated to be independent of fabrication process, temperature, and supply voltage variations. Driver circuitry consists of multiple stacked transistors to handle large voltage swings necessary to deliver up to 100 mW to each thermo-optical heater; (b) Routing of the control lines from the electronic circuitry to the 8 × 8 optical phased array transceiver, using top-level metalization, is shown. The common electrical terminal of all the 128 thermo-optical heaters are connected together using bottommost metal lines.
5. Experimental results
5.1. Operation of OPA in the transmit mode
The performances of individual optical components, namely, the tunable optical phase shifter, the tunable optical attenuator, the grating couplers, and the nano-photonic antennas were individually characterized in stand-alone structures concurrently fabricated in the same process. Array measurements, in the transmit mode, are conducted in near and far fields as the chip is placed on a temperature-controlled chuck.
Due to the inevitable fabrication process variations and mismatches as well as the nonuniform temperature profile across the OPA, the amplitude and phase values of different elements are unequal even for the same nominal settings. Fig. 4 shows the measured far-field image of the un-calibrated OPA in the transmit mode when all the elements have the same nominal amplitude and phase settings. The actual relative phase and amplitude values, derived in the aforementioned calibration process, are also shown in Fig. 4. The amplitude values for each setting can be measured in the near field as the field emanating from each element does not interfere with those of others. Far field measurements are used to extract the phase information. Initially, the amplitudes and phases of all the array elements, for all programmable amplitude and phase settings, are characterized and stored. The stored values are later used to create the desired near and far-field optical fields and radiation patterns. Amplitude measurements, for initial characterization, are conducted in near-field. Phase measurements, for initial characterization, are conducted in far-field and in pairs. The interference pattern of two radiating elements in far-field, while no other element is radiating as all other attenuation levels are set to be at maximum, is indicative of the relative optical phases. It is important to note that the phase measurements are conducted for all the attenuation levels as the changes in the tunable optical attenuators affect the optical phase as well. After the initial measurements, a complete lookup table including the relative phases of all the elements for every possible amplitude setting is created that is used to create arbitrary near and far field patterns. Examples of generated near-and far-field radiation patterns that are measured and compared with simulations are reported in Fig. 5.
Fig. 4 The phase and amplitude profile is un-uniform across the OPA (left) even for the same nominal setting due to natural fabrication process variations, mismatches, and temperature gradient leading to incoherent far-field radiation pattern (right).
Fig. 5 Examples of generated near- and far-field radiation patterns using the Optical Phased Array in the transmit mode with the corresponding amplitude and phase profile settings; (a) Far field of focused beam generated by uniform phase and amplitude profiles shows 12 dB peak to first side-lobe ratio; (b) Gaussian amplitude profile and uniform phase profile create a wider far-field main lobe with 11 dB side-lobe suppression. The main beam generated in (b) is steered 1.6° to the edge of each interference order in horizontal, (c) and vertical, (d), directions by progressing π phase shift between columns and rows, respectively; (e) Far field image nearly resembling a straight line is created by setting proper amplitude and phase profiles.
5.2. Operation of OPA in the receive mode
A fiber with angle θ ≈ 20° with respect to the normal axis of the OPA is placed far enough to create a plane wave impinging on the monolithic OPA (Fig. 6). Each array element collects a portion of the impinged light through a nano-photonic antenna, modifies the amplitude and phase through the tunable optical phase shifter and attenuator, and feeds the processed light to a combining waveguide. The total collected field is detected through another fiber located at the top of the output grating coupler. The received optical field by each antenna is coupled to a row waveguide through a directional coupler. Optical termination is used at the input waveguide of the coupler to dissipate the received power residue and eliminate the unwanted reflections. The coupling regions are designed judiciously to ensure near-equal power transfer from each element to the combining waveguide bus. To show the spatial selectivity of the OPA in receive mode, a three-step experiment was performed. First, phases and amplitudes of all the cells were measured and calibrated similar to what the approach in the transmit mode. To calibrate the amplitudes, all the tunable optical attenuators were set to the maximum attenuation levels; then, by detecting the received power from only one cell, the amplitude response of that cell is calibrated. Phase calibration is done sequentially in pairs when the attenuation levels of all elements, except for two, are set at their maximum values. The power of the fiber-coupled laser is around 15 mW at 1550 nm wavelength. The peak measured received power at the output fiber after phase and amplitude calibration, when all the array elements are in phase, is around 65 μW. Therefore, the total fiber-to-fiber loss is measured 23.6 dB. The loss is a function of fiber angles with respect to the normal axis of the OPA, scattering and substrate losses of impinging light to OPA, and waveguide bend and propagation losses. In the second step, after amplitude and phase calibration, the attenuations and phases are set so that all elements (1) contribute the same amount of optical power to the combining waveguide, and (2) adjust the phase so that all the optical fields coupled to the combining waveguide are in phase. This ensures that maximum power is received at the specified incident angle. In the third step, in order to demonstrate OPA rejection of signals at unwanted incident angles, the phase shift settings of all the elements with odd column number are changed by 180° while all other settings remain intact. It is expected that, in this new setting, the field collected by 32 elements (odd columns) will cancel the field collected by the other 32 elements (even elements). The output field in this new setting (null of the spatial pattern) is measured to be around 2.3 μW that is 14.5 dB below that of the original setting (peak of the spatial pattern).
Fig. 6 (a) A fiber with angle θ with respect to normal axis of OPA is placed far enough to create an impinging plane wave on the OPA. Each nano-photonic antenna collects a portion of the incident field, adjusts its amplitude and phase, and couples it to the combining waveguide. The total collected field is detected through another fiber located at the top of the output grating coupler; (b) Received field by each nano-photonic antenna is coupled to the combining waveguide through couplers judiciously designed to ensure near-equal coupled signal values across the array. Optical termination is used to dissipate the received power residue and eliminate the unwanted reflections.
6. Conclusion
The presented work is the first monolithic integration of a two-dimensional optical phased array transceiver, including the complete integrated control electronics, with independent amplitude and phase control for every element capable of arbitrary radiation pattern generation in a commercial CMOS SOI process. Monolithic electronically-controlled optical phased array transceivers enable low-cost solutions for various sensing, imaging, ranging, and quantum-secured wireless information exchange in future hand-held devices.
Acknowledgments
The authors would like to thank Nankyung Suh Cockerham and Wes Hansford from MOSIS for supporting the chip fabrication. The authors also thank F. Rezaiefar for designing the nanophotonic grating couplers, A. Samiei for designing the band-gap reference current and electro-static discharge protection circuitries, and M. Yamagata, A. Imani, A. Goel, R. Chen, and S. Subramanian for feedback and comments about various aspects of this project.
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