This Small Business Innovation Research Phase I project will develop an Electronic-Photonic System on Chip (EPSOC) for millimeter wave imaging. Millimeter-wave (mmW) imaging is an enabling technology for imaging and detection in degraded visual environments. However, the energy emitted in the mmW spectrum is approximately eight orders of magnitude lower than the energy emitted in the infrared spectrum. Consequently, passive mmW imaging is only viable with mmW receivers with very high sensitivity and inter-channel stability. Our proposed approach overcomes the limitations of traditional mmW receivers by using photonic technologies to up-convert the mmW radiation to optical frequencies, enabling a dramatic increase in sensitivity. The proposed system is integrated on a 3D hybrid electronic-photonic chip and maintains the phase information from the received mmW signals, thereby eliminating the need to integrate mechanical scanners, which reduces the overall system size and weight. Furthermore, our EPSOC leverages advances in the scaling of silicon technologies to provide high performance (fT and fmax frequency up to 300 and 400 GHz), integration density, and favorable economies of scale. Consequently, the proposed EPSOC is compact (2mmx2mm) and provides 5X better performance (0.1K-0.2K temperature sensitivity at a 33Hz rate) compared to existing mmW imagers on the market today.
The broader impact/commercial potential of this project is the development of a low-cost Silicon Electronic Photonic Network on Chip (EPSOC) for millimeter wave imaging. Traditional mmW imagers are based on high gain III-V amplifiers that are noisy and difficult to integrate densely into focal plane array systems. These systems do not maintain phase information, which necessitates the use of scanners, making them bulky and slow with high power requirements. In contrast, our hybrid silicon electronic-photonic approach ensures dense integration, and uses photonic phase shifters, which will eliminate the need to integrate mechanical scanners. The EPSOC will have an impact on a broad range of application areas that require mmW sensitivity including astronomy, aerial reconnaissance, stand-off threat detection, portal screening, persistent surveillance, situational awareness, and video imaging navigation in the absence of GPS signals. Furthermore, it will be particularly useful for applications that require high resolution imaging through smoke, fog, sandstorms, clouds and dielectric materials including plastic and clothing. Lastly, this project will demonstrate the feasibility of 3D electronic-photonic systems on chip with high sensitivity, stability and integration density. This would have applications to other areas as well, such as, RF communications and signal processing.
The objective the NSF SBIR Phase I Project was to design, fabricate, and characterize the photonic mmW receiver components as well as to simulate and fabricate the electronic front-end millimeter wave receiver. The photonic mmW receiver consists of three devices, the electro-optic modulator, the phase shifter and the optical bandpass filter. During the Phase I period, we have prototyped two electro-optic modulator devices. The first electro-optic modulator employs a novel design based on Lithium Niobate (LiNbO3). This design overcomes the traditional disadvantages of Lithium Niobate oscillators. This approach was able to achieve the same voltage sensitivity, using an order of magnitude smaller device dimensions (millimeters as opposed to several centimeters). In addition, it has the potential to realize ultra-high bandwidth operation (>100GHz). The second electro-optic modulator is based on a depletion-mode Silicon diode design. We expect that this modulator will operate with <3V and a bandwidth >20GHz. We have completed fabrication of this modulator and are now characterizing its performance. We have also demonstrated a novel innovative phase shifter design which is able to achieve a full π phase shift of an optical/mmW signal with less than 10mW of electrical power in a footprint of only 5 microns. Low loss, fabrication tolerant, bandpass filters were also realized. Specifically, we have optimized the fabrication process of Silicon waveguides. We were able to realize high extinction filters (>15dB) with low insertion loss (0.5dB). The electronic mmW receiver consists of a mmW antenna, a Low Noise Amplifier (LNA), a Dicke Single Pole Double Throw (SPDT) switch, a Voltage Controlled Oscillator (VCO), and a mixer. The thermal resolution of a passive mmW system is primarily dependent on its first component in the receiver chain. The first component of the mmW receiver system is the Low Noise Amplifier (LNA). Therefore, it is critically important for the success of the proposed EPSOC chip to optimize the Noise Figure (NF) and gain of the LNA. The simulations of the system performance showed a Noise Equivalent Temperature Difference (NETD) of 0.09-0.12K for 30 frames per second rates. The main objectives of the proposed LNA design were to achieve the highest gain, highest bandwidth, and at the same time minimize the Noise Figure (NF). The Dicke switch is employed to eliminate any gain variations within the RF frond-end of the electronic chip. The design adopted for the Voltage Controlled Oscillator (VCO) operates at a fundamental frequency of 77 GHz and is integrated with a tunable inductor to tune the center frequency from 67.3 GHz to 77.8 GHz. This design provides an output power of -4.5 dBm, requires a power of 14.3 mW to operate and has a phase noise of -108 dBc/Hz at 77 GHz. For the 3D integration of the electronic and photonic chips, pure gold-to-gold interconnects provide the best approach for the flip-chip bonding of the electronic and photonic 3D integration. Pure gold-to-gold interconnects minimize the noise, the parasitics, and maximize the bandwidth. The pure gold-to-gold interconnect will be accomplished through thermosonic bonding.
