Jason Chou (15-ERD-056)
Electronic signal processors are essential to military, industrial, and experimental needs in radar, communications, and diagnostics. Today's electronic processors used to convert analog signals into digital format are limited by a fundamental trade-off between resolution and speed, known as the "digital bottleneck." This digital bottleneck is rooted in device physics of the electronic converter (i.e., timing jitter), resulting in high-amplitude noise with frequency. Hence, electronic analog-to-digital conversion can achieve either high resolution or high speed, but not both. We propose an entirely new processor—one based on photonics—that can break through these barriers and provide fivefold to tenfold higher performance for applications in national security, research, and telecommunications. Our patented photonic processor utilizes optical mapping technology that converts amplitude information into the time domain, where it can be quantized with significantly higher resolution. For example, a standard 6-GHz oscilloscope offers only 45 effective levels of resolution, while our technique promises to offer over 500 effective levels across the same bandwidth.
We expect to demonstrate a photonic processor that can digitize continuous or single-shot analog input signals with 512 effective levels of resolution at a sample rate of 12 GHz. This goal is significant because, not only will it set a new record in resolution-bandwidth products, it will provide a high-fidelity, real-time temporal diagnostic needed to measure laser pulse characteristics delivered to the target in advanced fusion-class lasers. Moreover, this goal has direct application to military systems that require high-resolution and high-speed analog-to-digital converters for radar and electronic warfare. Finally, the technology has licensing opportunities in the telecommunications industry that can enable more sensitive and cost-effective wireless networks. Our photonic processor concept can be described in three basic steps. First, amplitude information is encoded in time by an ultrafast tunable optical filter, which maps analog information into wavelength. Second, a low-loss dispersive element maps wavelength information into time. Third, a data-processing step decodes the input amplitude by analyzing the time-encoded data.
Our proposed capability to achieve a fivefold to tenfold improvement in resolution for high-fidelity temporal diagnostics to measure laser pulse characteristics is well aligned with LLNL's core competency in lasers and optical science and technology by contributing to the design of future high-energy and high-peak-power laser systems.
FY15 Accomplishments and Results
In FY15 we verified our approach by successfully demonstrating a single-channel, low-repetition-rate analog-to-digital converter with a high spurious-free dynamic range. This range indicates that we obtained a strong primary signal compared to the most prevalent spurious or noisy signal. In addition, we scaled the system to a multiple-channel, high-repetition-rate analog-to-digital converter, with 128 effective resolution levels at a 4-GHz sample rate.