, HUJI, School of Computer Science and Engineering, Applied Physics
Innovative photonics applied to boost Analog-to-Digital conversion capabilities
Dr. Dan Marom, Head of Photonic Devices Laboratory, Department of Applied Physics,
The Selim and Rachel Benin School of Computer Science and Engineering
Information is created in most cases as analog electrical signals from the physical world.
Yet information is typically processed, stored, and transmitted in digital form. Analog signals must therefore be faithfully sampled and converted to digital representation for computational processing, information storage, data transfer etc., using Analog–to-Digital Converters (ADC).
The two key performance metrics for ADC are the conversion fidelity, required for accurate digital representation of analog signal levels, and high operating speeds, measured in signal sampling frequency, required for digitizing wideband information signals.
Conventional electronic ADC experience decreased conversion fidelity with increased sampling speeds. The steady information bandwidth growth of analog channels is not backed by similar progress in ADC electronic technology, making high sampling rate analog to digital conversion the limiting factor in the acquisition of wideband analog signals.
Our solution is based on introducing photonic signal processing components to tackle the difficult-to-realize elements of the ADC operation, namely the low-jitter high-bandwidth sampling, operating bandwidth reduction through wavelength-division multiplexed (WDM) parallelization, and spatial oversampling for noise reduction (see figure on next page). Taken together, these solutions lead to a photonically-assisted ADC that can scale to 100GSamp/sec and beyond at high conversion fidelity.
Mode locked lasers are extremely favorable for optical sampling as they operate at extremely low jitter statistics. We use such wideband laser pulses as a source for WDM pulse stream generation via our photonic processors. Having pulse streams with unique wavelengths allows them to be separated after the sampling process and independently detected and processed at reduced electronic rates. The accurate sampling clock and reduced rate processing is the first contributor to the photonically-assisted ADC.
The second contributor to our photonically-assisted ADC is our spatial oversampling concept.
The phase modulated optical signal is detected via a self-coherent receiver that provides multiple interference terms that constitute an over-complete basis. As the interference intensity terms are independently acquired with accompanied noise, we use simple signal processing across the entire measurement set to reduce the noise terms and provide a more faithful phase estimation value.
The spatial oversampling concept is similar to temporal oversampling that is employed in lower rate electronic ADC, but is impractical in high-speed applications where the electronics is already stressed.
Top: Photonically-assisted ADC system comprised of three subsections that can operate jointly or remotely
Left: WDM sampling pulse generator via optical slicing of a periodic ultrashort pulse source. Center: integrated photonic circuit including optical modulator and spatial oversampler in a self-coherent arrangement.
Right: Detection and signal processing module that can be remoted from sampling head.
Bottom-Left: photonic module performing wideband modulation and proprietary self-coherent spatial oversampling.
Bottom-Right: Noise reduction via spatial oversampling concept.
The system can be optimize by commercial parties interested in bringing the photonically-assisted ADC to market and is projected to provide sampling rates configurable to 100 GSamp/sec or higher, with 6-7 Effective Number of Bits (ENOB). As each additional bit of resolution constitutes an improvement of factor two, this is far better than any known alternatives that provide 3-5 ENOB.
Photonic building blocks scale to create the fastest and most accurate ADCs ever built
Based on existing technologies available in the photonic and electronic domains
Bridges the gap between electronic ADC abilities and high-speed sampling rate requirements
Increases the accuracy of the conversion beyond the electrical ADC capabilities
Optical sampling head can be remote from signal detection and processing unit for applications such as wideband antenna arrays.
Demonstrated ×8 WDM pulse sequence generation from ultrashort pulse source.
Demonstrated hybrid integrated photonic module performing wideband phase modulation and proprietary self-coherent spatial oversampling.
Tested photonically-assisted ADC operation of RF signals up to 18 GHz with 7.5 ENOB with undersampling due to repetition rate of available mode-locked laser rate.
Further progress will require partnership with optoelectronic component company / system integrator that can provide the advanced electronics and high-repetition-rate mode locked laser required to build full scale system.
Applications in all areas where need is constantly growing for high-speed, high-fidelity ADC conversion: communications, radar, defense, avionics and others.
Granted US 8,963,751