Wideband RF Photonic Link with Real-time Digital Post Processing
Navy STTR FY2014A - Topic N14A-T023
ONR - Steve Sullivan - steven.sullivan@navy.mil
Opens: March 5, 2014 - Closes: April 9, 2014 6:00am EST

N14A-T023 TITLE: Wideband RF Photonic Link with Real-time Digital Post Processing

TECHNOLOGY AREAS: Sensors, Electronics

ACQUISITION PROGRAM: FY15 Scalable, Integrated RF System for Undersea Platforms (SIRFSUP)

OBJECTIVE: Develop an integrated real-time electronic backend processor providing digitization and linearization of an analog photonic link. The analog photonic link and real-time backend processor shall provide an instantaneous bandwidth of at least 2 GHz and achieve spurious free dynamic range >120 dB Hz^2/3.

DESCRIPTION: Complex military communications, sensing and surveillance systems require distribution of high fidelity analog signals. Due to their wide bandwidth, low weight, and immunity against electromagnetic interference, analog fiber optic links have attracted ample attention. However, meeting the dynamic range requirements of communication and radar systems has proven to be challenging. In particular, nonlinearities of the electro-optic modulator, photodiodes, and electronic amplifiers have prevented the true potential of analog photonic links to be realized. In addition for wideband links the dynamic range of the analog-to-digital converter (ADC) poses another bottleneck. While significant research has been conducted on improving the components and on analog linearization, a wideband analog optical link that meets the stringent performance metrics of a military system has remained elusive.

In recent years, there has been unprecedented progress in commercial programmable devices in terms of both logic density and processing speed. Especially Field Programmable Gate Arrays (FPGAs) and Graphics Processor Units (GPU) offer a combination of high performance processing and programming flexibility without the high cost of complex application-specific integrated circuit (ASIC) development. Terabit real time processors may now be constructed using commercial off the shelf (COTS) FPGAs and GPUs. However, the challenges posed by handling the massive amount of high speed data at the board level in real time is a formidable barrier that has held back the use of COTS processors’ tremendous computational power in many applications. This program aims to leverage the progress in digital electronics to improve the dynamic range of analog optical links.

The envisioned RF to digitized data system would consist of a wideband (RF) photonic link covering from very high frequency (VHF) up to at least the K super high frequency (SHF) band for transport of the signal from a remote antenna over several hundred meters to the receiver system consisting of photo-detection, electronic interface to an ADC, and real time digital signal processing (DSP) for linearization of the system. In addition to linearization of the electro-optic modulator response, linearization of the photo-detector and ADC may be considered as well in order to meet/exceed the stated goals. It is well known that the ADC in such a wideband system represents a bottleneck because of its decreasing dynamic range as a function of analog bandwidth. Approaches to circumvent the ADC bottleneck and achieve wideband high-effective number of bits (ENOB) performance beyond the performance attainable with available electronic digitizers have been investigated—e.g., approaches employing front end optical pre-processing and backend signal reconstruction. While these approaches offer to achieve unprecedented bandwidth and dynamic range, they also suffer from a high level of complexity. A down-converting photonic link approach has been demonstrated whereby an intermediate frequency (IF) sub-band from the wideband RF signal is digitized and linearized in digital post processing. The goal of this STTR is real-time digitization and linearization of a ~2 GHz IF sub-band from the wide bandwidth of interest; therefore, down-converting link approaches are appropriate. As the focus of the STTR is the development of the real time DSP linearization algorithms and hardware, proof of concept demonstrations using a 2 GHz photonic link without down-conversion may be acceptable if the DSP linearization approach would be applicable to the ultimately envisioned design. Proposals should discuss the DSP linearization in terms of the proposed photonic link design and applicable link component non-linearities. ADC linearization techniques to aid in meeting/exceeding the 120 dB Hz^2/3 project goal are also within the scope of the STTR. Approaches should minimize the size, weight and power (SWaP) required by the electronic back end subsystem. SWaP requirements should be detailed in the proposal.

Development of new photonic devices, electro optic devices, and electronic ADC devices are not contemplated under this project.

In addition to extending the dynamic range of analog links, real-time digital processing of wideband RF spectrum is critical in achieving efficient utilization and dominance of the electromagnetic spectrum in C4ISR systems. For example, systems must be able to perform Fourier transform, beam forming and Doppler processing in real time. They must be able to capture elusive transients, trigger on them, capture them into memory without loss of information and analyze them in the frequency, time, modulation, statistical and code domains. It is anticipated that the real-time processors contemplated under this STTR will become one component of digital backend in C4ISR systems.

PHASE I: Photonic link and electronic backend design concept shall be developed addressing the goals in the description. Approach to linearization of the system shall be detailed. Detailed electronic backend designs to provide real-time linearization shall be developed. Modeling and simulation of the system and analysis of required power for DSP approach shall be performed. Proof-of-concept demonstrations are encouraged.

PHASE II: Build and test prototype packaged digitally enhanced analog optical link with 300 meter length, >120 dB Hz^2/3 dynamic range and ~2 GHz IF instantaneous analog bandwidth incorporating real-time DSP.

PHASE III: Transition the demonstrated wideband RF transmission technology into military systems and platforms that benefit from high dynamic range RF signal distribution and remoting via optical fiber. Pre-production engineering prototypes shall be developed with performance specifications satisfying targeted acquisition program requirements.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Wideband real-time processing is a ubiquitous dual-use technology that is needed in a variety of commercial applications. The technology plays an important role in applications such as test and measurement instruments, optical and wireless communications, imaging for security and surveillance, environmental monitoring, and biomedical diagnostics such as high throughput screening of biological fluids. The volume of these potential commercial markets is extremely large compared to the defense market this technology enables.

REFERENCES:
1. R.H. Walden, "Analog-to-digital converter survey and analysis," IEEE Journal on Selected Areas in Communications, Vol. 17, No. 4, p. 539, (1999).

2. P.W. Juodawlkis, et al., "Optically sampled analog-to-digital converters," IEEE Transactions on Microwave Theory and Techniques, vol. 49, no. 10, p. 1840, (2001).

3. J. Chou, et al., "Photonic Bandwidth Compression Front End for Digital Oscilloscopes," Journal of Lightwave Technology, Vol. 27, no. 22, p 5073, (2009).

4. T.R. Clark, Jr., S.R. O’Connor, and M L. Dennis, "A Phase-Modulation I/Q-Demodulation Microwave-to-Digital Photonic Link," IEEE Transactions on Microwave Theory and Techniques, vol. 58, no. 11, p. 3039, (2010).

KEYWORDS: Wideband Photonic Links; Analog Transmission; High Dynamic Range; RF Signal Distribution; Microwave Photonics; Digital Signal Processing

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