The maturity of RF photonic components has reached the point where fiber optic links are being system-tested to replace traditional copper coax links on avionic platforms. Many demonstrations of RF photonic links have been made with traditional and non-traditional modulation formats to improve the RF performance of the link. While the advantages of RF photonic links in regard to size and weight are important, the large instantaneous bandwidth of the fiber optic links is a key driver for the use of this technology in the airframe.

A traditional RF photonic link with electronic tuner and digitizer, showing the electro-optic modulator (EOM), photodetector (PD), local oscillator (LO), and intermediate frequency (IF).
As signals of interest evolve to higher frequencies, the use of RF photonics provides a path to identify and catalog these new signals. With the acceptance of the fiber optic link as a replacement to coax, the next step is to move the photonic technology to provide signal processing capability in the optical domain before converting back to an electrical signal. The current challenges are to continue to explore new photonic technologies for improved RF performance at the receive end of the link. The Air Force Research Laboratory is testing several methods to solve these problems.

Starting with a traditional RF photonic link shown in the figure, the RF signal is received at the antenna and then modulated onto an optical carrier provided by the laser. The RF modulated light is then passed down a fiber optic link to a photodetector, which then recovers the RF electrical signal. The RF signal would then be passed through a tuner to frequency down convert it to an intermediate frequency (IF) chosen by the local oscillator (LO). The IF signal is then digitized at an analog-to-digital converter (ADC) and processed. Since the RF signal is already in the optical domain, the photodetector and electronic tuner can be replaced with an optical tuner that can down convert the signal to an electrical IF.

An optical tuner can reduce mixing spurs in RF frequency down conversion. Such a system is important as it can provide better spur free performance than current electronic tuners and over a larger bandwidth. In addition, work continues on photonic ADCs that can digitize 10 GHz of instantaneous bandwidth with a resolution greater than 8 effective number of bits. A combination of these technologies can provide an RF-to-bits architecture without an optical-to-electronic conversion in the middle.

In support of the frequency down converter and ADC technologies, work continues on the optical clock that is used either for the LO or the sampler in these architectures, respectively. Optoelectronic oscillators continue to be investigated for improved performance; however, the limited frequency tunability is still a drawback to these types of low-phase-noise clock sources. New methods have been shown to make tunable RF sources using the injection locking of two lasers. These sources continue to be further explored in order to determine their noise performance over their tunable range.

Other signal processing applications include signal discrimination in cluttered RF environments. Once the RF signals have been modulated onto the optical carrier, a portion of the light can be split off for spectrum analysis. Demonstrations have already been made with a rare-earth doped crystal in a spectrum analyzer with an RF instantaneous bandwidth of 10 GHz with the potential for even higher bandwidths. Another demonstration has been made with a multimode optoelectronic oscillator for selective amplification of RF signals with an input sensitivity less than 15 dB above the thermal noise limit. These systems are sensitive to frequency agile signals and can be used to cue a narrowband receiver to analyze the signal.

Work continues on using RF photonic solutions for signal processing at the receive end of the fiber optic link on an avionics platform. Multiple applications exist that can benefit from the large instantaneous bandwidth of photonic systems. However, to compete with electronic solutions, integrating photonic solutions into small form factors will continue to be emphasized.

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Aerospace & Defense Technology Magazine

This article first appeared in the December, 2014 issue of Aerospace & Defense Technology Magazine.

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