Analog Angle Blog

A short primer on RF-over-fiber designs and applications

Sometimes, you discover than despite what you think is your supposedly broad knowledge of technology and applications, you have a “blind spot.” In other words, you are barely aware of something that’s fairly close to what you do know about. Or perhaps you have a foolish misconception about something that everyone else knows. I was about 12 years old when I discovered that contrary to my unspoken assumption, a “pediatrician” is a children’s doctor and not a foot doctor.

This kind of “whoops” happened to me when I recently came across a story about RF over fiber (RFoF). Yes, I had heard the term a few years back but never bothered to see what it’s about, other than think that it was a catchy phrase with a nice acronym. Then last month I saw two unrelated references to it, and decided I should dig deeper. What I found is that RFoF is a widely used, mature technology with many impressive installations and is supported by at least half a dozen vendors.

What is RFoF? In simplest terms, it’s an all-analog way to transfer extremely high-frequency signals—what we conveniently call RF—from point A to a point B many meters away by using optical fiber rather than the more-common coaxial cable, which is called RF over coax/cable, or RFoC. The RF signal of interest can be in the multi-gigahertz range and the link budget has many of the same parameters as a coaxial-cable link.

Figure 1 The benefits of using optical fiber instead of coaxial cable include the well-known ones such as EMI/RFI immunity, but also far less attenuation versus distance at GHz-class frequencies of interest. Source: ViaLite Communications

Although we usually think of an optical fiber link as a medium for digitized signals, it’s used as a linear, analog signal path for RFoF. For the gigahertz signals, direct digitization and then modulation of a laser signal onto fiber is not a viable option in most cases due to the bandwidth requirements and the lack of suitable standard A/D converters.

Applications for RFoF include linking satellite dishes and dish “farms” to control rooms, connecting broadcast booths to communications nodes at major events, conveying GPS and precision timing signals, and providing communications in deep mines or underwater.

Why switch from well-known coaxial cable to optical fiber? There are many reasons, of course, but the first one is that the loss at these frequencies over long distances—from tens to even hundreds of kilometers—is much higher (Figure 2).

Figure 2 The high-level signal chain for an RFoF link resembles the one for coaxial cables, with the transmitter electro-optical modulated laser and corresponding receiver photodiode as major hardware differences. Source: ViaLite Communications

Another low-loss alternative is to use rigid waveguides rather than coaxial cables, which are flexible waveguides, but these are harder to place and route or re-route. Other advantages of optical fiber are the well-known ones: immunity to EMI/RFI, security against signal interception, lighter weight, and overall cost-effectiveness for a given level of performance and distance.

When you think about it, using optical fiber in place of coaxial cable to convey electromagnetic energy in a confined path is just another manifestation of Maxwell’s equations; after all, whether the energy is in the “radio spectrum” or in the optical spectrum, it is still governed by those equations.

The RFoF signal chain looks straightforward but with two unusual blocks: the laser-diode electrical-to-optical converter (E/O modulator) at the transmit side and the complementary phototransistor-based optical-to-electrical converter (O/E demodulator) at the receiver side. The RF signal intensity modulates the laser directly without any frequency shifting, creating a linear optical signal which represents the original RF signal.

Note that it’s intensity modulation rather than amplitude modulation. That’s because you can’t change the amplitude of a photon, which is a constant and function of the photon’s frequency/wavelength. What you do control and modulate is the number of photons created, with each photon having that pre-defined energy value.

Among the many vendors who offer standard RFoF products are ViaLite Communications, RFOptic, Huber+Suhner, Amtele Communication AB, Optical Zonu Corp., and DEV Systemtechnik. Their websites show their product listings and datasheets for the electro-optical and optical-electrical modules, generally packaged in standard form-factor PCBs or enclosures, supporting bandwidths as high as 10 GHz and even higher (Figure 3).

Figure 3 These small, ordinary-looking enclosures provide a complete RFoF link, ranging up to 40 GHz. Image source: RFOptic

They also offer any auxiliary functions and blocks needed for a complete end-to-end system. Reading through their product and applications literature was an interesting and refreshing experience, as I was not really aware of the current state-of-the art, nor the commercial availability of these RFoF products which can help solve the gigahertz-link challenge.

Seeing these specifics also reinforces a trend we’ve seen, with increased system-level analog and digital integration of electronic and optical functions, and especially increased integration at the component and even chip levels. We now have MEMS-like optical waveguide, filters, interferometers, and other functions in materials such as lithium niobate, for example.

In addition, there is a lot of R&D effort toward developing active, integrated optical components as hybrid devices and ultimately as monolithic ones (there are some tough basic physics barriers to overcome). Perhaps we are at the early stages of an inflection point, roughly analogous to how discrete transistors became integrated circuits and mixed-signal devices, which subsequently changed everything.

Have you ever uncovered a “blind spot” of your own, where you were unaware of a technology or application that was close to what you do know about? Have you heard of or even used RFoF? What do you think the future of integrated electro-optics will bring, and when?

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