Analog optical fiber forges RF link
Co-axial copper cables have long been used as conduits of analog RF signals but there is now a medium for electromagnetic energy that is starting to compete with coax: optical fiber. A recent article in Microwaves & RF, Why More RF Engineers Are Choosing Fiber Connectors, explains the whys and hows of using fiber for both short distance as well as in-building longer runs, in small-cell applications and distributed-antenna systems (DAS).
The much-lower attenuation of fiber (around 1 dB/km) compared to coax (roughly tens of dB/km) makes it especially attractive in the latter cases. Fiber’s immunity to EMI/RFI is a big plus, too, especially in wide-area installations that have to deal with building motors and equipment, well-known and notorious interference sources.
Key to using fiber is the RF-to-optical (aka electro-optical, or E/O) converter at the source end and the complementary optical-to-RF converter at the receiver. For the transmitter, a distributed-feedback (DFB) semiconductor laser is used when wide dynamic range and low noise are critical, while for applications with lower-performance requirements, a Fabry-Perot (FP) laser is generally chosen. At the receiver, a PIN diode captures the photons and coverts them into electrical signals.
Avionics is a place where RF-over-fiber is essential in a system that needs light weight and to be immune to EMI.
Next: the element of irony
There’s an element of irony here: although we associate fiber-optic links with high-speed digital signals, as in the Gbit/sec cables and links which are the foundation underpinning the physical layer of the Internet and its data flow, this use of optical fiber for RF is entirely in the analog domain – as so much of RF still is. Thus, the traditional analog issues of noise, linearity, distortion, clipping, limiting, and similar play their usual roles. Once again, analog circuitry and functions are necessary and unavoidable, and the optical drivers and receivers need to be optimized for their analog-performance and parameters rather than the digital ones. Goodbye to eye-pattern woes, hello again to linearity headaches.
Of course, using fiber is not a trivial exercise. For engineers and installers whose experience and expertise are only with coax, there’s a new world of optical-cable specifications, the fiber plus the protective jacketing, optical connectors, bend radius, E/O converters and components, and more. In addition, it’s often makes sense to run a fiber-optic cable assembly which has unused fibers inside for additional capacity in the future, known as dark fiber. In contrast, running extra coax in parallel with the in-use coax is much more costly and occupies much more space.
Have you ever wished you had a performance- and cost-effective alternative to coaxial cable? Have you explored using analog fiber for RF? What concerns and issues would you have from a technical as well as personal standpoint?
Bill Schweber, is an electronics engineer and author who has written for EE Times, was analog editor at EDN and prior to that worked in marketing communications for Analog Design and was also editor of its technical journal.
This article first appeared on EE Times’ Planet Analog website.
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