原创 Analogue optical fibre strengthens RF link

The much-lower attenuation of fibre (around 1 dB/km) compared to coax (roughly tens of dB/km) makes it especially attractive in the latter cases. Fibre'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 fibre 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.

 

There's an element of irony here: although we associate fibre-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 fibre for RF is entirely in the analogue domain – as so much of RF still is. Thus, the traditional analogue issues of noise, linearity, distortion, clipping, limiting, and similar play their usual roles. Once again, analogue circuitry and functions are necessary and unavoidable, and the optical drivers and receivers need to be optimized for their analogue-performance and parameters rather than the digital ones. Goodbye to eye-pattern woes, hello again to linearity headaches.

Of course, using fibre 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 fibre 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 fibre-optic cable assembly which has unused fibres inside for additional capacity in the future, known as dark fibre. 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 analogue fibre for RF? What concerns and issues would you have from a technical as well as personal standpoint?