RF Over Fiber and Photonic Links Analog Photonic Links Informational

What is an analog RF over fiber link and when would I use it instead of a coaxial cable run?

Q: Is digital fiber better than analog?

Digital RFoF (digitizing the RF signal and transmitting digital data over fiber) offers: better dynamic range (limited by ADC/DAC resolution, not laser linearity), immunity to fiber nonlinearities, and ability to multiplex many channels on one fiber. However: it adds latency (ADC/DAC conversion time: 1-10 μs), requires high-speed ADCs/DACs (which are expensive and power-hungry), and adds quantization noise. Analog RFoF is preferred when: latency must be minimized (radar, EW), the signal bandwidth exceeds the ADC capability, and simplicity is important.

Q: What about wavelength division multiplexing?

WDM (Wavelength Division Multiplexing) allows multiple RF signals to share a single fiber by assigning each signal to a different optical wavelength. CWDM (Coarse WDM): 18 channels at 20 nm spacing (1270-1610 nm). DWDM (Dense WDM): 40-80 channels at 0.8 nm spacing. Each channel carries a separate RF signal. This is used for: multi-element phased array remoting (one fiber per array face), distributed antenna systems (multiple sectors on one fiber), and multi-band receivers (each band on a separate wavelength).

An analog RF over fiber (RFoF) link transports an RF signal through optical fiber by modulating a laser with the RF signal at one end and recovering the RF signal with a photodetector at the other end. It is used instead of coaxial cable when: (1) Distance: fiber has near-zero loss (0.2 dB/km at 1550 nm) versus coaxial cable that loses 10-50 dB/100m at microwave frequencies. For runs > 100 m at frequencies above 1 GHz, fiber is the only practical option. (2) Weight: fiber weighs ~30 g/m versus 100-500 g/m for coaxial cable. Critical for aerospace and shipboard installations where cabling can comprise a significant fraction of system weight. (3) EMI immunity: fiber is completely immune to electromagnetic interference. No coupling from nearby transmitters, no ground loop issues, and no lightning vulnerability. Essential for high-EMI environments (radar sites, transmitter facilities, manufacturing floors). (4) Bandwidth: a single fiber can carry signals from DC to 40+ GHz with minimal dispersion. A single coaxial cable type typically covers only a few octaves effectively. (5) Architecture: a basic analog RFoF link consists of: a transmitter module (laser diode or external modulator driven by the RF signal), optical fiber (single-mode for long distances, multimode for short runs), and a receiver module (photodetector that converts the optical modulation back to an RF electrical signal). The link is transparent: the RF signal goes in one end and comes out the other with some gain/loss and added noise.

Category: RF Over Fiber and Photonic Links

Updated: April 2026

Product Tie-In: Fiber Components, Modulators, Photodetectors

Analog RF Over Fiber Links

RF over fiber technology has become essential infrastructure for distributed antenna systems, radar remoting, satellite ground stations, and electronic warfare signal transport.

Link Performance

(1) Link gain: an RFoF link can have positive or negative gain (the RF output power divided by the RF input power). Simple directly-modulated links: typically -20 to -30 dB of loss (the link attenuates the RF signal). High-performance links with optical amplification or high-power lasers: can achieve +10 to +20 dB gain. (2) Noise figure: typical RFoF NF: 20-40 dB (much higher than a coaxial cable, which has NF equal to its loss in dB). The high noise figure is the main disadvantage of RFoF. For receive applications: place an LNA before the RFoF transmitter to overcome the link noise figure. (3) Dynamic range: SFDR: 100-120 dB·Hz^(2/3) for typical analog links. Limited by laser RIN (noise floor) and modulator nonlinearity (distortion ceiling). (4) Frequency range: DC to 40+ GHz with appropriate components. Wideband links covering 0.5-18 GHz are standard commercial products.

RFoF Link Performance

Fiber loss: 0.2 dB/km (1550nm) vs coax 10-50 dB/100m
RFoF link gain: -30 to +20 dB
NF: 20-40 dB (high; use LNA before link)
SFDR: 100-120 dB·Hz^(2/3)
BW: DC to 40+ GHz on single fiber

Common Questions

Frequently Asked Questions

When should I use coax instead of fiber?

Coaxial cable is preferred when: the distance is short (< 30 m at microwave frequencies, < 100 m at HF/VHF). The link noise figure must be very low (coax NF = cable loss; fiber NF = 20-40 dB). The cost must be minimal (RFoF modules cost $500-5000 per link; coax cable costs $5-50/m). Wideband dynamic range is not critical. Below 1 GHz and < 50 m: coax is almost always the better choice.

Is digital fiber better than analog?

Digital RFoF (digitizing the RF signal and transmitting digital data over fiber) offers: better dynamic range (limited by ADC/DAC resolution, not laser linearity), immunity to fiber nonlinearities, and ability to multiplex many channels on one fiber. However: it adds latency (ADC/DAC conversion time: 1-10 μs), requires high-speed ADCs/DACs (which are expensive and power-hungry), and adds quantization noise. Analog RFoF is preferred when: latency must be minimized (radar, EW), the signal bandwidth exceeds the ADC capability, and simplicity is important.

What about wavelength division multiplexing?

WDM (Wavelength Division Multiplexing) allows multiple RF signals to share a single fiber by assigning each signal to a different optical wavelength. CWDM (Coarse WDM): 18 channels at 20 nm spacing (1270-1610 nm). DWDM (Dense WDM): 40-80 channels at 0.8 nm spacing. Each channel carries a separate RF signal. This is used for: multi-element phased array remoting (one fiber per array face), distributed antenna systems (multiple sectors on one fiber), and multi-band receivers (each band on a separate wavelength).

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