How do I select the optical wavelength for an analog RF over fiber link?
Wavelength Selection for RFoF
Wavelength selection is one of the first design decisions in an RFoF system, as it determines the fiber type, component selection, maximum distance, and system cost.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Frequently Asked Questions
Can I use 1550 nm with multimode fiber?
Not recommended. Multimode fiber at 1550 nm has higher attenuation than at 850 nm (due to the fiber design optimization). More importantly: multimode fiber suffers from modal dispersion (different modes travel at different speeds), which limits the bandwidth-distance product to < 500 MHz·km. For RFoF at microwave frequencies: single-mode fiber is always used for 1310/1550 nm. Multimode is only practical at 850 nm for short distances.
What about 1060 nm?
1060 nm is an emerging wavelength for specialty applications: low fiber loss (0.5-1 dB/km in specialty fibers). VCSELs and DFB lasers available. Ytterbium-doped fiber amplifiers (YDFA) provide optical amplification at 1060 nm. Advantage over 1550 nm: lower stimulated Brillouin scattering threshold (allows higher optical power in narrow-linewidth applications). Used in: fiber-optic gyroscopes, some military RFoF systems, and coherent photonic links.
Does eye safety matter for RFoF?
Yes, especially for deployable systems. 1550 nm: Class 1M eye safety for optical powers up to ~10 mW (the eye cornea absorbs 1550 nm light before it reaches the retina, providing natural protection). 850 nm and 1310 nm: more hazardous to the eye (the light can reach and damage the retina at lower power levels). Class 1 limit is lower. For military and field-deployable RFoF: 1550 nm is preferred for eye safety (higher power can be used while remaining in a safe laser class).