Multi-Mode (MMF) vs Single-Mode (SMF)
Multi-Mode (OM3, OM4, OM5): The core of the glass is relatively wide (50µm). Light bounces off the walls of the core at multiple angles (modes) as it travels. Because the photons hit the walls, the signal degrades over distance through modal dispersion.
- Wavelength: 850nm (typically cheaper VCSEL lasers).
- Distance: Max 300 to 400 meters depending on 10GbE vs 25GbE speeds.
- Cable color standard: Aqua (OM3/OM4) or Lime Green (OM5).
Single-Mode (OS2): The core is brutally small (9µm). The laser fires straight down the middle in a single path. No bouncing means practically no dispersion.
- Wavelength: 1310nm (standard) or 1550nm (long haul).
- Distance: Starts at 10km (SFP-LR) and goes up to 120km (SFP-ZR).
- Cable color standard: Yellow.
- Note: Standard in OB truck broadcast engineering due to stadium distances.
SFP Transceivers & BiDi
A standard SFP requires two strands (LC duplex connector): one glass fiber for Transmit (Tx), one for Receive (Rx). However, in OB live sports, cable weight matters. BiDi (Bi-Directional) SFPs utilize Wavelength-Division Multiplexing (WDM) to shoot two different colors of light down a single glass strand. (e.g., Side A transmits at 1310nm and receives 1270nm. Side B transmits at 1270nm and receives 1310nm). This halves your required cable cores.
In a broadcast environment, 90% of fiber failures are due to dirty connectors. Even a single microscopic dust particle on the tip of an LC connector can physically scratch the glass or block the laser. Always use a One-Click Cleaner (Cletop) before every single plug-in. Never blow on the fiber with your mouth.
Optical Link Budgets (Beware the overload)
You cannot plug a 40km 1550nm Single-Mode laser directly into an receiver with a 2-meter patch cable. The laser power output (-0dBm) will physically overload and blind the receiver diode (which expects -14dBm).
Optical link budgeting is basic subtraction. Take the Transmitter Launch Power (e.g. 0 dBm), subtract the cable attenuation loss (0.35 dB/km for SMF), subtract splice and connector losses (0.5 dB each). The resulting number MUST fall within the Receiver's allowable sensitivity range.
If the signal arrives too hot, use Optical Attenuators (pads) to dampen the laser before it hits the receiver chip — this is common when using "LR" (Long Range) SFPs for short jumper runs.