Fiber optic communication primarily uses Wavelength Division Multiplexing (WDM), Time Division Multiplexing (TDM/OTDM), and Space Division Multiplexing (SDM) to transmit multiple data streams over a single fiber.

Wavelength Division Multiplexing (WDM)

WDM is the most widely deployed multiplexing technology in optical networks. It combines multiple optical signals onto a single fiber by assigning each signal a distinct wavelength (color) of light, allowing simultaneous transmission without interference . WDM enables bidirectional communication over a single fiber and increases network capacity without laying additional fibers. There are two main types of WDM:

• Coarse WDM (CWDM): Uses fewer channels with wider wavelength spacing, suitable for shorter distances and cost-effective deployments .
• Dense WDM (DWDM): Supports many closely spaced channels, often up to 96, and is used for high-capacity, long-haul networks . Optical add-drop multiplexers (OADMs) can insert or extract specific wavelengths at intermediate points, enabling flexible network topologies like rings or multi-site connections .

Time Division Multiplexing (TDM/OTDM)

TDM divides the transmission time into slots, with each data stream assigned a specific time slot. In optical networks, Optical Time-Division Multiplexing (OTDM) uses ultrafast optical pulses to combine multiple low-bit-rate channels into a single high-speed channel . OTDM can achieve extremely high data rates but requires precise synchronization and complex demultiplexing at the receiver.

Space Division Multiplexing (SDM)

SDM is an emerging technology that uses multiple spatial channels within a single fiber or multiple fibers in parallel to increase capacity . This includes:

• Multi-core fibers (MCFs): Multiple cores within a single fiber, each carrying independent signals.
• Few-mode fibers (FMFs): Different propagation modes act as separate channels. SDM offers dramatic capacity increases but faces challenges like crosstalk management and mode coupling.

Summary

• WDM: Multiplexes by wavelength; widely used; scalable; supports CWDM and DWDM.
• TDM/OTDM: Multiplexes by time; enables ultra-high-speed transmission; requires precise timing.
• SDM: Multiplexes by spatial channels; emerging technology for massive capacity growth. These multiplexing technologies are often combined in modern optical networks to maximize bandwidth, improve spectral efficiency, and meet the growing demand for high-speed data transmission .