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Wavelength Division Multiplexing (WDM)

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1. Concept of Wavelength Division Multiplexing

Wavelength Division Multiplexing (WDM) is an optical multiplexing technique designed to harness the immense transmission capacity of fiber optic cables. It combines multiple light beams of different wavelengths ($\lambda$) (colors of laser light) onto a single optical fiber strand.

Fundamentally, WDM is the optical equivalent of Frequency Division Multiplexing (FDM) operating at optical frequencies ($> 190\text{ THz}$), following the wave relationship:

(where $c = 3 \times 10^8\text{ m/s}$ is the speed of light, $f$ is optical frequency, and $\lambda$ is wavelength).

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2. Optical Combining & Splitting Mechanism

At the physical layer, combining and splitting optical wavelengths is accomplished using optical passive devices such as prisms, diffraction gratings, or Arrayed Waveguide Gratings (AWG):

• Snell's Law of Refraction: Different wavelengths of light refract at slightly different angles when passing through an optical prism.
• Combining: Beams entering at calculated angles merge into a single coaxial output beam.
• Splitting: A composite beam passing through the prism disperses into its constituent wavelengths, each focused onto an individual optical photodiode.

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3. Types of WDM: CWDM vs. DWDM

Optical networking standardizes two main implementations based on channel spacing:

1. CWDM (Coarse Wavelength Division Multiplexing)

• Standard: ITU-T G.694.2.
• Channel Spacing: Wide $20\text{ nm}$ spacing across the spectrum ($1270\text{ nm}$ to $1610\text{ nm}$).
• Channel Capacity: Up to 18 channels.
• Transceivers: Uses uncooled, inexpensive distributed feedback (DFB) lasers.
• Distance: Typically $\leq 50 - 80\text{ km}$ (metro campus networks, cable TV headends).

2. DWDM (Dense Wavelength Division Multiplexing)

• Standard: ITU-T G.694.1.
• Channel Spacing: Ultra-dense narrow spacing: $0.8\text{ nm}$ ($100\text{ GHz}$ grid) down to $0.4\text{ nm}$ ($50\text{ GHz}$) or $0.2\text{ nm}$ ($25\text{ GHz}$).
• Channel Capacity: 40, 80, 96, or up to $160+$ optical channels on a single fiber strand!
• Transceivers: Requires precision temperature-stabilized cooled lasers to prevent frequency drift.
• Optical Amplifiers (EDFA): Directly amplifies light waves across the C-band ($1530 - 1565\text{ nm}$) without converting light to electricity.
• Distance: Long-haul submarine and trans-continental carrier backbones ($> 1{,}000\text{ km}$).

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4. Comprehensive Comparison: FDM vs. TDM vs. WDM

Feature	FDM	TDM	WDM
Domain	Frequency (Hz)	Time (Seconds)	Optical Wavelength (nm)
Signal Type	Analog	Digital	Optical (Light pulses)
Transmission Medium	Copper (Twisted Pair, Coax), Air	Copper, Microwave, Fiber	Strictly Fiber Optic Cables
Operating Principle	Signals modulate different RF carriers	Dedicated time slots in round-robin	Light beams of different colors merge
Channel Separation	Guard Bands (Hz)	Time Framing / Guard intervals	Channel spacing ($\Delta\lambda$ in nm)
Typical Capacity	Megabits per second	Megabits to Gigabits/sec	Multi-Terabits per second (Tbps)

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