Introduction: The Building Blocks of the Light Highway

Modern optical networks, especially those using Wavelength Division Multiplexing (WDM), function like a multi-lane highway for data. Each lane is a distinct color (or wavelength) of light, carrying an independent stream of information. To manage this traffic of light, specialized components are needed to act as the "on-ramps," "off-ramps," and "lane dividers."

This page delves into the fundamental passive optical components that make WDM possible. We will explore how they combine and separate different colors of light, filter them with incredible precision, and enable the creation of flexible and high-capacity network nodes. Understanding these components is key to understanding how modern global communication works.

Multiplexers and Demultiplexers: The On and Off-Ramps

The most fundamental devices in any WDM system are the multiplexer and the demultiplexer. They perform opposite but complementary functions.

• Multiplexer (MUX): A MUX acts as an "on-ramp." It takes multiple independent optical signals, each at a unique wavelength (e.g., $λ_1, λ_2, ..., λ_N$), from separate input fibers and combines them into a single, multi-colored beam of light that is then sent down a common output fiber.
• Demultiplexer (DEMUX): A DEMUX acts as an "off-ramp." It takes a composite WDM signal arriving on a single fiber and separates it back into its constituent wavelengths, directing each color to a separate output fiber for processing.

The operation of these devices can be compared to a prism: a prism can split white light into a rainbow (demultiplexing), and in reverse, it can combine a rainbow of colors back into white light (multiplexing).

Technologies for (De)multiplexing

Several different physical principles and technologies are used to build MUX/DEMUX devices, each with its own advantages and disadvantages.

1. Thin-Film Interference Filters (TFF)

This technology relies on the principle of optical interference. TFFs are constructed by depositing dozens or even hundreds of extremely thin, alternating layers of two different dielectric materials onto a glass substrate.

The thickness of each layer is precisely controlled (often a quarter of the target wavelength). When light hits the multi-layer stack, a portion of it reflects at each layer boundary. Due to constructive and destructive interference, only one very narrow band of wavelengths is transmitted through the filter, while all others are reflected. To demultiplex a signal, these filters are used in a cascade: the first filter reflects wavelength $\lambda_1$ and passes the rest; the second filter reflects $\lambda_2$ and passes the rest, and so on.

2. Diffraction Grating-Based Devices

This technology works like a more precise, engineered version of a prism. A diffraction grating is a surface etched with thousands of microscopic, parallel grooves per millimeter. When a WDM light beam strikes the grating, it is diffracted, meaning each wavelength is bent or reflected at a slightly different angle. This spatially separates the colors.

The typical architecture involves a collimating lens to make the light parallel, the diffraction grating to separate the wavelengths angularly, and a focusing lens to focus each separated wavelength onto the end of a specific output fiber. This technique is highly effective for separating many channels at once.

Specialized Components for Control: FBG and Circulators

In addition to simply combining and splitting all wavelengths, advanced networks require components that can manipulate individual channels. Fiber Bragg Gratings and optical circulators are key enablers for this functionality.

Component Application: Building an OADM

We can now see how these specialized components are combined to create a functional Optical Add-Drop Multiplexer. Two common architectures are presented below.

1. OADM with Circulators and Tunable FBGs

This design uses two circulators and a tunable FBG to manage a single wavelength channel.

Drop Operation:

1. The multi-wavelength WDM signal enters circulator 1 at port 1 and exits at port 2.
2. The signal hits the TFBG, which is tuned to reflect the desired wavelength ($\lambda_i$).
3. The reflected signal $\lambda_i$ re-enters circulator 1 at port 2 and is directed out of port 3, which serves as the DROP port.
4. All other wavelengths pass through the TFBG unaffected.

Add Operation:

1. The unaffected wavelengths continue on their path towards the main output.
2. A new signal on the same wavelength $\lambda_i$ is injected into the ADD port. This is often routed to combine with the pass-through channels before leaving the node.
3. The combined signal of pass-through channels and the new added channel exits the OUT port.

2. OADM with Mach-Zehnder Interferometer (MZI) and FBGs

This more advanced architecture uses a Mach-Zehnder Interferometer composed of two 3dB couplers and two identical FBGs (one in each arm) to separate and add channels without needing a circulator. It relies on the precise interference of light waves.

The incoming WDM signal is split. The target wavelength is reflected by the FBGs in both arms. Due to phase shifts in the couplers, the two reflected signals interfere constructively at the DROP port and destructively at the main path. Conversely, the transmitted signals interfere constructively at the OUT port. The same principle in reverse allows for adding a new channel. This design can offer lower loss and better performance.