A wavelength division multiplexer (WDM) separates or combines wavelengths along a spatial axis determined by the optical design, typically corresponding to the dispersion direction of a diffraction grating or arrayed waveguide, rather than a fixed “x” or “y” axis.

How WDM Works

A WDM multiplexer (MUX) combines multiple optical signals of different wavelengths into a single fiber, while a demultiplexer (DEMUX) separates them at the receiver . The separation of wavelengths is achieved using optical components such as:

• Diffraction gratings: These disperse light into its constituent wavelengths at different angles. The spatial separation occurs along the dispersion axis, which is defined by the grating orientation, not necessarily labeled as x or y .
• Arrayed waveguide gratings (AWGs): Integrated optical devices that route different wavelengths to different output waveguides. The wavelength-dependent path differences create spatial separation along the device's designed output plane .
• Thin-film filters (TFFs): These selectively transmit or reflect specific wavelengths, often in a fixed optical path, without scanning in a traditional x-y sense .

Implications for “Axis” Scanning

In practical terms:

• WDM devices do not scan dynamically along an x-axis or y-axis like a mechanical scanner. Instead, each wavelength is directed to a specific output port or fiber based on its optical properties.
• The concept of x-axis or y-axis is only relevant in optical layout diagrams, where the dispersion direction of a grating or the output plane of an AWG is conventionally drawn horizontally or vertically. The actual physical axis depends on the device design .
• In applications like spectroscopy or confocal microscopy, WDMs may be integrated with scanning systems, but the WDM itself is a static wavelength separator, not a scanning device .

Summary

A WDM separates wavelengths along a dispersion-defined spatial axis, determined by the optical components used (gratings, AWGs, or filters). It does not inherently scan along a conventional x-axis or y-axis; the axis labeling is a matter of optical layout convention rather than device function. Each wavelength is routed to a fixed output port or fiber, enabling simultaneous multi-wavelength transmission or reception .