Many people struggle to understand the difference between Wavelength Division Multiplexing and optical splitters.

First, we need to understand the composition of a light source. Generally, light sources have a certain bandwidth, meaning they contain multiple wavelength components.
What we commonly refer to as "laser light" theoretically should contain only a single wavelength, also known as monochromatic light. However, achieving truly monochromatic light is practically impossible; lasers always have a certain linewidth and contain multiple wavelength components.

In communications, we need different wavelengths of light to carry different information. During signal transmission, wavelengths need to be combined (multiplexed) to facilitate transmission over a single optical channel. During signal demodulation, the different wavelengths of light need to be separated (demultiplexed). This is the function of a Wavelength Division Multiplexer (WDM).

So, simply put: A WDM is a device that combines (multiplexes) or separates (demultiplexes) light of multiple wavelengths, as shown in Figure 1. A splitter (PLC), however, divides light containing multiple wavelengths into different channels according to a specific optical power ratio. The light split into each channel remains broadband light containing all the wavelengths, as shown in Figure 2.

 

Figure 1: WDM

Figure 2: PLC

How do we test these two types of components? For example:

1. How to      test the optical spectrum of each channel of a WDM?

2. How to      test the delay difference between the various branches of a PLC?

3. How to      test distributed loss, i.e., the loss at various points inside an optical      component?

For the above tests, we can use an Optical Component Analyzer based on OFDR (Optical Frequency Domain Reflectometry) principles for high-precision testing. It can achieve a spatial resolution of up to sub-millimeter level and an optical delay resolution of up to sub-picosecond level.