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Multiplexing: What You Need to Know

Jul 14th, 2015

What is a MUX?

In electronics, a multiplexer (MUX) is a device that selects one of N inputs to be representative of the output. The output is connected to one of the N inputs to the device, and the selection of the input is typically handled by log2(N) select lines:

In the optical space, unlike the electrical space, the MUX is a device that acts similar to a combiner. For the purposes of this blog, we will focus on passive multiplexers; these devices do not require a power source or electronics to operate. Passive multiplexers take N separate optical signals and combine them from separate fibers to a single fiber. This method relies upon the principle of superposition; it requires that all signals are orthogonal and that their adjacent channel does not interfere. CWDM and DWDM signals meet this criterion but multiplexer specifications will differ.

In addition to a wavelength combiner, the multiplexer serves as a device that increases fiber efficiency. For example, we can take N lower bit rate signals and combine them into a single higher bit rate stream. This allows more efficient use of the single fiber.

The generation of acceptable orthogonal wavelengths is typically executed by utilizing carefully selected CWDM or DWDM transceivers. The transceivers selected will generate optical signals that are not only independent, but also matched with the multiplexers input channels. Because the multiplexer is a passive device, the input ports are fixed wavelengths and will be labeled with the expected channel.

In the Mux shown above, each input signal is marked as a fixed wavelength. The combined output is provided on “COM”

Power and Compatibility

Multiplexers have a limit to the input power that they can handle. When this limit is exceeded, product nonlinearities in the combined signal may occur. Nonlinearities result in distortion, performance loss, and can even damage the inner MUX workings.

Passive multiplexers are highly reliable devices as there are no electronic parts to fail or drift. This passive nature allows them to be protocol agnostic. Any given passive MUX will combine SONET, SDH, GigE, 10GigE, and other protocols without bias. These multiplexers are also signal rate independent, and will combine any signal up to the specification limit of the device. As with any device that is connected in an optical network, some loss of signal power should be expected. Typical values of insertion loss are on the order of 2dB to 3dB on each port. Multiplexers are available in multiple port configurations. Examples of port configurations include: 4,8,16,32 and 40.

Grating Technologies

 There are 4 competing technologies by which multiplexers are built on. These include:

1) Fiber Bragg Grating (FBG)

2) Array Waveguide Grating (AWG)

3) Thin Film Filter (TFF)

4) Diffraction Grating Filter (DG)

To Be Continued...

Stay tuned for Part II of our three series Mux/Demux Blog!

For more information on Precision's extensive line of Mux/DeMux products, contact sales@precisionot.com

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