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How To Select Wavelength Division Multiplexers

Image Credit: Microwave Photonic Systems Inc.

 

Wavelength division multiplexers (WDM) are electronic devices that combine light signals with different wavelengths, coming from different fibers, onto a single fiber. They are a cost effective method to expand the capacity of existing fiber optic cables.  WDMs use current electronics and fibers and shares fibers by transmitting different channels at different wavelengths (colors) of light.

 

How Wavelength Division Multiplexing Works

Wavelength division multiplexing starts with the phenomenon of light waves. Many different colors of light can be seen at the same time and the colors are transmitted together through the air. The colors may mix, but they are easily separated using a device such as a prism. Light waves can also travel through optical fiber. Many wavelengths can be sent through a cable together and with the use of couplers and filters, the waves are sorted to the intended detectors.

 

The input end of a WDM is a coupler that combines all the inputs into one output fiber. WDMs often have many inputs. Demultiplexing involves taking the input fiber, and collimating the light into a narrow, parallel beam of light. The light shines on a grating, which separates the light into the different wavelengths by sending them off at different angles. The grating is a mirror like device that works like a prism. The optics capture and focus each wavelength into a fiber to create separate outputs for each wavelength of light.

 

WDM demultiplexer. Image Credit: Fiber Optic Association

There are several advantages to using WDM.

  • Individual wavelengths can be from a normal link so current equipment can be used. Laser transmitters must be chosen to match the WDM demultiplexer so each channel is properly decoded at the receiving end.
  • Future enhancements are expected to offer 80-128 channels.

  • Digital signals can be mixed and matched.

Types

There are two types of wavelength division multiplexers.

  • Dense wavelength division multiplexers (DWDM): These devices use optical (analog) multiplexing techniques to increase the carrying capacity of fiber networks beyond levels that can be accomplished via time division multiplexing (TDM). With DWDM, different wavelengths of light can transmit multiple streams of information along a single fiber with minimal interference. Typically, four or more wavelengths in the 15nnnm erbium-doped fiber amplifier (EDFA) region are multiplexed. Most DWDM systems for long-distance transmissions offer 16 to 40 wavelengths at 2.5 Gbps (OC-48 SONET or STM-16 SDH), or 10 Gbps (OC-192 SONET or STM-64 SDH) per wavelength. They are deployed as point-to-point, static overlays for TDM networks and represent a precursor to optical networking. In reducing the cost of transport, dense wavelength division multiplexers require fewer electrical generators and share a single optical amplifier over multiple signals.  

  • Course wavelength division multiplexer (CWDM): These devices increase channel spacing to allow less sophisticated and thus cheaper transceiver designs. To provide 8 channels on a single fiber CWDM uses the entire frequency band between second and third transmission window (1310/1550 nm respectively) including both windows (minimum dispersion window and minimum attenuation window) but also the critical area where OH scattering may occur, recommending the use of OH-free silica fibers in case the wavelengths between second and third transmission window should also be used. They work on low water peak fibers because the high water absorption band has been eliminated by the fiber manufacture.

Specifications

Specifications for wavelength division multiplexers (WDM) include

  • Cable type
    • Single mode fibers allow one mode to propagate.
    • Multimode fibers support the propagation of multiple modes.
  • Wavelength range is the range of wavelengths the WDM will operate in.
  • Bandwidth, also called channel spacing, is the size of the windows in nm.
  • Channel spacing is the center-to-center difference in frequency between neighboring channels. Two hundred GHz corresponds to .4 nm spacing, 100 GHz corresponds to .2 nm spacing, and 50 GHz corresponds to .1 nm spacing.
  • Number of channels is the number of channels the device has to send information. This number can range from 4 to 160 with design enhancements adding more channels.  
  • Insertion loss is the attenuation caused by the insertion of wavelength division multiplexers (WDM) in an optical transmission system.
  • Polarization dependent loss is the maximum deviation in insertion loss across all input polarization states.

  • Isolation is a measure of light at an undesired wavelength at any given point.

  • Operating temperature and operating humidity are additional considerations when selecting wavelength division multiplexers (WDM).  

WDM Connectors

Wavelength division multiplexers (WDM) use many types of connectors.

  • FC and D4connectors are durable, zirconia-ceramic ferrules with a keyed body for repeatability.
  • MU connectors are 1.25 mm ferrules designed for high-speed data communications, voice networks, telecommunications, and dense wave division multiplexing (DWDM).
  • FDDI connectors, 2.5 mm ferrules with a fixed shroud, are used in fiber distributed data interface (FDDI) applications.
  • ESCON connectors, 2.55 mm ferrules with an adjustable shroud, are used in enterprise system connection (ESCON) applications.
  • LC connectors are high-precision, zirconia-ceramic ferrules that feature an RJ-45 push-pull housing and latching.
  • MT-RJ connectors hold two fibers with a ferrule that is smaller than the one used in MTP connectors, devices that are threaded and well-suited for high-density applications.
  • SC and ST connectors are easy-to-assemble and feature a bayonet mounting system.
  • SMA connectors include a low-cost, multi-mode coupling that is suitable for military applications.
  • Biconic connectors have precision-tapered ends for low insertion loss.

  • Loopback connectors are used to test transceiver systems.

Features

Wavelength division multiplexers (WDM) have features including:

  • Rack-mountable
  • Ability to add or drop channels during operation.

  • Light emitting diode (LED) indicators for monitoring power or synchronization.

Applications

Wavelength multiplexers can be used in submarine cables, to extend the lifetime of fiber cables, and reduce the cost of all land-based long distance communications links.

 

Resources

The Fiber Optic Association - Tech Topics