Mux and Demux Used in FTTH (Fiber To The Home) network

This transition from copper to fiber optic cabling is called FTTx (Fiber To The x)where x is determined by the distance between the optical fiber and the end user.

There are five most known FTTx architectures:

- FTTN Fiber To The Node

- FTTC Fiber To The Curb

- FTTB Fiber To The Building

- FTTH Fiber To The Home

In FTTH configuration the optical signal reaches the subscriber’s home or office and it is distributed by copper wiring or wireless connection. The replace of copper wires by optical cables offer to customers:

- Very wide bandwidth (1013 to 1016 Hz)

- minimal interference from neighboring sources

- Low transmission losses

- Increased signal security, because it is harder for an “adversary” to “eavesdrop” a light signal

Wavelength Division Multiplexing
 

WDM (Wavelength Division Multiplexing) is a technique similar to FDM (Frequency Division Multiplexing) which is used widely in Fiber To The Home networks While FDM combines various frequency carriers and sends them through a single copper cable, WDM combines various wavelengths and couples them into a single optical fibre. The two multiplexing techniques are similar because frequency and wavelength are connected by the equation λ=V/F , where V is the monochromatic beam's speed. The adoption of WDM enables the operator to expand the network's capacity without deployment of extra fiber cable. Furthermore A WDM optical channel can carry any transmission format.

 

The wavelength multiplexer at transmitter's side merges the input signals and passes them through the optical fiber. The signal is amplified by an EDFA or Raman amplifier and is separated back into individual output wavelengths at the wavelength demultiplexer at receiver's side. The separation of wavelengths at the demultiplexer is achieved by the use of interference Fabry–Pérot optical filters. These filters are two plane reflecting surfaces which contain air or other material between them. The separation of wavelengths is based on interference between the reflected beam and the two surfaces. The interference can be either constructive or distructive. Constructive interference occurs when the phase difference of transmitted waves is zero while distructive when δ≠0, where δ is the phase difference.

 

CWDM multiplexer
 

The Coarse wavelength division multiplexer has a channel spacing of 20nm and using the wavelengths from 1270 up to 1570nm. It features low insertion loss and low polarization depended loss (PDL)<= 0.1dB. When the polarization state of an incident wave changes, the output power is also changes. This is the concept of PDL and its calculated by the equation:

PDL = 10log(Pmax/Pmin)

where the fraction Pmax/Pmin is the peak to peak difference in power of an optical signal with respect to its polarization state. CWDM multiply the capacity of existing singlemode fibers by combining up to 16 ITU-T G.694.2 compatible channels in metropolitan access and enterprise networks and for CATV applications. They are a low cost approach for systems that use imprecise laser sources and are an alternative to more expensive DWDM components.
 

DWDM multiplexer
 

Dense wavelength division multiplexer (DWDM) has channel spacing of 100 or 200 GHz. It uses precise laser source which creates channels with very close spacing. It operates within the 1550 nm band and can drastically increase the capacity of an optical network. The equipment which is used in a DWDM system consists of high precision lasers, multi wavelength optical repeaters (MOR) and Erbium doped Fiber Amplifiers (EDFA). Nowadays the MOR became obsolete and EDFA are used instead, for signal amplification.

 

Add/drop

Both CWDM and DWDM multiplexers supports the add/drop function which adds or removes a single wavelength from an existing data stream.

 

For more WDM information, please visit http://www.optoroute.com.cn