xDSL, CATV, video on demand and other high bandwidth-demand applications cannot be supported by some existing old network infrastructures. This inneficiency by the old access network brings the need to connect the end users with the high performing optical WAN 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
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
- Very wide bandwidth (1013
- minimal interference from neighboring sources
- Low transmission losses
- Increased signal security, because it is harder for an “adversary” to “eavesdrop” a light signal
PON is a point to multipoint network which means that the incoming signal is distributed among several subscribers. It is called passive because it uses components like splitters and couplers which just manage the incoming and outcoming light power without changing its amount. The OLT (Optical Line Terminal) is the interface that links the MAN (Metropolitan Area Network) and the end users. The single light beam from the OLT is separated in the optical splitter into multiple beams, like a demultiplexer does. The use of an optical coupler enables the system to act as data multiplexer and demultiplexer
. Finally the PON is terminated on several ONTs (Optical Network Termination) which distribute the light signal to the customer equipment.
Alfaphonet offers splitters and couplers for Fiber To The Home and PON networks. The SFT-P planar optical splitter series is designed for FTTH and PON networks. It splits the input optical signal into 4, 8, 16, 32, 64 or 128 outputs and features:
- Very low insertion loss
- ultra wide bandwidth
- Polarization independence
- Excellent channel uniformity
- Compact size
- High reliability and stability
The SFT splitter/coupler series are ultra reliable devices that feature low backreflection, low insertion loss and high port isolation over wide temperature and wavelength ranges. They are an important element of FTTH or PON netwoks and can both split and couple light signals with splitting/coupling ratios from 1/99% to 50/50%. They operate in two modes: The single input with wavelength range from 1270 to 1630 nm (CWDM wavelength range) and the dual input wavelengths, 1310±40 nm and 1510±80 nm.
Apart from SFT couplers/splitters, which are for singlemode fibers, there are also the SFM couplers/splitters for multimode fibers.
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.
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.
Dense wavelength division multiplexer
) 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.
Alfaphonet offers the SFW-D DWDM multiplexer
- Wide channel bandwidth
- High adjacent channel isolation
- Low PDL
- Low chromatic dispersion
- Compact size
- Many packaging options
Both CWDM and DWDM multiplexers
supports the add/drop function which adds or removes a single wavelength from an existing data stream. The optical filtering can be done by the use of Fabry–Pérot optical filters