The need to split and branch optical signals in modern networks arises quite frequently. This may be due to the need to conserve fiber resources when a group of subscribers is located at a considerable distance from the central unit, or to the need to monitor and measure the parameters of fibers carrying the signal. The coupling and splitting of optical signals is also used in WDM systems.
To solve such problems, fiber optic splitters are used – passive multi-port devices designed to split the signal across several fibers in specified proportions.
Based on their operating principle, fiber splitters are conventionally divided into three main types:
- Directional splitters – splitters with output ratio between ports. The output ratio depends on the direction of optical radiation propagation. They are widely used in modern reflectometers.
- Non-directional splitters – splitters without output ratio between optical ports. They are used in cable television and data transmission networks.
- Spectral-selective splitters (wavelength-selective) – splitters that are sensitive to the input wavelengths, meaning that they “transmit” certain wavelengths while blocking others. Such components are used to split or couple optical signals of different wavelengths in multiplexers and demultiplexers.
Fiber splitters are supplied ready-to-use in a compact housing. If the splitter’s inputs and outputs have already been terminated with connectors during manufacturing, no additional tools are required for installation. In cases where an unterminated optical fiber exits the splitter housing, installation will require the same equipment and tools used for fiber splicing.
Based on their manufacturing technology, optical splitters can be divided into planar splitters (PLC, Planar Lightwave Circuit) and fused-type splitters (FBT, Fused Biconical Taper).
FBT splitters
FBT splitters are produced by splicing the end faces of standard optical fibers (pic 1).
Pic 1. Operation principle of fused splitters
The main feature of such splitters is the ability to distribute the signal unevenly with a specified ratio. This feature has found application in cable television networks, where it was necessary to compensate for signal differences between subscribers located at varying distances from the light source, and in passive optical networks (PON) built using “bus” topology, an example of which is shown in pic 2.
Pic 2. Fused splitters application in PON network
At the same time, the low quality and limited number of wavelengths used in FBT splitters led to the creation of an alternative – planar optical splitters.
PLC Planar Optical Splitters
The production of PLC splitters is more complex and consists of several stages. The first involves depositing a reflective cladding layer onto a quartz substrate, onto which the waveguide material is applied, where a mask for etching is subsequently formed. The etching process results in a system of waveguide channels. Next, a second reflective layer is applied. As a result, the splitter’s structure resembles a simple optical fiber: the reflective layer acts as the fiber cladding, and the etched channels form the fiber core. To create optical outputs, optical fibers are bonded to the ends of the etched channels. Finally, the resulting splitter is placed in a metal housing for reliability. Some of the manufacturing steps for planar splitters are shown in pic 3.
Pic 3. Manufacturing steps for planar splitters
Planar technology allows for the manufacture of splitters with an even number of output fibers ranging from 2 to 64 outputss. Thanks to a more complex and precise manufacturing process, planar splitters possess more stable and accurate optical characteristics and operate in the broad wavelength range of 1260–1650 nm.
Planar splitters are most widely used in the construction of passive optical networks using “star” or “tree” topologies in apartment buildings (pic 4) or in dense residential areas, where the uniformity of the output signal is not compromised by large variations in cable lengths to the subscriber.
Pic 4. Use of planar splitters in passive optical networks
It is important to remember that the main characteristic of optical splitters is insertion loss. When optical power is distributed among the splitter’s outputs, the signal attenuates. This feature must be taken into account when designing the network and calculating the optical budget. The more outputs a splitter has, the greater the insertion loss. Attenuation values of each type of splitter can be found in corresponding datasheets. A finished product always comes with a data sheet indicating the results of actual measurements.
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