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Splitter Types and Configurations

Today’s designers use optical splitters to passively tap, split, and multiplex optical signals. Since their introduction in the 1970s, splitters have seen their share of technological development, including the use of bulk optics, lenses, and optical fibers. As fiber optic communication systems expanded from conventional point-to-point networks into more complex point-to-multipoint FTTH systems, these developments resulted in products with better specifications, better environmental performance, and physical durability, while decreasing component size and costs.
Optical splitters — also known as couplers — have a number of characteristics that determine their function and application. These include: the number of input and output ports, the level of signal attenuation, the coupling ratio, the directionality of the light transmission through the splitter, wavelength selectivity, and whether it will be for single-mode or multimode operation.
Different applications require different numbers of input and output ports. To split one input between two outputs, a 1:2 splitter is required. Most splitters divide signals equally among all output ports, but devices can be made that divide the light unequally. For example, a splitter can be placed at the output of an optical amplifier that taps off 1% of the light for a performance monitor that verifies that all optical channels are present in the amplifier output.

Fused biconical tapered (FBT) splitters are fabricated by heating two optical fibers until they coalesce into a composite waveguiding structure. While the fibers are being heated, they are slowly stretched and tapered. This causes the light in the fiber to spread out into the composite structure to the point where it can be coupled to the other fiber. During this process, light is launched into the input fibers while a process control computer monitors the outputs and adjusts pull tension, fusion temperature, and duration. In this way, the desired coupling ratio between fibers is achieved.


FBT splitter

Image courtesy of JDSU

The completed splitter is then sealed in a protective enclosure. This usually consists of a metal tube placed around the fused section of bare fibers and held in place with an adhesive, with 250-micron-coated fiber pigtails passing through the ends of the package. With this compact packaging arrangement, the splitters can be mounted inside electronic equipment or modules where their pigtails can be terminated by connectors. FBT splitters are normally available in smaller splitter counts such as 1:2 and 1:4 configurations and can also easily fit into existing splice trays.
Planar lightwave circuit (PLC) splitters were the direct result of semiconductor technology associated with the production of computer processors and surface-mount electronics, and PLC splitters are mass produced in the same manner.
Like fibers, planar waveguides confine light in a region of high refractive index surrounded by material with a lower refractive index. The waveguide could be a strip embedded in a flat substrate, or it could be a strip deposited on the surface of the substrate.


PLC splitter

Image courtesy of Teem Photonics

Waveguide patterns are written using the same techniques that write integrated circuits onto semiconductor wafers. They can branch or merge, or can be cascaded to form splitters with more ports. Multiple parts can be written to each wafer, and then cut into individual components. Attaching fibers to the planar splitters requires that V-grooves be etched into the substrate material. The fibers are placed into these grooves, aligned, and then fixed in place with a cover block made from the substrate material.
The PLC splitter is smaller, more compact, and shows slightly lower losses than FBT splitters and can accommodate higher split ratios including the 1:32 used in FTTH passive optical networks. Various ITU PON standards specify wavelength independent splitters for transmitting bidirectional signals. The standard specifies 1490 and 1550 nanometers for downstream signals and 1310 nanometers for upstream signals through a variety of splitter combinations with the same attenuation value regardless of wavelength or direction.

Splitter Configurations

Splitter configurations, defined by the number of input and output ports, dictate the function of the device. There are four main configurations.
Tap splitters, also known as T and Y splitters, are three-port devices that split one input to a pair of outputs. The signal may be split evenly between the outputs or split in some other ratio. They are often directional devices. Simple 1-by-2 splitters are widely used in test equipment and network monitoring applications to tap off a small portion of a signal for analysis. In cable television applications, a variety of split ratios (10/90, 20/80, 30/70, and 40/60) are used for signal monitoring based on system attenuation values.
Other uses include signal drops on long fiber spans, such as in rural networks, where different coupling ratios allow for simple integration for remote access by using tap splitters along the span. At points close to the transmitter, optical power in the fiber is high, so only a small percentage of the signal needs to be dropped to the customer. At points further along the span where optical power in the fiber is lower, taps with higher split ratios ensure that each customer receives a roughly equal amount of optical power.
Tree or 1:N splitters divide a single input among multiple output ports. Some have a pair of inputs that are equally divided among all of the outputs. Others may combine multiple inputs to one or two outputs, and are generally directional.
Star splitters derive their name from the geometry used to show their operation. They have multiple inputs and outputs, often equal in number and are available in two distinct types. The first type is directional, mixing the signals from all inputs and distributing them to all outputs. The second type is non-directional, taking inputs from all ports, mixing them and distributing them back to all ports, both input and output.
Wavelength-selective splitters distribute signals according to their wavelength and are mainly used to route WDM signals to the appropriate ports, or to separate wavelengths travelling on a single fiber for different purposes. They also block wavelengths from reaching the wrong destination.


This article is excerpted from Fiber Optic Passive Devices.