How Do Different Fiber Optic Couplers Work?

Posted on November 12, 2014
July 5, 2020
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How Do Different Fiber Optic Couplers Work?

Fiber optic couplers (not to confuse with fiber optic adapters) cover a wide range of fiber optic devices that you may have used or not, including optical splitters, optical combiners and optical couplers. They function in different applications that require other than point-to-point links. How do they redistribute optical signals in data links?

What Is Fiber Optic Coupler?

Fiber optic coupler is one type of fiber optic component that allows for redistribution of optical signals. A fiber optic coupler is a device that can distribute the optical signal from one fiber among two or more fibers. A fiber optical coupler can also combine the optical signal from two or more fibers into a single fiber. A signal is attenuated more in a fiber optic coupler than in a connector or a splice, because the input signal is not directly transmitted from one fiber to another, but divided among the output ports. For instance, with a 1 x 2 fiber optic coupler, each output is less than one-half the power of the input signal (over a 3dB loss).

Different Fiber Optic Coupler Types

A basic fiber optic coupler has N input ports and M output ports. N and M typically range from 1 to 64. The number of input ports and output ports vary depending on the intended application for the coupler.

basic passive fiber optic coupler design Figure 1: basic passive fiber optic coupler design.

Under different principles, fiber optic couplers can be categorized differently. We will introduce fiber optic coupler types from two aspects.

Passive Coupler vs. Active Coupler

Fiber optic couplers can be either passive or active devices. Passive fiber optic couplers require no power for operation. The difference between passive and active couplers is that a passive coupler redistributes the optical signal without optical-to-electrical conversion. Active fiber optic couplers require an external power source. They are electronic devices that split or combine the signal electrically and use fiber optic detectors and sources for input and output. Most fiber optic couplers we’ve seen in optical links are passive devices, such as passive optical splitter and multiplexer (Mux) or de-multiplexer (Demux) in wavelength-division multiplexing (WDM) networks.

Fiber Optic Coupler Types

If we see fiber optic couplers by shape, there is Y coupler, T coupler, X coupler, star coupler and tree coupler, which split the optical signal based on the power.

Y Coupler

A Y coupler resembles the letter Y. The input signal is split evenly into the two output fibers.

Y coupler Figure 2: Y coupler.

T Coupler

Unlike the Y coupler, a T coupler has an uneven distribution. The power of one output signal is greater than the other output signal. Popular splitting ratios include 90%-10%, 95%-5% and 99%-1%.

T coupler Figure 3: T coupler.

X Coupler (2x2)

X couplers carry out the function of a splitter and a combiner in one package. The X coupler combines and divides the optical power from the two input fibers between the two output fibers. Another name for the X coupler is 2 x 2 coupler.

X coupler or 2 x 2 coupler Figure 4: X coupler or 2 x 2 coupler.

Star Coupler

Star couplers have more than two input or two output ports. A star coupler distributes optical signal from more than two input ports among several output ports.

star coupler Figure 5: star coupler.

Tree Coupler

A tree coupler is also a multiport coupler. It splits optical power from one input fiber to more than two output fibers. A tree coupler may also be used reversely to combine the optical signal from more than two input fibers to one output fiber.

tree coupler Figure 6: tree coupler.

Wavelength Selective Couplers-WDM Mux/Demux

Wavelength selective couplers are actually WDM Mux/Demux. Instead of splitting the signal based on their power, wavelength selective couplers split the signal based on their wavelengths, such as coarse wavelengths (CWDM) channels (1270 nm, 1290 nm, 1310 nm, etc.) and dense WDM (DWDM) channels (C band or L band). Signal of different wavelength channels are routed to different ports.

Optical Coupler vs. Optical Splitter vs. Optical Combiner

Optical coupler, optical splitter and optical combiner are all devices belonging to fiber optic couplers. Optical splitters are usually Y couplers, T couplers or tree couplers that have only one input port and two or more output ports, and on contrary, optical combiners have only one output port and two or more input ports. So you can also see the numeral “1” in the descriptions of optic splitters and combiners, like 1 x 2 fiber optic splitters/combiners, 1 x 4 splitters/combiners, and 1 x 64 splitters. However, for optical couplers, they can be in any form we have mentioned above, such as star couplers, 2 x 2 couplers, 1 x 4 couplers, tree couplers, etc. Each device is designed for its purpose.

Fiber Optic Coupler Manufacturing Technologies

Technologies used for constructing fiber optic couplers can be complex and difficult to understand. Three major manufacturing techniques are micro-optics, fused-fiber and planar waveguide. Micro-optics use individual optical elements such as prism, lens, mirrors etc. to construct an optical route. These elements divide the input optical signal into two or more separated light beams.

Fused-fiber couplers used the most basic material–optical fiber. Two or more fiber cores are twisted, fused and tapered together in a length. This type of fiber optic coupler is fused biconical taper (FBT) coupler.

fabrication of a fused biconical taper coupler (star coupler) Figure 7: fabrication of a fused biconical taper coupler (star coupler).

Planar waveguides are more like a semiconductor. A planar wafer is used to make a waveguide coupler, and the reflections occur only in y-directions. Planar lightwave circuit (PLC) couplers use this technology. Planar waveguides are more often used to make high port count couplers, such as 1 x 12 PLC splitter, 1 x 24 PLC splitter.

Applications of Fiber Optic Couplers

In local area network (LAN) applications, fiber optic couplers are used in either bus architecture or star architecture.

In a star network topology, the stations branch off from a central hub, much like the spokes on a wheel. Each network device connected by the star coupler can communicate with each other. Besides, the star coupler makes it easy to expand the number of workstations. For example, changing from a 4 x 4 to 8 x 8 doubles the system capacity.

star couplers are used in star network topology Figure 8: star couplers are used in star network topology.

Bus architecture utilizes T couplers to connect a series of stations to a single backbone cable. In a typical bus network topology, the T coupler at each node splits off part of the power from the bus and carries it to the attached equipment.

T couplers are used in bus network topology Figure 9: T couplers are used in bus network topology.

Tree coupler are typically used in cascaded PON architecture. The first tree coupler is directly connected to the optical line terminal (OLT) port in the central office, then each of the output fibers is routed to a tree coupler in other sites (outside enclosure/terminal box). If there’s need to further divide the signal, more tree couplers can be added. Of course other amplification or compensation modules are required to ensure the transmission.

tree couplers are used in cascaded PON architecture Figure 10: tree couplers are used in cascaded PON architecture.

Summary

Fiber optic couplers are important fiber optic devices that comprise several common passive optical devices such as optical splitter, optical combiner and optical coupler, as well as active couplers. These devices are widely used in fiber optic data links whether it is WDM systems or PON systems. In a word, fiber optic couplers contribute a lot in applications beyond point-to-point links.