In many applications, it may not be possible to have a design of many point-to-point connections. In these cases, optical couplers are used. A fiber optic coupler is a device that combines or splits optical signals. A coupling device may combine two or more optical signals into a single output, or the coupler may be used to take a single optical input and distribute it to two or more separate outputs. The Figure below shows an example of a basic four-port coupler, generally named fiber coupler 1 × 4.
Many couplers are designed bidirectionally, which enables the same coupler to be used to combine signals or split signals. An optical coupler being used to split a signal may be referred to as an optical splitter. Couplers are available with a wide range of input and output ports. A basic coupler may have only one input port and two output ports. Today’s technology supports couplers with up to 64 input and 64 output ports, as shown below.
There are many different types of couplers, and the number of input and output ports is dependent on the intended usage. Some of the types of optical couplers are optical combiners, Y couplers, star couplers, tee couplers, and optical splitters. In this article, we will only focus on the tee coupler and the star coupler and give you a comparison between them.
A tee coupler is a three-port optical coupling device that has one input port and two output ports, as shown below.
The tee coupler is a passive device that splits the optical power from the input port into two output ports. The tee coupler is in essence an optical splitter. The uniqueness of the tee coupler is that this type of coupler typically distributes most of the optical input power to one output and only a small amount of power to the secondary output. Note that when the outputs are evenly distributed, the coupler is called a Y coupler. The tee coupler is also referred to as an optical tap, due to the nature of the device. A majority of the power continues forward, but a portion of the signal (determined by the splitting ratio) is tapped to be used for an output port.
The tee coupler is a 1 × 2 coupler or 1 × 2 fiber splitter, meaning that it has one input port (or connection) and two output ports. As previously stated, the optical output power of the two output ports is typically not evenly distributed. Common splitting ratios are 90:10, 80:20, 70:30, 60:40, and 50:50 (a Y coupler). Not all manufacturers follow the convention of placing the larger value to the left of the colon and the smaller to the right. Some manufacturers simply reverse this and place the smaller value to the left of the colon and the larger to the right.
A typical use for a tee coupler would be to supply optical signals to a bus type network of in-line terminals. Assuming ideal conditions and a 90:10 split on the tee coupler, the first terminal would receive 10 percent of the optical signal and 90 percent of the optical signal would go forward to the next tee coupler.
The star coupler is used in applications that require multiple ports (input and/or output). The star coupler will distribute optical power equally from one or more input ports to two or more output ports. Here is a basic star coupler with four input ports and four output ports. Star couplers are available in 1 × 64 up to 64 × 64 dimensions.
A special version of the star coupler, called a tree coupler, is used when there is one input port and multiple output ports or when there are multiple input ports and one output port.
Star couplers are frequently used in network applications when there are a large number of output terminals. In our tee coupler example, we had to account for interconnection insertion loss and coupler insertion loss at each tee connection. However, with the star coupler there is only one coupler insertion loss regardless of the number of ports. With only one coupler insertion loss, a multiple port star coupler is more efficient than a series of tee couplers. So the larger the network is, the more efficient the star coupler becomes.
Two types of star couplers are commonly used: the reflective star and the transmissive star. Couplers are typically considered to be a black box, that is, only the manufacturer knows what’s inside. However, many star couplers are made of fused optical fibers as a type of fused fiber coupler.
Advantages of Star Coupler compared to Tee Coupler
The key advantage to the star coupler is that there is only one insertion loss caused by the coupler. The only remaining insertion losses are from the interconnections. The advantage of the star coupler becomes very apparent as the number of ports increases. Here is a simple loss-comparison chart that reveals the significance in the number of terminals versus loss for the tee and star couplers.
A star coupler has another advantage over a series of tee couplers. If one of the tee couplers in the series is disconnected, none of the other terminals down the line will receive an optical signal. However, disconnecting a terminal from the star coupler will not impact the operation of the other terminals.
Related Article: Overview of Bi-Directional Transceiver Modules