The various ways of using couplers are shown in Figure 13.1.
Physically, they look almost the same as a mechanical splice, in fact in some cases we would need to count the number of fibers to differentiate between them. If there is one fiber at each end, it is a mechanical splice, any other number and it is a coupler.
Coupler sizes — Figure 13.2
A coupler with a single fiber at one end and two at the other end would be referred to as a 1 X 2 coupler (read as one by two). Although 1 X 2, and 2 X 2, are the most common sizes they can be obtained in a wide range of types up to 32 X 32 and can be interconnected to obtain non-standard sizes. Splitters are more common than combiners and this has made it more natural to refer to the single fiber end as the input.
The numbering of the ports is shown in Figure 13.3 (port is just a fancy word used in electronics to mean a connection).
Splitting ratio or coupling ratio
The proportion of the input power at each output is called the splitting ratio or coupling ratio. In a 1 X 2 coupler, the input signal can be split between the two
outputs in any desired ratio. In practice however, the common ones are 90:10 and 50:50. These are also written as 9:1 and 1:1. In the cases where the splitting ratio is not 1:1, the port which carries the higher power is sometimes called the throughput port and the other is called the tap
Even when the splitting ratio is quoted as 1:1, it is very unlikely, due to manufacturing tolerances that the input power is actually shared equally between the two outputs. The acceptable error of between 1% and 5% is called the coupling or splitting tolerance.
A gloomy note before we start.
When consulting trade publications, we find that the terms used to describe coupler losses, the naming of the ports and even the numbering of the connections have not been totally standardized. This makes it difficult to avoid meeting several different versions of the formula for each loss.
The only way to combat this is to understand the nature of the losses and then to be fairly flexible when it comes to the way it has been expressed.
We may recall that, generally, the loss in decibels is derived from the standard formula:
Excess loss is a real loss. If 10 mW goes into a device and only 9 mW comes out, then it is reasonable enough to think of the other 1 mW to be a loss. The light energy has been scattered or absorbed within the coupler and is not available at the output. So what we are really saying is that the loss is dependent on the total output power compared to the input power. In the case of the coupler in Figure 13.4, the output power is the sum of ports 2 and 3 and the input is at port 1.
So excess loss would look like this:
where P1, P2, P3 are the power levels at the respective ports.
Directionality loss or crosstalk or directivity
A typical figure is –40 dB.
Generally, insertion loss for any output port could be written as:
As an example, the insertion loss at port 2 is:
Insertion loss = 10log(P2/P1) dB
If, in the above example, the splitting ratio was not 1:1, then port 2 may be referred to as the throughput port and so the formula above becomes the throughput loss. Similarly, if ports 3 and 1 were used, the loss could be called the tap loss.
This is often overlooked. Whenever a coupler is used, it has to be joined to the rest of the circuit. This involves two pairs of connectors and a splice at each end. The losses caused by these connectors or splices must be added to the losses introduced by the coupler.
Calculate the output power at each port in the coupler shown in Figure 13.5.
Output power at port 4
The directionality loss is quoted as –40 dB.
Starting with the standard formula for decibels.
Take the antilog of each side:
60 x 10–6 x 10–4 = power out
So power out of port 4 = 60 x 10–10 = 600 nW.
As the output from port 4 is so small, it is often ignored.
Output power at port 2
Port 2 is the throughput port i.e. the port with the largest output power. With a splitting ratio of 3:1, for every 4 units of power leaving the coupler there are three at port 2 and only 1 at port 3. This means that 0.75 of the power leaving the coupler goes via port 2.
But how much power is leaving the coupler? This is the input power minus the excess loss. Port 4 output can be ignored since it is so slight compared with the other power levels.
Into the standard formula we put the –1 dB (minus, as it is a loss) and the input power:
Divide both sides by 10:
–0.1 = log(powerout/60x10-6)
Take the antilog of each side:
60 x 10–6 x 10–0.1 = power out
By calculator, 10–0.1 = 0.794
So total power out of the coupler = input power x 0.794, or:
60 x 10–6 x 0.794 = 47.64 x 10–6 W
Of the 60 μW that entered the coupler, 47.64 μW is able to leave. Of this amount 0.75 leaves via port 2 so:
power out of port 2 = 0.75 x 47.64 μW = 35.73 μW
Output power at port 3
We have already calculated the power remaining after the excess loss to be 47.64 μW and since we are dealing with port 3, the tap port, the proportion of power leaving by this port is only 0.25 of the total.
Thus the output power at port 3 is 0.25 x 47.64 μW = 11.91 μW.
The results are shown in Figure 13.6.
The tee-coupler — Figure 13.7
This is simply a 1 x 2 coupler used to convey a single signal to a number of different work stations. Such stations are said to be connected on a network. It would use a high splitting ratio of 9:1 or similar to avoid draining the power from the incoming signal.
Advantages and disadvantages of a tee network
The main advantage is its simplicity. The couplers are readily available and, if required, can be supplied with connectors already fitted. This means that the network can be on-line very quickly indeed.
The disadvantage is the rapid reduction in the power available to each of the workstations as we connect more and more terminals to the network. As the power is reduced, the number of data errors increases and the output becomes increasingly unreliable. At first glance we could solve this problem by simply increasing the input power level. However we run the very real risk of overloading the first workstation.
Power levels in a tee network
- Specification for our example system:
- incoming power = 1 mW
- splitting ratio of each coupler = 9:1
- excess loss of each coupler = 0.3 dB. The couplers are joined by connectors with an insertion loss of 0.2 dB each.
The incoming power level is reduced by 0.2 dB by the first connector, and 0.3 dB by the excess loss.
Total power reduction is 0.2 + 0.3 = 0.5 dB.
By inserting the values into the standard decibel formula, remembering to use
–0.5 dB as it is a loss, we have:
–0.5 dB = 10log(powerout/1x10-3)
Divide both sides by 10:
–0.05 dB = log(powerout/1x10-3)
So, the input power is:
0.8913 x 1 x 10–3 = power out = 891.3 μW
The 891.3 μW is the power just before it is split into the two output ports. As the splitting ratio is 9:1, the throughput power at port 2 is 0.9 of the available power or 802.17 μW. Similarly, the tap power is 0.1 of 891.3 μW or 89.13 μW.
Step 3 — Figure 13.9
The throughput power, 802.17 μW, is actually the input power to the next section of the network and is simply a replacement for the 1 mW input in Step 1. This new input power suffers the same connector insertion loss, coupler excess loss and splitting ratio and so the calculations would involve exactly the same steps as we have already used.
The results we would obtain are throughput loss = 643.47 μW and the tap power going to terminal 2 = 71.49 μW.
The next section would decrease the powers by the same proportions and it would result in a throughput loss of 516.2 μW and a tap power of 57.4 μW. The same proportional loss would occur at each section of the network.
The star coupler
This is an alternative to the tee coupler when a larger number of terminals is involved as shown in Figure 13.10. The star coupler takes the input signal to a central location, then splits it into
many outputs in a single coupler. Styles of up to 1 x 32 and up to 32 x 32 are commonly available.
Advantages and disadvantages
The main advantage of using star couplers is that the losses are lower than a tee coupler for networks of more than three or four terminals as in Figure 13.11. This is because the star coupler requires only one input connector and suffers only one excess loss. The larger the number of terminals, the more significant are the benefits.
This is the most popular method of manufacturing a coupler. It is, or appears to be, a very simple process.
The fibers are brought together and are then fused just like in a fusion splicer as seen in Figure 13.12. The incoming light effectively meets a thicker section of fiber and spreads out. At the far end of the fused area, the light enters into each of the outgoing fibers.
Mixing rod couplers — Figure 13.14
If several fibers are connected to a short length of large diameter fiber, called a mixing rod, the incoming light spreads out until it occupies the whole diameter of the fiber. If several fibers are connected to the far end they each receive some of the light.
A reflective coupler can be produced by putting a mirror at the end of the mixing rod. The light traveling along the mixing rod is reflected from the end mirror and all the attached fibers receive an equal share of the incoming light.
Variable coupler — Figure 13.15
This is more of an experimental or test laboratory tool than for the installation environment. It enables the splitting ratio to be adjusted to any precise value up to 19:1, which allows us to try out the options before the final type of coupler is purchased.
The design principle is very simple. A vernier adjustment allows precise positioning of the incoming fiber so that the light can be split accurately between the two output fibers to provide any required splitting ratio. This form of variable coupler is available for all plastic as well as glass fibers, singlemode and multimode.
Kevin M Contreras H