Calculating and Understanding Optical Power Budgets

When calculating optical power budgets, organizations are dependent on two statistics from manufacturers: minimum transmit power and minimum receive sensitivity. Companies calculating their optical power budget simply subtract the minimum receive sensitivity from the minimum transit power. Minimum receive sensitivity is usually a negative number, making the calculation the same as adding the absolute value of the minimum receive sensitivity to the minimum transit power.

	Min Transmit (dBm)	Min Receive (dBm)	Calculation	Power Budget (dBm)
Device 1	-2	-23	-2-(-)23 = 21	21
Device 2	-3	-23	-3-(-)23 = 20	20

However, there are a few nuances in this relatively simple equation. One is that many device manufacturers provide the average minimum receive sensitivity or average minimum transit power. As a result, the calculation will not accurately show the least amount of optical power you will need because the actual minimum value will, at times, fall below the average value provided. It is imperative that the true minimum value, not the average, is used in the calculation.

There are also a few complications that can arise from the actual configuration of the network setup. When equipment from multiple vendors is used, or different models from the same vendor, the power required to carry light between media converters can vary depending on the direction of traffic. Therefore, the minimum transit power and receive sensitivity has to be calculated as if light was flowing in both directions, as the different transit power and receive sensitivity will lead to different results. Whichever results end up being smaller should be used as the minimum optical power budget. In the case below, we would use a Power Budget of 20dBm.

Min Transmit (dBm) Device 1	Min Receive (dBm) Device 2	Calculation	Available Power (dBm)
-2	-23	-2-(-)23 = 21	21

Min Transmit (dBm) Device 2	Min Receive (dBm) Device 1	Calculation	Available Power (dBm)
-3	-23	-3-(-)23 = 20	20

This initial optical power budget, however, is only the first step. Cable attenuation, splices and connectors also contribute to power loss while light travels through the fiber optic line. To identify the true minimum optical power budget, organizations also have to evaluate the amount of light energy that could be lost due to these factors.

Cable attenuation tends to be the largest contributor to power loss, creating between .22dB and .5dB in loss per kilometer. Cable attenuation can be determined by getting the exact number off of the cable you are installing. If you don’t have the cable yet, the manufacturer should be able to provide you with a worst case number of the type of fiber you plan to install. Multiply this factor by the number of kilometers in the installation. A fiber with .3db per kilometer of loss will lose 6dB over a 20km distance.

Because fiber does not come in 20km spools, the installation will contain several splices. Splices commonly generate .1dB of loss, though companies can find the true worst-case-loss scenario from the installers. If a typical distance between splices is 6km then a 20km installation would have 4 splices and .4dB (.1x4) of loss.

Connector loss also impacts the network, with statistics typically available from manufacturers or installers. Multiply the number of connectors in the installation by the loss for each connector to get total connector loss. The TIA standard for connector loss is .75dB. In a typical installation with 6 connectors you could expect to a 4.5dB connector loss.

Subtract all of the above from your available power. If this number is negative, there is no need to continue, as there is not enough power to drive the network. If this number is positive, there are two more things to consider before declaring the network to be fit.

Available Power	Cable attenuation loss	Splice loss	Connector loss	Remaining Available Power
20	6	.4	4.5	9.1dB

The first is what happens if the fiber gets cut and I have to splice it back together? In a proper installation, an estimation of the number of anticipated repairs over the life of the fiber needs to be made and accounted for in the power budget. These repairs will add splice loss, so we must multiply the number of anticipated splices by the loss of each splice (same number we used above), and subtract this from the remaining power. The number should still be positive.

Available Power	Repair splice loss	Remaining Available Power
9.1	.4	8.7dB

A final source of power loss stems from extreme temperatures and other unforeseen factors. Excess heat or cold can create optical power loss that varies within each deployment. Organizations often have to work with manufacturers and installers to identify how much loss they can expect under certain environmental circumstances and use those statistics to create a "safety factor" estimate. To guarantee error free operation, a value between 1.7dB and 3dB, is generally used.

Available Power	Safety factor loss	Remaining Available Power
8.7	1.7 to 3	5.7 to 7 dB

When the absolute minimum values are calculated with the network complications taken into account, businesses can determine an accurate Optical Power Budget for their fiber network. This allows them to accurately identify how long they can extend a fiber-optic cable between media converters. If too much power is lost during transmission, the light signal will be disrupted, creating major performance complications. If, after following the steps described in this paper, the remaining available power budget number is positive, you can be assured that your fiber network will deliver the required performance over the life of the installation.