In-Depth Look at Tx Power and Rx Power of a Fiber Transceiver

ponedjeljak , 27.03.2017.

When choosing fiber transceiver, you may usually hear the description like Tx (transmit) power and Rx (receive) power for a fiber transceiver. Do you know what do these two phrases stand for? If we want to choose a proper kind of fiber transceiver, do we have high requirements on its Tx power and Rx power? In fact, these two phrases are the important parameters of the fiber transceiver which mainly determine the transmission distance of the fiber transceiver. To better know the Tx power and Rx power of a fiber transceiver, let’s take them in the 10GBASE SR SFP and 10GBASE LR SFP transceivers as reference.

Tx Power and Rx Power in 10GBASE SR and 10GBASE LR SFP Transceivers
In general, the optical Tx power is the signal level leaving from the device which should be within the transmitter power range, while the optical Rx power is the incoming signal level being receiving from the far end device that should fall within the receive power range. Both of the two kinds of parameters are very important factors that affect the transmission distance of a fiber transceiver. The following will study the two factors in 10GBASE SR and 10GBASE LR SFP transceivers in details.

As for the 10GBASE SR SFP transceiver, it is a kind of multimode fiber transceiver that can support the distance of 300 m over OM3 multimode fiber patch cable. As for the 10GBASE LR SFP transceiver, it is a single mode type of fiber transceiver which enables the 10G network at lengths up to 10 km over single mode fiber patch cable. Why the former transceiver can be only used for short distance transmission but the latter one is able to support the same signal for a much longer distance? Except the differences of fiber patch cables they always work with, it is also relevant to their Tx power and Rx power parameters.

From the figure below, we can easily learn the different Tx power and Rx power between 10GBASE SR and 10GBASE LR SFP transceivers. As for the 10GBASE SR one, Tx power is between -7.3 dBm and -1 dBm and the maximum Rx power is below -11.1 dBm. With regard to the 10GBASE-LR one, Tx power is from -8.2 to 0.5 dBm and its maximum Rx power is -14.4 dBm. Hence, when choosing 10GBASE fiber transceiver, you are highly suggested to acquire these two information for better knowing the performance of the transceiver.

Optical Power Budget for 10GBASE SR and 10GBASE LR SFP Transceivers
We have noted before that the Tx power and Rx power affect the transmission distance of a fiber transceiver. How does the influence happen? In fact, the Tx power and Rx power determine the optical power budget (maximum allowable loss) of a fiber transceiver, which can have a direct influence on the transmission distance. In short, the bigger the optical power budget is, the longer the fiber transceiver will transmit the signal. How to calculate the optical power budget of a fiber optic transceiver? Here is the formula: Optical power budget = Min Tx power – Max Rx power

Let’s calculate the optical power budget of 10GBASE SR and 10GBASE LR SFP transceivers according to the formula. For the 10GBASE SR SFP transceiver, its optical power budget is 3.8 dBm. And the optical power budget of the 10GBASE LR SFP transceiver is 6.2 dBm, bigger than the former one. Hence, there is no doubt that the 10GBASE LR SFP transceiver can support a much longer transmission than the 10GBASE SR SFP transceiver.

Conclusion
Tx power and Rx power of the fiber transceiver are two main factors which can mainly determine the transmission distance. When choosing the fiber transceiver, you can use the Min Tx power to minus the Max Rx power, and then you would get the optical power budget of the transceiver. The bigger the optical power budget is, the better the transceiver will performs.