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

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 module. The following will study the two factors in 10GBASE SR and 10GBASE LR SFP modules in details.

As for the 10GBASE SR SFP module, it is a kind of multimode fiber module that can support the distance of 300 m over OM3 multimode fiber patch cable. As for the 10GBASE LR SFP module, it is a single mode type of fiber module which enables the 10G network at lengths up to 10 km over single mode fiber patch cable. Why the former module 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 modules. 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 module, you are highly suggested to acquire these two information for better knowing the performance of the module.

We have noted before that the Tx power and Rx power affect the transmission distance of a fiber module. How does the influence happen? In fact, the Tx power and Rx power determine the optical power budget (maximum allowable loss) of a fiber module, which can have a direct influence on the transmission distance. In short, the bigger the optical power budget is, the longer the fiber module will transmit the signal. How to calculate the optical power budget of a fiber optic module? 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 modules according to the formula. For the 10GBASE SR SFP module, its optical power budget is 3.8 dBm. And the optical power budget of the 10GBASE LR SFP module is 6.2 dBm, bigger than the former one. Hence, there is no doubt that the 10GBASE LR SFP module can support a much longer transmission than the 10GBASE SR SFP module.

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

Posted by: katherinewangfs at 10:28 AM | Comments (5) | Add Comment
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