What is the Tx and Rx Power of an SFP Optical Transceiver?

Transmitted optical power and received optical power are important parameters that affect the transmission distance of optical fiber links. When designing a new optical system, it is necessary to calculate the power budget of the transceiver in advance. Receiver sensitivity is also an influencing factor. Do you know what these two mean? This article will show you how to calculate and what they mean by examples.

What are SFP Tx and Rx Power?

Two important factors affect the performance of SFP modules: Tx power (Output power) and Rx Power (receiver sensitivity). We called the optical Tx power the signal level that leaves the optical equipment and the Tx power should be within the transmitter power range. The Rx power is the incoming signal level received from the far end device, and its numerical value is within the received power range. We will introduce how to calculate the optical power value of the 10GBASE-SR SFP+ and 10GBASE-LR SFP+ modules. Single-mode optical modules are used for long-distance optical fiber transmission over 2km, while multi-mode modules are usually used for short-distance data transmission under 2km.

The 10GBASE-SR multimode SFP+ module supports a maximum transmission distance of up to 300m over OM3 multimode fiber patch cable. And the 10GBASE-LR SFP Single Mode module can support a distance of 10km over single-mode fiber patch cords. Before designing a fiber optic link, we should read the product parameters carefully, as we can see the Cisco compatible 10GBASE-SR SFP+ Tx power is from 6.5dB to 0.5dB. The Rx power is under -11.1dB. And the receiver sensitivity of the Cisco compatible 10GBASE-LR SFP transceiver is under -12.6dB and the Tx power range is between -6.5 to 0.5dB

Figure 1: Cisco compatible 10GBASE-SR SFP+ parameter

Figure 2: Cisco compatible 10GBASE-LR SFP+ parameter

How to Know the Optical Power Budget by Tx and Rx Power?

Before we design a fiber optic link, we need to know the optical power budget (maximum allowable loss) to calculate the specific distance of the optical transceiver.

Optical power budget = Tx power – Rx power 

We can calculate the SR and LR modules’ optical power budget according to the aforementioned parameters and this equation.

Optical Power Budget of 10GBASE-SR SFP+ Module = Min Tx Power (-6.5dB) - Max Rx Power (-11.1 dB) 

Optical power budget (maximum allowable loss) = (-6.5 dB) – (-11.1 dB)=4.6 dB

Optical Power Budget of 10GBASE-LR SFP+ Module = Min Tx Power (-6.5dB) - Max Rx Power (-12.6 dB) 

Optical power budget (maximum allowable loss) = (-6.5 dB) – (-12.6 dB)=6.1 dB

How to Calculate the Transmit Distance by Tx and Rx Power?

In addition to the optical power budget, other factors that need to consider, including some optical fiber signal strength, or optical fiber connection components may take the attenuation, the fusion splicing points, and attenuation that we can't unpredictable may be caused by optical fiber patch cords bending (the attenuation is usually about 3 dB). The loss of each connector is 0.6 dB, and each fusion point's loss is 0.1 dB. If we use a 10GBASE-LR SFP+ module to design a fiber optic link that includes 2 connectors and 4 splicing points. And run over the LC Singlemode 1310nm wavelength fiber optic patch cord with a loss of 0.35dB per kilometer.

Figure 3: Optic Fiber Link

Then we can get the worst optical loss = The power budget - Total optical power loss = 6.1dB - 2×0.6 dB - 0.1×4dB - 3dB (reserved safety factor) = 1.5dB. The worst transmission distance = {Worst case optical power budget, in dB} / [Cable Loss, in dB/km]=1.5dB/0.35dB/km=4.28km. Through calculation, we conclude that the maximum transmission distance supported by this LR module in the designed optical fiber link is 4.28km. 

Meanwhile, through this series of calculations, we can know that the fiber signal strength is affected by the optical fiber budget and optic fiber cable loss; the more the optical power budget, the further the fiber transceiver can support. In the last part, we calculated the optical fiber budget of 10GBASE-LR and 10GBASE-SR modules. We can see that the optical power budget of 10GBASE-LR is much more than that of 10GBASE-SR. In addition, in terms of cable loss, the optical fiber loss of multi-mode optical cable is less than that of single-mode optical cable. Therefore, we can infer that the 10GBASE-LR transceiver supports a bigger maximum transmission distance than the 10GBASE-SR.

We need to note that we can only calculate the approximate optic power budget after these calculations, before designing the optical fiber link. What we need to know is that these are only theoretical data for reference. Engineers should pay attention to the accurate value obtained from the test on the switch and the cable loss value should also be tested to obtain the accurate number.

Conclusion

After introducing what the optical fiber budget and calculating the real case, I believe you already know what the SFP Tx and Rx power of an SFP module and how to correctly calculate the budget by Tx and Rx power in an optical fiber link and calculate the maximum fiber signal strength. If you have any questions about how to calculate the distance by Tx and Rx power range, you can contact customer service via [email protected]. QSFPTEK provides various optical transceiver modules. Check QSFPTEK 10GBE SFP+ transceiver module compatible with Cisco SFP-10G-LR specs(SFP+, 10GBASE-LR, 1310nm 10km DOM). Best SFP module price! 100% tested. Order now!