What is the SFP Tx power and Rx sensitivity of an SFP module?

Optical modules have several essential parameters. They are transmit power, receiver sensitivity, receiver overload, power consumption, and operating temperature. The parameters of the first two items determine the operating performance of the module and the maximum distance for transmitting optical signals under normal BER. This article will introduce you to the meaning of tx power and rxsensitivity, and how these two parameters affect the transmission distance of optical modules.

What are SFP Tx and Rx Power?

The TX power represents the intensity of the optical signal sent by the optical module. The RX receiving sensitivity represents the lowest optical signal intensity that the optical module can receive at the normal allowable bit error rate. Once the optical signal falls below the receiving sensitivity, the bit error rate increases sharply. In addition to receive sensitivity, another important parameter regarding received optical power is receive overload. Once the received optical power is higher than the overload, the receiver's device will be burned by the optical signal, causing the optical module to fail to work. Bit error rate represents the percentage of transmitted erroneous data and is the most important parameter for evaluating the quality of signal transmission. 

The transmission distance of the optical module is mainly determined by the luminous power and the receiving sensitivity. In addition, the dispersion tolerance also needs to be considered, but then we ignore the dispersion first, and mainly consider the influence of the optical power on the distance. I take 100GBASE-SR4 QSFP28 and 100GBASE-LR4 QSFP28 modules as an example, we will take both parameters. QSFP-100G-SR4 optical module is used for short-distance multi-mode fiber transmission, and is often used with OM3/OM4 fiber. The QSFP-100G-LR4 module is often used for long-distance single-mode fiber transmission and is used with OS2 fiber.

The 100GBSE-LR4 module supports transmission via G.652 OS2 with a maximum distance of 10km. We can see from the parameter table that the Cisco compatible QSFP-100G-LR4 TX power is from -7~-2.3dBm, and its receiving sensitivity is -11.5dBm. Therefore, we can conclude that the power budget of this optical module is a minimum of 4.5dBm, minus the insertion loss of the connectors at both ends of 0.5dB. According to the normal power attenuation of single-mode fiber at 1310nm 0.4dB/km, the distance that can be transmitted is about 10km.

Figure 1: Cisco compatible 100GBASE-LR4 QSFP28 parameters

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

When choosing an optical module in the transmission scheme, we need to choose according to the power budget of the optical module. The optical power budget represents the maximum optical power loss that the optical module can endure during transmission. Calculated as follows

Optical power budget（dB） = Mini Tx power(dBm) – Rx sensitivity(dBm) 

We calculate according to the parameters of the QSFP-100G-LR4 module, and we can get the following results

Optical Power Budget of 100GBASE-LR4 Module = Min Tx Power (-7dB) - Max Rx Power (-11.5 dB) 

Optical power budget (maximum allowable loss) = (-6.5 dB) – (-11.1 dB)=4.5 dB

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

When we calculate the transmission distance, in addition to the power budget of the optical module, we also need to consider the additional loss that may be generated by the optical fiber connector and the optical fiber fusion joint. Even bending of the fiber has the potential to cause additional power loss. Generally speaking, we will use 0.25 or 0.3dB attenuation for each fiber optic connector, and 0.1dB loss at the splice point. For the actual link loss, if it is a long-distance link or the link condition is relatively bad, it is recommended to use an OTDR device to perform the actual loss test and then purchase a suitable optical module.

Figure 2: QSFP28–LR4 transceiver links

Conclusion

If you have questions about calculating the distance by optic transceiver Tx power and Rx sensitivity range, you can contact customer service via sales@ecloudlight.com. Ecloudlight provides various optic modules at cost effective prices.