Understanding the TX/RX Optical Power on the Transceiver

Posted on April 14, 2020
July 8, 2020
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The TX and RX optical power are of vital importance to ensure the normal communication of the transceivers. But how much do you know about the TX/RX optical power? And how to test the optical power of the fiber optic transceiver? Find the answers in this article.

What Is the TX/RX Optical Power of the Transceiver?

The TX (transmit) optical power refers to the output optical power of the transceiver at the transmitting end, and the RX (receive) optical power means the input optical power at the receiving end. Both of them are important parameters that affect the signal transmission distance. Generally, only when the transmitted and received optical power are within the thresholds, can the transmission distance or quality of the modules be ensured. However, the optical modules with different wavelengths, transmission rates and transmission distances have various transmitted and received optical power. For instance, the following figure shows the TX/RX optical power of the 1G SFP modules with different transmission distances and wavelengths.

Parameter Transmission Distance Wavelength TX Optical Power RX Optical Power
1G SFP Transceiver 500m 850nm -9.5 dBm~-2.5 dBm -17 .0dBm~0 dBm
10km 1310nm -9.5 dBm~-3.0 dBm -9.5 dBm~-3.0 dBm
40km 1310nm -4.5 dBm~3.0 dBm -22.5dBm~-3.0dBm
40km 1550nm -4.0 dBm~1.0 dBm -21.0dBm~-3.0dBm
80km 1550nm -2.0 dBm~5.0 dBm -23.0dBm~-3.0dBm
100km 1550nm 0dBm~5.0 dBm -30.0dBm~-9.0dBm

How to Measure the TX/RX Optical Power of the Module?

For a transceiver with high availability and reliability, the transmitted and received optical power must be within a normal range. But how to test whether the transmitted and receive optical power of the transceiver are within the normal range?

Generally speaking, the clients can directly monitor the module’s TX/RX optical power through the DDM (digital diagnostic monitoring) function. However, when there is something wrong with the DDM information or the DDM is not supported by the transceiver, it is recommended to troubleshoot the optical transceiver first and then solve the issues. After that, you can get the TX/RX optical power of the modules. More information about troubleshooting the DDM faults and solutions, please visit How to Troubleshoot A Fiber Optic Transceiver?

Additionally, using the optical power meter is another method to measure the TX/RX optical power of transceivers. Taking the 10GBASE-LR SFP+ module as an example, the test steps are as follows:


  • First, insert the 10GBASE-LR SFP+ transceiver into the SFP+ port of the 10G switch.

  • Then, connect the optical transceiver to the optical power meter using LC-FC single mode fiber patch cables.

  • Finally, press the switch of the optical power parameter, and press the "λ" key to modulate the wavelength of the 10GBASE-LR SFP+ optical module (that is to say, to modulate the wavelength at 1310nm). When completed, the number displayed on the screen is the transmitted optical power of the modules. Also, the transmitted optical power is equivalent to the received optical power of the transceiver on the other side over short distance link.

Average Output Optical Power Detection.png

Figure 1: TX/RX Optical Power of Transceiver Tested by Optical Power Meter

Note:

1. If the transmitted optical power or received optical power is less than -30dBm (including -30dBm), it is often because the real signals haven't been sent or received by the modules.

2. If the received optical power is less than -14dBm (including -14dBm), there might be something wrong with the fiber optic patch cables, such as bad connectors and contaminated end face which results in excessive signal loss.

How to Troubleshoot the Optical Power Issues of Modules?

When there is something wrong with the optical power of fiber optic transceivers, such as the large/small optical power, unstable optical power, low side-mode suppression ratio and so on, which will affect the performance and transmission distance of the modules. So why do these undesirable phenomena occur? There must be something wrong with the hardware of the fiber optical transceivers and the issues are presented below:


  • The transmitted component TOSA (Transmitter Optical Sub Assembly) has a bad performance.

  • The PD and PIN components of the TOSA are not welded firmly.

  • The LD+ and LD- PINs of the TOSA are not welded firmly.

  • There is a bad performance with the chip of the module.

  • The components (such as capacitors, resistors, inductors, or magnetic beads) under the pins of the storage chip are missing or with bad performance.

  • The optical power does not match with the resistance.

If the above-mentioned undesirable situations occur in the optical transceivers, it is recommended that you directly contact the transceiver supplier for return or warranty.

Conclusion

In a word, the transmitted and received optical power are one of the important parameters to ensure the communication quality of fiber optic transceivers. It is very necessary to keep that the optical power is within the normal range. Together, the optical power is not the only parameter needed to consider, the extinction ratio, OMA, bit error rate, wavelength, etc. are also significant to the modules. Know more information about the transceiver tests, please refer to What Kinds of Testing Are Needed for Transceivers?