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How to Check the Optical Signal Strength of the SFP Optical Module?

Jan. 13, 2023

As we all know, SFP optical modules support various applications, such as Fibre Channel (FC) switches, SONET/SDH networks, Gigabit Ethernet, high-speed computer links, CWDM, and DWDM interfaces. When the SFP optical module is connected to the switch, the optical signal strength is a key parameter to ensure the normal operation of the entire connection. The SFP optical module supplier will introduce the optical signal measurement method of the SFP optical module and how to check the optical signal strength of the SFP optical module.

SFP optical module's optical signal strength and its importance

Generally, the signal strength of an SFP optical module includes Tx power and Rx power. The former represents the transmit power signal, and the latter represents the received power signal. For a general SFP optical module, the value of Tx and Rx power within a specific range means that the SFP optical module can operate normally. Take the Cisco GLC-SX-MM optical module as an example. Its transmit power range is -9.5~-3dBm, and the received power range is 0~-17dBm. If Tx or Rx is in the range of -30dBm or lower, it means that no signal is transmitted or received.

The strength of the optical signal directly determines whether the network connection is normal. If the Rx power is not strong enough, there will be no signal in the optical link. This is why long-distance optical modules or optical amplifiers are needed in long-distance transmission. If the Rx power is too large, the SFP optical module will be damaged. Therefore, choosing a high-quality SFP optical module is the basic guarantee to ensure that the network connection can work normally.

SFP Optical Module

Optical signal strength measurement of the SFP optical module

Generally speaking, there are two commonly used methods for measuring the intensity of optical signals: milliwatts (mW) and dBm. The former measures the intensity of optical signals by power, while the latter measures the intensity of optical signals in decibels. Different manufacturers may use one of them to measure the power of the optical signal. For example, Cisco switches use dBm to measure, while Brocade switches are used to measure in mW. Therefore, there is an equation between these two methods:

dBm->mW: mW=10dBm/10

mW->dBm: dBm=10*(Log mW)

How to check the optical signal strength of the SFP optical module?

To determine whether the SFP optical module (transmitter and receiver) is working at the appropriate signal level, you can view the status information of the SFP optical module. This information is very critical, including fiber type, transmit optical power range, and optical receive power.

In addition, switches such as Cisco and Brocade SAN switches will provide CLI (Command Line Interface) reference. This function is useful for viewing detailed information about SFP optical modules, including speed, serial number, part number, and receiving/sending direction optical signal strength.

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