Before shipment of the fiber optic transceiver, several tests are necessary to ensure transceiver can provide reliable operation, including the testing on the parameters of the transceiver module, such as output power, receiver sensitivity, working temperature, bias current, etc,. The output power, for instance, is a critical parameter to ensure the normal working of fiber optic transceivers when connected to the network switch. Here introduces the definition of the transceiver output power and the ways to measure it.
Output power refers to the average optical output power of the transmitter in the transceiver module. The unit of output power is W, mW or dbm.
The output power of transmitter is an important parameter of transceiver, which can affect the transmission distance. When the output power is too small, the input power at the receiving end of the module will be less than the receiving sensitivity of the module, so the module cannot normally receive signal light. When the output power is too large, the attenuator can be added to the receiving end according to the range of input optical power, but the bias current required will become large accordingly, which will affect the signal quality and the service life of the module.
Transceivers with different wavelengths, transmission speeds and distances may differ in the output power. There are four ways to judge whether the output power of the transceiver is within the normal range.
DDM is short for Digital Diagnostic Monitoring, which provides users with critical information concerning the status of the transmitted and received signals. Through real-time monitoring the module internal operating voltage, temperature, receiver power, transmitter bias current, and transmitter power, this approach allows for better fault isolation and error detection.
*The commands to show the DDM information of transceiver optics may vary among different switches. Refer to the specification before operation.
Figure 1. Specific information of an SFP transceiver plugged into the Cisco switch
As shown in Figure 1, the Tx power (output power) of the SFP module is -3.55 dBm, indicating normal as it falls within the alarm range: 1 dBm~-13.5 dBm.
The output optical power can also be viewed from the eye diagram. Below is the eye diagram testing of a 100G QSFP28 transceiver.
Figure 2. Eye Diagram Measurement Process
1.The test board converts electric signal from the error detector into optical signal, then send it into the transceiver. And finally that signal will flow along the fiber cable into the oscilloscope;
2. The error detector sends the synchronous clock signal into the oscilloscope on which the eye diagram is produced;
3. The optical oscilloscope shall set the same filter speed and central wavelength as the optical module. The eye diagram information generated (output optical power, extinction ratio, etc.) will be delivered to the computer.
Bias current can be adjusted to ensure the 100G QSFP28 transceiver being tested maintains the normal output power.
Figure 3. QSFP-40G-ER4 tested on an optical spectrum analyzer
1. Choose from FC-LC and FC-SC connector according to the fiber optic transceiver;
2. Connect the FC end of the simplex fiber cable to the “Optical Input” interface on the optical spectrum analyzer;
3. Connect the LC end of the simplex fiber cable to the Tx interface of the transceiver;
4. Press the “Auto” button to make the analyzer read the core parameters (including output optical power), then read the central wavelength and the Side-Mode Suppression Ratio (SMSR); Figure 3 shows that the output power of the tested transceiver is 2.8dBm.
5. Unplug the cable and the fiber transceiver when finish reading.
Figure 4. Optical power measurement with optical power meter
1. Confirm the fiber type, connector, central wavelength of the transceiver;
2. Select the congruent connector for the optical power meter;
3. Select the well-matched fiber optic cable;
4. Power on the network switch, and keep the switch port in normal state of admin up;
5. Alter the testing wavelength of power meter in line with the central wavelength of the transceiver;
6. Insert the fiber optic transceiver into the port of the switch, then connect the transceiver with the adapter of power meter through the fiber optic cable;
7. Read the result on the screen. Press the “dBm/w” button to change display unit.
If the output power is unusual, find out the specific reasons. Here lists some causes and countermeasures for the abnormal output power.
1. The optical transceiver itself breaks, then replace it;
2. The end face of the fiber cable or the optic interface of the transceiver gets contaminated, then clean with a fiber optic cleaner;
3. The connection fails because of the poor contact, then troubleshoot step by step.
The parameter test of fiber optic transceiver is of great importance, which can check the output power of the transceiver, as well as the other crucial parameters. The output power of the transceiver module can be acquired through different ways such as DDM Information reading, eye diagram measurement, optical spectrum analyzer and optical power meter.