The best way to Measure the Output Power of Transceiver Module

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The
best way 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.

 

The
Output Power of Transceiver Module

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.

 

Four
Ways to Measure the Output Power of Transceiver Module

1.
DDM Information Reading Through Switch

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.

 

2.
Eye Diagram Measurement

The
output optical power can also be viewed from the eye diagram. Below is the eye
diagram testing of a 100G QSFP28 transceiver.

 

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.

 

3.
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.

 

4.
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.

 

Causes
of Abnormal Output Power and the Countermeasures

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.

 

Conclusion

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.