Reference Guide to Optical Transceiver Output Power Testing - NADDOD Blog

Reference Guide to Optical Transceiver Output Power Testing

NADDOD Jason Data Center Architect Jan 9, 2023

Good quality optical transceivers are generally subjected to strict hardware and optical tests before shipment. The optical test mainly detects the compatibility of the optical transceiver, while the hardware test is mainly a parameter test, which contains the transmitting optical power, receiving sensitivity, operating temperature, bias current, etc.

The Importance of Optical Transceiver Output Power

Emitted optical power (Output Power) refers to the average output optical power of the light source at the transmitting end of the optical transceiver, also called output optical power. Unit: W or mW or dBm, unit conversion formula: P (dBm) = 10Log (P / 1mW).

Optical transceiver transmitting optical power is an important parameter affecting the transmission distance of the optical transceiver, when the transmitting optical power is too small, the receiving optical power at the receiving end of the transceiver will be less than the receiving sensitivity of the transceiver at this time, the transceiver can not normally receive the signal light; when the transmitting optical power is too large, although the receiving end can be increased by increasing the attenuator to achieve the receiving optical power range at the receiving end of the transceiver, but the required bias current will also be large, which will affect the signal This will affect the quality of the signal transmission and the lifetime of the transceiver.

You can check the transmitting optical power of the optical transceiver in the following four ways to determine whether it is in a normal state.

The Method of Detecting Output Power Emitted by The Optical Transceiver

1. Switch read DDM information

Digital diagnostic function (DDM) is like a “doctor”, can diagnose the “condition” of the optical transceiver, it can diagnose the working status of the optical transceiver, but also real-time monitoring of the internal parameters of the optical transceiver is normal. Through a comprehensive analysis of the detected transmit and receive optical power, voltage and bias current alarm information, it is possible to predict faults, locate the location of faults, shorten the repair time and reduce losses.

The commands for viewing DDM information on the switch vary and can be operated according to the specific model you are using.

DDM Information
The transmit optical power of the SFP optical transceiver in the above figure is -3.55 dBm, which is between the warning range of 1 dBm and -13.5 dBm, with normal data.

2.Eye chart meter test

The optical power emitted by the optical transceiver can be represented by the optical eye diagram information at the transmitting end. The following diagram shows the eye diagram test process of the 100G QSFP28 optical transceiver.

Eye char meter test
(1) The electrical signal provided by the BER is converted into an optical signal through the test board and transmitted to the 100G QSFP28 optical transceiver, which is connected to the optical oscilloscope through a fiber optic patch cable.

(2) The Trigger end of the BER connects the synchronous clock signal to the oscilloscope to achieve signal synchronisation and form an eye diagram on the optical oscilloscope.

(3) The optical oscilloscope needs to select the filter rate and central wavelength corresponding to the 100G QSFP28 optical transceiver, select a suitable eye diagram template to match the formed eye diagram, and the generated eye diagram information will be (emitted optical power, extinction ratio, eye diagram rise and fall time and eye diagram crossover point, etc.) sent to the computer.

In order to keep the test 100G QSFP28 optical transceiver emitted optical power within the normal emitted optical power range, the bias current can be adjusted so that the optical power is within the appropriate range.

3.Spectrometer test

(1) Select the FC-LC/SC connector type according to the optical transceiver type.

(2) Connect the FC connector end of the FC-LC simplex patch cable to the “Optical Input” interface of the spectrum analyser.

(3) Connect the LC connector end of the FC-LC simplex patch cable to the TX connector of the optical transceiver.

(4) Press the “Auto” button on the spectrum analyser; the core parameters (including the transmit optical power) will be read automatically, wait until the image display area of the spectrum analyser is fully displayed, and then proceed to read the central wavelength value and the side mode rejection ratio (SMSR) of the transceiver. The following diagram shows the spectrometer test of a 40G ER4 optical transceiver with an emitted optical power of 2.8dBm.

(5) After the information reading is completed, pull out the jumper, place the transceiver pull ring parallel to the transceiver port and use the transceiver pull ring to gently pull out the transceiver.

4.Optical power test

(1) Confirm the type of optical fiber, interface type and central wavelength used with the optical transceiver.

(2) Confirm the type of optical power meter interface.

(3) according to the optical transceiver, optical power meter interface type and optical transceiver with the use of fiber type to select the corresponding jumper.

(4) power up the switch equipment and ensure that the switch interface is normal and in admin up state.

(5) turn on the optical power meter and adjust the detection wavelength to the same wavelength as the centre of the optical transceiver.

(6) Insert the optical transceiver to be tested into the switch interface, and connect the optical transceiver transmitting port with the optical power meter adapter interface with a jumper.

(7) Read the data on the display of the optical power meter, that is, the optical power test results; you can switch the display unit through the dbm button.

If you detect the presence of abnormal transmitting optical power, you can investigate the specific causes, the following describes several causes of abnormal transmitting optical power and solutions for your reference.

Three, caused by the optical transceiver emitted light power is not normal reasons

1.the optical transceiver itself failure, the light-emitting power is not qualified, try to replace the optical transceiver.

2.optical transceiver core end face dirty or test fiber end face dirty, try to clean the optical transceiver end face and fiber end face.

3.there are defects at the connection point, such as flange head, fusion fiber or equipment interface problems, you can try to step-by-step troubleshooting.

Conclusion

The hardware parameters of the optical transceiver test is very important, not only to detect the optical transceiver’s transmitting optical power, but also to detect whether the other core parameters are normal, the regular supplier can provide the relevant test report of the optical transceiver, you can ask to see the relevant test results if you have doubts about its quality when purchasing the optical transceiver.