What testing procedures should high-quality optical modules go through?

The optical modules will undergo strict testing and quality inspection procedures before shipment, such as incoming material inspection, parameter inspection, aging test, real machine test, end face inspection, etc. The results of all test items must reach the standard level, otherwise the optical modules will return to the production line for readjustment. Understanding these tests will help us to distinguish the quality of optical modules. This paper will reveal the important tests that high-quality optical modules must pass, and the impact of these test results on the quality of optical modules.

1. Incoming inspection and patch detection

Incoming inspection and patch inspection will be carried out before assembling the optical module. Among them, incoming inspection refers to the inspection of incoming components by the manufacturer before assembling the optical module, such as the inspection of whether the optical emission module (TOSA), optical receiving module (ROSA) and optical emission and receiving module (BOSA) are qualified, so as to ensure the quality of the optical module, reduce the rework rate and defective rate, and avoid the damage of expensive optical components. Patch detection is mainly used to detect whether the PCB patch is correct and whether there is pollution, so as to ensure the performance of the optical module.

2. Optical power detection

The average output optical power is an important parameter of the optical module, which directly affects the communication quality.

3. Extinction ratio and optical modulation amplitude (OMA) test

Extinction ratio is one of the parameters used to measure the quality of the optical module. Extinction ratio refers to the ratio of the high level (i.e. all "1" codes) and low level (i.e. all "0" codes) optical power output by the laser. Through testing, it can be detected whether the laser operates within the optimal bias point and the optimal modulation efficiency range. In addition, the optical modulation amplitude (OMA) is also a measure of the power difference when the laser is turned on and off.

Both are tested by mainstream optical oscilloscope. At the same time, the relative amplitude of the "1" level and the "0" level of the optical signal can be seen in the extinction ratio detection. The greater the extinction ratio, the greater the relative amplitude, the stronger the optical signal can be received and identified, and the higher the reception sensitivity. At the same time, the extinction ratio is inversely proportional to the light power. During the test, it can be found that the greater the extinction ratio, the smaller the transmitted light power.

4. Bit error rate and receiving sensitivity test

Bit error rate is one of the parameters to measure the ability of optical modules to transmit symbols correctly. Bit error rate refers to the ratio of the number of bit error symbols received after photoelectric conversion at the receiving end within a specified time to the number of symbols given by the output end of the bit error tester. The bit error rate test needs to receive the optical signal with pseudo-random signal output by the tested optical module through the standard receiver tube test unit. At the same time, the standard receiver tube test unit is used for demodulation and comparison to complete the bit error rate test.

5. Eye chart test

Eye chart testing and adjustment is an important stage to ensure that the optical module can obtain the best signal. The so-called eye diagram is formed by superimposing and accumulating all the captured waveforms according to every three bits through the afterglow function of the oscilloscope. The digital signal quality of the optical module can be seen from the eye diagram test results. The performance of the optical module can be judged by carefully observing the eye height, eye width, jitter, duty cycle, etc. of the eye diagram. The larger the eye, the smaller the intersymbol interference, and the better the performance of the optical module.