Due to the increasing demand for bandwidth, the demand of large capacity optical communication devices become more and more urgent. Therefore, as the key part of optical communication devices, optical modules are becoming more and more popular, and the rate of them is higher and higher. 40G QSFP (Small Form-factor Pluggable Quad) optical module: four channel SPF (QSFP) optical module is designed specifically for high density, high speed and the need to be hot-pluggable. Such as a QSFP optical module, the total data transmission speed arrives at 40Gbps of all 4 channels together. It is located in the physical media related layer of the optical fiber communication Ethernet protocol and the core component of photoelectric and electro-optical conversion. The 40G QSFP+ optical module can be
adjusted to different transmission speeds, and can be applied to Ethernet, Channel Fiber and InfiniBand standards to solve the data transmission in higher efficiency.
In order to verify the performance of the 40G QSFP+ optical module, this paper introduces the test scheme and platform designed by QSFPTEK company for different test indexes of 40G QSFP+ optical module and carries out a large number of tests according to the parameter indexes of 40G QSFP+ optical module.
Test Specs of 40G QSFP+ Transceiver
The tests of optical transceivers are usually divided into transmitter test and receiver test. The test at the transmitting end usually includes average transmitting optical power, jitter, eye diagram, and extinction ratio. The test at the receiving end usually includes the bit error rate, jitter, and receiver sensitivity.
Average Transmit Optical Power
The average output power of the optical fiber module under the condition of normal optical fiber transmission is the average output power of the optical fiber module. Although the average transmitted optical power index should be larger in theory to increase the margin of transmitted optical power to reduce the impact of loss in the optical fiber on the transmission signal, in fact, it is not the bigger the better, but should be considered according to the specific transmission distance, cost, maintenance time and other aspects. In addition, in the test process, it should be noted that the input code type, the wavelength of the optical power meter, the change of working voltage and current will affect the value of average transmitted optical power.
QSFPTEK 40G QSFP+ SR4 and 40G QSFP+ LR4 transceivers fully comply with IEEE 802.3ba standard. The transmitter power of the 40GBASE-SR4 transceiver is -7~1dBm and the 40GBASE-LR4 transceiver is -7~2.3dBm.
Extinction Ratio
Theoretically, when modulating the laser, if the input is a “0” level signal, the average emitted light power of the laser should be 0, that is, no laser is generated. However, due to the inherent characteristics of the laser device, when the input is the “0” level signal, the laser will still produce a laser, so there will be corresponding average emission light power. The larger the average transmitted optical power is, the more it affects the signal level judgment, and then affects the reception sensitivity of the receiving end. The extinction ratio is a parameter used to describe this effect. Theoretically, it is necessary to improve the extinction ratio as much as possible, reduce the impact on the receiving sensitivity, and save the cost of increasing the transmitted optical power, but it is not that the larger the extinction ratio is, the better. Because the extinction ratio is too large, the bias current decreases, and the eye pattern-related jitter at the transmitter becomes larger. Therefore, it is best to keep the extinction ratio at a relatively limited value.
Eye diagram
Eye diagram refers to the figure after the superposition of various periodic random sequences is collected on the oscilloscope after the period of the oscilloscope and the transmitted periodic random pseudo sequence is adjusted to be the same. Because the final figure is very similar to human eyes, it is vividly called an “eye diagram”. Large “eye”: the gap between the eyes is large, that is, the eye diagram is relatively correct, indicating that the inter symbol crosstalk of the signal is small and the transmission characteristics of the system are good; Small “eye”: the gap between the eyes is small, that is, the eye pattern is not correct, which indicates that there is inter symbol interference between signals and the transmission characteristics of the system are not very good. Many parameters of the signal can be analyzed by using the eye diagram. For example, the receiving sensitivity can be judged by the time of the rising edge and falling edge of the eye diagram, and the jitter and noise can be judged by the thickness of the eye diagram. Therefore, an oscilloscope is often used to observe eye diagrams to analyze the advantages and disadvantages and performance of transmission systems. This method of analyzing signal quality through an eye diagram is often called an eye diagram analysis method. A typical eye diagram is shown in the figure below, which is generated by the superposition of various combinations between various random signals “0” and “1”.
Eye Pattern Formation
Receiving sensitivity
Receiving sensitivity is the key factor to measure the quality of an optical module. Reception sensitivity refers to the minimum received optical power Pmin that the receiving end can receive a modulated signal without bit error or limit to a certain bit error rate. Its unit is DBM. Firstly, the bit error meter sends a pseudo-random signal of a certain code type. The signal is connected to the electrical interface of the optical module through the high-speed line. Through the electro-optical conversion of the optical module, the light generated at the transmitting end is transmitted back to the receiving end through the optical fiber, and then transmitted back to the bit error meter through the high-speed line to monitor the bit error rate. At the same time, an adjustable optical attenuator will be added at the output end of the transmitting end to adjust the transmitting optical power, so that the transmitting optical power will continue to decrease until it reaches the critical point of bit error, but ensure that the bit error rate is within the limited bit error rate, and then measure the attenuated transmitting optical power value through the optical power meter, which is the receiving sensitivity.
QSFPTEK 40G QSFP+ System Test Platform
According to the five basic test indexes of average transmitted optical power, optical eye diagram, electric eye diagram, sensitivity, and bit error rate, the construction of the test system is mainly divided into two parts. One part detects the transmitted optical power, sensitivity, and bit error rate, and the other part detects the optical eye diagram and electric eye diagram. Generally speaking, the 40G QSFP+ optical module produced in the industry needs to be fully inspected for the three indexes of transmitted optical power, sensitivity and bit error rate, while the optical eye diagram and electric eye diagram only need to spot inspect the 40G QSFP+ optical module after the three indexes of transmitted optical power, sensitivity and bit error rate are fully inspected, so it is called detecting transmitted optical power The test part of sensitivity and bit error rate is the full inspection test system. The testing part of detecting optical eye diagrams and electric eye diagram is called sampling testing system.
40G QSFP+ Full Inspection Test System
The equipment required for the 40G QSFP+ full inspection test system includes 4×10G bit error meter, 16PCS 25G high-frequency lines, 1PC AOC high-speed test board, 1PC MPO-FC jumper, 4PCS optical branches, 4PCS handheld adjustable optical power meters, and 4PCS adjustable optical attenuators. The full inspection test system is shown in the figure below.
40G QSFP+ Sampling Test System
The equipment required for 40G QSFP+ sampling test system includes 4PCS 10G bit error meter, 16PCS 25G high-frequency lines, one AOC high-speed test board, one MPO-FC jumper, 4PCS optical branches, one high-speed electric switch, one optical switch, and one 12.5G oscilloscope with optical board card and electric board card. The sampling test system is shown in the figure below.
Conclusion
This paper introduces the main test indexes of the 40G QSFP+ optical module and QSFPTEK 40G QSFP+ optical module test system in detail. It can be learned that QSFPTEK has a rigorous optical module test process and platform, and is able to provide 40G QSFP+ series optical modules with high quality and high reliability.
Lucy Adam