With the popularity of fiber optical networks and the increasing development of optical communication technology, the requirements for the quality of optical modules are becoming more and more strict. Generally, optical modules will undergo rigorous testing to ensure quality and performance before shipment. So, what kinds of testing are needed for transceivers? Finding the answers in this article.
Incoming Quality Control (IQC) and surface mounted component inspection are significant to fiber optic transceivers before they are assembled. The IQC is the process to control the quality of fiber optic materials and parts for manufacturing a product before production begins. In terms of the fiber optic transceivers manufacturing field, the suppliers must test the optical emitting module (TOSA), optical receiving module (ROSA), and optical transmitting and receiving module (BOSA) to ensure the quality and performance of transceivers. As for the inspection of surface mounted component, it is mainly used for checking whether the Printed Circuit Board (PCB) is correct and whether there is pollution, so as to ensure the performance of the fiber optic transceivers.
After the assembly of the optical module is completed, a number of parameter tests are needed to test the signals at the transmitting end (TX) and receiving ends (RX). Only when the parameters like average output optical power, extinction ratio, optical modulation amplitude (OMA), bit error rate (BER) tests, etc. are compliant to the MSA standard, the performance and the quality of modules can be justified.
The average output optical power is an important parameter for transceivers which directly affects the communication quality of the module. It is the average optical power of transceivers under normal operating conditions. The average output optical power measurement can be achieved by the optical power meter to test the optical power of the transmitting end. For transceivers used for long-distance transmission, the average optical power is greater than the maximum input optical power.
Figure 1: Average Output Optical Power Detection
Extinction ratio refers to the ratio of the optical power at the high level and low level of the laser output. The test detects whether the laser is operating within the optimal bias point and optimal modulation efficiency.
The relative amplitude of the level “1” and level “0” of the optical signal can be detected in the extinction ratio measurement. The larger the extinction ratio and the relative amplitude, the stronger optical signal can be, and the higher the receiving sensitivity. Additionally, the extinction ratio is inversely proportional to the optical power. The larger the extinction ratio, the smaller the emitted optical power.
Figure 2: Extinction Ratio Measurement
The optical modulation amplitude (OMA) is used for measuring the difference between two optical power levels generated by the power source, let's say, P1 (when the light source is on) and P0 (when the light source is off). With OMA，it is possible to use a low or high extinction ratio, provided that eye safety is okay at the transmitter and does not overload the receiver.
Figure 3: OMA in a stressed eye diagram of an optical signal.
The bit error rate (BER) is one of the parameters in characterizing the performance of data channels. It is used to measure the number of bit errors that occur in a given number of bit transmissions and it is an indication of how often a packet or other data unit has to be retransmitted because of an error. If the BER is higher than expected, it means that a slower data rate will occur and the transmission time will be extended. In order to keep the stability and reliability of the transmission, the fiber optic transceiver must go through the BER test to keep the quality.
Figure 4: Bit Error Rate Test
The receiving sensitivity is defined as the signal optical power required at the receiver to achieve the targeted bit error rate. Typically, the RF power level received by an antenna on the ground will be between -125 dBm and -150 dBm depending on environmental factors. Generally speaking, the better the receiving sensitivity, the smaller the minimum received optical power (verse versa). If the receiving sensitivity is poor, the requirements on the receiving device of the optical module are higher.
In order to ensure the signal quality of the transceiver, the eye pattern test is very essential. The eye pattern is formed by superimposing and accumulating all captured waveforms according to the three bits of the oscilloscope's persistence function. The digital signal quality of the optical module can be seen from the eye pattern test results. The performance of the optical module is judged by carefully observing the eye height, eye width, jitter, and duty cycle of the eye diagram. The larger the eye, the smaller the crosstalk between codes, which means the optical transceiver will have better performance.
In addition, the multi-source agreement (MSA) clearly specifies the standard eye pattern of the transceiver (the purple part of the following figure). Only when the eye pattern of the optical transceiver is located in the grey part of the following part, the transceiver is qualified for the MSA standard. If the fiber optic transceiver can’t pass the eye pattern test, the additional calibration must be performed to improve the performance of transceivers.
Figure 5: Eye Pattern Test
When connecting two devices, fiber optic transceivers at both ends should work on the same wavelength. So the manufacturers must test the wavelengths of the transceiver to ensure that the wavelengths within the normal range. Generally, the spectrum analyzer is applied for measuring the center wavelength of the transceiver. Usually, it is normal to have a deviation for the center wavelength of the transceiver, and the deviation is recognized within a certain range. For example, the central wavelength of the SFP-10G-LR optical module is 1310nm with a deviation of ±50nm, and the central wavelength of the SFP-10G-SR optical module is 850nm with a deviation of ±10nm. If the tested result is inconsistent with the standard specifications, the optical module is considered a defective product.
Figure 6: Wavelength Testing
Manufacturers generally use optical aging boxes to simulate the extreme working conditions of the optical module and check whether the performance of the module is compliant with the standard. After the aging test is completed, the transmitter and receiver need to be tested to check whether the optical power, extinction ratio, and sensitivity parameters meet the requirements.
It is mainly used for testing the compatibility of the fiber optic modules. The optical transceiver is inserted into various corresponding brand switches to test whether the module can work well on the switches. The compatibility test can help you avoid pitfalls, and assist you to ensure and verify that all those third-party components meet the system-level requirements.
After passing through each test, the module must be inspected on the microscope to check if there are dirt and scratches. The transceiver is easily contaminated when the module is plugged into or removed from the network devices frequently. Therefore, the end-face inspection is needed before shipment to ensure that the module is clean.
From what we have presented above, a qualified fiber optic transceiver must be tested vigorously to ensure high-availability and reliability. The fiber optic manufacturers must have comprehensive optical modules standardized production lines and comprehensive quality test systems to keep the quality of the optical modules. For more information on how to choose reliable optical module suppliers, please visit How to Choose A Reliable Optical Transceiver Supplier?