Optical modules can realize end-to-end signal transmission, and it performs optical communication through optical fibers. The manufacturing technology of optical modules is also constantly improving, and the manufacturing process has become faster and less error-prone over time. The optical transceiver manufacturing process goes through the same rigorous testing and quality-checking procedures as other high-tech appliances. These procedures test the individual performance of the optical transceiver to ensure that every optical module sold gets the best performance possible. Every module of QSFPTEK has undergone rigorous testing, if it has some problem, it will go back to the production line for modulation, if there is a serious failure, then this transceiver will even be dismantled and disassembled.
Calibration – Tx, Rx, Eye-diagram, Voltage measurements
Tuning of the transmitter and receiver, eye-diagram, and voltage-level setting are the key steps in the optical transceiver fabrication process, by which the optimal operating parameters of the module are set to meet the requirements of quality and MSA standards. Figure.1 shows the transmitter power, receiver sensitivity, transmitter eye diagram, voltage, and temperature calibration and commissioning process. In this process, the optical module is called the Device Under Test (DUT), which is attached to a test board with a specific electrical interface for a certain form factor of transceiver SFP, XFP, QSFP, etc. The transmitter of the DUT is connected to a de-multiplexing assembly, which classifies the optical wavelength signal from the QSFP LR4 optical transceiver of the DUT with the help of an optical prism (the transceiver is using four CWDM lines in 1270, 1290, and 1310 and 1330 nm). If the DUT is an SFP/SFP+ transceiver with a single output wavelength, the DEMAX unit with an optical path control switch is not used.
The optical path control switch is a device enabling the selection of incoming wavelengths from the input port and forwarding it to the output port. This device utilizes the optical switching principle to achieve low insertion loss. The signal is then split so that part of it is sent to an oscilloscope for drawing an eye diagram and the other part is sent to a power meter unit, which measures it and transfers it to the receive port of the DUT transceiver.
The power meter is used to measure the transmit power of the DUT and to control it in the required range. The transceiver voltage measurement is performed directly on the test board and the results are displayed on the controller PC.
The temperature test unit is positioned parallel to any other measurement unit and is responsible for performing and monitoring the DUT temperature variation. This unit is responsible for heating the DUT to the maximum operating temperature and for the measurement of Tx, Rx operation, and eye diagram at that temperature. Similarly, the temperature test unit will also cool down the transceiver to the lowest operating temperature and perform the same measurements. These test procedures allow testing the operation of the DUT at extreme temperatures and recalibrating the part when the module does not pass this test.
The electrical signal portion of the test board is linked to a bit error rate tester. This tester produces a random signal pattern and then sends it through the DUT and analyzes the eye diagram with an oscilloscope.
The measurement and adjustment of the eye mask is an important stage of the transceiver path, through which the process is performed to ensure the best signal quality of the optical module, in line with the MSA standard requirements. The definition of the eye mask illustrates the output performance of the transmitter in terms of normalized amplitude and time, thus ensuring that the far-end receiver can consistently discern the difference between 1 and 0 levels in the presence of timing noise and jitter. Eye-diagram measurements provide an indication of the quality of the digital signal but do not indicate protocol or logic problems. The quality of the digital signal is easily seen by the eye diagram. The bit error rate (BER) decreases as the eye mask is turned off. A yellow arrow in Figure.2 denotes the open eye.
The MSA standard defines a precise eye diagram mask for optical modules (the gray rhomb located under the yellow arrow in the figure), and this part should not be crossed by the respective 0 and 1 signals (blue lines), as well as their translation. When the test signal line crosses the eye mask, this transceiver is not qualified and must be re-calibrated additionally. The more open the eye is the less probability that the receiver will mistake a logical 1 bit for a logical 0 bit, and vice versa.
Wavelength and Spectrum Test
All optical modules must emit precise wavelengths in order to successfully communicate with the corresponding transceiver. For example, 10GBASE-SR optical modules use 850nm wavelength with a possible deviation of +/- 10nm, while some optical modules, such as the DWDM SFP transceiver, use 1560.61nm wavelength, their deviation is only +/- 0.8nm, because the DWDM SFP transceiver has more precise laser output wavelength, for the module transmits signals without crosstalk with other side channels.
The spectrometer is used to measure the optical wavelength's accuracy. The transceiver is usually plugged into a factory environment with a specially powered PCB as a power supply, or a network switch device as used in Figure.3. Figure.3 shows an SFP LR optical module. the spectrometer shows the wavelength on the X-axis and the power on the Y-axis. We can see that the sharp point where the power peak is formed is at 1310.56nm (-3.16nm), and the difference between this wavelength and the 1310nm specified by MSA is within the operating error, so the module passes the spectral test.
Figure.3: Wavelength and Spectrum Test
In the optical module manufacturing process, after all the aforementioned processes are completed, the optical module lens needs to be checked for dirt or scratches, and when there is dirt, it needs to be cleaned because the optical module is connected to each test equipment, these procedures are prone to dust contamination of the optical module lens. This process is carefully examined using a microscope. Figure.4 shows the microscope test output screen.
Figure.4: Microscope Test Output Screen
In case there are no scratches and dust on the lens core and its cladding, then it means that this optical module passes the test. Otherwise, a cleaning procedure will be performed. After the cleaning procedure, dust, oil, and other foreign matter will be removed. After the cleaning procedure, a microscopic inspection is performed again to ensure the cleaning effect. If there are scratches in the cross-section, the module will undoubtedly fail the test and will be rejected and dismantled.
In the manufacturing process of optical modules, the test procedure cannot be ignored. After the key components of each device are soldered, they can be carefully calibrated to determine the future service life of the transceiver and its performance at work. If the recalibration phase cannot be accurately calibrated, then you are better off discarding the device. QSFPTEK has complete test equipment and a strict test process. Each optical module will go through a complete test process before it is delivered to customers, so as to avoid users suffering from various problems in use.