Today, many users apply optical network components from different suppliers. Thus, we need to test if the optical transceivers are compatible and interoperatable with other 100base-fx sfp transceiver components. Otherwise, components are possible to be broken. Meanwhile, the entire network can’t operate well.
As we know, a fiber optical transceiver has a transmitter and a receiver. The transceiver transmits data trough a fiber from transmitter to receiver. But the system doesn’t work and doesn’t get your desired bit-error-ratio. What’s wrong? Is there anything wrong with the transmitter? Or is the receiver at fault? Maybe both are faulty. A low-quality transmitter can compensate for by a low-quality receiver (and vice versa). So specifications should guarantee that any receiver can interoperate with a worst-case transmitter, and any transmitter will provide a signal with sufficient quality so that it will interoperate with a worst-case receiver.
But it’s difficult to define the worst case. The minimum power to achieve the system target will give minimum output power to the transmitter. If the receiver can only tolerate a certain level of jitter, this will be used to define the maximum acceptable jitter from the transmitter. Generally, to test an optical transceiver, there are four steps, including the transmitter testing and receiver testing.
Transmitter parameters may include wavelength and shape of the output waveform while the receiver may specify tolerance to jitter and bandwidth. The following are the steps to test a transmitter:
First, to test the transmitter, the input signal must be good enough. Measurements of jitter and an eye mask test must be performed to confirm the quality. An eye mask test is the common method to view the transmitter waveform and provides a wealth of information about overall transmitter performance.
Second, the optical output of the transmitter must be tested using several optical quality metrics such as a mask test, optical modulation amplitude, and Extinction Ratio.
To test a receiver, there are also two steps:
Third, different from the transmitter testing, which requires the input signal must be good enough, the receiver testing involves sending a signal that is poor. To do this, a stressed eye representing the worst case signal shall be created. This is an optical signal, and must be calibrated using jitter and optical power measurements.
Finally, testing the electrical output of the receiver must be performed. Three basic categories of tests must be performed:
a. A mask test ensures eye open. The mask test is usually accompanied by a bit error ratio depth.
b. Jitter budget tests for the amount of certain types of jitter.
c. Jitter tracking and tolerance tests the ability of the internal clock recovery circuit to track jitter within its loop bandwidth.