How to test an Optical Fiber Transceiver?

As networks grow and technologies move forward, many manufacturers of fiber optic components emerge on the market trying to grab their share of the networking world. As these manufacturers produce various components, they aim to manufacture them to be high quality and compatible with each other so the customers can mix various components from different manufacturers. This is mainly because of financial issues as many Datacenters are always looking for cost-effective solutions to implement in their network.

Optical transceivers are a crucial part of the fiber optic network. They are converting and driving the optical light across the optical cables. They are consisted of two main parts: transmitter and receiver. When it comes to maintaining and troubleshooting it is very important to be able to predict, test and find where the issue might occur or have already occurred. Sometimes if the connection doesn’t achieve the expected bit-error-ratio, at first glance we won’t be able to tell which part of the connection is causing the issue. It may be the cable, transceiver, receiver or both. Generally specifications should guarantee that any receiver will normally operate with any, as known as, worst-case transmitter and vice-versa that any transmitter will provide a signal with enough quality to be picked up by any worst-case receiver. The worst-case criteria is most often the hard part to define. However generally there are four steps in testing both transmitter and receiver part of the transceiver.

When it comes to testing the transmitter part, the testing consists of testing the wavelength and shape of the output signal. There are two steps in testing the transmitter:

• The optical output of the transmitter must be tested with the help of several optic quality metrics such as a mask test, optical modulation amplitude (OMA) and Extinction Ratio.
• Testing with an eye mask test which is the common method to view the transmitter waveform and provide an information about the overall transmitter performance. In the eye diagram, all the combinations of data patterns are superimposed on each other over a common time axis, usually less than two bit periods in width.

Testing the receiving part is the more complex part of the process, however there are also two steps of the testing:

• First part of the testing is to confirm that the receiver can receive a poor quality of signal and convert it. This is done by sending a poor quality of optical light sent to the receiver. As this is an optical signal it must be calibrated using jitter and optical power measurements.
• The other part of the test is testing the electrical input of the receiver. Within this step a three categories of tests must be performed: an eye mask test to ensure a large enough eye opening, jitter test to test the amount of certain types of jitter and jitter tolerance test which tests the ability of the receiver to track the jitter within its loop bandwidth.

Some form-factors, for example SFP+ transceivers, define some important tests. These tests are critical and are shown in the table below:

Testing an optical transceiver is a complicated and vital job to ensure it is always operating at maximum performance. Even the most basic eye-mask test is widely used today and enough to ensure the transmitter performance. Testing the receiver is a more complex process however testing it using complex "stress" signals is becoming more common because of the precise results.