Transceiver testing and quality requirements

Optical transceiver modules are the main end-to-end components that allow optical communication through optical fiber. Transceiver manufacturing technologies have developed over time to become faster and less susceptible to errors. Optical transceivers, like any other high-tech electrical appliance, during their manufacture process is undergoing firm testing and quality check procedures. These procedures are involved at each stage of the transceiver manufacturing process to ensure the best outcome. And if any of the procedures fail, then the item is rejected and returned to the production line for repeated adjustment (in case of critical failure, the transceiver is removed and dismantled).

The optical transceiver manufacturing process involves a lot of steps, but the most important ones (not all accounted for), which greatly affect the quality of the end product, are as follows:

1. Calibration – Tx, Rx, Eye-diagram, Voltage measurements;
2. Aging test;
3. Switch test;
4. Wavelength and Spectrum check;
5. Lens cleaning.

Calibration – Tx, Rx, Eye-diagram, Voltage measurements:

Transmitter and receiver tuning, eye diagram, and voltage level adjustment are the most crucial steps in the optical transceiver manufacturing process. These steps initially set up the transceiver for its best working parameters, which comply with quality and MSA standard requirements. Transmitter power, receiver sensitivity, transmitter eye-diagram, voltage and temperature calibration, and debugging process are shown in diagram 1-1. In this process Optical transceiver is described as a device under test (DUT), it is connected to the testing board with an appropriate electrical interface for a specific form factor transceiver SFP, XFP, QSFP, etc. DUT transmitter is connected to the de-multiplexing component (DEMAX) – this unit, with the help of optical prisms, separates optical wavelength signals which are sent from DUT QSFP LR4 optical transceiver (this transceiver uses four CWDM lines at 1270, 1290, 1310, and 1330nm (EDGE Optics, PN: 40G-QSFP-10). If DUT is an SFP/SFP+ transceiver with one output wavelength, then the DEMAX unit with optical path control switch is not used.

Optical Path Control Switch is a unit that simply allows to selection of any necessary wavelength from the input port and forwards it to the output port. This equipment uses an optical switching principle, which induces low (well-known) insertion loss. The signal is then split, where one portion of the signal is sent to the oscilloscope (for eye diagram), and the other portion to the power meter unit, which, after measurement, forwards it to the DUT transceiver receiving port.

DUT transmit power is measured with a power meter and controlled so that it is at the necessary range. Transceiver voltage measurements are done on the test board directly, and the results are shown on the controller PC.

The temperature (hot and cold impact) testing unit is located parallel to all other measurement units and performs DUT temperature change scenarios. What this unit practically is heats the DUT till the maximum operating temperature so that Tx, Rx, and eye-diagram measurements can be taken at the transceiver's maximum operating temperature. And cool the transceiver till the minimum operating temperature so that Tx, Rx, and eye-diagram measurements can be taken again. These tests can potentially indicate problems related to DUT temperature, which, if not passed, then additional calibration is performed on the affected part.

Diagram 1-1

However, the testing board's electrical signaling part is connected to the Bit Error Rate Tester. This tester generates a random signal pattern which is sent through the DUT, and later analyzed with an oscilloscope for eye-diagram purposes.

Eye-mask measurements and adjustments are an important phase of the transceiver's path to best signal quality guarantee, which coincides with MSA standard requirements. Eye-mask definitions specify transmitter output performance in terms of normalized amplitude and time in such a way as to ensure far-end receivers can consistently tell the difference between one and zero levels in the presence of timing noise and jitter. Eye-mask measurement results indicate the quality of the digital signal, but do not indicate protocol or logic problems. The quality of digital signals is simple to see with an eye diagram: Bit-Error-Rate (BER) degrades with eye closure. Eye-opening is indicated with yellow arrows in diagrams 1-2.

MSA standard for optical transceivers indicates a precise eye diagram mask (Grey rhomb drawing under yellow arrows) which should not be crossed by respective 0 and 1 signals (blue lines), and their transitions. If the test signal lines cross the eye mask, the transceiver fails the test and has to undergo additional calibration. In general, the more open the eye (yellow arrows), the lower the chance that the receiver in the transmission system will mistake a logical 1 bit for a logical 0 bit, or vice versa.

Wavelength and Spectrum Test:

Optical transceivers have to emit a precise wavelength to successfully communicate with their counterpart transceivers. For example simplest 10Gbps 10-kilometer (EDGE SKU: 10G-SFP-10) SFP transceiver has a transmission wavelength of 1310nm with a possible deviation +/- 50nm, and the multi-mode version 10Gbps 300-meter (EDGE SKU: 10G-SFP-300) SFP 850nm with possible deviation +/- 10nm. But transceivers, like, for example, 10Gbps 40 kilometer (EDGE SKU: DWDM-10G-SFP-40-21) DWDM SFP transceiver, have 1560.61 nm wavelength with +/- 0.8nm deviation. This DWDM transceiver especially has to be with a precise laser output wavelength, because it should not crosstalk with other side channels due to the system design principle.

During optical transceiver manufacturing, wavelength precision is measured with a spectrometer (Drawing 1-3). The transceiver is plugged into an electrical power source – mostly in a factory environment, it is a special powered PCB or a network device switch, as used in this example. This graph represents a 1Gbps 10-kilometer transceiver (EDGE SKU: 1.25G-SFP-10D). At the spectrograph, we can see X X-axis as wavelength, and Y Y-axis as power. The power peak forms a sharp form and is located at 1310.56nm (at -3.16dBm), it is very close to the MSA standard regulation 1310.00nm – so we can say this module has passed the spectral test.

In case DUT has a power peak at a different wavelength which is not consistent with MSA regulation, then this transceiver is discarded as defective!

Diagram 1-3

Lens Cleaning:

During the transceiver manufacturing process, after each testing step, optical transceiver lenses are checked for dirt and scratches and cleaned if necessary. It is due to the fact that each testing procedure involves connecting equipment to the transceiver optical parts, and therefore, it is susceptible to dirt. Before the leaning procedure, each transceiver lens is firmly checked by a microscope. A picture of the microscope test output can be seen in diagrams 1-4.

Diagram 1-4

If there are no scratches or dirt on the lens core and its cladding, then this test is positive. Otherwise cleaning procedure is performed. The cleaning procedure removes dirt, oils, and other foreign bodies/substances. So that after cleaning, another microscope test is performed. Of course, if the core has damage like for example, scratches, then this transceiver is rejected and dismantled.

Summary:

The optical transceiver manufacturing process involves a lot of important steps. The most important ones are at the transceiver manufacturing first stage, when the key elements are soldered and powered on for the first time, for calibration purposes. The calibration stage is crucial because it determines how the transceiver will perform for the rest of its life. If the transceiver delivers bad performance at the calibration stage, then the safest step would be to discard this unit. Other tests, which are performed on the transceiver after the calibration stage, can indicate potential problems and weakest points. Aging test and switch test are the perfect tests for indicating that the exact transceiver will have problems in the long term. Transceiver lens cleanness is an important factor because, if the lens is damaged by dirt, oil, or scratches, then this can cause issues in the long term. Like for example, increased laser deterioration and burnout.