Understanding the Optical Transceiver Quality Testing

It is well known that optical transceiver is an important part of optical fiber communication network. Usually, everyone is very concerned about the performance of the Ethernet switch, however, they may ignore the quality of the important components – optical transceiver, the price has become the only factor in the purchase. But the truth is that the market is now flooded with low-quality transceivers, and it is hard for ordinary users to discern the high-quality optical transceivers.

The quality of the optical transceiver determines network transmission performance, which can be significantly reduced once the work is cut. Like other high-tech appliances, the optical transceiver is subjected to rigorous testing and quality inspection procedures in its manufacturing process, such as optical power testing, sensitivity testing, eye diagram testing, aging testing, real machine testing, fiber end face detection, etc. These processes involve each stage of the production process to ensure the best results. If any program fails, the optical transceiver will be rejected and returned to the production line for heavy work.

So how do you test the performance parameters of the optical transceiver?

Transceiver Average Output Optical Power Measurement

The transmitting port of the optical transceiver consists of a light source and a related electronic circuit. Semiconductor-based light-emitting diodes (LEDs) and laser diodes are used as light sources in optical transistors. LED and vertical-cavity surface-emitting lasers (VCSEL) are typically used for transmitters on local and local networks, while Fabry-Perot (FP) lasers and distributed feedback (DFB) lasers are used for transmitters for Metro and long-distance networks.

In optical communication, light sources are intensity-modulated, which is a process of applying varied current to the laser to change the output power level. As illustrated in Figure 1, a finite power level represents logic zero rather than a true complete absence of power.

The average output optical power is an important parameter of the transmitter, which directly affects the communication quality of the module. It is the average optical power of the receiver under normal operating conditions. The optical power meter enables an average output optical power measurement to test the optical power at the transmission end. For transmitters for long-distance transmission, the average optical power is greater than the maximum input optical power.

Average optical power is measured with an optical power meter. The measurement unit is usually expressed in dBm, a logarithmic ratio of power level to 1mW.

Transceiver Extinction Ratio Measurement

The extinction ratio, when used to describe the performance of an optical transmitter used in digital communications, is simply the ratio of the energy (power) used to transmit a logic level ‘1’, to the energy used to transmit a logic level ‘0’. For a graphical description, the eye-diagram is commonly used as shown in Figure 2.

Optical Modulation Amplitude Measurement

The optical modulation amplitude (OMA) is used to measure the difference between the two optical power levels generated by the power supply, for example, P1 (when the light source is turned on) and P0 (when the light source is turned off). With OMA, a low or high light-down ratio can be used, provided that the transmitter’s eyes are safe and do not overload the receiver.

The figure-3 depicts OMA in a stressed eye diagram of an optical signal.

Receiver Sensitivity Test

Receive sensitivity is one of the key parameters for measuring the performance of optical transceiver receiving devices. The reception sensitivity test requires the power attenuation of the signal through a programmable optical attenuator, which enables the optical transceiver receiving signals of different power to be completed by comparing the error rate of different optical power by the error meter. Among them, the better the reception sensitivity, the smaller the minimum receiving light power. Conversely, if the reception sensitivity is poor, the higher the requirements are for the optical receiver device.

Transceiver Eye diagram Test

Eye diagrams are a common tool for viewing transmitter outputs. It provides a wealth of information about the overall transmitter performance. In an eye diagram, all combinations of data patterns are superimposed on a common timeline, usually less than two-bit periods. Figure 1 shows a signal with good amplitude and low jitter. You can imagine how the eye diagram is constructed by drawing eight possible sequences of three-bit waveforms (000,001,… 110, 111) overlaps on a common timeline.

Instead of multiple measurements, you can use the eye mask test to determine the quality of the eye. The mask consists of several polygons placed in the eye diagram and in the eye diagram, indicating areas where the waveforms should not intersect. The “good” waveform will never intersect the template, and the “bad” waveform will cross or violate the template. Step back and view the system-level view, opening one eye indicates that the receiver will easily distinguish between Logic 1 and Logic 0. If you close your eyes, the likelihood of error (error) increases. Figure 2 shows the waveforms that are easy to pass the eye mask test.

The broadband oscilloscope allows you to perform optical eye mask tests. These instruments have several names, including digital communication analyzers. The oscilloscope can perform tests and determine if any waveform samples have landed on the template.

Laser manufacturers want their lasers to pass mask tests without any irregularities, and they want to find measurements with sufficient margin. Expanding the mask size as much as possible provides maximum balance without incurring any mask hits.

Transceiver Eye Cross-Ratio (Crossing)

The cross-ratio of the eye chart is the relationship between the measuring amplitude of the intersection and the signal “1” and “0” bits, so different cross-scale relationships can convey different signal bits. The standard SFP optical transceiver transmitter has a cross-over ratio of 50%, which means that the optical signal logic ‘1’ code and the logic ‘0’ code account for one-half of the bits, respectively.

TransceiverJitter Time (RMS)

Jitter time is the period during which the timed noise generated by the transmission of optical signals at the SFP optical transceiver transmitter is transmitted. Minimize this associated jitter time in SFP optical transceiver and improve overall system performance.

Transceiver Bias current Test

In order for the laser LD high-speed switch to work properly. It must be added to the DC bias current I BIAS, which is slightly larger than the threshold current ITH directly represented by bias. If the BIAS is too large, the accelerator components will age, and if the BIAS is too small, the laser will not function properly.

Transceiver Wavelength Test

Since the optical modules used on the devices at both ends must emit the same wavelength to establish communication, the manufacturer must test the wavelength of the optical module before shipment to ensure that it is within the deviation range. Generally, manufacturers use optical spectrum analyzers and other instruments to measure the central wavelength of the optical module, and the measured central wavelength of the optical module usually deviates from the standard value. Different types of optical modules have different deviations, but as long as the deviation is within the allowable range, For example, the central wavelength of the SFP-1G-LX optical module is 1310nm, and its deviation is ±50nm, and the central wavelength of the SFP-1G-SX optical module is 850nm, and its deviation is ±10nm. The center wavelength of the 10G-CWDM-SFP-ER optical module is 1470 nm, and its deviation is ±7.5 nm. If the tested value is inconsistent with the standard specification, the optical module is regarded as defective.

Transceiver Compatibility Test

The compatibility testing is mainly for compatible optical modules. The optical module is inserted into the switch of the corresponding brand devices for testing. Normal communication means that the optical module has passed the test. If it is unable to communicate, it means that the optical module is not compatible with it.

Transceiver Optical Endface Inspect

The optic transceivers are highly variable in design and type: e.g. SFP, SFP+, XFP, XENPAK, GBIC, QSFP+, etc. The SFP, SFP+, and XFP have an LC connector interface; XENPAK, 1*9, GBIC has an SC interface. QSFP or QSFP28 transceivers typically have an MPO/MTP or LC interface.

Regardless of the type of fiber, application, or data rate, the transmission of light requires a clear pathway along with a link, including through any passive connections or splices along the way. A single particle on the core of a fiber can cause loss and reflections, resulting in high error rates and degraded network performance. Contamination on a fiber end face as shown in Figure 1 can also adversely impact the interface of expensive optical equipment, and in some cases even render equipment inoperative.

By inspecting the transceiver optical end face, we will check if there are dirt and scratches. Since contamination is the single biggest cause of fiber failure, every optical module is properly checked before shipping, even if it adds time and money.

Appearance check

It involves inspecting the optical modules before shipped for quality control purposes. Check the case of each module for scratches, dirt, color, smoothness, gold fingers for scratches, and labeling. Usually, poor optical transceiver module appearance will also be defective, while high-quality transceiver appearance is good.

This article describes what tests a high-quality optical transceiver needs to pass, and what these test links and parameters mean. By reading, I hope you can quickly distinguish the quality of an optical transceiver. Be sure to choose a high-quality optical transceiver that determines your network stability and transmission quality.

Optical Transceiver