Optical Transceiver
Optical transceivers need to be compatible with a variety of communication standards, as well as optical fiber type, transmission distance, data rate, and transmission format such as digital coherent or PAM4. The performance requirements for optical transceivers vary depending on the type of the communication standard. Optical transceivers must be verified, through testing, to ensure that they satisfy the performance requirements of Ethernet, OTN, and other communication standards.
Challenges
Challenge 1: Severe signal performance requirements for higher speeds
The PAM4 waveform can convey two bits of information in a single signal (symbol). On the other hand, the performance requirements for the signal waveform are more severe because of the narrower spacing between each PAM4 signal level. Moreover, PAM4 waveforms have three eyes, such that there are many evaluation items such as amplitude and noise, while the calculation of the eye aperture indices for PAM4 waveforms is complex.
Challenge 2: Error correction to ensure communication quality
As the transceivers supporting 800GbE and 400GbE with PAM4 are sensitive to signal quality degradation, forward error correction (FEC) technology must be adapted for data transmission. Therefore, verifying both the conventionally evaluated bit error rate and FEC operation must be done using a test signal including jitter and noise.
Challenge 3: Wavelength accuracy of signal light
Fiber-optic networks employ wavelength division multiplexing (WDM) technology, which uses signal light of different wavelengths. Optical transceivers must precisely match the wavelength of the signal light to the ITU grid specified in 12.5 to 100 GHz spacing.
Solutions
Solution 1: Quality check of signal waveforms
A sampling oscilloscope allows you to evaluate, from the histogram, the amplitude, jitter, skew (time difference), and other values for each eye of the PAM4 waveform. BERTWave MP2110A provides Transmitter and Dispersion Eye Closure for PAM4 (TDECQ) which is an indicator of the eye closure ratio.
Solution 2: Counting Symbol Errors
400GbE and 800GbE process error correction on PAM4 signals in units of blocks called Codewords.
The MP1900A Signal Quality Analyzer counts the number of symbol errors in the Codeword and determines how many of those errors can be corrected.
Solution 3: Optical Spectrum Measurement
An optical spectrum analyzer displays the wavelengths of the optical transceiver. The optical signal-to-noise level ratio (OSNR) is also an important measurement, since the spacing between signal optical wavelengths is narrower in DWDM.
