Test and Measurement for Coherent Optical Transceivers

Coherent technology was traditionally used for long-haul and metro networks. However, over the years, this technology has been increasingly adopted for shorter reach applications, such as Data-Center Interconnect (DCI) and 5G/6G front/backhaul, to overcome physical limitations of Intensity-Modulation/Direct-Detect (IM/DD) as those applications demand higher throughput.

The IEEE802.3 standardization group is working on defining 800GBASE-LR1 (≤ 10 km), 800GBASE-ER (≤ 20 km) and 800GBASE-ER1 (≤ 40 km) to be interoperable interfaces based on coherent technology. The OIF (Optical Internetworking Forum) is also defining Implementation Agreements (IAs) for Data-Center Networks (DCNs), DCIs and metro applications based on coherent technology. After the successful completion of the 400ZR IA used in coherent pluggable transceivers for edge DCI applications (≤ 80-120 km), the group is now working to define higher-speed coherent solutions for 800 Gb/s (800LR/ZR) and 1.6 Tb/s (1600LR/ZR/ZR+) data transmission.

Test and Measurement (T&M) equipment can be used to address the requirements of standards using coherent technology. The characterizations of coherent transmitters and receivers are notably different from DD technologies: for coherent transmitters, a reference receiver (optical modulation analyzer) is required which includes a significant amount of Digital Signal Processing (DSP) to assess the transmitter signal quality. For the coherent receiver, characterization depends on applying stressed patterns/signals from a reference or calibrated transmitter.

Keysight Technologies’ test solutions portfolio addresses the entire design cycle, from research, design validation, conformance test, protocol test and manufacturing. State-of-the-art equipment and test solutions enable early research into emerging technologies well before measurement standards are available. Later, as standards develop, close association and participation in the standards organizations enables Keysight to provide first-to-market test solutions allowing timely characterization and validation of component and systems designs to run concurrently with the standards development cycle.

The design cycle starts testing electro/optical devices such as dual-polarization IQ modulators, coherent receivers, amplifiers, TIAs and photodiodes. During this phase the components are characterized by measuring the frequency domain parameters. Components can be packaged and connectorized or can be probed on wafer with a probing station as is frequently the case with high-speed chips. Once the required components and sub-assemblies are available, proof of concept of the target generation of optics is performed. Keysight offers a complete range of AWGs and real-time oscilloscope configurations for the various bandwidth needs. The last stage shown is the validation and characterization of the complete coherent pluggable modules at physical and protocol layers (e.g. 400ZR, 800ZR designs).