# 400ZR – Optical Coherent Moves to Data Center Interconnects

Optical coherent transmission technology, initially used in long-haul transmission, is evolving into metro networks and expanding to data center interconnects. Significantly faster data transmission rates and higher levels of spectral efficiency can be achieved using higher order quadrature amplitude modulation (QAM) signaling.

QAM is a two-dimensional modulation format that modulates both the phase and amplitude of the signal. QAM combines two carriers with independently modulated amplitudes at the same optical frequency. The carriers are called in-phase (I) and quadrature-phase (Q). The standard notation is 2n QAM, where n-bits are transmitted per symbol.

For example, 16 QAM sends 4 bits per symbol, and 64 QAM sends 6 bits per symbol. The following formula is used to determine the bit rate:

*Bitrate = Baud rate (symbols/second) X Coding (bits/symbol) X Polarization*

Polarization is typically two. Therefore, a transceiver using 16 QAM and 64 GBaud raw symbol rate (approximately 50 GBaud without the overhead) could transmit 400 gigabits per second (Gb/s) on a single fiber.

The Optical Internetworking Forum (OIF) is developing an implementation agreement (IA), 400ZR, for transmitting 400 gigabit Ethernet (GE) over data center interconnect links up to 100 km using dense wavelength division multiplexing (DWDM) and higher order modulation such as 16 QAM. The 400ZR standard will reduce the cost and complexity for high bandwidth data center interconnects and promote interoperability among optical module manufacturers.

Many data centers within a campus or a metropolitan area need to significantly increase the bandwidth of connections between them to support exploding data growth. 16 QAM 64 GBaud is the target operating point for next-generation data center interconnects. 400G transceivers require dense electronic and photonic integration with tighter specifications and performance margins for all components. Find optical and photonics solutions here you need to accurately and efficiently test your 400G optical coherent components and transceivers so you can design the next generation of high-speed data center interconnects.