Optical Networking, Redefined.
March 24, 2023

400G ZR & ZR+: A Systems Engineering Company’s Perspective

400G Pluggable Coherent

“400G pluggable coherent optics like ZR and ZR+ variants have a lot of complexity built-in with their immense functionality.”

Introduction to 400G Pluggable Coherent Optics

Coherent technology has been the standard for fiber optic networks for transmissions of 100Gbps and higher at distances >= 40km for more than 10 years.  Data connections from the edge back to the core rely heavily on coherent optics.  In The Rise of 400G Coherent Pluggable Optics, we talked about the bifurcation of the coherent optical market into two distinct segments focused on: 

  1. Highest performance capacity and reach (typically embedded) 
  2. Pluggability with a small footprint, low power, and low cost 

Around 2017, the OIF (Optical Internetworking Forum) started forming an implementation agreement (400ZR) with the goal of reducing the cost of data center interconnects.  It was determined that the way to do that would be to move the coherent transponder functions into the router. While there had been coherent optics plugging into the router before, it was recognized that to truly reduce costs and increase efficiency, the client optic and the coherent optic must be in the same form factor.  The OIF 400ZR Implementation Agreement standardized the way forward using form factors such as QSFP-DD and OSFP which can be installed directly into a router or switch in a data center.  Among other advantages, this type of pluggable revolutionizes IP over DWDM (IPoDWDM) by eliminating the previous footprint penalty of DWDM optics that have historically been ~2x larger in size than client optics. 

The OIF 400ZR further defines the 400G pluggable coherent interface for use on point-to-point links up to 120 km. The 400ZR target applications include interconnecting local data center caching sites to metro point of presence (POP or backhaul) offices and interconnecting multiple data centers across metro areas.  Advantages of pluggable coherent optics include: 

  1. Lower cost 
  2. Reduced network complexity 
  3. Multivendor interoperability 
  4. Longer reach 

While these sub 120km DCI applications driven by hyperscalers were the main focus of 400ZR, demand continues to drive and expand the technology into other market segments and applications such as providing 400G interconnection to large enterprise networks and in metro rings, eliminating the need for expensive transport/line systems.   Both enterprise data centers and metro networks are facing increasing data traffic which puts pressure on network performance.  Next-generation DCI and metro network solutions are centered around medium-range, high-capacity DWDM links with support for 400G.  However, cable and telecom operators and transport network service providers have differing needs, which has led to the multiple standards related to the original 400ZR including OpenZR+, OpenROADM, and CableLabs P2P.  We discuss some of these in our white paper “The Network Operator’s Guide to the Latest Advancements in 400G and 800G”. 

Standards, Form Factors & Transmit Powers – Oh my! 

Within the industry, you’ll see 400ZR/400G ZR and OpenZR+/400G ZR+ used interchangeably.  These are the standard name/pluggable name, respectively. At an organization level, the difference between the 400ZR and the OpenZR+ standards is that the 400ZR is managed by the OIF and the OpenZR+ is organized as an MSA group both with all the main players in the industry working together.  The 400ZR is only rated to 120km distances and does not have multiple modulation or rate options, whereas the OpenZR+(400ZR+) is open to configuration for different modulation and data rates which presents a lot more options as well as some added complexities. 

Similar to what has been happening in the rest of the 400G market in the last 2-3 years, the QSFP-DD form factor is becoming the market leader for coherent optics of this type.  The main features of 2 basic product forms for 400G ZR and 400G ZR+ are shown below: 


  • OIF Implementation Agreement
  • Tunable DWDM C-Band (100 and 75 GHz spacing)
  • 400GbE
  • DP-16QAM 
  • C-FEC (Concatenated Forward Error Correction)
  • TX power > -10 dBm 
  • up to 120 km reach (with amplification) 

Note: the OIF TX spec defines –10 dBm as the minimum value, however, the TX power typically will be higher than that at –8/–9 dBm. Furthermore, a 400G ZRHT QSFP-DD version is also becoming available which will be a “high transmit” variant around 0 dBm. 

400G ZR+ QSFP-DD  

  • OpenZR+ MSA
  • Tunable DWDM C-Band (100 and 75 GHz spacing)
  • 400G-100G Multi-rate
  • DP-16QAM (400G) / DP-8QAM (300G / DP-QPSK (200G/100G) 
  • O-FEC (Open Forward Error Correction) and C-FEC
  • TX power: 
    • -10 dBm => minimum value from OpenZR+ 400G specification 
    • ~0 dBm  => expected value for many operators 
    • >0 dBm  => ongoing development 
  • 500 km+ reach (with amplification) – True reach will vary depending on the network design

While a TX output power of -10 dBm is the minimum spec for either industry standard, this presents limitations in terms of distance as we have seen in early deployments of 400G ZR and also brownfield deployments with existing Transport network design.  Most OLSs (optical line systems) or ROADMs (reconfigurable optical add-drop multiplexers) typically have an input requirement of 0 dBm. So if there’s a need to connect into other transport type of equipment such as an OLS or ROADMs along the network–either because there’s an existing route there or you need to go ultra long haul distances–a minimum of 0 dBm input is required for those systems versus a – 10 dBm.  A TX output power of -10 dBm is too low of an optical signal and will not be able to interface to an OLS or a ROADM due to this power input requirement.  The higher transmit power (0 dBm) products provide increased link budget and reduce dependence on amplification.   The more you reduce dependence on amplification, the better the OSNR (optical signal to noise ratio) you’ll achieve in a link and the further you can go.   

On that note, it seems that 0 dBm is becoming the unofficial new minimum requirement as the market trends towards 400G ZR+ and all the flexibility and capability that the Open ZR+ brings–including requests for +3 dBm, +5 dBm, and so on.  How is that higher transmit power achieved? Essentially with a mini EDFA(Erbium-doped fiber amplifier) built right into the transceiver that is capable of boosting the transmit levels from -10 to 0 ,+3 or greater.  Of course, the trade-off is that higher transmit power comes from higher power consumption and a very complex design to fit the amplification component in an already tight space with the current QSFP-DD form factor.  So -10 dBm optics will still have a sweet spot with 120km or shorter distance applications that are fine with a lower power budget. 

They’re Pluggable: Not Plug-n-Play 

With all the excitement and promise afforded by this new coherent technology, it bears keeping in mind that these are not just a transceiver one can buy off the shelf, plug in and expect to work.  Unlike 10G LR and 100G LR4 for example, which are fairly straightforward in terms of deployment, 400G ZR and ZR+ variants have a lot of complexity built-in with their immense functionality.  These optics must be considered within the context of an entire network design: the TX power, the amplification, the type of Mux/Demux, fiber span, OSNR, ROADMs, etc…, all of these factors matter in terms of successful network implementation of 400G ZR/ZR+ technology.   It is for that reason that Precision OT wrote this white paper, and what you can’t find in there you can certainly find out by giving our network experts a call.  We have experience with IP networks and Transport networks and everything in-between and along the periphery.  Our systems engineering, development and integration teams are working hard on external solutions to make all 400G ZR / ZR+ options more deployable and easier to use for all levels of network providers. 

A Systems Engineering and Integration Approach 

When it comes to deploying 400ZR in your network, there’s not a one-size-fits-all management approach.  Depending on how much integration you need to do with existing network infrastructure vs greenfield new deployment, your network must be flexible enough to handle each unique situation.  Whether your world is IP or Transport or somewhere in the middle, Precision Optical Technologies has you covered with a broad and deep understanding of all things optical fiber networks.