II-VI shrinks an optical line system into an OSFP module

II-VI has developed an optical line system that fits inside a pluggable module. 

The advent of coherent pluggable modules implementing the 400ZR standard allows switches and routers to be linked across separate data centres. Now, with a pluggable optical line system, a dedicated line-system platform is no longer needed to send the 400ZR signals over a fibre. 

Sanjai ParthasarathiIn turn, the network operating system on the switch manages the optical line system directly such that a separate optical management software is no longer needed.

“We have shrunk an entire pizza-box line system into a small pluggable,” says Sanjai Parthasarathi, chief marketing officer at II-VI.

Indeed, one customer refers to the II-VI pluggable product - dubbed the OSFP-LS - as a ‘zero-rack-unit’ platform.

Origins  

A dense wavelength-division multiplexing (DWDM) system comprises transponders and an optical line system. 

The elements used for a line system include an optical multiplexer and demultiplexer, amplifiers, and monitoring equipment that assess the quality of the DWDM link such as an optical channel monitor (OCM) and an optical time-domain reflectometer (OTDR).

Traditionally, a single vendor provided the optical line system and the transponders. Now line systems are ‘open’, part of the service providers’ demand to disaggregate equipment to avoid vendor lock-in. Accordingly, optical transponders from one vendor can now operate over another vendor’s line system. 

II-VI supplies the components used for line systems: amplifiers, multiplexers and demultiplexers, OCMs, and OTDRs.  

The idea of a pluggable optical line system came about following II-VI’s ongoing work to shrink the components used in line systems such as micro-pumps that are used in amplifiers. This requires not only shrinking the micro-pump but developing the required fibre and packaging technologies. 

“We have been miniaturising and reducing the power consumption of pumps for years, primarily for embedded applications in transceivers,” says Parthasarathi. 

Working with partners, II-VI has embedded its micro-pumps to fit within the CFP2-ACO and CFP2-DCO coherent modules.

“Knowing that 400ZR was coming and that it would use OSFP and other compact pluggables, and that many of the links will need amplification, we started looking at how our amplifier technology for the CFP2 could be shrunk further,” he says.

Networking equipment maker, Arista Networks, saw an ECOC 2018 Market Watch session given by II-VI and started a discussion. 

Arista was interested in whether an entire line system pizza-box could fit in a pluggable. "That way, you wouldn’t require another box that needs to be powered and cooled,” says Parthasarathi. “That was the genesis of the idea.”  

Parthasarathi says II-VI does not have an exclusive partnership with Arista but that the vendor is an early customer of the OSFP-LS.

OSFP-LS

Two amplifiers fit inside the OSFP: a booster amplifier used on the signal before transmission and a pre-amplifier at the receiver end. Uncooled micro-pumps are used so that no thermo-electric cooler is needed. 

The multiplexing and demultiplexing circuitry is housed in the cabling, external to the module, and are II-VI custom designs.

Sara Gabba

Not everyone operating a data centre switch is skilled in optical engineering so the pluggable line system is designed to configure itself. 

“It automatically detects the number of channels and sets the amplifiers, monitoring and adjusting them,” says Parthasarathi.

“A big part of the design is the implementation to optimise everything for this application,” adds Sara Gabba, strategic marketing manager at II-VI. “The gain-flattening and controlling the amplifiers’ power are technically demanding and therefore far from trivial.”     

The current version of the OSFP-LS, supporting up to eight wavelengths, has been demonstrated by Arista to customers. 

Source: Arista Networks

II-VI is looking to extend the pluggable optical line system architecture to higher channel counts thereby further exploiting a pluggable slot. 

II-VI’s micro-pumps are rated above 400mW so amplifying 16 channels within a pluggable module is not an issue given that the power is shared across the channels.

As for multiplexing and demultiplexing more channels, II-VI is exploring using a separate cartridge as well as integrating the filtering functions inside the module.

The OSFP-LS will be generally available towards the end of 2020. 

Future work 

“400ZR is just the beginning,” says Gabba. “It will support the future 800G-ZR modules and can be adapted for other applications, wherever you need amplification and multiplexing-demultiplexing, including longer reach, cable, and mobile backhaul.” 

The company is also looking to integrate the line system into the smaller QSFP-DD pluggable. 

Extra features could also be added to the OSFP optical line system through further miniaturisation such as OCM and OTDR functions.  

The initial module design uses photo-detectors for monitoring but does not have full optical channel monitoring. If a transceiver fails, the line system will continue to amplify the remaining channels but customers want to know if one of the wavelengths has gone down, says Parthasarathi.

After that, the only remaining feature is the reconfiguration of wavelengths. But Parthasarathi says the degree of miniaturisation needed is years off.

The majority of links data centre operators use are point-to-point; a pluggable reconfigurable optical-add drop multiplexing (ROADM) technology is not on their near-term wish list.