One of the major problems currently facing network operators is how to get more bandwidth from existing fibre. Simply adding more fibre is costly, energy-hungry and time-consuming, so operators are looking to optical systems manufacturers such as II-VI to come up with ever-more efficient designs, writes Dr. Sanjai Parthasarathi and Sara Gabba.
Network convergence is enabling new business models designed to scale rapidly by leveraging cloud computing and artificial intelligence. New applications in virtual, augmented and mixed reality as well as the autonomous car are driving projections of increasing demand for higher bandwidth requirements over next-generation low-latency 5G wireless networks. To keep pace with these innovations, carriers are evolving their optical networks, boosting capacity and network intelligence to provide services with greater bandwidth, flexibility and operational efficiency. These network upgrades require optical systems that can significantly decrease the equipment cost, power consumption and size per unit of bandwidth (Figure 1).
High bit rate DWDM transmission is a key enabler of higher network capacity. According to telecoms market analyst Dell’Oro, coherent transmission at 100 Gbps or greater will represent about 90% of the WDM equipment market by 2021. Bit rates will also climb from 100 Gbps to 200 Gbps, 400 Gbps, 600 Gbps and 1.2 Tbps, while the size of DWDM transmission equipment will shrink, from discrete solutions on line cards to smaller and smaller pluggable devices, enabling significant increases in bandwidth density. For transmission rates of 100 Gbps and above, coherent DWDM transmission is now established as the technology of choice for distances spanning from a few tens of kilometres to thousands of kilometres, while direct detect DWDM transmission has become a viable alternative for links shorter than 80 km.
Optical amplification is a critical enabler of high bit rate systems. Complex modulation formats employed in such systems introduce loss and noise impairments that need to be mitigated through amplification and filtering. While optical amplifiers boost the signal power, they also introduce amplified spontaneous emission (ASE) noise. Compact and low cost tunable filters have recently been introduced on the market to filter out this noise and significantly increase the optical signal to noise ratio (OSNR). These filters need to be tunable so they may be aligned to the desired DWDM channel to which the transmitter is set. (Figure 2)
Erbium-doped fibre amplifiers (EDFAs) and tunable filters are increasingly integrated directly into DWDM transceiver modules even as these modules are becoming much smaller. These new transceiver-embedded EDFAs are enabled by three new critical technology developments which are (a) the miniaturisation of 980 nm pump laser packaging, (b) the integration of multiple micro-optics, essential to the EDFA’s functions, into compact hybrid components and (c) the design of novel space-saving fibre management solutions.
A few leading pump laser manufacturers have recently introduced uncooled pumps specifically designed for DWDM transceivers with compact CFP2 form factors and potentially CFP4 in the future (Figure 3). These designs represent a leap in packaging miniaturisation unseen since the original introduction of 980 nm pump lasers more than 25 years ago. Most EDFA designers are familiar with the butterfly package that typically house pump lasers, some of which are trusted to operate reliably for decades, even on the ocean floor. These new miniature pumps are instead adopting much smaller packages in rectangular or cylindrical form. The thin, flat rectangular packages in particular are the smallest and are more easily air cooled and heatsinked.
EDFA designs require multiple micro-optic components including isolators to prevent unwanted back reflections, tap couplers and detectors for optical power monitoring and couplers to couple DWDM signals with pump lasers into erbium doped fibre where the optical amplification takes place. Hybrid device designs combine multiple micro-optics and perform multiple functions in one compact package. Other micro-optic components have been designed with input and output fibres on the same side of the package to further minimise the space required for fibre management. These hybrid and single-sided passive components (Figure 4) greatly facilitate integration within compact DWDM transceivers.
Advances in pump laser and micro-optics packaging technologies are necessary but not sufficient to shrink transceiver-embedded EDFAs to their smallest possible size. Transceiver-embedded EDFAs must fit within transceiver modules. Therefore, EDFA assemblies must be designed without enclosures. An effective solution recently introduced employs flexible substrates to simultaneously hold components, erbium doped fibre spools and fibre splices. These flexible substrates enable EDFAs to be custom fit to the narrow spaces within DWDM transceiver modules. II-VI’s FlexSOM EDFA platform is an example of an EDFA design leveraging such flexible substrates (Figure5).
Optical amplifiers were originally designed to amplify a band of DWDM channels and have been part of what is known as “common equipment”, which is required for initial turn up. In contrast, DWDM transceivers follow a “pay as you grow” model with longer deployment cycles. The increasing implementation of amplifiers into transceivers represents an exciting new opportunity as it expands the addressable market for amplifiers and sustains it over longer demand cycles.