Optical communications: Supporting colossal growth with superior lasers

As lasers with traditional architectures are helping to deliver ever higher data rates, newer designs with even more impressive characteristics are promising to gain traction.

BY RICHARD STEVENSON, EDITOR, CS MAGAZINE

There are times when anecdotal evidence obscures the big picture. That, though, is rarely the case. Take the youth of today – they have little interest in watching TV, but when the mood takes them they devour hour-after-hour of video clips on their devices. So, based on this observation, it is anything but surprising when market analysts forecast that the global transfer of data is on a continuous rise that shows no sign of abating.

Supporting the watching habits of the young, as well as the lives of all of us, is a rather complex infrastructure that enables the transmission of an astonishing level of data, involving systems working in both the RF and the optical domain. The capability of both is on the rise. For the RF, there’s the roll-out of 5G, while capacity of optical communication is increasing through the combination of sources with faster data rates and the laying of more fibre.

Like many of its predecessors, at this year’s European Conference on Optical Communication (ECOC) many speakers described efforts to hike data rates with optical technologies. This included the use of more sophisticated modulation schemes for propagating light through an optical fibre – a collaboration between researchers in Japan and Europe unveiled communication at an eye-watering 22.9 Pbit/s by extreme space-wavelength multiplexing – as well as increases in the modulation rate of many forms of laser. And in addition to the traditional approach to data transfer, involving optical fibre, a number of forms of free-space communication were discussed. They included transmission of data from Earth to satellites with 100 W sources emitting at around 1.55 µm, the use of blue and green lasers for underwater communication, and turning to light for the final link in homes and offices. Note that in many applications, one of the key benefits of switching from the RF to the optical is the far higher bandwidth.

Speaking in the Market Focus session, Coherent’s CEO Julie Eng gave an excellent overview of the deployment of various forms of laser in optical communications.

At the heart of all these forms of optical communication is the laser. In some cases, this device is directly modulated to deliver the data, and at other times it’s combined with some form of modulator.

Those attending ECOC 2023 got to hear a great deal about this vital source of light. In the Market Focus session, held in the exhibition hall, volumes of various forms of laser used in optical communication were discussed by Julie Eng, CTO of Coherent; and Gunter Larish, Product Manager Datacom at Trumpf, spoke about VCSELs for interconnects at more than 100 Gbit/s. Meanwhile, in the conference sessions, Chang Ge from Tokyo University of Technology claimed a record-breaking speed for 1060 nm VCSELs with a metal aperture; and Takuro Fujii from NTT Device Technology Labs, Japan, outlined efforts at developing a directly modulated membrane laser array on silicon. Another novel laser taking prominence at ECOC was the PCSEL, short for the photonic crystal surface emitting laser. Its trailblazer, Vector Photonics, discussed the capabilities of this source in the Market Focus session, the conference, and an off-site meeting held at the company’s headquarters – this is just a short drive from the Scottish Exhibition Centre, the location for this year’s ECOC.

A Coherent overview
One of the biggest suppliers of lasers to the optical communications market is Coherent. Speaking on its behalf, Eng explained that sales of transceivers are undergoing tremendous growth, while life cycles are typically just 2-3 years. More than half of the products sold today are operating at 200 Gbit/s or more, a sector that will be worth $10 billion by 2028.

The Coherent CTO also offered an insight into the types of materials that dominate, and where they are used. She explained that InP dominates optical communications, accounting for around three-quarters of the market. Of the remainder, GaAs has the largest share, with silicon photonics responsible for just a few percent.

According to Eng, GaAs lasers, in the form of the VCSEL, are widely used in short-reach links of less than 100 m. Meanwhile, technologies based on InP and silicon photonics are deployed for transmission over 500 m to 10 km, and also at more than 10 km, where narrow laser linewidths are needed.

Eng remarked that the GaAs VCSEL is only capable of low powers, but at low cost, and its sales will benefit from the rise in AI and machine learning. Modulation speeds for this class of VCSEL are increasing, with 1 x 4 and 1 x 8 arrays providing 400 Gbit/s and 800 Gbit/s sources. There is now much interest in a 200 Gbit/s VCSEL. “We and the rest of the industry will be working on that,” added Eng.

For silicon photonics, which offers modulation and routing, there’s a need to combine this chip with a high-power InP laser. Options include 100 mW uncooled and 200 mW cooled sources, according to Eng.

Similar capabilities may be provided with electro-modulated lasers, produced using monolithic integration. Coherent has demonstrated 200 Gbit/s per lane, using a source with 7 dBm (5 mW) of power that’s claimed to be suitable for transmitting data over distances from 500 m to a few kilometres.

For longer distances, Eng advocates combining an InP laser emitting in CW with a Mach-Zehnder modulator, a pairing that offers 200 Gbit/s per lane. She encouraged delegates to go to a live demo of this at the Coherent booth, where data transfer took place over 6 km of fibre.

VCSEL virtues
VCSELs are now a high-volume product, with billions shipped every year, partly due to their incorporation in cell phones. The performance of this class of laser continues to advance, with the VCSELs of tomorrow expected to combine an increased channel capacity with reduced power consumption and enhanced functionality, according to Larish.

The spokesman for Trumpf told those attending the Market Focus session that his company has been developing VCSELs for many years, and is currently working on increasing the spectral range of this device beyond 850 nm to 880 nm, 910 nm, 940 nm, 980 nm and 1060 nm, for sources operating at 100 Gbit/s. Larish added that the company is already shipping VCSELs with data rates of up to 56 Gbit/s.

Significant progress in combining higher speeds with longer wavelengths, which increase the transmission distance and minimise dispersion, was claimed by Ge, who has been working with colleagues at the University of Tokyo and Ambition Photonics. This team has been advancing VCSELs with a metal aperture, a design that is said to increase bandwidth and improve temperature stability (see Figure 1).