Innolume is in the money

The company believes that its InnoComb device has the potential to take the bandwidth advantages of wavelength division multiplexing (WDM) from telecoms systems and apply them to low-cost, short-reach applications. Ultimately, this technology could provide the driver for high-speed WDM interconnects in a photonics switching layer for future multicore computer processors.

"Optical communications are more and more approaching the short-reach," said Guido Vogel, Innolume's business development manager. "Optical interconnects have clear advantages in terms of requiring lower energy, exhibiting less crosstalk and producing less heat."

Innolume has demonstrated that its devices can achieve 10 mW per channel over 16 channels or better than 1 mW per channel over 100 channels with exceptionally low relative intensity noise (RIN ~0.1%) on each lasing line.

Dotty designs

But what exactly are quantum dots anyway? As Vogel explains, quantum dots are self-organizing nanoscale regions of a crystal structure that form on a lattice-mismatched III-V semiconductor substrate during epitaxial growth. In Innolume's case the quantum dots are made from indium arsenide (InAs) grown on gallium arsenide (GaAs) with aluminium gallium arsenide (AlGaAs) barriers.

The quantum dots confine electron-hole pairs (a hole being the space in the crystal lattice where the electron used to sit before it jumped up to start conducting electricity), which then recombine to emit a photon of a particular frequency (wavelength) that depends on the size of the quantum dot. The intrinsic size distribution of the quantum dots enables a very broad lasing spectrum (>80 nm) with a highly uniform intensity.

This ability of quantum dots to confine the charge carriers also dramatically improves the lasing characteristics, reducing the threshold current and increasing the temperature stability.

What's more, the low noise on the laser lines makes the devices ideal for high-speed external modulation. In datacom, for example, the comb laser's channels can be demultiplexed, modulated externally at 10 Gbit/s or higher, and recombined onto a single physical channel.

There's a catch, or a bonus, depending on which way you look at it. While most telecoms systems operate in the C- or L-bands around 1550 nm, InnoComb operates between 1064 and 1320 nm, which encompasses the O-band (1260–1360 nm). Thus, while components for existing telecoms systems cannot be reused, InnoComb opens up a new wavelength range for transmission.

"What makes us unique is the wavelength range. The quantum dot technology allows us to shift the GaAs wavelength up to 1320 nm," said Vogel. Depending on the quantum-dot size, spatial distribution and indium content, Innolume's comb laser effectively spans the wavelength gap between devices made from GaAs (<1100 nm) and indium phosphide (>1300 nm) quantum wells.

Cost is likely to be one of the key factors in InnoComb's adoption by the market. The fact that the InnoComb device is based on conventional Fabry-Perot edge-emitter laser design means that it costs 10 times less than integrated DFB lasers, says Innolume's senior optical engineer, Dongliang Yin.

Adding to the cost, however, is the need for an external demultiplexer (typically an integrated arrayed waveguide grating) to separate the channels prior to modulation. Despite this, the InnoComb device could make life much simpler, says Vogel, by eliminating the need for suppliers to stock multiple single-wavelength lasers or expensive tunable lasers.

Innolume began shipping prototype quantities of the device to customers earlier this year and is now developing strategic partnerships with blue chip semiconductor companies to take the technology to the next stage.

However, Innolume is the first to admit that the market for computer optical interconnect devices is at least 6–9 years away, and the company is exploring other markets while it waits. High-power lasers that operate over a broad wavelength range are proving attractive to the medical market, in particular for diagnostics, imaging and therapy.

Innolume's roots can be traced back to the research group of Nobel Laureate Zhores Alferov at the Ioffe Institute in St Petersburg, Russia, from where the company has drawn many of its core technical team over the years. Originally founded as NSC Semiconductor in 2003 (later known as NL Semiconductor), Innolume renamed itself in January 2007 after acquisition of rival quantum-dot developer Zia Laser of New Mexico in the US.