INDUSTRY LASERS
efficiency in data centers. Deployment of this technology is underpinned by the efforts of researchers, who are devising ways to slash the cost of large-scale silicon photonics development.
A key component in any photonic circuit is the light source. This tends to be an InP laser, because its spectral range is transparent to silicon. Today, three formats of this device are being used for this application: the cleaved facet laser, the hybrid silicon laser and the etched facet laser.
By far the most mature of these three is that based on a cleaved facet – chipmakers have manufactured hundreds of millions of them. As its name suggests, this type of laser is produced by cleaving an epiwafer to form an atomically flat surface. During the production process, engineers make edge- emitting lasers by cleaving wafers into bars, applying mirror coatings to the facets and then separating the bars into discrete chips. These are often hermetically sealed to ensure reliable operation.
One company that has fabricated a cleaved facet laser in a hermetic enclosure for use with silicon photonics chips is Luxtera. Its package, which also features a ball lens and a reflector, provides a reliable source of light that can be coupled into the silicon photonics chip.
An alternative light source, the hybrid silicon laser, is formed using a glass glue to fuse an InP gain chip to silicon. This relatively new approach, which has been championed by Intel for silicon photonics applications, uses the InP-based structure for light generation and amplification, with the laser cavity formed via the hybrid integration of the InP-based structure to the silicon-based waveguide.
The third approach – etched facet technology – has been pioneered by our team at BinOptics of Ithaca, NY. Its merits include the definition of facets through high-precision photolithography, rather than imprecise, hit-or-miss, mechanical cleaving. This ensures unprecedented uniformity and yield, as well as the capability to build structures that cannot be made with conventional techniques. Since our founding in 2000, we have fabricated over 50 million etched-facet lasers with this technology.
Figure 2. BinOptics’ horizontal cavity surface emitting laser (HCSEL) features an etched facet at an angle off from 45°. This ensures that the laser beam emerges at an angle to the gratings on the silicon photonics chip. The HCSEL has its electrical contacts on its surface and is flip-chip mounted onto the silicon photonics chip.
Ticking the boxes For wide-scale adoption in silicon photonics, there are three key requirements for the light source: it must be available as known good die; it has to survive without a hermetic package; and it must be able to be passively aligned to the silicon photonics chip, rather than requiring active alignment.
To meet the known good die criteria, the laser must deliver a high level of performance, even at temperature extremes. Device performance is evaluated by measuring light-current-voltage characteristics and spectral profiles at various temperatures. Benchmarking may include a certain threshold current for the laser to start lasing; a minimum light output at a certain current; and a side-mode-suppression-ratio – extracted from the spectral measurements – that exceeds a minimum value.
One of the weaknesses of a silicon hybrid laser is that the InP chip cannot be assessed for whether it conforms to known- good-die criteria until after it has been integrated with the silicon platform. If a bad chip is to blame for a faulty final package, it is costly, if not impossible, to replace this sub-standard gain chip.
Applying a hermetic seal is a common approach to increasing the reliability of an InP laser. But this is not an option for the entire silicon photonics circuit, due to cost and size requirements. To reap the size benefits that silicon photonics offer, the InP laser must be flip-chip mounted to the silicon photonics chip or wafer. And if the InP chip normally requires a hermetic package, this has to be applied to the integrated package of the InP chip and silicon photonics chip — and that would create a component that would be too expensive and bulky for most applications of silicon photonics. So, given all this, it is easy to see why InP light sources that do not require a hermetic package are very attractive for InP photonic applications.
Hermetic sealing is widely used with cleaved facet edge- emitting lasers, which typically feature a waveguide between the front and back facet to ensure the guiding of the light within the semiconductor. Deposition of dielectrics aids waveguiding and can tune the reflectivity of the facets, leading, for example, to a highly reflective back facet and a low reflectivity front facet.
Figure 1. BinOptics facet-etching technology enables passive alignment of the laser to the waveguide. This is possible because the location of the InP chip relative to an alignment mark or a fuducial can be known to within 0.1 µm
Degradation can occur in this class of laser if it is housed in a non-hermetic environment. If there is a discontinuity between the dielectrics on the facets and the waveguide then, over time, moisture will penetrate through this discontinuity and compromise the quality of the semiconductor. And if the dielectric is permeable to moisture, this will be absorbed,
March 2014
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