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Nitrel Ion Beam Treatment Increases Laser Lifetimes

Swedish start-up Comlase has developed an ingenious technique for protecting the surface of semiconductor laser chip facets. Oliver Graydon reports on the company's Nitrel treatment, which promises to boost the durability, output power and reliability of all types of laser diode.
A small Swedish firm believes that it has found a way to fabricate semiconductor lasers that can be driven, without failing, at much higher temperatures and output powers than ever before. If the claim of Stockholm-based Comlase turns out to be true, it could have dramatic consequences in fields ranging from data storage and telecoms to fiber pumping and laser welding.



Until now, the maximum output power and temperature of operation of high-power laser
diodes has been severely limited by the onset of a failure mechanism called catastrophic
optical mirror damage (COMD). This irreversible thermal damage to the surface of the
laser chip facets, which act as mirrors, is often caused by facet oxidation or
manufacturing defects that absorb light and act as "hotspots". It is the principal reason for
the poor manufacturing yield of high-power lasers.



Today, edge-emitting laser diodes are made by cleaving a semiconductor wafer into
individual chips. The cleaved ends are then covered with thin-film coatings - one end
with a high-reflectivity (HR) coating, the other end, antireflection (AR) - to form the
laser's rear mirror and front output coupler.



Dangling bonds

The problem tends to be that, after cleaving, the exposed facets are highly chemically
reactive because the bonds of the atoms in the surface layer have been physically torn
apart, creating so-called "dangling bonds". On contact with even low concentrations of
moisture or oxygen, these areas oxidize to create a light-absorbing hotspot that can lead
to COMD.

Although the conventional thin-film coatings that are applied to the facets do offer some
protection against moisture and oxygen, they are not a durable solution. For example,
aluminum oxide, which is often used as a coating, is relatively permeable to water and is
prone to moisture ingress over time.

To date, approaches to overcoming the problem have focused on depositing a passivation
layer, such as silicon (in Bookham's E2 process), on top of the raw facets to protect them
prior to depositing the thin-film reflection coatings. However, this process is only
effective at certain wavelengths.

"Applying a thin layer of silicon works very well for lasers operating at 900-980 nm, but
as soon as you go to shorter wavelengths, you get negative effects," explained Alfred
Feitisch, Comlase's CEO. "Our "˜Nitrel' passivation process is applicable to any
semiconductor material - going all the way from GaN in the blue to the infrared."

The Comlase Nitrel process uses nitrogen to tidy up all the dangling bonds on the raw
facet surface so that they cannot oxidize and act as a catalyst for COMD. Conventional
HR and AR thin-film coatings are then applied to the treated facets.

"What Comlase does is atomically seal the surface by taking off the oxygen and
substituting it with nitrogen atoms," said Feitisch. "This renders the surface chemically
stable so that it cannot react."

The process is performed in a specially designed ultra-high-vacuum chamber (reactor)
that features two electron guns and an ion gun for bombarding the facets of the laser with
a stream of nitrogen ions. The nitrogen ions polish the surface smooth, remove any
oxides and then seal it with a nitride layer. The electron guns are then used to deposit the
HR and AR thin-film coatings onto the rear and front facets by e-beam evaporation.

"The reactor has a carousel that puts a stack of 50 laser bars in front of the ion gun at a
time. We then flip them over and do the reverse side. Then we lay down the HR and AR
coatings," said Feitisch. "We are now building a new high-volume chamber that will
have at least three electron guns and will be able to process 800 laser bars in a single
run." Heading up this development is Olof Sahlén, Comlase's new vice-president of engineering.

Improved degradation

The benefits of the process are substantial, according to lifetime test data collected by
Comlase. It says that multimode 805 nm AlInGaAs laser diodes fabricated with the Nitrel
process showed no degradation in performance over a period of 9000 h when driven at
60-80 W with a junction temperature of 90°C. In comparison, untreated diodes driven
under the same conditions degraded rapidly, with three-quarters of the batch failing
before reaching 6000 h.

Tests with InGaAs diodes at the longer wavelength of 980 nm also showed great
improvements in performance. "We've put them on life tests at 180 mW/μm [emitter
width] and seen no degradation over thousands of hours," said Feitisch. "In short, what
this means is that if you take a 100 μm-wide single-chip emitter, [it] would be a reliable
18 W laser chip, which is enormously important for pumping high-power fiber lasers."

As far as applications for the process are concerned, Feitisch says that it would be ideal
for making high-power infrared (IR) lasers for pumping solid-state lasers, such as the US
military's proposed mobile 100 kW laser weapon. The ability to run diode lasers at much
higher temperatures (90°C, instead of room temperature) and higher output powers
should dramatically simplify the cooling requirements for the pumps and minimize the
number that are needed.

"In theory, you could run everything on a car-engine cooler, rather than using
refrigeration equipment," explained Feitisch. "We have set up a US subsidiary in
Delaware so that we can deal more easily with the US military and government."


Fiber lasers and thin-disc lasers could also benefit from diodes with improved reliability
and power levels. Ultimately, the technology could result in bright, compact diode bars
that are ideal for performing direct diode welding and surface treatment.

The improved reliability of the lasers could also be a big benefit for other mission-critical
applications. "We are in talks with the European Space Agency to design and qualify a
special laser bar for spaceborne applications, and hope to have a project started next
year," said Feitisch. "After all, if a laser fails on a satellite in space, it's a big problem."


Other potential applications include raising the output power of blue and red lasers used
in the optical storage industry. The result could be faster writing and reading of data onto
CD-ROMs, DVD and BluRay discs. Comlase says that it has already received enquiries
from Sony and Hitachi, who are both interested in the technology.

Although the company is initially focusing on applying its process to semiconductor
lasers, Feitisch says that there is no reason why the Nitrel process couldn't be applied to
semiconductor photodetectors, such as avalanche photodiodes (APDs) and pin diodes, to
help reduce their noise. "These devices have dark current [noise] issues that our process
should be able to eliminate," said Feitisch. "Nitride is an electrical insulator and can
prevent electrons from flowing over the surface of these devices, which causes the dark
current. We want to investigate this idea further."

In the case of APDs, the Nitrel process allegedly lowers the risk of voltage breakdown
over the junction region, thereby reducing the likelihood of device failure. And that's not
all - Feitisch says that the process could potentially be used to protect glass components
such as fibers and crystals. "For example, you could treat the end of optical fibers before
laying down an AR coating, in order to get extremely good adhesion and remove any
surface imperfections."

Another possibility is treating borate crystals, which are used for nonlinear optics, but are
very sensitive to moisture (hygroscopic). By creating a thin, protective film of nitride on
the surface of a crystal, it could be protected from water vapor.

Comlase has no shortage of possibilities for its technology, but with a staff of just 20, the
firm is having a tough time deciding which applications to pursue first. "Before we
explore other opportunities, we need to get some traction with laser manufacturers and
secure revenue," said Feitisch. No firms have licensed the Nitrel process yet, but he says
that it is currently being trialed by three parties and he is optimistic about the outcome.



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