technology nitride lasers
rhombus4
Fig. 1 Osram’s The problems with nitrides
520 nm ridge If nitride lasers are to replace the more complex diode-
waveguide pumped, solid-state lasers and frequency doubled
laser, which semiconductor lasers, then researchers must overcome
has a 2 µm the challenge of routinely producing high-quality InGaN-
broad stripe quantum wells with sufficiently high indium content.
with a 600 µm Extending emission from the blue to the green demands
resonator an increase in indium content in the InGaN wells, but this
length, can is hampered by a deterioration of the thermal stability of
deliver an this layer, alongside higher compressive strain due to
optical output larger lattice mismatch [2, 3].
power of 50
mW in pulse Our team of researchers at Osram has made an important
operation. To breakthrough in this area - we were the first to break the
limit thermal 500 nm barrier [4]. Since then we have progressed to
effects the laser even longer wavelengths, including the realization of
was measured The other form of commercial green laser features either 515nm and 520 nm pulsed laser operation from broad,
in pulse mode intra-cavity or extra-cavity frequency doubling. Both gain-guided test laser structures [5, 10]. Other
with a duty variants deliver fast switching times thanks to a 1060 nm, researchers have also enjoyed success, such as Nichia,
cycle of 1 GaAs-based semiconductor laser source that is either which has produced 8 mW continuous-wave operation at
percent and a electrically or optically pumped. Stable laser output over 515 nm from small ridge waveguide lasers.
pulse length of the operating temperature range results from maintaining
1 µs the semiconductor emission at the frequency conversion Our lasers, and those developed by Nichia, are produced
wavelength. by growth on the c-plane of gallium nitride. One weakness
of this approach is that high internal piezoelectric fields in
This can be realized by adding a DBR structure to the the polar growth direction hamper the device
electrically pumped laser, or using filter elements for the performance. These fields can be either minimized or
optical pumped laser. For intra cavity frequency doubling a eliminated by turning to semi-polar and non-polar planes,
periodically poled LiNbO
3
bulk crystal can be used. For respectively. Rohm has adopted this approach, and
the conversion outside of the cavity a several mm-long reported the longest lasing wavelengths for a laser on a
periodically poled SHG-crystal with additional narrow non-polar m-plane GaN-substrate. It has developed a
channel waveguide structure is used in order to reach the 499.8 nm laser with a very high junction temperature and
necessary power density for efficient laser light 97% out-coupling mirror reflectivity [7]. Progress has also
conversion. been realized by Sumitomo, which announced a 531nm
broad gain guided test laser in summer 2009 that was
Intra-cavity and extra-cavity approaches can produce MHz driven in pulsed operation and grown on the semi-polar
modulations speeds and operate over a wide temperature plane [2021]. Later that year this company reported
range. However, complex driving electronics are needed continuous-wave operation up to 2mW from small ridge
to drive these lasers efficiently. waveguide laser at 520nm on [2021]-plane [9].
Regardless of the growth plane, producing high material
quality with a low defect density is a big challenge.
More recently we have managed to push our lasers to
even longer wavelengths, and realized 526 nm emission
from broad-area, test laser structures on c-plane GaN.
Fig. 2 Ridge Additional results include a 520 nm ridge waveguide laser
laser emission with a record optical output power of 50 mW in pulse
can support operation (Figs. 1 and 2). This optical power level is
several lasing suitable for second-generation, red-green-blue scanning
modes at projection technology that can deliver an illumination level
520nm in of about 10 lumen on the screen.
pulsed
operation at Our direct green 520 nm ridge laser was processed as a
50mW output 2 µm broad stripe with a 600 µm resonator length, and it
power at room includes natural, cleaved laser facets with dielectric mirror
temperature coatings. The epitaxial structure consists of AlGaN
22
www.compoundsemiconductor.net January/February 2010
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