he latest addition to the family of semiconductor
laser diodes is the green laser, which is based on the
material system (Al,In)GaN. In 1996, this system
enabled the first laser diode on the short wavelength side of the
visible spectrum—405 nm, the “sweet spot” for light emitters
based on InGaN quantum wells (QWs). Since then, research-
ers have found it extremely difficult to push toward longer
wavelengths. Due to the physical and material properties of
the InGaN QWs, efficiency drops drastically as diodes move
towards green. And, unfortunately, this spectral region cannot
be reached with other III-V materials, such as AlGa As or
InGaAlP from the long wavelength side.
For this reason, the term the “green gap” has been coined
for the wavelength region around 500 nm. In 2009, however,
three companies—Osram, Nichia and Sumitomo—
simultaneously demonstrated the first true green laser diodes in the
range of 515 nm to 530 nm, where the human eye is most
sensitive. This article describes how researchers addressed the
fabrication challenges of adding green to the semiconductor
laser diode rainbow; the implications and applications of that
work; and the future outlook for the green semiconductor
laser diode market.
Why is the green semiconductor laser diode so important?
After all, frequency-doubled diode-pumped solid state (DPSS)
lasers are an affordable and rather efficient alternative, as
demonstrated by the omnipresent green laser pointer. Unfortunately, however, these complex modules are considerably more
expensive; they need active alignment to guarantee long-term
stability; they have a fixed wavelength; and they have a low
maximum modulation frequency.
Many applications rely on small, cheap, rugged and versatile
semiconductor lasers. Think of applications in biophotonics
and the life sciences, such as special microscopy techniques,
where laser diodes across the visible spectrum are needed as
excitation sources.
In the near-term, the first large-scale application for green
laser diodes is expected to be laser-based miniature projectors.
Green diodes are opening a huge market for a new class of
consumer electronic devices—just as violet laser diodes did
for the Blu-ray disc.
T
Semiconductor-based pico-projectors
Several miniature projectors are currently on the market.
Nikon offers an LED-based projector in one of its consumer
cameras, allowing users to immediately share images on any
flat white surface. Microvision manufactures laser-based projectors that are compatible in size to the iPhone; the company
coined the term “pico-projector” with this product line.
Because laser projectors have an infinite focus, they can
project images on three-dimensional surfaces. To fit into a cell
phone or PDA, the height of the projector must be limited
to a few millimeters, and the efficiency must be high enough
to be compatible with battery operation. At the same time,
[ Green laser diode ]
Green laser diode developed by
Sumitomo Electric Industries, Ltd.
The 6 Steps of Green Laser Diode Projection [ Flying spot projection ]
Illustration by Phil Saunders
4 Dichroitic mirror/prism beam combiner
5
Oscillating
MEMS-mirror
1
Analog input
data for the
three color
channels
2
Blue, green
and red
laser diodes
3
Lenses for the
collimation of
the laser beams 6
Projected image
The six steps of the green laser diode projection.
high-pixel resolution and brightness are desirable, as they
would be for any good projector.
However, there is a fundamental optical quantity limiting
the miniaturization of light projectors—the étendue, or the
Huygens-Helmholtz invariant of an optical system. This product of area and the solid angle of a light beam is a constant,
which may only increase along the optical beam path, much
like the Heisenberg uncertainty principle of a wave package. In
a projector based on an LED and digital micromirror device
(DMD), the étendue is given by the area of a single mirror and
half of the tilt angle between the “on” and “off” states of the
tiny mirrors in the DMD array.
Increasing the resolution—i.e., the number of mirrors—
and reducing the size of the array will reduce the étendue.
Since the étendue of the LED, which is a Lambertian emitter,
must match that of the DMD, a fundamental limit is set to the
LED chip size and therefore the brightness of an LED-based
pico-projector.
For the laser diode—with its nearly diffraction-limited
beam—this restriction does not apply. For this reason, true
pico-projectors, which fit into a cell phone or PDA, must be
realized as laser projectors, in which red, green and blue lasers
can be used in combination with a single moveable mirror
for a flying spot projection of the image. For the three-color
