1. Red Laser with a Short
635 nm Wavelength |
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There are many shades of red, from reds
that are strongly tinged with orange to the
deep red of blood. Figure 1 shows the graph
of the human luminosity factor, which is
human visual sensitivity as a function of
wavelength. Luminosity factor is an index
of how we perceive brightness for light of
different colors, and for the human eyes, a
green color with a wavelength of 555 nm
is perceived as the brightest. Taking this
color as the center, the shorter wavelengths
to the left are blue colors, and in the other
directions, the longer wavelengths to the
right are red colors. As can be seen from
the graph in figure 1, luminosity factor
for both reds and blues falls relative to the
peak of 555 nm, which is a green color,
and these colors are seen as less bright
than green. For example, of the laser
pointers used in meetings, green pointers
appear brighter than red pointers. This is
due to the difference in luminosity factor.
Even when we just consider the red area,
the difference with wavelength is clear.
Although both 680 nm and 640 nm are
both red light, the shorter wavelength, 640
nm, is perceived as brighter.
Sony developed a 645 nm red laser diode in preparation for Aichi Expo 2005. Sony
has now developed a new red laser diode
that is 1.6 times brighter by reducing the
wavelength by 10 nm to 635 nm. |
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| 2. Success in Achieving 7 W: the Industry's Highest Optical Output *1 |
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Even higher power is seen as necessary
for use as a light source in display
applications. This value is another aspect
that determines display brightness. Sony
succeeded in increasing the optical power
output from the 4.4 W of the device used at
Aichi Expo 2005 to the 7 W of the current
device. This output corresponds to an
optical power output about 1000 times that
of the red laser diodes commonly used for
DVD playback.
Increasing the optical power output is
not the only thing that is important in
achieving higher power. When acquiring
a high optical power output by insertion of
a large current, the heat generated in the
active region becomes enormous. While
this is related to the high-temperature
operation described in the next section,
how efficiently this high optical power
output is acquired is a critical point.
Sony achieved optical power output at a
high efficiency by using the following 5 technologies: (1) adoption of a broad area
stripe structure, (2) adoption of a laser
array structure (see figure 2 for the above
technologies), (3) adoption of a thin-film
cladding layer, (4) conductive type control
using Si and Mg doping, and (5) high-precision
mounting (see figure 3).
*1: As of August 21, 2008 (according to a Sony survey) |
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| 3. Aiming for Stable
Operation at 35°C |
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Laser diodes also generate heat by Joule
loss, which occurs when current flows.
When a laser generates heat, that heat
increases the temperature of the light
emission area and the laser diode may
become unable to convert the inserted
current to light easily. Since current that
cannot be converted to light is converted
to heat, this heat increases the temperature
even further. When this cycle continues, at
some point the laser will become unable to
emit light. Although lasers are normally
cooled appropriately using a cooling
structure, when the limits of the structure
are exceeded, the laser falls into the above
cycle and becomes unable to emit light.
In the worst case, the laser itself may be
degraded by its own heat generation. This is
the reason that operation and reliability are often reported for the low emperatures
of 10 to 15°C in currently published
reports on the development of super
high power red laser diodes. If it were
possible to achieve operation at 35°C,
systems that require cooling could be
simplified and the amount of energy
required for cooling could be reduced.
Such a device would be extremely easy
to use as a light source in displays.
The data shown in figures 4 and 5
is for a red laser diode device with a
wavelength of 642 nm. These show
the laser output characteristics at a
variety of temperatures (figure 4) and
long-term reliability characteristics
at 35°C (figure 5). Figure 6 shows
the characteristics for the 635 nm
wavelength device currently being
developed. Sony is proceeding with
development with stable operation
at 35°C as the goal for this 635 nm
wavelength device as well. |
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| Future Issues |
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Further development efforts in this area
are still required, since short wavelengths,
high optical power outputs, and high
operating temperatures are indispensable
when using laser diodes in display light
sources. In particular, even higher
operating temperatures are desired to
use laser diodes in display light sources.
Furthermore, a reduction in image
roughness (noise in the laser speckle) and
verification of the safety of laser light are
also issues for the use of laser diodes in
display light sources.
While there are still difficult issues that
must be solved, Sony is committed to
continuing development efforts aimed at
making extremely large displays practical
for home use. |
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 Providing the Excitement of Aichi Expo 2005 in the Home |
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See
all articles with figures and tables.  |
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Vol.55 |
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