Fall MRS Meeting Special Session on Industrial Applications of GaN Diode Lasers
At the Fall MRS Meeting (Boston, Dec. 1-5, 1997), a tutorial session entitled "Industrial Applications
of GaN Diode Lasers" was held.
The tutorial comprised three invited talks given by
experts in the printing, optical recording and display industries.
Ross Bringans of the Xerox Palo Alto Research Center, whose group
recently demonstrated a GaN laser, presented an overview of the printing
industry and its motivation for moving to GaN laser diodes. He
reported that the the worldwide digital printing market is expected to reach $100B by the year 2000.
Future printer technologies will be driven by the need for speed, color,
resolution, power consumption and multi-functionality (e.g. scan/fax/print
functions in a single unit). The major motivation for short wavelength
laser diode development for printers is improving the beam spot size while
maintaining an adequate depth of field (DOF) and the optical aperture.
Dr.
Bringans gave the example of a mid-range 600 dpi laser printer operating at
60 pages per minute (ppm), and observed that present 
=780 nm laser diodes in a
typical optical system need 6 mm aperture optics to achieve the 35 µm FWHM
spot size required in the scanning direction. For true 1200
dpi (17 µm spot size), =780 nm lasers would require more expensive and
bulky 12 mm aperture optics and larger polygon scanners, while producing a
depth of field of only 0.5 mm. A submillimeter depth of field requires a more costly mechanical
system capable of maintaining the photoreceptor surface within the laser
focus. Alternatively, a =390 nm GaN laser diode could operate at 1200 dpi with a 1
mm depth of field using 6 mm optics.
The performance requirements for the laser diode are modest, not far from
what has been demonstrated in the laboratory.
To discharge the photoreceptor at the speeds necessary for 60 ppm printing, a
GaN laser diode producing 6 mW of single mode CW output power would be required. Higher
power would enable increased printing speeds. Rise/fall times of about 1 ns are
desired to enable image enhancement. Finally, a GaN laser diode wavelength >430 nm
is preferred to avoid decomposition of commonly used organic photoconductor
materials.
Akito Iwamoto of the Toshiba
Multimedia Laboratory reviewd the development of DVD.
DVD-ROM products are now shipping in volume using
laser diodes in the 635-650 nm
range. These drives can read 0.4 µm pits separated by 0.74
µm tracks to store a total of 4.7 GB, readable at a rate of 11 MB/sec. Dr.
Iwamoto reported that Toshiba would begin shipping 2 GB DVD-RAM
drives based on writable phase-change media in January 1998. The Toshiba
DVD roadmap predicts 50 GB DVD-ROM and 20 GB DVD-RAM drives shipping in the
year 2005. The 7X increase in DVD-ROM storage
relative to CD-ROM has been achieved via improvements in the laser diode wavelength,
system numerical aperture and signal processing. The signal processing
contribution is roughly twice as important as the other two. Although
the recent shift from 780 to 650 nm has had a moderate effect on the
recording density, the shift to GaN laser diodes scheduled for 15 GB DVD-ROM drives
shipping in the year 2000 is expected to yield a much larger gain. These
drives will have 0.2 µm minimum pit lengths on a 0.4 µm track pitch.
The laser diode performance requirements for DVD-RAM are more difficult than those for printing.
The ideal GaN laser diode for DVD-RAM in the year 2000 would have
a 60°C CW operating point as low as 3V and 100 mA, producing a single
mode CW 30-40 mW beam with an aspect ratio <4. DVD-ROM has reduced beam
power requirements of 3-5 mW, but otherwise similar specifications.
Ultraviolet absorption in the glass substrate becomes an issue with GaN
laser diodes, resulting in an optimal wavelength of 400-430 nm.
Shorter wavelengths would have an optical transmission of less than
80%.
Robert Melcher of the IBM Watson Research
Laboratory discussed the impact of GaN laser diodes on future projection display
technology. The upshot of his talk was that displays are a potentially important
application of GaN laser diodes, if some major problems can be overcome.
The major issue is cost. Present day halide lamp technology
provides efficient (60 L/W) white light for $100-300 including power
supply. Consequently, displays based on gas and YAG laser technologies are
shipping in low volume in only the extreme high end of the display market.
The recently announced IBM reflective
LCD/Si projection display is a system in which the potential advantages of replacing
the halide lamp with laser diodes can be seen. In the present system,
the white light is divided into
primary colors using dichroic mirrors. Because liquid crystal spatial light
modulators (SLM) are used, the beams must be polarize. This results in
additional losses and system complexity. After reflecting off the SLMs, the three
images are recombined in a prism element and projected onto the viewing
screen. Although the
result is an industry leading 2048x2048 pixel projected color image,
the optics of collimating, splitting, polarizing and recombining
three broadband primary color beams is elaborate and costly.
If the lamp could be replaced by three diode lasers, the packaging
would immediately benefit from a simplified power supply and cooling
system, since laser diodes operate at higher overall efficiency and require
only a few volts (vs. kilovolts for halide lamps). Polarized, monochromatic
laser light eliminates the need for polarizing optics, eases the
specifications on the other optical elements and SLMs, and makes it easier
to filter out ambient light. If the laser diodes were pulsed in rapid succession, a
single, simplified optical system could replace the three beam approach of
the current system and a more saturated image spanning a broader
color gamut could be
built up from overlaid red, green and blue images using a scheme called
field sequential color.
Melcher sketched out the specifications of
a hypothetical projection TV based on 656, 532 and 457 nm laser diodes operating at a 6500K white
point color balance. To achieve a screen brightness of 500 Cd/m2 over a 55"
diagonal 16:9 aspect ratio screen, and assuming 33% optical efficiency and a 3X
screen gain, the laser diodes must produce 6.6 W, 1.8 W, and 1.2 W in the red, green
and blue, respectively. Existing broad area red laser diode arrays readily achieve
tens of Watts. If the blue and green wavelengths are to be provided by laser diodes,
GaN laser diodes must be extended to longer wavelengths and their output power
drastically improved.
Although Nichia has supplied blue ~450 nm and green ~525 nm GaN LEDs since
1995, extending laser diodes wavelengths into the visible will be more difficult
due to more stringent quality requirements for the InGaN active layer in lasers.
The high power requirements are also a formidible challenge.
Once Watt-class blue GaN laser diodes are
available, perhaps by 2000-2001?, some high end projection display platforms
might begin to migrate to red and blue laser diodes, while using diode-pumped,
doubled YAG technology to supply the green.
Toby Strite and Eric Hellman
last updated December 31, 1997 11:57:07 AM.
© 1997 The Materials Research Society
