Components
Biaxial MEMS scanner
The biaxial MEMS scanner is made
using standard bulk silicon MEMS
fabrication processes. The WVGA
pico-projector scanner has a scan mirror
diameter of approximately 1 mm, and
it produces an active video scan cone of
43. 2° by 24. 3°.
The scanner uses moving-coil actuation with a single drive coil, which can
be seen on the vertical scan frame in the
photo with just two drive lines as shown.
The single coil design simplifies the
fabrication of the MEMS scanner and
reduces the number of required interconnects. The MEMS die is housed in a
package with small magnets that provide
a magnetic field oriented at approximately
45° to the scan axes. A single composite
drive signal is applied that contains the
superposition of the fast-scan horizontal
drive at the resonant frequency of the
horizontal mirror motion and the 60-Hz
vertical drive sawtooth waveform.
The mechanical design of the MEMS
scanner allows motion along only the two
orthogonal scan directions. Mechanical
filtering, resulting from the different mass
and flexure stiffness governing horizontal
and vertical motion, sorts the drive signals by frequency content, inducing the
18-KHz resonant motion of the horizontal axis and the 60-Hz sawtooth motion
of the vertical axis. Piezo-resistive sensors
provide scan mirror position feedback to
the MEMS controller ASIC to maintain
closed loop accuracy of the desired scan
mirror motion.
Lasers
The technology for the red and blue
lasers in the PicoP leverages the technology of similar lasers that are used for
the optical disk storage industry. The
wavelength requirements are shifted
somewhat, but the basic technology is
the same—Ga AlInP red laser diodes
and GaN blue laser diodes.
Pico-projectors incorporate green
lasers as well. Prior to the push for
laser projectors, green lasers had not
been used for any similar high-volume
applications. The technology for the
[ Bi-axial MEMS scanner ]
Vertical scan
frame with
drive coils
Piezo
resistive
sensor
Only two
drive
lines
Scan
mirror
green laser in the PicoP is based on
infrared lasers developed for the telecom
industry, the other massive market for
laser technology. Robust near-infrared
laser diodes with very high modulation bandwidths are combined with
a frequency-doubling crystal, usually
periodically poled lithium niobate, to
produce a green laser that can be directly
modulated. With an eye toward the
burgeoning pico-projector industry, several companies, including Corning and
OSRAM, are ramping up production of
suitable green lasers.
The choice of which wavelength to
use for the lasers is based on two considerations. First is the response of the
human eye (the photopic response) to
different wavelengths. This response is a
somewhat Gaussian-looking curve that
peaks in the green-wavelength region
and falls off significantly in red and
blue. The amount of red and blue power
needed to get a white-balanced display
varies rapidly with wavelength.
For example, eye response increases
by a factor of two when the wavelength
is changed from 650 nm (the wavelength used for DVD drives) to 635 nm.
This allows the required laser power
to drop by the same factor, making a
projector that is lower power. Similarly,
the blue laser should be chosen to have
as long a wavelength as possible. Currently, blue lasers in the range of 440 to
445 nm are the best practical choice. As
the industry grows, longer wavelengths
in the range of 460 to 470 nm may
become a better option.
The second consideration is color
gamut. Since the photopic response is
peaked all through the green wavelength
range, the green wavelength should
be chosen where it will be most useful
for enhancing the color of the display.
Green lasers at 530 nm are a good choice
for maximizing the color gamut.
The ability to directly modulate the
lasers is at the heart of the scanned laser
pico-projector technology. Pixel times
at the center of the WVGA scanned
display are on the order of 20 ns. Lasers
therefore need modulation bandwidths
on the order of 100 MHz.
A path forward
The first generation of pico-projectors
are being launched this year. These
super-small, low-power projector systems
will open up the display bottleneck for
mobile devices, allowing information to
be accessed and shared more easily from
portable devices. With the high volumes
expected, there is great market opportunity for key components such as red,
green and blue lasers.
The scanned laser projector paradigm
provides a path forward to higher-resolution projectors without growth
in size. Unlike SLM-based projector
technologies—in which increased resolution means growth in the number of
pixels in the SLM—the single-pixel,
single-scan-mirror nature of the engine
remains the same, even as the resolution
of the projected display increases. t
Visit OPN online ( www.osa-opn.org) to
view videos that demonstrate the technical
concepts behind scanned laser projection and
how the MEMS scanner works.
Mark Freeman (mark_freeman@microvision.
com) is a principal engineer in optics at
Member Microvision Inc., in Redmond, Wash.,
U.S.A. Mark Champion is a principal engineer
in electronics and Sid Madhavan is a vice president of engineering at Microvision.
[ References and Resources ]
>> R. Sprague et al. “Mobile Projectors
Using Scanned beam Displays,” from
Mobile Displays, Technology and Applications, Bhowmik, Li and Bos, eds.,
John Wiley and Sons, Chapter 21, 2008.
>> W. Davis et. al. “MEMS-Based Pico
Projector Display,” Proceedings of IEEE/
LEOS Optical MEMS & Nanophotonics,
Freiburg, Germany, 2008.
