Red LD
Green SHG
laser
Blue LD
PicoP Integrated Photonics
Module scan engine.
MEMS
scanner
Miguel Khoury/Microvision
Scanned laser: A simple
projector design
The figure above shows the basic layout
of the pico-projector being developed by
Microvision Inc., called PicoP. The architecture is quite simple, consisting of one
red, one green, and one blue laser, each
with a lens near the laser output that collects the light from the laser and provides
a very low NA (numerical aperture) beam
at the output. The light from the three
lasers is then combined with dichroic
elements into a single white beam.
With a beamsplitter or basic fold-mirror optics, the beam is relayed onto
a biaxial MEMS scanning mirror that
scans the beam in a raster pattern. The
projected image is created by modulating
the three lasers synchronously with the
position of the scanned beam. The complete projector engine, also known as the
Integrated Photonics Module, or IPM, is
just 7 mm in height and less than 5 cc in
total volume.
The essence of the design is that,
with the exception of the scanner, the
rest of the optics engine deals with
making a single pixel. All three lasers
are driven simultaneously at the levels
needed to create the proper color mix
With pico-projectors,
you can project a
full-size image onto
whatever is near at
hand, whether it be the
wall, your shirt, or a
piece of paper.
for each pixel. This produces brilliant
images with the wide color gamut available from RGB lasers.
Direct-driving of the lasers pixel-by-pixel at just the levels required brings
good power efficiency and inherently
high contrast. The efficiency is maximized, since the lasers are only on at
the level needed for each pixel. The contrast is high because the lasers are completely off for black pixels rather than
using an SLM (spatial light modulator)
to deflect or absorb any excess intensity.
The single-pixel collection optics are
optimized to take the particular beam
properties of the red, green, or blue
laser and relay it through the scanner
and onto the screen with high efficiency
and image quality. The pixel profile is
designed to provide high resolution
and infinite focus with a smooth non-pixellated image.
However, with the simple opto-mechanical design, the electronics
design becomes more challenging. This
is because some of the display complexity is moved to the electronics. This
approach requires that engineers be able
to accurately place pixels and to modulate the laser at pixel rates.
Infinite focus
In a raster-scanned laser projector, there
is no projection lens. The projected beam
directly leaves the MEMS scanner and
creates an image on whatever surface it
is shone upon. Because of the scanned
single pixel design, light-collection
efficiency is kept high by placing the
collection lenses near the output of the
lasers, while the output beam NA is very
low. By design, the rate of expansion of
the single-pixel beam is matched to the
rate that the scanned image size grows.
As a result, the projected image is always
in focus.
