In our May issue, OPN brought you a feature article on scanned laser pico-projectors for mobile devices. On Thursday at CLEO/IQEC and PhAST, attendees learned about the opposite end of the projection industry: a display so big and detailed that its manufacturer had to devise its own laser.
How “big” is this display? About as big as the four walls and ceiling of one of the session rooms at the Baltimore Convention Center, according to Forrest L. Williams, a scientist at Evans & Sutherland (Salt Lake City, Utah, U.S.A.). The company specializes in digital planetariums and digital cinema projects – setups that dwarf even the most ambitious home theater gadgets.
To match the resolution of the human eye, a pixel should be about 1 minute of arc (arcmin) in angular size. But with large-field formats approaching 1,000 square meters, the display would be require tens of megapixels.
For decades, anyone who wanted to design a “synthetic environment” – to make planetarium-goers feel as if they’re standing in the middle of the Stonehenge circle, for example – had to rely on a multiplicity of separate projectors with overlapping images. Each projector has slightly different optical characteristics, leading to image distortion.
Williams’ company wanted to design a single large-field projector with an 8,192 x 4,096 display field, or 33.2 million pixels. (By contrast, your HDTV set at home has a display of 1,920 x 1,080 or 2.1 million pixels, and Sony’s SXRD technology uses 4,096 x 2,160 or 8.8 million pixels.)
The system uses scanned-column projection – instead of projecting one image all at once, it projects a narrow column of pixels back and forth very fast, and the persistence of human vision creates the appearance of a complete image.
Using regular lamps in such a huge projector would require many kilowatts of power, perhaps even megawatts, Williams said. Around the year 2000, when the company was initially designing large digital projectors, the team looked for high-power (greater than 5 W) lasers with cw output in red, green and blue – but they couldn’t find anything had enough power or efficiency.
The company ultimately designed and patented a laser system that starts with a “seed” 1,064- or 1,550-nm distributed feedback fiber laser. With a phase modulator, free-space resonator and nonlinear crystal, the laser puts out red, green or blue light from a box about 56 by 36 cm in size and a conversion efficiency approaching 90 percent for green light.
The next challenge is to extend the life expectancies of these lasers from six months (blue) and two-plus years (red) to 10 years by incorporating new materials into the components.
The laser projector also incorporates a grating light valve (GLV), which acts as an analog light modulator. The GLV is a micro-electrical-mechanical systems (MEMS) device with two microscopic silicon nitride ribbons per pixel, and three pixels per device. One remains stationary and the other moves up and down. When the ribbons are out of alignment, the GLV acts as a tiny square-well diffraction grating. Each device is about the size of a human red blood cell, Williams said.
The 33-megapixel projectors are scheduled for release in the second half of 2009. In the next few years, Evans & Sutherland may offer these high-efficiency compact visible lasers as a standalone product.
At another Thursday session at CLEO, Stuart J. McNaught of Northrop Grumman Corp. (Redondo Beach, Calif., U.S.A.) described the architecture of the first solid-state laser to break the 100-kW power barrier, which the company built for a U.S.-military-funded program.
The device consists of seven modular “laser chains” that are coherently combined with active phase locking into a single-aperture output beam. (Passive phase-locking methods don’t scale well to large numbers of beams, according to McNaught.) Each chain contains a 15.8-kW wavefront-corrected master-oscillator-power-amplifier (MOPA) laser, and the seven chains fit together in a single cabinet.
So far this year, McNaught and colleagues have completed 48 tests of the laser system with more than 2.75 hours of accumulated run time in the laboratory, and they hope to begin field tests soon.
CLEO/IQEC wrapped up today (Friday, June 5) with the final sessions on biomedical microscopy, surface plasmon polaritons, fiber lasers and other topics. However, we at OPN still welcome your comments on this series of blog posts. We moderate comments to thwart spammers, but please don’t let that deter you.
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