Single laser delivers full color

With its pure spectral hues and extremely large depth of focus, a laser projection display is in many ways the ideal image projector. The red-green-blue (RGB) output needed for full-color image projection can be produced by diode-pumped solid-state (DPSS) lasers in combination with nonlinear optical frequency converters. Neodymium-doped lasers of various types emit at approximately 900, 1000, and 1300 nm-wavelengths well suited for frequency doubling to RGB. However, obtaining full-color output

With its pure spectral hues and extremely large depth of focus, a laser projection display is in many ways the ideal image projector. The red-green-blue (RGB) output needed for full-color image projection can be produced by diode-pumped solid-state (DPSS) lasers in combination with nonlinear optical frequency converters. Neodymium-doped lasers of various types emit at approximately 900, 1000, and 1300 nm-wavelengths well suited for frequency doubling to RGB. However, obtaining full-color output in this fashion requires the use of three separate lasers. In addition, such a system is not efficient t producing red and blue light.

Researchers at Q-Peak Inc. (Bedford, MA) have developed an RGB laser source that avoids these problems. The source is based on a lone frequency-doubled DPSS oscillator-amplifier laser that is combined with an optical parametric oscillator (OPO). When pumped with the green output of the laser, the OPO produces a signal wavelength of 896 nm and an idler wavelength of 1256 nm. The signal and idler are then frequency-doubled and combined with the unused doubled laser output, resulting in a RGB source with wavelengths of 448, 523, and 628 nm (see photo here and on cover).

The repetitively Q-switched neodymium-doped yttrium lithium fluoride (Nd:YLF) oscillator produces 10-W average power at 1047 nm, an output that is amplified by a multipass diode-pumped Nd:YLF module to 20-W average power. The 2-mJ pulses are focused into a noncritically phase-matched type-I lithium triborate (LBO) crystal to generate 12.2 W of green light. Also based on LBO, the OPO contains three of the crystals in series, all mounted in a temperature-controlled oven at 150°C to achieve noncritical phase matching for the signal and idler wavelengths. To increase the generation of red light, the researchers frequency-double the idler by inserting a noncritically phase-matched type-II LBO crystal-mounted in a temperature-controlled oven at 40°C-into the cavity, thus shifting the cavity resonance from the signal to the idler wavelength. The signal is frequency-doubled outside the cavity using a critically phase-matched LBO harmonic generator.

The resulting output includes 0.65 W of blue light and 1.66 W of red light (green light is produced in excess of requirements). The light source is currently blue-limited because of the low peak power of the pulses and unfavorable conditions for critical phase-matching. Use of noncritical phase-matching for blue generation would result in full-power color-balanced RGB output, the researchers point out. The light source converts infrared (IR) laser light to visible light with an efficiency of 44%. More important, it converts IR to color-balanced RGB light with an efficiency of 32%, producing 1500 ANSI lumens of white light. Wall-plug efficiency is 7.5 lm/W.

The source is intended for large projection displays, says Peter Moulton, Q-Peak's chief technology officer. Potential applications include flight simulators for use by the US Air Force, as well as commercial movie projectors, although Moulton notes that a theater projector would require approximately 10 W per color to be sufficiently bright. "We are developing the light source," says Moulton. "The rest of the hardware has to be developed elsewhere." There is interest in the RGB source from companies involved in commercial laser projection displays, he adds.

John Wallace

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