Matsushita Electric Industrial Co Ltd of Japan has prototyped a green laser with significantly higher efficiency, and announced it at the 33rd Optical Symposium held July 2008. For continuous oscillation of green laser light at 4.5W to 7W, the wall-plug efficiency (or WPE, a measure of optical output to input power) is about 20%. Other researchers agree this is excellent efficiency. The WPE of the design developed jointly with Panasonic Shikoku Electronics Co Ltd of Japan and sample-shipped was about 7% at 1.7W continuous oscillation.
Matsushita Electric
Industrial has developed a number of high-efficiency green lasers with
several watts of output optical power, for use in applications
including image display, medical electronics and analysis instruments.
The new laser is primarily intended for use in image display equipment
such as rear-projection systems. Of the three (red, blue and green)
light sources, efficiency has been most difficult to improve for the
green, and higher efficiency has been a pressing need.
The difficulty in improving efficiency lies in the fact that the wavelength of infrared laser light is changed to produce green laser light. The new device uses roughly the same procedure, injecting the 1,064nm infrared light emitted by the fiber laser into the wavelength converter to create a 532nm wavelength green light. This time, though, the efficiency of the wavelength converter is about 70%, or double that of prior art, which directly boosts WPE. The conversion efficiency of the wavelength converter used with green lasers in the past was only about 34%, for 3W continuous oscillation.
Higher efficiency made it possible for the prototype to achieve continuous oscillation at 7W with green laser light. A rear-projection system demands green laser output levels of 3W for continuous oscillation, and 6W to 9W for intermittent operation. The prototype seems to fulfill this requirement nicely.
Frequency conversion efficiency was improved by placing the conversion device between two concave mirrors (see Fig). Infrared light that has not been converted into green is reflected by the two mirrors, and injected into the conversion device again. Prior designs used no mirrors, so infrared light only passed through the wavelength conversion device once. In the new design there are at least 20 passes, increasing conversion efficiency.
Conversion devices have periodic polarity reversal structures in the longitudinal direction. In the new design, infrared light uses a different path each reflection, so the structures must also be spaced regularly in the lateral direction to ensure efficient wavelength conversion for all paths. The larger the conversion device the more difficult it is to obtain uniform reversal structures, but the company developed a manufacturing technology to make it possible even with larger profiles. The device used this time measures 25 x 10 x 1mm.
Researchers plan to continue to improve efficiency and output power. The speaker, from Matsushita Electric Industrial, said, "There is still ample room to improve optical output, and we believe we can further increase efficiency as well." No commercialization is planned yet, but few technical problems remain.
by Tadashi Nezu
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