At last laser diodes are available in all the
light source colors needed in a projector: red, green and blue.
Sumitomo Electric Industries Ltd of Japan has achieved pulse
oscillation with green laser light at 531nm (Fig 1). Red and blue laser
diodes are already available on the market, but there hasn't been a
laser diode capable of directly lasing at about 530nm for green laser
light. Instead, 1064nm red laser light was wavelength-converted with
second harmonic generation (SHG) crystals to produce 532nm green laser
light.
Successful lasing of green laser light makes frequency conversion and related processing unnecessary, and will probably mean lower cost for laser-driven projectors, along with reduced power consumption and smaller size.
The green laser diode developed by Sumitomo Electric Industries has an InGaN active layer, just like the blue-violet laser diodes used in Blu-ray Disc. The difference is the growth plane, which is a GaN crystal semi-polar plane, and directly responsible for making the green light possible.
InGaN laser diodes normally use the c-plane, the GaN crystal polar plane, as the growth plane, with crystal growth along the axis defined by its normal direction. The semi-polar plane used, at an angle to the c-plane, is the {2021} plane. Using a semi-polar plane, or the non-polar plane perpendicular to the c-plane, weakens the effects of the piezoelectric field, in theory making it easier to lengthen the wavelength of an InGaN laser diode from blue toward green.
The laser diode cannot be immediately commercialized, though, because there is still a major problem with laser diodes using GaN semi- or non-polar planes: the GaN wafer.
GaN wafers that can be used with semi- or non-polar growth planes are still in the research and development stage, making them small and expensive. Even the largest, according to one university professor familiar with GaN crystal, is only "... about 10 x 30mm, and costs tens of thousands of yen." The c-plane GaN wafers used for blue-violet laser diodes, on the other hand, cost hundreds of thousands of yen but are available in diameters up to about two inches.
Research groups around the world are working on manufacturing large GaN crystals. In Japan, a group - including Tohoku University and Mitsubishi Chemical Corp, both of Japan - is using a hydrothermal synthesis technique called the "ammonothermal method" in an attempt to grow GaN crystals as large as silicon. These crystals could then be sliced to make large-diameter semi- and non-polar plane wafers.
The same professor mentioned above adds that these large-diameter GaN crystals will not be available commercially until at least 2013. Volume production of green laser diodes using semi- and non-polar planes seems unlikely until then.
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