White Light from a Near-UV Laser Diode

Anne L. Fischer

Techniques for producing white light from LED methods vary, but most involve mixing blue LED light with yellow phosphor to achieve what the human eye perceives as white. However, the white light is not stable, and its luminous efficacy decreases as forward-bias current increases.

A group from the Institute of Semiconductors at the Chinese Academy of Sciences, from the General Research Institute for Nonferrous Metals and from Grirem Advanced Materials Co. Ltd., all in Beijing, has produced stable white light by combining a laser diode with blue and yellow phosphors.

This figure shows the chromaticity coordinates of the fabricated white light in a CIE1931 color diagram. The X,Y label is the CIE coordinate of any color light, and the black “+” indicates the measured coordinates of the fabricated white light.

The researchers had employed near-ultraviolet wavelengths before and knew of work that had been done with laser diodes at Nichia Corp. in Tokushima, Japan, so they tried to further the idea by using near-UV wavelengths as the excitation source to see whether there were some unique characteristics in the white light it generated.

Human eyes are not as sensitive to light with 0.5 lm/W at 405 nm as they are to 683 lm/W at 555 nm; therefore, white light generated using this method is composed mostly of the blue and yellow phosphor light.

Using GaN-based laser diodes and blue/yellow phosphors, the researchers found that peak wavelength and full-width half-maximum of blue and yellow phosphor did not change with the current, so the parameters of the white light did not change with the injection current. This indicated that the fabricated white light is very stable. They also found that, as they increased the injection current, the luminous flux and efficacy also increased. There was no change in color temperature, in color-rendering index or in CIE coordinates.

One of the downsides to this method is the price of GaN-based laser diodes, which is higher than that of LEDs. Also the wall-plug efficiency of RGB LEDs is higher than that of laser diodes. Still, the group intends to keep working with this laser diode method to increase the efficiency. Yun Xu and Lianghui Chen, lead researchers, expect laser diode prices to drop, which they believe will increase the commercial application of this method.

The researchers also have taken this work a step further by attempting to fabricate white light with a high color-rendering index using red/green/blue phosphors. They are also using laser light to fabricate white light with high luminance.

Applied Physics Letters, Jan. 18, 2008, 021129.