University of California Santa Barbara has been exploring the use of laser diodes (LDs) in combination with phosphors as a means to produce white light [Kristin A. Denault et al, AIP Advances, vol3, p072107, 2013].
Commercial 'white' light-emitting diodes (LEDs) generally use a blue or near-ultraviolet (near-UV) III-nitride LED to excite a phosphor target that converts the relatively narrow spectrum of the LED into something that approaches a white light spectrum of varying quality.
UCSB believes that using laser diodes instead of LEDs could have a number of advantages such as avoiding the 'efficiency droop' problem of nitride semiconductor LEDs at high current injection. Further, the use of laser diodes could allow better thermal management, with the exciting laser diode being separated from the phosphor target material, due to the higher directionality of laser light. In LEDs, self-heating effects cause shifts in wavelengths that alter the color-rendering quality, along with reducing the efficiency.
Table 1: Parameters of commercial LDs used.
Peak wavelength Full-width at half-maximum Threshold current Wall-plug efficiency Near-UV 402nm 2.6nm 30mA 20% Blue 442nm 2.7nm 150mA 30%
Commercial near-ultraviolet (UV) and blue LDs (Table 1) were used to excite phosphors in disk form (Figure 1). The UV LD was used in combination with two types of red-green-blue phosphor mixed in silicone resin with respective weight ratios of 1.65:1:3.45 (RGB1) and 3.3:1:2.3 (RGB2). The phosphors that were used were a proprietary formulation developed by Mitsubishi Chemical Corp.
Figure 1: Experimental schematic for laser-excited phosphor samples in an integrating sphere for (a) near-UV-excited RGB phosphors and (b) blue-excited YAG:Ce.
The blue LD was used on standard yellow-emitting cerium-doped yttrium aluminium garnet phosphor, as used for low-cost white LEDs (Y3Al5O12:Ce3+/YAG:Ce = Y2.94Ce0.06Al5O12). The powdered YAG:Ce was formed into pellets and heated at 1500°C for 5 hours to make the phosphor target.
The measurements were taken with the phosphor mounted in an integrating sphere. The laser light came in through a side port, hitting the phosphor disk at a slight angle to avoid reflection back out of the side port. The measurements (Table 2) included values for correlated color temperature (CCT), color rendering (Ra), luminous flux (φν) and luminous efficacy (ην) of the resulting 'white' light.
Table 2: Measured correlated color temperature (CCT), color rendering (Ra), luminous flux (φν) and luminous efficacy (ην).
Sample CCT (K) Ra φν (lm) ην (lm/W) RGB1 3600 91 47 16 RGB2 2700 95 53 19 YAG:Ce 4400 57 252 76
The near-UV LD was operated at peak efficiency with an injection current of 450mA. The near-UV results are described as showing excellent color temperature and color-rendering capabilities. "The luminous flux produced is comparable to current commercially available bright white LEDs of similar color temperatures," the researchers write. The use of near-UV allows a range of color temperatures to be achieved with high color rendering.
However, the LDs have relatively low wall-plug efficiency (WPE), impacting luminous efficacy. The researchers hope that future development in laser technology may lead to efficacies exceeding those of commercial LEDs.
Figure 2: Spectral power distribution for phosphor samples (a) RGB1 and (b) RGB2 excited using a near-UV LD and (c) corresponding CIE (Commission Internationale de l'éclairage) chromaticity coordinates. Photographs of RGB2 phosphor sample (d) without and (e) with laser excitation.
White-light emitting devices using near-UV LD excitement would presumably contain filters that would remove the unconverted laser light, avoiding concerns about eye safety.
The blue-LD setup with a peak efficiency injection current of 750mA had much better efficacy, but the cost was a cool color temperature and poor color rendering due to the absence of a strong red component. LEDs that only use YAG:Ce phosphors suffer from a similar problem. A further problem for blue laser excitation is that the blue component covers a smaller range of wavelength due to narrower width of the laser spectral line compared with that resulting from spontaneous emissions from an LED.
The researchers performed calculations to gauge the possible improvements in efficacy for a blue-LD on YAG:Ce phosphor setup if the WPE could be increased from 30% to 75%. At 30% WPE, the calculations gave an efficacy of 78lm/W for a device of CCT of 4555K and Ra of 58. The efficacy at 75% WPE was almost 200lm/W. The calculated performance with 30% WPE is close to the experimental measurements achieved with the blue LD.
