Small-divergent-angle uniform illumination with enhanced luminance of transmissive phosphor-converted white laser diode by secondary optics design

Abstract With the advantage of low etendue and no efficiency droop, phosphor-converted white laser diode (pc-WLD) has been a very promising candidate in future high-luminance solid-state lighting. To obtain high brightness, the blue laser beam is usually collimated or focused onto a yellow phosphor layer, which may result in poor angular color uniformity (ACU). In this study, we observed the blue spot and yellow ring phenomenon of the transmissive pc-WLD, which is attributed to the mismatch between the focused intensity distribution of the blue light and the divergent intensity distribution of the yellow light. Then, we proposed a secondary optics consisting of a parabolic reflector coupled with a freeform lens to enhance the ACU by collimating all the rays emitting from the phosphor, which is located at the focus of the reflector. Through this design, we achieved the match of the intensity distribution of the blue and yellow lights, resulting in a high ACU of 0.85 with a view angle of ±10° by simulation. Moreover, the luminance of the pc-WLD can be enhanced by about four times using this method. Finally, we fabricated a prototype of the designed secondary optics and validated the effectiveness of the proposed method in achieving small-divergent-angle uniform illumination with enhanced luminance of the transmissive pc-WLD.