Design Of Direct-lit Laser Backlight Unit

Liquid crystal display technology has become the mainstream flat panel display technology.A backlight unit is necessary for a liquid crystal display as the fact that liquid crystal can't emit light.The performance of the backlight determines the image quality partly,so improving the performance of the backlight is a direct mean for displaying a better image.Compared with traditional light sources,laser has high color purity which makes it has good color performance because of its narrow spectrum.However when a laser diode is used in a direct-lit backlight unit,the beam emitted from the laser diode has to be enlarged and shaped to from a satisfied illumination with certain size and high brightness.In this thesis,a diffractive optical element(DOE)is exploited to shape the beam from a laser diode and a laser backlight unit based on the DOE is designed.the phase distribution of DOE designed by The Kirchhoff diffraction formula and the GS algorithm is simulated.The pincushion distortion in diffractive spot has been found and optimized and the spot become the desired uniform rectangular spot.When the diffraction distance is short,the sampling pitch of DOE is very small,which makes the manufacture of DOE expensive and time-consuming.In order to improve the preparation efficiency and reduce the cost,a laser backlight unit with new structure is designed.Two diffractive elements have been used,beam shaping DOE and expanding grating,to obtain a large diffraction spot at small distance.The diffractive elements used in the new structure have the characteristics of large spacing and easy preparation,and can effectively reduce the preparation cost and improve the preparation efficiency.The beam shaping DOE and the expanding grating are manufactured by maskless direct writing lithography based on the design.Two diffractive elements with good shape have been obtained by optimizing the manufacture parameters based on the manufacture conditions.The laser backlight unit was built and tested.The result shows that this backlight structure can effectively shape the Gaussian-distributed laser beam into a uniform rectangular beam.The average brightness of the backlight can reach 34427 cd/m~2 and the uniformity is 74.2%.