Laser, as a light source with high brightness and strong directivity, can emit monochromatic coherent beams. And its various merits have given rise to an ever increasing utilization of laser as a light source in the field of projection display technologies in recent years. A pure laser source, however, is confronted with a grave speckle issue and high cost, which accounts for the currently broad application of mixed laser sources of fluorescence and laser.
Fluorescence is generated by exciting fluorescence powder with laser after coating the fluorescence powder on a wavelength conversion device. For instance, three-primary-color light results from excitation of green and red fluorescence powder by blue laser, in which green fluorescence powder and red fluorescence powder are coated on a rotating wavelength conversion device, which is typically a fluorescent wheel structure, and sequentially outputs three primary-color light in accordance with time sequence through rotation. As another option, a red-blue double-color laser source is employed to generate the three primary-color light, where blue laser excites green fluorescence powder on a fluorescent wheel to generate green fluorescence, which, along with blue laser and red laser, forms the three primary-color light.
At present, the fluorescent wheel may be divided into transmission-type ones and reflection-type ones.
For a transmission-type fluorescent wheel, a substrate where fluorescence powder is coated is made of transparent material, allowing fluorescence resulted from excitation by excitation laser to diverge in an omnidirection covering 360 degrees, in which part of beams diverging towards the laser incident side can be reflected back off a reflective film coated on the laser incident side of the substrate, and transmit through the transparent substrate of the fluorescent wheel, before emitting outwards in the same direction in which the excitation laser is incident. As shown in a diagrammatic cross-section of the outer circumference of a fluorescent wheel in FIG. 1, 3b is a fluorescence powder layer, usually formed through mixing binder with fluorescence powder and secured between two layers of glass 3a on the front and the rear sides, in which the inner surface, where the front-side (i.e., the laser incident side) glass comes into contact with the fluorescence powder layer, is coated with a film 3c for transmitting laser and reflecting fluorescence, and the rear-side (i.e., the opposite side of the laser incident side) glass is used for transmitting the reflected fluorescence and fluorescence which is directly incident onto the glass surface.
In a reflection-type fluorescent wheel, as shown in FIG. 2, fluorescence powder is normally arranged on the outer circumference of an aluminum substrate, so that after fluorescence is generated from excitation by laser, part of the fluorescence is reflected by a mirror surface of the aluminum substrate at the opposite side of the laser incident side, while part of the fluorescence is allowed to directly emit out after reflection in a direction opposite to the laser incident direction, and the transmitted laser, in a direction opposite to that of the fluorescence, requires to travel through a circuit design for optical axis conversion, so as to be eventually optically combined with the fluorescence.
Both of the fluorescent wheels of the above two types are provided with a laser transmission region, which sequentially outputs laser and fluorescence in accordance with the rotation sequence of the fluorescent wheel, and the outputted laser via transmission can only serve as a light source after passing through a speckle-removing optical path for speckle removal.
In addition, the overall brightness of a laser source has been heightened since red laser devices took the place of red fluorescence, as red-blue double-color laser finds application, whereas the enrichment in the variety of laser devices further deteriorates the speckle phenomenon, calling for a special speckle-removing component to be added in the optical design, for instance, an optical fiber, a random phase plate and auxiliary components are introduced to design a speckle-removing optical path, guaranteeing the quality of a light source for projection and illumination, while leading to an increase in the volume and complexity of an optical architecture, which in turn goes against miniaturization of laser projection equipments.