Projection and display optics applications usually require light sources with low étendue to efficiently couple into a given optical system or provide a specified beam pattern. One way to accomplish this is by utilizing a laser in combination with a photoluminescent phosphor. This approach may be referred to as laser-activated remote phosphor (LARP) technology. The shorter wavelength primary light from the laser excites (pumps) the phosphor to emit a longer wavelength secondary light (wavelength conversion.) A significant advantage of using wavelength conversion is that the phosphor composition can be chosen so that the system emits a white light. Moreover, such a system can have a much lower étendue than incoherent sources such as high-power light emitting diodes (LEDs).
In LARP applications, the high pump fluxes that are needed to attain a high radiance of converted light from the phosphor have the unintended consequence of locally heating the phosphor in the pump region. This heating reduces the quantum efficiency of the phosphor, and thereby places severe limits on the final radiance of converted light. To address this problem, several approaches have been used. One solution is to use a wavelength converter in the form of a high thermal conductivity ceramic in combination with a high thermal conductivity substrate. Ceramic wavelength converters are formed by sintering a mass of inorganic phosphor particles at high temperature until the particles diffuse and stick together to form a monolithic piece. Typically, the sintered piece has a density that approaches the theoretical density for the material although in some applications it is desirable to maintain some porosity to enhance scattering. Ceramic wavelength converters have a thermal conductivity that is much greater than wavelength converters formed by dispersing individual phosphor particles in a silicone resin.
In the case of transmissive LARP geometries wherein the primary laser light is incident on one side of the wavelength converter and secondary light from the converter is emitted from the opposite side, a sapphire substrate is preferred since the substrate needs to be optically transparent as well as thermally conductive. Transmissive LARP configurations are desirable in many LARP applications because they require fewer optical components and have less complicated optical configurations. This makes them advantageous for applications that require compact LARP sources such as automotive, mobile phone, and other projection/illumination applications.
In order to increase the radiance of the light source, a dichroic reflector may also be added to a transmissive LARP system so that more light is emitted in the forward direction. This may effectively double the radiance of the converted light due to light recycling. However, a dichroic reflector may also have a negative affect on étendue. For example, if the dichroic reflector is placed on the substrate, the recycled secondary light may appear to have a larger effective spot size, increasing source étendue significantly. Even if one could eliminate scattering in the wavelength converter, the recycled light may appear at a different depth-of-field than the forward directed emission, again effectively increasing source étendue.
Another issue that arises when using a ceramic wavelength converter and a transparent substrate is the loss of secondary light that is trapped by total internal reflection (TIR). The fraction of radiation that becomes trapped depends on the relative indices of the substrate and propagation medium and is normally very large. In the case of sapphire-air, only 18% of the emitted radiation will exist within the critical angle cone and exit directly into air. To increase extraction, scattering is required to recycle this radiation, but this will result in an increased source spot size due to the multiple cycles of TIR and scattering into the propagation medium. In the absence of scattering, trapped emission from the wavelength converter will eventually leave through the sides to be lost completely.