The present disclosure relates to an optical cavity, comprising a first non-concave reflector positioned at a first end of the optical cavity and a second non-concave reflector positioned at a second end of the optical cavity that receives and reflects light reflected from the first non-concave reflector. The first non-concave reflector is configured to focus light that reflects off of the reflector back upon itself to avoid diffraction losses from the optical cavity. In one embodiment of the invention, the first non-concave reflector includes a layer of material that has a thickness that vanes as a function of radial distance out from an axial center of the layer. In another embodiment of the invention, the first non-concave reflector includes a layer of material that has an index of refraction that varies as a function of radial distance out from an axial center of the layer.