emitting through the back dbr mirror 120 passes across the interface in the recess 152 providing a transition from the substrate material 150 with a higher index of refraction n s to the material between the substrate 150 of the vcsel and the photodiode 158 with a lower index of refraction n a in the embodiment of fig1 the lower refractive index material may be vacuum air or another gas with an index of refraction of approx n a 1 at this interface due to the transition from material with high index of refraction to material with low index of refraction the curved surface of the recess has the function of a dispersive lens so that off axis rays of light are refracted to diverge from the optical axis at a greater angle thereby and considering the very large thickness of the substrate 150 the figures are not to scale relative to the width of the lasing cavity which is typically narrower than 20 u03bcm while the substrate has a thickness of approximately 500 u03bcm even slightly deflected central rays of the laser will once reflected at a surface of the photodiode or layers within the photodiode are no longer able to couple back into the cavity of the laser thereby back reflection of the laser light from the photodiode into the laser cavity is greatly reduced off axis rays of a high angle of divergence which are associated with spontaneously emitted light such as ray 176 strike the surface 153 of the interface of the recess 152 at a relatively large angle to the perpendicular and are subject.