Thermoreflectance Imaging of Back-Irradiance Heating in High Power Diode Lasers at Several Operating Wavelengths

In optical systems employing high-power diode lasers, back-irradiance of emission onto the laser facet has been found to contribute to catastrophic optical damage. In this paper, thermoreflectance imaging has been used to measure quantum well temperature rise at the facet for diode lasers emitting at several wavelengths under a wide range of back-irradiance beam positions. For TM-polarized diode lasers operating near 800 nm, the quantum well temperature at the facet is found to reach a maximum when back-irradiance is positioned at the cladding-substrate interface.  For TE-polarized lasers operating near 900 nm, a similar effect is observed, albeit with lower magnitude. Interestingly, a second maximum of similar magnitude is observed when back-irradiance is centered on the metal-semiconductor ohmic contact interface.  For TE-polarized lasers operating near 1000 nm, the cladding-substrate critical point disappears and the critical point at the metal-semiconductor interface intensifies. The dependence of the critical back-irradiance spot location on operating wavelength originates in the spectral absorptivities of the device's constituent materials. In addition, the incident light's polarization can affect its absorption in the metallic solder. These measurements provide key insights into the potential thermal contribution of back-irradiance to catastrophic optical damage in diode lasers at various operating wavelengths.