The junction temperature inside a VCSEL is a crucial factor to determine the light output of a VCSEL. Since a direct measurement is not possible one needs to adopt a measurement to qualify the capability of heat transfer from the VCSEL to the cooling. This can be quantified in a parameter, the thermal resistance.
The thermal resistance of a VCSEL device is a measure for the heat coupling and the capability of the cooling. To measure this parameter a test stand containing a temperature control and a system to operate the laser has been developed.
Measured is the wavelength spectrum depending on the temperature inside the VCSEL controlled through either the environmental temperature or the driving current of the device.
The spectrum of the VCSEL shifts with temperature. Therefore the change in wavelength of a given peak can be connected to the change in temperature or power. Using the same change in wavelength the ratio between power change and temperature change can be derived, which forms the thermal resistance.
By optimizing the thermal resistance of the package, the heat introduced by irradiation can be reduced. This in turn enables the laser to withstand higher irradiation damage and still provide light output.