Transient thermal measurement of laser diodes

In this research, the transient thermal response of laser diodes is measured using light power as the temperature sensitive parameter. It is well known that the light power of a laser diode decreases as temperature increases. When the laser is driven by a long pulse signal, the light power decreases within the pulse duration as a result of temperature increase. From the percentage of light power decrease versus time, the temperature rise as a function of time is deduced. In determining the temperature rise, a calibration curve of power versus temperature is used. This curve is produced by measuring the peak light power of the laser driven by a short pulse over a range of case temperature. Under short pulse condition, the laser junction temperature is approximated by the case temperature since the junction does not have enough time to heat up. For the approximation to be valid, the short pulse width must be much shorter than the thermal time constant of the laser. Several commercial laser diodes are studied. In the experiment, the laser case temperature is measured by a thermal coupler-based digital thermometer with an accuracy of +/-0.1 degrees C. The accuracy is verified by boiling water and ice water temperatures.