Transient thermal study of laser diodes

The transient thermal response of laser diodes is measured using the optical power as the temperature sensitive parameter. Commercial laser diodes in TO packages are chosen for this study. The monitoring photodiode in the package is used to measure the optical power. When the laser is driven by a long pulse signal, the optical power decreases within the pulse duration as a result of temperature increase. From the amount of optical power decrease, the temperature rise is determined. For comparison, a finite element technique is employed to model the laser device for both steady and transient analyses. The complete model includes the laser chip, the sub-mount, and the heatsink. The calculated transient response shows clearly how the temperature changes with time as step power is applied. The measured thermal resistance is significantly lower than the calculated one because the circuit is not fast enough to capture the true initial optical power when the laser is turned on. As a result, the initial optical power value is underestimated. This also underestimates the percentage decrease in optical power within the pulse duration and consequently the temperature rise. To correct this problem, a faster circuit is required