Athermal operation of slotted lasers in which the same laser wavelength is maintained even as the temperature is changed is presented. We show results with wavelength stability of ± 0.04 nm/5 GHz over a temperature range of 8 - 47°C without mode-hops. A greater range from 10 up to 85°C is found but this time with mode-hops occurring. These slotted lasers use a surface high-order grating of 37^th order and do not require re-growth. They contain a gain and grating sections and semiconductor optical amplifier at the output. Changes in the currents to these sections while the temperature is changed allow for the athermal operation. An analytical model and thermoreflectance measurements show that we can fully understand this behaviour and work towards improved laser designs that will allow a greater range of athermal operation while avoiding mode-hops.