Usually, semiconductor laser diodes are operated from current drives. Unfortunately, The output power is strongly dependent on the device temperature – a rise of 15° causes a fall in optical power of (typically) 17% for a given current. Using a feedback loop and monitor the output beam intensity using photodiode is one method to stabilizing the operating condition. Not an easy design problem and if the laser is to be modulated, that becomes much more difficult. Operate the laser at a constant temperature is the simplest way to stabilize the light output for any given drive current. The circuit describe below was designed to control the current through a thermoelectric heat pump to maintain the diode heat sink at a constant temperature.
Because the heat pump is reversible so it’s allowed to control the temperature both, above and below ambient. We can stabilized the temperature in the approximate range (0 – 45°C) determined by the setting of VR1. The circuit will be of use in other applications where thermal stabilization of small electronic components is required; long term stability is of the order of +/- 0.5°. [Source: http://graffiti.virgin.net/ljmayes.mal/]