TEMPERATURE STABILISATION OF LASER DIODES
Semiconductor laser diodes are usually operated from current drives; unfortunately for a given current the output power is strongly dependent on the device temperature - a rise of 15° causes a fall in optical power of (typically) 17%. One method of stabilising the operating conditions is to use a feedback loop and monitor the output beam intensity using a photodiode - not an easy design problem and one that becomes much more difficult if the laser is to be modulated. The simplest way to stabilise the light output for any given drive current is to operate the laser at a constant temperature. The circuit described here was designed to control the current through a thermoelectric heat pump to maintain the diode heat sink at a constant temperature.
The heat pump utilises the Peltier effect: a series of alternating P-N and N-P junctions are electrically connected in series. The input and output faces are beryllium oxide and the junctions sandwiched between; the P-N junctions are arranged on one face and the N-P on the opposite. Heat is absorbed at the N-P junctions and dissipated at the P-N junctions(1); the rate at which heat is pumped is approximately proportional to current.
The laser is mounted on a copper heat sink together with the AD590 monolithic temperature sensor. The voltage developed across R6 is proportional to the absolute temperature; this voltage is compared with a stable reference voltage derived from the bandgap reference i.e. (ZN425) and the error voltage determines the drive applied to the pump. A monolithic audio power amplifier (IC4) drives the heat pump; the amplifier's built-in thermal and current overload protection guarantees safe operation of the amplifier and also the heat pump. Since a large current may be taken by the circuit it is desirable to keep the resistance of interconnections as low as possible; good power supply decoupling is also advised (fig. 2).
The heat pump is reversible and this allows the temperature to be controlled both above and below ambient; the temperature can be stabilised in the approximate range; 0 - 45°C determined by the setting of VR1. The circuit will be of use in other applications where thermal stabilisation of small electronic components is required; long term stability is of the order of +/- 0.5°.
Figure 1: Temperature Stabiliser
Figure 2: Recommended supply decoupling
Reference
(1) "Guide to Thermoelectric Heat Pumps"; Marlow Industries, Inc. 1021 South Jupiter Road, Garland, Texas.