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Control for a laser diode mount

A control circuit for a laser diode mount LDM21 from Thorlabs was developed. There is a thermistor and a thermo-electric cooler (TEC) inside the mount. It accepts all 9 mm and 5.6 mm laser diodes. An AlGaInP laser diode type HL6314 from Opnext (Hitachi) was used, which has a wavelength of visible light output of 635 nm (red) with an optical output power of 3 mW.

The wavelength of laser diodes is a function of both the temperature and the operating current. Therefore, it is necessary to regulate these parameters to ensure precise measurement. Laser diodes are very sensitive to electrostatic discharges and current surges, therefore special precautions are needed.

A microcontroller ATmega 8 from Atmel and adequate circuits were used to solve the regulating problem. The ATmega 8 has enough in- and output pins and an SPI- Bus (Serial Peripheral Interface) which is used to communicate with analog/digital converters and other devices.

The thermistors value of resistance is analysed in a bridge circuit, the difference voltage is measured with the analog/digital converter afterwards the temperature is calculated. The control system in the microcontroller forces a current through the TEC as a function of the temperature difference between current and set-point temperature over a digital/analog converter.
  An IC (Integrated Circuit) from iC-Haus was used to drive the laser diode with a constant current as a voltage-controlled current source. It also has the advantage of being able to avoid current peaks while switching. All pins are protected from electrostatic discharges. The optical power is measured over a photodiode which is integrated to the laser diode.

Figure 1: Fully mounted device

Figure 1: Fully mounted device

The circuits that are necessary to regulate the laser diode were built on a printed circuit board. This board was attached in a case. The fully mounted device (see Figure 1) has four buttons for handling, a display and connectors for the laser diode mount and power source. There is a possibility to connect it to a personal computer via a USB-interface.

It runs as a stand-alone or as PC-controlled device. In the first case, the set-point temperature and the value of the laser current are selectable with the buttons, in the second case an automatic measurement and control, for example via Matlab, is possible.

July 03, 2008