Laser diodes differ from most “classical” lasers in two distinct ways:
► Firstly, they do not posses an inherently defined wavelength. Instead of two defined energy levels, the lasing transition occurs between two energy bands.
► Secondly, the pn junction of the diode defines the lasing volume, instead of the resonator in a classical laser.
These experiments investigate the variation of the emitted laser wavelength versus temperature and current. The divergence and polarisation of the laser emission are examined.
These experiments investigate the variation of the emitted laser wavelength versus temperature and current. The divergence and polarisation of the laser emission are examined. The set-up comprises a modern 30 mW green (525 nm) emitting laser diode, with integrated Peltier cooler, mount and driver. Collimating optics, lenses and a polarisation analyser are provided, along with a photodiode detector. A spectrum analyser is available as an option. All optical mounts and positioners are included. The laser diode is mounted in a rotational stage which allows the independent rotation around the beam propagation axis as well as perpendicular to this axis to measure the spatial distribution of the emitted laser light. The polarisation for different values of the injection current is analysed by means of a polarizer.
The spectrum analyser will be used to measure the change of wavelength by varying the temperature and injection current. The shift is app. 0.05 nm per °C. The temperature range of the diode laser controller can be varied from 10 to 60° C which results in a shift of 2.5 nm. Temperature and injection current are stabilised and displayed by the controller. The use of an oscilloscope is recommended to suppress disturbing environmental light. In this case some of the measurements are carried out with modulated diode laser light.
LE-0400 Diodelaser Characterisation
A laser diode emitting visible green radiation at a wavelength of 525 nm is used as probe laser. The laser diode is attached to a Peltier cooler which allows a controlled temperature change from 10-50 °C. Furthermore, the injection current can be set from zero to the maximum permissible value. The laser diode and the attached Peltier cooler are integrated into a round housing which is mounted into a twofold rotary stage for horizontal and coaxial rotation. This allows the measurement of the spatial intensity distribution of the emitted visible green light. The incident light from the laser diode is detected and measured with a fixed photodiode.
The emission of a laser diode is in general strongly divergent and asymmetric concerning the spatial propagation. The light appears to have two points of origin (astigmatic difference) and two orthogonal axes, each with different divergence. Most application however, require a round beam. To achieve this, a pair of cylindrical lenses are used as shown in the figure on the left. The case (A) shows the collimation of one direction using the collimator to create an almost parallel beam which is not affected by the cylindrical lenses. The orthogonal emission direction (B) is treated by the cylindrical lenses to obtain an almost round beam