Additional modulated supply

If your DC supply cannot be modulated (or not fast enough), you will have to add an additional modulated source, e.g, a waveform (ramp) generator. Alpes Lasers SA does not supply such generators nor the needed interfacing, but in the following, a scheme for setting up the interfacing is explained which has proven to work. Please refer to the drawings shown.

Connectors for the LLH with a BNC plug are available for CW laser operation. This is always center positive, shield negative, and can directly be connected to a stabilized DC supply; make sure polarity is correct and limits as shown in the datasheet are set.

To modulate the QCL, an AC signal needs to be added to the DC current. To prevent current modulation to go back to the DC source, an RLC circuit needs to be added. The values of ${R_{AC},L,C}$ depend on the modulation frequency, the AC source and (to some extent) on the QCL.