Diode Laser Absorption

Diode laser absorption

Another active diagnostic technique is Diode Laser Absorption (DLA). With this technique, not the scattered photons are detected, but the laser beam itself after the plasma is detected to measure the absorption coefficient of the plasma. If the frequency of the laser is tuned to an optically allowed transition in the plasma, this absorption will increase. By tuning the frequency of the laser, the line profile can be measured. From the line width quantities like the electron density (by Stark broadening), or the heavy particle temperature (by Doppler broadening) can be measured. From the absolute value of the absorption coefficient the density of the (excited) atomic state that absorbs the laser radiation can be determined.

Schematical layout of the setup for Diode Laser Absoption (in this case on the QL lamp)

The detector for Diode Laser Absorption used to be a photo diode, being able to measure at one position in the plasma at the same time. In the future we will utilize the iCCD camera for this purpose. The advantage is that a spatially resolved picture of the plasma can be taken in one measurement. An advantage of the photodiode however is the possibility of scanning the line profile relatively quickly (for one position in the plasma). In order to improve the sensitivity of this technique one could think of a multi-pass system by putting mirrors on either side of the plasma. This technique is known as cavity ring down, and has been successfully applied elsewhere in our group.