Maintenance Manual: SP2 Laser Alignment

Optimum performance of the SP2 is obtained when its laser beam is operating in the TEMoo mode, in which the beam cross-section has a Gaussian profile. Higher-order modes will have multiple spots, which may not affect the measurement of black carbon incandescence but will almost certainly limit the utility of the scattering data collected. In cases where very high-order modes are present, the incandescence data can also be compromised.

Intracavity laser power is also essential for peak measurement performance and depends heavily upon proper alignment and the surface integrity of the optics which bound the cavity.

A minor adjustment of the Nd:YAG laser alignment can frequently restore instrument performance. If the signal shape from a purely scattering particle is significantly non-Gaussian, such an adjustment can usually restore the peak shape. Cleaning the delicate surfaces of the laser’s optical components can restore lost power but is often a last resort prior to their replacement.