Electrode sputtering and pin material preionization over the life of the laser generates minute particles of dust within the resonator cavity. This dust is pushed into the resonator windows by the photon pressure of the beam, especially in the central portion of the optics. Dust on the windows interferes with lasing, resulting in the dip in the profile seen here.
Profile of beam from laser with dirty resonator optics
UV absorption of the dust eventually causes damage to the interior surfaces of the resonator optics. Therefore, a regular window cleaning is critical to proper laser operation. Even well-maintained optics eventually produce the profile seen above due to losses associated with damage to the internal crystal of the optical material.
Beam profile of beam from laser with misaligned resonator optics
For proper lasing to occur, resonator optics must be aligned perpendicular to the beam axis. Window misalignment causes the optical feedback to skew with respect to the gain medium, resulting in lower overall gain. Misalignment along the long axis results in reduced power and hot spots (in either direction) in the beam profile, as seen here.
Beam profile from a laser with worn electrodes or preionization pins
Misalignment along the short axis causes a dramatic drop in total power, often without discernible hot spots.
As preionization pins wear, spark gap increases, adversely affecting gas preionization. As the cathode electrode wears and becomes more flat, the discharge becomes non-uniform. Worn electrodes or preionization pins can cause a trapezoidal or split beam, as shown above. This effect cannot be corrected by resonator alignment and becomes more pronounced as the gas fill ages. The resulting non-uniform beam power density is unacceptable for critical applications. This problem can only be corrected by refurbishing the discharge assembly.