Continuum Surelite EX 10, Operation And Maintenance Manual

Page: 87 / 114

Section 6: Troubleshooting
996-0207 77
4. Activate P01 and press
SHUTTER
button.
5. While viewing the pulsewidth on the scope adjust the vertical position of the back
mirror M#1. Make very small adjustments at first to be able to determine effect of the
adjustment.
When making adjustments to the back mirror the pulse will change in size (pulsewidth)
as well as move in time. Generally, when adjusting the mirror in the correct direction
the pulse will move to the left indicating reduced build-up time.
6. Optimize the vertical position.
7. Now repeat the process while adjusting the horizontal axis.
8. Once the alignment is optimized, take single shot burns to verify oscillator alignment.
Note: A very small adjustment on the back mirror can have an effect on the
pulsewidth though it may not be noticeable on the burn paper.
Eliminating free-running
Free-running is a term used to describe the phenomenon of a Q-switched oscillator actually
lasing before the Q-switch is open. This effect can be damaging to optics and result in power
fluctuations and reduced harmonic conversion efficiency.
There are generally three ways in which free-running can occur:
Misalignment. If the polarization optics, especially the
λ
/4
plate and the Pockels cell,
become misaligned, then the polarization within the cavity becomes slightly undefined.
This means that the dielectric polarizer does not receive linearly polarized light.
Therefore, when the polarizer is supposed to be rejecting energy out of the cavity, a
certain amount “ leaks” through. That “ leakage” energy then builds up and becomes
lasing. Since this lasing occurs before the Q-switch is opened, it is called free-running.
Increased pump. If the flashlamp voltage is increased, the oscillator will reach a level
at which, due to the increased gain, it begins to lase. This occurs because the
polarization optics cannot reject enough energy out of the cavity.
Optical feedback. Optical feedback from an external reflective surface, can increase
the amount of energy in the oscillator cavity. This increased energy, just as with
turning up the pump voltage, can exceed the polarization optics ability to reject enough
energy out of the cavity.
During normal laser operation free-running will manifest itself in the form of erratic
shot-to-shot stability or even a snapping sound coming from the laser. If this occurs,
stop the Q-switching immediately. Then follow the procedure listed below to check
for and eliminate free-running.