Chapter V – Troubleshooting
5-17
C
996-0255 rev. b
10) A plus or minus shot-to-shot stability of between 1% and
2.5% is normal. If stability is poor, refer to troubleshoot-
ing guide.
G. 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
extremely damaging to all of the optics within the laser system because of
the very high peak powers generated.
Equipment needed:
• 1 oscilloscope (1 MΩ input) fast scope not
required
• 1 photodiode risetime of ~1 ns
• 1 Styrofoam high density beam block
• 1 metric Allen wrench set
• 1 English Allen wrench set
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 unde-
fined. 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 once again occurs because the
polarization optics cannot reject enough energy out of the
cavity.
• Optical feedback: Optical feedback from amplifiers, or
any 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.
The list above are all examples of “passive” free-running. It is considered
passive because the Q-switch is OFF when it occurs.
Free-running can also occur, especially in higher repetition rate systems
(30 Hz and higher), when the Q-switch is ON (or opening). This is called
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