Figure 6.3 illustrates an arrangement of cavity mirrors that will allow lasing, but with reduced output. A slight tilt of the high reflector compensates for a similar tilt of the output coupler. The resulting beam is skewed with respect to the resonator axis and the plasma tube bore. Under these conditions the laser can be peaked," but the output will be less than optimum because part of the beam is obstructed by the bore walls.

Walking the mirrors is a trial and error procedure that assures optimum mirror alignment. The goal is to align the intracavity beam with the resonator axis by making small adjustments of the high reflector and matching them with adjustments of the output coupler. By observing the change in output power as you move the mirrors, you will find the optimal alignment positions.

Once the high reflector has been adjusted for maximum power, detune one of its coarse controls until the output is about half its maximum value. Move to the other end of the laser and turn the corresponding output coupler control in the same direction.

Be careful! Use the same controls on both ends of the laser and turn them the same direction: if you turn the high reflector vertical control clockwise, turn the output coupler vertical control clockwise (keeping the same point of view) . If you lose lasing, reverse the direction of mirror movement until lasing is restored.

2 Observe the change in output power as you turn the output coupler control. If the output maximum exceeds the original value , walk the mirrors in the same direction again.Repeat until the power reaches its maximum.

3 If the output fails to reach the original value, return both controls to their original positions.

4 Adjust the high reflector for maximum power.

5 Walk the mirrors with the other pair of controls. If you first walked the vertical mirror controls, do the same with the horizontal controls or vice-versa.

Remember, always walk the mirrors in the direction of increased output power. If the power starts to decline, go the other way. And always find the maximum power with one set of controls before moving to the other set: finish with the vertical controls before you move the horizontal controls and vice versa. Always adjust the high reflector for maximum power before changing from one set of controls to the other.

6 Repeat the walking process several times, first with one set of controls, then with the other. Continue until the output power is as high as it can go.