Alignment
6-11
5. Iterate Steps 2 through 4 until the beam is centered on M
4
, M
5
and M
10
and power is maximized.
Avoid parallax error when noting beam position on the mirrors by put-
ting the line-of-sight normal to the mirror axis. It is important that the
beam is centered on M
5
and M
10
because this restores the alignment of
the prism sequence so that the beam passes through all the prisms at
the angle of minimum deviation. This minimizes beam walk and angu-
lar change throughout the wavelength tuning range.
6. Install the tuning slit assembly.
a. Verify the slit width is about 2.0 mm wide.
b. Place the tuning slit assembly into the laser head and onto the
micrometer shaft. Refer to Figure 6-8.
c. Rotate the slit width adjustment screw until the dowel pin on the
mating shaft can slide into the slot.
d. Seat the slit assembly against the micrometer stop.
e. Tighten the screw.
7. Set up the spectrometer to view the wavelength and, later, the mode-
locked spectrum. Use ND filter(s) and/or a variable OD attenuator to
attenuate the signal into the instrument to avoid saturation.
8. Tune the wavelengths to the center of the tuning range for the optic set
installed, but set it for 890 nm for the Long mirror set in order to stay
away from the water absorption region. Verify the beam is within the
two metal strips that make up the slit.
9. Align the photodiode pc board.
a. Loosen the two photodiode pc board mounting screws.
b. Move the pc board around until a maximum signal is observed on
the
PHOTODIODE bar graph on the front of the Model 3955, then
tighten the mounting screws. If the LED bar graph is saturated,
reduce the pump power to bring the signal below saturation before
aligning the pc board.
To avoid damaging nearby optics when installing the slit, carefully
move it into place and use your hand to support it while you tighten the
mounting screw.
Warning!
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