Complete laser head.
The laser must be cooled down and its temperature must be precisely
controlled to stabilize the wavelength. The stability of the laser
wavelength is important for Raman spectroscopy as the Raman lines are
moving according to the laser wavelength. To keep the temperature
constant, a PID regulator is used as shown in the figure. Two holes were
drilled into the aluminum cube, one for the thermistance connected to
the regulator and one for the sensor of a small digital thermometer to
display the temperature. The temperature stability is closed to 0.1°C.
To position the laser beam at the center of the microscope stage, 3
different mechanical devices are used. From the top to bottom: the
vertical and horizontal tilt with a telescope positioning system, the
vertical movement with a modified lab jack bellow and the horizontal
movement of the whole laser head with a Proxxon table.
The tube at the output of the laser is fitted with a beam expander
optics and the Semrock laser cleaning filter at the end of the tube.
This filter can be removed during the raw positioning of the laser so
that a faint red spot (spontaneous transitions of the diode) can be
seen. When the filter is in place, the laser beam is completely
invisible. It is very important to wear good safety goggles even
if the beam is not visible.