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7
these degrees of freedom, a perfect superimposition of the back reflected laser
spot with the incoming beam should be reached. When the alignment is perfect,
some laser sources could start having large power fluctuations on the output: if
this happens, rotate slightly the head so that the back reflected beam does not
fool the laser source feedback circuitry. Such a small misalignment will not
affect the power output.
After alignment, the centring tool must be placed again on the top of the support
post. There should be no need to change the alignment unless the incoming
laser beam is moved or additional components are placed on the light path
before the TCF.
Temperature tuning and device operation
The main TCF device does not have a power switch: by plugging the power
supply into the device socket, the etalon temperature will be changed and
stabilised to a set point, ranging approximately from 10 to 41 °C. During this
phase, the indicator on the top of the control box should gradually move towards
middle of its range, which indicates a stable condition. On units equipped with
an RGB LED indicator, the latter will instead change from red (or blue) to green
when the stability is approached.
The etalon set point is fixed by rotating the device knob: the lower the setting,
the lower is also the set-point temperature. Each subdivision on the multi-turn
knob scale corresponds theoretically to about 6·10
-2
°C.
The etalon is such that the temperature interval among transmission maxima is
about 17.5 °C: given the temperature range accessible to the device, up to two
maxima can thus be reached. The position of these transmission maxima (in
terms of temperature and with reference to the knob scale) and their
transmission intensity, as measured at factory after production, are reported as
calibration at the end of this manual. The position and spacing of the maxima
will change slightly using a different laser source due to small differences in the
wavelength. For this reason, the user should consider the reported values as a
first guess and will need to find the correct maxima positions for his source.
Many modern laser sources are specified to operate within a few tenths of a
nanometer from the nominal wavelength, and could slightly change the
operating frequency in this range (usually between a session of work and
another). When this happens a decrease in the output power from the etalon
will result and the device will need to be adjusted on the new laser operating
frequency.
The optimization of the set-point value should be done only after allowing a
sufficient warm-up time to the laser source, so that it has reached a stable
condition. The laser filter can initially be positioned at one of the factory
calibrated set-point. After a relevant change in temperature, the device is
usually get close to a stable enough condition within 10 minutes. A power meter
or photodiode sensor should be placed after the TCF and monitored in order to
verify the changes in transmittance of the device. By moving the set-point knob
by small steps, the user should be able to follow the variation of light
transmission and search for a position of maximum. While doing so, it is useful
