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9
If the knob set point gets too far from the maximum output setting, the stabiliser
could be unable to reach the maximum output. In this case, switch it off and
adjust the knob manually.
It is possible that the RGB LED indicator turns red or blue if the correction
voltage provided by the stabiliser is relatively large: this is not an indication of
problems in the TCF circuitry, but could also be a reason to adjust the static set
point position.
Device transmittance optimization
When the device is perfectly tuned to the laser frequency, the transmittance
should be quite high and comparable to the values obtained in the test sheet.
The most common cause for a decreased performance transmittance at
maximum are an insufficient beam collimation or a slightly skew incidence of
the beam on the etalon. Both these effects are never completely avoidable, due
to the dependence of the etalon transmittance on the light’s incidence angle,
and the non-ideality of the light source.
If the beam divergence is not low enough to obtain a satisfying transmission,, a
simple attempt to improve can be the use of a positive achromatic lens before
the TCF, having the focus approximately coincident with the laser source
’s
virtual point. A 400÷500 mm focal length lens could reduce enough the
divergence without requiring too much free space on the experimental setup.
Setting a normal incidence is always possible by means of the alignment
procedure described in this manual. In this configuration the device will attain
the best transmission, but the light back reflected by the etalon will be fed back
to the laser unit, and the interference of this light beam with the out coming one
could perturb the stability of some laser sources. In this case, it might be
necessary to accept a small misalignment of the TCF or use an optical isolator
to avoid the effect.
Etalon replacement and wavelength ranges
Three different types of etalon, coated for the most common green, red and blue
wavelengths ranges, are available from stock at Tablestable. Each etalon is
usable within ±20 nm from the design wavelength and is mounted in a sealed
cylindrical container.
It is possible to replace the TCF etalon in order to use the same device
alternatively on different laser wavelengths.
In the case of a TCF-2 unit, it is necessary to remove the stabiliser unit in order
to access the etalon. The stabiliser unit is held in position by the friction of two
O-rings: to remove the stabiliser, pull it very gently and slowly outside, easing
the movement by mean of an alternate rotation. On more recent units, an
additional grub screw on the TCF head keeps the stabiliser locked in position:
this must be loosened by means of a 1.5 mm hex key before pulling the
stabiliser out.
