MOGlabs DLC202, Руководство пользователя, Страница: 44 / 82

34 Chapter 4. Optimisation
produce clear (low-noise) dispersive error signals as shown in the
upper trace of fig. 3.7. Note that the error signal should be about
0.5 to 1.0 V p-p. While the signal looks cleaner at larger ampli-
tude relative to background oscilloscope noise, in fact the overall
performance will deteriorate. Other important factors to consider:
Probe power The probe power should be about 250 µ W. Higher
power will increase the photosignal, but the detector saturates
at about 500
µ W.
Probe intensity The probe intensity should be low to reduce power-
broadening. Thus, the probe beam should be expanded to 5
or 10 mm diameter, to allow high power and low intensity, as
discussed in section 3.5.
Polarisation The frequency discriminator (
ERROR
) signal is sensitive
to the pump and probe polarisations. Good polarisers and
careful alignment can be very helpful.
Coil design See appendix C.
Shielding The Zeeman coil produces substantial magnetic fields,
oscillating at 250 kHz. These fields can readily induce prob-
lematic potentials and currents in the laser head and/or main
circuit board. In particular, it is quite possible to produce a
larger frequency modulation from induced currents in the laser
diode than from the Zeeman modulation of the reference. It is
vital that the coil be located far from the main unit and from
the laser, or that it be shielded. A layer of high-permeability
material (soft iron or mu-metal) is probably adequate. To test
this, simply reverse the polarity of the coil connection. If the
error signal is also reversed, but otherwise similar, then the
shielding is probably adequate.