32
SolsTiS User Manual v10.0
Fine Frequency Tuning
– Locking to and Adjusting the Reference Cavity
(SolsTiS-SRX Only)
Once SolsTiS has been fine tuned to the required wavelength and the étalon
has been locked successfully to the nearest SolsTiS longitudinal mode, the
(reference)
Cavity Lock
should be applied by pressing the corresponding
Apply Lock
button on the control page of the UI. Engaging
Cavity Lock
results
in the SolsTiS cavity length being fine tuned via a PZT-mounted mirror until
the SolsTiS frequency matches that of the reference cavity, and then locking
(slaving) SolsTiS to the reference cavity frequency by a ‘side of fringe’ locking
scheme.
1.
Once the reference
Cavity Lock
has been engaged, confirm that the lock
has engaged successfully. The indicator on the left-hand status window
will confirm the lock condition.
Note!
When engaging the reference
Cavity Lock
, the
SolsTiS output wavelength may jump. Typically
engaging
Cavity Lock
may result in a wavelength
jump as the locking process involves fine tuning
SolsTiS’ longitudinal mode frequency (cavity length)
to match the nearest reference cavity mode (fringe).
Any wavelength jump should therefore not exceed
the reference cavity mode spacing (~3GHz).
2.
Typically, the reference cavity lock should engage on the first attempt. If
the
Cavity Lock
does not engage after several attempts, the reference
cavity voltage may be too close to 0 or 100, i.e. near the end of the cavity
lock circuit capture range. The reference cavity (length) may therefore
need to be tuned slightly until locking is achieved. If needed, the reference
cavity voltage may be monitored on a CRO by selecting
Ref Cavity V
from
the drop down menu of one of the two user configurable Monitor outputs
(as shown in the expanded view of the laser control page above). Ideally,
to ensure the reference cavity locking circuit has the widest possible
capture range, the reference cavity voltage should be centred at
~50% (~2V). If this is not the case, and especially if the voltage
is close to the edge of its range, try disengaging the
Cavity
Lock
, fine-tuning the reference cavity (length) and then
relocking. If the target wavelength cannot be reached after fine-
tuning the reference cavity, fine tuning of the étalon (after first
disengaging the etalon lock) and possibly also the BRF may
enable this.
3.
Once the
Cavity Lock
has been engaged, the laser frequency
can then only be tuned (offset) relative to the reference cavity
frequency by adjusting the
Ref Cavity
Coarse
and
Ref Cavity
Fine
+/- control buttons. The position of the
Ref Cavity
adjustments are indicated as a percentage.
4.
The reference cavity is fine tuned by altering the
Reference
Cavity Coarse
percentage up to four decimal places and fine-
tuned further by altering the
Reference Cavity Fine
percentage
up to two decimal places. This is done by moving the position of
the black underscore beneath the percentage values to the
number of decimal places required and using the +/- control
buttons.
Note!
In general there is no benefit to adjusting the
Ref Cavity
offset before locking the laser to the reference cavity. Similarly,
SolsTiS’ frequency is fine-tuned via the
Ref Cavity
offset,
SolsTiS’ frequency may relax back (jump) when the reference
cavity lock is disengaged. This jump may also be observed if
the SolsTiS has been locked to the reference cavity for an
extended period, even without adjusting its output frequency
via the
Ref Cavity
offset.