MOGlabs DLC202, Manual, Page: 14 / 82

4 Chapter 1. Introduction
BS
PD
M M
BS
ECDL
BS
Servo
Vapour cell
Offsets
λ/4 λ/4
Figure 1.4: Schematic setup for
DC
locking to an atomic transition. PD
is the
DLC
photodetector. BS beamsplitter, M mirror, λ/ 4 a quarter-wave
retarder.
and
LOCK OFFSET
controls. Feedback can be via one or both piezo
actuators, or the diode injection current, or all three.
1.4
AC
locking to an atomic transition
With
AC
locking (
FM
demodulation or “lock-in amplifier” detection),
the laser frequency can be locked to a peak centre. The
AC
ap-
proach offers the advantage of inherently lower detected noise and
thus the potential for improved laser frequency stability. The setup
is similar to that for
DC
locking, but modulation of the laser fre-
quency, or the reference frequency, is required. The
MOG
labs
DLC
provides an internal 250 kHz oscillator which can directly dither the
diode current, or drive an external modulator. In particular, it is
designed to drive a Zeeman-shift modulation coil surrounding the
atomic reference vapour cell; see appendix C.
Figure 1.5 shows a simplified
AC
locking setup, using a coil to
Zeeman-modulate the atomic reference, or an acousto-optic mod-
ulator (AOM) for modulating the frequency of the beam through the
vapour cell. If preferred, the modulator oscillator can be set to dither