Optical
Chopper
System
18488-D02 Rev G, September 24, 2012
Page 13
www.thorlabs.com
For example, the MC2000 is set to run the outer blade set at 70 Hz. The inner blade, by virtue of the
ratio of inner to outer slots will be running at 50 Hz. If a light path is common to both the inner and
outer blades, the signal will see a sum frequency of 120 Hz and a difference frequency of 20 Hz.
Setting the Chopping Speed in the Two Frequency Mode
The chopping frequency for the two-frequency blade is set the same way as described above for the
single frequency blades.
Note: The MC2000 synchronizes the outer portion of the two-frequency blade to the internal synthesizer
or external reference. The inner portion is chopping at a rate given by, 5/7 * reference
7
5
REF
Inner
f
f
=
,
(eq. 2)
For example, if the internal reference (or external reference) frequency is 100 Hz, the outer portion of
the blade would be chopping at 100 Hz and the frequency of the inner portion could be found using
equation 2,
Hz
Hz
f
Inner
42
.
71
7
5
100
=
=
.
5.5. Two-Slot
Blade
The 2-slot blade provides very stable low frequency chopping up to a frequency of 99 Hz. To
accomplish the high stability required, an outer blade frequency with a 50:1 ratio is used to minimize the
jitter. The optical chopper uses this outer frequency to lock the blade to either the internal or external
frequency generator. In applications where phase is important, the MC2000 uses a dual photo-interrupter
arrangement on the head to track the phase of the inner slot.
Setup for Two-Slot Blade Reference Frequency
Follow the setup procedures for external operating mode in Part 4: Basic Operation on page 9.
