THORLABS MC2000, Руководство по эксплуатации

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Optical Chopper System
18488-D02 Rev G, September 24, 2012 Page 12 www.thorlabs.com
5.3. Harmonic Reference Generation
Harmonic generation is a special feature available when operating in the external reference mode. The
on board microprocessor is used to multiply the external reference up to the 15
th
harmonic or to divide
the reference down to the 15
th
sub-harmonic. By combining both the frequency multiplication and
division together, a fractional harmonic can be obtained. The new frequency is used as the chopper
reference for chopping at a variety of frequency combinations, all derived from the external reference.
Setup for External Reference Frequency
Follow the setup procedures for external operating mode in Part 4: Basic Operation on page 9.
Setting the Reference Harmonic
Highlight and select the reference harmonic N menu item and enter a scroll bar value between 1 and 15.
Setting the Sub-Harmonic
Highlight and select the reference harmonic D menu item and enter a scroll bar value between 1 and 15.
Combining the Harmonic and Sub-Harmonic Modes
The sub-harmonic divider follows the harmonic multiplier. Therefore, the generated reference frequency
will always be
D
N
f f
REF EXT
=
. (eq. 1)
There are no special steps to combine these two features since they are always active.
Selecting the REFERENCE OUT signal
In the Harmonic frequency mode, a fractional harmonic reference output signal
D
N
ACTUAL ) ( ,
or the internal frequency synthesizer (TARGET) can be selected. Most cases in this mode, only the
(ACTUAL) fractional harmonic reference output signal will ever be used.
5.4. Two-Frequency Chopping
A special two-frequency blade is available from Thorlabs, which has seven slots on the outer portion of
the wheel and five slots on the inner part of the wheel. This unique prime number combination allows
the same chopper to discriminately chop two different light paths. This can be used in ratio metric
measurements and pump-probe type experiments.
Selecting the REFERENCE OUT signal
The MC2000 provides two additional reference outputs in this mode. The sum (SUM), and the
difference (DIFF) reference frequencies track the combined signal paths when they are frequency mixed
together. The example below illustrates this: