Electro-Optic Amplitude Modulator
Page 5
15956-D02 Rev.B, 1/26/2012
Chapter 2
Setup
WARNING
Do not remove cover. High Voltages are used to drive the crystals. Contact with these high voltages
may be harmful to the user.
2.1. Unpacking
The modulator is shipped form the factory ready to be placed into your setup. Carefully unpack the modulator and inspect the
optics by looking through the aperture. If the modulator is shipped with protective tape over the optical apertures, it will need to
be
removed
prior to operation.
WARNING
Do not drop or bang modulator. While Thorlabs supplies a rugged enclosure, excessive mechanical
shock will damage the LiNbO3 crystal.
2.2. Alignment
The EO-AM can be used as a variable waveplate without additional optics. Maximum amplitude modulation
requires both polarized input and a polarizer/analyzer at the output. The optical input may be polarized either
vertically or horizontally. The output polarizer may be aligned either parallel or perpendicular to the input
polarization. Parallel alignment of the output polarizer will give maximum transmission at low applied voltage and
minimum transmission at high applied voltage. Conversely, perpendicular alignment of the output polarizer will
give minimum transmission at low applied voltage and maximum transmission at high applied voltage. Best
results are obtained when the optical propagation is precisely aligned along the principal axis of the crystals. With
the input beam polarized either vertically or horizontally, direct the beam through the center of the modulator
aperture. Place a card several inches from the output of the modulator and mark the location where the beam
strikes the card. Insert a polarizer with polarization perpendicular to the input polarization between the card and
the modulator. Cover the input aperture with a piece of frosted adhesive tape (Scotch
TM
Magic Mending Table No.
810 or similar material).
As shown in Figure 3 on page 7, a geometric pattern (isogyre) similar to Isogyre “A” will be projected on the
viewing card. Adjust the modulator angle of incidence such that the center of the geometric pattern is coincident
with the mark previously made on the card indicating the beam location.
Figure 3 Geometric Pattern (Isogyre “A” and Isogyre “B”)
If voltage is applied to the modulator in this configuration, the pattern will be observed to change from Isogyre “A”
to Isogyre “B” as the voltage increases. If the isogyre is not fully closed with zero applied voltage (i.e.,
transmission of a centered beam through the analyzer is not at a minimum) the pattern can be adjusted using a
bias voltage (for broadband modulators only) or by using a quarter-waveplate at the modulator entrance.
Alternatively, the modulator can be tilted slightly so that the location of one of the dark lines corresponds to the
mark on the card denoting the position of the beam without the tape (or alternative diffuser).
