Electro-Optic Light Modulators

If you already read in the last section “The Electro-Optic Effect” or, more specifically, the subsection “Electro-Optic Modulation” then you already basically know the workings of an electro-optic modulator. If you have not already read through that section, then I would recommend you do so now as we will be only going over some of the results in this section. We will expand on our previous lessons and look at longitudinal vs transverse modulators.

Table of Contents:

  1. Longitudinal Electro-Optic Modulation
  2. Transverse Electro-Optic Modulation
  3. Electro-Optic Modulators in the Lab


As can be seen in the figure below, longitudinal electro-optic modulators have the voltages applied parallel to the direction of the light beam; as such, in order to increase the delay, higher voltages must be used. This, then, is one of the main problems with the longitudinal electro-optic modulator: high voltages are required. The voltages for longitudinal electro-optic modulators can in fact reach up to the multi-kV range. One of the main advantages of the longitudinal configuration, though, is that the modulators can have a wide acceptance aperature, so the confinements on needing a well-defined beam (i.e. well collimated) are not as strict. The main applications of longitudinal electro-optic modulators is in applications that do not require a high frequency response, or in non-laser applications.

Course 4, Module 7: Electro-Optic and Acousto-Optic Devices


Free-Space Electro-Optic Modulators


If you are a lab tech or simply a grad student trying to get things to work in the lab, you might be saying “This is great, Optics Girl, but it also doesn’t help me in any practical way.” Which would be fair, because the above descriptions are the classroom descriptions of EO modulators. In this part, we will try to demystify the EO modulators.

When you are going to order or work with an EO-modulator, there are three main types of EO-modulators you will find commercially available: phase, amplitude, and polarization modulators. Phase and polarization modulators are very similar, with the main difference being how the polarized beam is aligned into the EO-modulator. With the amplitude modulator, things can get a bit more complicated with the inclusion of polarizers and other bulk optics. We will go through each of these types 1 by 1!

Phase Modulators: Phase modulators

Polarization Modulators:

Amplitude Modulators: Very simple amplitude modulators are essentially phase modulators with the addition of a couple of polarizers. However, when you are going to order EOMs, you may see that some types of amplitude EOMs also employ Mach-Zehnder interferometers.

A very useful video showing how amplitude EOMs are used in the lab is here:

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