Magneto-Optic Effect and Modulator Basics

This page covers the basics of the Magneto-Optic Effect and the Magneto-Optic Modulator. It describes the magneto-optic modulator’s working operation, particularly its use as an optical isolator based on the magneto-optic effect.

Introduction

Light modulation is the process by which its properties, such as amplitude, phase, pulse width, and direction, are changed during passage through a medium. Two different schemes are employed for light modulation: internal modulation and external modulation.

• Internal modulation uses a simple circuit to modulate the injected or driving current of the light sources (e.g., LED, Laser, etc.), as the light output from these sources is directly controlled by it.
• External modulation is the process by which the amplitude and phase of the light source can be modulated externally by electro-optic, magneto-optic, acousto-optic, or elasto-optic processes.

What is the Magneto-Optic Effect?

Similar to how an electric field can affect optical properties, the presence of a magnetic field can also influence the optical properties of selected materials through variations in the refractive index. This phenomenon is referred to as the magneto-optic effect.

Materials that exhibit this effect include quartz, ZnS, NaCl, crown glass, and flint glass. When a beam of light (with plane polarization) passes through these materials subjected to a magnetic field, its polarization rotates by an angle ($\theta$) as expressed below:

Where:

• $V$ = Verdet constant
• $B$ = Magnetic flux parallel to the direction of light propagation
• $L$ = Length of the magneto-optic material

This effect is referred to as the Faraday effect, and a modulator based on this effect is known as a Faraday rotator.

Verdet constant values for selected magneto-optic materials at a wavelength ($\lambda$) of 590 nm are listed in the table below:

Let’s understand the working operation of the magneto-optic effect with the following diagram. The magneto-optic effect finds applications in fiber optic current sensors, high-power lasers, memories, etc.

Magneto-Optic Modulator Working Operation

The modulator based on the magneto-optic effect is known as a magneto-optic modulator. The following steps describe the magneto-optic modulator or Faraday rotator working operation:

1. As shown in the figure, a Faraday rotator and one pair of polarizers are used. In this arrangement, the left polarizer is vertically polarized, whereas the right polarizer is placed at 45° with respect to the vertical.
2. When light passes through the left polarizer, it becomes vertically polarized.
3. This vertically polarized light is then passed through the Faraday rotator, and it gets rotated by 45°, as shown.
4. This 45° tilted light passes through the right polarizer without any further rotation, as it is already placed at a 45° tilt.
5. The light gets reflected and made to pass again through the right polarizer. It does not undergo any rotation here, but when it enters the Faraday rotator, it goes through a 45° rotation.
6. Now, the final return beam of light will have a total rotation of 90°. The plane of polarization of this return beam is at a right angle to the initially transmitted beam of light.
7. Hence, the return beam gets blocked by the system.
8. This arrangement is used to pass the light beam in one direction and block it in the other direction. Such a system is also known as an optical isolator.