Magneto optic effect basics | Magneto optic modulator working

This page covers basics of Magneto Optic Effect and Magneto Optic Modulator. It describes magneto optic modulator working operation used as optical isolator based on magneto optic effect.

Introduction:
The light modulation is the process by which its properties such as amplitude, phase, pulse width, direction etc. are changed during the passage of the medium. There are two different schemes employed for the purpose of light modulation viz. internal modulation and external modulation. Internal modulation uses 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 are directly controlled by it. The external modulation is the process by which amplitude and phase of the light source can be modulated externally by electro-optic, magneto-optic, acousto-optic or elasto-optic processes.

What is Magneto Optic Effect?

• Like electric field, presence of magnetic field can also affect the optical properties of selected materials through variation of refractive index. This phenomenon is referred as magneto optic effect. The materials include quartz, ZnS, NaCl, crown glass, flint glass etc.
• When beam of light (with plane polarization) passes through these materials, its polarization rotates by angle (θ) as expressed below. Here magnetic materials are subjected to magnetic field.
➨ θ = V * B * L
Where,
V = Verdet constant
B = Magnetic flux parallel to direction of light propagation
L = Length of magneto-optic material

This effect is referred as faraday effect and modulator based on this effect is known as faraday rotator. Verdet constant values for selected magneto optic materials at λ of 590nm are listed in the table below.

Materials	Verdet constant (rd * m^-1* T^-1)
Crown Glass	6.4
Flint Glass	23.0
NaCl	9.6
SiO2 (Quartz)	4.0
ZnS	82.0

Let us understand working operation of magneto-optic effect with following diagram. The magneto optic effect find applications in fiber optic current sensor, high power laser, memories etc.

Magneto optic modulator working operation

Optical Isolator as per Magneto Optic Effect

The modulator based on magneto optic effect is known as magneto optic modulator. Following steps describes Magneto optic modulator or faraday rotator working operation:
➨As shown in the figure-1, faraday rotator and one pair of polarizers are used. In this arrangement, left polarizer vertically polarized where as right polarizer is placed 45^o with the vertical.
➨When light passes through the left polarizer, it becomes vertically polarized.
➨Then this vertically polarized light is passed through the faraday rotator and it gets rotate by 45^o as shown.
➨This 45^o tilted light passes through the right polarizer without any further rotation as it is already placed 45^o tilted.
➨The light gets reflected and made to pass again through the right polarizer. It does not have any rotation here but when it enters faraday rotator it goes through 45^o rotation.
➨Now final return beam of light will have total rotation of 90^o. Plane of polarization of this return beam is at right angle to the initially transmitted beam of light.
➨Hence return beam gets blocked by the system.
➨This arrangement is used to pass light beam in one direction and to block it in the other direction. Such system is also known as optical isolator.