Optical Modulators.
In solid state modulators 3 types are there. They are Bulk, Integrated Optical and All-Fiber Modulators. 1. Bulk
In these modulators the signal propagates through a uniform block of material. They lack wave guiding.
They require high electrical drive power and external optics to couple in of optical fibers. They require high electrical drive power and external optics to couple out of optical fibers.

2. Integrated Optical
How IO materials are formed? : By fabricating waveguides directly into modular material. For this modulators Low external power is needed. No external optics needed.

3. All-fiber
The optical signal never leaves the fiber. In All-Fiber Modulators low external power is necessary.
There is no need of external optics or precision alignment. Modulation capabilities of the glass are very poor.

Electro-optic Effect
Electro optic effect is the base for most of the optical modulators. In linear electro- optic Effect,
The refractive indices of crystals are linked to external electric field. In this Modulator the light propagating in an anisotropic medium can be decomposed into its normal modes, or Eigen waves.

It can be determined by the index ellipsoid

In certain crystals the application of an external electric field will redistribute the charges in the molecules. It results in a size change and orientation change of the index ellipsoid. This is the electro-optic effect. This effect relates the dimensions of the index ellipsoid to an applied electric field.

The electro optic tensor explains the electro-effect mathematically. Electro-optic effect is best described via a induced deformation of the index ellipsoid. In a Cartesian co-ordinate system It is defined by

An electric field can now change the co-efficient as per the equation
Where is a electro optic tensor. ,and are the x, y, z components of the applied electric field

The electric field changes the size, shape and orientation of the index ellipsoid. In most of the electro optic devices Lithium Niobate(LiNbO3 ) is used. In Lithium Niobate when the electric field is not there, it is a birefringent.

Its index ellipsoid is given by

When the light is travelling in x direction, then the allowed states of linear polarization are in parallel direction to the other two axis. When the light is travelling in z direction the same thing will happen. When the field is in the Z direction. For 3m crystal symmetry class its electro-optic tensor is given by

In z direction is applied to Lithium Niobate. The equation for the index ellipsoid becomes
.

Then if we write the new index ellipsoid as

Bulk Modulators

Bulk modulators are used mostly in optical system. They are electro-optic and accousto-optic devices.
In this modulator by varying the refractive index of the crystal optical phase modulation is achieved.
By applying voltage between the electrodes phase modulation is achieved.

The index variation is achieved by applying an electric field across the crystal. The change in the the electro-optic refractive index is directly proportional to the applied voltage.

L/d is called as geometric factor. The equivalent electronic circuit for the electro optic modulator system is given below. In this model, the modulator is modelled as a capacitor.

Electro-Optic Intensity Modulation

One suitable example for this Intensity modulator is LiNbO3. In this a phase modulator is inserted between cross polarizers. Polarization is varied by changing the phase modulator voltage V(t).
In this the polarization of the beam incident on the polarizer is varied. This leads to intensity modulation. In this setup the input polarizer is oriented 45 deg with respect to modulator axis. The output polarizer is oriented 90 deg with respect to input polarizer.

Bulk Acousto-optic Frequency Shifting

These devices are made up of LiNbO3. This modulator can be used for controlling the power, frequency or spatial direction of a laser beam. This is based on the acoustic optic effect. Refractive index is modified by the oscillating mechanical pressure of a sound wave. Acoustic Optic modulator also called Bragg Cell.

Integrated optical modulators

These devices utilize the fast electro-optical response of photonic crystals. High Modulation frequencies and low modulation voltages are the characteristics of these modulators.