Non Linear Optics

NON LINEAR OPTICS

The field of study in which the matter responds in a non linear manner to the incident light radiation is known as non linear optics. The term nonlinear refers to а situation where the cause and effect are not linearly proportional to each other.

In certain material, the intensity and frequency of the incident light input is not linearly proportional to that of the output. (ie., they exhibit non-linear effect). This behaviour is known as non linear effect. The material which exhibit this non linear effect are called non linear materials.

The dependence of optical properties such as refractive index on the electric and magnetic fields associated with light is also known as non linear effect.

We know that a light wave is electromagnetic in nature ie., it consists of electric and magnetic fields. When the light propagates through a material, it changes the properties of the medium, such as the refractive index. The charge depends on the electric and magnetic fields associated with the light.

For example, nonlinear effects cannot be observed with the ordinary light beam of low intensity. It is due to the reason that the electric and magnetic fields associated with the light beams is very weak.

With the invention of laser, it is now possible to have electric fields which are strong enough to observe interesting non linear effects.

Non linear properties

Few of the nonlinear phenomena observed are

1. Second harmonic generation

2. Optical mixing

3. Optical phase conjugation

4. Soliton

5. Parametric amplification and

6. Self focussing

Second Harmonic Generation

In a linear medium, polarization P is directly proportional to the electric field E that induces it

P ∝ E

P = ɛo χE

where ɛo permittivity of free space

χ - electrical susceptibility

In nonlinear medium for higher fields ie., higher intensities of light the relationship between the electric polarisation P and the electric field fails to be linear and non linear effects are observed.

where χ1 is the linear susceptibility and

χ2, χ3 … are higher order non linear susceptibilities. With increase of field, the higher order terms come into play.

Modulation of light

Modulation is the process of varying one of the parameters such as amplitude, intensity, frequency, phase and polarisation of a carrier wave in accordance with signal to carry the signal information.

But the optical detectors respond only to the intensity or irradiance of the light. Thus, only intensity modulators at optical frequencies are used.

Demodulation means the reverse process of modulation i.e. extraction of the original signal from the modulated carrier, detected at the receiver.

The two schemes used to modulate the optical signals in LED or LASER diodes are:

(i) Direct modulation

(ii) External modulation

(i) Direct modulation

• In the direct modulation an electronic circuit is designed to simply modulate the current inject into the device (LED, LASER diode).

• The optical output is controlled by the injected current, the desired amplitude (Intensity) modulation is obtained.

• The driver for this direct modulation may be a FET or an HBT hetero bipolar transistor. The structures of the electronic and opto-electronic devices (LED, LASER diodes) are different hence, these devices cannot be fabricated on the same chip.

These circuits are usually based on the hybrid technology. Hence the driver and the source should be fabricated on the same substrate and develop the Opto Electronics Integrated Circuit (OEIC) technology.

• The direct modulation has several problems. Limit in upper modulation frequencies (nearly 40 GHz). There is a shift in emission frequency.

External Modulation

In the external modulation scheme, the light passes through a material whose optical properties can be modified externally.

The electro-optic, acousto-optic, or magneto-optic modulators are example for the external modulator.

• The electro-optic effect is most widely used for high speed applications. It is most comparable with modern electronics. The electro-optic effect involves the change in the refractive index of the material by an electric field.

• In most of the semiconductors, the electro-optic effect is quite small. Hence extremely high fields are needed to cause optical modulation. Lithium-niobate is the most widely available electro-optic material. But it is not a semiconductor.

• It is found that the electro-optic effect is very strong in quantum wells made from GaAs/AlGaAs. Hence the quantum-well modulators plays a prominent role in the optical modulation.