Editorial Feature

An Introduction to Nonlinear Optics

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Nonlinear optics is the basis of many essential technologies used today that require an electromagnetic radiation source from a variety of wavelengths. Nonlinear optical technology is generally used in conjunction with laser technology, and the fundamental useful aspect of this technology is the capacity to change or expand the limited wavelength range directly obtainable by a laser source.

As a field of research, nonlinear optics is the examination of phenomena triggered by changes in the optical qualities of a material system caused by light. Nonlinear optical phenomena are deemed “nonlinear” when the material system reacts to an applied optical field based on the power of the optical field and in a nonlinear fashion. Normally, only laser light is powerful enough to affect the optical qualities of a material system.

Many experts consider the discovery of second-harmonic generation in 1961 to be the origin of nonlinear optics. This occurred just after the first working laser was demonstrated in 1960.

History of the field

In 1875, Scottish scientist John Kerr showed the refractive index in many solids and liquids is somewhat altered by the use of a strong DC field. This development, now referred to as the (DC) Kerr effect, is now considered the first nonlinear optical effect to be witnessed.

In 1894, German scientist Friedrich Pockels reported on a related effect where the refractive index change is relative to the field. While the Kerr effect is seen in liquids and amorphous solids, the Pockels effect arises only in crystalline materials without a centre of symmetry.

Further progress was made decades later, after sufficiently intense light sources, such as the laser, were invented.

Use in fibre optics

Fibre optic technology is one of the most common applications of nonlinear optics. Fibre optics technology has been around in one form or another since the early 20th century, but the significance of nonlinear optic principles like Stimulated Raman and Brillouin scattering wasn’t recognized until the 1970s. In the 1980s, the technology incorporated nonlinear effects for pulse compression and optical switching. In the past 20 years or so, researchers developed new kinds of fibre-optic amplifiers based on stimulated Raman scattering and four-wave mixing (FWM).

Key nonlinear optical phenomena and applications

Nonlinear optical interactions for properties such as frequency, polarization or path of incident light lead to a range of optical phenomena. Some of these phenomena have been harnessed for various purposes.

Second harmonic generation (SHG), also known as frequency doubling, is a nonlinear optical phenomenon involving two photons with the identical frequency combining after interacting with a nonlinear material. The process produces a new photon with double the power of the initial photons; double the frequency and half the wavelength. It is a unique instance of sum frequency generation. Using SHG, certain lasers can be transformed into visible light.

Optical phase conjugation is a technique that involves reversing the propagation direction and phase variation of a beam of light. This nonlinear optical phenomenon could be considered a real-time holographic process.

In this phenomenon, interacting beams connect in a nonlinear optical material to create a dynamic hologram in two of the three input beams, or real-time diffraction pattern within the material itself. In essence, all three incident beams interact at the same time to create a number of real-time holograms, while also creating a group of diffracted output waves that align as a "time-reversed" beam.

Optical phase conjugation has been used successfully in a range of different applications, including in long-distance, high bitrate fibre optic communications, laser target-aiming systems. The phenomenon also shows significant promise for use in optical data storage and processing systems.

Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.

Brett Smith

Written by

Brett Smith

Brett Smith is an American freelance writer with a bachelor’s degree in journalism from Buffalo State College and has 8 years of experience working in a professional laboratory.

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