# Training Videos

## Population Inversion and Optical Gain

This is a continuation from the previous tutorial - optical absorption and amplification. From the discussions in the optical absorption and amplification tutorial, it is clear that population inversion is the basic condition for the presence of an optical gain. In the normal state of any system in thermal equilibrium, a low-energy state is always more populated than a high-energy state, hence no population inversion. Population inversion in a system can only be accomplished through a process called pumping by actively exciting the atoms in a low-energy state to a high-energy state. If left alone, the atoms in a system...

## Optical Absorption and Amplification

This is a continuation from the previous tutorial - optical transitions for laser amplifiers. Optical absorption results in attenuation of an optical field, while stimulated emission leads to amplification of an optical field. To quantify the net effect of a resonant transition on the attenuation or amplification of an optical field, we consider the interaction of a monochromatic plane optical field at a frequency $$\nu$$ with a material that consists of electronic or atomic systems with population densities $$N_1$$ and $$N_2$$ in energy levels $$|1\rangle$$ and $$|2\rangle$$, respectively. Because the spectral intensity distribution of the monochromatic plane optical field that...

## Optical Transitions for Laser Amplifiers

This is a continuation from the previous tutorial - guided-wave all-optical modulators and switches. The word laser is an acronym for light amplification by stimulated emission of radiation. However, the term laser generally refers to a laser oscillator, which generates laser light without an input light wave. A device that amplifies a laser beam by stimulated emission is called a laser amplifier. Laser light is generally highly collimated with a very small divergence and highly coherent in time and space. It also has a relatively narrow spectral linewidth and a high intensity in comparison with light generated from ordinary sources....

This is a continuation from the previous tutorial - guided-wave optical frequency converters. As discussed in the (bulk medium) nonlinear optical modulators and switches tutorial, an all-optical modulator can be either of refractive type, which utilizes $$\boldsymbol{\chi}^{(3)'}$$, or of absorptive type, which utilizes $$\boldsymbol{\chi}^{(3)''}$$. For a guided-wave nonlinear optical device, however, any absorptive loss in the waveguide is detrimental to the device function due to the fact that the primary advantage of using an optical waveguide for the device is the long interaction length possible by the waveguiding effect. Therefore, all practical guided-wave all-optical modulators and switches are of refractive...