Gain

Gain may refer to:

Increase of quantity

People

Products

Music

Acronyms

Other uses

Gain (singer)

Ga-In (hangul: 가인) (born Son Ga-In (hangul: 손가인), September 20, 1987) is a South Korean singer, actress, and entertainer. She is best known as a member of the Korean pop music girl-group Brown Eyed Girls and for her appearances alongside Jo Kwon from 2AM for TV shows We Got Married and All My Love. As a solo artist she has released five EPs.

Music career

2005-09: Brown Eyed Girls & solo projects

Son Ga-In was noticed by the existing members of Brown Eyed Girls after she was eliminated during auditions for the popular South Korean reality series Let's Coke Play! Battle Shinhwa! She was approached by composer Ahn Jung Hoon and invited to audition at Brown Eyed Girls' company, eventually joining the group. The four members performed several small shows before officially debuting as Brown Eyed Girls in 2006. In the same year, she performed a duet "Must Have Love" with SG Wannabe's leader Kim Yong Jun, which became her first number one hit during the Christmas season.

Despite having initial successes, it was not until 2009 that her home group made a breakthrough into Korean mainstream with their hit song "Abracadabra", which was notable for the group's shift of their image and their new direction. After the unexpected success of the song, the group went on to become one of the most mentioned names of K-Pop girl-groups.

Gain (laser)

In laser physics, gain or amplification is a process where the medium transfers part of its energy to the emitted electromagnetic radiation, resulting in an increase in optical power. This is the basic principle of all lasers. Quantitatively, gain is a measure of the ability of a laser medium to increase optical power.

Definition

The gain can be defined as the derivative of logarithm of power as it passes through the medium:

where is the coordinate in the direction of propagation. This equation neglects the effects of the transversal profile of beam.

In the quasi-monochromatic paraxial approximation, the gain can be taken into account with the following equation

where is variation of index of refraction (Which is supposed to be small),

is complex field, related to the physical electric field with relation , where is vector of polarization, is wavenumber, is frequency, is transversal Laplacian; means real part.

Gain in Quasi two-level system

In the simple quasi two-level system, the gain can be expressed in terms of populations and of lower and excited states: