ABSTRACT

The laser is an amplifier as well as an oscillator, and like any other oscillator, requires positive feedback to operate. In a laser there is gain: a small optical signal traveling down the amplifier increases in amplitude with each pass; however, there are also sources of loss. In the gain threshold equation losses that occur at a single point, such as mirrors, are expressed as a loss distributed across the entire length of the amplifier. The threshold gain equation expresses all losses in the laser as a distributed loss. The single most important equation in laser work is the gain threshold equation. This equation allows computation of the minimum gain the laser medium must exhibit to allow the laser to oscillate. In reality, this equation represents the loss of all optical elements in the laser: absorption, cavity optics, and other intra-cavity losses.