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The transient response of a semiconductor laser is calculated by solving the coupled rate equations, which are two differential equations involving the electron and the photon densities. The normalized electrical current ... injected into the laser suddenly increases from ... to ... ; the electron and photon normalized densities are shown (in purple and red). Time is normalized by the lifetime of the electron. Photons are created by electron-hole recombination in the laser. The interaction between electrons and photons, described by the coupled rate equations, leads to damped relaxation oscillations. The normalized time required to reach photon and electron density equilibrium values is around 1-3, and the actual time a few nanoseconds, since the typical value of the lifetime of the electron is around 1 nanosecond. Therefore, current modulated semiconductor lasers can be used to emit optical digital signals in lightwave communication systems up to a few gigabit/second. For simplicity, the coupled rate equations used have been derived with reasonable approximations and the qualitative predictions can be trusted and are verified by experiments. A semiconductor LASER (Light Amplification by Stimulated Emission of Radiation) is a very compact light source (0.2 mm long) in which electric current (a few mA) is converted into coherent optical power with high efficiency. In 1970, the successful Continuous Wave (CW) operation of the Double-Heterostructure laser at room temperature coincided with the achievement of low loss (1 dB/km) optical fiber. These high improvements of both laser and optical fiber created strong excitement worldwide.
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