Cross Saturation
Content
CROSS SATURATION
Gain saturation: the phenomenon that the gain of an amplifier is reduced for high input signal powers. An amplifier device such as a laser gain medium cannot maintain a fixed gain for arbitrarily high input powers, because this would require adding arbitrary amounts of power to the amplified signal. Therefore, the gain must be reduced for high input powers; this phenomenon is called gain saturation (or gain compression). In the case of a laser gain medium, the gain does not instantly adjust to the level according to the optical input power, because the gain medium stores some amount of energy, and the stored energy determines the gain. For example, a sudden increase in the input power of a laser gain medium will reduce the gain only within a certain time, because the population of excited laser ions is only reduced with a certain finite rate. Mode competition: the phenomenon that different resonator modes experience laser amplification in the same gain medium, leading to cross-saturation effects. The modes of a laser resonator all experience optical amplification in the same gain medium, e.g. a laser crystal, in which they spatially overlap to a significant extent. This leads to the phenomenon of mode competition or gain competition. The fact that different modes experience amplification in the same gain medium, and that this leads to cross-saturation effects, where stimulated emission by one mode causes gain saturation not only for itself (self-saturation), but also for the other modes. If the intensity distributions of the modes are identical, so that self- and cross-saturation are equally strong mode competition is then reduced. In a linear resonator, each resonator mode forms a standing-wave pattern in the gain medium, which differs for modes with different frequencies. Therefore, if one mode acquires a large power and strongly saturates its gain, another mode may see a reduced amount of gain saturation, as its standing-wave pattern is different.(spatial hole burning).
Gain saturation: the phenomenon that the gain of an amplifier is reduced for high input signal powers. An amplifier device such as a laser gain medium cannot maintain a fixed gain for arbitrarily high input powers, because this would require adding arbitrary amounts of power to the amplified signal. Therefore, the gain must be reduced for high input powers; this phenomenon is called gain saturation (or gain compression). In the case of a laser gain medium, the gain does not instantly adjust to the level according to the optical input power, because the gain medium stores some amount of energy, and the stored energy determines the gain. For example, a sudden increase in the input power of a laser gain medium will reduce the gain only within a certain time, because the population of excited laser ions is only reduced with a certain finite rate. Mode competition: the phenomenon that different resonator modes experience laser amplification in the same gain medium, leading to cross-saturation effects. The modes of a laser resonator all experience optical amplification in the same gain medium, e.g. a laser crystal, in which they spatially overlap to a significant extent. This leads to the phenomenon of mode competition or gain competition. The fact that different modes experience amplification in the same gain medium, and that this leads to cross-saturation effects, where stimulated emission by one mode causes gain saturation not only for itself (self-saturation), but also for the other modes. If the intensity distributions of the modes are identical, so that self- and cross-saturation are equally strong mode competition is then reduced. In a linear resonator, each resonator mode forms a standing-wave pattern in the gain medium, which differs for modes with different frequencies. Therefore, if one mode acquires a large power and strongly saturates its gain, another mode may see a reduced amount of gain saturation, as its standing-wave pattern is different.(spatial hole burning).
