The wall-plug efficiency of a laser system is its total electrical-to-optical power efficiency, i.e., the ratio of optical output power to consumed electrical input power as measured at the wall plug. For example, if the optical output power of a laser is 5 W while its power supply consumes 100 W of electrical power, the wall-plug efficiency is 5%.
Taking the term seriously, the electrical power should be measured at the wall plug, so that this efficiency includes losses in the power supply and also the power required for a cooling system, which can be significant for high-power lasers. However, it is common that the wall-plug efficiency is calculated based on the electric power delivered to the laser diodes (e.g. in a diode-pumped solid-state laser system), ignoring losses in power supplies (which can be quite small for modern switched-mode power supplies).
Using the term in that common way, values of the order of 25% result for many diode-pumped laser systems (→ all-solid-state lasers), e.g. Nd:YAG lasers. Even values above 30% are possible, e.g. with thin-disk lasers based on Yb:YAG and efficient laser diodes. It is to be expected that within the next few years laser diodes could become even more efficient, further raising the wall-plug efficiency of such systems. Pure laser diode systems (→ direct-diode lasers) can reach the highest efficiencies, sometimes well above 60%, but they can not always be used, e.g. because of their poor beam quality and their inability to generate intense pulses. When using a high-power fiber laser as a brightness converter, one can obtain high output beam quality and (to some extent) intense light pulses, while the overall wall-plug efficiency can in the best cases be of the order of 50%. On the other hand, argon ion lasers, and even more so titanium–sapphire lasers and the like when they are pumped with argon ion lasers, generally have wall-plug efficiencies around or below 0.1%.
Particularly for high-power lasers, a high wall-plug efficiency is a very important quality. It reduces the electrical power consumption and also the amount of heat which has to be removed. Therefore, it not only cuts down the electricity bill but also reduces the demands on electrical installations and on the cooling system, and in turn often also the size of the laser system. Even for low-power lasers, the efficiency can be important in certain application areas, where the power budget is tight. Examples are telecom devices with a large number of transmitters, and lasers for space applications.
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