Definition: operation mode of a laser with continuous light emission
German: CW-Betrieb, kontinuierlicher Betrieb
Category: laser devices and laser physics
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Author: Dr. Rüdiger Paschotta
Continuous-wave (cw) operation of a light sources means that it is continuously operated, i.e., not pulsed. The term is most frequently applied to lasers but also to gas discharge lamps, for example.
For a laser, continuous-wave operation implies that it is continuously pumped and continuously emits light. The emission can occur in a single resonator mode (→ single-frequency operation) or on multiple modes.
The first continuous-wave laser was a helium–neon laser operating at 1153 nm . A version working with the now common emission wavelength of 632.8 nm was demonstrated soon after that. Later on, many other types of lasers were developed which can also be operated continuously: other gas lasers, many types of solid-state lasers (including semiconductor lasers), and dye lasers.
For many lasers with low-gain laser transitions, continuous-wave operation is difficult to achieve, while operation with pulsed pumping is easy to obtain. In some cases, continuous-wave operation is only possible with fiber lasers, but not with bulk lasers, as the fiber geometry greatly increases the gain efficiency. Some so-called self-terminating laser transitions are not suitable at all for continuous-wave operation, or only with additional measures.
The output power of a continuous-wave laser is more or less constant on longer time scales, but it can exhibit substantial power variations e.g. due to mode beating (if single-frequency operation is not achieved) and other kinds of laser noise. Various techniques for the stabilization of lasers concerning output power and/or optical frequency can be applied, often involving additional optical elements in or around the laser resonator.
In continuous-wave operation, some lasers exhibit too strong heating of the gain medium. The heating can then be reduced by quasi-continuous-wave operation, where the pump power is only switched on for limited time intervals, but long enough to reach steady-state optical conditions. The latter implies that the on time is much longer than the relaxation oscillation damping time.
Continuous-wave Mode Locking
Some lasers are called continuous-wave mode-locked. This means that the laser is mode-locked, and the pulse energy (or average power) is constant, i.e. there is no Q-switched mode locking. Despite the emission of pulses, mode locking can be considered as a kind of continuous-wave operation in the sense that the resonator modes are excited with constant powers.
Questions and Comments from Users
I am aware of Ti:sapphire lasers that operate with narrow band filters and are CW, and of Ti:sapphire lasers that have mode-locked the entire output spectrum and are short-pulse lasers. But if a Ti:sapphire laser is not modelocked and does not have filters to select a narrow-band output, what is the proper way to describe it? Is it continuous-wave?
Answer from the author:
Yes, it will emit continuously, although perhaps not with a narrow bandwidth, and with some power fluctuations. So it definitely a continuous-wave laser.
If we were able to chop a narrow band CW source with a 20-fs chopper, could it be considered as a femtosecond laser? Would the bandwidth of these pulses increase to meet the requirements of the Fourier limit?
Answer from the author:
You might in principle called that a femtosecond laser, although the method is certainly not practical, because you cannot chop a beam with that speed, and also you would utilize only a very tiny part of the laser energy.
Yes, the optical bandwidth would be increased by that chopping.
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|||A. Javan, W. R. Bennett Jr., and D. R. Herriott, “Population inversion and continuous optical maser oscillation in a gas discharge containing a He–Ne mixture”, Phys. Rev. Lett. 6 (3), 106 (1961), doi:10.1103/PhysRevLett.6.106|
See also: modes of laser operation, quasi-continuous-wave operation, resonator modes, self-terminating laser transitions
and other articles in the category laser devices and laser physics
When operating a laser cutter at 5000 Hz and 85% duty cycle, is that considered CW?
Answer from the author:
Maybe the laser is indeed running in continuous-wave mode, and only its output beam is blocked during 15% of the time. But if the laser is switched off during those times, it is not continuous-wave mode.