continuous-wave operation (original) (raw)

Author: the photonics expert (RP)

Definition: operation mode of a laser with continuous light emission

Alternative term: cw operation

Category: laser devices and laser physics

Related: modes of laser operationquasi-continuous-wave operationresonator modesself-terminating laser transitions

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DOI: 10.61835/n8h Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What is Continuous-wave Operation of Lasers?

Continuous-wave (cw) operation of a light source 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 in multiple modes.

The first continuous-wave laser was a helium–neon laser operating at 1153 nm [1]. 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.

Figure 1: A simple continuous-wave laser consists only of a diode-pumped laser head and two mirrors around it. Source: Cutting Edge Optronics.

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 approximately 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.

Quasi-continuous-wave Operation

In continuous-wave operation, some lasers would 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.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is continuous-wave (CW) operation of a laser?

Continuous-wave (CW) operation of a laser means that it is continuously pumped and continuously emits light, in contrast to pulsed operation.

Why can't all lasers be operated in continuous-wave mode?

Some lasers have low-gain or self-terminating laser transitions, or would get too hot for prolonged pumping, which makes continuous-wave operation difficult or impossible. For such lasers, pulsed pumping is often required.

What is quasi-continuous-wave operation?

Quasi-continuous-wave (QCW) operation is used for lasers that would overheat in true CW operation. The laser is pumped in intervals long enough to reach steady-state optical conditions, which reduces the average thermal load.

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