Which mode of laser operation is continuous over time?

The mode of laser operation that is continuous over time is Continuous Wave (CW) mode. In this mode, the laser emits a constant output of light without interruption, allowing for a steady and continuous delivery of energy. This characteristic is particularly useful in applications where a constant laser output is essential, such as in laser cutting, welding, and certain types of therapeutic procedures in medicine.

In contrast, repetitively pulsed and single pulsed modes involve the emission of light in distinct bursts, which means they do not provide a continuous output of energy. While pulsed operation can be beneficial for specific applications, it inherently lacks the uninterrupted nature of CW operation.

Single Pulsed Q-switched lasers also produce light in the form of pulses, allowing for very high peak powers in each pulse. However, similar to other pulsed modes, they do not deliver a continuous output over time. The pulse duration is typically quite short, and there are intervals between pulses where no laser output occurs.

Thus, Continuous Wave is distinguished by its uninterrupted operation, making it the correct answer to the question regarding a laser mode that is continuous over time.