What is the difference between a continuous-wave and a pulsed laser?

A continuous-wave laser emits a steady beam of constant power, while a pulsed laser concentrates its energy into brief bursts, achieving much higher peak power than its average power for applications that need intense, short exposures.

How are ultrashort femtosecond pulses produced?

Mode-locking forces many of the laser's longitudinal modes to oscillate with a fixed phase relationship, so they interfere to form an extremely short pulse that circulates in the cavity and is emitted as a regular train.

Laser Types and Operation

Lasers are classified by their gain medium and pumping, and operated in continuous or pulsed regimes produced by Q-switching and mode-locking.

Definition

The categorization of lasers by gain medium and excitation mechanism, and the modes of operation, continuous-wave or pulsed, by which their output power and temporal structure are controlled.

Scope

This topic surveys the principal classes of laser and how they are run. It includes gas lasers such as helium-neon and carbon-dioxide, solid-state lasers such as ruby, neodymium, and titanium-sapphire, semiconductor diode lasers, dye and fibre lasers, and their characteristic wavelengths and pumping methods. It also covers operating regimes: continuous-wave output, and the generation of short and ultrashort pulses by Q-switching and mode-locking, together with the resulting peak powers and pulse durations. It connects the underlying gain physics to the diverse practical devices.

Core questions

What gain media are used in lasers and what wavelengths do they produce?
How are different lasers pumped to create population inversion?
How do Q-switching and mode-locking generate short pulses?
What trade-offs distinguish continuous-wave from pulsed operation?

Key concepts

gas laser
solid-state laser
semiconductor diode laser
fibre laser
continuous-wave operation
Q-switching
mode-locking
ultrashort pulses

Key theories

Classification by gain medium: Lasers are grouped as gas, solid-state, semiconductor, dye, and fibre lasers; the gain medium fixes the available wavelengths, efficiency, and power, and dictates suitable pumping by light, electric current, or discharge.
Pulsed operation by Q-switching and mode-locking: Q-switching spoils and then suddenly restores the cavity quality factor to release stored energy as an intense nanosecond pulse, while mode-locking phases many longitudinal modes together to produce a train of picosecond or femtosecond pulses.

Clinical relevance

Different laser types suit different medical uses: carbon-dioxide lasers for soft-tissue cutting, neodymium and erbium lasers for ophthalmic and dermatological procedures, excimer lasers for corneal reshaping, and diode lasers for photocoagulation, with pulsed operation enabling precise, low-collateral-damage tissue interaction.

History

Maiman built the first laser, a pulsed ruby device, in 1960; Javan and colleagues demonstrated the continuous helium-neon gas laser shortly after, and Hall's group produced the first semiconductor diode laser in 1962. Subsequent decades added carbon-dioxide, dye, fibre, and titanium-sapphire lasers and the techniques of Q-switching and mode-locking.

Key figures

Theodore H. Maiman
Ali Javan
Robert N. Hall

Seminal works

svelto2010
salehteich2019

Frequently asked questions

What is the difference between a continuous-wave and a pulsed laser?: A continuous-wave laser emits a steady beam of constant power, while a pulsed laser concentrates its energy into brief bursts, achieving much higher peak power than its average power for applications that need intense, short exposures.
How are ultrashort femtosecond pulses produced?: Mode-locking forces many of the laser's longitudinal modes to oscillate with a fixed phase relationship, so they interfere to form an extremely short pulse that circulates in the cavity and is emitted as a regular train.