Terminology and History
- The first device using amplification by stimulated emission operated at microwave frequencies and was called a maser.
- When similar optical devices were developed, they were first known as optical masers.
- Today, all devices operating at frequencies higher than microwaves are called lasers (e.g. infrared lasers, ultraviolet lasers, X-ray lasers, gamma-ray lasers).
- Devices operating at microwave or lower radio frequencies are called masers.
- The back-formed verb to lase is frequently used in the field, meaning to give off coherent light.
- The first laser was built in 1960 by Theodore Maiman at Hughes Research Laboratories.
- Theoretical work by Charles H. Townes and Arthur Leonard Schawlow contributed to the development of the first laser.
- The word 'laser' originated as an acronym for light amplification by stimulated emission of radiation.
- The acronym LOSER (light oscillation by stimulated emission of radiation) has been humorously noted as a more correct option.
- The terms 'laser' and 'maser' are also used for naturally occurring coherent emissions in astrophysical and atom lasers.
- Albert Einstein established the theoretical foundations for the laser and the maser in 1917.
- Rudolf W. Ladenburg confirmed the existence of stimulated emission and negative absorption in 1928.
- Valentin A. Fabrikant predicted the use of stimulated emission to amplify short waves in 1939.
- Willis E. Lamb and R.C. Retherford found apparent stimulated emission in hydrogen spectra in 1947.
- Alfred Kastler proposed the method of optical pumping in 1950.
- Joseph Weber presented the idea of using stimulated emissions to make a microwave amplifier in 1951.
- Charles H. Townes and his team produced the first microwave amplifier in 1953.
- Nikolay Basov and Aleksandr Prokhorov independently solved the problem of continuous-output systems in the Soviet Union.
- Prokhorov and Basov suggested optical pumping of a multi-level system as a method for obtaining population inversion in 1955.
- Charles H. Townes, Nikolay Basov, and Aleksandr Prokhorov shared the Nobel Prize in Physics in 1964.

Fundamentals of Laser Physics
- A laser produces a narrow beam of light in a single wavelength.
- Light and other forms of electromagnetic radiation are described as the group behavior of fundamental particles known as photons.
- Photons are released and absorbed through electromagnetic interactions with other fundamental particles.
- The release of a photon in a laser is triggered by the nearby passage of another photon, known as stimulated emission.
- This process allows for the possibility of a chain reaction, where photons trigger stimulated emission in other atoms.
- Electrons and their interaction with electromagnetic fields are important in understanding chemistry and physics.
- Discrete energy levels of electrons in an atom play a role in absorption and emission of photons.
- Stimulated emission and its characteristics are explained by quantum mechanics.

Design and Characteristics of Lasers
- Lasers are characterised by their coherence, both spatial and temporal.
- Spatial coherence is expressed through the output being a narrow beam, which can be focused to tiny spots or have low divergence.
- Temporal coherence implies a polarised wave at a single frequency, with phase correlation along the beam.
- Lasers can emit a broad spectrum of light or different wavelengths simultaneously.
- Certain lasers are not single spatial mode and have light beams that diverge more than the diffraction limit.
- Gain medium, laser pumping energy, high reflector, output coupler, and laser beam are components of a laser.
- Mode-locked lasers are capable of emitting extremely short pulses.
- The pulses repeat at the round-trip time of the resonator.
- Mode-locked lasers are used in femtosecond physics, femtosecond chemistry, and ultrafast science.
- Titanium-doped, artificially grown sapphire (Ti:sapphire) is a suitable gain medium for mode-locked lasers.
- Consecutive pulses from a mode-locked laser are phase-coherent and identical.

Applications of Lasers
- Lasers are used in optical disc drives, laser printers, barcode scanners, DNA sequencing instruments, and fiber-optic communication.
- They are also used in semiconducting chip manufacturing, laser surgery and skin treatments, cutting and welding materials, and military and law enforcement devices.
- Lasers are employed in laser lighting displays for entertainment purposes.
- Semiconductor lasers in the blue to near-UV are used in car headlamps.
- Lasers have replaced light-emitting diodes (LEDs) in some applications to excite fluorescence as a white light source.

Operation and Types of Lasers
- Lasers can operate in continuous or pulsed mode.
- Continuous-wave lasers have constant power output over time.
- Many lasers lase in multiple longitudinal modes simultaneously.
- Pulsed lasers produce optical power in pulses at a certain repetition rate.
- Some lasers cannot be operated in continuous mode due to limitations.
- Pulsed pumping is another method of achieving pulsed laser operation.
- Flash lamps and other pulsed sources are used to pump the laser material.
- Pulsed pumping was historically used with dye lasers.
- Three-level lasers require pulsed pumping.
- Excimer lasers and copper vapor lasers cannot be operated in continuous wave mode.