Abstract

A free-electron laser (FEL) is a fourth-generation light source that produces short bursts of extremely intense radiation.Similar to a laser, a FEL uses relativistic electrons as a gain medium rather than stimulated emission from atomic or molecular excitations.[22] [23] In a FEL, a collection of electrons moving through a magnetic device called an undulator or wiggler produces radiation.After then, the radiation interacts with the electrons to produce coherent emission, which exponentially increases the radiation\'s intensity.Because electron kinetic energy and undulator parameters can be changed as needed, free-electron lasers are more adjustable than other laser types and can be built for a wider frequency range. [24]At the moment, their wavelengths span the visible spectrum, ultraviolet, X-ray, terahertz radiation, infrared, and microwaves.[25]

Introduction

A laser is a device that emits light through optical amplification based on stimulated emission of radiation. The term laser originally stood for Light Amplification by Stimulated Emission of Radiation. The first working laser was built by Theodore Maiman in 1960, based on theoretical work by Charles Townes and Arthur Schawlow. Lasers emit coherent light, meaning the light waves are highly synchronized. This coherence makes lasers unique, allowing them to be focused tightly, travel long distances with very low spread, and operate at narrow wavelengths or produce extremely short pulses.

Because of these special properties, lasers are used in:

• Cutting and welding

• CD/DVD players, laser printers, barcode scanners

• Fiber-optic communication and photolithography

• Medical treatments and surgery

• Entertainment lighting, military targeting, and speed-range measurements

• Blue/UV semiconductor lasers in automobile headlights

History and Development

The concept of stimulated emission was first introduced by Einstein (1916). In 1928, Rudolf Ladenburg observed stimulated emission experimentally.
In the 1950s, Charles Townes developed the first maser (microwave amplification device). Townes, Basov, and Prokhorov received the 1964 Nobel Prize for this work.

Townes and Schawlow later proposed the optical maser, now known as the laser. A patent conflict involved Gordon Gould, who eventually received several patents and coined the term laser.

Major milestones:

• 1960: Maiman creates the first ruby laser

• 1960: Javan, Bennett, Herriott develop the first He–Ne gas laser

• 1962: Hall demonstrates the first semiconductor laser

Early lasers found applications in alignment, eye surgery, and holography. By the 1970s–80s, they entered daily life through laser scanners, printers, and CD players.

Principle of Laser Operation

Atoms have discrete energy levels. When electrons move from higher to lower levels, they emit photons. This can happen in two ways:

1. Spontaneous emission

2. Stimulated emission (key to laser action)

For stimulated emission to dominate, a population inversion must be created—more atoms in the excited state than in the lower state. This is achieved through pumping (light or electric current).

Different laser systems:

• Three-level lasers (e.g., ruby laser, pulsed)

• Four-level lasers (can operate continuously)

Laser Components and Beam Characteristics

A laser typically includes:

• A gain medium (gas, solid, semiconductor)

• A pumping source

• An optical resonator (two mirrors)

Light bounces between mirrors, amplifying with each pass until part of it exits as a laser beam.

Laser light is:

• Highly collimated (narrow beam)

• Monochromatic (nearly single wavelength)

• Coherent (phase-synchronized)

• Highly intense and focusable

Beam divergence depends on the wavelength and size of the emitting aperture. Lasers may produce continuous-wave output or high-intensity pulses (down to femtosecond durations).

Free-Electron Lasers (FEL)

A free-electron laser is a type of fourth-generation light source that uses relativistic electrons, not atoms, as the gain medium. As electrons pass through an undulator (a magnet array), they emit radiation that becomes coherent through a process called microbunching.

Key features:

• Extremely bright, short pulses

• Tunable over a very broad wavelength range (microwave → X-ray)

• First demonstrated in 1971 by John Madey

FELs work by accelerating electrons to near light speed, passing them through magnetic fields, and amplifying the emitted radiation through electron–photon interaction.

Conclusion

In summary, FELs can produce an output with significantly higher spectrum brightness and coherence when compared to other synchrotron radiation sources (pure undulators and wigglers). Numerous domains, including atomic and molecular physics, ultrafast X-ray science, advanced material studies, ultrafast chemical dynamics, biology, and medicine, can benefit greatly from this.A few of FELs also demonstrate exceptionally high precision, such as when it comes to temporal pulse positioning or beam position stability and concentrating of generated X-rays down to spots with only a few nanometers in diameter [31].The main disadvantage of FELs is that only a small number of large facilities worldwide can employ them due to the high cost and size of their setups. In Hamburg, a very ambitious free electron laser project (European XFEL, once part of the TESLA project, now part of a European project) is being pursued [32]. With wavelengths as low as 0.05 nm, pulse durations as short as 100 fs, and exceptionally high brightness, the 3.4 km long XFEL produces hard X-ray output with previously unheard-of performance qualities. Lasing wavelengths below 0.15 nm, or a photon energy of 10 keV, have already been attained by the LCLS at SLAC.

References

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Copyright

Copyright © 2025 Muhammad Arif Bin Jalil. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.