LASER stands for Light Amplification by Stimulated Emission of Radiation. A laser is a device that makes a narrow, intense beam of light of (almost) one wavelength, in which all the light waves are in step with each other. It does this with three things: a gain medium, a pump that feeds it energy, and a pair of mirrors that bounce the light back and forth so it builds up. The first working laser, built by Theodore Maiman in 1960, used a ruby crystal and gave red light at 694 nm.
What is the full form of LASER?
Each word in the acronym describes one part of the physics:
- Light: the output is electromagnetic radiation, from ultraviolet through visible to infrared.
- Amplification: the light gets stronger as it passes through the gain medium.
- Stimulated Emission: the process that does the amplifying (explained below).
- Radiation: the energy leaves as photons.
Today “laser” is written as an ordinary word in lower case, and it also gave us the verb “to lase”, meaning to produce laser light.
How does a laser work? Stimulated emission and population inversion
Atoms (or molecules, or electrons in a semiconductor) sit in definite energy levels. Light interacts with them in three ways:
- Absorption: a photon with exactly the right energy lifts an atom from a lower level to a higher one.
- Spontaneous emission: an excited atom drops back on its own and releases a photon in a random direction at a random time. This is how a bulb or an LED gives light.
- Stimulated emission: a passing photon of the right energy triggers an excited atom to drop down early. The atom releases a second photon that is a copy of the first: same wavelength, same direction, same phase. One photon in, two identical photons out.
Stimulated emission only wins over absorption when more atoms are in the upper level than in the lower one. This condition is called population inversion. In normal thermal equilibrium the lower level is always more crowded, so an inversion has to be forced by pumping energy in. It cannot be done with only two levels, which is why practical lasers use three-level or four-level schemes: the pump lifts atoms to a high level, they fall quickly to a longer-lived upper laser level, and they lase down to a lower level that empties fast.
The three essential parts of a laser
| Part | What it does | Examples |
|---|---|---|
| Gain (active) medium | The material whose atoms or molecules are inverted and amplify light by stimulated emission. It sets the wavelength. | Helium-neon gas mix, CO2 gas mix, Nd:YAG crystal, ytterbium-doped glass fibre, a semiconductor junction |
| Pump (energy source) | Supplies the energy that creates population inversion | Electrical discharge in gas lasers, flash lamps or laser diodes for solid-state lasers, electric current in diodes |
| Optical resonator (cavity) | Two mirrors facing each other send the light back through the medium many times so it keeps growing. One mirror reflects almost fully; the other (the output coupler) lets a small fraction out as the beam. | In a He-Ne tube, one mirror reflects about 99.9% and the output coupler transmits about 1% |
Lasing starts from a few spontaneous photons that happen to travel along the cavity axis. Each pass through the gain medium multiplies them, and once the gain per round trip exceeds the losses (mirror transmission, scattering, absorption), the beam builds up to a steady power.
Properties of laser light
- Monochromatic: the output covers a very narrow band of wavelengths. A He-Ne laser emits at 632.8 nm; a white LED spreads over roughly 400 to 700 nm.
- Coherent: the waves keep a fixed phase relationship over a long distance and time. This is what makes interferometry and holography possible.
- Collimated (directional): the beam spreads very little. Because the cavity only amplifies light travelling along its axis, the divergence is tiny compared with a torch.
- High intensity: since the power is packed into a narrow beam, a lens can focus it to a spot a few micrometres to a fraction of a millimetre wide. That power density is what lets lasers cut, weld and engrave.
Types of laser by gain medium
| Type | Gain medium | Wavelength | Typical use |
|---|---|---|---|
| Gas: helium-neon | He-Ne mixture (about 5:1 to 10:1), electrical discharge | 632.8 nm (red) | Alignment, barcode scanners, physics labs; output usually 0.5 to 50 mW |
| Gas: carbon dioxide | CO2 with nitrogen and helium, electrical discharge | 10.6 µm (far infrared) | Cutting and engraving wood, acrylic, sheet metal |
| Solid-state: Nd:YAG | Neodymium-doped yttrium aluminium garnet crystal, pumped by lamps or diodes | 1064 nm (near infrared) | Welding, marking, medicine, range-finders |
| Solid-state: ruby | Chromium-doped aluminium oxide crystal, flash lamp | 694 nm (red) | Historical first laser, some holography |
| Fibre | Glass fibre doped with rare earths such as ytterbium or erbium, diode-pumped | About 1.03 to 1.12 µm for ytterbium | Metal cutting, welding, marking |
| Semiconductor (diode) | p-n junction, electric current | 405 nm (Blu-ray), 445 to 465 nm (blue), 650 nm (red pointers, DVD), 808 nm (pumping), 1310 and 1550 nm (optical fibre links) | Pointers, optical drives, fibre-optic communication, pumping other lasers |
| Excimer | Noble gas plus halogen, e.g. ArF, KrF, XeCl | 193 nm, 248 nm, 308 nm (ultraviolet) | Chip lithography, eye (LASIK) surgery |
| Dye | Organic dye in a liquid solvent | Tunable over a wide band | Spectroscopy, research |
Continuous-wave vs pulsed lasers
A continuous-wave (CW) laser gives a steady output, which suits cutting and welding where a constant heat input is wanted. A pulsed laser releases its energy in short bursts. Techniques such as Q-switching and mode-locking squeeze the energy into pulses of nanoseconds down to femtoseconds, so the peak power can be thousands of times the average power. Short pulses remove material before the heat spreads, which is why they are used for marking and fine machining.
Worked example: energy of a laser photon
The energy of one photon is
E = hc / λ
where h = 6.626 × 10-34 J s (Planck’s constant), c = 2.998 × 108 m/s and λ is the wavelength in metres. Divide by 1.602 × 10-19 J/eV to get electron-volts.
He-Ne laser, λ = 632.8 nm = 632.8 × 10-9 m
- E = (6.626 × 10-34 × 2.998 × 108) / (632.8 × 10-9) = 3.14 × 10-19 J
- In eV: 3.14 × 10-19 / 1.602 × 10-19 = 1.96 eV
CO2 laser, λ = 10.6 µm = 10.6 × 10-6 m
- E = (6.626 × 10-34 × 2.998 × 108) / (10.6 × 10-6) = 1.87 × 10-20 J
- In eV: 0.117 eV, about one-seventeenth of a He-Ne photon
How many photons per second? A 1 mW He-Ne laser emits 1 × 10-3 J/s, so it sends out 1 × 10-3 / 3.14 × 10-19 = about 3.2 × 1015 photons every second.
A quick shortcut for exams: E (in eV) is about 1240 / λ (in nm). For 632.8 nm, 1240 / 632.8 = 1.96 eV, matching the long calculation. Shorter wavelength means higher photon energy, which is why ultraviolet lasers can break chemical bonds directly while CO2 lasers work mainly by heating.
Laser safety classes (IEC 60825-1)
The international standard IEC 60825-1 groups laser products by how much radiation a person can be exposed to:
- Class 1: safe in all normal use (often because the laser is fully enclosed, as in a DVD drive).
- Class 1M: safe unless viewed through magnifying optics.
- Class 2: visible lasers up to 1 mW continuous; the blink reflex protects the eye. 2M is the same but unsafe through optics.
- Class 3R: visible continuous lasers up to 5 mW; low but real risk of eye injury.
- Class 3B: up to 0.5 W continuous (315 nm to far infrared); direct beam is hazardous to the eye.
- Class 4: above Class 3B limits; direct and scattered beam can injure eyes and skin and start fires. Industrial cutting and engraving lasers are Class 4 sources.
Lasers in manufacturing
In a workshop the laser is a heat source that can be focused to a tiny spot and moved by a computer. What changes from machine to machine is the source and how much material it removes:
- A laser cutting machine melts or vaporises right through sheet and plate, with a gas jet blowing the molten metal out.
- A laser engraver removes material to leave a recess you can feel.
- A laser etching machine changes only a very thin surface layer to mark serial numbers, logos and codes.
- The fibre laser is the source behind most modern metal cutting and marking machines.
Beyond manufacturing, lasers are used in fibre-optic communication, surgery, measurement and spectroscopy. Lasers are part of the first-year B.Tech engineering physics syllabus at most Indian universities, and the NPTEL physics and optics courses cover the rate equations in more depth.
FAQs
What is the full form of LASER?
LASER stands for Light Amplification by Stimulated Emission of Radiation. It describes how the device works: light is amplified inside a gain medium by stimulated emission.
What is population inversion in a laser?
Population inversion is the state in which more atoms are in a higher energy level than in the lower level below it. It is needed so that stimulated emission outpaces absorption, and it is created by pumping energy into the gain medium.
Why is laser light monochromatic and coherent?
Stimulated emission produces photons that copy the triggering photon in wavelength, direction and phase, and the resonator only sustains a narrow set of wavelengths. The result is light of nearly one colour with waves in step.
What is the wavelength of a He-Ne laser and a CO2 laser?
A helium-neon laser emits red light at 632.8 nm. A carbon dioxide laser emits far-infrared light at 10.6 micrometres, which the eye cannot see.
Who invented the laser?
Theodore Maiman built the first working laser in 1960 at Hughes Research Laboratories, using a ruby crystal pumped by a flash lamp. It emitted red light at 694 nm.