What is a Laser?

A laser is an intense light source that is coherent, directional, and monochromatic. This means that the phases of the light waves are aligned, the waves travel in a single direction, and they emit a narrow range of wavelengths1 .

This is different to other light sources, such as lamps or light-emitting diodes (LEDs), which are typically less intense, non-coherent (phases of light waves are out of sync with each other), and emit a wider range of wavelengths in many directions (Figure 1). 

Figure 1: Differences between lasers and standard light sources (a) Coherent and directional emission from a laser source. The dashed lines indicate the alignment of phases of the light waves. Lasers typically emit more intense light with a narrower range of wavelengths. (b) Non-coherent and non-directional emission of light from a lamp. These types of light sources can emit a wide range of wavelengths.

Lasers can be categorised as continuous wave (CW) or pulsed based on the duration of light emission. CW lasers output a constant beam, whereas pulsed lasers output a periodic pulse, and can achieve higher intensities. The width of the pulse may range from seconds to femtoseconds (10^-15 s). The repetition rate describes how many pulses fire per second, in Hz.

How Do Lasers Work?

Lasers work by stimulated emission, one of three main processes that Einstein used to describe how matter exchanges energy with electromagnetic radiation, or light. This process is what gave rise to the word “laser”, which is an acronym meaning Light Amplification by Stimulated Emission of Radiation2 .

To explain this concept, we can consider an atom that has a ground energy level, E1, and a higher energy level, E2. We can call the difference in the energy between these levels ΔE (Figure 2).

• If a photon, with energy, hν ≥ ΔE is incident to an atom in E1, then it can be promoted to E2. This is called absorption.
• An atom in E2 can drop back down to E1 and emit that energy as another photon with energy = ΔE. This is called spontaneous emission.
• Let us imagine a third case where an atom in E2 is incident to another photon with energy = ΔE. This can force the atom to drop back down to E1, in the process emitting a photon of light, whilst the incident photon is conserved. This is called stimulated emission. Crucially, the emitted photon will have the same energy (wavelength), the same phase (coherence), and will retain the same direction as the incident photon.

Figure 2: The three main processes that may occur when atoms exchange energy with light. Spontaneous and stimulated emission are classed as radiative processes (as they involve the emission of radiation, or light).

Population Inversion and Pumping

To achieve lasing, the rate of stimulated emission needs to be higher than the rate of absorption. For this to happen, the number of atoms (N), or population,  in a higher energy state needs to be greater than the population in the ground state. This is known as population inversion. At thermal equilibrium, inversion is disfavoured, so we need to provide the material with energy to populate the higher energy states until N₂ > N1. This is known as “pumping” and may be achieved by irradiation with a lamp, another laser, or by electrical current.

In a two-energy level system it is not possible to achieve population inversion. This is because the pumping energy is equally likely to stimulate emission as it is to be absorbed, which at best will result in N1 = N2. As such, at least three levels are required.

In a three-energy level system, the following stages occur (Figure 3):

1. Pumping with energy ΔE = E3-E1 excites some atoms from E1 to E3.
2. Atoms populating an unstable E3 level quickly decay to E2, which generates more space in E3 for further pumping. The population in E2 is much more stable (metastable) and is therefore able to stay longer in E2 than in E3.
3. An incident photon with energy ΔE= E2-E1 induces stimulated emission back to E1, releasing light.

Figure 3: The stages of generating laser emission with a three-energy level laser system. Population of higher energy states with respect to the ground state (N_2,3 > N₁) results in population inversion.

Because the pumping and stimulated emission photons are of different energies, the pump does not induce stimulated emission. Also, the metastable state ensures that population of E2 and E3 will be greater than E1 (N2,3> N1), resulting in population inversion! Four-level systems also exist and work under a similar principle.

What is a Laser Made of?

All lasers comprise three essential components: a gain medium, a pump, and a cavity3 (Figure 4):

Table 1: Essential components of a laser and their functions.

Figure 4: Schematic of a typical laser. Pump energy raises the gain medium to higher energy state, where a population inversion is reached. Light generated by stimulated emission bounces back and forth in the laser cavity, reflected by mirrors at either end, self-stimulating more emission. The laser beam is released via partially reflective mirror.

To start the laser, the following steps occur:

1. Pumping energy raises the gain medium to a higher energy level.
2. The gain medium spontaneously emits a photon.
3. Photons travel along the cavity and are reflected back, stimulating more emission (amplification) in a chain reaction on every round trip.
4. Once the number of photons emitted exceeds photons lost through the partial reflector or as heat, the laser threshold is reached.
5. The partial reflector releases a portion of the light as the output laser beam.

Laser Applications

There are several types of different lasers that can be used for a wide range of applications. A short list of some well-known lasers and some of their uses is given below (Figure. 5). 4 :

Solid-state lasers:

• Titanium-Sapphire (Ti:Sa) – spectroscopy and research.
• Neodymium: Yttrium Aluminium Garnet (Nd:YAG) – laser eye surgery, cancer treatment, laser hair removal, manufacturing, spectroscopy.
• Semiconductor laser diodes – barcode readers, telecommunications, DVD players, spectroscopy.

Gas-state lasers:

• Helium-Neon (HeNe) lasers – spectroscopy.
• Copper Vapour lasers (CVL) – laser light shows, laser cutting.
• Carbon dioxide (CO2) lasers – laser cutting and welding, laser surgery.
• Nitrogen (N2) lasers – pumping of liquid-dye lasers, mass spectrometry, lidar.

Dye lasers:

• Organic fluorescent dyes dissolved in solvent.
• Examples include Rhodamine 6G and Coumarin 485 – used in dermatology to treat skin conditions and tattoo removal.

Figure 5: Applications of lasers: Tattoo removal (top left), spectroscopy and research (top right), laser cutting (bottom left), laser light shows (bottom right).

Edinburgh Instruments Lasers

At Edinburgh Instruments, we produce lasers for spectroscopy. This includes CO₂ lasers, our range of pulsed solid state lasers, and our picosecond wavelength tunable AGILE laser (Figure 6).

We also integrate pulsed Nd:YAG lasers into our LP980 spectrometers for transient absorption and phosphorescence spectroscopy, as well as Ti:Sa lasers into our FLS1000 spectrometers for fluorescence spectroscopy. CW laser diodes are used in our RMS1000 and RM5 Raman microscopes.

Figure 6: Edinburgh Instruments’ pulsed diode EPL lasers (top), the PL5 CO2 laser (bottom left), and picosecond tunable wavelength white light AGILE laser (bottom right).

If you are interested in find out more about our lasers, get in touch with our friendly team by completing this form.

 References

1. Svelto, O. Principles of Lasers (2010). doi:10.1007/978-1-4419-1302-9.
2. Young, H. & Freedman, R. University Physics with Modern Physics. Recherche (2007).
3. Corner, L. Introduction to Laser Physics, in Proceedings of the 2019 CERN – Accelerator – School course on High Gradient Wakefield Accelerators, Sesimbra, (Portugal) (2020).
4. Biswas, D. J. Different Types of Lasers, in A Beginner’s Guide to Lasers and Their Applications, Part 1 (2023). doi:10.1007/978-3-031-24330-1_10.