What are three types of lasers


Here you can find out what the word laser actually stands for and why you shouldn't look directly at it.

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Laser explained simply

Laser light is coherent, strongly bundled electromagnetic radiation with high intensity and a very narrow frequency band. Moved on the electromagnetic spectrum, the laser radiation ranges from the far infrared, through the visible, to the X-ray spectrum.

The word laser denotes the device and the physical effect at the same time. Laser is an acronym that stands for "Light Amplification by Stimulated Emission of Radiation". Translated, this means “light amplification through stimulated emission of radiation”.

To be able to produce light of this quality, your laser needs at least three components.
First of all, you need a laser medium that largely determines the properties of the laser. Through optical transitions of excited atoms or molecules into energetically preferred states, you generate photons here. There are different types of laser media, such as gases, crystals or diodes.

What is needed next is a pumping mechanism. With this you supply the medium with the necessary energy to stimulate the transitions. This can be, for example, a flash lamp or an electrically operated gas discharge.

Finally you need a laser resonator. This is a more or less complex system made up of mirrors and other optical elements. With the resonator you provide the feedback and thus the stimulated emission.
Depending on the choice of these individual components, there are different types of lasers, which differ in the achievable power and frequency properties.

Laser construction

The three components laser medium, pump and laser resonator are common to every laser. You determine the type of laser and what you can achieve with it. In the following we will explain all three components to you more clearly.

Laser medium

You generate photons in the laser medium. This emission arises from optical transitions in excited atoms or molecules. Through these transitions, the particles are transferred into energetically more favorable states. The most important requirement of a laser medium is that a population inversion can be produced. To do this, it must have at least three energy levels. Energy levels are energy eigenvalues ​​of quantum mechanical systems. It is only ever possible that an atom or molecule is in such a level. The lowest level is the ground state and all others are excited states.
Population inversion here means that the upper state of an optical transition is more likely to be occupied than the lower state. Such media can be either gaseous, liquid or solid.


Optical pumping describes the process with which you add energy to the medium. This happens when the laser medium is excited by an external energy source, such as other lasers or a flashlight. In this way you achieve the population inversion without the pumping process competing with the stimulated emission. Therefore, a different quantum mechanical transition is pumped than is ultimately used for photon emission.

Laser resonator

With a laser resonator you determine the emission rate and the properties of the photons. Through reflections, you let individual photons pass through the medium several times. This stimulates further emissions in the direction you want and enables the light to be amplified. To do this, the photons must propagate perpendicular to the reflecting media. The photons emitted in this way have the same quantum numbers as the triggering photons. Any spontaneous emissions that may occur do not themselves generate any further photons, since it is very likely that they do not radiate perpendicular to the reflecting media.
Due to this selection, you can achieve a very narrow beam direction with your laser beams.

Laser function

You excite the atoms or molecules of the laser medium to higher energy levels. With this you generate a laser beam. These energy levels have the longest possible mean decay time. This way you keep the probability of spontaneous emissions as low as possible and the energy of the pumping process is retained longer. Continuous pumping creates the desired population inversion. That means more particles are in one of their excited states than in their ground state.

Now only a stimulation by a photon has to take place so that an excited atom falls back from its excited state to its ground state. It emits a photon in the same direction and with the same energy as the generating photon. In this case, the same energy means that the new photon has the same frequency and wavelength as the original photon. The phase position of both photons is also the same.

As previously described, the photons are reflected in the resonator and pass through the medium several times. This process leads to a chain reaction in which more and more photons are generated, which in turn generate more and more photons.
One side of the resonator is partially transparent in order to enable the laser beam to be deflected. As a result, the reflective property of the resonator is retained and further emissions take place.

Spontaneous emission

The spontaneous emission is a quantum mechanical phenomenon. It occurs when atoms or molecules emit photons on transitions from higher to lower energy levels; it is not possible to predict this type of emission. It is a disintegration process whose occurrence can only be provided with a certain probability.
Mathematically, you can express it as follows:

The formula says the number the spontaneous emissions or the excited particles per volume and time , proportional to the particle number density is in the excited state.

Stimulated emission

The function of the laser is based on the stimulated emission. In this case, the emission of the photon does not occur spontaneously. Instead, another photon stimulates the transition to an energetically more favorable level, which leads to the emission of a photon.

In order for this transition to take place, a photon with an energy that corresponds to the energy difference between the excited level and the ground state must interact with the excited atom. As a result, the excited atom changes to the lower energy state and emits a photon of the corresponding energy difference. This is coherent with the original photon and flies in the same direction as this.

Coherence means that the new photon has the same energy (wavelength and frequency), the same direction, the same polarization and the same phase position as the incident photon.

Longitudinal modes

Depending on the design of the resonator, different numbers of standing waves, of certain wavelengths, can develop in it. Thus certain wavelengths and their multiples can be particularly amplified by such a resonator. You call such different waveforms modes. With the number of longitudinal modes, you know how many waves can oscillate in the resonator. You describe vibrations along the direction of propagation of the radiation as longitudinal. These are intensity peaks and valleys at a distance of half a wavelength.
When it comes to lasers, you differentiate between single-mode lasers, which oscillate at almost one frequency, and multi-mode lasers.

Transverse modes

A transverse mode describes the distribution of the phase position of the wave perpendicular to the direction of propagation. Consequently, a mode which is not perpendicular to the resonator mirrors leads to a shift in the laser frequency. The reason for this is the increased resonator length, which now leads to the formation of standing waves with nodes in the laser profile.

If you use a cylindrical resonator, your beam ideally forms a Gaussian profile. In the case of modes that are not perpendicular to the resonator mirrors, profiles with radial and angular dependencies are formed instead. This changes the length of the resonator, since the length of the path between the mirrors is changed. As a result, the longitudinal mode spectra can be falsified, since different transverse modes are superimposed.

Laser properties

It is not possible to make a general statement about the properties of a laser. These are determined by various aspects. The resonator of a laser primarily determines its qualities. It is also incorrect in this context that lasers are always tightly bundled beams with a narrow frequency range.
But what is true is that light can be manipulated extremely well with lasers and its properties make it possible to bundle the rays very closely. This enables you to achieve very high power densities.

The most relevant properties of lasers are coherence, polarization and frequency or wavelength.

In contrast to other light sources, the light from a laser does not consist of just one wavelength. The waves are also almost in phase with one another. This is where the term coherence length comes from. This term gives you a statement about the distance over which the waves of a laser are in phase.

The polarization of a transverse wave describes the direction of its oscillation. With lasers, all waves have the same polarization. This is mostly linear, but other polarizations are also set depending on the area of ​​application. Different polarizations are achieved by optical components in the resonator or in the beam path.

The wavelength of the laser is determined by the laser medium. Depending on its energy transitions, the medium can be excited for lasing at different wavelengths or only over a very narrow bandwidth.

Laser dangers

Depending on the power, lasers cause damage to biological tissue.
Even outputs in the milliwatt range damage the eye. The lens focuses the parallel laser beam on the retina. This causes damage to the retina that leads to partial blindness.

Higher performance leads to sunburn-like damage to the skin, which can also cause skin cancer. This damage can range up to severe burns.

You should especially watch out for stray light. Laser light already reflected on a wall or other surface leads to the corresponding damage. Therefore, when working with lasers, the laser safety precautions must always be taken into account.

Laser application

The development of the laser has changed our world significantly. It has found its way into all areas of our life.

In everyday life you will find lasers in laser printers and from CD to Blue-Ray disc players in every optical drive. But you probably also know the laser pointer, which already has the word laser in its name. Lasers are also used for every purchase at the checkout to identify the barcodes on the goods.

There are of course a number of other areas of application in everyday life.

But lasers are also constantly used for data acquisition, in industry, medicine, science and the military.

So you see, lasers are not just science fiction devices. The laser is an essential part of our daily life.