Laser : Fundamentals

Some important dates

Here are the important steps that led to the discovery of the laser:

  • 1887: Heinrich Hertz accidentally discovers the photoelectric effect. This lucky breakthrough will allow Albert Einstein to introduce the notion of photons.

  • 1901: The scientific conundrum known as “the ultraviolet catastrophe” (spectral energy densities diverge at high frequencies) is solved by Planck. He hypothesises that the energy of a type of frequency ν is not a random continuous variable but a random discrete set of variables, represented by the values nh ν. Interestingly, Planck and his contemporaries at first find it very difficult to accept this idea of discrete leaps in energy. However, subsequent experiments prove that the theory is entirely correct.

  • 1905: Einstein introduces a means to quantify electromagnetic energy. The photon is born. Unfortunately, the arrival of the photon cannot take into account the phenomenon of black body radiation (the spectral density of electromagnetic energy emitted by an enclosed area at a temperature T and at thermal equilibrium). However, shortly afterwards, Born devises a means to quantify the energy levels of electrons (1913). This in turn allows Einstein to prove that photons and black body radiation are in fact compatible thanks to the notion of stimulated emission.

  • 1949: Kastler and Brossel develop the first optical pumping and the first population inversion. By 1950, the first MASERs appear (Microwave Amplification by Stimulated Emission Radiation), devices that are capable of amplifying an electromagnetic wave in the microwave region (Weber, Townes and Basov).

  • 1954: The first MASER is built (an ammonia maser with a 13 mm wavelength). The electromagnetic wave is confined in three dimensions by a “box” and is reflected off its sides. However, this is still in the microwave rather than the optical domain. In fact, scientists at the time thought it was impossible to make an optical laser because the cavity would have to be incredibly small (of the order of magnitude of a wavelength i.e. only tens of μm at the most!).

  • 1958: Schawlow and Townes decide to use an open Fabry-Pérot cavity for their experiments. The idea is to confine the electromagnetic field like in a closed box but in only one dimension: the main axis of light propagation in the cavity. This means that only certain specific electromagnetic waves are amplified, but the resulting beam is much more powerful than when using a closed cavity.

  • 16th May 1960: Maiman demonstrates the first ever optical laser effect. The amplifying medium is a ruby, the crystal most used in early lasers because it was already well known from its application in MASERs. This is a pulsed operation laser with a wavelength of 694.3 nm.

  • 1961: Javan, Bennet and Herriot build the first gas helium-neon laser operating continuously at 1.15 . In fact, this laser can emit over a whole range of discrete wavelengths, from green to infrared via orange and red (633 nm).

  • 1962: First red helium-neon laser.

  • 1965: First semiconductor lasers.

  • 1966: First coloured pulsed lasers (red, orange, yellow).

  • 1970: First coloured continuous-wave lasers (red, orange, yellow).

Since the discovery of the first real laser in 1961, many others have been developed each year. Current research is focused on the development of solid state lasers (diode lasers, crystal or amorphous solids doped with active ions, optic fibre lasers) with the aim of obtaining much shorter pulses (the present limit is 4.5 fs or 4.5 10-15 seconds) and much greater power (emissions of about 10 kilowatts are now common).

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