Guest post by Rachel Won International Editor, Nature Photonics
This week’s set of experiments featured in our poll are all about the advent of the maser (microwave amplification by stimulated emission of radiation) and the optical maser, now known as the laser, and the remarkable wide impact these inventions had in science, technology and society.
The concept of stimulated emission was introduced in 1917 by Einstein, who found that the process of absorption by atoms must be accompanied by an amplification process such that the received radiation can stimulate the emission of the same kind of radiation. It was not until 1953 that the effect was experimentally demonstrated by Charles Townes and his two graduate students at Columbia University in New York. Their maser used stimulated emission in a stream of energized ammonia molecules to produce amplification of microwaves at a frequency of about 24.0 GHz. The development of a maser was simultaneously carried out by Nikolay Basov and Alexander Prokhorov at the Lebedev Institute in Moscow.
The achievements led to the award of the Nobel Prize in Physics in 1964 to Townes, Basov and Prokhorov.
The invention of the maser kicked off a race to create a similar device for visible light, now known as a laser (with ‘microwave’ replaced by ‘light’). In 1958 Townes, together with Arthur Schawlow, then at Bell Labs, published a paper extending the maser techniques to the infrared and optical region. The first working laser, however, was built by Theodore Maiman at Hughes Research Laboratories in 1960. His laser used a solid-state synthetic ruby crystal pumped by a flashlamp to produce red laser light at 694 nm.
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Theodore Maiman and his invention, the first laser. (Photo credit: HRL Laboratories, LLC)
With the invention of laser, the possibility of high-density data transmission with light emerged, and optical fibres have been the pre-requisite for this development. When light travels from a medium with a higher refractive index to one with a lower refractive index, light can be totally reflected, a phenomenon known as total internal reflection. Based on this principle, optical fibres are composed of a high-refractive-index core surrounded by a low-refractive-index cladding layer. The principle of light transmission through optical fibres was known early on, but high optical losses had limited long-distance light transmission.
In 1966, Charles Kao and George Hockham from the English company Standard Telephones and Cables suggested that impurities in the glass were the cause of the fibre attenuation. They proposed that, for high-purity silica glass, the attenuation of light could be reduced to 20 dB km−1 from 1,000 dB km−1 at that time. Nowadays, optical losses are at ~0.2 dB km−1 at the telecommunication wavelength of 1.55 μm, facilitating a plethora of applications. The combined development of lasers and low-loss optical fibres has directly enabled the availability of huge amounts of information at our fingertips via internet. Kao was awarded the Nobel Prize in Physics in 2009 for his groundbreaking achievements in fibre optic communications.
As a final note on this week’s experiments, looking back it is interesting to see that the path from Einstein’s original theory of stimulated emission to the present-day wide-spread use of lasers has not been straightforward. When the first working laser was reported in 1960, it was described by Maiman as “a solution
looking for a problem.” Now, not only are lasers used in almost any technology including, for instance, data storage, healthcare and manufacturing, but they also play an essential role in science. Lasers can be made as tiny as a single atoms, which can produce nonclassical light that could have applications in quantum information technology, whilst on the other extreme, National Ignition Facility (NIF), the world’s largest and most energetic laser facility, is working on using powerful lasers to produce fusion energy on a scale which can solve global energy problem and even to re-create the Big Bang.
Experiments covered this week:
1953 Demonstration of the maser by Townes, Gordon and Zeiger
1960 Demonstration of the laser by Maiman
1960s Fibre optics for data transmission
Next week Amos Martinez writes about the discovery of cosmic microwave background from faint radio waves and the laser frequency combs for precision metrology.