Lasers, what are they and how do they work?

Earlier this year I celebrated the fiftieth anniversary of the Moon landing of Apollo 11 by publishing a series of eight articles about the ‘Space Race’ of the 1960s. I enjoyed that task so much that I decided to write a few more posts about some of the other cool technologies that made the news during that decade; I hope you’ve been enjoying them.

In this post I’ll be talking about lasers, those intense beams of light that can not only cut through steel but also read the data off of DVDs, print our documents, are used to measure distances with extreme accuracy and are even used in medical surgery, especially eye surgery. Most people know that the word LASER is an acronym standing for ‘Light Amplification through Simulated Emission of Radiation’ and many have heard that the property that makes a laser beam different is something called ‘coherence’.

In the 1960s Lasers were devilish weapons the bad guys used to threaten James Bond! (Credit: Pinterest)

This coherence actually comes in two forms, spatial and temporal. In spatial coherence the photons, the particles of light are emitted in very precisely the same direction resulting in the very narrow beam that lasers are best known for. Temporal coherence means that those photons all have very precisely the same frequency. It also allows laser pulses to be very accurately timed, On and Off along with the amount of time a pulse lasts.

The property that makes a laser beam different from ordinary light is called Coherence. (Credit: Sciencewise.info)

The phenomenon that produces Laser light is a purely quantum mechanical effect that was first recognized by Albert Einstein in 1917. Einstein was working on the problem of how an atom with its electrons in the minimum energy or ‘ground state’ can be excited into higher energy state by absorbing a photon of just the right energy. At the same time an atom that is in an excited state can also decay back into its ground state by emitting a photon with the exact same energy.

Einstein was calculating the probabilities of a Atom absorbing or emitting a photon when he discovered the possibility of Stimulated Emission. (Credit: Sciencewise.info)

As he calculated the probability per unit time of that decay process occurring Einstein also noticed that the probability increased a lot if another photon of just the right energy happened to be near the excited atom. The photon stimulated the decay of the atom and the emission of a second photon identical to the first!

For the next several decades simulated emission remained nothing more than an interesting possibility. It wasn’t until 1928 that Rudolf W. Ladenberg even confirmed its existence but the idea of a practical usage of the phenomenon seemed almost impossible. That would require a material that had the majority of its atoms in the excited state, at condition called a population inversion and which was thought to never occur in nature.

It wasn’t until 1951 that Joseph Weber suggested that a microwave cavity could be used to produce a population inversion, by confining the atoms and pumping in energy the atoms could be ‘supercharged’. A small microwave signal could then simulate all of the excited atoms to decay greatly amplifying that small initial signal. The device was first build two years later and called a Maser for Microwave Amplification through Simulated Emission of Radiation. Masers are still used today by radio astronomers to amplify the extremely weak signals they study.

Charles Townes and the first MASER, a radio version of a laser that was actually invented first! (Credit: IEEE Spectrum)

With the Maser showing how it could be done the hunt was on for a material that could create a population inversion at optical wavelengths. As often happens nowadays there were several teams of researchers who came close, Bell Labs and Columbia University among others. It was Theodore H. Maiman at Hughes Research Laboratories who produced the first laser by employing a synthetic ruby crystal pumped by a flashlamp to produce a pulse of red laser light at a wavelength of 694 nanometers.

Theodore Maiman holding the parts of the first Laser. (Credit: SciHi Blog)

It wasn’t long before continuous lasers were also developed using helium or neon as the lasing material. Then in 1970 Zhores Alferov in the USSR along with Izuo Hayashi and Morton Panish at Bell Labs demonstrated that semiconductor material could also be employed as a lasing material. Before long lasers were being manufactured cheaply and in mass quantities.

The workings of a semiconductor laser. (Credit: Google Sites)

Today lasers are everywhere; they are in the checkout scanners at supermarkets, our DVD and CD players and laser printers and if you get your TV signal on optical fiber it’s a laser that transmits the signal going through the fiber. Research into new types of lasers is ongoing and laser manufacturing is a big industry. The non-semiconductor laser industry is today valued at more than $2 billion dollars while semiconductor lasers total more than $3 billion.

Today Lasers are so cheap you can buy one for just a few bucks! (Credit: Thorlabs)

Lasers seemed almost magical back in the 1960s, a symbol of how far our science and technology had come in its control of nature. Today we pretty much take them for granted. That’s progress I suppose.

World’s Most Powerful X-ray Laser generates its first Light (and just what is a Laser anyway?)

The European XFEL, a powerful new scientific instrument based at Germany’s nuclear research institute DESY near Hamburg has produced its first light pulses. The light produced by the XFEL (which stands for X-ray Free Electron Laser) has a wavelength of 0.8 billionths of a meter, that’s about 500 times shorter than the wavelength of visible light.

Although the XFEL only produced a single pulse of light as a test of it’s performance, when it is fully operational in September the instrument will produce 27,000 pulses every second. Also, while the X-ray photons produced by the XFEL are only considered “soft” X-rays, with an individual photon energy of around 10,000eV, the intensity of the light, that is the number of photons produced will be greater than any other X-ray source on Earth.

The research planned for the XFEL includes taking photographic images of individual atoms, investigations into just what is going on during chemical reactions, especially bio-chemical reactions, and even studies of conditions existing in the interior of planets

Now a Free Electron Laser (FEL for short) is a very different kind of Laser from the Laser pointers or Gas Lasers people are more familiar with. I happen to know a lot about the differences because I wrote a paper describing those differences for my course in Quantum optics back in grad school. My professor for that course was Lorenzo Narducci, a well known and highly regarded researcher in Quantum Optics. Doctor Narducci adamantly insisted that those differences were such that Free Electron Lasers were not actually Lasers. Which of course begs the question; what is a Laser.

Many people know that the acronym Laser stands for Light Amplification by Stimulated Emission of Radiation and they know that a Laser’s light is special because it is a very narrowly focused beam of only a single frequency or colour. However, the way that a Laser produces that light isn’t commonly understood, so I’m gonna tell ya.

Laser Emission

Looking at the picture above we can see how an atom in its ground state can absorb a photon of light at a certain wavelength (spontaneous absorption) becoming excited in the process. The atom will then emit that photon again at a later time (spontaneous emission).

Funny thing is though, if while the atom is excited by the first photon a second photon comes along similar to the first (same wavelength) the second photon can stimulate the atom to emit its stored photon (hence the Stimulated Emission in Laser) and the two photons will fly off in the same direction together in step with each other, this is called coherence.

Do this with a lot of atoms all at once (something known as a population inversion) and you get the powerful flood of coherent light we call a Laser. It’s kind of like the difference between a lot of people just walking around and an army of men marching in step. A Laser is more powerful just as the army is more powerful.

Now a Free Electron Laser produces its light by a completely different mechanism. A beam of electrons is accelerated to close to the speed of light. This is usually done using what is known as a Linear Particle Accelerator and the Linear Accelerator at DESY for their FEL is 3.4 kilometers in length.

This beam of high energy electrons is then directed through the center of a device called an undulator where thousands of permanent magnets are arranged with alternating magnetic poles, north-south then south-north back to north-south then south-north and on and on. See picture below.

Free Electron Laser Undulator

The charge on the electrons in the beam interacts with the magnets causing the electrons to undulate back and forth, switching direction every time the poles of the magnets flip and this switching back and forth produces a high intensity beam of light whose wavelength is determined by the spacing of the magnets and the velocity of the electron beam.

Now the light from a FEL does have several characteristics in common with the light from a Laser, the output beam is both very narrowly focused and the photons produced are almost exactly the same wavelength. However the light is certainly not produced by Stimulated Emission of Radiation, which is why Professor Narducci refused to consider FELs to be true Lasers.

Whether or not you decide that Free Electron Lasers are real Lasers the world will soon have powerful new instrument for the study of the interaction of matter and light. I look forward to the results that will come from the European FEL. By the way Professor Narducci gave me an A in quantum Optics!