Lasers: The benign third eye
Shivanand Kanavi
On May 16, 1960 Maiman demonstrated the Ruby Laser. Here is an excerpt from my book "Sand to Silicon: The amazing story of digital technology" on Lasers, their basic science and everyday applications outside of James Bond's Goldfinger and George Lukas' Starwars !
ಸಮಸ್ತ ಕತ್ತಲೆಯ ಮಸಕವ ಕಳೆದಿಪ್ಪ ಇರವ ನೋಡಾ !
ಬೆಳಗಿಗೆ ಬೆಳಗು ಸಿಂಹಾಸನವಾಗಿ,
ಬೆಳಗು ಬೆಳಗ ಕೂಡಿದ ಕೂಟವ
ಕೂಡಲಸಂಗಯ್ಯ ತಾನೆ ಬಲ್ಲ !
--ಬಸವಣ್ಣ, 12th C
“A splendid light has dawned on me about the absorption and emission of radiation.”
—ALBERT EINSTEIN, in a letter to Michael Angelo Besso, November 1916
Photo: Palashranjan Bhaumick
In popular perception, lasers are still identified with their destructive potential—the apocalyptic ‘third eye’ of Shiva. It was no coincidence that the most powerful neodymium glass laser built for developing nuclear weapon technology at the Lawrence Livermore Laboratory in the US (back in 1978) was named Shiva.
Villains used lasers in the 1960s to cut the vaults of Fort Knox in the James Bond movie, Goldfinger. In 1977 Luke Skywalker and Darth Vader had their deadly duels with laser swords in the first episode of Star Wars. As if to prove that life imitates art, Ronald Reagan poured millions of dollars in the 1980s, in a ‘ray gun’, a la comic-strip super-hero stories, with the aim of building the capability to shoot down Soviet nuclear missiles and satellites.
In our real, daily lives, lasers have crept in without much fanfare:
• All sorts of consumer appliances, including audio and video CD players and DVD players use lasers.
• Multimedia PCs are equipped with a CD-ROM drive which uses a laser device to read or write.
• Light emitting diodes (LED)—country cousins of semiconductor lasers—light up digital displays and help connect office computers into local area networks.
• LED-powered pointers have become popular in their use with audiovisual presentations.
• Who can forget the laser printer that has revolutionised publishing and brought desk top publishing to small towns in India?
• Laser range finders and auto-focus in ordinary cameras have made ‘expert photographers’ of us all.
• The ubiquitous TV remote control is a product of infrared light emitting diodes.
• The bar code reader, used by millions of sales clerks and storekeepers, and in banks and post offices, is one of the earliest applications of lasers.
• Almost all overseas telephone calls and a large number of domestic calls whiz through glass fibres at the speed of light, thanks to laser-powered communications.
• The Internet backbone, carrying terabits (tera = a million million) of data, uses laser-driven optical networks.
C.K.N. Patel won the prestigious National Medal of Science in the US in 1996 for his invention of the carbon dioxide laser, the first laser with high-power applications, way back in 1964 at Bell Labs. He says, “Modern automobiles have thousands of welds, which are made by robots wielding lasers. Laser welds make the automobile safer and lighter by almost a quintal. Even fabrics in textile mills and garment factories are now cut with lasers.”
Narinder Singh Kapany, the inventor of fibre optics, was also the first to introduce lasers for eye surgery. He did this in the 1960s along with doctors at Stanford University. Today’s eye surgeons are armed with excimer laser scalpels that can make incisions less than a micron (thousandth of a millimetre) wide, on the delicate tissues of the cornea and retina.
LASER BASICS
So what are lasers, really? They produce light that has only one wavelength and a high directionality and is coherent. What do these things mean, and why are they important? Any source of light, man-made or natural, gives out radiation that is a mixture of wavelengths—be it a kerosene lantern, a wax candle, an electric bulb or the sun. Different wavelengths of light correspond to different colours.
When atoms fly around in a gas or vibrate in a solid in random directions, the light (photons) emitted by them does not have any preferred direction; the photons fly off in a wide angle. We try to overcome the lack of direction by using a reflector that can narrow the beam, as from torchlight, to get a strong directional beam. However, the best of searchlights used, say, outside a circus tent or during an air raid, get diffused at a distance of a couple of miles.
The intensity of a spreading source at a distance of a metre is a hundred times weaker than that at ten centimetres and a hundred million times weaker at a distance of one kilometre. Since there are physical limits to increasing the strength of the source, not to mention the prohibitive cost, we need a highly directional beam. Directionality becomes imperative for long-distance communications over thousands of kilometres..
Why do we need a single wavelength? When we are looking for artificial lighting, we don’t. We use fluorescent lamps (tube lights), brightly coloured neon signs in advertisements or street lamps filled with mercury or sodium. All of them produce a wide spectrum of light. But a single wavelength source literally provides a vehicle for communications. This is not very different from the commuter trains we use en masse for efficient and high-speed transportation. Understandably, telecom engineers call them ‘carrier waves’. In the case of radio or TV transmission, we use electronic circuits that oscillate at a fixed frequency. Audio or video signals are superimposed on these carrier channels, which then get a commuter ride to the consumer’s receiving set.
With electromagnetic communications, the higher the frequency of the carrier wave, the greater the amount of information that can be sent piggyback on it. Since the frequency of light is million times greater than that of microwaves, why not use it as a vehicle to carry our communications? It was this question that led to optical communications, where lasers provide the sources of carrier waves, electronics enables your telephone call or Internet data to ride piggyback on it, and thinner-than hair glass fibres transport the signal underground and under oceans.
We have to find ways to discipline an unruly crowd of excited atoms and persuade them to emit their photons in some order so that we obtain monochromatic, directional and coherent radiation. Lasers are able to do just that.
Why coherence? When we say a person is coherent in his expression, we mean that the different parts of his communication, oral or written, are connected logically, and hence make sense. Randomness, on the other hand, cannot communicate anything. It produces gibberish.
If we wish to use radiation for communications, we cannot do without coherence. In radio and other communications this was not a problem since the oscillator produced coherent radiation. But making billions of atoms radiate in phase necessarily requires building a new kind of source. That is precisely what lasers are.
Like many other ideas in modern physics, lasers germinated from a paper by Albert Einstein in 1917. He postulated that matter could absorb energy in discrete quanta if the size of the quantum is equal to the difference between a lower energy level and a higher energy level. The excited atoms, he noted, can come down to the lower energy state by emitting a photon of light spontaneously.
On purely theoretical considerations, Einstein made a creative leap by contending that the presence of radiation creates an alternative way of de-excitation, called stimulated emission. In the presence of a photon of the right frequency, an excited atom is induced to emit a photon of the exact same characteristics. Such a phenomenon had not yet been seen in nature.
Stimulated emission is like the herd effect. For example, a student may be in two minds about skipping a boring lecture, but if he bumps into a couple of friends who are also cutting classes, then he is more likely to join the gang.
A most pleasant outcome of this herd behaviour is that the emitted photon has the same wavelength, direction and phase as the incident photon. Now these two photons can gather another one if they encounter an excited atom. We can thus have a whole bunch of photons with the same wavelength, direction, and phase. There is one problem, though; de-excited atoms may absorb the emitted photon, and hence there may not be enough coherent photons coming out of the system.
What if the coherent photons are made to hang around excited atoms long enough without exiting the system in a hurry? That will lead to the same photon stimulating more excited atoms. But how do you make photons hang around? You cannot slow them down. Unlike material particles like electrons, which can be slowed down or brought to rest, photons will always zip around with the same velocity (of light of course!)—300,000 km per second.
THE BARBERSHOP SOLUTION
Remember the barber’s shop, with mirrors on opposite walls showing you a large number of reflections? Theoretically, you could have an infinite number of reflections, as if light had been trapped between parallel facing mirrors. Similarly, if we place two highly polished mirrors at the two ends of our atomic oscillator, coherent photons will reflected back and forth, and we will get a sustainable laser action despite the usual absorptive processes.
At the atomic level, of course, we need to go further than the barber’s shop. We need to adjust the mirrors minutely so that we can achieve resonance, i.e., when the incident and reflected photons match one another in phase, and standing waves are formed. Lo and behold, we have created a light amplification by stimulated emission of radiation (laser).
In the midst of disorderly behaviour we can see order being created by a laser. Physics discovered that the universe decays spontaneously into greater and greater disorder. If you are a stickler, ‘the entropy—measure of disorder—of an isolated system can only increase’. This is the second law of thermodynamics. So are we violating this law? Are we finally breaking out of thermodynamic tyranny?
It should be noted, however, that the universe becomes interesting due to the creation of order. Evolution of life and its continuous reproduction is one of the greatest acts of creating order. However, rigorous analysis shows that even when order is created in one part of the universe, on the whole, disorder increases. Lasers are humanity’s invention of an order-creating system.
Charles Townes, a consultant at Bell Labs, first created microwave amplification through stimulated emission in 1953. He called the apparatus a maser. Later work by Townes and Arthur Schawlow at Bell Labs, and Nikolay Basov and Aleksandr Prokharov in the Soviet Union led to the further development of laser physics. Townes, Basov and Prokharov were awarded the Nobel Prize for their work in 1964. Meanwhile, in 1960, Theodore Maiman, working at the Hughes Research Laboratory, had produced the first such instrument for visible light—hence the first laser—using a ruby crystal.
Since then many lasing systems have been created. At Bell Labs C K N Patel did outstanding work in gas lasers and developed the carbon dioxide laser in 1964. This was the first high power continuous laser and since then it has been perfected for high power applications in manufacturing.
THE SEMICONDUCTOR REVOLUTION IN LASERS
What made lasers become hi-tech mass products was the invention of semiconductor lasers in 1962 by researchers at General Electric, IBM, and the MIT Lincoln Laboratory. These researchers found that diode devices based on the semiconductor gallium arsenide convert electrical energy into light. They were highly efficient in their amplification, miniature in size and eventually inexpensive. These characteristics led to their immediate application in communications, data storage and other fields.
Today, the performance of semiconductor lasers has been greatly enhanced by using sandwiches of different semiconductor materials. Such ‘hetero-junction’ lasers can operate even at room temperature, whereas the older semiconductor lasers needed cooling by liquid nitrogen (to around -77 0C). Herbert Kroemer and Zhores Alferov were awarded the Nobel Prize in physics in 2000 for their pioneering work in hetero-structures in semiconductors. Today, various alloys of gallium, arsenic, indium, phosphorus and aluminium are used to obtain the best LEDs and lasers.
One of the hottest areas in semiconductor lasers is quantum well lasers, or cascade lasers. This area came into prominence with the development of techniques of growing semiconductors layer by layer using molecular beam epitaxy. Researchers use this technique to work like atomic bricklayers. They build a laser by placing a layer of a semiconductor with a particular structure and then placing another on top with a little bit of cementing material in between. By accurately controlling the thickness of these layers and their composition, researchers can adjust the band gaps in different areas. This technique is known as ‘band gap engineering’.
If the sandwich is thin enough, it acts as a quantum well for electrons. The electrons confined in this way lead to quantum systems called quantum wells (also known as particle in a box). The gap in the energy levels in such quantum wells can be controlled minutely and used for constructing a laser. Further, by constructing a massive club sandwich, as it were, we can have several quantum wells next to each other. The electron can make a stimulated emission of a photon by jumping to a lower level in the neighbouring well and then the next one and so on. This leads to a cascade effect like a marble dropping down a staircase. The system ends up emitting several photons of different wavelengths, corresponding to the quantum energy staircase. Frederico Capasso and his team built the first such quantum cascade laser at Bell Labs in 1994.
Once a device can be made from semiconductors, it becomes possible to miniaturise them while raising performance levels and reducing their price. That’s the pathway to mass production and use. This has happened in the case of lasers too.
We can leave the physics of lasers at this point and see how lasers are used in appliances of daily use: A bar-code reader uses a tiny helium-neon laser to scan the code. A detector built into the reader detects reflected light and the white-and black bars are then converted to a digital code that identifies the object.
A laser printer uses static electricity; that’s what makes your polyester shirt or acrylic sweater crackle sometimes. The drum assembly inside the laser printer is made of material that conducts when exposed to light. Initially, the rotating drum is given a positive charge. A tiny movable mirror reflects a laser beam on to the drum surface, thereby rendering certain points on the drum electrically neutral. A chip controls the movement of the mirror. The laser ‘draws’ the letters and images to be printed as an electrostatic image.
After the image is set, the drum is coated with positively charged toner (a fine, black powder). Since it has a positive charge, the toner clings to the discharged areas of the drum, but not to the positively charged ‘background’. The drum, with this powder pattern, rolls over a moving sheet of paper that has already been given a negative charge stronger than the negative charge of the image. The paper attracts the toner powder. Since it is moving at the same speed as the drum, the paper picks up the image exactly. To keep the paper from clinging to the drum, it is electrically discharged after picking up the toner. Finally, the printer passes the paper through a pair of heated rollers. As the paper passes through these rollers, the toner powder melts, fusing with the paper, which is why pages are always warm when they emerge from a laser printer.
Compact discs are modern avatars of the old vinyl long-playing records. Sound would be imprinted on the LPs by a needle as pits and bumps. When the needle in the turntable head went over the track, it moved in consonance with these indentations. The resultant vibrations were amplified mechanically to reproduce the sound we heard as music. Modern-day CDs and DVDs are digital versions of the old Edison’s phonograph. Sound or data is digitised and encoded in tiny black or white spots corresponding to ones and zeros. These spots are then embedded in tiny bumps that are 0.5 microns wide, 0.83 microns long and 0.125 micron high. The bumps are laid out in a spiral track much as in the vinyl record. A laser operating at a 0.780-micron wavelength lights up these spots and the reflected signal is then read by a detector as a series of ones and zeroes, which are translated into sound.
In the case of DVDs, or digital versatile discs, the laser operates at an even smaller wavelength, and is able to read much smaller bumps. This allows us to increase the density of these bumps in the track on a DVD with more advanced compression and coding techniques. This means we can store much more information on a DVD than we can on a CD. A DVD can store several GB of information compared with the 800 MB of data a CD can store.
A CD is made from a substratum of polycarbonate imprinted with microscopic pits and coated with aluminium, which is then protected by a thin layer of acrylic. The incredibly small dimensions of the bumps make the spiral track on a CD almost five kilometres long! On DVDs, the track is almost twelve kilometres long.
To read something this small you need an incredibly precise discreading mechanism. The laser reader in the CD or DVD player, which has to find and read the data stored as bumps, is an exceptionally precise device.
The fundamental job of the player is to focus the laser on the track of bumps. The laser beam passes through the polycarbonate layer, reflects off the aluminium layer, and hits an opto-electronic device that detects changes in light. The bumps reflect light differently than the rest of the aluminium layer, and the opto-electronic detector senses the change in reflectivity. The electronics in the drive interpret the changes in reflectivity in order to read the bits that make up the bytes. These are then processed as audio or video signals.
With the turntables of yesterday’s audio technology, the vibrating needles would suffer wear and tear. Lasers neither wear themselves out nor scratch the CDs, and they are a thousand times smaller than the thinnest needle. That is the secret of high-quality reproduction and the high quantity of content that can be compressed into an optical disc.
C.K.N. Patel recalls how, in the 1960s, the US defence department was the organisation that evinced the greatest interest in his carbon dioxide laser. “The launch of the Sputnik by the Soviet Union created virtual panic,” he says. “That was excellent, since any R&D project which
the military thought remotely applicable to defence got generously funded.” ‘Peacenik’ Patel, who is passionate about nuclear disarmament, is happy to see that the apocalyptic ‘Third Eye’ has found peaceful applications in manufacturing and IT. Patel refuses to retire and is busy,
in southern California, trying to find more applications of lasers for health and pollution problems.
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