LASER - a story

 “The successful warrior is the average man, with laser-like focus.” – Bruce Lee

My papa showed me ‘saptarshi’ and Pole star when I was in school. I was interested in Astronomy. In fact everyone is at least gets charmed at least once by the celestial show that begins each night. This ignites curiosity in our mind about what these bright spots are, how they change, how distant are they, is there a heaven? The interest brought me to an amatuer astronomy meet at Madhavpur ghed (famous for its krishna temple, Osho ashram and a beautiful beach) in 2008. While sky gazing, I was shocked and amazed when Mr. Narendra Gor, the main organizer of the meet, indicated some of the stars using something in his hand. The light was coming out of it and seemed to reach to the stars. In an instant my interest shifted from astronomy to that thing in his hand. In the question answer sesion that followed, I asked him about the thing in his hand? He smiled and said it was 635 nm 5 mW red laser pointer. We get interested in things we are in awe of and I got an urge to own the pointer and asked more about it. Later I tried to find similarity between laser pointer, laser printer, laser beam, laser projector, laser treatment. So, many laser products we are engrossed with in real life.

 

(a)                                                                                     (b)

Figure 1 (a) a falling asteroid being shown by a parent to his sibling (taken from www,owlcation.com) (b) laser beam point to a star while sky gazing (taken from http://www.saao.ac.za)

Let us try to understand how it all happened. Believe me this is interesting. The story of laser is the story of spectroscopy. The discipline was born in 19th century and has played a fundamental role in the study of light emission which ultimately led us to having masers and lasers. Hooke, Hyugens and Newton had major contribution in understanding the behaviour of light, its wave motion or its corpuscular nature.

Then we were introduced to the problems of black body radiation, compton effect, photoelectric effect, the science of which could not be understood with existing laws of that time. Rutherford, Bohr, Planck, Pauli and Einstein identified fundamental behaviour of light and quantum particles. Though each of these were great in some way or the other, there was something in Einstein that others didn’t possess. He probably had the power of stimuli. He stimulated us towards gravitational waves – we are still in his direction with research using LIGO, towards relativity, towards radiation. LASER is an offshoot we invented 30 years after he has shown the direction.

Exactly a hundred years ago, when a majority of the people believed the light to have a continuous value, Einstein realized this belief to be a hindrance and proposed several postulates in order to understand the behaviour of light absorption and emission. One such proposal was to believe an atom can absorb photon of only certain wavelengths, depending upon its allowed energy levels. The atom, then, will be in a higher energy state and the photon disappears. Another proposal was that while passing through a substance, the light “stimulates” emission of light and that emitted photons prefer to travel together. The later postulate is truly a novel thought. Such emission of photons will, unbelievably, travel in the same direction, frequency and phase. Significance of Einstein’s contribution lies in this fact that he has genuinely identified how light (or photon) behaves. Just like quantum mechanics, its difficult to comprehend how laser light does not diverge as if it is somehow kept in a transparent tube. They follow quite a disciplined way – Coherence, Frequency, Directionality etc.

Figure 2: Diagram showing atomic absorption, spontaneous emission and stimulated emission.

(Image taken from http://www..aber.ac.uk)

Many newer scientific terms like Population inversion, Pumping, Stimulated Emission etc. came into existence after Einstein’s paper. These were key research issues during 1920s to 1950s. The journey of lasers began with the idea of Stimulated Emission proposed by Einstein progressed somewhat slowly. The idea was used to explain various phenomena in 1920s by theoretical physicists and later verified as well. In 1930s, microwaves and radar research dominated due to its implications in war. In this process magnetic resonance was discovered in which the first population inversion was obtained. Later in the 1950s, continued research into radio frequency spectroscopy and Weber’s efforts for recognition of concepts leading to the maser were important which later resulted into the making of the first operating maser (microwave amplification) building by a group of researchers headed by C H Townes. To be specific, Schawlow and Townes have proposed a technique for the generation of very monochromatic radiation in the infra-red optical region of the spectrum using an alkali vapour as the active medium.

Later stimulated emission was achieved for various frequencies of light, so a general term LASER was coined by Gordon Gould in 1959. The first working laser was built by Theodore Maiman in 1960. It was Ruby Laser. Ruby is a crystal of aluminium oxide added with few chromium atoms as impurities. So many people were involved this field that in a few years, solid state laser, He-Ne laser, Caesium laser, neodymium laser and laser diode were made. A 10 micro meter natural laser emission was also identified in the Mars and Venus atmosphere by M. Mumma in 1981.

Now we know that LASER stands for Light Amplification by the Stimulated Emission of Radiation and the construction of a laser source needs three things a pump source, laser medium and optical resonator. The pump source provide initial energy. Laser medium is the main part which defines wavelength of laser. The pump source excites the medium to get population inversion. The process of stimulated emission proposed by Einstein occurs in this medium. Two parallel mirrors between which multiple reflection of light results into emission of same frequency photon which comes out from one of the reflecting mirror.

Figure 3: Typical construction of a laser source.

Solution in Search of a Problem

As happens in many discoveries, theory precedes practical. Here in the case of lasers, we have invented how to produce laser. But we were not sure how to use this discovery. The light of laser was different, highly luminous, travels for kilometers; we even had short pulses of femtosecond. But we were not sure how to utilize these characteristics. It was initially thought of as a tool for making advanced weapons. Since military use was of prime concern in those years, range-finding was the first application of the laser. About 10 to 30 nano second laser pulse were sent and received from a target and from travel time, distance of the object was found.

In 1968, laser was tested for removing a membrane covering the pupil of the eye of a young girl. Lasers are using in the post-surgery treatment of cataracts. Crushing of urinary stones in a bloodless way is one of the most important areas of progress in which laser light is directed at the stone by means of an optical fibre in which the stone is vapourized and crushed. Similarly, in photocoagulation, laser light is converted to heat energy with a rise of temperature to about 65 C which results in the denaturing of protein and can destroy abnormal blood vessels, cysts, tumours, etc.

Figure 4: Photograph of laser eye surgery.

Invention of the semi-conducting diode laser has been important due their smallest size – a fraction of a millimeter. They can be incorporated into an integrated circuit board as well. The idea of total internal reflection of laser light inside fiber glass has progressed so much that silica fibres can transmit infrared pulses at the wavelength of 1.5 micro meter with a minimum loss of 5% per kilometer.

Figure 5: Diagram showing various parts of Semiconductor Diode Laser.

We used to measure a small displacement using an interferometer. This is called optical non-contact measurement system. Using laser light, application of interferometer is made much more accurate. Similarly laser gyroscope measures very small rotations. A group of astronomers in California sent a powerful laser beam from a ruby laser to the Moon and succeeded in receiving the reflection, which gave precise distance from the Earth to the Moon.

Figure 6: Image showing use of laser interferometer in Metrology.

Lasers lights are used for light shows which are very popular displays for entertainment. Patterns can be drawn on walls or on smog using narrow laser beam. Barcode reader is an application that uses laser light. We all have seen it in every supermarket and big shops. Here, black and white parallel lines of different amplitude reflected laser light of modulated amplitude, the reading of which is an information of the kind of goods, its price etc.

Figure 7: Display of laser lights to a public gathering.

Anti missile defense system used laser to dispose the energy of warhead by partially damaging it. The star wars programme of US defense agency is aimed at defending them against Inter-continental Ballistic Missile strikes. Laser spectroscopy technique is used to create a detective system to identify liquid explosives even if they are concealed. In quantum cryptography, laser and laws of quantum mechanics are blended for secure transmission of information. It is based on the principle that in quantum mechanics, any measurement perturbs a system in an unpredictable way.

Those who have used audio Cassettes know the importance of compact discs and how it has changed dynamics of music and video watching. It is one of the most popular applications of lasers – the recording and reading of compact discs. The laser writes pulse sequence in the form of holes on a glass disc. To read the stored information, the disc is rotated and read using the light of a semiconductor laser.

Figure 8: Image of compact disc being ‘written’ by a laser beam.

By 1990s, laser was used to solder parts, make holes, cut clothes and used in automotive industry. The laser beam reaches with great precision and is used for cutting metal, leather, paper or wood. Laser are used to engrave and clean printed circuit boards. An instrument called LIDAR which stands for Light Detection and Ranging, the light backscattered by molecules or particles in the atmosphere is measured to identifying presence of aerosol, water droplets, clouds and smoke particles. Concentration profiles as a function of altitude and their temporal evolution can help us understand atmospheric dynamics. Adaptive optics compensates for aberrations induced by the atmosphere. Distortion of the wavefront can be estimated by artificial source which used to be a powerful laser. Signal from astronomical source is then compensated using this reference wavefront information. In geophysics, satellites receive reflected laser light to measure the movement of the Earth’s crust for example continental drift can be measured in this way. For ultra cooling a laser photon is collided with individual atoms which absorbs the photon and gets pushed in the direction of the photon. After a while, the atom re-emits the photon, resulting into decreased temperature. This is method is used to obtain Bose-Einstein condensation, which was predicted by Einstein in 1924. Thus, Einstein’s one proposal has been useful to prove existence of his another prediction. So much power in Einstein’s stimulation that we all still going in the direction in science indicated by him.