The laser appears

1 An indication of the interest of the US Air Force at the time is that they were willing to support research for shorter millimetre waves at the French electronics company CSF while de Gaulle was delivering his anti-American tirades.

2 The new unit appearing here is T for Tera meaning lo12Hz = IOOOOOOOOO OOO Hz = I million MHz = 1000 GHz.

Advances usually come in small steps. A swimmer might be celebrated if he improves the world record from (say) 3 minutes 25.2 seconds to 3 minutes 24.3 seconds although the change in the time clocked is a mere 04 per cent. The largest ship, the tallest building, the fastest aeroplane, when they first appear might beat the previous record by a factor of two but not much more. Advances in communications can actually be much steeper. When Marconi experimented with short waves in the 1910s the jump in frequency was by a factor above 10. About the same factor of 10 was achieved by the war time efforts when microwave radar was introduced. So a new jump towards higher freqencies might well have been expected in the late 1950s. What was the state of the art then? The highest frequency available was about 150 GHz. At the time the wildest expectation might have been for a further increase by a factor of 7 or 8. The research was fuelled by the military requirements of electronic measures and countermeasures.1

And then out of the blue came a theoretical proposal to reach frequencies a couple of thousand times higher. Two American physicists, Arthur Schawlow and Charles Townes, claimed that it would be possible in principle to produce electromagnetic waves which have frequencies of about 500THz.2 The corresponding wavelength is o.6|J,m

3 In fact, the laser is more of an oscillator than an amplifier but the corresponding acronym 'loser' just did not sound right. I should perhaps mention here that according to some people the acronym laser has a much earlier origin, as suggested in Fig. 12.1.

which is well known to cause the sensation of red colour in the brain. In other words the new proposal envisaged producing visible light. OK, you might say, what's so great about that? We have had light of all colours ever since the creation. The difference is that the prediction was about producing coherent light.

Perhaps the simplest (although not quite accurate) way to define coherent waves is to say that the radiation is of a single frequency. The waves coming from the Sun contain a wide range of frequencies, as witnessed by rainbows. On the other hand the electromagnetic waves which bring us radio and television are close to a single frequency. For incoherent waves the 15000 channels mentioned above would have been a marvellous achievement. For coherent waves, for which our old calculations apply, it's just peanuts. Taking a 15 per cent bandwidth around 500 THz the number of telephone channels comes to about 18 billion with single sideband modulation, and still above a billion by pulse code modulation. This means that if we managed to concentrate all the European population in a phonebox in London and all the Americans (North and South) in a similar phonebox in New York then they could all talk to each other on a single line and there would still be some spare capacity.

Oscillators producing electromagnetic waves always worked on classical principles. The proposal by Schawlow and Townes was based on quantum physics which is an exceedingly obscure subject. Fortunately, for oscillators, the quantum theoretical aspects can be very simply summarized by the maxim: 'what goes up, must come down'. The 'going up' means that energy is given to the system. The 'coming down' has to be a little more specific. The energy gained has to be given out in the form of radiating electromagnetic waves.

In i960, less than two years after the appearance of the paper by Schawlow and Townes, the experimental proof was provided by Maiman in California. He managed to obtain coherent red light with the aid of a piece of ruby crystal illuminated by a flashlamp. The new device came to be called (not immediately, but a little later) a laser, which is an acronym for Light Amplification by Stimulated Emission of Radiation.3

It needs to be emphasized that the laser did not appear in response to industrial demand. At the time resources for scientific research were plentiful and no justifications were needed to prove the immediate or even eventual usefulness of the research. Perhaps the greatest spur was the human desire for exploration. Mount Everest had to be climbed because 'it was there'. The North Pole and the South Pole had to be discovered and space had to be explored in order to satisfy human curiosity. Well, the exploration of the electromagnetic spectrum was just part of this general zealousness; it was done for the greater glory of science.

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