First-Hand:The First Continuous Visible Laser

   It is difficult to recreate these days the sense of excitement and expectency and the enormous interest that arose during the early days of the laser in 1959-62.    Gordon, Zeiger and Towne's demonstration of gain and oscillation at microwave frequencies had shown inverted populations were a potent new source of coherent electromagnetic energy and Townes and Schalow had pointed the way to extend this result to optical frequencies.   But the sticking point was picking the right medium to use.   Numerous suggestions for suitable media appeared in the literature.   Often, these were contingent on one or more difficult-to-measure parameters being known with some precision.   Ruby, caesium, xxxx were all proposed and some rejected.   Picking the right medium was important; no one wanted to waste time studying a medium unless there was some indication that inversion was possible.  Inversion seemed to be a rare phenomena presumably because nature favored thermal equilibrium everywhere.   In retrospect, this seems a quaint idea.   Now we know if one dumps enough energy into a system with distinct energy levels, one or more pairs of them will usually invert.   

One of the seemingly poorer candidates, ruby, later turned out to be the first visible laser to operate successfully;  Theodore Maiman was the first to discover the advantages of using a giant pulse of energy to invert a pair of energy levels, and thereby was awarded the honor of being the first over the finish line.  One of the better candidates (proposed by Javan), was an electrical discharge in a mixture of neon and helium gases, producing inversion in a pair of neon levels by excitation transfer from excited helium atoms.  Gas discharges are quite complicated mixtures of relatively simple electronic  excitation processes whose light producing properties have been studied for a long time.  One of the earliest  publications was by Duffenback who reported the enhancement of certain neon spectral lines in a HeNe dischage through collisions of the second kind.   It was this enhancement that caught Javan's attention.   Indeed, Javan's intuition was correct; measurements by Bennett confirmed the possibility of inversion between the neon 2s2 and 2p4     levels of neon resulting from enhanced transfer of excitation energy to the neon 2s2 level from the metastable helium level at almost the exact same energy level.   The transition wavelength  was in the near infra-red at 1.15 microns.   The experimental confirmation of inversion and oscillation was a tour de force.   Don Herriott's optical expertise was crucial in the experiment.   Curved mirror optical cavities with their large angle tolerance  where not well understood those days, so Herriott had to struggle with the problem of aligning plane mirrors enclosed within a vacuum tube to within a fraction of an arc-second to find a high Q cavity resonance.   Patience paid off in the end and oscillation at 1.15 microns was achieved in Dec 1960.   It wasn't visible light, but from the Bell Labs point of view it was coherent and continuous, both telephone company imperatives.

Like Maiman's ruby laser, the infra-red HeNe laser created a stir thruout Bell Labs and the scientific cmmunity.   The era of coherent light energy had begun.  To many, it was the Holy Grail.  Herwig Kogelnik, responsible for putting a firm foundation under much that we presently know about laser cavities and laser beams put it well.  "Think of all that bandwidth!"    Strangely enough, given it's wide usage today in so many fields, the laser was first publicized in the media as a 'solution looking for a problem'.

Like most other laboratories, the US Army Signal Corps was fascinated by the potential of lasers for military communications.   Soon after the infrared HeNe made it's public appearance, the Signal Corps decided it needed one of  it's own to play with, so they placed a request with Bell Labs to have one built and shipped down to the Signal Corps facility near Red Bank, NJ.