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The first laser, Theodore Maiman's ruby laser of 1960, worked in flashes. Within months a second kind of laser was running at Bell Labs, and it did something the ruby could not: it stayed on. Its inventor was Ali Javan. This post explains the idea he had, what his team's 1961 paper actually reported, and how to work with its numbers.
Who was Ali Javan?
According to MIT's obituary, Javan was born in Tehran in 1926 and came to the United States in 1949. At Columbia University he worked with Charles Townes, inventor of the maser, and earned his PhD in 1954 with Townes as his adviser, without having taken a bachelor's or master's degree. He joined MIT as an associate professor in 1961, held the Francis Wright Davis chair from 1978 to 1996, and died in Los Angeles on 12 September 2016, aged 89. He was inducted into the National Inventors Hall of Fame in 2006, and his original 1960 laser is at the Smithsonian's National Museum of American History.
For the maser that came first, read our post on Charles Townes and the maser.
What problem did Javan solve?
Every laser needs a population inversion: more atoms in an upper energy level than in the lower level of the transition you want. The ruby laser got one by optical pumping, a bright flash that lifted chromium ions. MIT's obituary quotes Javan's former student Irving Herman: this "was difficult and not applicable to all systems".
Javan proposed a different route in Physical Review Letters in July 1959, "Possibility of Production of Negative Temperature in Gas Discharges". (A population inversion is sometimes called a negative temperature, because it is the reverse of the normal thermal spread of atoms over levels.) The APS Physics Landmarks article summarises the idea. An electric discharge excites helium atoms to a long-lived state that happens to lie close in energy to an excited state of neon. When an excited helium atom collides with a neon atom, it hands over its energy, filling that neon level.
What did the 1961 paper report?
Javan, Bennett and Herriott's letter, received on 30 December 1960 and published on 1 February 1961, gives the details:
A quartz tube 80 cm long and 1.5 cm across, holding 1 mm Hg of helium and 0.1 mm Hg of neon.
Mirrors inside the gas chamber, one metre apart, coated with 13 dielectric layers to reflect 98.9% between 1100 and 1200 nm.
A 28 MHz radio-frequency generator to run the discharge, dissipating an estimated 50 watts.
Continuous oscillation at five near-infrared wavelengths. The strongest, at 11 530 angstroms (1153 nm), gave 15 milliwatts.
A linewidth of 10 to 80 kHz and a beam spread of less than one minute of arc.
The team did not simply build the tube and hope. The paper describes months of measurement first: lifetimes of the neon levels, the cross section for energy transfer from helium, and gain in the afterglow of a pulsed discharge. Only then did they find "an optimum condition for the maser action prior to the observation of oscillation".
When did the beam first appear?
MIT records that the breakthrough came on 12 December 1960, after a snowstorm had closed Bell Labs early. At 4:20 pm, by Javan's watch, a continuous beam came out of the apparatus. On 13 December the team used the beam to carry a telephone call. About six months later, the APS article notes, other Bell Labs researchers ran a helium-neon laser at the red 632.8 nm wavelength, the version most people have seen.
Worked example: reading the 1961 numbers
Use h = 6.63 × 10^-34 J s, c = 3.00 × 10^8 m/s and 1 eV = 1.60 × 10^-19 J.
Photon energy at 1153 nm. E = hc/λ = (6.63 × 10^-34 × 3.00 × 10^8) / (1.153 × 10^-6) = 1.73 × 10^-19 J, which is about 1.08 eV. The shortcut E (eV) = 1240/λ (nm) gives 1240/1153 = 1.08 eV.
Photons per second. At 15 mW: N = P/E = 0.015 / 1.73 × 10^-19 ≈ 8.7 × 10^16 photons per second.
Efficiency. Output over input power = 0.015 W / 50 W = 3 × 10^-4, or 0.03%. Gas lasers based on atoms waste most of the energy they take in, a problem our post on Kumar Patel and the CO2 laser explains.
Colour purity. The frequency is f = c/λ = 3.00 × 10^8 / 1.153 × 10^-6 ≈ 2.60 × 10^14 Hz. A 10 kHz linewidth is 10^4 / 2.60 × 10^14 ≈ 4 × 10^-11 of that frequency.
Beam spread. One minute of arc is 1/60 degree, or about 2.9 × 10^-4 rad. After 1 km the beam widens by at most about 1000 × 2.9 × 10^-4 ≈ 0.29 m.
Mistakes to avoid with laser questions
Using nanometres in hc/λ. Convert to metres, or use the 1240 shortcut only with λ in nm and E in eV.
Calling the first He-Ne laser red. The 1960 laser was infrared, at 1153 nm. The red 632.8 nm line came later.
Thinking helium lases. In this laser neon emits the beam; helium stores and transfers the energy.
Mixing up power and energy. Photons per second need power (J/s) divided by energy per photon (J).
Level diagrams can feel abstract on paper. The Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
What can a student learn from how Javan worked?
Javan started from a question about atoms, not from a device: which collision could fill one level faster than another? He wrote the idea down and published it in 1959. Then his team measured each step of the process before expecting a beam. When a long project depends on one result, check each link in the chain first, as they did.
Try it now: find the photon energy for the 1199 nm line of the same laser, and say whether it is larger or smaller than the 1153 nm photon.
