Optics & Photonics News

which could reach up to 99 percent reflectivity, but that was barely enough—and the coatings would have to be exposed to the laser discharge. Further problems came from the need to align the two flat mirrors forming the Fabry-Pérot resonator without a laser interferometer to assure their parallelism.

Javan and Bennett worked long hours on gas-discharge physics and enlisted the help of Donald Herriott, one of Bell’s few in-house optics experts, who had worked with multilayer coatings at Bausch & Lomb. Progress was slow because they wanted to measure gain before trying to demonstrate oscillation. Experimental malfunctions caused further delays.

Javan thought that microwave frequencies of 2 to 4 GHz would excite the gas more efficiently than the 10-MHz radio source used in their gain experiments, but the microwaves melted the laser tube. When they tried baking out impurities in a tube with their best mirrors installed, the heat destroyed the reflective coatings. Department head Al Clogston assured upper management that progress was being made.

Bennett, Herriot and Javan mounted their second-best mirrors on an 80-cm tube and tried a milder bake-out process. Although the mirrors showed crazing, they remained intact. On December 13, 1960, they filled the tube with a helium-neon mixture and fired up the discharge. Having seen no sign of lasing by 4 p.m., they stopped, discouraged, and discussed what to do next. As Herriott fiddled with a mirror, Javan spotted something new on their oscilloscope. They had finally hit the sweet spot and generated a beam. Bennett later estimated that Bell Labs had spent a million dollars on instrumentation and equipment. It was the kind of project that, as Bloembergen had foreseen, was beyond the reach of any university laboratory.

Bell colleagues also contributed new ideas. Gary Boyd and Jim Gordon showed that curved cavity mirrors could make stable resonators that greatly relaxed mirror alignment requirements, easing development of low-gain lasers. Alan White and

Dane Rigden of Bell’s exploratory development group built
an enhanced copy of the original 1.15-µm helium-neon laser
for the Army Signal Corps, then demon-
strated laser action on the now-standard
632.8-nm helium-neon line in 1962.

Peter Sorokin (left) and Mirek Stevenson at IBM, adjusting their uranium laser.

Courtesy of IBM

microwave resonances in solids but switched to optically-pumped solid-state lasers after they saw the Schawlow-Townes paper.

Expecting low gain, Sorokin devised a square resonator based on total internal reflection to avoid losses. They chose calcium fluoride as a host material because its refractive index was just above the square root of two; thus, it would have total internal reflection at a 45° angle. They dug through the literature to identify uranium and samarium as suitable light emitting ions. The crystals were custom-grown, cut into squares, and polished. When they heard the news about Maiman’s laser, they were stunned.

They quickly ordered their own flashlamps and had spare crystals cut and polished into rods. Their crystals required cryogenic cooling to act as four-level lasers, so they put a uranium-doped rod into a special dewar with a window on the bottom and fired the flashlamp. “It worked the first time,” Sorokin recalls. “Bill Smith supported us with anything we needed, but we didn’t spend a lot of money.” IBM already had much equipment, including the special windowed dewars.

Bell Labs and the gas laser

Deep pockets and strong management support were crucial to Bell Labs’ success in developing the helium-neon laser.

The helium-neon system posed huge challenges. Gain was low, so the laser would need a long tube to develop enough gain to offset cavity losses. The best cavity mirrors were multilayer dielectric coatings,

Those first diode lasers required high-current pulses and cryogenic cooling to reach threshold.

The problem was that they were homojunction devices, with nothing to keep carriers near the junction.

The dawn of diode lasers

Semiconductors were hot after the invention of the transistor, and through the 1950s, developers explored new materials, moving from the initial germanium devices to silicon and later III-V compounds such as gallium arsenide. In early 1962, Sumner Mayburg at the General Telephone and Electronics Laboratories in Bayside, N. Y., and Jacques Pankove at RCA Labs separately reported strong recombination radiation from GaAs junctions. But it took a report of nearly 100 percent emission efficiency from a