Memorial Tributes: Volume 27
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  • LINN F. MOLLENAUER (1937-2021)
    LINN F. MOLLENAUER

     

    BY ERICH P. IPPEN

    LINN FREDERICK MOLLENAUER, an innovative researcher who pioneered ultra-high-capacity short-pulse fiber-optic communication systems at Bell Laboratories in Holmdel, New Jersey, died on July 28, 2021, at the age of 84.

    Linn was born on Jan. 6, 1937, to Alice and Donley Mollenauer and raised in Mt. Lebanon, Pennsylvania, a town of lovely homes outside of Pittsburgh. An excellent student with a gift for math and science, he went to Cornell and graduated with a bachelor’s degree in engineering physics in 1959. From there he went to Stanford, where he was a graduate student under Arthur Schawlow (NAS 1970). For his thesis entitled “Exchange-coupled Chromium Ion Pairs in Ruby,” Linn developed new methods for studying optical materials. The stress-plate optical modulator and tilted-plate interferometer that he constructed for those purposes went on to assist future graduate research students.

    Linn received his Ph.D. in physics from Stanford in 1965 and accepted a position as assistant professor in the Physics Department at the University of California, Berkeley. At Berkeley, in addition to teaching quantum mechanics, he further developed his stress-plate measurement method for studies of circular dichroism and demonstrated the effectiveness of optical pumping for achieving nuclear polarizations in solids. On the side, in a workshop at home, Linn constructed an ultra-stable, high-resolution scanning Fabry Perot interferometer for laser spectroscopy that became a valued instrument in many early laser laboratories. When he left Berkeley in 1972 for Bell Laboratories in Holmdel, he licensed the fabrication of his interferometer to colleagues at a new company, Burleigh Instruments, that continued to produce it.

    At Bell Laboratories, Linn’s interest in optical pumping was further peaked by a recent demonstration of laser action in so-called color-center (or F-center) materials by Fritz Lüty’s group at the University of Utah. Color-center crystals provided the potential for lasers with wide ranges of wavelength tunability, similar to those of dye lasers but over a wider range in the near infrared. Within a year, Linn had created the first continuous wave (cw) color center using a Li-doped KCl crystal. He followed in quick succession with cw lasers based on F2+ centers in LiF, NaF, and KF as well as KCl to cover wavelengths from 0.82 to 1.8µm. Then, with the achievement of mode locking via synchronous pumping, he and collaborators added the capability to generate picosecond pulses and demonstrate distortionless propagation of such pulses over kilometers of fiber near the zero-dispersion wavelength of 1.33µm. These advances spurred the next several decades of seminal research into the nonlinear and dispersive properties of optical fibers at wavelengths of importance to optical communications and methods for transmitting data at ultra-high rates.

    Realizing he could use his mode-locked F-center laser to follow up on Akira Hasegawa’s proposal for generating optical solitons, Linn created and propagated the first solitons in fiber in 1980. He then showed that these solitons could be transmitted over long distances by overcoming fiber losses with periodic all-optical amplification via Raman gain. Subsequent innovations over the next decade included soliton lasers, filtering methods to suppress noise, timing-jitter and self-frequency shifts, and schemes for managing group-velocity dispersion. By 1988 he had demonstrated soliton transmission over 4,000 km of fiber; by 1991, two wavelengths over 9,000 km; and in 1993, 10 Gb/s over 20,000 km.

    Linn remained at the forefront of this field until he retired, regularly refining transmission techniques for solitons and pushing the limits of time-division and wavelength-division multiplexing for ever-increasing data capacity — up to 109 10 Gb/s channels (a total of 1.09 Tb/s) over 18 km in 2003. When he retired that year, he donated the equipment in his lab to Cornell and became a visiting scientist there. He also served as a guest professor at the University of Arizona, Tucson. In 2006 he co-authored the book Solitons in Optical Fibers: Fundamentals and Applications (Academic Press) with his longtime Bell Labs colleague and friend Jim Gordon (NAE 1985, NAS 1988).

    Linn was a fellow of the Optical Society of America, the American Physical Society, and the IEEE. He received both the R.W. Wood Prize (1982) and the Charles Hard Townes Medal (1997) from Optical Society of America, the 2001 Quantum Electronics Award of the IEEE Photonics Society, the Stuart Ballantine Medal (1986) from the Franklin Institute, and the prestigious Rank Prize for Optoelectronics (1991). At Bell Labs he became a fellow and then distinguished member of staff. In 1993 he was elected to the National Academy of Engineering.

    Those who knew Linn personally and watched him work were in awe of his multivaried abilities and talents as an experimentalist. He covered all the bases. He had his own small machine shop where he himself fabricated the components he needed; he had his own electronics shop for special devices; he created color centers, for himself and others, via electron radiation; he developed apparatus for maintaining laser crystals cryogenically; and he carried out amazingly complicated experiments, like circulating fiber loops to emulate long-distance transmission by remote personal control. At home, too, Linn had his own shop for home-improvement projects. He was an early adopter of hybrid and electric cars and of solar panels on the roof of his Colts Neck, New Jersey, home.

    While a graduate student, Linn met and married the lovely and charming Marjorie Lynn Trammel, who had just graduated from Stanford with a B.A. and M.A. in music. Marjorie went on to a distinguished career as a harpist, performing extensively with orchestras around the United States and internationally, as well as on radio and television. Music, especially chamber music, always played an important role in their home and in their community throughout the years. Linn and Marjorie were married for 58 years. She died on Dec. 7, 2020, just seven months before Linn did. They are survived by their sons, David and James, and three grandchildren.