Expanding Optical Horizons

　　Russian and Finnish researchers, collaborating on a proof-of-concept project to expand the uses for large-core-diameter, multimode optical fiber, used high-power lasers and anisotropic materials to develop a fiber that preserves the coherent properties of the light passing through it. Such preservation is a necessity for enabling quantum computing and sensor networks as well as for replacing costly single-mode fiber in longer-range communications applications.

　　The researchers have published their results in the Optical Society (OSA) journal Optics Express.

　　Optical fiber is the backbone of modern communications. Single-mode fiber predominates in long-distance applications because of its reliability; but such fiber, with an inner diameter measuring just 10 microns, is expensive. Lower-cost, multimode fiber, with an inner diameter as wide as 100 microns, is used today for communications over short distances, typically 1,000 meters for 1-Gbit/second transmissions. Researchers are working to expand the utility of multimode fiber, not only to replace single-mode fiber for long-range communications, but also to enable quantum computers and to build distributed sensor networks that would require little or no power to run.

　　The proof-of-concept research into coherence-preserving multimode optical fibers drew investigators from the Moscow Institute of Physics and Technology (MIPT), the Kotelnikov Institute of Radio-engineering and Electronics of the Russian Academy of Sciences (IRE RAS), and the Optoelectronics Research Center at Finland’s Tampere University of Technology. “Quantum computing can be one possible application; however, in this work, the purpose of our investigations was for high-power applications, wherein we could add powers of different waves inside the one fiber and observe the results due to nonlinear processes,” project lead Sergey Nikitov, IRE RAS director and deputy head of MIPT’s Section of Solid State Physics, Radiophysics, and Applied Information Technologies, told EE Times. Nikitov’s co-authors included MIPT professor Vasily Ustimchik, who is a senior research scientist at IRE RAS and the Russian Quantum Center, and Tampere professor emeritus Jorma Rissanen, a winner of the IEEE Richard W. Hamming Medal.

　　Coherence-preserving optical fibers have advantages over semiconductor sensors in that they have little need for electrical power and can handle the results from distributed sensor systems unaided. Their utility not only in high-powered laser systems, but also as sensors, comes from the observed fact that a change in their polarization characteristics enables accurate sensing of changes induced by environmental factors.

　　Fiber lasers use an optical resonator to induce lasing by reflecting light back and forth. Today such lasers only utilize the purely fundamental mode (upper left in the first figure), limiting the power to what can be carried in a 10-nm fiber. Increasing the transmission power for larger lasers results in uncontrolled variations in the refractive index of the fiber, giving rise to parasitic nonlinear effects. The solution used by the Russian and Finnish investigators, as well as by others, is to vary the core and inner cladding (see the second figure).

　　Using this technique, the Russian and Finish researchers proved the concept that less than 1 percent of the energy from high-power laser transmission is lost in 100-micron fibers. And by making the inner cladding of the large fiber anisotropic (meaning it propagates only in the direction of the length, because the inner cladding is elliptical), the researchers faithfully maintained the fiber’s polarization properties.