Lawrence Livermore National Laboratory



Eyal Feigenbaum

Overview

We explored the feasibility of controlling the light propagation in an optical fiber by modifying a short segment of it using a plasmonic waveguide. The proposed geometry, aimed at maximizing the optical coupling between the light propagating at the fiber core and the plasmonic structure, is a "D-shaped" fiber. We experimented with the coupling and with the plasmonic segment deposition using different existing designs, which led us to design and fabricate a new and optimized D-shaped fiber (see figure). We experimented with both metal and R6G dye, which present broad-band and narrow-band options for the lossy channel. To calibrate more precisely the coupling of the core and the lossy channels, we devised a methodology based on in-situ etching. We implemented a coupled-mode theory model in Matlab, which enabled a better understanding of the underlying physics of the coupling. We tested the energetics of this design and found that at least 40 joules could flow within the fiber-optic segment with only 12 percent energy loss and no significant change in performance, indicating that this technology could meet the energetic requirements for a high-power front-end system.

Background and Research Objectives

In the last two decades, it has been repeatedly demonstrated that the strong light–matter interactions in metal optic devices mediated by propagating plasmonic waves open new scientific and technological avenues (Barnes et al. 2003, Ozbay 2006). Plasmonic waves (i.e., surface plasmon polaritons) are electromagnetic waves propagating on metal surfaces coupled with surface electron oscillations. These plasmonic waves with strong surface localization are enabled by the negative permittivity of the metal and are accompanied by attenuation from material absorption, as well as sub-wavelength confinement.

We have conducted a feasibility study of plasmonic fiber devices for laser-pulse control in front-end systems. We developed a methodology for designing and fabricating plasmonic-fiber devices and evaluating their energetic limits. We fabricated the devices by depositing metal on D-shaped fiber segments and then optically characterized them. We developed semi-analytical design methods and validated them by comparing them to numerical simulations.



Figure 1.
(a): D-shaped fiber illustration; (b): cross-section micrograph of the fabricated D-shaped fiber; (c): schematic diagram of the spectral transmission setup.

The feasibility of merging plasmonic and optical-fiber technologies was previously explored mainly for the purpose of developing sensing applications associated with surface-wave enhanced sensitivity to molecules (Caucheteur 2015). The goal of this study was to develop a methodology for designing and fabricating linear D-shaped plasmonic-fiber devices and evaluating the limits of their energetics. This study will serve as the foundation for designing fiber-based spectral filters and for designing nonlinear optical-transmission devices for light-pulse reshaping.

Impact on Mission

This research advances the science, technology, and engineering competencies that are the foundation of the NNSA mission. Specifically, it enhances Lawrence Livermore National Laboratory's core competencies in lasers and optical science, as well as advanced materials and manufacturing.

Conclusion

This study examined a hybrid platform consisting of a fiber optic segment and a plasmonic device to control the transmission through the fiber optic by designing the modal coupling in the device. The focus of this study was designing the linear transmission and establishing the energy levels under which such a device could operate. The results of this study indicate that the system's energetics and controlled performance may be appropriate for use in high-power laser front-end systems.

To examine the feasibility of using this hybrid plasmonic-optical fiber technology to control optical pulses in front-end systems, we developed the fabrication, characterization, and modeling techniques for this type of device and tested its energetic limits. Examining the commercially available alternative for side-polished D-shaped fibers revealed that it is not optimal as a basis for this hybrid technology because the variations of the minimal metal-to-core gap are too large and the gap varies along the axis of propagation direction. We fabricated a nominal D-shaped fiber geometry for this technology. We found that at least 40 joules could flow in the fiber segment with substantial interaction with the segment's cross-section (12-percent loss on the segment itself) and without a significant change in performance. That result indicates that this technology could meet the energetic requirements for a high-power, front-end fiber-optics system. The experiment was halted due to damage incurred by the input-fiber facet when the output energy reached 40 joules. We concluded that the energy output could be even higher with the use of improved end-caps, coatings, or larger fiber cores.

Potentially, the results of this study could be applied to designing nonlinear coupling methods to control short-pulse ultra-fast light propagation and light-pulse reshaping in high-energy front-end systems. This addresses the need for novel fiber-laser architectures and methods for expanding the operational envelope of fiber lasers. This study also contributes to improving and enhancing the technology of short-pulse lasers. We intend to extend the research and development of this design to the control of the nonlinear coupling of the two transmission modes.

References

Barnes, W. L., et al. 2003. "Surface Plasmon Subwavelength Optics." Nature 424(6950), 824–830. doi:10.1038/nature01937.

Caucheteur, C., et al. 2015. "Review of Plasmonic Fiber Optic Biochemical Sensors: improving the Limit of Detection." Analytical and Bioanalytical Chemistry 407(14), 3883–3897. doi: 10.1007/s00216-014-8411-6.

Ozbay, E. 2006. "Plasmonics: Merging Photonics and Electronics at Nanoscale Dimensions." Science 311, 189–193. doi: 10.1126/science.1114849.

Publications and Presentations

Faruk, M. O., et al. 2017. "Toward Plasmonic Control of Light Propagation in an Optical Fiber." SPIE Laser and Photonics 2017 Proceedings. LLNL-PROC-756212.

Feigenbaum, Eyal, et al. 2018. System and Method for Plasmonic Control of Short Pulses in Optical Fibers. U.S. patent 16/037,837, filed July 17, 2018.