ECE PhD Thesis Defense: Purva Bhumkar

ECE PhD Thesis Defense: Purva Bhumkar

Title: Power-Scalable Sources Leveraging Nonlinear Angular Momentum Exchange Between Photons

Presenter: Purva Bhumkar

Advisor: Professor Siddharth Ramachandran

Chair: Professor Wei-Lun Chao

Committee: Professor Siddharth Ramachandran, Professor Anna Swan, Professor Tianyu Wang, Professor Jeffrey Demas

Google Scholar: https://scholar.google.com/citations?user=m686C4AAAAAJ&hl=en

Abstract: Lasers enable applications ranging from consumer technologies, such as barcode scanners, laser points, and displays, to advanced systems for remote sensing, precision manufacturing, and biomedical imaging. Many scientific applications require high-power lasers in the visible spectral region, including blue lasers for underwater LiDAR or communications, yellow guide-star lasers for astronomical adaptive optics, and red lasers for biomass remote sensing. Although semiconductor laser diodes offer compact and cost-effective sources with broad spectral coverage, they cannot always deliver the kilowatt-level peak powers required by these applications at the desired wavelengths. Mature high-power laser gain media exist, but they are largely restricted to the 1-, 1.5-, and 2-µm spectral bands. Nonlinear frequency conversion therefore provides a practical route to extending these high-power laser platforms to the visible portion of the spectrum. Fiber-based nonlinear frequency conversion offers a more compact and robust alternative to bulk free-space optical parametric oscillators.

Four-wave mixing (FWM) in an optical fiber is a nonlinear process in which two pump photons interact to generate a lower-energy Stokes photon and a higher-energy anti-Stokes photon while conserving energy and momentum (phase matching). A single FWM step pumped at 1 µm (the shortest wavelength at which high-power lasers are widely available), however, limits the shortest attainable anti-Stokes wavelength due to the long-wavelength transparency limit for the Stokes wave. In addition, conventional single-mode fibers impose dispersion-dependent phase-matching constraints, restricting output to only certain visible wavelengths. Instead, this work bridges the gap between 1 µm and visible wavelengths by using sequential steps and intermodal FWM interactions mediated by the orbital angular momentum (OAM) modes of a ring-core multimode fiber. An intermediate near-infrared wavelength (

Efficient power transfer from a 1064-nm pump to an intermediary wavelength near 800 nm is demonstrated using seeded intermodal FWM, producing multi-kW peak-power pulses. This high-power intermediate wave then drives a second FWM stage to generate red light in the optical fiber. The phase-matching landscape is further analyzed to identify pathways for yellow- and blue-light generation, and a numerical model is used to predict performance and power scalability. Overall, sequential intermodal nonlinear interactions enabled by stable OAM modes in large-mode-area multimode fibers provide a compact, single-pass, and power-scalable platform for frequency conversion from widely available 1-µm laser sources to tailored wavelengths in otherwise inaccessible spectral regions.