Engineered Fiber for Mitigating Thermal Mode Instability
Abstract
Engineered Fiber for Mitigating Thermal Mode InstabilityAbstract:Although fiber lasers have the capability to exceed 10kW out of" a single aperture, the ability to coherently scale has historically been limited by several factors, including pump brightness and" stimulated Brillouin scattering (SBS). There are currently several complimentary approaches for mitigating SBS without compromising" the ability to coherently combine sources, and pump brightness is nearly at the requisite brightness levels that taken together all""ow for 5 kW singleaperturefiber laser elements. However, the relatively recent discovery of thermal mode instability (TMI) prohibit"s the achievable coherently scalable power levels with good beam quality at this power level. This singular physical phenomenon has completely blocked the finaldevelopment and deployment of DEW-class fiber laser systems.This proposal focuses on the development of a fundamentally new fiber type specifically designed to increase the TMI threshold. The fiber was invented under a previous HEL-"JTO seed program, whose aim was to scale the core diameter of LMA fibers. The seed effort revealed new underlying physics that led t"o new concepts for mitigating TMI. Preliminary modeling from this program indicates that TMI can be increased by at minimum a factor of 3.5 even with unoptimizeddesigns. The development and testing of this new fiber type is the focus of this program.The proposed" research and development program described here is specifically designed to:~ Provide a focused effort in developing a new, robust"", commercializable fiber type~ Ensure tight coupling between modeling, fabrication, and measurements~ Develop the new fiber type t"o increase the TMI threshold by a factor of 5-10~ Validate the new fiber type at multi-kW power levels up to 5 kW~ Educate a new w"ave of directed energy laser expertsThe new fiber type exploits both index tailoring and gain tailoring in conventional fibers, wi"thout relying on the complex physics and environmental susceptibility inherent to photonic crystal fiber types. This strategy in par"ticular ensures that the fiber can be made by same commercial vendors that make conventional ytterbium-doped fibers, resulting in ro"bust manufacturingmethods and performance for military-grade applications.One of the key strengths of this program is the tight connection between simulation-based design and experimental measurements. Generated fiber designs will be fabricated at nLight in an iterative fashion; measured data will feed into the design simulations to generate modified designs that guide the fabrication. The University of California San Diego (UCSD) will develop a full-spectrum SBS-suppression system based on pseudo-random bit patterns (PR"BS), and a fullmulti-kW testbed will be developed exploiting Optical Engines~ commercial high-brightness pump drive modules. Finall""y, the Air Force Research Laboratory (AFRL/RDLTS, Kirtland AFB) will perform an independent evaluation of the new fibers in their st"ate-of-the-art fiber amplifier testbed.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- May 05, 2017
- Source ID
- N000141712534
Entities
People
- John Marciante
Organizations
- Office of Naval Research
- United States Navy
- University of Rochester