Power scaling of combinable fiber lasers to beyond 5kW
Abstract
The first active deployment of a laser weapon on the USS Ponce in 2014 opened a new chapter for DEWs. Many DEWs are currently planned for deployment on a wide range of platforms in the next few years. The current systems are still limited in their capabilities and a further reduction in SWaP is critical for their practical deployment on smaller weapons platforms. Further power scaling of current systems to >100kW and improved beam quality would significantly improve DEWÕs capabilities in range, lethality and versatility. The key approaches are coherent and spectral combining of high-power fiber lasers. Both would require narrow spectral linewidth at GHz levels for combining a large number of lasers. Higher power from each laser is critical to minimize the overall system complexity and SWaP. SBS presents the most significant nonlinear limit for the power scaling of a fiber laser with narrow spectral linewidth. Average power is also limited by Transverse Mode Instability (TMI), caused by mode coupling resulting from the traveling thermal grating driven by modal interference in fiber lasers. The current power of combinable fiber lasers, broadly defined here as having GHz-level linewidth, is well below 2kW, limited by SBS and TMI. Current record single-mode powers of ~3kW for directly diode pumped fiber lasers were achieved in LMA fibers with reduced NAs. There is little room for conventional LMA fibers to go much beyond the current levels. It is increasingly clear that a significantly different approach has to be taken to go beyond 5kW. There is a system-level trade-off between SBS and TMI. Additional SBS suppression techniques can ease TMI and vice versa. First of all, this demands roughly equal SBS and TMI thresholds at the system level and second, multiple SBS and TMI mitigation techniques can be combined both at the system level and in the fiber design at the same time. We are proposing a system-centric approach, where phase modulation for the mitigation of SBS and shorter seed wavelength (<1035nm) for the mitigation of TMI by lowering quantum defect are deployed at the system level, and an acoustically-segmented core for the suppression of SBS and a multiple-cladding-resonance all-solid photonic bandgap fiber design for the suppression of TMI are incorporated in the fiber designs. In addition, an optimum core diameter is chosen to balance SBS and TMI thresholds at the system level. The key objectives are i) to study advanced fibers which incorporate the best SBS and TMI mitigation techniques, ii) to study the optimum core diameter to balance the requirements of SBS and TMI at the system level, iii) to study and optimize the wavelength of operation for optimum TMI mitigation and iv) a system- level study of the combined approach.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Oct 06, 2018
- Source ID
- W911NF1710454
Entities
People
- Liang Dong
Organizations
- Army Contracting Command
- Clemson University
- United States Air Force