Er:ZBLAN Fiber-based Ultra-short Pulse Generation, Amplification and Wavelength Tuning in Mid-IR and LWIR

Abstract

The development of intense ultrashort laser sources in the mid-infrared (mid-IR, 2-8 μm) and long-wave infrared (LWIR, 8-15 μm) is of great interest in various important research directions, including vibrational spectroscopy of the absorption lines of gases localized in the "fingerprint" wavelength region (2-20 μm), remote sensing and chemically sensitive LIDAR measurements, utilizing the atmosphere transparency windows (3–5 μm, and 8-15 μm), and fundamental sciences, such as laser particle acceleration, attoscecond pulse generation, and strong-field physics.

For those applications, the use of fiber lasers provides the advantages such as low cost, compactness, maintenance-free operation, and diffraction-limited beam output, competing with other solid state laser sources. Fluoride fibers are used instead of silica fibers to build mid-IR lasers, benefiting from the much broader transmission window reaching 4 μm. Reaching LWIR using mid-IR lasers has the intrinsic advantage of smaller quantum defect compared with near-IR lasers, and thus potentially higher conversion efficiency, consequently leading to the generation of higher energy/power LWIR signals.

In this thesis, key aspects for the development of the pump and signal laser sources of a LWIR OPCPA system are presented, which is recognized as a promising pathway for efficient intense few-cycle pulse generation in LWIR. First, ultrashort pulses centering at 2.8 μm were generated from an Erbium-doped fluoride fiber mode-locked oscillator. Its performance was characterized experimentally, and its mode-locking mechanism was numerically investigated. Second, these pulses were amplified and simultaneously compressed in an Erbium-doped fluoride fiber nonlinear pre-amplifier, without using any external dispersion management arrangements. Its characteristics were investigated, and the physics of its operation was thoroughly studied via numerical simulations. The combination of the first two experiments constitutes the front end for seeding both pump and signal pathways of the LWIR OPCPA system. Third, mid-IR soliton pulses, tunable from 2.8 μm to beyond 4 μm were generated, using Raman-induced soliton self-frequency shift (SSFS). This experiment was also analyzed via numerical simulations. This process has high conversion efficiency and stability, and was implemented as a compact and practical laser system. Fourth, ultrashort LWIR pulses were generated by difference frequency generation (DFG), using mid-IR lasers as pump and signal sources (2.8 μm and 4 μm) for the first time. Lastly, a novel phase stabilization technique for mode-locked pulses using a Gires-Tournois interferometer was demonstrated. It has the potential to be used for the phase stabilization of the LWIR OPCPA system.

To summarize, ultrashort LWIR pulses were generated, using DFG pumped and seeded with mid-IR fluoride fiber lasers for the first time. This research is the first step in the proof-of-principle demonstration of the signal source for a LWIR OPCPA system. It also provides with a seed source for the high-energy large-core Er:ZBLAN fiber based CPA system, which is currently being developed. This mid-IR fiber CPA will serve as the OPCPA pump source. This thesis research opens new paths for developing intense mid-IR and LWIR ultrashort pulse sources, useful for a wide range of potential applications.