Generation and confinement of microwave gas-plasma in photonic dielectric microstructure

Hollow-Core Fiber Technology: The Rising of “Gas Photonics”

Since their inception, about 20 years ago, hollow-core photonic crystal fiber and its gas-filled form are now establishing themselves both as a platform in advancing our knowledge on how light is confined and guided in microstructured dielectric optical waveguides, and a remarkable enabler in a large and diverse range of fields. The latter spans from nonlinear and coherent optics, atom optics and laser metrology, quantum information to high optical field physics and plasma physics. Here, we give a historical account of the major seminal works, we review the physics principles underlying the different optical guidance mechanisms that have emerged and how they have been used as design tools to set the current state-of-the-art in the transmission performance of such fibers. In a second part of this review, we give a nonexhaustive, yet representative, list of the different applications where gas-filled hollow-core photonic crystal fiber played a transformative role, and how the achieved results are leading to the emergence of a new field, which could be coined “Gas photonics”. We particularly stress on the synergetic interplay between glass, gas, and light in founding this new fiber science and technology.

Spectral-temporal dynamics of high power Raman picosecond pulse using H2-filled Kagome HC-PCF

We report on the spectral-temporal characterization of a 1.8 μm wavelength and high power picosecond pulse Raman source. It is generated via frequency conversion to the first-order Stokes of a 27 ps chirped pulse Yb-doped fiber laser inside a molecular hydrogen-filled Kagome hollow-core photonic crystal fiber (HC-PCF). Depending on the average power and chirp of the pump laser, the average power of this Raman source can be as high as 9.3 W, and its pulse duration can be as short as ∼17  ps. In agreement with stimulated Raman scattering under the very high gain transient regime, the experimental results show the Stokes spectral structure to change following a three-stage sequence when the average pump power is increased. For a pump with a chirp corresponding to a bandwidth of 200 GHz, we found that for a pump power lower than 7 W, the Stokes spectrum is generated from the blue side of the pump spectrum, and then it exhibits a spectral replica of the pump spectrum for 7-14 W pump power range. Finally,the Stokes spectrum is chiefly generated from the red side of the pump spectrum when the pump power is further increased. Conversely, the Stokes pulse temporal profile shows a strong dependence with the pump power. For a low pump power range, the Stokes pulse exhibits a single peak with a full width at half-maximum of ∼17  ps. For higher pump powers, the Stokes pulse presents a double-peak structure with each peak having a duration of less than 15 ps. The present results can be used to develop compact and efficient frequency down-convertors to the increasingly widespread Yb-based picosecond lasers.

Gas mixture for deep-UV plasma emission in a hollow-core photonic crystal fiber

We report on the first deep-ultraviolet/ultraviolet (DUV/UV) emission using a highly compact microwave-driven plasma-core photonic crystal fiber. The latter consists of a few centimeter long micro-plasma column of a gas mixture in the core of Kagome hollow-core photonic crystal fiber. The plasma is generated by nonintrusively exciting a ternary gas mixture of argon, nitrogen, and oxygen (Ar/N₂/O₂) with a microwave resonator. Several spectral lines in the wavelength range of 200-450 nm were produced, guided by an Ar-N₂-O₂ plasma-filled fiber, and controlled by simply varying the gas ratio of this gas mixture. An optimum gas mixture ratio was experimentally and theoretically identified for the strongest emission in the DUV range of 200-275 nm. The developed DUV emitting plasma-core fiber represents an important milestone towards the development of tunable and miniaturized DUV/UV laser sources.

Generation of surface-wave microwave microplasmas in hollow-core photonic crystal fiber based on a split-ring resonator

We report on a new and highly compact scheme for the generation and sustainment of microwave-driven plasmas inside the core of an inhibited coupling Kagome hollow-core photonic crystal fiber. The microwave plasma generator consists of a split-ring resonator that efficiently couples the microwave field into the gas-filled fiber. This coupling induces the concomitant generation of a microwave surface wave at the fiber core surround and a stable plasma column confined in the fiber core. The scheme allowed the generation of several centimeters long argon microplasma columns with a very low excitation power threshold. This result represents an important step toward highly compact plasma lasers or plasma-based photonic components.