Dan Rhonehouse

Mid-IR supercontinuum generation in ultra-low loss, dispersion-zero shifted tellurite glass fiber with extended coverage beyond 4.5 μm

Mid-infrared sources are a key enabling technology for various applications such as remote chemical sensing, defense communications and countermeasures, and bio-photonic diagnostics and therapeutics. Conventional mid-IR sources include optical parametric amplifiers, quantum cascade lasers, synchrotron and free electron lasers. An all-fiber approach to generate a high power, single mode beam with extremely wide (1μm-5μm) and simultaneous wavelength coverage has significant advantages in terms of reliability (no moving parts or alignment), room temperature operation, size, weight, and power efficiency. Here, we report single mode, high power extended wavelength coverage (1μm to 5μm) supercontinuum generation using a tellurite-based dispersion managed nonlinear fiber and an all-fiber based short pulse (20 ps), single mode pump source. We have developed this mid IR supercontinuum source based on highly purified solid-core tellurite glass fibers that are waveguide engineered for dispersion-zero matching with Tm-doped pulsed fiber laser pumps. The conversion efficiency from 1922nm pump to mid IR (2μm-5μm) supercontinuum is greater than 30%, and approaching 60% for the full spectrum. We have achieved > 1.2W covering from 1μm to 5μm with 2W of pump. In particular, the wavelength region above 4μm has been difficult to cover with supercontinuum sources based on ZBLAN or chalcogenide fibers. In contrast to that, our nonlinear tellurite fibers have a wider transparency window free of unwanted absorption, and are highly suited for extending the long wavelength emission above 4μm. We achieve spectral power density at 4.1μm already exceeding 0.2mW/nm and with potential for higher by scaling of pump power.

Monolithic fiber chirped pulse amplification system for millijoule femtosecond pulse generation at 1.55 µm

A monolithic fiber chirped pulse amplification system that generates sub-500 fs pulses with 913 µJ pulse energy and 4.4 W average power at 1.55 µm wavelength has recently been demonstrated. The estimated peak power for the system output approached 1.9 GW. The pulses were near diffraction-limited and near transform-limited, benefiting from the straight and short length of the booster amplifier as well as adaptive phase shaping for the overall mitigation of the nonlinear phase accumulation. The booster amplifier employs an Er(3+)-doped large mode area high efficiency media fiber just 28 cm in length with a fundamental mode (LP(01)) diameter of 54 µm and a corresponding effective mode area of 2290 µm(2).

Low loss, wide transparency, robust tellurite glass fibers for mid-IR (2-5 μm) applications

Mid Infrared (MIR) fiber optics has gained a great deal of interest over the past several decades. Applications range from passive transport to fiber lasers and nonlinear applications. These fibers have found use in a wide array of fields such as sensing, military countermeasures, scientific instrumentation, medical instrumentation, and in research laboratories. As with all fiber development there is a continual urge to seek better performance characteristics including transparency over a wide wavelength range, corrosion resistance, high power handling and low loss. We report on development of tellurite glass fibers displaying exceptionally high performance for various applications including wide band, low loss passive transport for mid IR, high efficiency, wide wavelength range and high power supercontinuum generation from visible to MIR wavelengths >4.5um, and active doping in fibers for use in laser cooling. High performance in each of these areas of interest has been brought about by development of a stable glass formulation and advanced processing techniques to remove impurities ions, entrapped hydroxyl, and scatter centers which allow fibers to be made with exceptionally low losses ~0.2dB/m.

Towards all-fiber optical coolers using Tm-doped glass fibers

We present our experimental and theoretical results of optical cooling in Tm-doped glass fibers as optical cooler pumped by single-mode, high efficiency and high power Tm-doped glass fiber lasers. The effects of impurities including OHabsorption and transition metals have been investigated systematically using different purified glasses for fiber fabrication. Our experimental results of spectroscopic measurements show temperature drops of more than 30 degrees from room temperature with pump powers of less than 3W. The results are in good agreement with theoretical simulation.

Infrared materials and fiber optics

At the Naval Research Laboratory (NRL), we are developing infrared glasses, ceramics and optical fibers for many active and passive applications. The chalcogenide glasses transmit from approximately 1 μm to 12 μm in the important infrared wavelength region. Fibers have been developed for passive applications, including light-pipes for remote chemical sensor systems for environmental pollution monitoring, exo-planet discovery, scanning near field optical microscopy and aircraft protection systems. Active applications have also been developed to exploit the high optical nonlinearity of the fibers to generate broadband IR supercontinuum sources and Raman wavelength shifters. In addition, rare earth doping has been used to create bright sources in the IR for applications such as dynamic IR scene projection systems. More recent developments have focused on fabricating hollow core negative curvature fiber with waveguide loss 100X lower than the material loss in the infrared. Additionally fiber devices and components have been designed and fabricated. Examples include IR fiber combiners utilized to couple the output from several QCLs in the MWIR and LWIR into a single output fiber, thereby enabling efficient power and wavelength scaling. Robust splicing between different types of fibers has also been demonstrated.

Solitonic supercontinuum of fs mid-IR pulses in W-type index tellurite fibers with two zero dispersion wavelengths

We are able to generate red-shifted dispersive waves up to a wavelength of 5.1 µm by pumping a W-type index tellurite fiber in the anomalous dispersion regime between its two zero dispersion wavelengths.

All fiber approach to solid-state laser cooling

An all fiber approach to optical cooling is being investigated experimentally and theoretically using Tm-doped fiber laser and Tm-doped fiber cooler. A single mode, high efficiency and high power Tm-doped fiber laser is used to pump at the absorption edge of Tm-doped fiber coolers, one made by germanate and the other by tellurite glasses. The glass characterization shows that the quenching effect, which is negative for cooling processes in the fiber, in germanate glass is much stronger than that in tellurite glass. The preliminary results of experiments indicate cooling effects could occur in the fiber, but net cooling in the system has not been achieved. A theoretical framework aimed at understanding the nature of cooling in this laser cooling system has been developed which shows that the temperature in the sample could increase even if the fiber core is indeed cooling. The details of the temperature dynamics depend on many factors such as background loss and absorption of scattered light by the heat spreader.