The absorption and emission properties of transition metal (TM)-doped zinc chalcogenides have been investigated to understand their potential application as room-temperature, mid-infrared tunable laser media. Crystals of ZnS, ZnSe, and ZnTe, individually doped with Cr/sup 2+/, Co/sup 2+/, Ni/sup 2+/, or Fe/sup 2+/ have been evaluated. The absorption and emission properties are presented and discussed in terms of the energy levels from which they arise. The absorption spectra of the crystals studied exhibit strong bands between 1.4 and 2.0 /spl mu/m which overlap with the output of strained-layer InGaAs diodes. The room-temperature emission spectra reveal wide-band emissions from 2-3 /spl mu/m for Cr and from 2.8-4.0 /spl mu/m for Co, Cr luminesces strongly at room temperature; Co exhibits significant losses from nonradiative decay at temperatures above 200 K, and Ni and Fe only luminesce at low temperatures, Cr/sup 2+/ is estimated to have the highest quantum yield at room temperature among the media investigated with values of /spl sim/75-100%. Laser demonstrations of Cr:ZnS and Cr:ZnSe have been performed in a laser-pumped laser cavity with a Co:MgF/sub 2/ pump laser. The output of both lasers were determined to peak at wavelengths near 2.35 /spl mu/m, and both lasers demonstrated a maximum slope efficiency of approximately 20%. Based on these initial results, the Cr/sup 2+/ ion is predicted to be a highly favorable laser ion for the mid-IR when doped into the zinc chalcogenides; Co/sup 2+/ may also serve usefully, but laser demonstrations yet remain to be performed.

Transition metal-doped zinc chalcogenides: Spectroscopy and laser demonstration of a new class of gain media

Quantum control of complex objects in the regime of large size and mass provides opportunities for sensing applications and tests of fundamental physics. The realization of such extreme quantum states of matter remains a major challenge. We demonstrate a quantum interface that combines optical trapping of solids with cavity-mediated light-matter interaction. Precise control over the frequency and position of the trap laser with respect to the optical cavity allowed us to laser-cool an optically trapped nanoparticle into its quantum ground state of motion from room temperature. The particle comprises 108 atoms, similar to current Bose-Einstein condensates, with the density of a solid object. Our cooling technique, in combination with optical trap manipulation, may enable otherwise unachievable superposition states involving large masses.

https://science.sciencemag.org/content/sci/early/2020/01/29/science.aba3993.full.pdf

Cooling of a levitated nanoparticle to the motional quantum ground state.

Infrared (2–12 μm) solid-state laser sources: a review

Transition metal (TM) doped II-VI chalcogenide laser materials offer a unique blend of physical, spectroscopic, and technological parameters that make them the gain media of choice for cost effective broadly tunable lasing in the Mid-IR. The II-VI semiconductor hosts provide a low phonon cut-off, broad IR transparency, and high thermal conductivity. When doped with transition metal ions, these materials feature ultrabroadband gain, low saturation intensities, and large pump absorption coefficients. This combined with the low-cost mass production technology of crystal fabrication by postgrowth thermal diffusion, as well as broad availability of convenient pump sources, make these materials ideal candidates for broadly tunable mid-IR lasing in CW, gain-switched, free running, and mode-locked regimes of operation. This review summarizes experimental results on optically pumped lasers based on Cr and Fe doped II-VI wide band semiconductors providing access to the 1.9-6 μm spectral range with a high (exceeding 60%) efficiency, multi-Watt-level (18 W in gain switch and 30 W in pure CW) output powers, tunability in excess of 1000 nm, short-pulse (<;50 fs) multi-watt oscillation, multi-Joule long-pulse output energy, and narrow spectral linewidth (<;100 kHz).

Progress in Mid-IR Lasers Based on Cr and Fe-Doped II–VI Chalcogenides

Commercial 0.5 kW Yb-doped fiber amplifiers have been characterized and found to be suitable for coherent beam combining. Eight such fiber amplifiers have been coherently combined in a tiled-aperture configuration with 78% combining efficiency and total output power of 4 kW. The power-in-the-bucket vertical beam quality of the combined output is 1.25 times diffraction limited at full power. The beam-combining performance is independent of output power.

Coherent combining of a 4 kW, eight-element fiber amplifier array.

A new, scalable coherent beam combination architecture has been developed and experimentally verified. Coherent Polarization Beam Combination (CPBC) relies on the combination of multiple beams through a coherent superposition of orthogonal polarization states. By actively controlling the phases and polarizations of individual beams, the creation of new polarization states at polarization beam combiners enables the coherent combination of any number of beams with arbitrary power ratios. A four-beam CPBC was experimentally demonstrated at low laser power level, with 96% combining efficiency, nearly diffraction-limited beam quality, and variable polarization of the output beam. The CPBC method will enable power scaling of master oscillator power amplifier laser systems.

Coherent Polarization Beam Combination

A new coherent polarization beam combination (CPBC) architecture was employed to combine four amplified beams and produce over 25 W of coherent output with 94% efficiency and nearly diffraction-limited beam quality. The individual beams were produced by a fiber master oscillator power amplifier system and their phases were controlled and locked using servo-controlled phase modulators. The CPBC approach relies on a coherent superposition of orthogonal polarization states and allows the combination of an arbitrary number of beams with arbitrary power ratios.

Power scaling of a fiber master oscillator power amplifier system using a coherent polarization beam combination.

We have demonstrated 50 W of output power from a free-running Tm:YAlO3 laser at 1940 nm. We have also obtained Q-switched operation with 7 mJ of output energy at a 5 kHz repetition rate.

High power Q-switched Tm:YAlO3 lasers

A principal difficulty of long dwell coherent imaging ladar is its extreme sensitivity to target or platform motion. This paper describes a motion compensated frequency modulated continuous wave 3D coherent imaging ladar method that overcomes this motion sensitivity, making it possible to work with nonstatic targets such as human faces, as well as imaging of targets through refractive turbulence. Key features of this method include scannerless imaging and high range resolution. The reduced motion sensitivity is shown with mathematical analysis and demonstration 3D images. Images of static and dynamic targets are provided demonstrating up to 600×800 pixel imaging with millimeter range resolution.

Motion compensated frequency modulated continuous wave 3D coherent imaging ladar with scannerless architecture.

A method for frequency modulated continuous wave LADAR imaging is provided. The method comprises the steps of generating a chirped laser signal and a constant-frequency laser signal, forming a target laser signal from the chirped laser signal and the constant-frequency laser signal and forming a local oscillator laser signal from the chirped laser signal and the constant-frequency laser signal. The method further comprises delaying or frequency shifting the local oscillator laser signal and directing the target laser signal to a target. The method further comprises combining a returned signal received from the target with the delayed or frequency shifted local oscillator laser signal to form a combined received signal, and capturing the combined received signal with a plurality of image sensors. The method further comprises performing a nonlinear operation on the captured combined received signal and Fourier transforming the captured combined received signal to determine a range corresponding to each of the plurality of image sensors in the imaging device.

Bruce G. Tiemann

Papers

Coherent Polarization Beam Combination

Power scaling of a fiber master oscillator power amplifier system using a coherent polarization beam combination.

High power Q-switched Tm:YAlO3 lasers

Motion compensated frequency modulated continuous wave 3D coherent imaging ladar with scannerless architecture.

FMCW 3-D LADAR imaging systems and methods with reduced Doppler sensitivity