Jonathan W. Evans

840 mW continuous-wave Fe:ZnSe laser operating at 4140 nm

We report the demonstration of high-power (840 mW) continuous-wave laser oscillation from Fe2+ ions in zinc selenide. The output spectrum of the Fe:ZnSe laser had a line-center near 4140 nm with a linewidth of 80 nm. The beam quality was measured to be M2≤1.2 with a maximum slope efficiency of 47%. Small shifts observed in output wavelength with increased output power were attributed to thermal effects. No thermal roll-off of slope efficiency was observed at the maximum of output power.

A Passively

We report the demonstration of high-average-power passively Q-switched laser oscillation from Fe2+ ions in zinc selenide. A semiconductor saturable absorber mirror was used as a passive Q-switch element. Using a 60% R outcoupler, the pump-limited output power was 515 mW. The spectral center of the laser was 4045 nm. The pulse repetition frequency (PRF) at maximum power was ~ 850 kHz with a corresponding minimum pulsewidth of 64 ns Full-Width Half-Maximum. The pulse energy and peak power were and 8.3 W, respectively. The average output power was limited only by available pump power and increased with a slope efficiency of 22%. No thermal rolloff of slope-efficiency was observed. The beam quality was measured to be M2 ≤ 2.6. The temporal stability of the pulsed output was characterized. Thermal effects were shown to play a significant role in determining the PRF of the output.

Q

It has been experimentally shown that a Cr:ZnSe optical amplifier can be successfully used for self-amplification of the cascade Er:Y2O3 laser. This is to our best knowledge the first demonstration of an optical amplifier where both signal and pump are delivered from a single laser source. Absorption and emission spectra of the Cr:ZnSe are perfectly positioned to amplify the output of the cascade Er:Y2O3 laser, when the 1.6 μm emission of the cascade laser serves as a pump, while 2.7 μm as a signal. We have also shown that this concept is valid for any bulk or fiber cascade erbium laser.

-Switched, CW-Pumped Fe:ZnSe Laser

We have achieved ≥ 840 mW continuous-wave (CW) output power from iron-doped zinc selenide (Fe:ZnSe).1 The beam quality was measured to be M2 ≤ 1.2. The laser exhibited a slope efficiency of 47% with no thermal roll-off at maximum output power. Various dichroic mirrors and other spectral filters were inserted into the cavity to discretely tune the output of the laser from 3843 nm to 4337 nm. Demonstration of arbitrary discrete tuning shows that, in principle, Fe:ZnSe is capable of efficient continuously-tunable CW lasing over nearly 500 nm of bandwidth.

Path to doubling the efficiency of mid-IR erbium lasers

Spectroscopic investigation of Co2+:CdTe was performed to evaluate it’s potential as a lasing medium. The sample had a targeted doping concentration of 2% and measurements were performed from 10 - 120K. Cross-sections for Co:CdTe were calculated using Füchtbauer-Ladenburg and reciprocity methods. Calculations suggest the potential for efficient lasing at ~3.7μm when pumped by a 3μm laser source on the 4A2 ↔ 4T2 transition. The fluorescence lifetime was measured to quantify the temperature dependence of the non-radiative relaxation rate. This work aims to characterize Co:CdTe as a novel gain medium for compact, tunable mid-infrared lasers operating within the atmospheric transmission window.

A broadly tunable continuous-wave Fe:ZnSe laser

A sample of Fe:ZnSe fabricated by post-growth thermal diffusion was purchased commercially. The sample was cooled to 80 K using liquid nitrogen and used as the gain element in a Watt-class continuous-wave laser with an output wavelength centered at 4050 nm. The sample was removed from the laser and treated using a hot isostatic press (HIP) technique. The crystal was then re-placed in the laser resonator. After the HIP treatment, and with no other changes to the laser resonator, the slope efficiency of the laser increased by 1.5×. The spectral output was red-shifted to 4122 nm and the output linewidth was narrowed by nearly two orders of magnitude, resulting in a 36× increase in power spectral density. The shift in wavelength and the increase in power scaling performance is consistent with the activation of previously inactive iron impurities in the sample by the HIP treatment.

Optical spectroscopy of cobalt-doped cadmium telluride

A sample of Fe:CdMnTe was grown from melt using the Bridgman technique and was polished and coated for use as a laser crystal. Optical spectroscopy techniques were used to determine the absorption and emission cross-sections of the sample at 80 K. A cryogenic free-running mid-IR laser was constructed in the X-cavity configuration. The laser output was centered at 5223 nm with a spectral width of 1 nm with a maximum average power of 810 mW. The laser exhibited a slope efficiency of 16.4% with respect to total pump power with a pulse repetition frequency of 400 Hz and a pulse duration of 220 μs.

Increasing the performance of an Fe:ZnSe laser using a hot isostatic press

Abstract. This editorial introduces the special section on Solid-State Lasers.

Demonstration and power scaling of an Fe:CdMnTe laser at 5.2 microns

ZnSe doped with Cr2+ was analyzed by EDS, XPS and Micro-Raman spectroscopy techniques. EDS and XPS analysis revealed that chromium concentration is more than 2% and there are additional impurities, Ga, Ti, and Ta. EDS measurements did not reveal any variation in chromium concentration when a line scan was performed over a 200 μm distance. XPS analysis indicated that the sample surface is inhomogeneous. Photoluminescence was acquired by exciting the sample with 325 nm laser beam. Photoluminescence revealed charge transfer bands. Micro-Raman study revealed the LO, TO and 2TA modes at 252, 205 and 140 cm-1. Under 488 or 514.5 nm excitation background luminescence was predominant due to excitation of Cr2+ electrons into the conduction band. However, 632.8 nm laser excitation revealed, strong Raman signals. Raman data were acquired by exciting the sample on the grain boundary and inside the domain. The ratio of LO and TO peak intensities changed randomly when data were acquired from different points on the grain boundary indicating the presence of random strain in the material. When Raman data were acquired from different points on the sample surface for comparison, it revealed that the LO mode was distorted as well as broadened whereas the TO mode intensity increased. This was due to the presence of local modes induced by the sample inhomogeneity and the interaction of the holes with the LO mode.

Special Section Guest Editorial: Solid-State Lasers

Fe:ZnSe lasers have been pumped by several types of diode-pumped solid state laser, including Cr:Er:YSGG (2800 nm),1 Cr:CdSe (2970 nm),2 and Er:YAG (2698 nm,3 2936 nm4). None of these sources has exceeded 1.5 W of true continuous-wave (CW) output power. In this work, we report demonstration of a CW Fe:ZnSe laser pumped by a 10 W Er:Y2O3 laser emitting at 2740 nm,5 which had not been previously attempted. The Er:Y2O3 pump laser was characterized with respect to propagation losses, beam quality, mode size, and pointing stability. It was determined that the limit of output power from the Fe:ZnSe laser was limited by the output stability of the pump laser. The Fe:ZnSe laser operated with < 22% slope efficiency and 800 mW output power was achieved at approximately 4050 nm.