Mid-infrared laser operation of Er3+-doped BaF2 and (Sr,Ba)F2 crystals.

We report on the first, to our knowledge, mid-infrared laser operation of two Er3+-doped barium-containing fluorite-type crystals, BaF2 and (Sr,Ba)F2, featuring a low-phonon energy behavior. A continuous wave 4.9 at.% Er:(Sr,Ba)F2 laser generated 519 mW at 2.79 µm with a slope efficiency of 25.0% and a laser threshold of 27 mW. The vibronic and spectroscopic properties of these crystals are determined. The phonon energy of (Sr,Ba)F2 is as low as 267 cm-1. The Er3+ ions in this crystal feature a broadband emission owing to the 4I11/2 → 4I13/2 transition and a long luminescence lifetime of the 4I11/2 level (10.6 ms) making this compound promising for low-threshold, broadly tunable, and pulsed 2.8-µm lasers.

Diode-pumped mode-locked Yb:BaF2 laser.

We report on a continuous-wave (CW) and passively mode-locked operation of a fluorite-type Yb:BaF2 crystal. Pumped with a spatially single-mode, fiber-coupled InGaAs laser diode at 976 nm, the Yb:BaF2 laser generated a maximum CW output power of 512 mW at 1054.4 nm, corresponding to a laser threshold of 36.5 mW and a slope efficiency of 65.0%. A continuous wavelength tuning across 85 nm (1007-1092 nm) was achieved. By implementing a semiconductor saturable absorber mirror for initiating and sustaining the soliton pulse shaping, near Fourier-transform-limited pulses as short as 52 fs were generated at 1058.2 nm with an average output power of 129 mW at a pulse repetition rate of ∼79.5 MHz. To the best of our knowledge, this is the first report on the passively mode-locked operation of the Yb:BaF2 crystal.

Thermal lensing, heat loading and power scaling of mid-infrared Er:CaF2 lasers.

Mid-infrared Er:CaF2 laser operating on the 4I11/2 → 4I13/2 transition is developed. Its power scaling capabilities and thermo-optics (fractional heat loading and thermal lensing) are compared under pumping into the 4I11/2 and 4I9/2 states. Using a 4.5 at.% Er:CaF2 crystal, a record-high continuous-wave output power of 0.83 W is achieved at 2800 nm with a slope efficiency of 31.6% and a laser threshold of 24 mW and the fractional heat loading is measured under lasing and non-lasing conditions, yielding the values of 52.0% and 71.7%, respectively (for pumping at 967.6 nm, into the 4I11/2 state). The thermal lens in Er:CaF2 is negative (divergent) owing to the negative thermo-optic coefficient and large and negative contribution of the photo-elastic effect. The sensitivity factors of the thermal lens are Mr = -4.84 and Mθ = -5.15 [m-1/(kW/cm2)] and the astigmatism degree is as low as 6%. When pumping into the higher lying 4I9/2 manifold, the thermal lens is enhanced owing to the additional heat generation from the multiphonon non-radiative path from this state, and the laser slope efficiency is deteriorated.

Er:KY3F10 laser at 2.80 µm.

We report on the mid-infrared laser operation of a cubic 15 at.% Er3+:KY3F10 crystal. In the quasi-continuous-wave regime, the peak power reaches 255 mW at 2.80 µm (the 4I11/2→4I13/2 transition) with a slope efficiency of 10.9% and a laser threshold of 58 mW. Two pumping schemes (to the 4I11/2 and 4I9/2 states) are compared. The emission properties of the Er3+ ions in KY3F10 are studied, indicating high stimulated-emission cross-section of 0.57×10-20cm2 at 2.80 µm, a large gain bandwidth of 40 nm, and a long 4I11/2 state lifetime of 4.64 ms.

Spectroscopy and diode-pumped laser operation of transparent Tm:Lu3Al5O12 ceramics produced by solid-state sintering.

A transparent Tm:Lu3Al5O12 ceramic is fabricated by solid-state reactive sintering at 1830 °C for 30 h using commercial α-Al2O3 and Lu2O3/Tm2O3 powders and sintering aids - MgO and TEOS. The ceramic belongs to the cubic system and exhibits a close-packed structure (mean grain size: 21 µm). The in-line transmission at ∼1 µm is 82.6%, close to the theoretical limit. The spectroscopic properties of the ceramic are studied in detail. The maximum stimulated-emission cross-section is 2.37×10-21 cm2 at 1749nm and the radiative lifetime of the 3F4 state is about 10 ms. The modified Judd-Ofelt theory accounting for configuration interaction is applied to determine the transition probabilities of Tm3+, yielding the intensity parameters Ω2 = 2.507, Ω4 = 1.236, Ω6 = 1.340 [10-20 cm2] and α = 0.196×10-4 cm. The effect of excited configurations on lower-lying interconnected states with the same J quantum number is discussed. First laser operation is achieved under diode-pumping at 792 nm. A 4 at.% Tm:Lu3Al5O12 ceramic laser generated 3.12 W at 2022-2035nm with a slope efficiency of 60.2%. The ceramic is promising for multi-watt lasers at >2 µm.