Source: http://aoot.osa.org/oe/abstract.cfm?uri=oe-27-6-9287
Timestamp: 2019-04-21 16:39:11+00:00

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Quasi-continuous-wave laser operation of 20 at.% Tm:LiYF4 thin films (84–240 μm) grown by Liquid Phase Epitaxy (LPE) on undoped LiYF4 substrates is achieved. The 240 μm-thick Tm:LiYF4 active layer pumped at 793 nm with a simple double-pass scheme generated 152 mW (average power) at 1.91 μm with a slope efficiency of 34.4% with respect to the absorbed pump power. A model of highly-doped Tm:LiYF4 lasers accounting for cross-relaxation, energy-transfer upconversion and energy migration is developed showing good agreement with the experiment. The pump quantum efficiency for Tm3+ ions is discussed and the energy-transfer parameters are derived. These results show that LPE-grown Tm:LiYF4 thin films are promising for ~1.9 μm thin-disk lasers.
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Fig. 1 Bright-field microscope images of 20 at.% Tm:LiYF4 / (001) LiYF4 epitaxy: (a) top surface of the as-grown layer, arrow indicates surface dendritic LiF structures; (b) laser-grade polished top surface of the active layer (thickness: 100 μm); (c) growth defects at the substrate / layer interface (indicated by an arrow) due to striation defects in bulk LiYF4 substrate; (d) growth defect at the layer surface (indicated by an arrow).
Fig. 2 Optical microscope image of the polished side facet of the 20 at.% Tm:LiYF4 / (001) LiYF4 epitaxy. The arrow indicates the crystallographic  direction.
Fig. 3 Spectroscopy of 20 at.% Tm:LiYF4 thin films: (a) Absorption spectra for the 3H6 → 3H4 and 3H6 → 3F4 transitions (in black) compared with the absorption cross-section, σabs, spectra for a 3 at.% Tm:LiYF4 single-crystal (in red), both for σ-polarization; (b) luminescence spectra for the 3F4 → 3H6 transition and π and σ polarizations, the excitation wavelength is 780 nm.
Fig. 4 Decay of luminescence from the 3H4 (a) and 3F4 (b) states of Tm3+ ions for 20 at.% Tm:LiYF4 thin films: circles – experimental data, black lines – single-exponential fits.
Fig. 5 (a) Scheme of the laser based on 20 at.% Tm:LiYF4 / LiYF4 epitaxy: P – Glan-Taylor polarizer, PM – pump mirror, OC – output coupler; (b) typical oscilloscope traces of the laser emission showing relaxation oscillations and the incident pump radiation; (c) typical laser emission spectrum (unpolarized output).
Fig. 6 Evaluation of the beam quality factors M2x,y for output beam of the laser based on 20 at.% Tm:LiYF4 / LiYF4 epitaxy: symbols – experimental data on the squared beam diameters, curves – their parabolic fits. Layer thickness: 240 μm, TOC = 5%, Pinc = 2.2 W. Inset – 2D profile of the laser beam in the far-field captured with a thermal imaging screen.
Fig. 7 Input-output dependences for the 20 at.% Tm:LiYF4 active layers (quasi-CW operation, duty cycle: 1:2): (a) layer thickness: 240 µm, various TOC; (b) TOC = 2%, various layer thickness. η – slope efficiency. The vertical axis corresponds to the averaged peak output power.
Fig. 8 (a) Simplified scheme of energy levels of Tm3+ ions in LiYF4 showing possible spectroscopic processes (pump, laser, CR – cross-relaxation, black arrows – radiative decay, NR – non-radiative relaxation, ETU – energy-transfer upconversion, EM – energy migration); (b) Quasi-three-level scheme of a Tm:LiYF4 laser used for modeling.
Fig. 9 Pump quantum efficiency ηq for 20 at.% Tm:LiYF4 well above the laser threshold as a function of TOC for energy-migration rates Wd varying from 28 to 280 × 103 s−1, l = 240 µm, L = 0.2%. Calculation with Eq. (14). CCR = 6.4 × 10−17 cm3s−1, τ30 = 1.51 ms, σLSE = 2.48 × 10−21 cm2 and σLabs = 0.31 × 10−21 cm2.
Fig. 10 Modified Findlay-Clay analysis for 20 at.% Tm:LiYF4: laser threshold Pth as a function of transmission of the output coupler TOC (a) for KETU varying from 0 to 6.0 × 10−18 cm3s−1 and fixed Wd = 170 × 103 s−1 and (b) for Wd varying from 28 to 280 × 103 s−1 and fixed KETU = 1.0 × 10−18 cm3s−1. Modeling parameters: l = 240µm, L = 0.2%, CCR = 6.4 × 10−17 cm3s−1, and wp = 17 µm.
Fig. 11 Modified Caird diagram for the laser based on 240 μm-thick 20 at.% Tm:LiYF4 active layer: inverse of the laser slope efficiency, 1/η, plotted as a function of inverse of the output coupling, 1/TOC. Black solid line and curve: no absorption saturation (ηabs = const, standard Caird plot) or absorption saturation (ηabs ≠ const) for KETU = 0 and Wd = 0; dashed colour curves - ηabs ≠ const, Wd varying from 28 to 280 × 103 s−1 and KETU = 0; dotted red curve - ηabs ≠ const, Wd = 170 × 103 s−1 and KETU = 1.0 × 10−18 cm3s−1; circles – experimental data.
Fig. 12 Modified Findlay-Clay diagram for the laser based on 240 μm-thick 20 at.% Tm:LiYF4 active layer: laser threshold, Pth, plotted as a function of output coupling, TOC. Black line: KETU = 0 and Wd = 0; dashed green curve - Wd = 170 × 103 s−1 and KETU = 0; dotted red curve - Wd = 170 × 103 s−1 and KETU = 1.0 × 10−18 cm3s−1; circles – experimental data. The passive loss L is 0.2%. For all curves, ηabs ≠ const.
Fig. 13 Optical-to-optical efficiency, ηopt, versus the number of pump passes Np for 5 at.% Tm: and 20 at.% Tm:LiYF4 active layers. Modeling parameters: 2wp = 200 µm, Pinc = 1 kW, l = 240 µm, L = 0.2% and TOC = 2%.
Table 1 Output Characteristicsa of Lasers Based on Highly-Doped Tm:LiYF4 Active Layers.
(2) g 0 l=( σ SE L N 2 − σ abs L N 1 )l≈ T OC +L 2 .
(3) η abs 1−pass ( T OC )=1−exp(− σ abs P N 1 l).
(4) η abs total ( T OC )= η abs 1-pass [1+ R p (1− η abs 1-pass )].
(6) η q = N 2 τ 2 +( σ SE L N 2 − σ abs L N 1 ) I L σ abs P N 1 I P .
(7) η q σ abs P N 1 I P =2 C CR N 1 N 3 −2 K ETU N 2 2 .
(8) N 3 = σ abs P N 1 I P + K ETU N 2 2 W d +1/ τ 30 + C CR N 1 .
(9) η q =2 C CR N 1 W d +1/ τ 30 − K ETU N 2 2 σ abs P N 1 I P C CR N 1 W d +1/ τ 30 +1 .
(10) η q =2 C CR N 1 W d +1/ τ 30 1+ C CR N 1 W d +1/ τ 30 + 2 K ETU ( N Tm − N 1 ) 2 N Tm − N 1 τ 2 + ( T OC +L) P out h ν L T OC π w p 2 l .
(11) η q =2 C CR N 1 W d +1/ τ 30 + C CR N 1 .
(12) η q = 1/ τ 30 +2 W CR −(1/ τ 3rad )(1− β 32 ) 1/ τ 30 + W CR .
(13) η q =1+ W CR 1/ τ 30 + W CR .
(14) η q =2 C CR W d +1/ τ 30 C CR W d +1/ τ 30 + ( σ SE L + σ abs L )l σ SE L N Tm l−( T OC +L)/2 .
Output Characteristicsa of Lasers Based on Highly-Doped Tm:LiYF4 Active Layers.

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