Dual-wavelength-pumping of mid-infrared Tm:YLF laser at 2.3 µ m: demonstration of pump seeding and recycling processes

: Upconversion pumping of thulium lasers emitting around 2.3 μm (the 3 H 4 → 3 H 5 12 transition) has recently attracted a lot of attention as it is compatible with the mature Yb-laser 13 technology. To explore this possibility, we built a mid-infrared Tm:LiYF 4 laser pumped by an 14 Yb:CaF 2 laser at 1.05 μm delivering an output power of 110 mW at 2.31 μm for a maximum 15 incident pump power of 2.0 W. A strong absorption issue appeared in the Tm laser: the slope 16 efficiency vs. the incident pump power was 7.6% while that vs. the absorbed pump power 17 reached 29%. To overcome this issue, a dual-wavelength pumping at 0.78 μm and 1.05 μm was 18 explored (combining both the direct and upconversion pumping schemes). The reciprocal 19 interplay between the two pumps was studied to evaluate their benefits in terms of the pump 20 absorption and laser efficiency. We observed an interesting decrease of the laser threshold for 21 upconversion pumping when adding a small fraction of the direct pump revealing a seeding 22 effect for the excited-state absorption from the metastable 3 F 4 level. A recycling process of this 23 manifold by excited-state absorption in the 3 F 4 → 3 F 2,3 loop was also observed. The pump


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Mid-infrared (MIR) lasers emitting at the wavelengths around 2.3 μm (falling into the 2.0-29 2.4 µm atmospheric window, the K band) are of practical importance for spectroscopy of 30 various atmospheric and biological species such as HF, CO, CH4, H2CO and C6H12O6 leading 31 to applications in the atmosphere gas sensing and pollutant detection, combustion studies and 32 non-invasive glucose blood measurements [1,2]. Such laser sources are also interesting for 33 pumping of mid-infrared optical parametric oscillators (OPOs) [3].

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There exist several approaches to address this spectral range, namely by using lasers based

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Finally, the thulium ions (Tm 3+ ) themselves can provide a direct generation of the 2.3 μm 39 radiation [7,8] according to the 3 H4 → 3 H5 electronic transition, Fig. 1(a). Note that Tm 3+ ions 40 can be efficiently pumped at 0.78-0.8 μm (directly to the 3 H4 state) using commercial high- regimes and high quality of the laser beam. In this way, Tm lasers operating on the 3 H4 → 3 H5 transition may represent a simple and cheap alternative to Cr 2+ -ion-based lasers at the expense 48 of much narrower gain bandwidth. The emission range of such Tm lasers is also filling the gap 49 between other direct emission of rare-earth-ions in the near-mid-IR, namely Ho 3+ ( 5 I7 → 5 I8, 50 ~2.1 μm) and Er 3+ ( 4 I11/2 → 4 I13/2, ~2.8 μm).

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During the last years, a great progress in developing continuous-wave (CW) [

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The photon avalanche process recycles the populations of Tm 3+ multipets in favor of the 3 H4, 72 and 3 F4 ones acting as the upper laser level and the "effective" ground-state, respectively.

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The FWHMs of these peaks are 2.5 nm and 4.3 nm, respectively. The 3 H6 → 3 H5 GSA is weak 103 between 1000 nm and 1100nm (σGSA ≈ 0.01×10 -20 cm 2 in this region) since it is located in the 104 tail of the peak at 1.20 µm (for -polarization) that has a value of σESA = 0.62×10 -20 cm 2 .

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The OC also provides HT at 1.9 μm to suppress the laser on the 3 F4 → 3 H6 transition. The

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The wavelength of the Yb:CaF2 laser is continuously tuned from 1032 to 1054 nm, Fig. 3(a).

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To justify the selection of the pump wavelength, we have studied the single-pass pump 157 absorption as a function of λP,UC and the incident pump power, Fig. 3(b). The highest pump 158 absorption is observed at 1050 nm in agreement with the ESA spectra of Tm 3+ ions in LiYF4 159 for σ-polarization, cf. Fig. 1(d). A certain pump level is needed to reach a kind of plateau for pump absorption (~30%). Thus, tuning the pump wavelength to match precisely one of the ESA 161 peaks is indeed critical as otherwise we are not able to reach reasonably high pump absorption 162 even for high pump levels.

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However, the slope efficiency vs. the absorbed pump power is relatively high, 29%, which then 181 rises the interest of using the Yb-laser technology for pumping Tm 3+ -doped materials emitting 182 at 2.3 µm. A typical spectrum of the laser emission is shown in Fig. 4(b). The laser emitted 183 linearly polarized radiation (π) and its polarization state is naturally selected by the anisotropy 184 of the gain medium. To illustrate the superiority of the short focal length L1 lens for pure UC 185 pumping, both the input-output dependences of the Tm-laser for f = 50 mm and f = 150 mm 186 are given in Fig. 4(a). Using the latter lens, the maximum output power of the MIR Tm-laser 187 was only 80 mW corresponding to an increased threshold of 1.2 W.

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The dual-wavelength pumping is studied by fixing the power level of one pump source and varying that of the second one. The laser output powers are plotted versus the total incident 196 pump power, PΣ = P0.78μm + P1.05μm and the slope efficiency and the laser threshold are 197 determined with respect to PΣfor fair comparison. To assist UC pumping, the P0.78μm value is 198 fixed at several different levels and P1.05μm is varied, as shown in Fig. 5(a). With increasing the 199 added P0.78μm power up to 2.0 W, both the maximum output power of the Tm-laser and its slope 200 efficiency gradually increase reaching 500 mW and 15.0%, respectively, Fig. 5(a,b).           Fig. 6(b), when using dual-pumping despite a lower absorption at 302 0.78 μm, cf. Fig. 9(b).

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The observed rapid increase of the pump absorption at 1.05 μm under co-pumping at 304 0.78 μm (even for relatively low added pump powers) is responsible for the useful decrease of 305 the laser threshold of the dual-wavelength-pumped Tm:LiYF4 laser, cf. Fig. 7. It is also partially 306 responsible of the increase of the laser slope efficiency, as shown in Fig. 5(b)