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Abstract
The laser diode (LD)-pumped Tm:YAP (a-cut, 3.5 at.%) laser generated a maximum ∼2.3 µm continuous wave (CW) laser output power of ∼3 W. The higher output power benefited from the positive effect of the cascade lasing (simultaneously operating on the 3H4 → 3H5 and 3F4 → 3H6 Tm3+ transition). It was the highest CW laser output power amongst the LD/Ti:Sapphire-CW-pumped ∼2.3 µm Tm3+-doped lasers reported so far. Under the cascade laser operation, the slope efficiency of the ∼2.3 µm laser emission versus the absorbed pump power increased from 13.0% to 21.4%.
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1. Introduction
Laser sources emitting at ∼2.3 µm were of practical importance for atmosphere gas sensing [1,2], optical metrology of combustion processes [3], and non-invasive blood glucose measurements [4]. In addition, they were also important sources for the pumping of mid-infrared optical parametric oscillators [5] and the generation of high-order harmonics [6]. Laser diode (LD)-pumped ∼2.3 µm solid-state lasers were attractive for applications because they provided a compact, robust, and all-solid design and relayed on the commercially available, and high-power LDs as the pump sources, and their power-scalability could reach multi-watt output [7]. Due to the broad and intense 3H6 → 3H4 absorption band of trivalent thulium ions (Tm3+) matched well with the emitting wavelength of the commercial AlGaAs LDs, the Tm3+ can be easily pumped [8]. Besides the well-known 3F4 → 3H6 Tm3+ transition at ∼2 µm [9], the Tm3+ may emit at ∼1.5 µm (3H4 → 3F4) [10] and ∼2.3 µm (3H4 → 3H5) [11], as shown in Fig. 1(a). Compared with the ∼2 µm laser transition, the ∼2.3 µm laser transition represented a quasi-four-level laser scheme with no reabsorption [7] and was not self-terminating [12]. However, limited by the severe thermal effects caused by the large quantum defect (about 65%) and the easily quenched lifetime of upper laser level (3H4) caused by the multi-phonon non-radiative relaxation (NR) and cross-relaxation (CR, Tm1 (3H4) + Tm2 (3H6) → Tm1 (3F4) + Tm2 (3F4), see Fig. 1(a)) processes [12], the laser operation at ∼2.3 µm was challenging. The NR depends on the host strongly and the CR processes rise fast with the Tm3+ dopant concentrations.
Fig. 1. (a) Partial energy level scheme of Tm3+ in YAP: red and pink arrows, pump absorption and laser transitions, respectively; green arrows, non-radiative relaxation (NR); blue arrows, cross-relaxation (CR) and energy-transfer upconversion (ETU) processes; τrad, radiative lifetime [20]. (b) Cascade lasing: |0>, ground-state; |1>, intermediate metastable lower-lying excited-state; |2>, higher-lying excited-state.
Yttrium aluminum perovskite YAlO3 (shortly YAP) was a mature and well-developed laser host material [13]. Due to the excellent thermal and thermo-mechanical properties (thermal conductivity: ∼11 W/(mK)) [14], the rare-earth ions (RE3+)-doped YAP crystals were widely used in high-power laser systems, such as Nd:YAP (121 W at ∼1.3 µm) [15], Yb:YAP (7 W at ∼1 µm) [16], Ho:YAP (107 W at ∼2 µm) [17], Er:YAP (6.9 W at ∼3 µm) [18], and Tm:YAP (315 W at ∼2 µm) [19] laser. Some studies have shown the Tm:YAP crystals were promising laser gain medium for ∼2.3 µm solid-state laser operating on the 3H4 → 3H5 Tm3+ transition [7,20–22]. The lower maximum phonon energy (552 cm−1 [20]) could diminish the NR processes from the 3H4 upper laser level and deliver a lower laser threshold. The maximum emission cross-section of the Tm:YAP crystal for the 3H4 → 3H5 Tm3+ transition was measured to be 0.86 × 10−20 cm2 at 2275 nm (for E||b) [22] higher than that of commonly used Tm:YLF crystal (0.57 × 10−20 cm2 at 2305 nm for π-polarization [12]) and Tm:YAG crystal (0.35 × 10−20 cm2 at 2324 nm [23]). However, limited by the available pump power supplied by the LD or Ti:Sapphire (TS) laser and the intrinsic challenges of the 3H4 → 3H5 Tm3+ transition, the obtained highest ∼2.3 µm CW laser output power of the continuous wave (CW)-pumped Tm:YAP laser was only 1.33 W [7]. In the quasi-CW regime, the peak output power reached only 2.69 W, featuring a complex quasi-CW pumping scheme [7].
Recently, the cascade lasing strategy allowing the Tm3+-doped laser to simultaneously operate on the 3H4 → 3H5 and 3F4 → 3H6 Tm3+ transition was recognized as a promising method to improve the performance of the ∼2.3 µm Tm3+-doped laser [24,25]. Cascade lasing was a phenomenon in which two laser transitions (|2> → |1 > and |1> → |0>, see Fig. 1(b)) could facilitate each other and was widely used in the RE3+ with a metastable lower-lying excited-state (|1>) [26–30]. For Tm3+, due to the strong NR from the 3H5 state, the 3F4 state was the metastable lower-lying excited state in cascade lasing.
In the present work, two Tm:YAP crystals with the Tm3+ dopant concentrations of 2.0 at.% and 3.5 at.%, respectively, were studied, and the positive effect of the cascade lasing on the ∼2.3 µm laser performance was demonstrated for the 3.5 at.% one. When the 3.5 at.% Tm:YAP laser simultaneously operated on the 3H4 → 3H5 and 3F4 → 3H6 Tm3+ transition, the maximum CW laser output power of ∼3 W at 2274 nm with the slope efficiency (versus absorbed pump power (Pabs)) increasing from 13.0% to 21.4% was obtained for 5% OC. It was the highest CW laser output power amongst the LD/TS-CW-pumped ∼2.3 µm Tm:YAP lasers reported so far.
2. Experimental setup
Two Tm:YAP crystals with different Tm3+ dopant concentrations (CTm) and sizes were employed as the laser gain medium. Such crystals were provided by Beijing Opto-Electronics Technology Co.Ltd. and grown by the conventional Czochralski method. The parameters of the two Tm:YAP crystals were CTm1 = 2.0 at.%, aperture: 4 × 4 mm2, thick: 12 mm, and CTm2 = 3.5 at.%, aperture: 3 × 3 mm2, thick: 10 mm, respectively. The actual Tm3+ density corresponds to NTm1 = 3.59 × 1020 cm−3 and NTm2 = 6.28 × 1020 cm−3, respectively. They were oriented for light propagation along the a-axis (a-cut) thus giving access to two principal light polarizations, E || b and E || c. Both their end faces were polished to laser-grade quality with good parallelism and uncoated. The schematic configuration of the LD-pumped Tm:YAP laser is shown in Fig. 2(a). As a pump source, we used a fiber-coupled AlGaAs LD (fiber core diameter: 200 µm, numerical aperture: 0.22) emitting up to ∼40 W at ∼790 nm (3H6 → 3H4 Tm3+ transition in absorption, see Fig. 1). The emitting wavelength of the LD red-shifted with the temperature, as shown in Fig. 2(b). To match well with the absorption peak of the Tm:YAP crystal (∼794 nm [20]), the temperature of the LD was fixed at ∼35 °C. The pump beam was collimated and focused into the Tm:YAP crystal by a pair of lenses (f = 50 mm) resulting in the pump spot radius of 100 µm. The Tm:YAP crystals were wrapped with indium foil and then mounted inside a water-cooled copper block with the temperature set at 12 °C. The laser cavity consisted of a flat pump mirror (PM) providing a high transmission at 0.79 µm (pump wavelength) and a high reflection at 1.8-2.4 µm (laser wavelength), and a set of flat output couplers (OCs) with the transmission of TOC = 0.5%-10% at 2.2-2.4 µm.
Fig. 2. (a) Schematic configuration of the LD-pumped Tm:YAP laser: LD - laser diode, PM - pump mirror, OC - output coupler, F - long-pass filter. (b) Relationship between the emitting wavelength and the temperature of the LD.
The residual pump beam was filtered out using a long-pass filter (FELH900, Thorlabs) with a cutoff wavelength of 1 µm and transmission of ∼85% at ∼2.3 µm. The laser output power and emission spectra were measured with an optical power meter (S425C + PM400, Thorlabs) and an optical spectrum analyzer (APE GmbH, Germany), respectively.
3. Experimental results and discussion
Firstly, the pump absorption efficiency (ηabs) of the two studied Tm:YAP crystals as a function of the incident pump power (Pinc) was examined under non-lasing (NL) conditions. The pump absorption efficiency decreased from 94.08% to 70.01% for 2.0 at.% Tm:YAP crystal and from 96.22% to 72.86% for 3.5 at.% ones with the increased incident pump power due to the absorption saturation (ground-state (3H6) bleaching), as shown in Fig. 3(a). The higher pump absorption efficiency of the 3.5 at.% Tm:YAP crystal benefited from the higher Tm3+ doping level. The pump absorption efficiency under the lasing (L) condition was determined for each OC according to the condition ηabs,L = ηabs,NL(Pth). Pth was the laser threshold.
Fig. 3. (a) Measured pump absorption efficiency of the two studied Tm:YAP crystals. (b) Evaluation of the beam quality parameter M2 factor, 3.5 at.% Tm:YAP, TOC = 5%.
The 90/10 knife-edge method was employed to measure the beam quality parameter M2 factor. In the horizontal (X) and vertical (Y) directions, the M2 factors were determined to be 2.49 ± 0.1 and 1.95 ± 0.1, respectively, indicating that the Tm:YAP laser operated close to the fundamental transverse mode, see Fig. 3(b).
The continuous wave (CW) laser experiments were performed in the laser cavity shown in Fig. 2(a). First, we described the results achieved with the 2.0 at.% Tm:YAP crystal. The input-output dependences and the laser emission spectra are shown in Fig. 4(a) and Fig. 4(b), respectively. Figure 4(b) shows that the Tm:YAP laser operated only on the 3H4 → 3H5 Tm3+ transition without any emission on the 3F4 → 3H6 ones (∼2 µm). The maximum ∼2.3 µm CW laser output power of 2.36 W with the laser slope efficiency (η, versus Pabs) of 14.3% was achieved with the 2% OC. The 0.5%, 1%, 5%, and 8% OC corresponded to 1.2 W, 1.63 W, 1.4 W, and 0.8 W CW laser output power, respectively. The ∼2.3 µm CW laser operation was not realized with the 10% OC due to the mismatch between the small gain and the large loss. Note that the power transfer curve of the Tm:YAP laser was nonlinear, and a larger laser slope efficiency was obtained at the higher Pabs. Therefore, the laser slope efficiency was fitted by segments. For example, the “η = 6.2% / 9.7% / 14.3%” in Fig. 4(a) indicated the laser slope efficiency of the fit in three segments. We believed it should be caused by the positive ETU (Tm1 (3F4) + Tm2 (3F4) → Tm1 (3H6) + Tm2 (3H4), see Fig. 1(a)) effect. More populations were accumulated in the metastable 3F4 state at the higher pump level, leading to the enhanced ETU processes which could refill the upper laser level (3H4). However, since all the studied OCs were coated for high transmission of above 90% at 1.9-2.1 µm, and the populations accumulated in the 3F4 state were not enough due to the lower Tm3+ doping level and the lower pump level, the ∼2 µm laser operation wasn’t obtained. It was possible to achieve the cascade laser operation by increasing the pump intensity. The laser emission spectra were independent of the pump power, so the typical ones with different OCs were measured at Pabs of about 27 W. The observed laser emission wavelength (2274 nm) was in agreement with the local maximum in the stimulated-emission cross-section spectra for the E||b light polarization, which was naturally selected by anisotropy of the gain [22]. It was not red-shifted or bule-shifted with the increase in the transmission of the OCs, which was expected for the quasi-four-level laser transition.
Fig. 4. (a) Input-output dependences of the 2.0 at.% Tm:YAP laser and (b) typical laser emission spectra achieved with different OCs captured at Pabs = 27 W: η - laser slope efficiency versus Pabs.
As shown in Fig. 5(a) (b), the 3.5 at.% Tm:YAP laser operated only on the 3H4 → 3H5 Tm3+ transition (2274 nm) and generated the maximum CW laser output power of 0.665 W, 0.675 W, and 0.580 W for the OCs with the lower transmission of 0.5%, 1%, and 2%, respectively. The improvement in laser slope efficiency due to the positive ETU effect wasn’t observed. We believe that this may be because the populations accumulated in the 3F4 state were not enough to generate the positive ETU effect at the lower pump power. The 3H4 state luminescence lifetime was easily quenched by the cross-relaxation which increased as the Tm3+ doping level (∼0.15 ms for 2.0 at.% Tm3+ doping level and ∼0.05 ms for 3.5 at.% one [20]). Therefore, the absorbed pump power was limited by ∼15 W to avoid severe thermal effects.
Fig. 5. Input-output dependences and typical laser emission spectra of 3.5 at.% Tm:YAP laser: (a),(b) TOC = 0.5%, 1%, and 2%; (c),(d) TOC = 5%; (e),(f) TOC = 8% and 10%; η - laser slope efficiency versus Pabs.
For the 5% OC, as shown in Fig. 5(c) (d), the 3.5 at.% Tm:YAP laser operated on the 3H4 → 3H5 Tm3+ transition (2274 nm) at Pabs < 23.22 W with the slope efficiency of 13.0%. As the Pabs continued to increase, a significant improvement in the slope efficiency was observed. The slope efficiency reached 30.5% close to the Stokes limit, ηSt,L = λP/λL = 34.8%. Here, λP and λL were the wavelengths of the pump beam and output laser, respectively. The laser emission spectra demonstrated, at Pabs > 23.22 W, the 3.5 at.% Tm:YAP laser simultaneously operated on the 3H4 → 3H5 (2274 nm) and 3F4 → 3H6 (1888nm) Tm3+ transition. Another long-pass filter coated for the transmission of ∼90% at ∼2.3 µm and almost zero transmission at ∼2 µm was used to separate the ∼2.3 µm power contribution. After filtering out the ∼2 µm power contribution, the maximum ∼2.3 µm CW laser output power of 2.97 W with a slope efficiency of 21.4% was obtained. This Tm:YAP cascade laser operated at a high pump level. Due to a combination of the decreased pump absorption efficiency by ground-state bleaching and the increased one by cascade lasing, the pump absorption efficiency under the cascade lasing condition was close to that measured at the laser threshold. No thermal roll-over or crystal thermal fracture was observed and both ∼2.3 µm and ∼1.88 µm laser power were stable during the Tm:YAP laser operating for several hours at least Pabs ∼31 W, indicating excellent thermo-mechanical properties. In addition, the strong natural birefringence of the Tm:YAP crystal dominates any thermally induced changes of the refractive index, which could greatly suppress the depolarization losses. To the best of our knowledge, 2.97 W was the highest one amongst the LD/TS-CW-pumped ∼2.3 µm laser operating on the 3H4 → 3H5 Tm3+ transition, as shown in Table 1. Allowing the simultaneous operation of the 3H4 → 3H5 and 3F4 → 3H6 Tm3+ transition drastically reduced the populations of the 3F4 state and also strongly reduced the ETU processes. Therefore, we determined that the improvement in laser slope efficiency (from 13.0% to 21.4%) only benefited from the positive effect of the cascade lasing. The Tm3+ stored in the metastable intermediate 3F4 level was recycled and forced down to the ground-state 3H6 level, which could avoid the bottleneck effect and help form population inversion for the 3H4 → 3H5 Tm3+ transition as well as avoid excessive bleaching of the ground-state. For the 8% and 10% OC, as shown in Fig. 5(e) (f), the 3.5 at.% Tm:YAP laser operated only on the 3F4 → 3H6 Tm3+ transition (1888nm) and generated the maximum CW laser output power of 4.64 W and 2.06 W, respectively, and the ∼2.3 µm CW laser operation was not realized.
Table 1. Performance comparisona of the reported LD/TS-CW-pumped ∼2.3 µm lasers.
The intracavity losses (L) of the Tm:YAP lasers were estimated by using the Findley-Clay method, $- \ln ({{R_{OC}}} )= 2{g_0}l - L$, where ${R_{OC}}$ is the OC reflectivity, ${g_0}$ is the small-signal gain, and l is the length of the laser crystal. This analysis yielded L of 3.7% and 8.9% for 2.0 at.% Tm:YAP laser and the 3.5 at.% one, respectively, as shown in Fig. 6.
Although all the studied OCs were coated for the high transmission at ∼2 µm to suppress the unwanted ∼2 µm laser emission, the 3F4 → 3H6 Tm3+ transition was still able to generate. We believe there could be two reasons. Firstly, the higher Tm3+ doping level could enhance the CR processes and thus promote the accumulation of more populations on the 3F4 state, which facilitated the 3F4 → 3H6 Tm3+ transition. Secondly, the lifetime of the 3H4 state (upper-laser level for 3H4 → 3H5 Tm3+ transition) was quenched by the enhanced CR process, which increased the laser threshold of the ∼2.3 µm laser transition. For this 3.5 at.% Tm:YAP laser, there was a possible method to suppress ∼2 µm laser emission, that is, the OCs were coated for a higher transmission of ∼100% at ∼2 µm. If this 3.5 at.% Tm:YAP laser operated only at ∼2.3 µm at high pump power, the ETU effect should be beneficial.
Note that when the Tm:YAP laser didn’t operate on the cascade state, all power was measured behind the first ∼85% transmission filter, not directly measured behind OCs. When the Tm:YAP laser operated on the cascade state, the total power was measured behind the first ∼85% transmission filter, and the 2.3 µm CW laser output power was measured behind the second ∼90% transmission filter. All measured power data was divided by the filter transmission to obtain more realistic power data. All the laser emission spectra in this work were measured after the ∼85% transmission filter.
The light polarizations for the 2.0 at.% and 3.5 at.% Tm:YAP laser at ∼2.3 µm and ∼1.88 µm were E || b, which were naturally selected by the anisotropy of the gain and consistent with the polarized stimulated-emission spectra measured in the Ref. [7].
The more output characteristics of the Tm:YAP laser are summarized in Table 2.
Table 2. Output performancea of the LD-CW-pumped Tm:YAP lasers.
4. Conclusion
To conclude, the ∼2.3 µm CW laser output power of about 3 W was obtained based on the 3.5 at.% Tm:YAP crystal. It was the highest one amongst the reported LD/TS-CW-pumped Tm3+-doped laser operating on the 3H4 → 3H5 Tm3+ transition. The laser operated at a cascade state at higher pump level with the 5% OC, which increased the laser slope efficiency from 13.0% to 21.4%. The bottleneck effect related to the accumulation of electronic excitations in the intermediate long-living 3F state was efficiently avoided. This behavior demonstrated the positive effect of the cascade lasing on the performance of the Tm:YAP laser operating on the 3H4 → 3H5 Tm3+ transition. In addition, another Tm:YAP crystal with the Tm3+ dopant concentrations of 2.0 at.% was studied. For the 2.0 at.% Tm:YAP laser, there was no cascade laser operation. The maximum ∼2.3 µm CW laser output power of 2.36 W was obtained with the 2% OC. The laser slope efficiency increased from 6.2% to 14.3% due to the refilling of the 3H4 state by the positive ETU processes.
In this work, the maximum ∼2.3 µm CW laser output power of ∼3 W was limited by the available pump power. Therefore, the further power scaling can be realized by employing a laser diode with a higher output power as the pump source. In addition, the ∼2.3 µm CW laser output power and the slope efficiency can be improved by changing the pumping scheme to dual-end pumping, selecting a more suitable Tm3+ dopant concentration, and bonding the Tm:YAP crystal to mitigate the thermal effects at the higher pump power, etc.
Funding
National Natural Science Foundation of China (52072351, 12004213, 12174223, 12274263, 21872084, 62175128); Qilu Young Scholar Program of Shandong University; Taishan Scholar Foundation of Shandong Province.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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