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Abstract
We report on the cascade continuous-wave operation of a diode-pumped Tm:YVO4 laser on the 3F4 → 3H6 (at ∼2 μm) and 3H4 → 3H5 (at ∼2.3 μm) Tm3+ transitions. Pumped with a fiber-coupled spatially multimode 794 nm AlGaAs laser diode, the 1.5 at.% Tm:YVO4 laser yielded a maximum total output power of 6.09 W with a slope efficiency of 35.7% out of which the 3H4 → 3H5 laser emission corresponded to 1.15 W at 2291-2295 and 2362-2371 nm with a slope efficiency of 7.9% and a laser threshold of 6.25 W.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The development of application-oriented mid-infrared solid-state lasers is an important and hot research topic for the laser community. A simple and cost-efficient method to obtain laser emission in the short-wave infrared spectral range around 2.3 μm is to employ thulium lasers operating on the 3H4 → 3H5 electronic transition [1], see Fig. 1 (a). In 1994, Pinto et al. reported on a Tm:LiYF4 laser continuously tunable between 2.20 μm and 2.46 μm [2]. Thulium ions (Tm3+) can be efficiently excited at 0.79 μm directly to the upper laser level (3H4), e.g., by commercially available high-power fiber-coupled AlGaAs laser diodes. Efficient and high-power Tm lasers operating on another transition, 3F4 → 3H6 (at ∼2 μm), have been realized by employing a variety of oxide and fluoride gain materials [3,4]. However, the power-scalable (multi-watt) continuous-wave (CW) operation of diode-pumped Tm lasers on the 3H4 → 3H5 transition still remains a challenge. So far, the majority of results on ∼2.3 μm Tm lasers, see, for example [5–7], were obtained under pumping by Ti:Sapphire lasers suffering from their complexity, high cost, and limited power scaling. Still, this approach allowed to prove the possibility of achieving high slope efficiencies from ∼2.3 μm Tm lasers exceeding the Stokes limit: Guillemot presented a Tm:KY3F10 laser generating 0.84 W at 2.34 μm by pumping at 773 nm, with a high slope efficiency of 47.7% (vs. the incident pump power) [5] owing to efficient energy-transfer upconversion (ETU, Fig. 1 (a)) leading to a pump quantum efficiency approaching 2 [8].
Fig. 1. (a) Partial energy level scheme of Tm3+ in YVO4: red and green arrows, pump and laser transitions, respectively; black arrows, non-radiative (NR) relaxation; blue arrows, cross-relaxation (CR) and energy-transfer upconversion (ETU) processes; τ3H4, τ3H5 and τ3F4, luminescence lifetimes of the 3H4, 3H5, and 3F4 states for 5 at.% Tm:YVO4, respectively [16]; (b) crystal-field splitting of the 3H4 and 3H5 multiplets, Гi (i = 1-5), irreducible representations for the Stark sub-levels, D2d site symmetry (after [15,21]), green arrows, observed laser lines.
To date, diode-pumped CW laser operation of Tm lasers at ∼2.3 µm has been realized only in a few Tm3+-doped laser crystals, such as Tm:Y3Al5O12 (Tm:YAG) [9], Tm:YAlO3 (Tm:YAP) [9,10], and Tm:LiYF4 (Tm:YLF) [9,11–14]. Kifle et al. presented a quasi-CW diode-pumped Tm:YAP laser delivering 2.69 W at 2.27 μm by pumping at 789 nm with a slope efficiency of 33.1% (vs. the absorbed pump power) and a laser threshold of 4.53 W [9].
Among the laser host materials for Tm3+ doping with the goal of developing diode-pumped lasers, yttrium orthovanadate (YVO4) is attracting attention [15–17]. This crystal is tetragonal (sp. gr. I41/amd) and optically uniaxial (positive). YVO4 can be grown by the conventional Czochralski method. As a host matrix, it offers high thermal conductivity (8.9 Wm-1K-1 and 12.1 Wm-1K-1 along the a and c axes, respectively) [18], weak anisotropy of thermal expansion, and positive thermo-optic coefficients [19] resulting in the positive thermal lens which is essential for microchip laser operation [20]. Tm3+ ions in the YVO4 crystal feature a relatively broad 3H6 → 3H4 absorption band which is advantageous for diode-pumping due to a weak sensitivity to the possible temperature drift of the diode wavelength, as well as polarized emission at ∼2 μm [16].
Dual-waveband lasers simultaneously emitting at ∼2 μm and ∼2.3 μm based on a single gain medium appear promising for several applications fields. (i) The ∼2 µm emission spectrally matches a strong absorption band of water and is relatively eye-safe, being relevant for clinical medical treatment [22–24]. The ∼2.3 µm emission is weakly absorbed by water while overlaps with a characteristic absorption band of glucose (C6H12O6), being very suitable for non-invasive glucose blood measurement [25,26]. In some surgeries such as cardiovascular surgery [27], insulinoma resection [28], cardiac surgery [29,30], and liver transplantation surgery [31], real-time continuous non-invasive glucose blood monitoring is essential. Especially for diabetic patients, such monitoring can greatly reduce the risk. (ii) The ∼2 μm and ∼2.3 μm emissions are located in the atmospheric transparency windows and are suitable for free-space telecommunications, laser imaging radar, and range-finding. The simultaneous dual-waveband operation of a laser could facilitate the expansion of information propagation capability by increasing the number of channels and could enhance the measurement accuracy as a result of double detection [32]. (iii) The ∼2 µm emission overlaps with strong absorption bands of water vapor and CO2 and a weak absorption band of CO, while the ∼2.3 μm one matches with the weak absorption bands of water vapor and CO2 [33] and strong absorption bands of CO, N2O, CH4, C2H2, and N2O. The detection of multiple gas components can be realized simultaneously by a single simultaneous dual-waveband laser operating at ∼2 μm and ∼2.3 μm. Therefore, the availability of dual-waveband ∼2 μm and ∼2.3 μm sources can provide greater flexibility, convenience, and selectivity for the above-mentioned applications. Cascade lasing on two transitions can also decrease the overall quantum defect [34].
In the present work, we aimed to demonstrate the first dual-waveband laser operation at ∼2 μm and ∼2.3 μm of a diode-pumped Tm:YVO4 laser based on cascade lasing on the 3F4 → 3H6 and 3H4 → 3H5 Tm3+ transitions. Multiple laser lines were observed around 1.95 μm, 2.29 μm, and 2.36 μm, overlapping with strong absorption bands of gas molecules H2O and CO2 (∼1.95 µm), N2O (∼2.28 µm), CO (∼2.36 µm), and CH4 (∼2.37 µm), indicating a great potential of such lasers for atmospheric environmental monitoring.
2. Laser set-up
As a gain material, we have employed Tm:YVO4. Two doping levels of 1.5 at.% and 3.0 at.% Tm3+ were tested. The crystals were grown by the Czochralski method. Rectangular laser elements were oriented for light propagation along the a crystallographic axis (a-cut) having a thickness of 10 mm and an aperture of 3.0 × 3.0 mm2. Both their end faces were polished to laser-grade quality and antireflection (AR) coated for 0.79 ± 0.01 μm (reflectance: R < 0.5%) and 1.85-2.36 μm (R < 1%). Figure 2 (a) shows the schematic of the laser set-up. A simple compact plano-plano laser cavity was employed with a geometrical length of 13 mm. It was formed by a flat input mirror (M1) coated for high transmission at 0.79 μm and high reflection at 1.8-2.4 µm and two different flat output mirrors supporting laser operation on two Tm3+ laser transitions of interest (M2): one providing a transmission TOC of 2% at 1.8-2.1 µm and 1% at 2.1-2.4 µm (labelled: OC1) and another one-TOC of 5% at 1.8-2.1 µm and 2% at 2.1-2.4 µm (labelled: OC2). The Tm:YVO4 crystal was wrapped with indium foil and mounted in a water-cooled copper block with a temperature set at 12 °C. The crystal was placed close to the input mirror. The pump source was a ∼794 nm fiber-coupled AlGaAs laser diode (fiber core diameter: 200 µm, numerical aperture (N.A.): 0.22). The pump beam was collimated and focused into the laser crystal through the input mirror using a 1:1 collimation system (focal length: f = 50 mm) resulting in a waist diameter of 200 µm. The residual pump was filtered out using a long-pass filter (F1) with a cutoff wavelength of 1 µm. Another long-pass filter (F2) with a transmission of ∼90% at ∼2.3 µm and almost zero transmission at ∼2 μm was used to separate the ∼2.3 µm power contribution. The laser emission spectra were measured using an optical spectrum analyzer (APE GmbH, Germany).
Fig. 2. (a) Scheme of the diode-pumped Tm:YVO4 laser: LD-laser diode, M1-input mirror, M2-output coupler, F1 and F2-long-pass (LP) filters; (b) pump absorption under non-lasing conditions as a function of the incident pump power.
Even though the high reflections were provided by both studied output couplers at 794 nm, the back-reflected divergent pump makes little contribution to the gain. Thus, the pumping was in single pass. The pump absorption efficiency under non-lasing conditions ηabs,NL for both Tm:YVO4 crystals was initially determined in pump-transmission measurements [35]. The ηabs,NL as a function of the incident pump power is plotted in Fig. 2 (b). It was weakly dependent on the pump level indicating almost negligible bleaching of the ground state and amounted to 71% (1.5 at.% Tm) and 94% (3.0 at.% Tm). Subsequently, the pump absorption efficiency under lasing conditions was estimated from the ηabs,NL value at the threshold pump power.
3. Results and discussion
First, we studied the laser performance of the 1.5 at.% Tm:YVO4 crystal using both output couplers (OC1 and OC2), see Fig. 3 (a, b). Here, we use the following notations: PΣ-the total output power, PΣ = P2μm + P2.3μm, where P2μm and P2.3μm are the output powers for the 3F4 → 3H6 and 3H4 → 3H5 Tm3+ transitions, respectively, and ηΣ (η2μm and η2.3μm) are the corresponding laser slope efficiencies vs. the pump power absorbed in the laser crystal. and Pth,2μm and Pth,2.3μm denote the laser thresholds for both transitions. We also define the power fraction of the laser emission on the 3H4 → 3H5 Tm3+ transition, X = P2.3μm/PΣ.
Fig. 3. Diode-pumped cascade Tm:YVO4 lasers simultaneously operating on the 3F4 → 3H6 and 3H4 → 3H5 transitions: (a-c) input-output dependences, η-slope efficiency: (a, b) the 1.5 at.% Tm:YVO4 crystal, output couplers: (a) OC1 and (b) OC2; (c) the 3.0 at.% Tm:YVO4 crystal with both OC1 and OC2; (d) power fraction X of the 3H4 → 3H5 emission.
The laser operation in the plano-plano cavity was ensured by the positive (focusing) thermal lens of the Tm:YVO4 crystal arising from a positive thermo-optic coefficient dn/dT of this material [19].
The output characteristics of the Tm:YVO4 laser are summarized in Table 1.
Table 1. Output Performancea of CW Cascade Diode-Pumped Tm:YVO4 Lasers
Higher total output power, as well as that for the 3H4 → 3H5 Tm3+ transition, were achieved when using higher output coupling (OC2, TOC = 5%/2%), see Fig. 3 (b). For the maximum absorbed pump power of 22.24 W, the 1.5 at.% Tm:YVO4 laser generated a maximum total output power PΣ of 6.09 W with a slope efficiency ηΣ of 35.7% and a laser threshold Pth,2μm of 4.81 W. The optical efficiency reached 27.4%. The emission on the 3F4 → 3H6 transition corresponded to higher output power (P2μm = 4.94 W), higher slope efficiency (η2μm = 28.1%), and lower laser threshold, i.e., first, only the emission at ∼2 μm appeared and for pump powers exceeding Pth,2.3μm of 6.25 W, both 3F4 → 3H6 and 3H4 → 3H5 laser emissions were observed, and the two laser emissions were time-domain synchronized. For Tm lasers operating on the 3H4 → 3H5 transition with a direct pumping to the upper laser level (3H4), the characteristic laser establishment time (i.e., the time interval between switching on the pump and the first laser oscillations) is faster than 1 ms [36]. The highest P2.3μm reached 1.15 W with a slope efficiency η2.3μm of 7.9%. The input-output dependences for both the total output power and the power contributions on both laser transitions were linear within the studied range of pump powers, further power scaling was limited by the available pump power.
A similar behavior was observed for smaller output coupling (OC1, TOC = 2%/1%), as shown in Fig. 3 (a). For the maximum absorbed pump power of 17.63 W, the maximum total output power PΣ was 4.47 W with a slope efficiency ηΣ of 32.0% and a laser threshold Pth,2μm of 3.72 W. The optical efficiency was 25.4%. For the 3H4 → 3H5 transition, the laser generated an output power P2.3μm of 1.10 W with a higher slope efficiency η2.3μm of 9.0% and a lower laser threshold Pth,2.3μm of 5.08 W (as compared to OC2). More details can be found in Table 1.
For the 3.0 at.% Tm:YVO4 crystal, the laser performance deteriorated, see Fig. 3 (c). Cascade lasing was achieved only for small output coupling (OC1). For the maximum absorbed pump power of 13.08 W, the maximum total output power PΣ of the cascade laser was 2.16 W with a slope efficiency ηΣ of 27.1% and a laser threshold Pth,2μm of 4.96 W. The optical efficiency was 16.5%. For the 3H4 → 3H5 transition, the corresponding output power P2.3μm reached 0.56 W with a slope efficiency η2.3μm of 7.1% and a laser threshold Pth,2.3μm of 4.96 W. For a Tm3+ dopant concentration greater than 2 at.%, the 3H4 manifold fluorescence lifetime will be significantly quenched by the CR process, which is favorable for the 3F4 → 3H6 Tm3+ transition but unfavorable for the 3H4 → 3H5 one. Therefore, the deteriorated laser performance of the heavily doped 3.0 at.% Tm:YVO4 crystal is probably due to the stronger competition between the high-gain 3F4 → 3H6 transition and the low-gain 3H4 → 3H5 one. For higher output coupling (OC2), the laser operated solely on the 3F4 → 3H6 transition, as we were unable to reach the second laser threshold for the 3H4 → 3H5 transition.
The power fraction of the 3H4 → 3H5 laser emission as a function of absorbed pump power is plotted in Fig. 3 (d) for the two studied output couplers. It rapidly increases above the laser threshold Pth,2.3μm and saturates for pump levels well exceeding the threshold pump power, reaching X about 24% (OC1) and 20% (OC2).
The typical spectra of cascade laser emission achieved with two output couplers and the 1.5 at.% Tm:YVO4 crystal are shown in Fig. 4 (a, b). Here, the spectral intensities for the two observed laser transitions were normalized for better visibility, while the corresponding output powers can be seen in Fig. 3 (a, b). The spectra were measured at different absorbed pump powers.
Fig. 4. The spectra of output emission from the diode-pumped cascade 1.5 at.% Tm:YVO4 laser captured at different absorbed pump powers Pabs: (a) OC1 and (b) OC2.
Table 2 summarizes the observed laser wavelengths (as the main focus of the present work is the 3H4 → 3H5 Tm3+ transition, exact values for each laser line are given while for the 3F4 → 3H6 transition, only the spectral range is indicated).
Table 2. Laser Wavelengths of the Cascade Diode-Pumped Tm:YVO4 Laser at Different Pump Powers
For the 3F4 → 3H6 Tm3+ transition, with increasing the output couplers (from 2% for OC1 to 5% for OC2), the laser wavelength experienced a noticeable blue-shift, from 1944-1974nm to 1922-1929nm at the absorbed pump power of 16.6 W. This behavior is typical for quasi-three-level laser transitions with inherent reabsorption at the emission wavelength and is related to decreasing reabsorption losses at higher inversion in the gain medium associated with the higher transmission of the output coupler [16]. It also agrees with the gain spectra of Tm3+ in the YVO4 crystal [15]. At the threshold pump power, the laser operated on a single line (1955nm/1918nm for OC1/OC2, respectively). With increasing the pump power, multiple longitudinal laser modes were generated. They are due to the etalon effect at the air gap between the input mirror M1 and the laser crystal, as well as the relatively broad gain spectra of Tm3+. This was verified by varying the pump power, which, due to the positive thermal expansion of the crystal in the longitudinal direction, affected the laser spectra.
For the quasi-four-level 3H4 → 3H5 Tm3+ transition, as expected, the spectral position of the laser lines was almost independent of the output couplers. For OC1 and OC2, at the threshold pump powers, the laser operated at single lines of 2363 nm and 2364 nm, respectively. With increasing the pump power, multiple laser lines appeared within two spectral ranges, at ∼2.29 μm and ∼2.36 μm, cf. Table 2. The mechanism of appearance of these lines is similar to that for the 3F4 → 3H6 Tm3+ transition.
Cascade laser emission from rare-earth ions provides a dual-waveband output with a large wavelength separation, which can help to simultaneously address different applications (see the Introduction section). In our case, well above the laser thresholds for both Tm3+ laser transitions, the ratio between the corresponding power components P2.3μm/P2μm was almost constant (a non-competitive laser operation).
Simultaneous laser operation of Tm3+ on the 3H4 → 3H5 and 3F4 → 3H6 transitions is a bit different from cascade lasing of Er3+ [37] or Ho3+ [38] where an intermediate metastable level |1 > plays two roles: a terminal laser level for a transition from a higher-lying excited-state |2> → |1 > and an emitting level for a transition to the ground-state |1> → |0 > . By allowing the simultaneous operation on both transitions, the metastable level |1 > can be fast depopulated avoiding the bottleneck effect. In our case, the efficient multiphonon non-radiative relaxation from the 3H5 Tm3+ state (the terminal laser level for the 3H4 → 3H5 transition) naturally helps to avoid the bottleneck effect. However, the long lifetime of the 3F4 Tm3+ state may play a significant role, especially at high pump intensities by accumulating the electronic excitations in this manifold and causing a non-negligible ground-state bleaching thus reducing the pump absorption efficiency. Cascade lasing can remove this limitation.
Huang et al. reported on cascade lasing from a diode-pumped Tm:LiYF4 laser delivering a maximum total output power of 5.49 W of which 1.12 W was generated at 2305 nm with a slope efficiency of ∼8.6% [34]. Obviously, Tm:YVO4 allows to access longer wavelengths of the mid-infrared emission.
4. Conclusions
To conclude, Tm:YVO4 is a promising crystal for efficient, multi-watt CW diode-pumped lasers simultaneously emitting at ∼2 μm and ∼2.3 μm due to a cascade laser scheme, 3H4 → 3H5 and 3F4 → 3H6. We report on a non-competitive (in terms of the relative power fractions) cascade laser operation of a diode-pumped 1.5 at.% Tm:YVO4 laser delivering a total output power of >6 W at ∼1.95 μm, ∼2.29 μm, and ∼2.36 μm with a high slope efficiency of 35.7% out of which the mid-infrared laser emission owing to the 3H4 → 3H5 transition corresponded to 1.15 W with a slope efficiency of 7.9% and a laser threshold of 6.25 W. We also analyzed the effect of the output coupling and Tm3+ doping level on the dual-waveband laser operation. Simultaneous dual-waveband laser sources emitting at ∼2 μm and ∼2.3 μm based on a single laser medium are attractive for applications in cardiovascular surgery, free-space telecommunications, and gas composition detection.
Funding
National Natural Science Foundation of China (52072351, 12004213, 12174223, 12274263, 21872084, 62175128); “RELANCE” Chair of Excellence project funded by the Normandy Region; Qilu Young Scholar Program of Shandong University.
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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