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Abstract
Efficient diode-pumped continuous-wave (CW) and wavelength tunable Tm:YAP lasers based on the vibronic and electronic transitions are investigated. A total maximum output power of 4.1 W is achieved with multi-wavelength output around 2162 nm and 2274 nm, corresponding to a slope efficiency of 29.8% for a 3 at. % Tm:YAP crystal. A maximum output power of 2.48 W with a slope efficiency of 25.4% is obtained at 2146 nm for a 4 at. % Tm:YAP crystal. Using a birefringent filter (BF), the emission wavelengths of the Tm:YAP laser are tuned over spectral ranges of 59 nm from 2115 nm to 2174 nm and 127 nm from 2267 nm to 2394 nm, respectively, which is the first demonstration of wavelength tunable Tm:YAP laser based on the electronic transition 3H4→3H5 and vibronic transition 3F4→3H6, to the best of our knowledge. The results show great potentials of the Tm:YAP crystal for realizing efficient lasers in the spectral range of 2.1-2.4 µm.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Lasers emitting at ∼2.3 µm have attracted considerable attention due to the practical and potential applications in the fields of gas detection, non-invasive glucose measurements, laser LIDAR and coherent high-harmonic generations [1–4]. Particularly, the Tm3+ ions doped crystal is regarded as one kind of the most promising material to achieve laser emission at ∼2.3 µm based on the 3H4→3H5 transition, due to the direct pumping by commercial laser diodes (LDs) around ∼800 nm [5,6]. However, the cross-relaxation (CR) process related to 3H4 + 3H6 →3F4 + 3F4 between adjacent Tm3+ ions would prevent the population of 3H4 manifold [7]. Although certain crystals with low phonon energies can partially compensate the CR process and improve the laser efficiency by energy-transfer up-conversion (ETU, 3F4 + 3F4→3H4 + 3H6), the power scaling at ∼2.3 µm is still limited [8]. Moreover, the multiphoton non-radiative (NR) relaxation and severe thermal effect would also hinder the improvement of the ∼2.3 µm laser efficiency.
To date, various ∼2.3 µm bulk lasers have been investigated by using different Tm3+ doped oxide, fluoride and vanadate crystals, including Tm:YAlO3 (YAP), Tm:Y3Al5O12 (YAG), Tm:KLu(WO4)2, Tm:YLiF4 (YLF), Tm:KY3F10 (KYF), Tm:BaY2F8 (BYF), Tm:YVO4 and Tm:GdVO4, and etc. Laser operations in CW, Q-switching, and mode-locking regimes have also been widely investigated under ∼0.8 µm or ∼1 µm up-conversion (UC) pumping schemes [9–19]. Among the crystals mentioned above, Tm:YAP crystal exhibits excellent physical and chemical properties, such as high thermal conductivity (∼11 Wm−1K−1), weak anisotropy of thermal expansion, low phonon energy (552 cm-1) and large emission cross-section (0.86 × 10−20 cm2 at 2278 nm), which is considered as promising gain medium suitable for highly intense laser pumping and efficient ∼2.3 µm lasing [20]. Very recently, Yu et al. have reported a CW Tm:YAP laser with a highest output power of 2.97 W at 2274 nm, which is based on the electronic transition 3H4→3H5, and no other emission wavelength is observed [21]. As is well known, the electronic transition happens among the energy levels of an electron in an atom or molecule. However, in some laser gain media, there is strong electron-phonon coupling induced vibronic transition due to the strong interaction between the electrons in the electronic transition and inherent lattice vibrations (phonons) in the crystalline host, leading to the laser transitions where not only a photon but one or several phonons are emitted. Thus, strong homogeneous broadening of the electronic transition and large gain bandwidth are achieved. For Tm3+ ions, the vibronic transition related to 3F4→3H6 can extend the emission spectra into a wavelength region of 2.1-2.2 µm [22]. Based on the electronic transition 3H4→3H5 and the vibronic transition related to 3F4→3H6, a highest output power of 1.03 W is obtained from a Tm:YAP laser with multi-wavelengths of ∼2.14 µm and ∼2.27 µm [23], however, no report is found on a higher output power or single wavelength operation at ∼2.14 µm from the Tm:YAP laser.
In this work, highly efficient diode-pumped Tm:YAP lasers are demonstrated based on the vibronic and electronic transitions. When employing a 3 at. % doped Tm:YAP crystal, a maximum output power of 4.1 W with multi-wavelength at 2162 nm and 2274 nm is achieved, corresponding to a slope efficiency of 29.8%. For the case of using a 4 at. % Tm:YAP crystal, a maximum output power of 2.48 W is obtained at 2146 nm, which is the highest output power ever achieved from the Tm:YAP laser related to the emission in 2.1-2.2 µm wavelength region. Moreover, broadly wavelength tunable Tm:YAP lasers are realized with a birefringent filter (BF), which deliver spectral regions of 2115-2174 nm and 2267-2394 nm, respectively.
2. Experimental setup and results
Three Tm:YAP crystals with different Tm3+ doping concentrations, dimensions and cutting orientations are employed as gain media in the experiment. The specific parameters of these Tm:YAP crystals are listed in Table 1. All the crystals are wrapped with indium foil and placed in copper block with water cooled to temperature of 15°C. A fiber-coupled 796 nm wavelength-locked LD (DILAS, Compact 30/200) with a fiber core diameter of 200 µm is used as the pump source. The emission wavelength shows nearly no variation with the output power increased to the maximum 30 W. The pump laser is focused into the Tm:YAP crystal by a 1:2 optical coupling system. The spot radii of the pump beam and the laser beam inside the crystals are both around 200 µm, which match well with each other.
Table 1. Parameters a of the employed Tm:YAP laser crystals.
2.1 CW Tm:YAP laser
Figure 1 shows the experimental setup of the diode-pumped CW Tm:YAP laser. The input mirror of the laser oscillator is a concave mirror (IM) with a curvature radius of 400 mm, high-reflection (HR) coated at 2100-2500 nm and anti-reflection (AR) coated at 770-1050 nm. The output couplers (OCs) are flat mirrors with different transmission of 1% and 2% in the spectral range from 2100-2500 nm. Both OC mirrors are high-transmission (HT) coated at around 1900 nm to suppress the strong laser oscillation at around 1900 nm. The total physical length of the laser cavity is about 13 mm. A laser power meter (PM100D, Thorlabs) is employed to measure the laser output power. An optical spectrum analyzer (OSA207C Fourier transform spectrum analyzer, Thorlabs Inc.) is used to measure the laser emission spectrum.
The pump light absorbance of the Tm:YAP crystal is measured under non-lasing conditions and shown in Fig. 2(a). With the increase of the pump powers, the absorbances decrease from 87.1% to 59.0%, 89.9% to 74.1% and 94.9% to 92.5% for the 2 at. %, 3 at. % and 4 at. % Tm:YAP crystals, respectively. This is attributed to the bleaching effect that occurs in the ground state energy level, where the Tm3+ ion is depopulated in the ground state while excited to the intermediate 3F4 level. The absorption coefficient can be generally expressed as a function of the pump intensity [24]:
where α0 denotes the small-signal absorption coefficient, Ip is the pump intensity and Isat is the saturation intensity. From formula (1), the absorption coefficient decreases with the increase of pump intensity, and the higher small-signal absorption coefficient is, the slower the absorbance decreases, which is consistent with the trend observed in Fig. 2(a).
Fig. 2. (a) Absorbances to the pump light of Tm:YAP crystals with different doping concentrations under non-lasing conditions, (b) Output powers of the 2 at. % Tm:YAP laser, Output spectra of the Tm:YAP lasers at different absorbed pump powers for (c) TOC = 1%, (d) TOC = 2%.
As shown in Fig. 2(b), the output powers of the 2 at. % Tm:YAP CW lasers are investigated with different OCs. When the OC of T = 1% is employed, the threshold absorbed pump power is measured to be 4.07 W. At low absorbed pump powers, only single wavelength oscillation at 2274 nm is observed, as shown in Fig. 2(c). With the increase of the pump power, multi-wavelength oscillations around 2152 nm and 2274 nm are observed, and a maximum output power of 3 W with a corresponding slope efficiency of 29.1% is achieved at the absorbed pump power of 16.82 W. For the case of T = 2%, the threshold absorbed pump power increases to 7.7 W and a maximum output power of 1.74 W with a slope efficiency of 23.8% is delivered at the absorbed pump power of 16.82 W. Multi-wavelength operations at around 2274 nm and 2383 nm are obtained, as shown in Fig. 2(d). The output light polarization direction is E∥c for the used a-cut Tm:YAP crystal.
For the Tm3+ ions doped in YAP crystal, the longest wavelength related to the electronic transition 3F4→3H6 is 1999 nm, however, the happening of vibronic transition would further red-shift the emission wavelength longer than 2000 nm as shown in Fig. 3(a), which gives the observed laser wavelength and the corresponding electronic or vibronic transitions. For the electronic transition 3H4→3H5 in Tm3+ ions, the shortest wavelength related to possible transition among the Stark level manifolds is about 2135 nm, corresponding to a very weak emission spectral peak near 2.14 µm as shown in the stimulated-emission cross-section spectra of Tm:YAP crystal in Ref. [23]. Thus, the laser wavelengths over 2135 nm should be generated by the electronic transition 3H4→3H5, however, the corresponding emission cross-sections in the spectral region from 2100 nm to 2200 nm are too small to support efficient laser emissions. So the efficient laser emissions from 2.1 µm to 2.2 µm for Tm:YAP crystal could be possibly obtained by the vibronic transitions 3F4→3H6 with highly Tm3+-doped crystals and appropriate coatings of OCs [22,25]. In this work, since the OCs are partially reflectivity (PR) coated in a broad spectral region of 2100-2500 nm, the possible laser emissions related to the vibronic transitions can be well supported. Since the emission cross-sections in the spectral region over 2200 nm can support efficient laser emissions, the achieved emission wavelengths of 2274 nm and 2383 nm are undoubtedly related to the electronic transition 3H4→3H5.
Fig. 3. (a) Energy level scheme of the Tm3+ ions in YAP crystal, solid red arrows for the obtained emission wavelengths in this work, dash blue arrows for the longest and shortest emission wavelengths related to the electronic3F4→3H6 and 3H4→3H5 transitions, respectively [26,27], (b) Output powers of the CW 3 at. % Tm:YAP laser, Output spectra of the Tm:YAP lasers at different pump powers for (c) TOC = 1%, (d) TOC = 2%.
As for 3 at. % Tm:YAP crystal, the threshold absorbed pump powers are 7.91 W and 12.41 W for OCs of T = 1% and 2%, respectively. The output light polarization direction is E∥c for the used b-cut Tm:YAP crystal. Under a maximum absorbed pump power of 22.33 W, the laser delivers a maximum output power of 4.1 W with OC of T = 1%, corresponding to a slope efficiency of 29.8% (see Fig. 3(b)). Compared with output power characteristics of 2 at. % Tm:YAP crystal, higher output powers are achieved by using the 3 at. % Tm:YAP crystal, since higher Tm3+ ion concentration enhances the pump light absorption and electron-phonon coupling effect. Furthermore, with the increase of the Tm3+ ions doping concentration, both the CR and ETU processes are strengthened. The CR process would depopulate the 3H4 manifold, while the ETU process has an opposite effect. ETU process would contribute to the improvement of the laser slope efficiency, which is increased from 29.1% of 2 at. % Tm:YAP crystal to 29.8% of 3 at. % Tm:YAP crystal. However, it is still inferior to the stokes limit (∼35%) possibly due to the more pronounced CR process. Figure 3(c) and (d) demonstrate the output spectra of the CW 3 at. % Tm:YAP lasers for different absorbed pump powers and output couplings. Multi-wavelength operations at 2162 nm and 2274 nm are obtained for T = 1% OC. According to the analyzed transitions in Tm3+ ions, the 2162 nm laser emission is attributed to the dominant vibronic transition 3F4→3H6 and the electronic transition 3H4→3H5. The 2274 nm emission is obviously related to the electronic transition 3H4→3H5. Single output wavelength at 2274 nm is obtained with OC of T = 2% for the 3 at. % Tm:YAP crystal.
When employing a 4 at. % Tm:YAP crystal, the laser can only oscillate with OC of T = 1%. As shown in Fig. 4(a), the threshold absorbed pump power is 6.58 W and a maximum output power of 2.48 W is obtained at the absorbed pump power of 16.69 W. The output light polarization direction is E∥b for the used c-cut Tm:YAP crystal. To avoid the damage to the laser crystal, the pump powers are not further increased. As shown in Fig. 4(b), only single wavelength of 2146 nm is observed for the 4 at. % Tm:YAP crystal, while there is no emission wavelength at 2274 nm which has been obtained for the other doping concentration Tm:YAP crystals. It can be attributed to that at high concentration of Tm3+ ions doping, the CR process results in a rapid depopulation of the 3H4 level, thus suppressing the laser emission at 2274 nm mainly related to the electronic transition 3H4→3H5. Normally, the related laser emission at 2146 nm would also be suppressed, however, the vibronic transition 3F4→3H6 under high Tm3+ ion doping concentration makes the 2146 nm laser emission efficient. This further confirms that the efficient Tm-doped laser emission in the spectral region from 2.1 µm to 2.2 µm, such as the obtained wavelengths of 2146 nm, 2152 nm, and 2162 nm should be mainly attributed to the vibronic transition 3F4→3H6. In addition, a highly Tm3+-doped crystal with a small transmission rate OC can introduce a low inversion ratio, which can further flatten and broaden the gain spectra towards longer wavelengths related to the vibronic transition 3F4→3H6 [10], as shown in the gain spectrum of the Tm:YAP crystal [28]. The similar situation can also be found for Tm:BYF crystal as shown in Ref. [25], in which the longest laser wavelength related to the electronic transition 3F4→3H6 is 1956 nm, while the lasers emitting at 1932 nm, 1930-2001 nm and 1990-2027 nm are obtained for 2 at. %, 8 at. % and 18 at. % Tm3+ doping concentrations, respectively. It well indicates that the high doping concentration promotes the electron-phonon coupling.
Table 2 summarizes the output laser characteristics of the three different concentration dopant Tm:YAP crystals in this work.
Table 2. Output characteristics b of LD-pumped Tm:YAP lasers.
To compare the output CW laser characteristics of the Tm3+ doped crystals at around 2.3 µm, Table 3 summarizes the results ever obtained in previous works, from which it can be seen that the laser operation at ∼2.3 µm from Tm3+ doped crystals are mainly based on the electronic transition 3H4→3H5. In combination with the vibronic transition 3F4→3H6, multi-wavelength operations can be achieved and extend the achievable spectra of Tm3+ doped bulk lasers. Under the combination of vibronic and electronic transitions, the highest output powers achieved from Tm:KLu(WO4)2, Tm:YAG, and Tm:YAP crystals are mainly limited to be around 1 W, however, in this work, a total maximum output power of 4.1 W at around 2162 nm and 2274 nm as well as that of 2.48 W at 2146 nm are achieved from the 3 at. % and 4 at. % Tm:YAP crystals, respectively.
Table 3. Output characteristics c of Tm3+-doped bulk lasers in the spectral region of 2.1-2.4 µm.
2.2 Wavelength tunable Tm:YAP laser
Due to the strong electron-phonon coupling effect, the Tm:YAP crystals are expected to achieve laser emissions in a broad spectral range. A 2 at. % doped Tm:YAP crystal is chosen to study the wavelength tunable characteristics due to its ability of realizing multiple wavelength operations. As shown in Fig. 5, the wavelength tunable Tm:YAP laser is realized with a V-type cavity. The folded cavity is comprised of two concave mirrors IM and M1 with radii of 400 mm and 200 mm, respectively, and a plane OC of T = 1%. In the laser cavity, a 2 mm-thick BF plate is placed near the OC with a Brewster angle to tune the laser wavelength.
Using a BF, the Tm:YAP lasers are wavelength tuned over a spectral range of 59 nm (2115 to 2174 nm) and 127 nm (2267 to 2394 nm), respectively, as shown in Fig. 6(a). However, the laser emission in the spectral range of 2175-2266 nm is not observed, which may be due to the small gain. A maximum output power of 1.8 W is obtained at 2142 nm under the absorbed pump power of 16.82 W, as shown in Fig. 6(b). Multi-wavelength operations at 2115 nm and 2389 nm, triple-wavelength operation at 2124 nm, 2125 nm and 2394 nm are also realized, as shown in Fig. 6(c) and (d). Table 4 shows the comparisons of wavelength tuning performance for different Tm3+ doped bulk lasers at ∼2.3 µm. The tunable Tm:YAP laser exhibits excellent wavelength tunability in a total wavelength range of 186 nm in two different spectral regions over 2000 nm. The tunable wavelength range of the Tm:YAP crystal is somehow narrower than that of 260 nm obtained by the Tm:YLF crystal, however, the output spectra are only located in the spectral range from 2200 nm to 2460 nm, and no laser output is realized from 2100 nm to 2200 nm.
Fig. 6. (a) Wavelength tuning curves of the 2 at. % Tm:YAP laser. (b) The relationship between output powers and absorbed pump powers at 2142 nm. (c) Dual-wavelengths and (d) triple-wavelengths output spectra from the 2 at. % Tm:YAP laser.
Table 4. Wavelength tunable abilities of the Tm3+-doped crystals.
3. Conclusion
In this work, highly efficient diode-pumped CW and wavelength tunable Tm:YAP lasers based on the vibronic and electronic transitions are realized and demonstrated. A total maximum output power of 4.1 W at around 2162 nm and 2274 nm with slope efficiency of 29.8% is obtained from a 3 at. % Tm:YAP laser. A maximum CW output power of 2.48 W at 2146 nm is achieved with the 4 at. % Tm:YAP crystal. Wavelength tunable Tm:YAP laser is realized by using a BF plate, which delivers a total wavelength tunable range of 186 nm consisting of two spectral regions of 2115 nm-2174 nm and 2267 nm-2394 nm. A maximum average output power of 1.8 W is obtained at 2142 nm. To the best of our knowledge, this is the first wavelength tunable Tm:YAP laser based on the electronic transition 3H4→3H5 and the vibronic transition 3F4→3H6, which demonstrates that the Tm:YAP crystal has excellent potential for realizing efficient laser emissions in a ultrabroad spectral range of 2.1-2.4 µm.
Funding
National Natural Science Foundation of China (62175130, 62375154, 62275144, 62322509); Natural Science Foundation of Shandong Province (ZR2022LLZ007); Distinguished Young Scholars from 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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