• Chinese Optics Letters
  • Vol. 21, Issue 3, 031401 (2023)
Jiaqun Zhao1、*, Yuantong Liu1, Ping Cheng2, and Rui Yu1
Author Affiliations
  • 1College of Science, Hohai University, Nanjing 211100, China
  • 2College of Computer and Information, Hohai University, Nanjing 211100, China
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    DOI: 10.3788/COL202321.031401 Cite this Article Set citation alerts
    Jiaqun Zhao, Yuantong Liu, Ping Cheng, Rui Yu. Continuous-wave three-wavelength operation of a diode-pumped Tm:YVO4 laser on the 3H43H5 and 3F43H6 transitions[J]. Chinese Optics Letters, 2023, 21(3): 031401 Copy Citation Text show less

    Abstract

    A diode-pumped continuous-wave Tm:YVO4 laser operating on the H34H35 and F43H36 transitions was demonstrated for the first time, to the best of our knowledge. An a-cut Tm:YVO4 crystal with 1.5% (atomic fraction) Tm3+ ion concentration was used to characterize the laser behavior. A common commercial laser diode with a central wavelength of 790 nm and a bandwidth of 3.2 nm was utilized as a pump source. With an output coupler for the H34H35 and F34H36 transitions, simultaneous three-wavelength laser operation was achieved. The laser emissions at 2292 and 2363 nm in π-polarization and at 2108 nm in σ-polarization were realized. With an incident pump power of 22 W, the total output power of 1.17 W at 2292, 2363, and 2108 nm was obtained. The output power at 2292 and 2363 nm was measured to be 750 mW, and the output power at 2108 nm was measured to be 420 mW.

    1. Introduction

    Mid-infrared laser sources emitting in the 2–2.5 µm special region have attracted a great deal of interest in recent years. Such emission falls in the atmospheric transparency window. Some important gas molecules such as N2O (2.28 µm), CO (2.3 µm), CH4 (2.35 µm), NH3 (2.1 µm), and HF (2.5 µm), exhibit strong absorption lines in this region. Especially, 2.3 µm lasers have been applied to gas sensing in the atmosphere[1,2] and noninvasive blood glucose measurements[3,4]. Various approaches have been used to obtain 2.3 µm laser emission. A solid-state Raman laser based on stimulated Raman scattering (SRS) can produce 2.3 µm laser emission[5]. Semiconductor lasers operating near 2.3 µm have been reported[6]. Cr2+:ZnS and Cr2+:ZnSe lasers have also directly generated laser emissions in the special region[7,8].

    A promising approach to generate 2.3 µm laser emission is using the H34H35 quasi-four-level transition of the Tm3+ ions in a host crystal. The energy-level scheme of Tm3+ ions in a host crystal is shown in Fig. 1. In the early 1970s, 2.3 µm laser emission based on the prediction of the Judd–Ofelt theory was realized in Tm-doped oxide host materials[9]. By now, lasers at 2.3 µm based on the Tm3+H34H35 transition have been reported and analyzed theoretically in various host crystals, e.g., Tm:YAG[9,10], Tm:YAP[9,1114], Tm:YLF[13,1518], Tm:KYF[1921], Tm:LuAG[22], and Tm:Klu(WO)4[23]. These lasers based on the Tm3+H34H35 transition in different host crystals can generate laser emissions with different wavelengths. Among the studies, Tm:YLF and Tm:YAP lasers with relatively low Tm3+ ion concentration were mainly demonstrated. Narrow-linewidth high-quality Ti:sapphire lasers were commonly used as pump sources to pump 2.3 µm Tm lasers[11,1821]. Recently, diode-pumped 2.3 µm Tm lasers have been reported[12,17].

    Energy-level scheme of Tm3+ ions in host crystal.

    Figure 1.Energy-level scheme of Tm3+ ions in host crystal.

    So far, continuous-wave (CW) 2.3 µm Tm lasers have also been reported by several groups. For example, Wang et al. reported an a-cut Tm:YAP laser generating a maximum CW output power of 1.12 W at dual-wavelength of 2274 and 2383 nm with a slope efficiency of 14.0%[12]. Guillemot et al. reported a CW Tm:YAG laser delivering a maximum output power of 1.07 W at 2.19 and 2.32 µm with a slope efficiency of 46.3%[10]. Muti et al. demonstrated a Tm:KYF laser operating at 2.34 µm and generating 0.12 W with a slope efficiency of 18%[19]. Loiko et al. reported a diode-pumped quasi-CW Tm:LiYF4 laser delivering a maximum peak output power of 2.4 W at 2306 nm with a slope efficiency of 11.5%[24].

    However, to our knowledge, laser emission on the H34H35 transition in Tm-doped vanadate crystals has not been reported. So far, studies on Tm-doped vanadate (e.g., Tm:YVO4[2528], Tm:GdVO4[2729], and Tm:LuVO4[27,28]) lasers have mainly focused on the F34H36 transition around 1.9 µm. Among the vanadate crystals, the Tm:YVO4 crystal with natural birefringence is an excellent laser gain medium for 1.9 µm laser due to its large absorption and emission cross sections. For the Tm:YVO4 crystal, the calculated lifetimes of F34, H35, and H34 excited states were 1208, 1237, and 224 µs, and the measured lifetimes of F34 and H34 were 1.9 ms and 176 µs, respectively[30]. For the H36H34 transition of the Tm:YVO4 crystal, the peak absorption cross section for π polarization reached 2.8×1020cm2 near 800 nm, and the peak stimulated emission cross section corresponding to the F34H36 transition was close to 2.8×1020cm2[30]. The strong absorption in Tm:YVO4 should make it more suitable for microchip lasers. Zayhowski et al. reported a 5% (atomic fraction) Tm:YVO4 microchip laser producing an output power of 150 mW at 1.92 µm with a slope efficiency of 20%[31]. Saito et al. demonstrated a room-temperature 5% Tm:YVO4 laser obtaining an output power of 48 mW at 1.94 µm with a slope efficiency of 25%[26]. Hu et al. reported a 3% Tm:YVO4 laser producing a maximum CW output power of 2.59 W at 1923.1 nm with a slope efficiency of 41.7%[25].

    The main goal of the work described here is to report laser emission on the H34H35 transition at 2.3 µm in a Tm:YVO4 crystal. In this study, to the best of our knowledge, we demonstrated a diode-pumped CW Tm:YVO4 laser simultaneously operating on the H34H35 and F34H36 transitions for the first time. A common commercial laser diode (LD) with a central wavelength of 790 nm was used as a pump laser source. A simple linear resonant cavity was adopted. With an output coupler for the H34H35 and F34H36 transitions, the output powers of 750 mW at 2292 and 2363 nm and 420 mW at 2108 nm were obtained at the same time.

    2. Experimental Design

    The scheme of a diode-pumped Tm:YVO4 laser is shown in Fig. 2. A common commercial fiber-coupled LD was used as a pump source, and the maximum output pump power was 22 W. The output-coupling fiber of the LD had a core diameter of 400 µm and a numerical aperture (N.A.) of 0.22. The central wavelength of the LD was measured to be 790 nm at 25°C. The full width at half-maximum (FWHM) of the LD was about 3.2 nm. The pump beam from the output-coupling fiber was coupled into the Tm:YVO4 crystal by two identical plano–convex lenses with focal lengths of 25 mm and high transmission at 780–810 nm. The waist radius of the pump beam within the laser crystal was estimated to be 200 µm. The pump light from the LD operating in the CW regime was unpolarized. The uncoated a-cut Tm:YVO4 crystal with 1.5% Tm3+ ion concentration was used as laser gain medium. Tm3+ ion concentration around 1.5% contributed to efficient laser emission on the H34H35 transition in Tm-doped crystals[22]. The end faces of the Tm:YVO4 crystal with the dimension of 3mm×3mm×20mm were parallel. The Tm:YVO4 crystal was wrapped with indium foil and mounted in a water-cooled copper heat sink. The heat sink temperature was controlled by a thermoelectric cooler system.

    Scheme of a Tm:YVO4 laser operating on the 3H4→3H5 and 3F4→3H6 transitions.

    Figure 2.Scheme of a Tm:YVO4 laser operating on the 3H43H5 and 3F43H6 transitions.

    The simple linear resonant cavity was composed of an input mirror (M1) and an output coupler (M2). M1 was a flat input mirror, which was designed to be high-transmittance coated at 780–810 nm and high-reflection coated at 1900–2400 nm (R>99.8%). M2 was a plano–concave output coupler with a curvature radius of 100 mm, which was designed to partial reflection at 1900–2150 nm (R=95%) and partial reflection at 2250–2400 nm (R=99%) to realize laser emissions on the H34H35 and F34H36 transitions simultaneously. Both of the cavity mirrors were made of CaF2. The physical cavity length was about 30 mm. A 45° flat dichroic mirror (M3) coated for high transmissivity at 2108 nm (T>95%) and high reflectivity at 2292 and 2663 nm (R>98%) was used to separate the laser emissions on the H34H35 and F34H36 transitions.

    3. Experimental Results and Discussion

    The feasibility of achieving laser emissions on the H34H35 and F34H36 transitions in the Tm:YVO4 crystal was studied. With the output coupler coated for partial reflection at 1900–2150 nm (R=95%) and partial reflection at 2250–2400 nm (R=99%), a three-wavelength Tm:YVO4 laser can be realized. Figure 3 shows the spectral information of the Tm:YVO4 laser, which was recorded at the maximum incident pump power of 22 W. Three spectral lines with central wavelengths of 2108, 2292, and 2363 nm were observed. The corresponding FWHMs of the three output wavelengths were about 14.5, 8.4, and 9.3 nm, respectively. The three output central wavelengths were almost unchanged when the incident power was different. The lasing spectrum was measured by a commercial grating monochromator (Zolix, Omni-λ3007) with a resolution of 0.1 nm. The two spectral lines of 2292 and 2363 nm corresponded to the H34H35 transition. The multiwavelength behavior on the H34H35 transition could be caused by the transitions between the Stark components of the H34H35 multiplets[23,30].

    Lasing spectrum of the Tm:YVO4 laser.

    Figure 3.Lasing spectrum of the Tm:YVO4 laser.

    In this experiment, the output powers and pump power were measured by a power meter (Coherent, PM30). The absorbed pump power of the Tm:YVO4 crystal can be measured by removing the mirrors M2 and M3, and the absorbed pump efficiency of the crystal was calculated to be around 92.5%. The output powers of the Tm:YVO4 laser were measured as a function of incident pump power; the results are plotted in Figs. 4 and 5. Heat sink temperature affected the oscillation threshold and output powers of the Tm:YVO4 laser. At a heat sink temperature of 20°C, the pump threshold of the Tm:YVO4 laser was about 11.7 W, and the maximum total output power was 802 mW. When the heat sink temperature was controlled at 12°C, the pump threshold power decreased to 9.8 W, and the maximum total output power increased to 1170 mW. Figure 4 shows that low heat sink temperature was beneficial to reduce pump threshold and enhance conversion efficiency.

    Output powers versus incident pump power for the Tm:YVO4 laser at the heat sink temperatures of 12°C and 20°C.

    Figure 4.Output powers versus incident pump power for the Tm:YVO4 laser at the heat sink temperatures of 12°C and 20°C.

    Output powers versus incident pump power for the Tm:YVO4 laser at the heat sink temperature of 12°C.

    Figure 5.Output powers versus incident pump power for the Tm:YVO4 laser at the heat sink temperature of 12°C.

    As seen in Fig. 5, the laser emissions on the H34H35 and F34H36 transitions had different pump thresholds. When incident pump power was 9.8 W, a laser spectral line of 2363 nm was observed. A spectral line of 2292 nm was also observed when the incident pump power was increased to 10.5 W. When incident pump power was increased to 11.7 W, a spectral line of 2108 nm appeared. At the maximum incident pump power of 22 W, the Tm:YVO4 laser generated a total output power of 1.17 W for the three wavelengths with an optical-to-optical conversion efficiency of 5.3% and a slope efficiency of 10.4% (both calculated relative to the incident pump power). The maximum output power at 2108 nm was measured to be 420 mW. To our knowledge, this is the first time laser emission of the Tm:YVO4 laser on the F34H36 transition beyond 2.1 µm has been obtained. The maximum output power of 2292 and 2363 nm was 750 mW. According to spectral intensity distribution, the output powers of 2363 and 2292 nm were estimated to be 340 and 410 mW, respectively. The saturation of output power was not observed. In this experiment, we found that the output power at 2292 nm increased more rapidly than the output power at 2363 nm when the incident pump power increased. A possible reason was that a competition existed between the two emission lines at 2292 and 2363 nm from H34 and H35 multiplets. Similar laser behavior also happened in the 2.3 µm Tm:Klu(WO4)2 laser[23].

    The polarization states of the output laser beams were also measured by a Glan prism. The laser emissions on the H34H35 transition at 2292 and 2363 nm were both linearly polarized along the c axis of the Tm:YVO4 crystal (Ec, π-polarization). The laser emission on the H34H35 transition at 2108 nm was also linearly polarized and perpendicular to the c axis (Ec, σ-polarization). The polarizations were naturally selected by the anisotropy of the laser gain.

    By using the knife-edge method, the output laser beam radius of Tm:YVO4 laser as a function of the distance from a focusing lens was achieved. The focusing lens (f=100mm) was placed 160 mm away from the output coupler M2. As shown in Fig. 6, for the maximum output powers of 750 mW at 2292 and 2363 nm and 420 mW at 2108 nm, the M2 beam quality factors were calculated to be 2.34, 2.27, and 1.61 at 2292, 2363, and 2108 nm, respectively.

    Measured beam qualities of the Tm:YVO4 laser at maximum output power: (a) M2 = 2.34 at 2292 and M2 = 2.27 at 2363 nm; (b) M2 = 1.61 at 2108 nm.

    Figure 6.Measured beam qualities of the Tm:YVO4 laser at maximum output power: (a) M2 = 2.34 at 2292 and M2 = 2.27 at 2363 nm; (b) M2 = 1.61 at 2108 nm.

    For efficient laser operation at 2 µm (F34H36), Tm3+ ion concentration was commonly greater than 2%. Low Tm3+ ion concentration (<2%) was not beneficial to improve 2 µm laser performance[22]. However, for the 1.5% Tm:YVO4 crystal used in this experiment, laser emission at 2108 nm was realized. To further study this laser behavior, the laser emission on the H34H35 transition was suppressed by increasing the transmittance of the output coupler in this experiment. So, the output coupler M2 was replaced by another output coupler, which was designed for partial reflection at 1900–2150 nm (R=95%) and high transmissivity at 2250–2400 nm (T>80%). Other parameters remained unchanged. With increasing incident pump power to 20 W, the laser emission on the F34H36 transition at 1900–2150 nm was not observed. This indicated that 2.3 µm laser oscillation on the H34H35 transition was beneficial to improve 2 µm laser operation on the F34H36 transition, since the laser level H35 was rapidly depopulated by multiphonon processes to the level F34[22].

    4. Conclusion

    In conclusion, a Tm-doped vanadate laser operating on the H34H35 transition was reported for the first time. For this, an a-cut Tm:YVO4 crystal with 1.5% Tm3+ ion concentration was employed. Laser emissions at 2292 and 2363 nm on the H34H35 transition and at 2108 nm on the F34H36 transition were realized. For the pump power of 22 W, the Tm:YVO4 laser generated a total output power of 1.17 W with a total optical-optical efficiency of 5.3% and a slope efficiency of 10.4% (versus the incident pump power). The output powers of 750 mW at π-polarized 2292 and 2363 nm and 420 mW at σ-polarized 2108 nm were simultaneously obtained. The 2.3 µm Tm:YVO4 laser has the potential to be applied to multigas detection. In this experiment, only a 1% transmission output coupler at 2.3 µm was used. If the proper output coupler was used, desired output wavelengths would be obtained. Output powers and conversion efficiency would be improved by proper Tm3+ concentration and crystal length. The Tm:YVO4 crystal as laser gain medium can be used in passively Q-switched regime to achieve short pulses.

    References

    [1] F. J. McAleavey, J. O’Gorman, J. F. Donegan, B. D. MacCraith, J. Hegarty, G. Mazé. Narrow linewidth, tunable Tm3+-doped fluoride fiber laser for optical-based hydrocarbon gas sensing. IEEE J. Sel. Top. Quantum Electron., 3, 1103(1997).

    [2] A. Garnache, A. Liu, L. Cerutti, A. Campargue. Intracavity laser absorption spectroscopy with a vertical external cavity surface emitting laser at 2.3 µm: application to water and carbon dioxide. Chem. Phys. Lett., 416, 22(2005).

    [3] J. T. Olesberg, M. A. Arnold, C. Mermelstein, J. Schmitz, J. Wagner. Tunable laser diode system for noninvasive blood glucose measurements. Appl. Spectrosc., 59, 1480(2005).

    [4] S. T. Fard, W. Hofmann, P. T. Fard, G. Bohm, M. Ortsiefer, E. Kwok, M.-C. Amann, L. Chrostowski. Optical absorption glucose measurements using 2.3 µm vertical-cavity semiconductor lasers. IEEE Photon. Technol. Lett., 20, 930(2008).

    [5] J. Zhao, Y. Li, S. Zhang, L. Li, X. Zhang. Diode-pumped actively Q-switched Tm:YAP/BaWO4 intracavity Raman laser. Opt. Express, 23, 10075(2015).

    [6] R. Wang, S. Sprengel, G. Boehm, M. Muneeb, R. Baets, M. Amann, G. Roelkens. 2.3 µm range InP-based type-II quantum well Fabry-Perot lasers heterogeneously integrated on a silicon photonic integrated circuit. Opt. Express, 24, 21081(2016).

    [7] S. Vasilyev, I. Moskalev, M. Mirov, S. Mirov, V. Gapontsev. Multi-watt mid-IR femtosecond polycrystalline Cr2+:ZnS and Cr2+:ZnSe laser amplifiers with the spectrum spanning 2.0–2.6 µm. Opt. Express, 24, 1616(2016).

    [8] U. Demirbas, A. Sennaroglu. Intracavity-pumped Cr2+:ZnSe laser with ultrabroad tuning range between 1880 and 3100 nm. Opt. Lett., 31, 2293(2006).

    [9] J. A. Caird, L. G. DeShazer, J. Nella. Characteristics of room temperature 2.3-µm laser emission from Tm3+ in YAG and YAlO3. IEEE J. Quantum Electron., 11, 874(1975).

    [10] L. Guillemot, P. Loiko, E. Kifle, J.-L. Doualan, A. Braud, F. Starecki, T. Georges, J. Rouvillain, A. Hideur, P. Camy. Watt-level midinfrared continuous-wave Tm:YAG laser operating on the 3H4→3H5 transition. Opt. Mater., 101, 109745(2020).

    [11] L. Guillemot, P. Loiko, A. Braud, J.-L. Doualan, A. Hideur, M. Koselja, R. Moncorgé, P. Camy. Continuous-wave Tm:YAlO3 laser at ∼2.3 µm. Opt. Lett., 44, 5077(2019).

    [12] F. Wang, H. Huang, F. Wu, H. Chen, Y. Bao, Z. Li, O. L. Antipov, S. S. Balabanov, D. Shen. 2.3–2.5 µm laser operation of LD-pumped Tm:YAP on the 3H4→3H5 transition. Opt. Mater., 115, 111054(2021).

    [13] E. Kifle, P. Loiko, L. Guillemot, J. Doualan, F. Starecki, A. Braud, T. Georges, J. Rouvillain, P. Camy. Watt-level diode-pumped thulium lasers around 2.3 µm. Appl. Opt., 59, 7530(2020).

    [14] L. Dong, H. Chu, S. Xu, S. Zhao, D. Li. Sulfur-doped graphitic carbon nitride for Tm:YAIO3 laser operation at 2.3 µm. Opt. Lett., 46, 2043(2021).

    [15] J. F. Pinto, L. Esterowitz, G. H. Rosenblatt. Tm3+:YLF laser continuously tunable between 2.20 and 2.46 µm. Opt. Lett., 19, 883(1994).

    [16] I. Yorulmaz, A. Sennaroglu. Low-threshold diode-pumped 2.3-µm Tm3+:YLF Lasers. IEEE J. Sel. Top. Quantum Electron., 24, 1601007(2018).

    [17] H. Huang, S. Wang, H. Chen, O. L. Antipov, S. S. Balabanov, D. Shen. High power simultaneous dual-wavelength CW and passively-Q-switched laser operation of LD pumped Tm:YLF at 1.9 and 2.3 µm. Opt. Express, 27, 38593(2019).

    [18] F. Canbaz, I. Yorulmaz, A. Sennaroglu. Kerr-lens mode-locked 2.3-µm Tm3+:YLF laser as a source of femtosecond pulses in the mid-infrared. Opt. Lett., 42, 3964(2017).

    [19] A. Muti, M. Tonelli, V. Petrov, A. Sennaroglu. Continuous-wave mid-infrared laser operation of Tm3+:KY3F10 at 2.3  µm. Opt. Lett., 44, 3242(2019).

    [20] L. Guillemot, P. Loiko, R. Soulard, A. Braud, J. Doualan, A. Hideur, P. Camy. Close look on cubic Tm:KY3F10 crystal for highly efficient lasing on the 3H4→3H5transition. Opt. Express, 28, 3451(2020).

    [21] A. Muti, F. Canbaz, M. Tonelli, J. E. Bae, F. Rotermund, V. Petrov, A. Sennaroglu. Graphene mode-locked operation of Tm3+:YLiF4 and Tm3+:KY3F10 lasers near 2.3 µm. Opt. Lett., 45, 656(2020).

    [22] V. Sudesh, J. A. Piper. Spectroscopy, modeling, and laser operation of thulium-doped crystals at 2.3 µm. IEEE J. Quantum Electron., 36, 879(2000).

    [23] P. Loiko, E. Kifle, L. Guillemot, J. Doualan, F. Starecki, A. Braud, M. Aguiló, F. Díaz, V. Petrov, X. R. Mateos, P. Camy. Highly efficient 2.3 µm thulium lasers based on a high-phonon-energy crystal: evidence of vibronic-assisted emissions. J. Opt. Soc. Am. B, 38, 482(2021).

    [24] P. Loiko, R. Soulard, L. Guillemot, G. Brasse, J. L. Doualan, A. Braud, A. Tyazhev, A. Hideur, F. Druon, P. Camy. Efficient Tm:LiYF4 Lasers at ∼2.3 µm: effect of energy-transfer upconversion. IEEE J. Quantum Electron., 55, 1700212(2019).

    [25] H. Hu, H. Huang, J. Huang, J. Deng, W. Weng, J. Li, W. Lin. Watt-level passively Q-switched Tm:YVO4 laser with few-layer WSe2 saturable absorber. Infrared Phys. Technol., 113, 103554(2021).

    [26] H. Saito, S. Chaddha, R. S. E. Chang, N. Djeu. Efficient 1.94-µm Tm3+ laser in YVO4 host. Opt. Lett., 17, 189(1992).

    [27] R. Lisiecki, P. Solarz, G. Dominiak-Dzik, W. Ryba-Romanowski, T. Lukasiewicz. Effect of temperature on spectroscopic features relevant to laser performance of YVO4:Tm3+, GdVO4:Tm3+, and LuVO4:Tm3+ crystals. Opt. Lett., 35, 3940(2010).

    [28] J. Šulc, P. Koranda, P. L. Černý, H. Jelínková, Y. Urata, M. Higuchi, W. Ryba-Romanowski, R. Lisiecki, P. Solarz, G. Dominiak-Dzik, M. Sobczyk. Tunable lasers based on diode pumped Tm-doped vanadates Tm:YVO4, Tm:GdVO4, and Tm:LuVO4. Proc. SPIE, 6871, 68711V(2008).

    [29] Y. Urata, S. Wada. 808-nm diode-pumped continuous-wave Tm:GdVO4 laser at room temperature. Appl. Opt., 44, 3087(2005).

    [30] R. Lisiecki, P. Solarz, G. Dominiak-Dzik, W. Ryba-Romanowski, M. Sobczyk, P. Černý, J. Šulc, H. Jelínková, Y. Urata, M. Higuchi. Comparative optical study of thulium-doped YVO4, GdVO4, and LuVO4 single crystals. Phys. Rev. B, 74, 035103(2006).

    [31] J. J. Zayhowski, J. Harrison, C. Dill, J. Ochoa. Tm:YVO4 microchip laser. Appl. Opt., 34, 435(1995).

    Jiaqun Zhao, Yuantong Liu, Ping Cheng, Rui Yu. Continuous-wave three-wavelength operation of a diode-pumped Tm:YVO4 laser on the 3H43H5 and 3F43H6 transitions[J]. Chinese Optics Letters, 2023, 21(3): 031401
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