- Email: [email protected]
Ho3+-SrLaGa3O7 crystal
Optics and Laser Technology xxx (xxxx) xxx–xxx
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Full length article
Investigations on the spectroscopic properties and laser performance of Tm3+/Ho3+-SrLaGa3O7 crystal Shufang Gaoa, Shan Xua, Chaoyang Tub, a b
⁎
School of Physics and Optoelectronic Engineering, Yangtze University, Jingzhou 434023, China Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China
H I GH L IG H T S
first demonstrates spectroscopic and laser properties of Tm /Ho -SLGO Crystal. • The stimulated emission cross-sections are larger than YAG and YAP. • The • The maximum output powers of 6.15 W for a-cut crystal have been obtained. 3+
3+
A R T I C LE I N FO
A B S T R A C T
Keywords: Czochralski (CZ) technique Spectroscopic properties 2.0 μm laser
The Tm3+/Ho3+-SrLaGa3O7 crystal was grown using the Czochralski (CZ) method. The absorption spectra, the fluorescence spectra around 2.0 μm and energy-transfer (ET) schemes between Tm3+ ions and Ho3+ ions has been studied. The lasing performance of Tm3+/Ho3+-SrLaGa3O7 crystal for a-cut and c-cut pumped by a fibercoupled laser diode laser are presented in this paper. The maximum output powers of 6.15 W for a-cut crystal at 2041 nm and 2.51 W for c-cut crystal at 2044 nm have been obtained, respectively. The maximum slope efficiency in terms of absorbed pump power nearly reached 36.5% for the a-cut crystal and about 18.8% for the ccut crystal. In addition, the width of tuning can reach about 450 nm, suggesting the potential of these crystals for ultrashort pulse generation by mode-locking.
1. Introduction 2.0 µm eye-safe laser sources are promising candidates for a variety of applications in the fields of remote sensing, high-resolution molecular spectroscopy, the monitoring of atmospheric and biomedical systems, as well as in frequency synthesis of mid-IR wavelengths [1–3]. Tm3+(3F4 → 3H6), Ho3+(5I7 → 5I8) doped and Tm3+−Ho3+ co-doped laser crystals are considered good for laser media for continuous wave (CW) and pulsed laser operation in the wavelength region around 2.0 μm. The tunable laser operation can extend to ∼2.1 μm with Ho3+ ions co-doping crystals however the tunable range can only extend from1.85 μm to 2.03 μm with Tm3+ ions co-doping crystals. Also, Ho3+ ions are generally characterised by higher emission cross-sections and longer upper laser level lifetimes compared to their Tm3+ counterparts and these features are especially desirable for low-threshold and efficient laser operation. However, due to the lack of suitable laser diodes, it is difficult to directly pump the Ho3+. Because of the efficient energy transfer (ET) from Tm3+ to Ho3+, Ho3+ is usually co-doped with Tm3+
⁎
as a sensitizer. Hence, Tm3+ and Ho3+ co-doped crystals can be pumped at 800 nm with commercial AlGaAs diode lasers and generate efficient 2.0 μm lasers. Tm3+, Ho3+ co-doped to allow for solid state lasers generating 2.0 µm radiation, has been investigated in a variety of crystalline materials, such as Tm3+, Ho3+ co-doped YAG [4], YLiF4 [5], LuLiF4 [6], etc. The most studied Tm3+and Ho3+ co-doped laser gain media were garnet and fluoride crystals. Unfortunately, these gain media are characterized by relatively narrow and weak absorption lines in the 780–790 nm wave-length range. By contrast, Tm3+, Ho3+-based vanadate crystals, such as Tm3+, Ho3+-GdVO4[7], exhibit significantly broader and stronger absorption and emission bands in general and especially around 800 nm where commercial AlGaAs-based pump lasers can be deployed. So materials which produce large absorption and emission cross-sections as a result of strong anisotropy in the crystalline matrices attracted much attention. Among the reported laser crystals, the gallate with a tetragonal melilite structure are characterized by their local disordered crystal structure [8–11]. The general formulation
Corresponding author. E-mail addresses: [email protected], [email protected] (C. Tu).
https://doi.org/10.1016/j.optlastec.2018.09.015 Received 3 December 2017; Received in revised form 21 August 2018; Accepted 8 September 2018 0030-3992/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Gao, S., Optics and Laser Technology, https://doi.org/10.1016/j.optlastec.2018.09.015
Optics and Laser Technology xxx (xxxx) xxx–xxx
S. Gao et al.
of these crystals can be expressed as ABC3O7 (where A = Ba, Sr, Ca; B = La, Gd and C = Ga, Al) and the space group is P421m [12,13]. In this system, indeed, the A2+ and the B3+ ions, thus the Tm3+, Ho3+ dopants which substitute for the B3+ ions, are distributed almost statistically between two large cation sites and this leads to structural disorder and inhomogeneous broadening of all the optical transitions [13,14]. Here we report on spectroscopy and room-temperature laser operation for Tm3+/Ho3+ co-doped SrLaGa3O7 (SLGO) crystal in the 2.0 μm spectral region. The lasing threshold, output power, optical-tooptical conversion efficiency and output wavelength have been studied experimentally.
Table 1 The concentration and segregation coefficient of the ions in crystals. Crystals
(wt%) 2+
Sr Tm
3+
/Ho
3+
:SLGO
17.08
(at.%) Ln
3+
3+
La Ga3+
Tm 38.63 11.34
3+
/Ho
3+
Nc(1020 cm−3)
K
5.32 1.21
0.85 0.995
3+
(Tm )4.24 (Ho3+)0.995
2. Experiments and discussions 2.1. Crystal growth The polycrystalline materials used for single crystal growth were obtained by classical solid-state reaction. The initial chemicals of SrCO3 (99.99%), La2O3 (99.99%), Ga2O3 (99.99%), Tm2O3 (99.99%) and Ho2O3 (99.99%) powders were mixed in an agate mortar in stoichiometric amounts. These materials were pressed into tablets and sintered to obtain single phase powder. The growth of the samples were performed with an apparatus growth consisting of a DJL-400 Czochralski furnace with conventional resistive heating. During crystal growth, the pulling rate was 2 mm/h and the rotation rate was 12.0–20.0 rpm. When the growth process was over, the crystals were drawn out of the melt and cooled down to room temperature at a rate of 30.0 °C/h. As is known, there is concentration quenching phenomenon with the increase in doping ion concentration, fluorescence peak in the rare earth element doped crystals. When the doping concentration of Tm3+ exceeds a certain value, the transverse relaxation process will occur, however with the increase of doping concentration, the fluorescence peak gradually decreased. So Tm3+/Ho3+ co-doped crystals were grown with 5at% Tm3+ and 1.0 at% Ho3+ concentration in the melt according to reference [15]. The as-grown Tm3+/Ho3+-SrLaGa3O7 crystal is shown in Fig. 1. Tm3+, Ho3+ actual content in the crystals was checked by inductively coupled plasma atomic spectroscopy. The average ion concentration in the crystal Nc, and the segregation coefficient of the doping ions in crystals K are shown in Table 1.
in this paper were grown by the Czochralski technique. To evaluate the absorption properties of Tm3+/Ho3+-SLGO crystal, the polarized absorption spectrum of the crystal were measured first at room temperature by using Perkin-Elmer UV–VIS-NIR Spectrometer (Lambda900). The measured room temperature absorption spectra are shown in Fig. 2. We can see that the absorption peak at 795 nm is due to the transition 3H6 → 3H4 of Tm3+ ions. The influence of Ho3+ ions on the absorption peaks at 795 nm can be ignored because Ho3+ ions have no significant absorption at this wavelength, which is suitable for AlGaAs LD laser pumping. According to absorption spectrum measurements, the absorption coefficient can be calculated by the equation:
2.2. Spectra properties of crystal
α = 2.303 × OD/L
The co-doping of Ho3+ with Tm3+ is normally chosen to enable efficient operation across 2.0 µm spectral region through energytransfer routes in the Tm–Ho system (3F4 → 5I7). The laser crystals used
where OD is the optical density and L is the thickness of the sample (cm). The relation between the absorption cross-section and the absorption coefficient can be expressed as σabs = α/Nc. The absorption peak value of 1.56 × 10−20 cm2 for σ-polarization at 795 nm, which are larger than those of LiYF4 (0.3 × 10−20 cm2)[16] and YAG (0.63 × 10−20 cm2)[17]. The absorption peak of π-polarization at 795 nm is weaker (6.2 × 10−21 cm2) and the absorption line is narrower. The full-width at half-maximum (FWHM) at the strongest absorption peak of these crystals amounts to 30 nm for σpolarization, and 26 nm for π polarization, which makes it more suitable for being pumped by high power diodes. The inset shows the infrared region of the spectra which are corresponding to the transition 3H6 → 3F4 of Tm3+ ions and 5I8 → 5I7 of Ho3+ ions respectively. The unpolarized fluorescence spectra of the Tm3+/Ho3+-SLGO wafers in the region 1600–2300 nm which was measured using an Edinburgh Instruments FLS920 spectrometer with the resolution of 0.5 nm is shown in Fig. 3. The emission band consists of 3F4 → 3H6 transition of Tm ions and 5I7 → 5I8 transition of Ho ions, and the width of the fluorescence band can reach about 450 nm. It follows that such gain media can also be employed for ultrashort-pulse generation using appropriate mode-locking techniques. Ultrafast laser sources around 2.0 μm are of particular interest for applications in time-resolved spectroscopy, nonlinear frequency up-conversion to the mid/far-
Fig. 2. Room temperature polarized absorption spectra from the UV up to the IR wavelength regions.
Fig. 1. Photograph of the Tm3+/Ho3+-SLGO crystal. 2
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Fig. 5. Energy transfer schemes between Tm3+ ions and Ho3+ ions.
2.3. Energy transfer Fig. 3. The fluorescence spectra of the Tm3+/Ho3+-SLGO crystal.
In order to illustrate the concept of Energy transfer (ET) between Tm3+ ions and Ho3+ ions in Tm3+/Ho3+-SLGO crystal definitely, schematic energy level diagrams of Tm3+ ions and Ho3+ ions are presented in Fig. 5. Under 795 nm excitation, Tm3+ ions are excited to 3 H4 states. Then the non-radiative transitions of Tm3+: 3H4 → 3F4 and the CR mechanism Tm3+: 3H4 + 3H6 → 3F4 + 3F4 take place and the 3F4 level was populated. The emissions at 1.9 μm emerge under the transitions of Tm3+: 3F4 → 3H6. Because that the 3F4 level of Tm3+ ions is close in the level 5I7 of Ho3+ ions, efficient ET from Tm3+: 3F4 to Ho3+: 5 I7 level may happen. Most ions in Tm3+: 3F4 will transfer to Ho3+: 5I7 level and return to Ho3+: 5I8 ground state producing strong 2.0 μm emission via 5I7 → 5I8 transition.
infrared spectral regions, mid-IR super continuum generation, optical communications and photomedicine. The emission of Tm ion at about 1800 nm is weaker than emission of Ho at about 2000 nm, which indicates the energy transfer from Tm to Ho is complete. As a result of the energy transition from Tm to Ho ions, 3F4(Tm) + 5I8 (Ho) → 3 H6(Tm) + 5I7(Ho), the bands corresponding to 5I7 → 5I8 transition of Ho ions were recorded. The fluorescence lifetime is a critical factor to estimate the laser material. However, the measurement of the fluorescence lifetime of RE ions is specially difficult because the reabsorption effect lengthens the measured lifetime seriously. In order to reduce that effect during lifetime measurement, the lifetime measurement was carried out with fine powdered Tm3+/Ho3+-SLGO crystal immersed in monochlorobenzene, which was used as the refractive index matching fluid to minimize the re-absorption in the particles and contained in a glass cuvette [18]. Fluorescence decay curves for the 3F4 → 3H6 (Tm3+) and 5I7 → 5I8 (Ho3+) are shown in Fig. 4. It can be seen that within early time, the luminescence from the 3F4 level of the Tm3+ ion exhibits fast decay whereas the observed fast rise of the 5I7 multiplet of the Ho3+ ion,which due to the energy transfer in the Tm-Ho system (3F4-5I7). By linear fitting the back-end decay curves The fluorescence lifetimes τf of 3 F4 (Tm3+) and 5I7 (Ho3+) in the Tm3+/Ho3+-SLGO crystal are 4.27 ms and 9.38 ms respectively. Such dynamic luminescence behavior is typical for Tm3+/Ho3+ co-doped crystals such as Tm3+/Ho3+: YAG, Tm3+/Ho3+: NYW and other crystals [19,20].
2.4. Laser performance of crystals To test the 2.0 μm laser performance of this system, the experiment setup was illustrated in Fig. 6. We have used the a-cut and c-cut oriented crystals with size of 3 × 3 × 5 mm3 respectively. Laser performance of a crystal cleaved wafer was demonstrated in an end-pumped plano-concave (concave-concave) resonator. The fiber-coupled laser diode arrays at 79 nm with a core diameter of 400 μm and a numerical aperture of 0.22. The pulse duration was 200 μs, and the duty cycle was 2%. The output from the fiber was imaged with 1:1 magnification into the laser crystal by a pair of anti-reflection-coated plane-convex lenses. An unpolarized pump beam passed through an input couplers (the radius of 200 mm) with a transmission (HT) > 95% at 795 nm and an output coupler reflectivity higher (HR) > 99.9% in the region of 1900–2100 nm. The length of the plane-concave cavity (concave-concave cavity) was 11 mm. Each crystal was measured in turn by using four output couplers (OCs) with different transmissions of TOC = 3.5%, 4%, 7% and 5.5% (concave lenses) at 2.0 µm to find the most efficient configuration.
Fig. 4. Fluorescence decay curves for the 3F4 → 3H6 (Tm3+) and 5I7 → 5I8 (Ho3+).
Fig. 6. Scheme diagram of experimental setup for the LD-end-pumping Tm3+/ Ho3+-SLGO crystal. 3
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Fig. 7. The average output powers versus the absorbed pump power under different output couplers and different cooling water temperatures.
Table 2 The laser performance parameters obtained at 283 K. a-cut
Absorbed pump power (W) Laser threshold (W) Output power (W) Slope efficiency (%) Wavelength (nm)
c-cut
T = 3.5
T = 4.0
T = 7.0
T = 5.5
T = 3.5
T = 4.0
T = 7.0
T = 5.5
36.9 21.92 1.74 11.2 2049
36.9 23.15 1.82 12.8 2047
36.9 25.0 1.50 13.0 2043
36.9 23.78 2.51 18.8 2044
36.9 17.74 3.57 18.5 2047
36.9 18.83 4.51 24.6 2044
36.9 22.54 3.32 24.2 2040
36.9 20.08 6.15 36.5 2041
The free-running Tm3+/Ho3+-SLGO laser with all the adopted output coupler transmissions was realized at 2.04 µm. Fig. 7 shows the laser performance obtained under these different conditions. Because the duty cycle of the quasi-CW pump laser was 2%, the values in the figure are the measured ones multiplied by 50 [21]. The laser performance parameters obtained at 283 K are shown in Table 2. Due to the quasi-three-energy-level property of Tm3+/Ho3+-SLGO, the output power and slope efficiency decrease with the operating temperature. As the crystal temperature increases, the population of the lower-state increases, which causes the laser gain to decrease [22,23]. A maximum slope efficiency of 36.5% for a-cut Tm3+/Ho3+-SLGO crystal was obtained with output coupler of T = 5.5%, corresponding to a maximum output power of 6.15 W and an optical-to-optical conversion efficiency of 16.7%. A maximum slope efficiency of 18.8% for the a-cut Tm3+/ Ho3+-SLGO crystal was obtained with output coupler of T = 5.5%, corresponding to a maximum output power of 2.51 W and an optical conversion efficiency of 7%. The laser spectra of the free-running laser are shown in Fig. 8 under different output couplers and different cooling water temperatures. An optical spectrum analyzer with wavelength emitted 633 nm (Bristol
instrument) is used to measure the laser spectrum at single-wavelength operation regimes. From Fig. 8(a), it is obviously seen that the lasing wavelength is red-shifted as the cooling water temperature rises. The central wavelength is 2041 nm at 283 K and red-shifted to 2049 nm at 298 K for a-cut as well as 2044 nm at 283 K and red-shifted to 2050 nm at 298 K for c-cut crystal respectively. Since the Tm3+/Ho3+-SLGO is quasi-three-level laser material. According to the Boltzmann distribution, the population of the lower-states decreases as the energy increases. As is known to all, the laser gain is proportional to the product of the emission cross section and the population density difference between the upper-states and lower-states. Thus, as the operating temperature rises, the laser has a higher gain when the electron transits to high sub-states, which results in the red-shifted of the laser spectrum at high operation temperatures. Whereas Fig. 8(b) shows that the lasing wavelength is blue-shifted as the TOC of output coupler increasing. The intrinsic intracavity loss increases with the TOC of output coupler increasing. The increased distribution for population inversion causes the short-wave gain to increase which results in the blue-shifted of the laser spectrum.
4
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Fig. 8. The laser spectra of the free-running laser under different output couplers (a) and different cooling water temperatures (b).
3. Conclusion
output power and optical conversion efficiency, although the a-cut orientation crystal gives higher maximum slope efficiency over the c-cut without considering emission wavelength. Considering this, the pulsed laser performance of diode-pumped cryogenic Tm3+/Ho3+-SLGO laser further demonstrate that it is excellent laser material for 2.0 µm
To our knowledge, this paper is the first to report on study of Tm3+/ Ho3+-SLGO crystal lasers. Output powers of 6.15 W for a-cut crystal at 2041 nm as well as about 2.51 W for c-cut crystal at 2044 nm were achieved. The output power increased heavily with the decrease of crystal temperature. At different temperatures and output couplers, the thresholds and the output powers were measured to be compared and it was the quasi-three-energy levels structure that influenced the output power and optical efficiency basically. The maximum slope efficiency in terms of absorbed pump power nearly reached 36.5% for the a-cut crystal and about 18.8% for the c-cut crystal. The crystals basically demonstrate similar performance in terms of
Acknowledgments This work has been supported by the National Natural Science Foundation of China (11804032, 11647065, 11604026, 11764014, 11647138).
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Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optlastec.2018.09.015. References [1] S. Wenk, S. Furst, V. Danicke, D.T. Kunde, Design andtechnical concept of a Tm laser scalpel for clinicalinvestigation based on a 60 W, 1.92 µm Tm fiber laser system, in: T.M. Buzug (Ed.), Advances in Medical Engineeringed, Springer, Berlin, 2007, pp. 447–452. [2] D. Creeden, P.A. Ketteridge, P.A. Budni, Mid-infrared ZnGePparametric oscillator directly pumped by a pulsed 2µm Tm-doped fiber laser”, Opt. Lett. 33 (2008) 315–317. [3] N.S. Nishioka, Y. Domankevitz, Comparison of tissue ablation with pulsed holmium and thulium lasers, IEEE J. Quantum Electron. 26 (1990) 2271–2275. [4] T.Y. Fan, G. Huber, R.L. Byer, Continuous-wave operation at 2.1μm of a diode-laserpumped, Tm-sensitized Ho: Y3Al5O12 laser at 300K, Opt. Lett. 12 (1987) 678–680. [5] B.M. Walsh, N.P. Bames, M. Petros, J. Yu, U.N. Singh, Spectroscopy and modeling of solid state lanthanide lasers: application to trivalent Tm3+ and Ho3+ in YLiF4 and Lu LiF4, J. Appl. Phys. 95 (2004) 3255–3271. [6] Y.X. Bai, J.R. Yu, M. Petros, B. Trieu, P. Petzar, N. Singh, A. Sato, K. Asai, K. Mizutani, Fully conductively cooled, diode-pumped Ho:Tm:LuLiF4 laser oscillator/amplifier, Opt. Lett. 37 (2012) 2562–2564. [7] A. Sato, K. Asai, K. Mizutani, Lasing characteristics and optimizations of a diodeside-pumped Tm, Ho:GdVO4 laser, Opt. Lett. 29 (2004) 836–838. [8] I. Pracka, M. Malinowski, M. Swirkowicz, J. Kisielewski, A.L. Bajor, Czochralski growth and characterization of SrLaGa3O7: Ho3+ single crystals, in: International Conference on Solid State Crystals 2000: Growth, Characterization, and Applications of Single Crystals, 4412, 2001, pp. 375–379. [9] S.I. Kubota, M. Izumi, Luminescence of Eu3+, Tb3+ and Tm3+ in SrLaGa3O7, J. Alloys Compd. 283 (1999) 95–101. [10] A. Agnesi, F. Pirzio, L. Tartara, E. Ugolotti, H. Zhang, 378 fs pulse generation with
6
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Investigations on the spectroscopic properties and laser performance of Tm3+/Ho3+-SrLaGa3O7 crystal Shufang Gaoa, Shan Xua, Chaoyang Tub, a b
⁎
School of Physics and Optoelectronic Engineering, Yangtze University, Jingzhou 434023, China Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China
H I GH L IG H T S
first demonstrates spectroscopic and laser properties of Tm /Ho -SLGO Crystal. • The stimulated emission cross-sections are larger than YAG and YAP. • The • The maximum output powers of 6.15 W for a-cut crystal have been obtained. 3+
3+
A R T I C LE I N FO
A B S T R A C T
Keywords: Czochralski (CZ) technique Spectroscopic properties 2.0 μm laser
The Tm3+/Ho3+-SrLaGa3O7 crystal was grown using the Czochralski (CZ) method. The absorption spectra, the fluorescence spectra around 2.0 μm and energy-transfer (ET) schemes between Tm3+ ions and Ho3+ ions has been studied. The lasing performance of Tm3+/Ho3+-SrLaGa3O7 crystal for a-cut and c-cut pumped by a fibercoupled laser diode laser are presented in this paper. The maximum output powers of 6.15 W for a-cut crystal at 2041 nm and 2.51 W for c-cut crystal at 2044 nm have been obtained, respectively. The maximum slope efficiency in terms of absorbed pump power nearly reached 36.5% for the a-cut crystal and about 18.8% for the ccut crystal. In addition, the width of tuning can reach about 450 nm, suggesting the potential of these crystals for ultrashort pulse generation by mode-locking.
1. Introduction 2.0 µm eye-safe laser sources are promising candidates for a variety of applications in the fields of remote sensing, high-resolution molecular spectroscopy, the monitoring of atmospheric and biomedical systems, as well as in frequency synthesis of mid-IR wavelengths [1–3]. Tm3+(3F4 → 3H6), Ho3+(5I7 → 5I8) doped and Tm3+−Ho3+ co-doped laser crystals are considered good for laser media for continuous wave (CW) and pulsed laser operation in the wavelength region around 2.0 μm. The tunable laser operation can extend to ∼2.1 μm with Ho3+ ions co-doping crystals however the tunable range can only extend from1.85 μm to 2.03 μm with Tm3+ ions co-doping crystals. Also, Ho3+ ions are generally characterised by higher emission cross-sections and longer upper laser level lifetimes compared to their Tm3+ counterparts and these features are especially desirable for low-threshold and efficient laser operation. However, due to the lack of suitable laser diodes, it is difficult to directly pump the Ho3+. Because of the efficient energy transfer (ET) from Tm3+ to Ho3+, Ho3+ is usually co-doped with Tm3+
⁎
as a sensitizer. Hence, Tm3+ and Ho3+ co-doped crystals can be pumped at 800 nm with commercial AlGaAs diode lasers and generate efficient 2.0 μm lasers. Tm3+, Ho3+ co-doped to allow for solid state lasers generating 2.0 µm radiation, has been investigated in a variety of crystalline materials, such as Tm3+, Ho3+ co-doped YAG [4], YLiF4 [5], LuLiF4 [6], etc. The most studied Tm3+and Ho3+ co-doped laser gain media were garnet and fluoride crystals. Unfortunately, these gain media are characterized by relatively narrow and weak absorption lines in the 780–790 nm wave-length range. By contrast, Tm3+, Ho3+-based vanadate crystals, such as Tm3+, Ho3+-GdVO4[7], exhibit significantly broader and stronger absorption and emission bands in general and especially around 800 nm where commercial AlGaAs-based pump lasers can be deployed. So materials which produce large absorption and emission cross-sections as a result of strong anisotropy in the crystalline matrices attracted much attention. Among the reported laser crystals, the gallate with a tetragonal melilite structure are characterized by their local disordered crystal structure [8–11]. The general formulation
Corresponding author. E-mail addresses: [email protected], [email protected] (C. Tu).
https://doi.org/10.1016/j.optlastec.2018.09.015 Received 3 December 2017; Received in revised form 21 August 2018; Accepted 8 September 2018 0030-3992/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Gao, S., Optics and Laser Technology, https://doi.org/10.1016/j.optlastec.2018.09.015
Optics and Laser Technology xxx (xxxx) xxx–xxx
S. Gao et al.
of these crystals can be expressed as ABC3O7 (where A = Ba, Sr, Ca; B = La, Gd and C = Ga, Al) and the space group is P421m [12,13]. In this system, indeed, the A2+ and the B3+ ions, thus the Tm3+, Ho3+ dopants which substitute for the B3+ ions, are distributed almost statistically between two large cation sites and this leads to structural disorder and inhomogeneous broadening of all the optical transitions [13,14]. Here we report on spectroscopy and room-temperature laser operation for Tm3+/Ho3+ co-doped SrLaGa3O7 (SLGO) crystal in the 2.0 μm spectral region. The lasing threshold, output power, optical-tooptical conversion efficiency and output wavelength have been studied experimentally.
Table 1 The concentration and segregation coefficient of the ions in crystals. Crystals
(wt%) 2+
Sr Tm
3+
/Ho
3+
:SLGO
17.08
(at.%) Ln
3+
3+
La Ga3+
Tm 38.63 11.34
3+
/Ho
3+
Nc(1020 cm−3)
K
5.32 1.21
0.85 0.995
3+
(Tm )4.24 (Ho3+)0.995
2. Experiments and discussions 2.1. Crystal growth The polycrystalline materials used for single crystal growth were obtained by classical solid-state reaction. The initial chemicals of SrCO3 (99.99%), La2O3 (99.99%), Ga2O3 (99.99%), Tm2O3 (99.99%) and Ho2O3 (99.99%) powders were mixed in an agate mortar in stoichiometric amounts. These materials were pressed into tablets and sintered to obtain single phase powder. The growth of the samples were performed with an apparatus growth consisting of a DJL-400 Czochralski furnace with conventional resistive heating. During crystal growth, the pulling rate was 2 mm/h and the rotation rate was 12.0–20.0 rpm. When the growth process was over, the crystals were drawn out of the melt and cooled down to room temperature at a rate of 30.0 °C/h. As is known, there is concentration quenching phenomenon with the increase in doping ion concentration, fluorescence peak in the rare earth element doped crystals. When the doping concentration of Tm3+ exceeds a certain value, the transverse relaxation process will occur, however with the increase of doping concentration, the fluorescence peak gradually decreased. So Tm3+/Ho3+ co-doped crystals were grown with 5at% Tm3+ and 1.0 at% Ho3+ concentration in the melt according to reference [15]. The as-grown Tm3+/Ho3+-SrLaGa3O7 crystal is shown in Fig. 1. Tm3+, Ho3+ actual content in the crystals was checked by inductively coupled plasma atomic spectroscopy. The average ion concentration in the crystal Nc, and the segregation coefficient of the doping ions in crystals K are shown in Table 1.
in this paper were grown by the Czochralski technique. To evaluate the absorption properties of Tm3+/Ho3+-SLGO crystal, the polarized absorption spectrum of the crystal were measured first at room temperature by using Perkin-Elmer UV–VIS-NIR Spectrometer (Lambda900). The measured room temperature absorption spectra are shown in Fig. 2. We can see that the absorption peak at 795 nm is due to the transition 3H6 → 3H4 of Tm3+ ions. The influence of Ho3+ ions on the absorption peaks at 795 nm can be ignored because Ho3+ ions have no significant absorption at this wavelength, which is suitable for AlGaAs LD laser pumping. According to absorption spectrum measurements, the absorption coefficient can be calculated by the equation:
2.2. Spectra properties of crystal
α = 2.303 × OD/L
The co-doping of Ho3+ with Tm3+ is normally chosen to enable efficient operation across 2.0 µm spectral region through energytransfer routes in the Tm–Ho system (3F4 → 5I7). The laser crystals used
where OD is the optical density and L is the thickness of the sample (cm). The relation between the absorption cross-section and the absorption coefficient can be expressed as σabs = α/Nc. The absorption peak value of 1.56 × 10−20 cm2 for σ-polarization at 795 nm, which are larger than those of LiYF4 (0.3 × 10−20 cm2)[16] and YAG (0.63 × 10−20 cm2)[17]. The absorption peak of π-polarization at 795 nm is weaker (6.2 × 10−21 cm2) and the absorption line is narrower. The full-width at half-maximum (FWHM) at the strongest absorption peak of these crystals amounts to 30 nm for σpolarization, and 26 nm for π polarization, which makes it more suitable for being pumped by high power diodes. The inset shows the infrared region of the spectra which are corresponding to the transition 3H6 → 3F4 of Tm3+ ions and 5I8 → 5I7 of Ho3+ ions respectively. The unpolarized fluorescence spectra of the Tm3+/Ho3+-SLGO wafers in the region 1600–2300 nm which was measured using an Edinburgh Instruments FLS920 spectrometer with the resolution of 0.5 nm is shown in Fig. 3. The emission band consists of 3F4 → 3H6 transition of Tm ions and 5I7 → 5I8 transition of Ho ions, and the width of the fluorescence band can reach about 450 nm. It follows that such gain media can also be employed for ultrashort-pulse generation using appropriate mode-locking techniques. Ultrafast laser sources around 2.0 μm are of particular interest for applications in time-resolved spectroscopy, nonlinear frequency up-conversion to the mid/far-
Fig. 2. Room temperature polarized absorption spectra from the UV up to the IR wavelength regions.
Fig. 1. Photograph of the Tm3+/Ho3+-SLGO crystal. 2
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Fig. 5. Energy transfer schemes between Tm3+ ions and Ho3+ ions.
2.3. Energy transfer Fig. 3. The fluorescence spectra of the Tm3+/Ho3+-SLGO crystal.
In order to illustrate the concept of Energy transfer (ET) between Tm3+ ions and Ho3+ ions in Tm3+/Ho3+-SLGO crystal definitely, schematic energy level diagrams of Tm3+ ions and Ho3+ ions are presented in Fig. 5. Under 795 nm excitation, Tm3+ ions are excited to 3 H4 states. Then the non-radiative transitions of Tm3+: 3H4 → 3F4 and the CR mechanism Tm3+: 3H4 + 3H6 → 3F4 + 3F4 take place and the 3F4 level was populated. The emissions at 1.9 μm emerge under the transitions of Tm3+: 3F4 → 3H6. Because that the 3F4 level of Tm3+ ions is close in the level 5I7 of Ho3+ ions, efficient ET from Tm3+: 3F4 to Ho3+: 5 I7 level may happen. Most ions in Tm3+: 3F4 will transfer to Ho3+: 5I7 level and return to Ho3+: 5I8 ground state producing strong 2.0 μm emission via 5I7 → 5I8 transition.
infrared spectral regions, mid-IR super continuum generation, optical communications and photomedicine. The emission of Tm ion at about 1800 nm is weaker than emission of Ho at about 2000 nm, which indicates the energy transfer from Tm to Ho is complete. As a result of the energy transition from Tm to Ho ions, 3F4(Tm) + 5I8 (Ho) → 3 H6(Tm) + 5I7(Ho), the bands corresponding to 5I7 → 5I8 transition of Ho ions were recorded. The fluorescence lifetime is a critical factor to estimate the laser material. However, the measurement of the fluorescence lifetime of RE ions is specially difficult because the reabsorption effect lengthens the measured lifetime seriously. In order to reduce that effect during lifetime measurement, the lifetime measurement was carried out with fine powdered Tm3+/Ho3+-SLGO crystal immersed in monochlorobenzene, which was used as the refractive index matching fluid to minimize the re-absorption in the particles and contained in a glass cuvette [18]. Fluorescence decay curves for the 3F4 → 3H6 (Tm3+) and 5I7 → 5I8 (Ho3+) are shown in Fig. 4. It can be seen that within early time, the luminescence from the 3F4 level of the Tm3+ ion exhibits fast decay whereas the observed fast rise of the 5I7 multiplet of the Ho3+ ion,which due to the energy transfer in the Tm-Ho system (3F4-5I7). By linear fitting the back-end decay curves The fluorescence lifetimes τf of 3 F4 (Tm3+) and 5I7 (Ho3+) in the Tm3+/Ho3+-SLGO crystal are 4.27 ms and 9.38 ms respectively. Such dynamic luminescence behavior is typical for Tm3+/Ho3+ co-doped crystals such as Tm3+/Ho3+: YAG, Tm3+/Ho3+: NYW and other crystals [19,20].
2.4. Laser performance of crystals To test the 2.0 μm laser performance of this system, the experiment setup was illustrated in Fig. 6. We have used the a-cut and c-cut oriented crystals with size of 3 × 3 × 5 mm3 respectively. Laser performance of a crystal cleaved wafer was demonstrated in an end-pumped plano-concave (concave-concave) resonator. The fiber-coupled laser diode arrays at 79 nm with a core diameter of 400 μm and a numerical aperture of 0.22. The pulse duration was 200 μs, and the duty cycle was 2%. The output from the fiber was imaged with 1:1 magnification into the laser crystal by a pair of anti-reflection-coated plane-convex lenses. An unpolarized pump beam passed through an input couplers (the radius of 200 mm) with a transmission (HT) > 95% at 795 nm and an output coupler reflectivity higher (HR) > 99.9% in the region of 1900–2100 nm. The length of the plane-concave cavity (concave-concave cavity) was 11 mm. Each crystal was measured in turn by using four output couplers (OCs) with different transmissions of TOC = 3.5%, 4%, 7% and 5.5% (concave lenses) at 2.0 µm to find the most efficient configuration.
Fig. 4. Fluorescence decay curves for the 3F4 → 3H6 (Tm3+) and 5I7 → 5I8 (Ho3+).
Fig. 6. Scheme diagram of experimental setup for the LD-end-pumping Tm3+/ Ho3+-SLGO crystal. 3
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Fig. 7. The average output powers versus the absorbed pump power under different output couplers and different cooling water temperatures.
Table 2 The laser performance parameters obtained at 283 K. a-cut
Absorbed pump power (W) Laser threshold (W) Output power (W) Slope efficiency (%) Wavelength (nm)
c-cut
T = 3.5
T = 4.0
T = 7.0
T = 5.5
T = 3.5
T = 4.0
T = 7.0
T = 5.5
36.9 21.92 1.74 11.2 2049
36.9 23.15 1.82 12.8 2047
36.9 25.0 1.50 13.0 2043
36.9 23.78 2.51 18.8 2044
36.9 17.74 3.57 18.5 2047
36.9 18.83 4.51 24.6 2044
36.9 22.54 3.32 24.2 2040
36.9 20.08 6.15 36.5 2041
The free-running Tm3+/Ho3+-SLGO laser with all the adopted output coupler transmissions was realized at 2.04 µm. Fig. 7 shows the laser performance obtained under these different conditions. Because the duty cycle of the quasi-CW pump laser was 2%, the values in the figure are the measured ones multiplied by 50 [21]. The laser performance parameters obtained at 283 K are shown in Table 2. Due to the quasi-three-energy-level property of Tm3+/Ho3+-SLGO, the output power and slope efficiency decrease with the operating temperature. As the crystal temperature increases, the population of the lower-state increases, which causes the laser gain to decrease [22,23]. A maximum slope efficiency of 36.5% for a-cut Tm3+/Ho3+-SLGO crystal was obtained with output coupler of T = 5.5%, corresponding to a maximum output power of 6.15 W and an optical-to-optical conversion efficiency of 16.7%. A maximum slope efficiency of 18.8% for the a-cut Tm3+/ Ho3+-SLGO crystal was obtained with output coupler of T = 5.5%, corresponding to a maximum output power of 2.51 W and an optical conversion efficiency of 7%. The laser spectra of the free-running laser are shown in Fig. 8 under different output couplers and different cooling water temperatures. An optical spectrum analyzer with wavelength emitted 633 nm (Bristol
instrument) is used to measure the laser spectrum at single-wavelength operation regimes. From Fig. 8(a), it is obviously seen that the lasing wavelength is red-shifted as the cooling water temperature rises. The central wavelength is 2041 nm at 283 K and red-shifted to 2049 nm at 298 K for a-cut as well as 2044 nm at 283 K and red-shifted to 2050 nm at 298 K for c-cut crystal respectively. Since the Tm3+/Ho3+-SLGO is quasi-three-level laser material. According to the Boltzmann distribution, the population of the lower-states decreases as the energy increases. As is known to all, the laser gain is proportional to the product of the emission cross section and the population density difference between the upper-states and lower-states. Thus, as the operating temperature rises, the laser has a higher gain when the electron transits to high sub-states, which results in the red-shifted of the laser spectrum at high operation temperatures. Whereas Fig. 8(b) shows that the lasing wavelength is blue-shifted as the TOC of output coupler increasing. The intrinsic intracavity loss increases with the TOC of output coupler increasing. The increased distribution for population inversion causes the short-wave gain to increase which results in the blue-shifted of the laser spectrum.
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Fig. 8. The laser spectra of the free-running laser under different output couplers (a) and different cooling water temperatures (b).
3. Conclusion
output power and optical conversion efficiency, although the a-cut orientation crystal gives higher maximum slope efficiency over the c-cut without considering emission wavelength. Considering this, the pulsed laser performance of diode-pumped cryogenic Tm3+/Ho3+-SLGO laser further demonstrate that it is excellent laser material for 2.0 µm
To our knowledge, this paper is the first to report on study of Tm3+/ Ho3+-SLGO crystal lasers. Output powers of 6.15 W for a-cut crystal at 2041 nm as well as about 2.51 W for c-cut crystal at 2044 nm were achieved. The output power increased heavily with the decrease of crystal temperature. At different temperatures and output couplers, the thresholds and the output powers were measured to be compared and it was the quasi-three-energy levels structure that influenced the output power and optical efficiency basically. The maximum slope efficiency in terms of absorbed pump power nearly reached 36.5% for the a-cut crystal and about 18.8% for the c-cut crystal. The crystals basically demonstrate similar performance in terms of
Acknowledgments This work has been supported by the National Natural Science Foundation of China (11804032, 11647065, 11604026, 11764014, 11647138).
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