Open Access
Add to BibSonomy
Abstract
A transparent Tm:Lu3Al5O12 ceramic is fabricated by solid-state reactive sintering at 1830 °C for 30 h using commercial α-Al2O3 and Lu2O3/Tm2O3 powders and sintering aids - MgO and TEOS. The ceramic belongs to the cubic system and exhibits a close-packed structure (mean grain size: 21 µm). The in-line transmission at ∼1 µm is 82.6%, close to the theoretical limit. The spectroscopic properties of the ceramic are studied in detail. The maximum stimulated-emission cross-section is 2.37×10−21 cm2 at 1749nm and the radiative lifetime of the 3F4 state is about 10 ms. The modified Judd-Ofelt theory accounting for configuration interaction is applied to determine the transition probabilities of Tm3+, yielding the intensity parameters Ω2 = 2.507, Ω4 = 1.236, Ω6 = 1.340 [10−20 cm2] and α = 0.196×10−4 cm. The effect of excited configurations on lower-lying interconnected states with the same J quantum number is discussed. First laser operation is achieved under diode-pumping at 792 nm. A 4 at.% Tm:Lu3Al5O12 ceramic laser generated 3.12 W at 2022-2035nm with a slope efficiency of 60.2%. The ceramic is promising for multi-watt lasers at >2 µm.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Transparent ceramics doped with rare-earth ions (RE3+) represent a promising alternative to the single-crystal development for laser applications [1–3]. Their advantages lie in the technological area, as they offer (i) easier fabrication methods, (ii) lower synthesis temperatures, (iii) size-scalable production, (iv) optical isotropy and (v) the possibility to control the volume of RE3+ doping or (vi) to fabricate composite structures [4]. Physically, ceramics may offer compositions which are not present in the single-crystal form and allow for higher RE3+ doping levels. When going from a single-crystal to polycrystalline ceramics, the spectroscopic properties of the RE3+ ions are typically preserved [5]. A proper fabrication also sustains the thermal and thermo-optical response of the material. As a result, RE3+-doped ceramics are of interest for efficient and power-scalable lasers.
So far, the most widespread material for fabrication of transparent ceramics is the cubic (sp. gr. Ia3¯d) yttrium aluminum garnet, Y3Al5O12. The first studies focused on RE:Y3Al5O12 transparent ceramics for emission at ∼1 µm based on the Nd3+ [1–3] and Yb3+ [6,7] active dopant ions and, more recently, the interest shifted to the ∼2 µm spectral range [8–10]. The 2-µm laser emission is eye-safe and primarily achieved with Tm3+ and Ho3+ doping. The lasers operating at ∼2 µm have multiple applications in remote sensing (LIDAR), wind mapping, spectroscopy, range-finding or medicine. They can also be used for nonlinear frequency conversion into the mid-IR [11]. In particular, the thulium (Tm3+) ion benefits from (i) large Stark splitting of the ground-state (3H6) leading to broadband emission, (ii) strong absorption at ∼0.8 µm (the 3H6 → 3H4 transition) allowing for diode-pumping and (iii) an efficient cross-relaxation (CR) process for adjacent Tm3+ pairs, 3H6 + 3H4 → 3F4 + 3F4, increasing the pump quantum efficiency up to 2 [12]. Tm3+-doped garnets are also known for long upper laser level lifetimes (∼10 ms) and emission wavelengths extending beyond 2 µm [13], where the unwanted structured absorption of water in the atmosphere is absent. The latter is in particular relevant for mode-locked (ML) lasers operating at ultrashort pulse durations (broad spectra) with specific applications [14].
Tm:Y3Al5O12 laser ceramics have been previously demonstrated [8,15–17]. Gao et al. reported on a continuous-wave (CW) diode-pumped Tm:Y3Al5O12 ceramic laser delivering 6.05 W at 2016nm with a slope efficiency of 65% [17]. Gluth et al. demonstrated a Tm:Y3Al5O12 ceramic oscillator ML by a GaSb-based SEmiconductor Saturable Absorber Mirror (SESAM) delivering 3-ps pulses at 2012nm at a repetition rate of 89 MHz [18].
Another compound in the cubic garnet family, i.e., lutetium aluminum garnet, Lu3Al5O12, is known for its better thermal properties as compared to its yttrium counterpart which is advantageous for the ceramic technology [19,20]. Considering Tm3+ doping, Lu3Al5O12 is attractive because of the closeness of the ionic radii of Tm3+ and Lu3+ (as compared to Y3+). Studies on Tm:Lu3Al5O12 single-crystals exist [21,22], however, there is a lack of information about such ceramics. Wang et al. reported on a ML Tm:Lu3Al5O12 ceramic laser delivering 2.7 ps pulses at 2022nm; a broad tuning range of 1808-2088nm was also demonstrated [23]. Zhou et al. reported on a CW Tm:Lu3Al5O12 ceramic laser delivering an output power of 2.64 W at 2016nm with a slope efficiency of only 35.6% [24]. Note that Lu3Al5O12 ceramics doped with other RE3+ ions are also known [25,26].
In the present work, we demonstrate the potential of Tm:Lu3Al5O12 transparent ceramics for highly-efficient multi-watt laser operation at ∼2 µm and study thoroughly their spectroscopic properties, which are of key importance for the development of CW and in particular ML lasers.
2. Synthesis of ceramics
Tm:Lu3Al5O12 transparent ceramics were prepared by solid-state reactive sintering of commercial α-Al2O3 (purity: 99.98%) and Lu2O3/Tm2O3 (5N) powders, using MgO (5N) and tetraethoxysilane (TEOS, 5N) as sintering aids [23]. The raw materials were mixed in a stoichiometric ratio of (Lu0.96Tm0.04)3Al5O12 and ball milled for 12 h with absolute ethyl alcohol as milling medium. Afterwards, we dried the slurry for 2 h at 80 °C in an oven, grounded and sieved it subsequently through a 200-mesh screen. Finally, we calcined it at 800 °C for 1 h to remove organic components. After the preparation, we pressed uniaxially the powders into pellets (diameter: 20 mm) at 50 MPa and cold isostatically pressed (CIP) at 250 MPa. The green-bodies were sintered at 1830 °C for 30 h in a tungsten mesh-heated vacuum furnace. In order to remove the oxygen vacancies, we annealed the samples at 1450 °C for 10 h in air. The Tm3+ doping level of 4.0 at.% corresponds to NTm = 5.65×1020 cm−3 (calculated using a density ρcalc of 6.695 g/cm3 determined from the XRD data).
The ceramics were transparent and colorless, as shown in Fig. 1. For spectroscopic studies, a sample with a thickness t of 3.08 mm was cut and both surfaces were polished to laser-grade quality.
3. Characterization of the fabricated ceramics
3.1 Structure and morphology
The structure and phase purity of the fabricated transparent ceramics were confirmed by X-ray diffraction (XRD), see Fig. 2(a). The ceramic sample was finely powdered for the XRD study. The ceramic has a cubic structure similar to that of undoped Lu3Al5O12 (ICSD card #23846, sp. gr. Ia3¯d – O10h, No. 230) [27]. No traces of other phases are observed in the XRD pattern. The lattice constant is a = 11.91 Å. It is slightly larger than that of undoped Lu3Al5O12 (a = 11.90 Å). Tm3+ ions substitute for the Lu3+ ones in a single type of sites (D2 symmetry) [28]. The difference in ionic radii of Tm3+ (0.994 Å) and Lu3+ (0.977 Å) for VIII-fold coordination by O2- [29] explains the observed lattice expansion.
Fig. 2. Structure and microstructure of the Tm:Lu3Al5O12 ceramic: (a) X-ray powder diffraction (XRD) pattern, black peaks stand for undoped Lu3Al5O12 (ICSD card #23846), numbers denote the Miller’s indices (hkl); (b) Field-Emission Scanning Electron Microscope (FESEM) image of a fractured surface, the scale bar is 50 µm.
The study of the microstructure by Field-Emission Scanning Electron Microscopy (FESEM), Fig. 2(b), reveals a close-packed structure with clean grain boundaries and lack of pores. The grain size distribution was analyzed using the ImageJ software counting >50 grains. The measured grain sizes were multiplied by the shape factor of 1.2. The mean grain size is 21 ± 4 µm.
3.2 Raman spectroscopy
The vibronic properties of the fabricated ceramic were studied with Raman spectroscopy, see Fig. 3, showing an unpolarized Raman spectrum for an excitation wavelength of 514 nm. For cubic Lu3Al5O12 containing 80 atoms in the unit-cell, the factor-group analysis predicts 97 lattice modes. They are characterized (at k = 0, the center of the Brillouin zone) by the following set of irreducible representations [30]: Г = 3A1g + 8Eg + 14F2g + 5A1u + 5A2u + 5A2g + 10Eu + 14F1g + 16F2u + 17F1u. Here, 25 lattice modes (A1g, Eg and F2g) are Raman-active. In Fig. 3, 14 bands are observed and assigned to the lattice vibrations.
Fig. 3. Unpolarized Raman spectrum of the Tm:Lu3Al5O12 ceramic; numbers indicate Raman frequencies in cm−1.
The most intense Raman band at 796 cm−1 (A1g) is due to the breathing mode of the [AlO4] tetrahedron [31]. Its full width at half maximum (FWHM) is 12.7 cm−1. Other intense bands appear at lower frequencies, at 259 cm−1 (T2g) and 374 cm−1 (A1g). The maximum phonon energy hνph for the studied ceramic is 868 cm−1 (T2g).
3.3 Optical absorption spectroscopy
The transmission spectrum of the ceramic is shown in Fig. 4. At 1.0 µm (out of the absorption of Tm3+ ions), the transmission T amounts to 82.6% which is close to the theoretical value set by the Fresnel losses, T0 = 83.6% (assuming a refractive index n = 1.826 [32]). This value confirms the very good optical quality of the fabricated ceramic.
Fig. 4. Transmission spectrum of a laser-grade-polished 4 at.% Tm:Lu3Al5O12 ceramic sample (t = 3.08 mm), green curve - theoretical transmission determined by Fresnel reflections.
The absorption bands are due to Tm3+ transitions from the ground-state (3H6) to the excited-states (from 3F4 to 1D2). The absorption cross-sections were calculated from the absorption coefficient αabs according to the equation σabs = αabs/NTm. The αabs values, in their turn, were determined from the transmission spectrum, cf. Figure 4, as αabs = -ln(T/T0)/t [cm−1] where the thickness t is expressed in cm. For the 3H6 → 3H4 transition which is used for conventional pumping of Tm lasers, Fig. 5(a), the maximum σabs is 5.0×10−21 cm2 at 787.6 nm and the FWHM of several closely located absorption peaks is 12.4 nm. For the 3H6 → 3F4 transition, which is spectrally overlapping the ∼2 µm emission of Tm lasers, σabs is 9.5×10−21 cm2 at 1631.9 nm, see Fig. 5(b). In the range of the laser emission (2.02 µm), the reabsorption losses σabs are < 0.04×10−21 cm2.
Fig. 5. Absorption cross-section, σabs, of Tm3+ in the Lu3Al5O12 ceramic for (a) the 3H6 → 3H4 transition and (b) the 3H6 → 3F4 transition. Arrow in (a) indicates the pump wavelength.
3.4 Optical emission spectroscopy
The stimulated-emission (SE) cross-sections, σSE, for the 3F4 → 3H6 transition of Tm3+ are shown in Fig. 6(a). They was calculated using a combination of the reciprocity method (RM) [33] and the Füchtbauer-Ladenburg (F-L) equation [34]:
Fig. 6. Stimulated-emission (SE) of Tm3+-doped Lu3Al5O12 ceramic at ∼2 µm (3F4 → 3H6 transition): (a) SE cross-sections, σSE, calculated using the RM and the F-L equation, and (b) gain cross-section, σgain, for various inversion ratios β, calculated using a combined SE cross-section spectrum (see the details in the text). Arrow in (a) indicates the laser wavelength.
The simultaneous use of the two methods allows one to avoid the detrimental effect of reabsorption on the measured W(λ) spectrum and to quantify σSE at long wavelengths where the RM gives high error due to the exponential term in Eq. (1a). A reasonable agreement between the two methods is observed for τrad = 9.8 ± 0.3 ms. To construct the combined SE cross-section spectrum further used for the gain calculations, we have selected the wavelength of 1.88 µm as a borderline. At shorter wavelength, the RM was applied owing to the notable reabsorption and at longer wavelength, we used the F-L equation.
Tm:Lu3Al5O12 features a broad emission spanning from 1.61 to 2.06 µm. The maximum σSE is 2.37×10−21 cm2 at 1749nm (as calculated using the RM). In the long-wavelength spectral range where the laser operation is expected, σSE is lower, namely, 1.03×10−21 cm2 at 2021 nm (the value obtained from the F-L equation).
The 3F4 → 3H6 Tm3+ lasers operate in a quasi-three-level scheme (exhibiting reabsorption). Thus, the gain at a distinct laser wavelength is quantified as σgain = σSE – (1 – β)σabs, where β = N2(3F4)/NTm is the inversion ratio, Fig. 6(b). Physically, an increase of β indicates an increased level of losses in the laser (e.g., high output-coupling) according to the rule “gain equals the losses”. The evolution of the gain spectra with β explains the variation of the laser wavelength when increasing the output coupling ratio. For Tm:Lu3Al5O12, a broad peak at ∼2020nm dominates in the spectra up to at least β = 0.30.
The luminescence decay curve measured at 1950nm (emitting state: 3F4) is shown in Fig. 7. It is single-exponential in agreement with a single type of sites for Tm3+ ions. The luminescence lifetime τlum = 9.68 ms is close to the estimated τrad value (9.8 ± 0.3 ms) which indicates weak non-radiative relaxation and quenching of luminescence from the 3F4 state in the ceramic.
Fig. 7. Luminescence decay curve for the 4.0 at.% Tm:Lu3Al5O12 ceramic: symbols – experimental data; red line – single-exponential fit for the luminescence lifetime τlum.
3.5 Judd-Ofelt analysis
To assign the transitions in the absorption spectra, we calculated the Stark splitting for the Tm3+ multiplets in Lu3Al5O12 using crystal-field parameters reported for bulk crystals [35]. The D2 group has four one-dimensional irreducible representations Гi (i = 1, 2, 3, 4) and each 2S+1LJ multiplet is split into 2J + 1 singlet Stark components. The energy-level scheme is shown in Fig. 8(a). The energies of the Stark sub-levels for all multiplets from 3H6 up to 3P2 are also listed in Table 1 (previously reported only for the upper and lower laser levels).
Fig. 8. Energy-level schemes of Tm3+:Lu3Al5O12: (a) energy-level scheme showing the pump and laser transitions, cross-relaxation (CR), radiative (R) and non-radiative (NR) decays; (b) details of the splitting of the upper and lower laser multiplets; green arrow – ZPL transition. The position of the Stark sub-levels is calculated using the crystal-field parameters from [35].
Table 1. Calculated Stark Splitting of Tm3+ Multiplets in Lu3Al5O12
The details for the 3H6 and 3F4 multiplets are shown in Fig. 8(b). The total splitting of the ground-state (3H6) is rather large (ΔE = 777 cm−1) defining a possible emission above 2 µm.
The 4f-4f transition intensities of Tm3+:Lu3Al5O12 were analyzed using the Judd-Ofelt (J-O) theory [36,37]. The squared reduced matrix elements U(k) in absorption and emission were taken from Ref. [38]. The magnetic-dipole (MD) contributions for J → J’ transitions with ΔJ = 0, ±1 were calculated separately within the Russell-Saunders approximation on wave functions of the Tm3+ ion under the assumption of a free-ion. More details about the calculation procedure for Tm3+ can be found elsewhere [39].
The standard J-O theory and its modification (denoted as the modified Judd-Ofelt (mJ-O) theory) [40,41] accounting for configuration interaction were used. For both approaches, the electric-dipole (ED) line strengths for a J → J’ transition can be written as:
The absorption oscillator strengths were determined from the measured absorption spectra (fΣexp) and calculated using the J-O and mJ-O theories (fΣcalc), as shown in Table 2. Here, the “Σ” superscript stands for the total (ED + MD) value. The latter approach gives much lower root mean square (rms) deviation between fΣexp and fΣcalc. The corresponding set of parameters - (Ωk) for the J-O theory and (Ωk and α) for the mJ-O one are given in Table 3.
Table 2. Experimental and Calculated Absorption Oscillator Strengthsa for Tm:Lu3Al5O12 Ceramic
Table 3. Parameters of the “Standard” and Modified Judd-Ofelt Theories for Thulium Ions in Various Ceramic Materials
A direct application of both J-O and mJ-O theories for the description of the transitions (emission) gives a notably underestimated radiative lifetime of the lower-lying excited state (3F4), namely, τrad = 5.60 ms (J-O) and 6.78 ms (mJ-O). This is a known problem for Tm3+-doped cubic garnets [42].
To support further considerations, we calculated the wave functions of the Tm3+ multiplets with the same total angular momentum quantum number J = 4, namely, 3F4, 3H4 and 1G4, see Table 4. Due to the strong spin-orbit interaction, they form a group of strongly interconnected states [45]. Indeed, the non-diagonal elements in Table 4 are comparable in magnitude to the diagonal ones. Thus, through the 1G4 excited-state of the 4fn configuration, the effect of excited configurations (e.g., 4fn-15d1) is transferred to the lower-lying states, 3F4 and 3H4. In a recent paper [46], this effect was described in terms of the hypersensitivity of the 3H6 → 3F4 Tm3+ transition to the local ligand field.
Table 4. Wave Functions of Tm3+ Multiplets with J = 4 in Lu3Al5O12
To avoid this problem, the mJ-O theory was applied to all the observed Tm3+ transitions except 3H6 → 3F4 to minimize the rms deviation between fΣexp and fΣcalc. The results are shown in Table 2 (the last column) following the same calculation procedure as described above. The corresponding intensity parameters are Ω2 = 2.507, Ω4 = 1.236, Ω6 = 1.340 [10−20 cm2] and α = 0.196×10−4 cm (or Δ = 25510 cm−1).
In Table 3, we also provide for comparison the intensity parameters reported for other Tm3+-doped transparent ceramics, i.e., Tm:Y3Al5O12 [43] and Tm:(Lu,Sc)2O3 [44].
By using such intensity parameters, we calculated the probabilities of spontaneous radiative transitions AΣcalc, the luminescence branching ratios B(JJ’) and the radiative lifetimes of the excited-states τrad for Tm3+ in Lu3Al5O12 ceramic, see Table 5. Such an approach gives good agreement between the radiative lifetime of the 3F4 state calculated with the mJ-O theory (τrad = 10.32 ms), its estimation from the calculation of the SE cross-sections by the RM and F-L methods (τrad = 9.8 ± 0.3 ms), and the reabsorption-free luminescence lifetime (τlum = 9.68 ms), see Section 3.4.
Table 5. Probabilities of the Radiative Spontaneous Transitionsa for Tm:Lu3Al5O12 Ceramic
4. Laser operation
4.1 Laser setup
Laser operation was achieved in a compact plano-plano (microchip-type) cavity, as shown in Fig. 9(a). It was formed by a flat pump mirror (PM) coated for high transmission (HT) at ∼0.79 µm and for high reflection (HR) at 1.80–2.08 µm, and a set of flat output couplers (OCs) with a transmission TOC = 1.5%–20% at the laser wavelength of ∼2.02 µm. The ceramic laser element (t = 3.08 mm, aperture: 5×5 mm2) was kept uncoated. It was mounted in a Cu-holder using In foil for better thermal contact from all four lateral sides. The holder was cooled down to 12 °C by circulating water. Both the PM and OC were placed as close as possible to the laser element resulting in a geometrical cavity length of ∼3.1 mm.
Fig. 9. (a) Scheme of the diode-pumped Tm:Lu3Al5O12 (Tm:LuAG) ceramic laser: LD – laser diode, PM – pump mirror, OC – output coupler, SA – saturable absorber; (b) transmission spectrum of the SWCNT-SA (Fresnel losses are subtracted).
As a pump source, we used a fiber-coupled (core diameter: 105 µm, N.A. = 0.22) AlGaAs laser diode (MTLC00031, BWT Beijing Ltd.) emitting up to 30 W of unpolarized output at 792 nm (3H6 → 3H4 transition of Tm3+, conventional pumping). Its emission bandwidth was 4.4 nm. The pump radiation was reimaged into the laser element through the PM by a lens assembly (1:1 reimaging ratio, focal length f = 30 mm). The OCs were partially reflective at the pump wavelength (R ≈ 40%), so that the pumping was in double-pass and the total pump absorption at the laser threshold ηabs(2-pass) amounted to 33.1 ± 0.5%. The waist of the pump beam in the focus was 2wP ≈ 100 µm.
For passive Q-switching, a transmission-type saturable absorber (SA) was inserted between the laser element and the OC at a minimum separation. It was based on purified arc-discharge single-walled carbon nanotubes (SWCNTs) [47]. The SWCNT/PMMA composite film (thickness: ∼300 nm) was spin-coated onto an uncoated 1-mm-thick glass substrate. In the small-signal transmission spectrum of the SA, Fig. 9(b), a broad absorption band spanning from 1.6 to 2.2 µm is observed. It is assigned to the first fundamental transition of semiconducting nanotubes (E11) with varying tube diameters between 1.5 and 2.2 nm. At the laser wavelength of ∼2.02 µm, the internal small-signal transmission (corrected for the Fresnel losses) TSA was 97.9%. According to our previous studies, the saturation intensity of a similar SWCNT-based SA Isat was 7 ± 1 MW/cm2, the fraction of the saturable losses α'S/α'SA = 0.21 (where α'SA = 1 – TSA), both specified for ns pulse durations [10], and the recovery times of the initial absorption are τrec = 0.25 and 1.16 ps for “fast” and “slow” components, respectively, as determined in a pump-probe experiment [48].
4.2 Continuous-wave laser operation
Continuous-wave laser operation was realized without the SA in the cavity. The input-output power characteristics are shown in Fig. 10(a). The maximum output power reached 3.12 W at 2022-2035 nm with a slope efficiency η of 60.2% (vs. the absorbed pump power, Pabs). The laser threshold was at Pabs = 0.25 W and the optical-to-optical efficiency ηopt was 18.8% (vs. the pump power incident on the crystal). These characteristics were measured for TOC = 5%. Using TOC > 10%, the output deteriorated probably due to increased upconversion losses and energy-migration to impurities associated with high inversion in the gain medium. Although such effects do not affect the slope efficiency directly, they cause stronger heat loading. For low output coupling (TOC ≤ 5%), a thermal roll-over was observed for Pabs > 5.5 W. The power scaling was limited to avoid fracture of the ceramic sample. The laser operated at the fundamental transverse mode, see the inset in Fig. 10(a).
Fig. 10. Continuous-wave (CW) diode-pumped Tm:Lu3Al5O12 ceramic laser: (a) input-output dependences, η – slope efficiency, inset – spatial profile of the laser beam (TOC = 5%, Pabs = 5.3 W); (b) typical laser emission spectra measured at Pabs = 3.0 W.
The laser output was unpolarized. The laser emission spectra are shown in Fig. 10(b). They are only weakly dependent on the output coupling, the emission occurred at ∼2.02 µm in agreement with the gain spectra, cf. Figure 6(b).
4.3 Passive Q-switching
Q-switching was realized by inserting the SWCNT-SA into the cavity. There existed an upper limit of stable passively Q-switched (PQS) operation, which is ascribed to unwanted heating of the SA by the residual (non-absorbed) pump leading to deterioration of its nonlinear properties. According to the stability of Q-switched operation and the pulse characteristics, a low output coupling was selected (TOC = 3%). The PQS laser delivered a stable pulse train up to Pabs ∼3.5 W. Above this pump level, irregular multi-pulse behavior was observed. The laser output was unpolarized. The laser performance is presented in Fig. 11.
Fig. 11. Diode-pumped SWCNT-SA PQS Tm:Lu3Al5O12 ceramic laser: (a) input-output dependence, η – slope efficiency; (b) typical laser emission spectrum; (c) pulse duration (FWHM), pulse energy and pulse repetition frequency (PRF) and (d) typical oscilloscope traces of a single Q-switched pulse and the pulse train both measured at Pabs = 2.6 W and TOC = 3%. (a,b) contain CW regime data for comparison.
The average output power of the PQS laser Pout reached 0.85 W at 2023 nm with η = 27.8%, Fig. 11(a). With respect to the CW performance, the Q-switching conversion efficiency is reduced to ηconv = 43.6%. This value is mainly determined by the insertion loss of the SA (Fresnel losses and non-saturable losses of the SWCNT / PMMA film). The laser threshold increased to Pabs = 0.95 W for the same reason. The laser emission spectrum, Fig. 11(b), became narrower as compared to the CW operation mode, because of the stronger mode competition while bleaching the SA.
The pulse duration (FWHM), Δτ, and the pulse repetition frequency (PRF), were measured directly, and the energy of a single Q-switched pulse was calculated as Eout = Pout/PRF, see Fig. 11(c). The pulse characteristics (duration and energy) were dependent on the pump level which is typical for “fast” SAs such as carbon nanostructures (graphene, SWCNTs) [49]. The latter is explained by the different degree of SA bleaching, which depends on the intracavity intensity. With an increase of the pump power, Δτ shortened from 322 to 256 ns and Eout increased from 4.8 to 5.6 µJ. The PRF increased nearly linearly, from 37 to 151 kHz. As a result, the highest peak power Ppeak = Eout/Δτ amounted to 22 W.
A typical oscilloscope trace of a single Q-switched pulse is shown in Fig. 11(d). It has a nearly Gaussian temporal shape, revealing a nearly-optimum ratio between the SA modulation depth and the output coupling. The pulse train exhibited relatively weak intensity instabilities, below 15%, as shown in Fig. 11(d). They are attributed to thermal effects in the SA [50].
5. Conclusion
Thulium-doped Lu3Al5O12 transparent ceramics are promising for continuous-wave, Q-switched and mode-locked lasers at ∼2 µm. As compared to their yttrium counterpart, Tm:Y3Al5O12, they are expected to offer better thermal properties, easier Tm3+ doping and advantageous spectroscopic properties (in particular, slightly longer emission wavelength of ∼2.02 µm). In the present work, we demonstrated efficient laser operation of a Tm:Lu3Al5O12 transparent ceramic. A diode-pumped continuous-wave ceramic laser delivered up to 3.12 W at 2022-2035 nm with a slope efficiency of 60.2% (vs. the absorbed pump power), which is attributed to the good optical quality of the ceramic and the efficient cross-relaxation for Tm3+ ions. Moreover, we characterized the spectroscopic properties of Tm3+ in the fabricated Lu3Al5O12 ceramic. A method to describe correctly the transition intensities from the lower-lying multiplets with J = 4 (3F4) is proposed. For power scaling and improvement of the slope efficiency, it would be promising to fabricate ceramics with even higher Tm doping (>5 at.%) leading to higher pump absorption and more efficient cross-relaxation among Tm3+ ions. On the other hand, high doping levels may lead to a decrease of the optical quality and thermal conductivity of ceramics. Thus, a search of an optimum Tm doping is needed. For mode-locked applications, it seems promising to fabricate compositionally “mixed” (Lu,Y)3Al5O12 ceramics featuring inhomogeneous broadening of the emission spectra.
Funding
Ministerio de Economía y Competitividad (MAT2016-75716-C2-1-R (AEI/FEDER, UE)); Agència de Gestió d'Ajuts Universitaris i de Recerca (2017SGR755); Horizon 2020 Framework Programme (grant agreement No. 739573); European Regional Development Fund (Grant No. CZ.02.1.01/0.0/0.0/15_006/0000674); Ministerstvo Školství, Mládeže a Tělovýchovy (Large Research Infras. Project No. LM2015086, NPU I Project No. LO1602); Chinese Academy of Sciences Key Project (QYZDB-SSW-JSC022).
Acknowledgements
P.L. thanks Dr. Olga Dymshits (St. Petersburg, Russia) for the help with the XRD study.
Disclosures
The authors declare no conflicts of interest.
References
1. A. Ikesue, T. Kinoshita, K. Kamata, and K. Yoshida, “Fabrication and optical properties of high-performance polycrystalline Nd:YAG ceramics for solid-state lasers,” J. Am. Ceram. Soc. 78(4), 1033–1040 (1995). [CrossRef]
2. J. Lu, K. I. Ueda, H. Yagi, T. Yanagitani, Y. Akiyama, and A. A. Kaminskii, “Neodymium doped yttrium aluminum garnet (Y3Al5O12) nanocrystalline ceramics – a new generation of solid state laser and optical materials,” J. Alloys Compd. 341(1-2), 220–225 (2002). [CrossRef]
3. H. Yagi, T. Yanagitani, K. Takaichi, K. I. Ueda, and A. A. Kaminskii, “Characterizations and laser performances of highly transparent Nd3+:Y3Al5O12 laser ceramics,” Opt. Mater. 29(10), 1258–1262 (2007). [CrossRef]
4. A. Ikesue and Y. L. Aung, “Ceramic laser materials,” Nat. Photonics 2(12), 721–727 (2008). [CrossRef]
5. J. Lu, M. Prabhu, J. Song, C. Li, J. Xu, K. Ueda, A. A. Kaminskii, H. Yagi, and T. Yanagitani, “Optical properties and highly efficient laser oscillation of Nd:YAG ceramics,” Appl. Phys. B 71(4), 469–473 (2000). [CrossRef]
6. K. Takaichi, H. Yagi, J. E. A. Lu, A. Shirakawa, K. Ueda, T. Yanagitani, and A. A. Kaminskii, “Yb3+-doped Y3Al5O12 ceramics – A new solid-state laser material,” Phys. Status Solidi A 200(1), R5–R7 (2003). [CrossRef]
7. J. Dong, A. Shirakawa, K. Ueda, H. Yagi, T. Yanagitani, and A. A. Kaminskii, “Laser-diode pumped heavy-doped Yb:YAG ceramic lasers,” Opt. Lett. 32(13), 1890–1892 (2007). [CrossRef]
8. W. X. Zhang, Y. B. Pan, J. Zhou, W. B. Liu, J. Li, B. X. Jiang, X. J. Cheng, and J. Q. Xu, “Diode-pumped Tm:YAG ceramic laser,” J. Am. Ceram. Soc. 92(10), 2434–2437 (2009). [CrossRef]
9. H. Chen, D. Shen, J. Zhang, H. Yang, D. Tang, T. Zhao, and X. Yang, “In-band pumped highly efficient Ho:YAG ceramic laser with 21 W output power at 2097nm,” Opt. Lett. 36(9), 1575–1577 (2011). [CrossRef]
10. R. Lan, P. Loiko, X. Mateos, Y. Wang, J. Li, Y. Pan, S. Y. Choi, M. H. Kim, F. Rotermund, A. Yasukevich, K. Yumashev, U. Griebner, and V. Petrov, “Passive Q-switching of microchip lasers based on Ho:YAG ceramics,” Appl. Opt. 55(18), 4877–4887 (2016). [CrossRef]
11. V. Petrov, “Frequency down-conversion of solid-state laser sources to the mid-infrared spectral range using non-oxide nonlinear crystals,” Prog. Quantum Electron. 42, 1–106 (2015). [CrossRef]
12. E. C. Honea, R. J. Beach, S. B. Sutton, J. A. Speth, S. C. Mitchell, J. A. Skidmore, M. A. Emanuel, and S. A. Payne, “115-W Tm:YAG diode-pumped solid-state laser,” IEEE J. Quantum Electron. 33(9), 1592–1600 (1997). [CrossRef]
13. R. C. Stoneman and L. Esterowitz, “Efficient, broadly tunable, laser-pumped Tm:YAG and Tm:YSGG cw lasers,” Opt. Lett. 15(9), 486–488 (1990). [CrossRef]
14. Z. Pan, Y. Wang, Y. Zhao, H. Yuan, X. Dai, H. Cai, J. E. Bae, S. Y. Choi, F. Rotermund, X. Mateos, J. M. Serres, P. Loiko, U. Griebner, and V. Petrov, “Generation of 84-fs pulses from a mode-locked Tm:CNNGG disordered garnet crystal laser,” Photonics Res. 6(8), 800–804 (2018). [CrossRef]
15. Y. Zou, Z. Wei, Q. Wang, M. Zhan, D. Li, Z. Zhang, J. Zhang, and D. Tang, “High-efficiency diode-pumped Tm: YAG ceramic laser,” Opt. Mater. 35(4), 804–806 (2013). [CrossRef]
16. Y. Wang, D. Shen, H. Chen, J. Zhang, X. Qin, D. Tang, X. Yang, and T. Zhao, “Highly efficient Tm:YAG ceramic laser resonantly pumped at 1617 nm,” Opt. Lett. 36(23), 4485–4487 (2011). [CrossRef]
17. W. L. Gao, J. Ma, G. Q. Xie, J. Zhang, D. W. Luo, H. Yang, D. Y. Tang, J. Ma, P. Yuan, and L. J. Qian, “Highly efficient 2 µm Tm:YAG ceramic laser,” Opt. Lett. 37(6), 1076–1078 (2012). [CrossRef]
18. A. Gluth, Y. Wang, V. Petrov, J. Paajaste, S. Suomalainen, A. Härkönen, M. Guina, G. Steinmeyer, X. Mateos, S. Veronesi, M. Tonelli, J. Li, Y. Pan, J. Guo, and U. Griebner, “GaSb-based SESAM mode-locked Tm:YAG ceramic laser at 2 µm,” Opt. Express 23(2), 1361–1369 (2015). [CrossRef]
19. Y. Fu, J. Li, C. Wang, T. Xie, W. Li, L. Wu, and Y. Pan, “Fabrication and properties of highly transparent Yb:LuAG ceramics,” J. Alloys Compd. 664, 595–601 (2016). [CrossRef]
20. H. Nakao, A. Shirakawa, K. Ueda, H. Yagi, and T. Yanagitani, “CW and mode-locked operation of Yb3+-doped Lu3Al5O12 ceramic laser,” Opt. Express 20(14), 15385–15391 (2012). [CrossRef]
21. N. P. Barnes, M. G. Jani, and R. L. Hutcheson, “Diode-pumped, room-temperature Tm:LuAG laser,” Appl. Opt. 34(21), 4290–4294 (1995). [CrossRef]
22. K. Scholle, E. Heumann, and G. Huber, “Single mode Tm and Tm,Ho:LuAG lasers for LIDAR applications,” Laser Phys. Lett. 1(6), 285–290 (2004). [CrossRef]
23. Y. Wang, R. Lan, X. Mateos, J. Li, C. Li, S. Suomalainen, A. Härkönen, M. Guina, V. Petrov, and U. Griebner, “Thulium doped LuAG ceramics for passively mode locked lasers,” Opt. Express 25(6), 7084–7091 (2017). [CrossRef]
24. Z. Zhou, X. Huang, X. Guan, J. Lan, B. Xu, H. Xu, Z. Cai, P. Liu, D. Yan, X. Xu, J. Zhang, M. Lei, and J. Xu, “Continuous-wave and passively Q-switched Tm3+-doped LuAG ceramic lasers,” Opt. Mater. Express 7(9), 3441–3447 (2017). [CrossRef]
25. D. Luo, J. Zhang, C. Xu, H. Yang, H. Lin, H. Zhu, and D. Tang, “Yb:LuAG laser ceramics: a promising high power laser gain medium,” Opt. Mater. Express 2(10), 1425–1431 (2012). [CrossRef]
26. L. Basyrova, R. Maksimov, V. Shitov, M. Baranov, V. Mikhaylovsky, A. Khubetsov, O. Dymshits, X. Mateos, and P. Loiko, “Effect of SiO2 addition on structural and optical properties of Yb:Lu3Al5O12 transparent ceramics based on laser ablated nanopowders,” J. Alloys Compd. 806, 717–725 (2019). [CrossRef]
27. F. Euler and J. A. Bruce, “Oxygen coordinates of compounds with garnet structure,” Acta Crystallogr. 19(6), 971–978 (1965). [CrossRef]
28. J. B. Gruber, M. E. Hills, R. M. Macfarlane, C. A. Morrison, G. A. Turner, G. J. Quarles, G. J. Kintz, and L. Esterowitz, “Spectra and energy levels of Tm3+:Y3Al5O12,” Phys. Rev. B 40(14), 9464–9478 (1989). [CrossRef]
29. R. D. Shannon, “Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides,” Acta Crystallogr. 32(5), 751–767 (1976). [CrossRef]
30. A. A. Kaminskii, H. Rhee, O. Lux, H. J. Eichler, S. N. Bagayev, H. Yagi, K. Ueda, A. Shirakawa, and J. Dong, “Stimulated Raman scattering in “garnet” Lu3Al5O12 ceramics – a novel host-material for Ln-and TM-lasant ions,” Laser Phys. Lett. 8(6), 458–464 (2011). [CrossRef]
31. K. Papagelis and S. Ves, “Infrared spectroscopy and lattice dynamical calculations of Gd3Al5O12, Tb3Al5O12 and Lu3Al5O12 single crystals,” J. Phys. Chem. Solids 64(4), 599–605 (2003). [CrossRef]
32. Y. Kuwano, K. Suda, N. Ishizawa, and T. Yamada, “Crystal growth and properties of (Lu,Y)3Al5O12,” J. Cryst. Growth 260(1-2), 159–165 (2004). [CrossRef]
33. S. A. Payne, L. L. Chase, L. K. Smith, W. L. Kway, and W. F. Krupke, “Infrared cross-section measurements for crystals doped with Er3+, Tm3+, and Ho3+,” IEEE J. Quantum Electron. 28(11), 2619–2630 (1992). [CrossRef]
34. B. Aull and H. Jenssen, “Vibronic interactions in Nd:YAG resulting in nonreciprocity of absorption and stimulated emission cross sections,” IEEE J. Quantum Electron. 18(5), 925–930 (1982). [CrossRef]
35. C. A. Morrison, E. D. Filer, and N. P. Barnes, “Energy levels and branching ratios of Tm3+ in ten garnet laser materials,” report No. HDL-TR-2196, Harry Diamond Labs Adelphi MD (1991).
36. B. R. Judd, “Optical absorption intensities of rare-earth ions,” Phys. Rev. 127(3), 750–761 (1962). [CrossRef]
37. G. S. Ofelt, “Intensities of crystal spectra of rare-earth ions,” J. Chem. Phys. 37(3), 511–520 (1962). [CrossRef]
38. B. M. Walsh, N. P. Barnes, and B. Di Bartolo, “Branching ratios, cross sections, and radiative lifetimes of rare earth ions in solids: Application to Tm3+ and Ho3+ ions in LiYF4,” J. Appl. Phys. 83(5), 2772–2787 (1998). [CrossRef]
39. L. Zhang, H. Lin, G. Zhang, X. Mateos, J. M. Serres, M. Aguiló, F. Díaz, U. Griebner, V. Petrov, Y. Wang, P. Loiko, E. Vilejshikova, K. Yumashev, Z. Lin, and W. Chen, “Crystal growth, optical spectroscopy and laser action of Tm3+-doped monoclinic magnesium tungstate,” Opt. Express 25(4), 3682–3693 (2017). [CrossRef]
40. A. A. Kornienko, A. A. Kaminskii, and E. B. Dunina, “Dependence of the line strength of f–f transitions on the manifold energy. II. Analysis of Pr3+ in KPrP4O12,” Phys. Status Solidi B 157(1), 267–273 (1990). [CrossRef]
41. P. Loiko, A. Volokitina, X. Mateos, E. Dunina, A. Kornienko, E. Vilejshikova, M. Aguilo, and F. Diaz, “Spectroscopy of Tb3+ ions in monoclinic KLu(WO4)2 crystal: application of an intermediate configuration interaction theory,” Opt. Mater. 78, 495–501 (2018). [CrossRef]
42. J. T. Thomas, M. Tonelli, S. Veronesi, E. Cavalli, X. Mateos, V. Petrov, U. Griebner, J. Li, Y. Pan, and J. Guo, “Optical spectroscopy of Tm3+:YAG transparent ceramics,” J. Phys. D: Appl. Phys. 46(37), 375301 (2013). [CrossRef]
43. B. J. Fei, J. Q. Huang, W. Guo, Q. F. Huang, J. Chen, F. Tang, W. C. Wang, and Y. G. Cao, “Spectroscopic properties and laser performance of Tm:YAG ceramics,” J. Lumin. 142, 189–195 (2013). [CrossRef]
44. W. Jing, P. Loiko, J. M. Serres, Y. Wang, E. Vilejshikova, M. Aguiló, F. Díaz, U. Griebner, H. Huang, V. Petrov, and X. Mateos, “Synthesis, spectroscopy, and efficient laser operation of “mixed” sesquioxide Tm:(Lu,Sc)2O3 transparent ceramics,” Opt. Mater. Express 7(11), 4192–4202 (2017). [CrossRef]
45. E. B. Dunina, L. A. Fomicheva, A. A. Kornienko, and M. V. Grigoreva, “Effect of configuration interaction of rare-earth ion states on the intensity of intermultiplet transitions,” J. Appl. Spectrosc. 85(3), 407–415 (2018). [CrossRef]
46. P. A. Ryabochkina, S. A. Antoshkina, E. V. Bolshakova, M. A. Ivanov, V. V. Kochurihin, A. V. Malov, S. N. Ushakov, N. V. Shchuchkina, and K. N. Nishchev, “Hypersensitive transitions of Tm3+, Ho3+ and Dy3+ rare-earth ions in garnet crystals,” J. Lumin. 132(8), 1900–1905 (2012). [CrossRef]
47. P. Loiko, X. Mateos, S. Y. Choi, F. Rotermund, J. M. Serres, M. Aguiló, F. Díaz, K. Yumashev, U. Griebner, and V. Petrov, “Vibronic thulium laser at 2131 nm Q-switched by single-walled carbon nanotubes,” J. Opt. Soc. Am. B 33(11), D19–D27 (2016). [CrossRef]
48. W. B. Cho, A. Schmidt, J. H. Yim, S. Y. Choi, S. Lee, F. Rotermund, U. Griebner, G. Steinmeyer, V. Petrov, X. Mateos, M. C. Pujol, J. J. Carvajal, M. Aguiló, and F. Díaz, “Passive mode-locking of a Tm-doped bulk laser near 2 µm using a carbon nanotube saturable absorber,” Opt. Express 17(13), 11007–11012 (2009). [CrossRef]
49. A. S. Yasukevich, P. Loiko, N. V. Gusakova, J. M. Serres, X. Mateos, K. V. Yumashev, N. V. Kuleshov, V. Petrov, U. Griebner, M. Aguiló, and F. Díaz, “Modeling of graphene Q-switched Tm lasers,” Opt. Commun. 389, 15–22 (2017). [CrossRef]
50. J. M. Serres, P. Loiko, X. Mateos, K. Yumashev, U. Griebner, V. Petrov, M. Aguiló, and F. Díaz, “Tm:KLu(WO4)2 microchip laser Q-switched by a graphene-based saturable absorber,” Opt. Express 23(11), 14108–14113 (2015). [CrossRef]
