A novel trigonal ordered langasite-type silicate crystal, Yb3+:Ca3NbGa3Si2O14 (Yb:CNGS), is characterized with respect to absorption and stimulated-emission cross-sections as well as Raman spectra using polarized light. Thanks to its excellent thermo-mechanical properties, highly-efficient multi-watt lasing is demonstrated. A compact a-cut 5 at .% Yb:CNGS crystal diode-pumped at 978 nm generated a maximum output power of 7.27 W at 1062-1068 nm with a slope efficiency of 78%. These values represent record parameters for ~1 μm lasers based on langasite-type crystals. The very broad and smooth gain cross sections of Yb:CNGS are attractive for ultrashort pulse and broadly tunable lasers at ~1 μm.
© 2017 Optical Society of America
Silicates are known as very suitable laser host crystals for doping with trivalent rare-earth ions (RE3+). A well-known example is the family of monoclinic RE2SiO5 silicates that allows for the incorporation of Nd3+, Yb3+, Er3+ or Tm3+ leading to efficient laser operation in the near-IR [1,2]. There exists a different class of langasite-type trigonal silicate crystals. Lanthanum gallium silicate (langasite) La3Ga5SiO14 (LGS) , and calcium niobium gallium silicate, Ca3NbGa3Si2O14 (CNGS)  belong to this class. These crystals are known for their good elastic and thermal properties and they were studied as piezoelectric materials . LGS has a disordered structure while CNGS is an ordered crystal that is mechanically stronger and possesses better elastic properties than LGS . CNGS exhibits weak thermal expansion (αa = 5.7 and αc = 5.5 × 10−6 K−1), high specific heat (Cp = 0.83 J/gK) and moderate thermal conductivity (κ = 1.82 W/mK) . Therefore, it is very interesting for RE3+ doping concerning power-scaling. The acentric (point group 32) CNGS crystals are suitable for self-frequency-doubling (SFD) .
To date, only few studies were devoted to RE3+-doped LGS and CNGS crystal lasers focusing mostly on Nd3+ ions and reporting modest output characteristics. Continuous-wave (CW) and passively Q-switched (PQS) Nd:LGS lasers were demonstrated [7,8]. Later on, the spectroscopic, elastic and laser properties of Nd:CNGS crystal were described [9,10]. The Nd:CNGS laser generated 1.63 W at 1065 nm with a slope efficiency η of 31%. Mode-locking of a Nd:CNGS laser was also demonstrated very recently .
Ytterbium (Yb3+) doping is very suitable for highly-efficient and power-scalable lasers at ~1 μm. This is due to the simple energy-level scheme of the Yb3+ ion eliminating parasitic mechanisms such as excited-state absorption and upconversion. Yb3+ ions are typically excited at the 2F7/2 → 2F5/2 transition by commercial InGaAs laser diodes emitting at ~960-980 nm leading to a low quantum defect. When embedded in anisotropic matrices, the Yb3+ ions offer broad spectral bands leading to wavelength-tunable laser emission . Yb3+-doped CNGS crystals have been grown only very recently . Besides the crystal growth, this first work reported on the thermal properties, preliminary laser operation and SFD characterization.
In the present paper, we demonstrate the potential of Yb:CNGS for highly-efficient multi-watt CW laser operation based on a polarization-resolved study of the spectroscopic properties.
2. Crystal growth and spectroscopy
CNGS melts congruently at ~1350 °C . High optical quality 1 at.%, 3 at.% and 5 at.% Yb:CNGS crystals were grown by the Czochralski pulling method in a N2 + 1 vol% O2 atmosphere (in order to avoid formation of color centers related to the oxygen defects) using -oriented CNGS seeds. The as-grown and polished 5 at.% Yb:CNGS crystal is shown in Fig. 1(a). It had a slight yellow coloration due to the weak absorption band at ~460 nm related to the residual O defects, Fig. 1(b), which can be partially removed by a proper annealing in air, see inset in Fig. 1(b).
The structure and phase purity of the crystals were confirmed by X-ray powder diffraction (XRD). Yb:CNGS is trigonal (sp. gr. P321, lattice constants a = b = 8.0779 Å, c = 4.9750 Å) and belongs to the family of A3BGa3Si2O14 crystals where A = Ca or Sr and B = Ta or Nb with a langasite-type but ordered structure . In the CNGS lattice, Fig. 2(a), the Ca2+ ions are located in a twisted Thomson cube (3e Wyckoff position, VIII-fold O2-–coordination, ionic radius RCa = 1.12 Å) with relatively large Ca2+ – O2- distances, 2.22 – 2.83 Å . The edge-sharing [CaO8] polyhedra form isolated planes parallel to the a-b plane linked by [GaO4] and [SiO4] tetrahedra. The Yb3+ ions (RYb = 0.985 Å) in CNGS will replace the Ca2+ ones providing a certain number of A vacancies to maintain the charge compensation [6,13]. Small Yb3+ ions in the A sites will further stabilize the CNGS structure .
CNGS is an optically uniaxial crystal. Its optical axis is parallel to the c-axis. The principal refractive indices, no = 1.772 and ne = 1.855 at 1.02 µm  (positive uniaxial crystal). The two principal light polarizations for Yb:CNGS are E || c (π) and E ⊥ c (σ).
The structure of Yb:CNGS was further studied by Raman spectroscopy. The polarized Raman spectra of a 5 at.% Yb:CNGS crystal for the , , and geometries are shown in Fig. 2(b). They were measured with a Renishaw inVia confocal Raman microscope, the excitation wavelength λexc = 514 nm. The standard notations are used (m and l are the propagation directions of the excitation and scattered light, and m ≡ l for the confocal geometry used, n and k are the polarizations of the excitation and scattered light, respectively). The spectra agree well with those for the undoped CNGS . At the center point (Г) of the first Brillouin zone, the irreducible representations can be written as Г = 10A1 + 13A2 + 23E, of which 1A2 + 1E modes are acoustic, 10A1 modes are Raman-active, 12A2 ones are IR-active and the rest of 22E ones are either Raman or IR-active. A total of 15 modes are clearly resolved in Fig. 2(b). The maximum phonon frequency of Yb:CNGS is 988 cm−1 (A1) and it is assigned to the Si-O stretching vibrations. The most intense bands in the Raman spectra are located at 583 (A1), 623 (A1) and 786 (A1) cm−1 and are related to the O-Si-O bending, O-Ga-O stretching and Si-O stretching vibrations, respectively.
At first, we studied the spectroscopic properties of a 5 at.% Yb:CNGS crystal. The Yb3+ doping concentartion NYb was calculated as 5.3 × 1020 at/cm3 assuming a segregation coefficient KYb ≈1 and a density ρ = 4.17 g/cm3 (measured with a hydrostatic method). The absorption spectra were measured using a Varian CARY-5000 spectrophotometer; the spectral bandwidth (SBW) was 0.3 nm. The absorption cross-sections, σabs, for π and σ-polarizations are shown in Fig. 3(a). A specific feature of the absorption spectrum is the presence of two closely located peaks centered at 977.2 and 979.1 nm and clearly resolved even at RT. It was not reported in  due to the low resolution spectral measurements. According to the low-temperature (6 K) spectroscopy (unpublished), they are related to the transitions from two closely located Stark sub-levels of the 2F7/2 ground-state (transitions 0 → 0' (zero-phonon line, ZPL), and 1 → 0', respectively). The maximum σabs is 1.3 × 10−20 cm−2 at 979.1 nm and the total full width at half maximum (FWHM) of the absorption peak around the ZPL is 4.2 nm (for σ-polarization).
The stimulated-emission cross-sections, σSE, were calculated with the modified reciprocity method :Fig. 3(a).
The Yb:CNGS crystal exhibits a strong polarization-anisotropy of the σSE spectra which is a prerequisite for the linearly polarized laser output. The larger σSE is observed for σ-polarization: the maximum is σSE = 2.5 × 10−20 cm−2 at 979.1 nm and in the spectral range of highest gain cross section at ~1018 nm, it is 0.97 × 10−20 cm−2. This is lower than the estimation from  due to the low resolution spectral measurements not accounting for the reabsorption effect on the emission spectra .
The Yb3+ ion represents a quasi-three-level laser scheme and the emission wavelength is determined by the level of inversion, β = N2(2F5/2)/NYb, in accordance with the gain cross-section, σg = βσSE – (1 – β)σabs, spectra, as shown in Fig. 3(b) for σ-polarization. For small β < 0.15, the gain spectrum is smooth and broad with a potential bandwidth of >80 nm. Increasing β, a local peak is formed in the spectrum centered at ~1018 nm (β > 0.15). Thus, this material may be of interest for broadband tunable and ultrashort pulse lasers. The luminescence decay curve was measured from an edge of a 1 at.% Yb:CNGS crystal (thickness: 3 mm) to avoid reabsorption effect and it is shown in Fig. 4. It is clearly single-exponential; the decay time τlum is 0.71 ms.
3. Microchip laser operation
The laser experiments were performed in a compact (microchip type) set-up . The uncoated crystal was mounted in a Cu-holder providing cooling from all four lateral sides and Indium foil ensured good thermal contact. The holder was water-cooled down to 14 °C. The plano-plano cavity consisted of a pump mirror (PM) antireflection (AR)-coated for 0.88-0.99 μm and high-reflection (HR)-coated for 1.01–1.23 μm, and an output coupler (OC) providing transmittance TOC = 0.5%, 1%, 5% or 10% for 1.02–1.1 μm. Both PM and OC were located as close as possible to the polished crystal faces, i.e. with minimum air gaps. Hence the geometrical cavity length was almost equal to the thickness of the Yb:CNGS sample.
As pump source, we used an InGaAs fiber coupled laser diode (fiber core diameter: 200 μm; numerical aperture N.A.: 0.22) emitting up to ~20 W at ~978 nm. The unpolarized pump radiation (M2 ~71) was collimated and focused into the crystal using a lens assembly (1:1 imaging ratio, focal length: 30 mm) resulting in a pump spot radius wp of ~100 μm and a Rayleigh length of 2zR = 1.65 mm (in the crystal). All four OCs provided partial reflection at the pump wavelength (~90%), so the crystal was pumped in a double-pass. The total pump absorption under lasing conditions (Abs = Pabs/Pinc, where Pabs and Pinc are the absorbed and incident pump power, respectively) was determined from the small-signal pump-transmission measurements and rate-equation modelling .
In all laser experiments, we studied a-cut samples in order to maintain linearly polarized laser output. At first, we studied the effect of the output coupling on the laser performance, using a 3 mm-thick 3 at.% Yb:CNGS crystal that provided an Abs of ~41%, see Fig. 5(a). The best performance was observed for TOC = 1%, namely 3.16 W at 1054-1066 nm with a slope efficiency η of 80% (with respect to Pabs). The laser threshold was at Pabs = 1.5 W and the optical-to-optical efficiency (with respect to Pinc) ηopt was 22%. For TOC = 5%, the laser performance only slightly deteriorated (η = 79%) and for TOC = 10%, the increase of the laser threshold and the drop of η were noticeable. This is attributed to upconversion losses arising from both Tm3+ impurities present in the Yb2O3 reagent and Yb3+-Yb3+ ion pairs. Indeed, the intensity of the blue (~480 nm) upconversion in the Yb3+-Tm3+ system increased with TOC. The laser output was linearly polarized (σ).
The typical laser emission spectra are shown in Fig. 5(b). With the increase of TOC (outcoupling losses), a blue-shift of the emission wavelength is observed in accordance with the σg spectra, Fig. 3(b). Remarkably, for TOC = 10%, laser oscillation in two spectral ranges, 1015-1019 nm and 1040-1047 nm, was observed, corresponding to two local maxima in the gain spectra. The multi-peak spectral behavior is due to the etalon effects arising from the small air gaps between crystal and resonator mirrors and is typical for microchip-type lasers.
Several Yb:CNGS samples with different Yb3+ doping and thickness t were studied using TOC = 1%, see Fig. 6. A summary of the output characteristics of these crystals and the corresponding values of Abs are listed in Table 1. The laser output in all cases was σ-polarized.
The maximum output power was achieved with a 4 mm-thick 5 at.% Yb:CNGS crystal, namely 7.27 W at 1062-1068 nm with η = 78%. The laser threshold was at Pabs = 1.7 W and ηopt = 39%. Even higher slope efficiency (η = 84%) at lower threshold (Pabs = 1.0 W) was reached with a 3 mm-thick 1 at.% Yb:CNGS sample but the output lower (1.52 W) was limited by low absorption. In general, the increase of Yb3+ doping leads to the decrease of η due to increased internal losses in Yb:CNGS. These losses are caused by stronger lattice distortion with the Yb3+ incorporation. The internal losses in Yb:CNGS were estimated from the output performance of a 4 mm-thick 5 at.% Yb3+-doped crystal delivering the maximum output power, see Fig. 6(a), using a model of a quasi-three-level lasers , resulting in δ = 0.004 ± 0.002 cm−1.
The Yb:CNGS lasers generated a nearly circular output laser beam (M2x,y < 1.15) for the whole range of studied Pabs, see Fig. 7. This is promoted by the weak anisotropy of thermal expansion, αa/αc = 1.04 . The input-output dependences (Fig. 5 and Fig. 6) are clearly linear indicating no detrimental thermal effects. No damage of the crystal is observed in our experiments up to, at least, Pabs = 11.2 W. This is a remarkable result when considering the modest κ of this crystal which is lower even compared to the monoclinic Yb:KLuW and Yb:YCOB [16,18]. This is an indication of high stress fracture limit of CNGS.
Laser operation with an a-cut Yb:CNGS crystal in the microchip laser cavity indicates a positive thermal lens for this crystal cut. A direct measurement of the thermo-optic coefficient, dn/dT, of Yb:CNGS for σ-polarization also indicated a positive value, namely dno/dT = 4.0 × 10−6 K−1 at ~1 µm (unpublished).
In conclusion, we report on highly-efficient (slope efficiency ~80%) multi-watt diode-pumped CW lasers based on a novel trigonal ordered silicate crystal, Yb:CNGS. Using a 5 at.% Yb:CNGS, a maximum output power of 7.27 W is extracted at 1062-1068 nm with η = 78%. These are record parameters for ~1 μm lasers based on this class of materials. CNGS offers relatively high Yb3+ doping levels, high transition cross-sections, and broad spectral bands. Moreover, a combination of the appropriate thermal and elastic characteristics makes Yb:CNGS a promising material for power-scalable lasers, in particular in microchip-like cavities due to its positive thermal lens . Taking into account the long upper-laser-level lifetime of Yb3+ ions (0.71 ms), this crystal is suitable for PQS lasers employing Cr:YAG or V:YAG as saturable absorbers. By using a special crystal cut for type I or II phase-matching, self-frequency doubled lasers seems to be feasible . Tunable and ultrashort pulse lasers at ~1 µm can benefit from the broad and smooth gain cross section of Yb:CNGS.
Spanish Government (MAT2016-75716-C2-1-R, (AEI/FEDER,UE), MAT2013-47395-C4-4-R, TEC 2014-55948-R); Generalitat de Catalunya (2014SGR1358); National Natural Science Foundation of China (51472147, 61178060, 51672161).
F.D. acknowledges additional support through the ICREA academia award 2010ICREA-02 for excellence in research. X.M. acknowledges support from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 657630. P.L. acknowledges financial support from the Government of the Russian Federation (Grant 074-U01) through ITMO Post-Doctoral Fellowship scheme.
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