Open Access
Add to BibSonomy
Abstract
We report on sub-50 fs pulse generation from a passively mode-locked Yb:SrF2 laser pumped with a spatially single-mode, fiber-coupled laser diode at 976 nm. In the continuous-wave regime, the Yb:SrF2 laser generated a maximum output power of 704 mW at 1048 nm with a threshold of 64 mW and a slope efficiency of 77.2%. A continuous wavelength tuning across 89 nm (1006 – 1095 nm) was achieved with a Lyot filter. By implementing a SEmiconductor Saturable Absorber Mirror (SESAM) for initiating and sustaining the mode-locked operation, soliton pulses as short as 49 fs were generated at 1057 nm with an average output power of 117 mW at a pulse repetition rate of ∼75.9 MHz. The maximum average output power of the mode-locked Yb:SrF2 laser was scaled up to 313 mW for slightly longer pulses of 70 fs at 1049.4 nm, corresponding to a peak power of 51.9 kW and an optical efficiency of 34.7%.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Strontium fluoride (SrF2, called strontiofluorite in the mineral form) is a cubic (sp. gr. $Fm\bar{3}m$) crystal belonging to the family of alkaline earth fluorides M2 + F2 (where M = Ca, Sr, Ba or their mixture) with fluorite (CaF2) structure. It features wide band gap (∼9.7 eV), broad transparency (0.13 – 10 µm), and low refractive index (n = 1.481 at ∼1 µm) [1], occupying an intermediate place between its CaF2 and BaF2 counterparts, and finds applications in UV optical components and thermo-luminescent dosimeters. Due to its congruent melting and a relatively low melting point (about 1477°C), the growth of large volume SrF2 crystals by the Bridgman-Stockbarger / Czochralski methods is well developed. SrF2 is a somewhat brittle material being sensitive to thermal shock.
SrF2 is a suitable laser host material for doping with rare-earth ions (RE3+) [2–5]. It benefits from high thermal conductivity (κ = 9.3 Wm-1K-1) [6], a negative thermo-optic coefficient (dn/dT = -1.2 × 10−5 K-1) [7] leading to a weak thermal lensing, and low phonon energy (hνph = 285 cm-1) relevant for suppressing the multi-phonon non-radiative path. Like other fluorite-type crystals, at low doping levels (<0.1 at.%), SrF2 accommodates RE3+ ions in several types of isolated sites (trigonal, C3v, tetragonal, C4v, and cubic, Oh) while at higher doping levels, the dopant ions tend to form clusters ranging from ion pairs (dimers) to large agglomerates leading to significant inhomogeneous spectral broadening of the absorption / emission spectral bands (a “glassy-like” spectroscopic behavior) [8,9]. Similar to other MF2 crystals, the RE3+ ions in SrF2 crystals exhibit long luminescence lifetimes of 2.24 ms leading to low laser thresholds and good energy storage capabilities.
Ytterbium ions (Yb3+) are known for laser emission around 1 µm owing to the 2F5/2 → 2F7/2 electronic transition. They offer a simple energy-level scheme leading to a high slope efficiency and weak heat loading, as well as relatively large Stark splitting of the ground-state (2F7/2) which determines the spectral bandwidths. Yb3+-doped SrF2 crystals with a doping level of a few at.% (i.e., with the majority of the dopant ions forming clusters) combine good thermal and thermo-optical properties of the host matrix, smooth and broad (“glassy-like”) emission / gain profiles of the dopant Yb3+ ions, as well as the long luminescence lifetime of the latter [9], making them suitable for high-power / high-energy ultrafast lasers / amplifiers at ∼1 µm.
Compared to Yb:CaF2, Yb:SrF2 benefits from much higher absorption cross-sections at the zero-phonon line at 976 nm, a slightly longer luminescence lifetime, as well as higher nonlinear refractive index n2. It is complimentary to Yb:CaF2 in terms of good thermal properties and a well-developed growth procedure. Camy et al. reported on a continuous-wave (CW) Yb:SrF2 laser pumped by a Ti:Sapphire laser delivering 165 mW at 1046 nm with a slope efficiency of 53% and a laser threshold of 95 mW [9]. The authors also achieved wavelength tuning between 1003 – 1062 nm. Siebold et al. developed a high peak power free-running Yb:SrF2 laser diode-pumped at 940 nm with a tuning range of 73 nm [10]. A Yb:SrF2 based femtosecond regenerative amplifier produced 325 fs pulses at a repetition rate of 100 Hz with a pulse energy of 1.4 mJ before compression [11]. In 2009, Druon et al. reported on a diode-pumped femtosecond Yb:SrF2 laser mode-locked (ML) by a SEmiconductor Saturable Absorber Mirror (SESAM) delivering 143 fs pulses at 1046.7 nm, with an average output power of 450 mW at a pulse repetition rate of 112.5 MHz [12]. Wu et al. developed a diode-pumped SESAM ML Yb,Gd:SrF2 laser generating 951 fs pulses at 1046 nm at an average output power of 296 mW [13].
Yb:SrF2 transparent ceramics are also known [14]. Basiev et al. reported on a pulsed diode-pumped Yb:(Ca,Sr)F2 ceramic laser [15]. Doroshenko et al. also indicated stimulated-emission from an Yb:SrF2 ceramic at 1.01 – 1.09 µm without providing details [16].
The promising spectroscopic and thermal properties of the Yb:SrF2 crystal motivated us to explore its potential for sub-50 fs pulse generation from passively ML lasers. Recently, we have achieved sub-50 fs pulses from a diode-pumped SESAM ML Yb:BaF2 laser [17]. Note that the growth of high quality Yb:SrF2 crystals is easier than that of their barium counterparts.
2. Crystal growth and spectroscopy
2.1 Crystal growth
A single-crystal of Yb3+:SrF2 was grown by the Bridgman-Stockbarger method using a graphite crucible (Ф25 mm, height: 30 mm). The starting reagents were SrF2 (purity: 4N, Sigma-Aldrich) and YbF3 obtained by fluorination of an Yb2O3 precursor (4N, Alfa Aesar) for attaining higher purity. The initial doping level was 3 at.% Yb3+ (with respect to Sr2+). To avoid oxygen pollution potentially causing the formation of O2- assisted optical centers and / or translucent oxyfluoride phases in the SrF2 crystals, the growth chamber was first sealed to vacuum (<10−5 mbar) and then refilled with a mixture of Ar + CF4 gases. The use of CF4 is critical to avoid the appearance of Yb2+ ions acting as quenching centers.
A mixture of SrF2 + YbF3 was introduced into the crucible which was first slightly heated (∼30 - 50°C) above the melting point and kept at this temperature for 3 – 4 h to homogenize the melt. The crystal growth started at ∼5°C below the melting point and was provided by translating the crucible in a vertical temperature gradient of 30 - 40°C/cm. Once the growth was completed, the crystal was cooled to room temperature (RT, 20°C) within 48 h. A photograph of an as-grown crystal boule is shown in Fig. 1(a). It had cylindric shape (Ф20 mm, length: 25 mm). The crystal was oriented by means of Laue diffraction. A polished crystal sample cut along the [111] crystallographic direction providing weak depolarization losses is shown in Fig. 1(b). The crystal was transparent and colorless. The actual Yb3+ concentration was determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to be 2.89 at.% (ion density: NYb = 5.87 × 1020 at/cm3).
Fig. 1. 2.89 at.% Yb:SrF2 crystal grown by the Bridgman-Stockbarger method: (a) an as-grown crystal boule; (b) a polished crystal sample oriented along the [111] direction.
2.2 Optical spectroscopy
The transmission spectrum in the 0.2 – 1.2 µm range of a crystal sample (2.6 mm-thick) cut from the central part of the as-grown Yb:SrF2 crystal boule is shown in Fig. 2(a). It presents an intense Yb3+ absorption band around 1 µm (the 2F7/2 → 2F5/2 transition). No signs of Yb2+ species possessing a characteristic absorption at 0.22 – 0.36 µm [18] are found in the spectrum. The crystal transmission at ∼1.2 µm is close to the theoretical one. Note that the UV absorption edge of pure SrF2, corresponding to its band-gap, appears at ∼0.13 µm.
Fig. 2. 2.89 at.% Yb:SrF2 crystal: (a) transmission spectrum of a polished crystal sample (2.6 mm thick), grey dashed curve – theoretical transmission (undoped crystal); (b) RT Raman spectrum, λexc = 514 nm.
The unpolarized Raman spectrum of the Yb:SrF2 crystal is shown in Fig. 2(b). It contains only one peak at 284.7 cm-1 (peak width: 10.5 cm-1) typical for the cubic fluorite-type structure (T2g mode) [19]. No other peaks frequently appearing in synthetic SrF2 crystals and related to defect / F-center modes are observed highlighting the good crystalline quality.
The RT absorption, σabs, and stimulated-emission (SE), σSE, spectra for the 2F7/2 ↔ 2F5/2 Yb3+ transition in SrF2 are shown in Fig. 3(a). The maximum σabs corresponding to the zero-phonon-line (ZPL) of Yb3+ amounts to 0.89 × 10−20 cm2 at 976.1 nm and the corresponding peak linewidth (full width at half maximum, FWHM) is 8.6 nm. This makes Yb:SrF2 attractive for pumping by commercially available high-power InGaAs laser diodes emitting at ∼0.98 µm. The σSE spectra were calculated using a combination of the Füchtbauer–Ladenburg (F-L) formula and the reciprocity method (RM). In the spectral range where laser operation is expected (at wavelengths well above the ZPL), σSE amounts to 1.3 × 10−21 cm2 at ∼1049 nm.
Fig. 3. Transition cross-sections for Yb3+ ions in 2.89 at.% Yb:SrF2 at RT: (a) absorption, σabs, and stimulated-emission (SE), σSE, cross-sections; (b) gain cross-sections, σgain = βσSE – (1 – β)σabs, β = N2(2F5/2)/NYb – population inversion ratios β = N2/NYb.
According to the quasi-three-level nature of the Yb laser scheme with reabsorption, the gain cross-section, σgain = βσSE – (1 – β)σabs, of Yb:SrF2 was calculated as shown in Fig. 3(b). Here, β = N2/NYb is the inversion ratio and N2 is the population of the upper laser level (2F5/2). The gain spectra are smooth and broad exhibiting a “glassy-like” spectroscopic behavior. This is due to the strong inhomogeneous spectral broadening in all fluorite-type crystals (including SrF2) originating from clustering of the Yb3+ dopant ions. With increasing the inversion ratio, the spectral maximum experiences a blue shift, from ∼1061 nm for small β = 0.03 to 1043 nm for higher β = 0.11. The gain bandwidth (FWHM) for an intermediate β = 0.06 is 32.5 nm. The broadband gain behavior indicates the high potential of Yb:SrF2 for broad wavelength tuning and sub-100 fs pulse generation from passively ML lasers.
The RT luminescence lifetime of the upper laser level (2F5/2) of Yb3+ ions in SrF2 was measured using a finely powdered crystalline sample to avoid the effect of radiation trapping (reabsorption), see Fig. 4. The Yb3+ ion luminescence exhibits a single-exponential decay with a characteristic lifetime τlum = 2.24 ms. This value agrees well with the radiative lifetime τrad = 2.25 ± 0.1 ms used for calculating the σSE spectra via the F-L formula.
Fig. 4. RT luminescence decay curve (blue) of the 2.89 at.% Yb:SrF2 crystal, λexc = 925 nm, λlum = 1020 nm, red line – single-exponential fit.
The low-temperature (LT, 10 K) absorption and emission spectra of Yb3+ ions in SrF2 are shown in Fig. 5. The electronic transitions of Yb3+ ions in clusters were assigned following the previous work on 5 at.% Yb:CaF2 [20]. The Stark sub-levels of the ground-state (2F7/2) were labeled as i = 0, 1, 2, 3 and those of the excited-state (2F5/2) – as j = 0’, 1’, 2’. The experimental crystal-field splitting is 2F7/2 = [0, 91, 490, 672] cm-1 and 2F5/2 = [10221, 10375, 10872] cm-1 corresponding to the partition functions Z1 = 1.766 and Z2 = 1.509, so that their ratio Z1/Z2 = 1.17. The ZPL corresponding to transitions between the lowest sub-levels of the two manifolds has an energy EZPL of 10221 cm-1. These parameters were used for calculating the σSE spectra via RM.
Fig. 5. LT (10 K) absorption (a) and emission (b) spectra of Yb3+ ions in the 2.89 at.% Yb:SrF2 crystal, “+” mark electronic transitions assigned to Yb3+ ion clusters.
3. Laser set-up
The schematic of the Yb:SrF2 laser is shown in Fig. 6. A cubic laser element was cut from the as-grown bulk 2.89 at.% Yb:SrF2 crystal for light propagation along the [111] direction [21] having an aperture of 3 mm × 3 mm and a thickness of 3 mm. It was polished to laser-grade quality from both sides with good parallelism and mounted in a passively cooled copper holder using Indium foil. The sample was placed at Brewster’s angle between the two concave folding mirrors M1 and M2 (radius of curvature, RoC = -100 mm) in an astigmatically compensated X-shaped standing-wave cavity. The pump source was a spatially single-mode, fiber-coupled InGaAs laser diode delivering a maximum output power of 1.29 W at 976 nm (the emission wavelength was locked with a fiber Bragg grating (FBG) and the spectral linewidth (FWHM) was ∼0.2 nm). The pump radiation was unpolarized. The measured beam propagation factor M2 of the pump radiation at the maximum output power was ∼1.02. The pump beam was collimated and focused into the laser crystal through the M1 mirror via an aspherical lens L1 (focal length: f = 26 mm) and a spherical focusing lens L2 (f = 100 mm) yielding a beam waist (radius) of 18.7 µm × 33.5 µm in the sagittal and tangential planes, respectively.
Fig. 6. The schematic of the Yb:SrF2 laser. LD: fiber-coupled laser diode; L1: aspherical collimating lens; L2: spherical focusing lens; M1, M2 and M4: curved mirrors (RoC = -100 mm); M3: flat end mirror for CW laser operation; DM1 and DM2: flat dispersive mirrors; OC: output coupler; SESAM: SEmiconductor Saturable Absorber Mirror.
The CW laser performance was evaluated in a four-mirror cavity without implementing the saturable absorber (SA) and the DMs. One cavity arm was terminated by a flat end mirror M3 and the other cavity arm – by a plane-wedged output coupler (OC) with a transmission at the laser wavelength TOC in the range of 0.4% - 7.5%. The radius of the fundamental laser cavity mode inside the Yb:SrF2 crystal was estimated by the ABCD method to be 22.7 µm × 33.0 µm in the sagittal and tangential planes, respectively. The measured single-pass pump absorption under lasing conditions decreased with the output-coupling in the range of 84% - 67% due to the ground-state bleaching.
For passively ML operation, the flat end mirror M3 was substituted by a curved mirror M4 (RoC = -100 mm) for creating a second beam waist on the SESAM with a calculated beam radius of ∼78 µm to ensure its efficient bleaching. A commercial SESAM (BATOP, GmbH) was implemented. Its parameters at ∼1 µm were as follows: a modulation depth of 3%, a non-saturable loss of 2%, a saturation fluence of 45 µJ/cm2 and a relaxation time of ∼1 ps. Two flat dispersive mirrors (DM1 and DM2) with a negative group delay dispersion (GDD) per bounce of -250 fs2 were implemented in the other cavity arm to compensate the dispersion from the SrF2 crystal (20.4 fs2/mm at 1050 nm) and to balance the self-phase modulation (SPM) for soliton pulse shaping. The geometrical cavity length of the ML Yb:SrF2 laser was 1.976 m, corresponding to a pulse repetition rate of ∼75.9 MHz.
4. Continuous-wave laser operation
In the CW regime, the maximum output power of the Yb:SrF2 laser amounted to 704 mW at 1048 nm at an absorbed pump power Pabs of 969 mW corresponding to a slope efficiency η of 77.2% and a laser threshold of 64 mW (for 2.5% OC), Fig. 7(a). The laser threshold gradually increased with the output coupling, from 36 mW (TOC = 0.4%) to 131 mW (TOC = 7.5%). The laser wavelength experienced a monotonous blue-shift with increasing the output coupling in the range of 1023.2 – 1058.7 nm, as shown in Fig. 7(b). This behavior is typical for quasi-three-level Yb lasers with inherent reabsorption at the laser wavelength.
Fig. 7. CW diode-pumped Yb:SrF2 laser: (a) input - output dependences for different OCs, η – slope efficiency; (b) typical spectra of laser emission.
The Caird analysis was applied by fitting the measured laser slope efficiency as a function of the output coupler reflectivity, ROC = 1 - TOC [22]. The total round-trip cavity losses δ (reabsorption losses excluded), as well as the intrinsic slope efficiency η0 (accounting for the mode-matching and the quantum efficiency) were estimated yielding η0 = 79.2 ± 3.8% and δ = 0.1 ± 0.06%, as shown in Fig. 8(a). The low value of δ evidences the good optical quality of the laser crystal.
Fig. 8. CW diode-pumped Yb:SrF2 laser: (a) Caird analysis: slope efficiency vs. ROC = 1 – TOC; (b) tuning curve obtained with a Lyot filter and TOC = 0.4%.
The wavelength tuning of the Yb:SrF2 laser in the CW regime was studied by inserting a 2-mm thick quartz plate acting as a Lyot filter at Brewster’s angle in the cavity arm terminated by the OC. A lower TOC of 0.4% was used and the incident pump power was 800 mW. The laser wavelength was continuously tunable between 1006 and 1095 nm, i.e., across 89 nm at the zero-power-level, Fig. 8(b).
5. SESAM mode-locked laser operation
The SESAM was implemented to start and stabilize the soliton ML operation of the Yb:SrF2 laser. A total round-trip negative GDD of -1000 fs2 was introduced by using the two flat DMs (DM1 – DM2), see Fig. 6. After careful cavity alignment, stable and self-starting ML operation was readily achieved with a 4% OC. The measured optical spectrum of the laser had a FWHM of 17 nm assuming a sech2-shaped profile at a central wavelength of 1049.4 nm, Fig. 9(a). The second-harmonic generation (SHG)-based intensity autocorrelation trace was well fitted with a sech2-shaped temporal profile giving a pulse duration (FWHM) of 70 fs corresponding to a time-bandwidth-product (TBP) of 0.324, see Fig. 9(b). An average output power of 313 mW was obtained at Pabs of 902 mW for a pulse repetition rate of 75.9 MHz, corresponding to an optical efficiency of 34.7% and a peak power of 51.9 kW. The inset in Fig. 9(b) shows the measured intensity autocorrelation trace on a long-time span of 50 ps indicating single-pulse CW-ML operation free of multiple pulse instabilities.
Fig. 9. SESAM ML Yb:SrF2 laser with TOC = 4%. (a) Optical spectrum and (b) SHG-based intensity autocorrelation trace. Inset: autocorrelation trace on a time span of 50 ps.
Shorter pulses could be achieved by using a smaller output coupling of 2.5%. The Yb:SrF2 laser delivered pulses with a spectral bandwidth of 21 nm at a central wavelength of 1055.2 nm (assuming a sech2-shaped profile), Fig. 10(a). The excellent fit of the recorded intensity autocorrelation trace using a sech2-shaped temporal profile resulted in an estimated pulse duration of 59 fs, see Fig. 10(b). The 50-ps long-time scale autocorrelation trace revealed single-pulse mode-locking, see the inset in Fig. 10(b). The corresponding TBP was 0.318 being very close to the Fourier-transform-limited value of 0.334. The average output power dropped to 262 mW at Pabs of 795 mW for the same pulse repetition rate of 75.9 MHz, corresponding to an optical efficiency of 32.9% and a peak power of 51.5 kW.
Fig. 10. SESAM ML Yb:SrF2 laser with TOC = 2.5%. (a) Optical spectrum and (b) SHG-based intensity autocorrelation trace. Inset: autocorrelation trace on a time span of 50ps.
The shortest pulses with ultimate stability were achieved using a 1% OC. Assuming a sech2-shaped spectral profile, an emission bandwidth of 25 nm was observed at a central wavelength of 1057 nm, see Fig. 11(a). Figure 11(b) shows the recorded intensity autocorrelation trace for the shortest pulses. The curve was well fitted with a sech2-shape temporal profile, yielding 49 fs for the pulse duration. This corresponds to a TBP of 0.329 again being very close to the Fourier-transform-limit. Single-pulse CW mode-locking was also confirmed by a long time-scale intensity autocorrelation trace, see the inset in Fig. 11(b). The average output power further dropped to 117 mW at Pabs of 856 mW, which corresponded to an optical efficiency of 13.7% and a peak power of 27.7 kW.
Fig. 11. SESAM ML Yb:SrF2 laser with TOC = 1%. (a) Laser spectrum and (b) SHG-based intensity autocorrelation trace. Inset: autocorrelation trace on a time span of 50 ps.
The radio-frequency (RF) spectra of the shortest pulses were recorded to confirm the stability of the ML operation in different frequency span ranges, as shown in Fig. 12. The recorded fundamental beat note located at 75.9 MHz exhibited a high extinction ratio of >76 dBc above carrier, see Fig. 12(a). The measured uniform harmonics on a 1-GHz frequency span again revealed high stability of the single-pulse ML operation, see Fig. 12(b).
Fig. 12. RF spectra of the SESAM mode-locked Yb:SrF2 laser: (a) Fundamental beat note at 75.9 MHz recorded with a resolution bandwidth (RBW) of 300 Hz, and (b) Harmonics on a 1-GHz frequency span recorded with an RBW of 100 kHz. TOC = 1%.
To confirm the pulse shaping mechanism, the far-field beam profiles of the Yb:SrF2 laser both in the CW and ML regimes were recorded with an IR camera placed at ∼1.2 m from the OC. It was relatively easy to switch between the CW and ML regimes just by a slight cavity misalignment. Almost unchanged beam diameters in the far-field were observed during such switching, see Fig. 13. This observation in combination with the excellent sech2-shaped spectral and temporal profiles of the shortest pulses indicates that soliton mode-locking was the dominant pulse shaping mechanism rather than Kerr-lens mode-locking. This conclusion is in line with the relatively small value of the nonlinear refractive index n2 of strontium fluoride at ∼1 µm (0.91 – 1.34 × 10−20 m2/W [23]), resulting in a weak nonlinear amplitude modulation for a 3 mm-thick laser crystal.
Fig. 13. Measured far-field beam profiles of the diode-pumped SESAM ML Yb:SrF2 laser: (a) CW and (b) SESAM ML operation.
The weak satellite peaks observed in the spectra of the mode-locked Yb:SrF2 laser around 1005 nm are assigned to uncompensated intracavity GDD at the short-wave spectral wing and non-optimized spectral reflectivity of the cavity mirrors. A similar phenomenon was previously observed for a SESAM mode-locked Yb:CaGdAlO4 laser with a prism pair for the dispersion management [24], and later explained by Sévillano et al. [25].
6. Conclusion
To conclude, ytterbium-doped strontium fluoride (Yb:SrF2) is a promising crystal for broadly tunable lasers and especially power-scalable ultrafast (sub-100 fs) oscillators emitting around 1 µm. In the present work, we report on the sub-50 fs pulse generation from a diode-pumped SESAM mode-locked 2.89 at.% Yb:SrF2 laser. By using a high-brightness pump source (a cost-effective single-mode fiber-coupled InGaAs laser diode), a commercial SESAM for initiating and sustaining the mode-locking operation, soliton pulses as short as 49 fs were generated at a central wavelength of 1057 nm corresponding to an average output power of 117 mW at a pulse repetition rate of ∼75.9 MHz. Our laser results indicate the potential of Yb:SrF2 crystals with optimized Yb3+ ion doping levels and thickness for further power scaling and pulse shortening via the Kerr-lens mode-locking technique as demonstrated previously for Yb:CaF2 [26,27].
Funding
National Natural Science Foundation of China (61975208, 61905247, U21A20508); Sino-German Scientist Cooperation and Exchanges Mobility Program (M-0040); National Key Research and Development Program of China (2021YFB3601504); Agencia Estatal de Investigación (PID2019-108543RB-I00).
Acknowledgment
Xavier Mateos acknowledges the Serra Húnter program.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. H. H. Li, “Refractive index of alkaline earth halides and its wavelength and temperature derivatives,” J. Phys. Chem. Ref. Data 9(1), 161–290 (1980). [CrossRef]
2. F. Druon, S. Ricaud, D. N. Papadopoulos, A. Pellegrina, P. Camy, J. L. Doualan, R. Moncorgé, A. Courjaud, E. Mottay, and P. Georges, “On Yb:CaF2 and Yb:SrF2: review of spectroscopic and thermal properties and their impact on femtosecond and high power laser performance,” Opt. Mater. Express 1(3), 489–502 (2011). [CrossRef]
3. W. Ma, X. Qian, J. Wang, J. Liu, X. Fan, J. Liu, L. Su, and J. Xu, “Highly efficient dual-wavelength mid-infrared CW laser in diode end-pumped Er:SrF2 single crystals,” Sci. Rep. 6(1), 36635 (2016). [CrossRef]
4. T. T. Basiev, M. E. Doroshenko, V. A. Konyushkin, and V. V. Osiko, “SrF2:Nd3+ laser fluoride ceramics,” Opt. Lett. 35(23), 4009–4011 (2010). [CrossRef]
5. A. Sottile, E. Damiano, M. Rabe, R. Bertram, D. Klimm, and M. Tonelli, “Widely tunable, efficient 2 µm laser in monocrystalline Tm3+:SrF2,” Opt. Express 26(5), 5368–5380 (2018). [CrossRef]
6. P. A. Popov, P. P. Fedorov, V. A. Konyushkin, A. N. Nakladov, S. V. Kuznetsov, V. V. Osiko, and T. T. Basiev, “Thermal conductivity of single crystals of Sr1−xYbxF2 + x solid solution,” Dokl. Phys. 53(8), 413–415 (2008). [CrossRef]
7. J. Boudeile, J. Didierjean, P. Camy, J. L. Doualan, A. Benayad, V. Ménard, R. Moncorgé, F. Druon, F. Balembois, and P. Georges, “Thermal behaviour of ytterbium-doped fluorite crystals under high power pumping,” Opt. Express 16(14), 10098–10109 (2008). [CrossRef]
8. V. Petit, P. Camy, J.-L. Doualan, X. Portier, and R. Moncorgé, “Spectroscopy of Yb3+:CaF2: from isolated centers to clusters,” Phys. Rev. B 78(8), 085131 (2008). [CrossRef]
9. P. Camy, J. L. Doualan, A. Benayad, M. Von Edlinger, V. Ménard, and R. Moncorgé, “Comparative spectroscopic and laser properties of Yb3+-doped CaF2, SrF2 and SrF2 single crystals,” Appl. Phys. B 89(4), 539–542 (2007). [CrossRef]
10. M. Siebold, J. Hein, M. C. Kaluza, and R. Uecker, “High-peak-power tunable laser operation of Yb:SrF2,” Opt. Lett. 32(13), 1818–1820 (2007). [CrossRef]
11. S. Ricaud, P. Georges, P. Camy, J. L. Doualan, R. Moncorge, A. Courjaud, E. Mottay, and F. Druon, “Diode-pumped regenerative Yb:SrF2 amplifier,” Appl. Phys. B 106(4), 823–827 (2012). [CrossRef]
12. F. Druon, D. N. Papadopoulos, J. Boudeile, M. Hanna, P. Georges, A. Benayad, P. Camy, J. L. Doualan, V. Menard, and R. Moncorge, “Mode-locked operation of a diode-pumped femtosecond Yb:SrF2 laser,” Opt. Lett. 34(15), 2354–2356 (2009). [CrossRef]
13. Y. Wu, Z. Zou, C. Wang, J. Liu, L. Zheng, and L. Su, “Broadly tunable and passively mode-locked operations of Yb3+,Gd3+:SrF2 laser,” IEEE J. Select. Topics Quantum Electron. 25(4), 1–5 (2019). [CrossRef]
14. Y. Gao, B. Mei, W. Li, Z. Zhou, and Z. Liu, “Effect of Yb3+ concentration on microstructure and optical properties of Yb:SrF2 transparent ceramics,” Opt. Mater. (Amsterdam, Neth.) 105, 109869 (2020). [CrossRef]
15. T. T. Basiev, M. E. Doroshenko, P. P. Fedorov, V. A. Konyushkin, S. V. Kuznetsov, V. V. Osiko, and M. S. Akchurin, “Efficient laser based on CaF2-SrF2-YbF3 nanoceramics,” Opt. Lett. 33(5), 521–523 (2008). [CrossRef]
16. M. E. Doroshenko, A. A. Demidenko, P. P. Fedorov, E. A. Garibin, P. E. Gusev, H. Jelinkova, V. A. Konyshkin, M. A. Krutov, S. V. Kuznetsov, V. V. Osiko, and P. A. Popov, “Progress in fluoride laser ceramics,” Phys. Status Solidi C 10(6), 952–957 (2013). [CrossRef]
17. W.-Z. Xue, Z.-L. Lin, H.-J. Zeng, G. Zhang, P. Loiko, L. Basyrova, A. Benayad, P. Camy, V. Petrov, X. Mateos, L. Wang, and W. Chen, “Diode-pumped mode-locked Yb:BaF2 laser,” Opt. Express 30(9), 15807–15818 (2022). [CrossRef]
18. B. Moine, B. Courtois, and C. Pedrini, “Luminescence and photoionization processes of Yb2+ in CaF2, SrF2 and BaF2,” J. Phys. (Paris) 50(15), 2105–2119 (1989). [CrossRef]
19. A. R. Gee, D. C. O’Shea, and H. Z. Cummins, “Raman scattering and fluorescence in calcium fluoride,” Solid State Commun. 4(1), 43–46 (1966). [CrossRef]
20. M. Ito, C. Goutaudier, Y. Guyot, K. Lebbou, T. Fukuda, and G. Boulon, “Crystal growth, Yb3+ spectroscopy, concentration quenching analysis and potentiality of laser emission in Ca1−xYbxF2 + x,” J. Phys.: Condens. Matter 16(8), 1501–1521 (2004). [CrossRef]
21. K. Genevrier, D. N. Papadopoulos, M. Besbes, P. Camy, J. L. Doualan, R. Moncorgé, P. Georges, and F. Druon, “Thermally-induced-anisotropy issues in oriented cubic laser crystals, the cryogenically cooled Yb:CaF2 case,” Appl. Phys. B 124(11), 209 (2018). [CrossRef]
22. J. A. Caird, S. A. Payne, P. R. Staver, A. Ramponi, and L. Chase, “Quantum electronic properties of the Na3Ga2Li3F12:Cr3+ laser,” IEEE J. Quantum Electron. 24(6), 1077–1099 (1988). [CrossRef]
23. Y. Guo, S. Lu, L. Su, C. Zhao, H. Zhang, and S. Wen, “Z-scan measurement of the nonlinear refractive index of Nd3+, Y3+-codoped CaF2 and SrF2 crystals,” Appl. Opt. 54(4), 953–958 (2015). [CrossRef]
24. Y. Zaouter, J. Didierjean, F. Balembois, G. L. Leclin, F. Druon, P. Georges, J. Petit, P. Goldner, and B. Viana, “47-fs diode-pumped Yb3+:CaGdAlO4 laser,” Opt. Lett. 31(1), 119–121 (2006). [CrossRef]
25. P. Sévillano, P. Georges, F. Druon, D. Descamps, and E. Cormier, “32-fs Kerr-lens mode-locked Yb:CaGdAlO4 oscillator optically pumped by a bright fiber laser,” Opt. Lett. 39(20), 6001–6004 (2014). [CrossRef]
26. G. Machinet, P. Sevillano, F. Guichard, R. Dubrasquet, P. Camy, J. L. Doualan, R. Moncorgé, P. Georges, F. Druon, D. Descamps, and E. Cormier, “High-brightness fiber laser-pumped 68 fs-2.3 W Kerr-lens mode-locked Yb:CaF2 oscillator,” Opt. Lett. 38(20), 4008–4010 (2013). [CrossRef]
27. P. Sevillano, G. Machinet, R. Dubrasquet, P. Camy, J.-L. Doualan, R. Moncorge, P. Georges, F. P. Druon, D. Descamps, E. E. D. H. G. Cormier, and P. Moulton, “Sub-50 fs, Kerr-lens mode-locked Yb:CaF2 laser oscillator delivering up to 2.7 W,” in Advanced Solid-State Lasers Congress (Optical Society of America, Paris), p. AF3A.6 (2013). [CrossRef]
