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Abstract
A Kerr-lens mode-locked laser based on a Yb3+-doped disordered gadolinium scandate (Yb:GdScO3) crystal is reported for the first time, to the best of our knowledge. The crystal with the perovskite structure was grown using the Czochralski method, and its room temperature (RT) and low temperature (LT) spectra were also investigated. Due to the crystal's multisite structure (Gd3+/Sc3+ site), Yb:GdScO3 offers broad and intense polarized emission spectra in the near-infrared range (975–1075 nm). The stimulated emission cross section σSE is 0.46 × 10−20 cm2 at 1000 nm with an emission band width of 75.7 nm for E // b polarization. The continuous wave (CW) laser was operated pumped by a 976 nm fiber-coupled LD laser, resulting in a maximum output power of 8.74 W with a slope efficiency of 76.1% was obtained. Additionally, a pulses as short as 74 fs are generated at ∼1061.7 nm via Kerr-lens mode-locking. The average output power amounts to 32 mW at a pulse repetition rate of 101.4 MHz. All results indicate Yb:GdScO3 a promising candidate for 1 µm ultrashort laser.
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1. Introduction
1 µm lasers have significant applications in high-power pump sources, laser radar, and medical and scientific research [1–4]. Yb3+ ions, with only two energy levels and short energy spacing, offer advantages over Nd3+ doped crystals by avoiding parasitic effects like excited-state absorption, energy-transfer upconversion, and cross-relaxation [5,6]. The low quantum defect (∼7%) resulting from the in-band pumping scheme enhances laser efficiencies and reduces thermal load. Furthermore, the wider emission bandwidth and longer fluorescence lifetime [7] support ultra-strong, ultra-short pulse generation. Yb-based femtosecond lasers capable of delivering sub-100 fs pulses have been demonstrated in various crystals, such as Yb:Y3Al5O12 (35 fs) [8], Yb:Lu2O3 (35 fs) [9], Yb:YSO (95 fs) [10], Yb:CaYAlO4 (17 fs) [11], Yb:YAlO3 (29 fs) [12], and Yb:SrLaAlO4 (30 fs) [13]. Host materials with smooth and broadband emission spectra, along with good mechanical, thermal, and optical properties, play a crucial role in further shortening the pulse duration.
GdScO3 crystal has an orthorhombic perovskite structure with the space group Pnma (No. 62) [14]. Known for its low phonon energy (∼452 cm−1) and high thermal stability [15], GdScO3 crystals have garnered significant attention in the advancement of solid-state lasers. For instance, Er:GdScO3 crystals grown using the edge-defined film-fed (EFG) method have demonstrated impressive results, achieving a high power continuous wave (CW) laser output of 0.863 W at approximately 2700 nm with a slope efficiency of 11.25% [16]. Tm:GdScO3 crystals have shown tunable laser operation at 321 nm and produced pulses as short as 44 fs [17,18]. Additionally, GdScO3 has proven to be a versatile host material for lasers operating in the 1 µm, 2 µm, and 3 µm regions. Notably, Nd:GdScO3 crystals have achieved a maximum output power of 3.16 W with a slope efficiency of 47.5% [19], while Yb:GdScO3 crystals have generated a maximum output power of 13.45 W with an optical-to-optical efficiency of 63.3% [20]. The broad emission bandwidth of 85 nm suggests the potential for ultrashort pulse lasers. However, no advancements in pulse laser technology have been reported yet. Therefore, further research is crucial to fully explore the capabilities of GdScO3 crystals in ultrafast laser applications.
In this research, a Yb3+ doped GdScO3 crystal has been grown by the Czochralski method. The polarized absorption and emission spectra of the Yb:GdScO3 crystal were analyzed at both room temperature and low temperature (77 K). Additionally, the laser performance was analyzed in both continuous wave (CW) and Kerr-lens mode-locked regimes. The Yb:GdScO3 laser was able to generate soliton pulses as short as 74 fs at 1061.7 nm, operating at a frequency of approximately 101.4 MHz.
2. Crystal growth and structure
The Yb:GdScO3 crystal was grown using the Czochralski method. The Yb2O3, Gd2O3, and Sc2O3 powders with the purity of 5N were prepared as raw materials and weighed according to the formula: Yb0.04Gd0.96ScO3. The mixed powders were pressed into bulks and then sintered in the air at 1400°C for 24 h. The treated blocks were then positioned within iridium crucibles under a nitrogen atmosphere for crystal growth. The single-crystal growth process comprises three principal steps: shoulder release, equalizing, and closing. The pulling speed for this process varies between 0.5 and 1.5 mm/h, with the rotational speed around to 15 rpm. High-purity nitrogen gas was introduced as a protective atmosphere during the crystal growth. The crystals were cooled to room temperature at a rate of 40 °C/h. The photograph of as-grown 4 at.% Yb:GdScO3 crystal is shown in Fig. 1.
Fig. 1. The XRD patterns of Yb:GdScO3 single crystal. The inset presents photograph of the as-grown Yb:GdScO3 bulk.
Figure 1 displays the XRD patterns of 4 at.% Yb:GdScO3 crystals compared to the standard card of GdScO3 crystal (PDF#27-0220). The diffraction peaks of Yb:GdScO3 crystals align well with the GdScO3 phase, indicating a shared crystal structure. Calculated lattice parameters for Yb:GdScO3 crystal are a = 0.5434 nm, b = 0.5724 nm, and c = 0.7911 nm, slightly smaller than those of pure GdScO3 (a = 0.5487 nm, b = 0.5756 nm, and c = 0.7926 nm) [21]. This reduction in lattice parameters is attributed to the substitution of Yb3+ with a smaller cation radius in place of Gd3+.
3. Spectroscopic properties of Yb:GdScO3
To determine the Stark splitting of the ground state and the excited-state of Yb3+ ions in GdScO3, we have measured the low temperature (77 k) absorption and emission spectra for E // a, E // b, and E // c. In Fig. 2(a), At the low temperature (77K) absorption spectrum shows a prominent peak at 985.0 nm (10142 cm−1), corresponding to the transition between the lowest Stark sub-levels of the 0 → 0′ multiplets (zero-phonon line). In cases where the structural symmetry is less than cubic, the ground state 2F7/2 and excited state 2F5/2 of Yb3+ split into four and three Stark levels, respectively. The peaks at 10,460 and 10,834 cm−1 are associated with the 0 → 1′ and 0 → 2′ transitions, respectively. At lower temperatures, the splitting peaks for different polarization directions are more distinct, nearly identical, and easily distinguishable.
Fig. 2. Low-temperature polarized spectroscopy of the Yb:GdScO3 crystal: (a) absorption and (b) luminescence spectra. (c) Energy-level diagram of Yb3+ in GdScO3 crystal. Z1(2) are the partition functions.
The energy level diagram of Yb3+ ions in GdScO3 was plotted based on the assignment of electronic transitions, and shown in Fig. 2(c). The ground-state exhibits a total splitting of 709 cm−1, which is conducive to wavelength-tunable laser operations. In addition, Fig. 2(c) also presents the calculated partition functions Z1(2), for the ratio of Z1/Z2 1.35.
The Yb3+ concentration in the initial head region of the as-grown crystal was found to be 1.6 at.% through inductively coupled plasma atomic emission spectroscopy (ICP-AES). Subsequently, the effective segregation coefficient of Yb3+ in the GdScO3 crystal can be determined as keff = Cs/C0 = 0.4. The room temperature polarized absorption cross section of the Yb:GdScO3 crystal is illustrated in Fig. 3(a). The results reveal that the absorption spectra display polarization-specific characteristics, with distinct absorption coefficients in each orientation. The absorption cross section of Yb3+ is computed using the formula [18]: σabs = α/N0 (where the corresponding Yb3+ ion densities are 2.25 × 1020 cm-3). It is evident that the absorption line shapes for E // a and E // b are quite similar, but notably different from that for E // c. The most intense absorption is observed at 985 nm for E // b, corresponding to the transition of 2F7/2 → 2F5/2, with a maximum absorption cross-section of 5.82 × 10−20 cm2. The full width at half maximum (FWHM) is 19.7 nm, indicating that this material is suitable for pumping with commercial high-power InGaAs diode lasers for power scalable operation.
Fig. 3. The room temperature polarized (a) absorption cross section, (b) emission cross section versus wavelength, and (c) fluorescence decay curves of the 2F5/2 multiplet of Yb:GdScO3 crystal.
The RT stimulated-emission (SE) cross-sections, σSE, were calculated using a combination of the reciprocity method (RM) [22] and the Füchtbauer–Ladenburg (F-L) equation [23].
For each polarization, represented by the variable i, with values a, b, or c, λ denotes the light wavelength, Ii(λ) represents the measured emission spectrum, n represents the average refractive index, and τ signifies the radiative lifetime of the emitting state (2F5/2), which was determined to be 0.85 ms by using a modified reciprocity method.
The emission cross sections of E // a, E // b and E // c calculated by the FL method are 1002 nm 1.40 × 10−20 cm2, 1000 nm 0.46 × 10−20 cm2 and 1000 nm 1.30 × 10−20 cm2, and the FWHMs are 79.5 nm, 75.7 nm, and 87.4 nm, respectively. Notably, Yb:GdScO3 exhibits a wide emission bandwidth in different polarization directions, which is higher than most well-studied materials, as shown in Table 1. Consequently, Yb:GdScO3 demonstrates potential use for generating a broad tuning range laser and ultrafast mode-locked laser pulses.
Table 1. Comparison of the main spectral parameters of different Yb-doped crystals
The gain characteristics of a laser can be characterized by the gain cross section of the laser crystal, which is given by the expression:${\sigma _g}(\lambda )= \mathrm{\beta} {\sigma _{em}}(\lambda )- ({1 - \mathrm{\beta} } ){\sigma _{abs}}(\lambda )$. β represents the proportion of inverted ions to the total Yb3+-ion density. The polarized σgain spectra of 1000 ∼ 1200 nm with different β values are shown in Fig. 4. In the case of a constant β, the generation of large laser gain is attributed to a larger emission cross section and a smaller self-absorption effect. The maximum σgain value (0.295 × 10−20 cm2 at 1061 nm when β = 0.32) corresponds to light polarization E // c, similar to the value of E // a (0.291 × 10−20 cm2 at 1060 nm). For both low β value and higher β value, the local peak centered at ∼ 1060 nm dominates in the spectra for the three polarization. Compared to the other polarizations, the bandwidth of the E // c polarization is the largest at β = 0.32, which is close to 80 nm, while the gain bandwidths of the E // b and E // c polarizations are 59 nm and 39.8 nm, respectively. Such a large, broad, and flat profile of the gain cross section supports a wide band tunable and ultrashort pulse laser for Yb:GdScO3 crystal. The experimental results indicate that Yb:GdScO3 exhibits a broad and very flat gain spectrum in all three polarization cases.
4. Laser set-up and continuous-wave (CW) operation
Firstly, we investigated the CW laser performance of the 3.5 mm-thick Yb:GdScO3 crystal, and the setup depicted in Fig. 5(a). A compact two-mirror laser resonant cavity for CW laser performance was employed in this work, where the pump source was a fiber-coupled 976 nm InGaAs diode laser with a core diameter of about 105 µm. After passing through a collimated lens (f = 50 mm) and a focused lens (f = 75 mm), the pump beam is focused onto the gain medium resulting in a spot size of about 157.5 µm. Initially, three different orientations (a-cut, b-cut, and c-cut)of Yb:GdScO3 were tested under an pump power of 10.28 W, resulting in output powers of 2.05 W, 1.15 W, and 1.06 W, respectively, using a 5% output coupler transmission (TOC). Subsequently, four different output couplers with varying transmission rates (TOC = 5%, 10%, 15%, and 20%) were employed to enhance the slope efficiency of the a-cut orientation. Eventually, the highest output power 8.74 W with a slope efficiency of 76.1% was achieved for TOC = 20%, as depicted in Fig. 5(b). The high slope efficiency is comparable to that of Yb:CLNGG (74%, [30]) and Yb:YLuGdCOB (70%, [31]), slightly lower than that of Yb:YSGG (80.1%, [32]) and Yb:CYA (92%, [7]), indicating the high quality of the Yb:GdScO3 crystal.
Fig. 5. (a) Scheme of the LD-pumped CW Yb:GdScO3 laser; (b) Output power versus absorbed pump power of a-cut samples; (c) Laser emission spectra of a-cut at different OCs.
The corresponding laser spectra with different OCs are shown in Fig. 5(c). With the change of output couplers from TOC = 5% to 20%, the cavity loss and the threshold become higher. In order to meet the requirements of laser oscillation, the beta value should be continuously increased, causing the maximum wavelength of σg(λ) to move towards the short end. Consequently, the spectral peak demonstrates a blue shift as the OC reflectance increases in varying directions.
5. Mode-locked soliton laser operation
Figure 6 shows a schematic of the Kerr-lens mode-locked (KLM) laser setup with a standard Z-fold cavity. A E // b Brewster angle-cut sample was mounted on a copper block heat sink with both end faces of the sample laser-graded polished, and a half-wave plate (HWP) was used to adjust the polarization of the incident pump for optimal absorption. The copper block heat sink was maintained at ∼14.0°C by circulating water to remove heat generated during laser operation. The pump source is a commercial single mode fiber laser (ALS-IR-75, Azur Light Systems), emitting at 979 nm (M2 ∼ 1.25) with beam diameter 27 µm and up to 10 W output power. A collimated defocusing system consisting of a concave lens (L1) with a focal length of 25 mm and a convex lens (L2) with a focal length of 200 mm is used for recollimation. The pump beam is then focused into the crystal through a convex lens (L3) with a focal length of 75 mm. M4 and M5 were dichroic concave mirrors with 75 mm radium of curvature (ROC), coated with high reflectivity over 1020-1100 nm (R > 99.9%), and high transmittance around 800-1000 nm (R < 5%). The optical dispersion was mitigated by employing diverse arrays of Gires-Tournois interferometer mirrors (GTIs) within the cavity, yielding a comprehensive group delay dispersion (GDD) ranging from -250 fs2 to -1000 fs2. And no extra-cavity pulse compression was employed. The total cavity length was about 1.5 m, corresponding to a repetition rate of 100 MHz.
Fig. 6. Experimental setup of the KLM Yb:GdScO3 laser. HWP: half wave plate; L1: concave lens, f = 25 mm; L2: convex lens, f = 200 mm; L3: convex lens, f = 75 mm; M4 and M5: concave mirrors with radius of curvature (ROC) of 75 mm; GTI1-GTI5: Gires-Tournois interferometer mirrors; OC: optical coupler.
The KLM operation of the Yb:GdScO3 laser was initially explored by employing an output coupler with a transmission of TOC = 0.4% to minimize intracavity losses and generate ultrashort pulses. Manipulation of the KLM operation was achieved by slightly misaligning the M5 mirror, thus bringing the cavity closer to the periphery of the stability range. Using a commercial optical spectrum analyzer (AvaSpec-ULS2048, Avantes), the mode-locked optical spectrum was measured as shown in Fig. 7(a). The laser’s optical spectrum, with a full width at half-maximum (FWHM) of 12.5 nm, exhibited a sech2-shaped profile centered at 1059 nm. The corresponding intensity autocorrelation trace was measured by a commercial intensity auto-correlator (APE Pulse Check USB), as shown in Fig. 7(b). The recorded second-harmonic generation (SHG)-based intensity autocorrelation trace was well-represented by a sech2-shaped temporal profile, estimating a pulse duration of 125 fs.
Fig. 7. (a) KLM Yb:GdScO3 laser with TOC = 0.4% Laser spectra; (b) SHG-based intensity autocorrelation trace; (c) KLM Yb:GdScO3 laser with TOC = 1.6% Laser spectrum; (d) SHG-based intensity autocorrelation trace. Inset: intensity autocorrelation trace measured on a time span of 40 ps.
Further reduction in pulse duration was accomplished by employing an output coupling of 1.6%. The Yb:GdScO3 laser produced pulses with a spectral bandwidth of 18.1 nm at a center wavelength of 1061.7 nm (assuming a sech2-shaped profile), as presented in Fig. 7(c). The fit of the recorded intensity autocorrelation trace with a sech2-shaped temporal profile yielded an estimated pulse duration of 74 fs, with an average of 37 mW, observed in Fig. 7(d). The time bandwidth product of the laser is 0.35 which is close to the value of 0.315 for the transform limited sech2-pulse, indicating the theoretically shortest pulse of 65.4 fs under the current conditions. The long-scale autocorrelation scan extending over 40 ps, depicted in the inset of Fig. 7(d), confirmed the operation of the laser in continuous-wave mode with mode-locked characteristics, devoid of any pedestals or multi-pulsing instabilities.
To ascertain the stability of the mode-locking operation, we conducted an analysis of the radio frequency (RF) spectrum using a commercial RF spectrum analyzer (Agilent 4407B). The signals were acquired within a frequency window of 0.2 MHz, utilizing a resolution bandwidth (RBW) of 100 Hz, and a frequency span of 0.9 GHz with a 1 kHz RBW, as illustrated in Figs. 8(a) and 8(b). The RF spectrum of the fundamental harmonic at 101.4 MHz exhibited a signal-to-noise ratio (SNR) of approximately 65 dBc. The absence of discernible side peaks in the higher-order harmonics suggests that the KLM operation remained stable, devoid of Q-switch instability. The observed fluctuations in the harmonic peaks within Fig. 8(b) may be attributed to the constrained sampling rate of the RF spectrum analyzer.
Fig. 8. Radio frequency (RF) spectra at: (a) the fundamental beat note with the resolution bandwidth (RBW) of 100 Hz, and (b) 0.9 GHz wide span range with 1 kHz RBW.
6. Conclusion
In summary, a Yb:GdScO3 crystal with high-quality was successfully grown using the Czochralski method. To the best of our knowledge, the low-temperature spectral performance was investigated for the first time. For E // b polarization, the maximum absorption cross section was determined to be 5.82 × 10−20 cm2 at 985 nm. The stimulated emission cross section at 1000 nm was calculated to be 0.46 × 10−20 cm2 with FWHM of 75.7 nm. Utilizing a 976 nm fiber laser as a pump source, a maximum output power of 8.74 W, accompanied by a slope efficiency as high as 76.1% was achieved. By using a mode-locking technique, the Yb:GdScO3 laser produces soliton pulses as short as 74 fs at 1061.7 nm with an operating frequency of ∼101.4 MHz. Through further appropriately managing the intracavity dispersion and mode-locked pulse intensity, shorter pulses could be generated directly from Yb3+-doped mode-locked GdScO3 lasers. All the results indicate that Yb:GdScO3 crystal are prospective for achieving few-optical-cycle pulse generation around 1 µm.
Funding
National Natural Science Foundation of China (61975071).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available.
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