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Abstract
Diode-pumped continuous-wave and passively Q-switched pulse Yb:Ca3TaGa3Si2O14 lasers are reported. At an absorbed pump power of 5.4 W, a 1046.4 nm continuous-wave laser with an output power of 3.24 W and a slope efficiency of 63.4% was obtained. When a Cr4+:YAG saturable absorber with an initial transmission of 95% was used, a 1015.2 nm passively Q-switched pulse laser with a repetition frequency of 5.43 kHz, energy of about 73.6 µJ, duration of 35 ns, and peak output power of 2.1 kW was demonstrated.
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1. Introduction
Due to the high physicochemical stability and easy growth of the large-size crystal by Czochralski method, Yb3+ doped langasite family A3BGa3Si2O14 (A = Ca, Sr; B = Nb, Ta) crystals have been widely investigated as laser materials at 1.0 µm [1–3]. A 1062–1068 nm continuous-wave (CW) laser with an output power of 7.27 W and a slope efficiency of 78%, as well as a 1015.3 nm passively Q-switched pulse laser with a repetition frequency of 22.5 kHz, duration of 4.4 ns, energy of 62.2 µJ and peak output power of 1.56 kW have been realized in a Yb:Ca3NbGa3Si2O14 (Yb:CNGS) crystal [1,2]. Based on a semiconductor saturable absorber mirror (SESAM), a 1052 nm pulse laser with an average power of 45 mW and a duration of 47 fs, as well as a 525 nm self-frequency-doubling (SFD) pulse laser with an average power of 40 mW and a duration of 100 fs have also been realized in a Yb:CNGS crystal [3]. Furthermore, a 1065 nm CW laser with an output power of 3.5 W and a slope efficiency of 38.6%, as well as a 1065 nm passively Q-switched pulse laser with an energy of 45.7 µJ, repetition frequency of 33.1 kHz and duration of 8.4 ns have also been obtained in a Nd:CTGS crystal [4].
Ca3TaGa3Si2O14 (CTGS) crystal has the same structure to CNGS crystal, which belongs to trigonal system with space group P321. The crystal exhibits weak anisotropic thermal expansion (8.49 and 8.17 × 10−6 K−1 along a and c axes, respectively), high specific heat (0.57 Jg-1K-1 at 300 K) and moderate thermal conductivity (1.5 and 2.4 Wm-1K-1 along a and c axes at 300 K, respectively) [5]. Room-temperature polarized spectroscopic properties of the Yb:CTGS crystal have been investigated [6]. The crystal has a peak absorption cross section of 3.69 × 10−20 cm2 at 977 nm and a peak emission cross section of 0.55 × 10−20 cm2 at 1046 nm for the σ polarization. Furthermore, the full width at half maximum (FWHM) of gain band around 1046 nm is 58 nm at an inversion parameter of 0.2 for σ polarization, which is larger than that (49 nm for σ polarization) of the Yb:CNGS crystal [6]. The fluorescence lifetime of the 2F5/2 multiplet of the crystal is 634 µs. Furthermore, the CTGS crystal has a non-centrosymmetric structure, and the effective nonlinear optical coefficients are 0.44 and 0.34 pm/V for type I (38.7°, 30.0°) and type II (61.1°, 0°) orientations, respectively [7], which indicates that the crystal doped with rare earth ions can be used as a SFD material. At present, efficient SFD lasers have been realized in some Yb3+-doped bifunctional crystals, such as the well-known Yb:YAl3(BO3)4 (Yb:YAB) and Yb:YCa4O(BO3)3 (Yb:YCOB) [8–10]. For example, a 1043 nm CW laser with an output power of 4.3 W and a slope efficiency of 48%, as well as a 532 nm SFD laser with an output power of 1.1 W have been achieved in a Yb:YAB crystal [8]. Furthermore, a 1040 nm CW laser with an output power of 8.35 W and a slope efficiency of 70%, as well as a 513 nm SFD laser with an output power of 6.3 W have also been realized in a Yb:YCOB crystal [9,10]. Compared with the Yb:YAB and Yb:YCOB crystals with the large effective nonlinear coefficients, the Yb:CTGS crystal has a broader gain band, which is favorable for the generation of ultrashort pulse laser [11–13]. Recently, a CW 1047 nm laser with an output power of 4.2 W and a slope efficiency of 30.4% has been demonstrated in a Yb:CTGS crystal [14]. Compare with that (78%) reported previously in the Yb:CNGS crystal [1], the slope efficiency realized in the Yb:CTGS crystal was still low. Furthermore, the passively Q-switched pulse laser performance of the Yb:CTGS crystal has not been reported. In this work, the CW and passively Q-switched pulse laser performances of the Yb:CTGS crystal were investigated in detail.
2. Laser experimental arrangements
An end-pumped linear plano-concave resonator was adopted for CW and passively Q-switched pulse laser operations, as shown in Fig. 1. A Yb:CTGS crystal grown by the Czochralski method in our lab was used as a gain medium. In this crystal, a part of Ca2+ ions are replaced unequivalently by Yb3+ doping ions, which leads to difficulty for growing the crystal with high doping concentration [15]. Under the current growth conditions, the optical quality of the crystal doped with the Yb3+ concentration higher than 0.9 at.% became worse. Furthermore, according to the polarized spectral investigation [6], the peak absorption and emission cross sections of the Yb:CTGS crystal for σ polarization are large than those for π polarization. Therefore, a 4.0-mm-thick, c-cut uncoated (0.9 at.%)Yb:CTGS crystal with a cross section of 3.0 × 3.0 mm2 was used in this work. Room-temperature absorption coefficient spectrum of the c-cut crystal in 875–1050 nm was recorded by a spectrophotometer (Lambda 950, Perkin Elmer), and is shown in Fig. 2. The peak absorption coefficient of the crystal is 3.2 cm-1 at 977 nm. A CW fiber-coupled laser diode (LD) with a core diameter of 100 µm and a numerical aperture of 0.22 was used as the pumping source. The emission wavelength of the LD is stabilized at 976 nm by the volume Bragg grating (VBG) technology and the FWHM of the emission band is less than 1.0 nm, as shown in Fig. 2. By recording the powers before and after the crystal, the single-pass absorptivity of the 4.0-mm-thick Yb:CTGS crystal was measured to be about 70%, which is close to the theoretical value of 67% calculated from the absorption spectrum of the crystal. By using a telescopic lens system (TLS) consisting of two convex lenses, the pump beam was focused into the crystal. By using the convex lenses with different focal lengths, the magnification ratio of the TLS can be changed and then the pump beams with different waist diameters in the crystal were realized. The crystal was mounted in a copper holder cooled by water at 20 °C. There is a hole with a diameter of 1.0 mm in the center of the holder to allow the passing of laser beams. An input mirror (IM) with a transmission of 91.6% around 976 nm and a reflectivity of 99.7% at 1046 nm was placed as close to the crystal as possible. Three output mirrors (OMs) with the same curvature radius (100 mm) but different transmissions (3.0%, 5.0% and 8.2%) at 1015 and 1046 nm were used. The cavity length was close to 110 mm. In the passively Q-switched pulse laser experiment, a 0.6-mm-thick AR-coated Cr:YAG crystal with an initial transmission of 95% was used as a saturable absorber and inserted as close to the output face of the Yb:CTGS crystal as possible. The cavity length was kept at 110 mm. The pulse profile was measured by a 5 GHz InGaAs photodiode (DET08C, Thorlabs) connected to a digital oscilloscope with a bandwidth of 1 GHz (DSO6102A, Agilent).
Fig. 2. Room-temperature absorption coefficient spectrum of the c-cut Yb:CTGS crystal in 875–1050 nm. The emission spectrum of the used LD is also shown.
3. Results and discussion
CW laser performances of the Yb:CTGS crystal for different pump beam waist diameters were investigated firstly, and the results at an OM transmission of 5.0% are shown in Fig. 3(a). It can be seen that the best laser performance was achieved at a pump beam waist diameter of 156 µm. Then, CW laser performances for different OM transmissions T were further investigated at a pump beam waist diameter of 156 µm, as shown in Fig. 3(b). Laser spectrum was recorded by a spectrometer (waveScan, APE) with a resolution of 0.2 nm, and is shown in the inset of Fig. 3(b). At an absorbed pump power of 5.4 W and an OM transmission of 5.0%, the laser wavelength was located at 1046.4 nm with a FWHM of 1.8 nm, which did not change with the variation of the OM transmission and pump power. When the OM transmission was 5.0% and pump beam waist diameter was 156 µm, a 1046.4 nm laser with a maximum output power of 3.24 W and a slope efficiency of 63.4% was obtained at an absorbed pump power of 5.4 W, and the threshold was 0.5 W. In order to avoid the fracture of the crystal during the laser experiment, the pump power was not further increased. The obtained maximum slope efficiency is about two times of that (30.4%) of the Yb:CTGS CW laser reported previously [14], but still lower than that (78%) of the Yb:CNGS CW laser with the similar resonator structure [1]. Based on the measured thresholds for different OM transmissions [16], the round-trip optical loss of the Yb:CTGS crystal was calculated to be about 1.6%, which is far larger than that (0.32%) of the Yb:CNGS crystal [1]. Under the current growth conditions, the as-grown Yb:CTGS crystal contains some impurities and defects, which leads to the high optical loss of the crystal. Therefore, when optical quality of the Yb:CTGS crystal is further improved by optimizing the growth technology, the laser performance may be enhanced.
Fig. 3. (a) CW output power versus absorbed pump power for different pump beam waist diameters at an OM transmission of 5.0%. (b) CW output power versus absorbed pump power for different OM transmissions at a pump beam waist diameter of 156 µm. The inset shows the laser spectrum at an absorbed pump power of 5.4 W and an OM transmission of 5.0%.
Polarization characteristic of the output laser was analyzed by a Glan-Taylor polarizer, and the results at the absorbed pump powers of 3.9 and 5.4 W are shown in Fig. 4(a). A circularly polarized laser was observed when the absorbed pump power was lower than 3.9 W. When the absorbed pump power was further increased, the polarization state of the output laser changed to the elliptical, which may be caused by the thermal effect of the Yb:CTGS crystal. The thermal focal length of the crystal at an absorbed pump power of 5.4 W was measured to be about 98 mm [17]. The laser beam was focused by a convex lens with a 100 mm focal length. Then, using a Pyrocam III camera from Ophir Optronic Ltd, the spatial profiles of the focused beam at a pump beam waist diameter of 156 µm and an OM transmission of 5.0% were recorded at different distances from the focusing lens. The beam radius was calculated by the 4-sigma method and the beam quality factor M2 can be estimated by fitting these data to the Gaussian beam propagation expression. As shown in Fig. 4(b), a nearly circular output beam was observed, and the M2 in the horizontal and vertical directions (called X and Y, respectively) were fitted to be 1.78 and 2.04 at the absorbed pump power of 5.4 W, respectively.
Fig. 4. (a) Laser polarization states at an OM transmission of 5.0% and a pump beam waist diameter of 156 µm, when the absorbed pump powers were 3.9 and 5.4 W, respectively. (b) Squared beam radius ω2 of the output laser as a function of the distance Z from the focusing lens at an absorbed pump power of 5.4 W, when the OM transmission and pump beam waist diameter were 5.0% and 156 µm, respectively. The inserts show 2D and 3D images of the output beam.
For the passively Q-switched Yb:CTGS pulse laser, when the pump beam waist diameter in the crystal was kept at 156 µm, the average output power versus absorbed pump power for different OM transmissions are shown in Fig. 5(a). A pulse laser with a maximum average output power of 0.4 W and a slope efficiency of 21.2% was obtained at an absorbed pump power of 4.58 W and an OM transmission of 8.2%. The inset shows that the spectrum of the pulse laser at an absorbed pump power of 4.58 W and an OM transmission of 8.2%. Compare with that of the CW laser, the wavelength of pulse laser blue-shifted to 1015.2 nm with a FWHM of 0.15 nm, which is caused by the increment of the cavity loss [18]. The higher cavity loss implies that larger value of population inversion of Yb3+, i.e. higher gain in the cavity, is required for achieving laser oscillation. Because only a c-cut Yb:CTGS crystal was used as the gain medium in this work, the output laser has the σ polarization characteristics. Therefore, only the σ polarized gain cross section spectrum is shown in Fig. 5(b) for analysis. As shown in Fig. 5(b), with the increment of inversion parameter β, the peak gain wavelength of the Yb:CTGS crystal blue-shifts from 1046 to 1015 nm. At an OM transmission of 8.2% and absorbed pump power of 4.58 W, the polarization state of the output pulse laser was measured to be elliptical, which may be caused by the thermal effect in the Yb:CTGS crystal. The beam quality factors M2 of the pulse laser in the horizontal and vertical directions were fitted to be 2.15 and 2.31 at the absorbed pump power of 4.58 W, respectively.
Fig. 5. (a) Average output power versus absorbed pump power for the passively Q-switched Yb:CTGS pulse laser for different OM transmissions T. The inset shows the laser spectrum at an absorbed pump power of 4.58 W and an OM transmission of 8.2%. (b) σ polarized gain cross section spectra in 975–1100 nm of the Yb:CTGS crystal for different inversion parameters β.
Figure 6 shows the dependence of pulse characteristics on the absorbed pump power for different OM transmissions. When the absorbed pump power was increased, the repetition frequency, pulse energy and peak output power increased, while the pulse duration kept unchanged. For OM transmission T = 8.2%, a 1015.2 nm pulse laser with a repetition frequency of 5.43 kHz, energy of 73.6 µJ, duration of 35 ns and peak output power of 2.1 kW was achieved at an absorbed pump power of 4.58 W. The obtained pulse energy is close to that (62.2 µJ) reported previously in the Yb:CNGS crystal [2]. Pulse profile of the output laser at an absorbed pump power of 4.58 W and an OM transmission of 8.2% is shown in Fig. 7. It can be seen that the amplitude between various pulses was changed obviously and the output pulse energy was unstable, due to the thermal effect of the crystal and the multimode laser operation. The amplitude variation between various pulses was generally kept within ±15% and the interpulse time jittering was less than ±10%, respectively. Because the plane OM is not available in our lab at present, the Q-switched pulse performance of the Yb:CTGS crystal in a linear plane-parallel cavity can not be investigated in this work.
Fig. 6. Pulse characteristics of the passively Q-switched Yb:CTGS laser versus absorbed pump power for different OM transmissions T. (a) Repetition frequency. (b) Pulse duration. (c) Pulse energy. (d) Peak output power.
Fig. 7. Pulse profile of the passively Q-switched Yb:CTGS laser at an OM transmission of 8.2% and an absorbed pump power of 4.58 W. (a) Repetition frequency. (b) Pulse duration.
Laser oscillation at 507.6 nm was also observed in the passively Q-switched Yb:CTGS pulse laser experiment, and the spectrum was recorded by a spectrometer (HR4000, Ocean Optics), as shown in Fig. 8. Because the CTGS crystal with the non-centrosymmetric structure is a nonlinear optical material [7], the 507.6 nm laser can be achieved by the second-order nonlinear frequency conversion of the 1015.2 nm fundamental laser. However, because the Yb:CTGS crystal used in this experiment was not cut according to the phase matching angle of the corresponding laser wavelength, only weak SFD signal can be observed. Furthermore, for the SFD laser experiment in the future, an a-cut Yb:CTGS crystal should be adopted to generate the linearly polarized laser appropriately for type I SHG process.
4. Conclusion
Efficient CW and passively Q-switched pulse lasers at 1.0 µm were successfully realized in the Yb:CTGS crystal. The passively Q-switched pulse laser performance of the Yb:CTGS crystal was investigated for the first time. Furthermore, the SFD phenomenon of the crystal was observed in the passively Q-switched pulse laser experiment. If the crystal is cut accurately according to the phase matching angle in the future, the high performance SFD laser can be expected.
Funding
National Key Research and Development Program of China (2021YFB3601504); National Natural Science Foundation of China (52272010); Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (2021ZR119, 2021ZZ118); Scientific Instrument Developing Project of the Chinese Academy of Sciences (YZLY202001).
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.
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