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Abstract
1060 nm continuous-wave and passively Q-switched pulse lasers were demonstrated in a 3.5-mm-thick X-cut Nd:GdPO4 crystal. At an absorbed pump power of 1.91 W, a 1060 nm continuous-wave laser with a maximum output power of 0.72 W and a slope efficiency of 42% was realized. When a Cr4+:YAG saturable absorber with an initial transmission of 85% was used, a passively Q-switched pulse laser with repetition frequency of 6.7 kHz, pulse energy of about 19 µJ, pulse duration of 58 ns, and peak power of 0.33 kW was obtained in a plano-concave cavity with a length of 95 mm and an absorbed pump power of 1.91 W. For a plano-plano cavity with a length of 9 mm and the same pump power, a pulse laser with repetition frequency of 7.2 kHz, pulse energy of about 14.1 µJ, pulse duration of 4.1 ns and peak power of 3.4 kW was obtained.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to the advantages of high physicochemical stability, high thermal conductivity, and strong capability of accommodating large amounts of dopant ions, rare earth orthophosphate LnPO4 crystals have been investigated as promising host materials of solid-state lasers [1–5]. According to the different rare earth ion elements, the structure of the orthophosphates can roughly be divided into two types, namely the monoclinic structure (Ln = La to Gd) and the tetragonal structure (Ln = Tb to Lu as well as Sc and Y) [6].
At present, the host crystals YPO4 and LuPO4 with tetragonal structure doped with Nd3+ and Yb3+ ions have been demonstrated as efficient gain media for solid-state lasers [2–5]. A 1063 nm continuous-wave (CW) laser with an output power of 2.16 W and a slope efficiency of 56.4%, as well as a 1034 nm CW laser with an output power of 1.61 W and a slope efficiency of 75% have been realized in an Nd:YPO4 and a Yb:LuPO4 crystals, respectively [3,5]. In addition, a 1005 nm passively Q-switched pulse laser with pulse energy of 151 µJ, repetition frequency of 19.2 kHz and pulse duration of 3 ns has also been obtained in a Yb:LuPO4 crystal [4]. However, due to the high volatility and toxicity of the Pb2P2O7 solvent, the YPO4 and LuPO4 crystals can only be grown by the spontaneously-nucleated flux method [5,7]. Then, only millimeter-size YPO4 and LuPO4 crystals have been grown [2–5], which limits research and application of the crystals.
Using the Li2CO3-2MoO3 as a solvent, an Nd:GdPO4 crystal with dimensions of 31 × 13 × 10 mm3 was successfully grown by the top-seeded solution growth method in our lab recently [8]. The crystal belongs to the monoclinic structure with space group P21/n. The detailed structure diagram of the crystal can be found in Ref. [8]. Room-temperature polarized spectroscopic properties of the Nd:GdPO4 crystal have been investigated in detail, and the crystal has a maximum absorption cross section of 2.13×10−20 cm2 at 800 nm and a maximum emission cross section of 7.11×10−20 cm2 at 1060 nm for the E//Z polarization [8]. The specific heat capacity, thermal conductivity, and the thermal expansion of the GdPO4 crystal are 0.41 J·g-1·K-1, 3.2 W·m-1·K-1 and 12×106 m·K-1 at room temperature, respectively [9]. Furthermore, the investigation shows that Nd:GdPO4 crystal has a weaker concentration-dependent fluorescence quenching effect compared with the Nd:YAG crystal [8]. Therefore, the GdPO4 crystal doped with a high Nd3+ concentration may be used as a promising microchip gain medium for 1.06 µm laser. In this work, CW and passively Q-switched pulse laser performances of the Nd:GdPO4 crystal were investigated for the first time.
2. Laser experimental arrangements
An end-pumped linear resonator was adopted and is shown in Fig. 1. A 3.5-mm-thick, X-cut uncoated (3 at.%)Nd:GdPO4 crystal with a cross-section of 2 × 3 mm2 was used as a gain medium. Room-temperature polarized absorption spectra of the crystal in 770-830 nm was recorded by a spectrophotometer (Lambda 950, Perkin Elmer), and is shown in Fig. 2. The peak absorption coefficient of the crystal for the E//Z polarization is 13.7 cm-1 at 800 nm. The width of absorption band around 800 nm of the crystal is about 16 nm, and broader than those of the Nd:YVO4 (about 1.9 nm) and Nd:YAG (about 1.0 nm) crystals [8]. The broad bandwidth indicates that the Nd:GdPO4 crystal may be more suitable to be pumped by diode laser, because the emission wavelength of diode laser may shift with the variations of the temperature and output power. Due to the lack of diode laser emitted at 800 nm in our lab at present, a fiber-coupled diode laser at 790 nm with a core diameter of 100 µm and a numerical aperture of 0.15 from Dilas Inc was used to pump the crystal. By using a polarizer, the pump beam delivered by the used laser diode is measured to be circularly polarized. It can be seen from Fig. 2 that the absorption coefficient at 790 nm of the crystal is 2.7 cm-1. The full width at half maximum (FWHM) of absorption peak at 790 nm was about 2.8 nm. The single-pass absorptivity of a 3.5-mm-thick Nd:GdPO4 crystal was measured to be about 62% by recording the powers before and after the crystal, which is close to the theoretical value of 60% calculated from the polarized absorption spectra of the crystal. By using a telescopic lens system (TLS) consisting of a 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 are realized. The crystal was mounted in a copper holder cooled by water at 20 °C. There is a hole with a diameter of 1 mm in the center of the holder to allow the passing of laser beams. An input mirror (IM) with a transmission of 90% around 790 nm and a reflectivity of 99.7% at 1060 nm was placed as close to the crystal as possible. Four output mirrors (OMs) with the same radius of curvature (100 mm) but different transmissions (0.7%, 1.6%, 3.4% and 5%) at 1060 nm were used. The cavity length was close to 95 mm.
Fig. 2. Room-temperature polarized absorption spectra of the (3 at.%) Nd:GdPO4 crystal in 770-830 nm.
3. Results and discussion
CW laser performances of the Nd:GdPO4 crystal for different pump beam diameters were firstly investigated, and the results at an OM transmission of 3.4% are shown in Fig. 3(a). It can be seen that the highest slope efficiency of the laser was achieved at a pump beam diameter of 156 µm. Then, CW laser performances for different OM transmissions T were investigated at a pump beam 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 1.91 W and an OM transmission of 3.4%. The laser wavelength was located at 1060 nm with a FWHM of 2.5 nm, which did not change with the variation of the OM transmission and pump power. When T was 3.4%, a 1060 nm laser with a maximum output power of 0.72 W and a slope efficiency of 42% was obtained at an absorbed pump power of 1.91 W, and the threshold was 0.12 W. In order to avoid the crack of the crystal during the laser experiment, the pump power was not further increased.
Fig. 3. (a) CW output power versus absorbed pump power for different pump beam diameters at an OM transmission of 3.4%. (b) CW output power versus absorbed pump power for different OM transmissions at a pump beam diameter of 156 µm. The inset shows the laser spectrum at an absorbed pump power of 1.91 W.
The comparison of CW laser performances between the Nd:GdPO4 and other similar Nd3+ doped crystals, such as GdVO4, YPO4, LuPO4 and KGd(PO3)4, is listed in Table 1. The highest slope efficiency of 42% presently realized in the Nd:GdPO4 crystal is lower than those of the Nd:LuPO4 (54%) and Nd:YPO4 (56.4%) crystals [2,5]. The difference may be induced by the more serious thermal effect in the Nd:GdPO4 crystal, because the thermal conductivity (3.2 Wm-1K-1 [9]) of the GdPO4 crystal is far lower than those (about 12 Wm-1K-1 [10]) of the LuPO4 and YPO4 crystals. However, the GdPO4 crystal belongs to the monoclinic structure, while the LuPO4 and YPO4 crystals belong to the tetragonal structure. At present, larger-sized Nd3+ doped LuPO4 and YPO4 orthophosphate crystals cannot be grown by the same approach used in the growth of Nd:GdPO4 crystal. In the following work, we will try to adjust and develop the growth technology, including the solvent and growth parameters, for realizing the growth of the large-sized other Nd3+ doped orthophosphate crystals with higher thermal conductivity, such as LuPO4, YPO4 and GdLuPO4. Furthermore, using the Findlay-Clay method [11], the round-trip loss coefficient of the Nd:GdPO4 crystal was calculated to be about 0.02 cm-1 based on the measured thresholds for different OM transmissions. Above value is far larger than those (0.001 cm-1 and 0.005 cm-1, respectively) of the well-known Nd:YAG and Nd:YVO4 crystals [12,13]. Therefore, when optical quality of the Nd:GdPO4 crystal is further improved by optimizing the growth technology, such as the adjustment of the flux, seed orientation and temperature field design, the laser performance can be enhanced in the future.
Fig. 4. (a) Squared beam radius ω2 of the output laser as a function of the distance Z from the focusing lens at an OM transmission of 3.4%, a pump beam diameter of 156 µm, and an absorbed pump power of 1.91 W. The inserts show the 2D and 3D images of the output beam. (b) Beam quality factors in the horizontal and vertical directions for different absorbed pump powers.
Table 1. Comparison of 1.06 µm CW laser performances for some Nd3+ doped phosphate and GdVO4 crystals
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 diameter of 156 µm, and an OM transmission of 3.4% 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. Figure 4 shows the spatial profile and quality factor M2 of the laser beam. A nearly circular output beam was observed. The beam quality factors M2 in the horizontal and vertical directions (called H and V corresponding to the E//Y and E//Z polarization directions, respectively) were fitted to be 2.96 and 3.03 at an absorbed pump power of 1.91 W, respectively. When the pump power was reduced to 0.52 W, M2 in the H and V directions were improved to 1.58 and 1.63, respectively, due to the reduction of the thermal effect. To investigate the polarization characteristics along the two directions, i.e. E//Y and E//Z, respectively, of the X-cut crystal, the output lasers under the absorbed pump powers of 1.5 and 1.9 W were analyzed by a Glan-Taylor polarizer, as shown in Fig. 5(a). It can be seen that an E//Z linearly polarized laser was always observed, which is caused by the larger emission cross section for the E//Z polarization for the X-cut Nd:GdPO4 crystal shown in Fig. 5(b) [8].
Fig. 5. (a). Laser polarization state at an OM transmission of 3.4%, a pump beam diameter of 156 µm, and the absorbed pump powers of 1.5 and 1.9 W. (b). Room-temperature polarized emission cross section spectra of the Nd:GdPO4 crystal in 1000-1150 nm [8].
According to the resonator transform circle theory [16], the thermal focal length of a laser medium in a plano-plano resonator is equal to the cavity length at which the output power vanishes. Then, the thermal focal lengths of the Nd:GdPO4 crystal at different absorbed pump powers were measured and are shown in Fig. 6 at the pump beam waist diameter of 156 µm. Based on the measured thermal focal length of 110 mm and the ABCD law of Gaussian beam propagation [17], the waist diameter of the fundamental laser in the Nd:GdPO4 crystal was calculated to be 242 µm, when waist diameter of the pump beam was 156 µm and the absorbed pump power was 1.91 W. In this case, the mode overlap efficiency (ωl/ωp)2 between the laser and pump beams was estimated to be 2.4 [18].
Fig. 6. Thermal focal length versus absorbed pump power in the Nd:GdPO4 crystal at the pump beam waist diameter of 156 µm.
In order to investigate the passively Q-switched pulse performance of the Nd:GdPO4 crystal, A 0.5-mm-thick AR-coated <100> Cr4+:YAG saturable absorber with an initial transmission of 85% was inserted into the laser cavity, and placed as close to the output face of the crystal as possible. The pump beam diameter in the crystal was also 156 µm and the cavity length was kept at 95 mm. Because the Cr4+:YAG crystal is placed as close to the Nd:GdPO4 crystal, the waist diameter of the fundamental laser in the saturable absorber is similar to that (about 242 µm) in the gain medium when the absorbed pump power was 1.91 W. 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). The pulse characteristics versus absorbed pump power for different OM transmissions are shown in Fig. 7. At an absorbed pump power of 1.91 W, a 1060 nm Nd:GdPO4 pulse laser with average output power of 0.13 W, repetition frequency of 6.7 kHz, pulse energy of about 19 µJ, pulse duration of 58 ns, and peak output power of 0.33 kW was obtained at an OM transmission of 5%. The wavelength of output laser did not change with the variation of the OM transmission and the pump power. To the best of our knowledge, it is the first passively Q-switched pulse laser demonstration in the Nd3+ doped orthophosphate LnPO4 crystals.
Fig. 7. Pulse characteristics of the passively Q-switched Nd:GdPO4 laser versus absorbed pump power for different OM transmissions in a plano-concave cavity. (a) Average output power. The inset shows the pulse laser spectrum. (b) Repetition frequency. (c) Pulse duration. (d) Pulse energy. (e) Peak output power.
Previous investigation has shown that a Q-switched pulse laser with narrower width and higher output peak power can be realized in a cavity with a shorter length and a higher OM transmission [19]. Therefore, the passively Q-switched Nd:GdPO4 pulse laser was also investigated in a plano-plano cavity at a pump beam diameter of 156 µm. The cavity length was reduced to 9 mm and OM transmission was increased to 8.6%. In this case, the waist diameter of the fundamental laser in the saturable absorber was estimated to be 208 µm when the absorbed pump power was 1.91 W. Figure 8 shows the pulse characteristics versus the absorbed pump power. At an absorbed pump power of 1.91 W, a 1060 nm Nd:GdPO4 pulse laser with average output power of 0.1 W, repetition frequency of 7.2 kHz, pulse energy of about 14.1 µJ, pulse duration of 4.1 ns, and peak output power of 3.4 kW was realized. The pulse profile of the output laser at an absorbed pump power of 1.91 W is shown in Fig. 9. The amplitude variation between various pulses was generally kept within ±10% and the interpulse time jittering was less than ±5%, respectively.
Fig. 8. Pulse characteristics of the passively Q-switched Nd:GdPO4 laser versus absorbed pump power in a plano-plano cavity with an length of 9 mm and an OM transmission of 8.6%. (a) Average output power. The inset shows the laser spectrum at an absorbed pump power of 1.91 W. (b) Repetition frequency and pulse duration. (c) Pulse energy and peak output power.
Fig. 9. Pulse profile of the passively Q-switched Nd:GdPO4 laser in a plano-plano cavity with a length of 9 mm, an OM transmission of 8.6%, a pump beam diameter of 156 µm, and an absorbed pump power of 1.91 W. (a) Repetition frequency. (b) Pulse duration.
4. Conclusion
1060 nm CW and passively Q-switched pulse lasers were successfully realized in the Nd:GdPO4 crystal. It is the first laser operation demonstrated in the Nd3+ doped orthophosphate LnPO4 crystal with monoclinic structure. A centimeter-size Nd:GdPO4 crystal can be grown by the top-seeded solution growth method. Therefore, the crystal may be a promising gain medium of solid-state laser.
Funding
Strategic Priority Research Program of Chinese Academy of Sciences (XDB200000000); Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (2021ZR119, 2021ZZ118).
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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