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Abstract
The laser properties of Nd:YPO4 crystal were demonstrated for the first time. For a 1.2 at.% doped Nd:YPO4 crystal, the absorption cross-section at 803 nm, stimulated emission cross-section at 1063 nm, and fluorescence lifetime was measured to be 8.1 × 10−20 cm2, 1.6 × 10−19 cm2, 156 μs, respectively. With an as-grown 0.6 mm thin slice which was unpolished and uncoated, efficient diode-pumped continue-wave (CW) laser operations were realized at 1.06 and 1.3 μm wavebands. The 1063 nm output power reached 2.16 W when the absorbed pump power was 4.07 W, corresponding to an optical-to-optical efficiency of 53%, and a slope efficiency of 56.4%. The 1.3 μm laser output exhibited the simultaneous operations of dual-wavelengths, i.e. 1338 and 1347 nm. The maximum output power was 800 mW at an absorbed pump power of 3.08 W, giving an optical-to-optical efficiency of 26% and a slope efficiency of 28.2%.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As a popular used laser material, the Nd:YVO4 crystal with tetragonal zircon structure possesses many advantages, like high absorption coefficient, broad absorption bandwidth, large emission cross-section, stable physical and chemical properties, and polarized output, etc [1–3]. Now it has played a leading role in solid-state laser fields of small and medium power levels.
In recent years, a new system of orthophosphates (LnPO4, Ln = Sc, Y, Tb, Dy, Ho, Er, Tm, Yb, and Lu) with the same tetragonal zircon structure gradually arouses people's attention, because of excellent laser, linear and nonlinear optical properties [4–6]. Comparing with YVO4 crystal, the transmission waveband of YPO4 is broader (150 – 3300 nm), at same time it has no crystal defects caused by the valence change of vanadium element at high temperature. In 2013, the ytterbium (Yb) doped LuPO4 crystal were successfully grown by the high-temperature solution method [7]. Its CW, passively and actively Q-switched laser properties were demonstrated successively. With a 0.3 mm thickness, 10 at.% doped Yb:LuPO4 crystal, 1.61 W CW laser around 1039 nm was achieved [8]. Then the output power was improved to 5.3 W with a 2 mm, 5 at.% doped Yb:LuPO4 crystal [9]. The passively Q-switched operation of Yb:LuPO4 presented a pulse duration of 3 ns and a peak power of 50.3 kW [10]. In 2017, the CW laser performance of Yb:YPO4 were reported [11], giving a maximum output power of 3.62 W and an optical conversion efficiency of 36%.
Neodymium (Nd) doped crystals are important solid-state laser materials. Compare with Yb3+ doped crystals, the four energy-levels system of Nd3+ is more easily to realize laser emission. However, to date, the only report on lasing properties of Nd-doped orthophosphate was at the end of the last century [4]. With a 1 mm long, 10% doped Nd:LuPO4 crystal, a maximum output power of 0.3 W was obtained at 1.06 μm, corresponding to a slope efficiency of 54%. For Nd:YPO4 crystal, O. Guillot-Noël et. al. reported its low temperature transmission spectrum of the 4I9/2→4F3/2 transition, as well as Electron Paramagnetic Resonance (EPR) spectrum in 2000 [12]. In present paper, the room temperature spectra and laser properties of Nd:YPO4 were reported for the first time. Efficient diode-pumped CW laser operations were demonstrated at 1.06 and 1.3 μm wavebands, with the highest output powers of 2.16, 0.8 W, respectively. This work shows that Nd:YPO4 is an excellent laser material for diode-pumped microchip laser.
2. Crystal growth
The YPO4 crystal belongs to the space group I41/amd and point group 4/mmm. Lanthanide ions Nd3+ are doped into the subject host by partially replacing the Y3+ ions, and occupying the sites with unique point-group symmetry D2d. For crystal growth, Re2O3 (Re = Nd, Y) and PbHPO4 are mixed with a certain proportion in a platinum crucible. The PbHPO4 is decomposed into Pb2P2O7, a part of Pb2P2O7 reacts with Re2O3 to synthesize RePO4, and the other is used as the flux. The temperature is kept at 1250 °C for 24 h. After the solution is stable, it is cooled down at a rate of 1 °C /h. When the temperature falls to 900 °C, the crystal basically no longer grows. Then the power is shut down until the room temperature. By soaking with strong HNO4 for 2 days, the YPO4 crystals are separated from the flux. With above high-temperature solution growth method, we have successfully obtained Nd:YPO4 crystal plates with millimeter sizes. The plate-like habit is not related to the natural cleavage, but originates from the growth rate difference of different crystal directions. The growth rate along c axis is much larger than those along a and b axes. At the beginning of the cooling, we will make an oscillation of growth temperature to control the number of nucleation, which will affect the supersaturation of the growth solution. In this way, the normal growth rate of the crystal cylindrical surface can be adjusted. Combining with the crystal growth time, we can control the sample thickness in a certain extent. The inset of Fig. 1 is an as-grown Nd:YPO4 crystal with dimensions of 3 × 2 × 0.6 mm3, which is used as the experimental sample for various measurements in this work. The crystallographic plane is (010), with long edge along c axis and short edge along a axis, respectively. Its nominal Nd3+ concentration, i.e. the Nd3+proportioning in the growth raw material, is 3 at.%. According to the electron probe microanalysis (EPMA) measurement, the practical Nd3+ concentration in the Nd:YPO4 crystal is 1.2 at.%. So the segregation coefficient of Nd3+ ions is 0.4.
3. Spectral properties
At room temperature (290 K), the absorption spectrum of Nd:YPO4 was measured by a Hitachi U-3500 spectrophotometer with a resolution of 0.2 nm. As seen in Fig. 1, the largest absorption cross-section is 8.1 × 10−20 cm2, which locates at 803 nm. The corresponding absorption coefficient is 13.78 cm−1. The full width at half maximum (FWHM) of this absorption peak is ~5 nm, which is very suitable for AlGaAs laser diode pumping.
The emission spectrum and luminescence decay curve were also measured at a room temperature of 290 K, and the results were shown in Figs. 2 and 3, respectively. The Fuchtbauer-Ladenbrug (F-L) formula was employed to calculate the stimulated emission (SE) cross-section σSE,
where I(λ) is the intensity of the emission spectrum, τrad is the radiation lifetime, c is the light speed and n is the refractive index at emission wavelength [5]. Three emission bands at ~900 nm, ~1063 nm and ~1340 nm were observed, corresponding to the Nd3+ energy level transitions of 4F3/2 → 4I9/2, 4F3/2 → 4I11/2 and 4F3/2 → 4I13/2, respectively. The strongest emission occurs at 1063 nm, giving the maximum σSE of 1.6 × 10−19 cm2. For the multi-peak emissions at 1320-1385 nm, the largest σSE is 6.0 × 10−20 cm2, which is 3/8 of the value at 1063 nm. The σSE at 900 nm waveband is 1.7 × 10−20 cm2, which is much smaller than those at 1.06 and 1.34 μm. It needs to be noted that in this paper we present unpolarized absorption and emission spectra, because the crystal sizes are so small that for polarized measurements the signals are very weak compared with the background noises, the corresponding experimental data cannot be used for the accurate calculation of spectral parameters.
Fig. 3 Luminescence decay curve of Nd:YPO4 crystal. The black curve is the measured data, and the red line is the fitting curve.
By single-exponential fitting the luminescence decay experimental curve in Fig. 3, the luminescence lifetime τ of Nd:YPO4 is determined to be 156 μs, which is much longer than that of Nd:YVO4 crystal (95 μs for 1.2 at.% Nd3+ ions concentration [13]). From the absorption spectrum of Fig. 1, the total radiative lifetime τrad is calculated to be 201 μs by J-O theory [14]. So the fluorescence quantum yield τ/τrad is 0.78 for Nd:YPO4 crystal. The fitted intensity parameters are Ω2 = 2.73 × 10−20 cm2, Ω4 = 5.26 × 10−20 cm2, Ω6 = 8.75 × 10−20 cm2, respectively. The emission probabilities A(J”→J’), luminescence branching ratios β(J”→J’), and radiative lifetimes τ’rad of different transitions are listed in Table 1. For the transition of 4F3/2→4I11/2 which emissions 1.06 μm laser wavelength, the β value of Nd:YPO4 is 0.624, which is comparable to Nd:YAG (0.60) and larger than Nd:YVO4 (0.467) [15, 16].
Table 1. Luminescence parameters of Nd:YPO4 crystal
4. Laser experiments
With a fiber-coupled laser diode (LD) as the pump source, the Nd:YPO4 laser experiments were performed under a plano-concave resonator, as shown in Fig. 4. The center emission wavelength of the LD is 805 nm, the fiber core diameter is 100 μm and the numerical aperture (N. A.) is 0.22. A lens assembly (LA) with 1:1 imaging ratio and 36 mm focal length is used to focus the pump beam into the laser crystal. The planar mirror at the pump end (M1) is anti-reflection (AR) coated at 805 nm and high-reflection (HR) coated at laser emission wavebands (1.06 or 1.34 μm). The concave mirror at the output end (M2) is partially transmitted at laser emission waveband, with a curvature radius of 100 mm. For 1.06 μm laser operation, the transmittances of M2 (TM2) are chosen to be 2%, 5%, 10% and 20%, respectively. For 1.34 μm laser operation, TM2 are 2%, 5%, 10%, and 15%. As demonstrated by the inset of Fig. 1, the Nd:YPO4 sample is a thin slice with a transmitting length of 0.6 mm. It is an as-grown crystal without polishing and coating. To relieve the thermal effect, the crystal slice is stuck on a copper block with thinned, heat conductive, double-sided adhesive. The copper block is cooled by circulating water of 15 °C. The total cavity length is about 10 mm.
The experimental results are shown in Fig. 5. With the elevating of the pump power, the absorption efficiency decreases from 49% to 31%. For 1.06 μm laser operation, the optimized output coupler (OC) is TM2 = 10%. Under this condition, the pump threshold is 0.22 W, and the maximum output power is 2.16 W when the absorbed pump power is 4.07 W, giving an optical conversion efficiency of 53% and a slope efficiency (ηs) of 56.4%, as shown in Fig. 5(a). From the inset of Fig. 5(a), it can be seen that the laser wavelength is 1063 nm. It does not change with the variation of experimental conditions. In all cases, the 1.06 μm laser output is π-polarized, i.e. its linear polarization direction is along the c-axis (long edge) of the Nd:YPO4 sample. In this experiment, the Nd:YPO4 crystal was not damaged under a high intensity, CW absorbed pump light of 50 kW/cm2. According to our measurements, for 800 nm CW absorbed pump light, the damage thresholds of 0.5~1 at.%, 1~1.5 mm Nd:YVO4, Nd:GdVO4, Nd:LuVO4 thin slices are ~10 kW/cm2. The high anti optical damage ability of Nd:YPO4 crystal are favorable for future high power laser applications. Beside of plano-concave resonator introduced above, we also attempted plane-parallel laser cavity. With a plane output coupler of TM2 = 2%, we achieved the highest output power of 1.33 W under the same experimental conditions. It is only a little lower than the result of TM2 = 2% concave output coupler, i.e. 1.53 W as demonstrated in Fig. 5(a). This experiment justifies the further microchip laser application of Nd:YPO4 crystal.
Fig. 5 1.06 (a) and 1.34 μm (b) laser characteristics obtained from different output transmittances. Insets: the corresponding laser spectra measured at the maximum output powers.
The 1.3 μm laser results are shown in Fig. 5(b). The optimized OC is TM2 = 5%, which is lower than the value (TM2 = 10%) of 1.06 μm. It can attribute to the smaller σSE at 1.3 μm, which need to be compensated by the smaller coupling transmittance. For the TM2 = 5% OC, the maximum output power is 800 mW at an absorbed pump power of 3.08 W, corresponding to an optical conversion efficiency of 26% and a slope efficiency of 28.2%. At the highest output power, the simultaneous operations of 1337.7, 1346.6 nm dual-wavelengths are observed, as seen in the inset of Fig. 5(b). Both of the wavelengths are σ-polarized, with their polarization directions along the a-axis (short edge) of the Nd:YPO4 sample. When the pump power is larger than 3.1 W, the present cooling structure is failure and the laser output power no longer increases. The similar experimental phenomenon occurs for 1.06 μm laser operation, i.e. the heat induced saturation of output power happens at high pump levels. Nevertheless, during the whole 1.3 μm laser operation processes, no optical damage is found from the Nd:YPO4 crystal.
With the Findlay-Clay method [17], we explored the intra-cavity losses of diode-pumped Nd:YPO4 lasers. The round-trip loss δ can be written as
where L is the double-pass passive loss and R is the reflectivity of the output coupler. Since the absorbed pump power at laser threshold (Pth) is proportional to δ [4], i.e. Pth ∝ δ, from the relationship between Pth and ln(1/R) the parameter L can be deduced. The experimental data are plotted in Figs. 6(a) and 6(b), for 1.06 and 1.34 μm, respectively. By linear fittings, the L values are obtained from the horizontal coordinate intercepts, which represent the situations of Pth = 0. The fitting results are L = 7.7% for 1.06 μm laser and L = 4.8% for 1.34 μm laser. Considering the Nd:YPO4 sample is unpolished and uncoated, we speculate that above losses mainly come from the reflection of the crystal surfaces. By crystal polishing and AR coating, the Nd:YPO4 lasers are hopeful to perform better. By referencing the Sellmeier equations of YPO4 crystal [5], it can be known that the refractive index of π-polarized 1.06 μm laser is 1.816 (ne), and the refractive index of σ-polarized 1.34 μm laser is 1.639 (no). Compared with 1.34 μm laser, 1.06 μm laser has higher refractive index which means larger surface reflection loss. It may be the primary reason for its larger L value.We also attempted the 4F3/2 → 4I9/2 transition, i.e. 900 nm laser operation, but no output was obtained. This result can attribute to three reasons: large crystal refractive indexes compared to those at 1.06 and 1.3 um, three-level transition system, and small stimulated emission-cross section. For the present uncoated crystal sample, these factors mean large surface reflection loss, high pump threshold, and increased operating difficulty.
Although the 0.9 μm laser oscillation and output have not been realized, the efficient 1.06, 1.3 μm laser operations of Nd:YPO4 thin plate still demonstrate that this crystal is a promising microchip laser material. With diode-pumped 1.06 μm CW laser as an example, we give a comparison between the current results with those previous reported typical microchip lasers based on other Nd-ion crystals in Table 2. Here, both the slope efficiency and the optical efficiency are relative to absorbed pump power. It can be seen that among all of the laser microchip materials listed in Table 2, Nd:YPO4 has exhibited prominent output performance including low threshold, large power, and high efficiency.
Table 2. A comparison of diode-pumped 1.06 μm CW laser results for different Nd3+ doped crystal microchips.
In Table 3, the spectroscopic and thermal parameters are listed for Nd:YPO4 and other several most excellent Nd3+ doped laser crystals, including Nd:YVO4, Nd:YAG and Nd:YLF.
Table 3. Spectroscopic, thermal parameters of Nd:YPO4 and several famous Nd3+ doped laser crystals
The emission cross-section and absorption cross-section of Nd:YPO4 crystal are comparable with those of Nd:YAG and Nd:YLF crystals. The high doping capability of Nd:YPO4 will bring high laser gain (per unit length) and larger absorption coefficient for pump light, which is favorable for diode end-pumped microchip laser operations. The excellent thermal and anti laser damage characteristics indicate that Nd:YPO4 is hopeful to be applied for high power laser output. It need to be noted that limited by the small sample sizes we are unable to give out our own thermal conductivity data of Nd:YPO4 crystal at present stage. In Table 2 we list the value of YPO4 polycrystalline ceramic instead. In material fields, it has been widely accepted that around the room temperature the thermal conductivity of single crystal is almost identical to that of the corresponding polycrystalline ceramic, at the same time a low level rare earth ions doping will not affect the thermal conductivity apparently. So, the value of YPO4 polycrystalline ceramic (12.0 W/m°C) in Table 3 is available for low-doping Nd:YPO4 crystal. In the reference [32], the laser damage threshold of YPO4 crystal was measured to be 2.4 GW/cm2 at 1064 nm, 17 ns conditions. According to the reference [27], this parameter is proportional to tp0.5 where tp is the pulse width, so the corresponding conversion value for 1064 nm, 10 ns is 1.85 GW/cm2, which is listed in Table 3. Compared with Nd:YVO4, Nd:YPO4 has small emission cross-section and long fluorescence lifetime, which represent good energy storage ability and passively Q-switching potential [35, 36]. For Nd-doped vanadate laser crystals, the mixing of multiple constituents is an effective approach to improve passively Q-switching and mode-locking performances, by broadening emission spectrum, decreasing stimulated emission cross-section, and elevating fluorescence lifetime [36–38]. Previously, several documents have proved that the mixed crystals LuxY1-xPO4 (0 < x < 1) are available [39, 40]. By referencing the experience of Nd-doped vanadate crystals, one can expect better pulse laser performance from mixed orthophosphate crystal Nd:LuxY1-xPO4 than that from single crystal Nd:YPO4.
5. Conclusions
By high-temperature solution growth method, we successfully obtained Nd:YPO4 crystal with millimeter sizes. With an as-grown, 0.6 mm thickness Nd:YPO4 thin plate, which was uncut, unpolished and uncoated, 2.16 W laser output was obtained at 1063 nm, and 800 mW laser output was obtained at 1338, 1347 nm dual-wavelengths, corresponding to optical conversion efficiencies of 53%, 26%, respectively. Our research manifests that Nd:YPO4 crystal is an excellent low threshold, high efficiency microchip laser material, which possesses good spectroscopic, thermal and anti laser damage properties. Its unprocessed characteristic is helpful for rapid use and saving cost.
Funding
National Natural Science Foundation of Shandong Province (ZR2017MF031); National Natural Science Foundation of China (11374170).
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