Single crystals of Ho-doped CaYAlO4 crystals with 1.0% and 0.5% holmium concentration have been grown by Czochralski technique in N2 atmosphere. Due to the randomly distributing of Ca2+ and Y3+ ions in the crystal structure, the doped Ho3+ ions show a broad absorption and fluorescence bands. So it can be efficiently utilized to obtain ultrafast pulse generation in time range of pico- or even femtoseconds. Moreover, Ho:CYA laser was operated pumped by Tm:fiber laser, and the maximum output laser power of 2.38 W for 1.0% Ho:CaYAlO4 was obtained.
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In the last few decades, holmium lasers operating on the 5I7 to 5I8 transition is of particular interest because of potential applications, including LIDAR system, materials processing, medical applications, optical communications, and so on . Unfortunately, it is difficult to find ideal laser diodes suitable for direct pumping of holmium ions, therefore, one traditional approach is to use Tm-Ho-codoped crystalline gain media. For such materials, Tm absorbs the pump light, and self-quenches with a quantum efficiency approaching 2, finally transfers its energy to Ho-Ho ions. Norman P. Barnes and associates have established a rate equation model to describe the dynamics of the lowest four manifolds of both the sensitizer Tm and activator Ho simultaneously , and many previous studies have been reported for generating 2-μm laser radiation by using Tm-Ho-codoped YAG [3–5], YLF [5–7], YVO4 , YAP . Especially, A. A. Lagatsky et al., demonstrated picosecond or femtosecond regime pulse laser operation of Tm:Ho-codoped tungstate crystal host pumped by Ti:sapphire . However, these Tm-Ho-codoped laser materials have a significant thermal population and a deleterious upconversion effect due to the Tm-Ho energy transfer processes. So, another solution to this problem is to in-band pump the singly Ho-doped crystals with Tm-doped crystal or fiber lasers. In earlier works, David W. Hart et al. demonstrated continuous wave and Q-switched Ho:LuAG laser pumped by Co:MgF2 laser or Tm:YLF lasers [10,11]. Moreover, many Ho-doped crystal lasers pumped by Tm fiber lasers have been reported by J.W. Kim, D.Y. Shen and so on [12–14]. More recently, Hao Chen and associates reported laser operation of Ho:YAG ceramic with high power as 21.4 W pumped by a Tm fiber laser at about 1907 nm .
CaYAlO4 (CYA) is an interesting laser crystalline host. It belongs to the tetragonal ABCO4 compounds with the K2NiF4 structure. In such crystal, the AlO6 octahedrons formed perovskite-type structure, and Ca2+ and Y3+ ions distribute randomly in the nine coordinated sites. One can see the three-dimensional structure of CYA in [16,17]. This matrix have disordered structure. This feature leads to broadening of absorption and emission lines of lanthanide ions embedded in this compound. In the previous works, many researches about rare earth doped CYA such as Nd:CYA [18,19], Yb:CYA [20,21], Er:CYA [22,23], Tm:CYA [17,22] have been reported. Moreover, Y. Zaouter and associates have obtained femtosecond laser operation using Yb:CaGdAlO4 crystal , which material has the same crystal structure as CYA. The thermal expansion coefficient of CYA crystal (20~1200K) is 8 and 11 × 10−6 /K along the a and c axes, respectively, while the thermal conductivity is 3.7 and 3.3 W/m/K along a and c axes, respectively [18,21]. For Re3+-doped CYA crystals, such as 1.0 at.% Yb3+:CYA, it has a relatively large specific heat value of 0.593 Jg−1 K−1 at 301 K (Nd:YAG, 0.59 Jg−1 K−1 at 300 K), and the room-temperature thermal conductivities along a and c axes are 3.6 and 3.2 W/m/K, respectively . Based on these points, we would like to investigate the optical and lasing properties of holmium doped CYA crystals.
To the best of our knowledge, there are no papers concerning the crystal growth and laser operation of Ho:CYA. Through the conventional Czochralski growth methods, undoped and Ho-doped CYA crystals were obtained. And, absorption and fluorescence spectra were investigated at room temperature. Ho:CYA crystal laser operation experiments was carried out. Tm:fiber laser was used as an excitation source. The maximum output laser power was 2.38 W for 1.0% Ho-doped CYA (slope efficiency was 48.3%).
2. Crystal growth
CaCO3, Al2O3, Y2O3 and Ho2O3 powders (5N purity) were used as starting materials for the crystal growth. Firstly, undoped CYA single crystal was grown by the Czochralski technique. According to , the equilibrium distribution coefficients of Ca and Y differs a lot, therefore obtaining crystal deprived of color centers are nontrivial. In our experiments, however, the undoped CYA crystal was transparent and colorless, as shown in . Crystals with nominal formula Hox:CaY1-xAlO4 (x = 0.01, 0.005) were grown by the Czochralski method. CaYAlO4 crystal rod cut along the <100> direction was used as a seed. The growth atmosphere was N2. The growth temperature was set to 1810°C. All the growth processes are similar to that when we grow neodymium or erbium doped CYA crystals [19,23].
Finally, Ho-doped CYA crystals with good optical quality were obtained, as shown in Fig. 1 . It can be seen that the crystal-melt interface is low arched, and the crystallization front is most flat. Moreover, the 1.0% Ho-doped CYA crystal well developed two planes along the <001> direction, and two (101) planes well developed at the end of the 0.5% Ho-doped CYA crystal, corresponding to the Hartman-Perdok theory (HPT) . Although a low pulling speed was used to prevent inclusions in the growing process, much iridium particles can be found in the as-grown crystal surface due to the high growth temperature. At last, for the 1.0% Ho-doped CYA crystal, the Ca/Y ratio is determined to be 0.98 through inductively coupled plasma atomic emission spectrometer (ICP-AES) experiments. Therefore, we believe the Ca and Y contents are not of much difference, although they occupy the same lattice position randomly.
3. Spectroscopy properties
For spectroscopic measurements, a disk sample was cut from the Ho-doped CYA crystal and optically polished with the thickness of about 1.0 mm. Two surfaces of the disk sample were perpendicular to the <100> axis. The polarized absorption spectra were measured by a Perkin-Elmer Lambda 900 spectrophotometer at room temperature. The unpolarized fluorescence spectrum was recorded on FLSP920 spectrometer. Xenon lamp coupled with monochromator was used as an excitation source. Polarized emission spectra were recorded on DongWoo Optron DM711 monochromator coupled with PbS semiconductor detector. 488 nm line of Ar+ laser was used as an excitation source.
Room temperature polarized absorption spectra of Ho-doped CYA are shown in Fig. 2 and Fig. 3 . It can be easy seen that there are two primary peaks in both polarized spectra. In the E||a polarized direction, the absorption peaks around 1922 nm is much higher than the other one at 1980 nm while in the E||c direction, there are much coincidence between the two absorption peaks at 1948 nm and 1990 nm. So, the pump laser wavelength was chosen at 1922 nm. Figure 4 shows the room temperature unpolarized fluorescence spectra of Ho3+ 5I7→5I8 transition, in which six minor peaks are pointed out. Compared with other Ho-doped crystals, such as YAG , LuAG , KLuW , the optical spectra curve of Ho-doped CYA are more smooth. Attributing to the crystal’s disorder structure, the fluorescence spectra are almost a smooth line with a wide FWHM (full width at half maximum). Moreover, the room temperature polarized fluorescence spectra of 1.0% Ho-doped CYA are shown in Fig. 5 .Strong dependence of crystal anisotropy on polarized emission can be clearly observed. Intensity of emission for light with electrical vector polarized parallel to crystallographic axis a is more than order times stronger contrary to light with electrical vector polarized parallel to axis c. Shape of the line differs with polarization. The most prominent line in E||c polarized spectrum is blue-shifted 25 nm in comparison to strongest line in E||a polarized spectrum. Such a broad emission bands can be efficiently utilized to obtain ultrafast pulse generation in time range of pico- or even femtoseconds.
4. Laser experiments
The Ho-doped CYA crystals were c-cut with the dimensions of 4 × 4 × 20 mm3, and antireflection-coated at 1.9~2.1 μm for laser experiments. The setup used in our experiments is shown schematically in Fig. 6 . The Tm-doped fiber laser was used as pumping source, and the operating wavelength of the fiber laser was turned to be 1922 nm by employing a volume Bragg grating. More detailed information about the pump laser could be found in . The pump fiber laser was collimated by a 30 mm focal length plano-convex lens and focused to a beam of ~300 μm in diameter at the center of Ho-doped CYA crystal with a 200 mm focal length lens. Moreover, a simple two mirror resonator was adopted. The input coupler (IC) was a plane mirror with high reflectively (>99.8%) at the lasing wavelength in the range of 2050~2250 nm and high transmission (> 95%) at the pumping wavelength around 1850~1960 nm. The output coupler (OC) was concave mirror (radius of curvature = 100 mm), with transmission of 5% at 2000~2250 nm and high reflectivity at the pump wavelength. Thecrystal samples were wrapped with indium foil and mounted on a water cooled heat sink maintaining at 20°C. In addition, the physical length of the resonator was 38 mm.
With these operating conditions described above, output characteristics of the Ho-doped CYA crystals were shown in Fig. 7 . The maximum output power were 2.38 W and 1.70 W under 5.0 W incident pump power for 1.0 at. % and 0.5 at.% Ho-doped CYA, respectively. And, it can be seen that the 1.0 at. % Ho-doped CYA laser has a higher slope efficiency (48.3%) than 0.5 at. % Ho-doped CYA crystal laser (35.1%). In another words, better performances have been shown in terms of both slope efficiency and the maximum output power in this experiment. It should be pointed out that, good beam quality of fiber pump sources allows the use of relatively low Ho3+ ion concentration in bulk crystals where the loss due to Ho:Ho upconversion is dramatically . Although better laser operation have been got by the 1.0 at. % Ho-doped CYA crystal than the lower doing one, we can’t sure higher doping concentration is better for laser operation. Besides the doping concentration, the Ho:Ho upconversion parameter is influenced by some other conditions, such as the pumping energy, and the sample length . Therefore, more crystals with various doping concentration should be grown, and different rod length should be selected in the next work.
In addition, output wavelength of the 1.0 at.% Ho-doped CYA laser was measured to be 2092 nm (as shown in Fig. 8 ) using a 0.55 m monochromator of 0.05 nm specified resolution at 438.8 nm (Omni-λ500, Zolix). The laser line width was 5.0 nm. Because of the broad absorption and emission spectra of Ho-doped CYA crystal, it was believed that much higher output power can be scaled, and Q-switched laser operation experiments are in progress.
High optical quality Ho-doped CYA crystals were grown by the Czochralski method. Polarized absorption spectra were investigated and analyzed. Emission spectra showed a wide band due to the randomly distributing of Ca2+ and Y3+ ions. At last, pumped by the Tm-doped fiber laser at 1922 nm, the maximum output power were 2.38 W and 1.70 W for 1.0% and 0.5% Ho-doped CYA crystal, respectively. More modified experiments are in progress, and high output power are believed to scaled. In conclusion, the results of our study of the optical properties and laser experiments permit us to conclude about the potential of this crystal for application as a laser material in infrared area.
This work is partially supported by Science and Technology Innovation Project of Shanghai Institute of Ceramics (Y24ZC5150G) and the Science and Technology Commission of Shanghai Municipality (No. 11DZ1140301). And the author Dahua Zhou sincerely thanks Dr. Yang Fei for the help of drawing the laser schematic diagram.
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