Tm, Mg co-doped LiTaO3 crystal is grown by the traditional Czochralski method. By analyzing the absorption and emission measurements of the Tm/Mg: LiTaO3 single crystal with the Judd-Ofelt analysis, the intensity parameters Ω2,4,6, transition probabilities, exited state lifetimes, branching ratios, and emission cross-sections were calculated. Non-photorefractive continuous wave laser operation with a Tm/Mg: LiTaO3 single crystal is demonstrated for the first time. We obtained 1.51 W output power at 1.92 μm with a slope efficiency of near 38.5%, which, to the best of our knowledge, are the largest output power and highest slope efficiency obtained for this crystal thus far. The long-term photorefractive effect was also quantitatively analyzed.
© 2014 Optical Society of America
Tm3+-activated materials have attracted much attention in the last years for their broad infrared luminescence emissions, from 1400 nm up to 2100 nm (associated with 3H4 → 3F4 and 3F4 → 3H6 transitions), because of their potential applications in atmospheric, medicinal, and space applications [1–3]. In particular, solid-state lasers near 1.9 μm using Tm doped crystalline host materials are of considerable interest because they can be used as efficient optical pump sources for optical parametric oscillator (OPO) near degeneracy where the parametric gain is significantly higher. It can also be used to pump active laser materials like Cr2+ doped ZnSe as already demonstrated [4, 5]. Furthermore, compared to long 2-μm (2.0–2.1 μm) wavelength lasers, the lasers around 1.9 μm have a lower absorption of vapor, making it more suitable for laser radar systems .
During the past few years, different Tm doped crystalline host materials have been already investigated for their favourable properties to obtain an efficient laser emission [7–12], and Table 1 presents an example of demonstrated laser action in common Tm-doped crystals. However, only the Tm: YLF, Tm:YVO4, and Tm/Mg: LN crystals [9, 11, 12] obtained an efficient laser action with a center wavelength below 1.91 μm. As a quasi-three level system, the laser emission near 1.9 μm is difficult to achieve due to the reabsorption effect in the Tm-ion laser. Recently, we have demonstrated an efficient continuous wave (CW) laser output at 1856 nm wavelength from a Tm, Mg codoped lithium niobate (LiNbO3, LN) crystal, which provides a great opportunity for efficient short 2-μm (1.8–2.0 μm) wavelength lasers . Lithium tantalate (LiTaO3, LT) crystal, which is structurally isomorphous to LN, is one of the most excellent and useful ferroelectric materials for piezoelectric, linear electric-optic, and nonlinear optic device applications [13, 14]. Compared to LN crystal, LT crystal has smaller birefringence and higher photorefractive damage resistance, which makes LT more suitable for high power laser operation. Furthermore, the photorefractive damage resistance of the LT crystal could be increased by the co-doping of Mg [15–17]. Compared with other laser crystals (YAG, YAP, YLF, LLF, YVO4), there has been a significant interest in the growth and investigation of the properties of LT crystal doped with rare earth, in which laser oscillation, electro-optic, and nonlinear optics effects can be integrated within just one crystal. In addition, the uniaxial character of LT crystal ensures natural polarization of the output laser beam. These properties make the rare earth doped LT crystals attractive for multi-functional photonic and integrated opto-electronic applications.
In this Letter, the Tm/Mg: LT crystal with high quality was successfully prepared. The absorption spectrum, the fluorescence spectrum, and the laser performance of the Tm/Mg: LT crystal were also investigated. Based on the absorption spectrum, Judd-Ofelt (J-O) theory was applied to calculate spectral parameters of Tm ions. Non-photorefractive continuous wave laser operation with a Tm/Mg: LT single crystal at 1.92 μm is demonstrated for the first time. The maximum output power of 1.51 W with good beam quality of M2=1.80±0.1 and the slope efficiency of 38.5% were realized in Tm/Mg: LT crystal laser with 1.5% output coupler. Moreover, the long-term photorefractive effect of the Tm/Mg: LT crystal was also quantitatively analyzed.
2. Crystal growth and experiment methods
The Tm, Mg: LiTaO3 crystal was grown by the traditional Czochralski method from the congruent LiTaO3 ([Li]/[Ta]=0.951) melt with a Tm concentration of 0.5 mol %. The MgO was added to the solution to suppress photorefractive damage and its concentration was 1 mol %. The Li2CO3, Ta2O5, Tm2O3, and MgO (all had the same purity of 99.99%) were used as the raw materials during the crystal growth. The mixture was completely mixed and pressed into disks, followed by heating in air for 12 hours at 1150°C. Then, the polycrystalline materials were synthesized and loaded into a 65-mm-diameter iridium crucible for crystal growth. The crystal was grown in nitrogen gas (with high purity of 99.99%) environment using  directed LiTaO3 seed. The rotation rate and pulling speed were 8 rpm and 0.6 mm/h, respectively. After the crystal growth, the crystal was polarized under an electric field to produce a single-domain crystal for reducing the scattering loss inside the crystal. The wafer used for study was cut parallel to the ferroelectric Z-axis, which had two polished optical surfaces.
The concentration of Tm3+ ion in Tm, Mg: LiTaO3 crystal was detected by inductively coupled plasma-atomic emission spectrometry (ICP-AES). The polarized absorption spectrum of the Tm, Mg: LiTaO3 wafer was recorded by Lambda 900, Perkin-Elmer in the 600–2000nm spectrum range at room temperature. The fluorescence spectrum and fluorescence decay curve at room temperature were acquired by Nikon G250 spectrophotometer with laser diode (LD) as the pump source (excited at 808 nm).
3. Results and discussion
A picture of the as-grown Tm/Mg: LT laser crystal with a size of Ø 30×20 mm3 is shown in the inset of Figure 1. Bubbles, and inclusions are not observed. The top and bottom parts of the Tm/Mg: LT crystal was cut to check the Tm ions concentration. The concentration of Tm3+ ion changed from 1.61 × 1020 cm−3 of the top part to 1.68 × 1020 cm−3 of the bottom part. Small differences in the axial distribution suggest that the distribution coefficient for Tm is close to unity.
Figure 1 presents the unpolarized absorption spectrum of Tm/Mg: LT crystal in the visible and infrared regions. In the range of 600 nm to 2000 nm, the absorption spectrum of Tm/Mg: LT crystal consists of four absorption bands centered at approximately 695, 794, 1194, and 1660 nm, which correspond to the transitions from 3H6 to 3F3+3F2, 3H4, 3H5, and 3F4, respectively. In the range of 760 nm to 820 nm, there is one strong absorption band located at 794 nm, the corresponding absorption cross section and full width at half maximum (FWHM) are 4.6 × 10−20 cm2 and 6 nm, which well match the emitting wavelength of high-power AlGaAs laser diodes.
The room-temperature absorption spectra was used to calculate the Judd-Ofelt intensity parameters Ω2,4,6 of Tm3+ (shown in Table 2). The radiative lifetime (τr) of the 3F4 → 3H6 transition can be calculated by using the Ω2 (7.17 × 10−20 cm2), Ω4 (1.09 × 10−20 cm2) and Ω6 (1.16 × 10−20 cm2) of Tm3+ in our crystal and are shown in Table 2. The radiative lifetime (τr) of the Mg: LT sample is 0.97 ms, which is higher than that of YAG and YAP crystals, but lower than that of YLF and LLF crystals. Moveover, the radiative transition probability (A), radiative lifetime (τr) and fluorescence branching ratio (β) of different upper levels can also be calculated by using Ω2,4,6 and the results are shown in Table 3. The energy band diagrams of Tm3+ ion corresponding to these transitions is shown in Figure 2. Emission wavelength of an AlGaAs laser diode matches perfectly an absorption band associated with the 3H6 → 3H4 transition whose intensity is sufficiently high to ensure efficient pumping. When the crystal is pumped by a 793 nm laser diode, on one hand, some ions in the 3H4 level decay radiatively to 3H5 with 2.3 μm emission or nonradiatively (NR) to the 3H5 level. Subsequently, ions in the 3H5 level undergo NR process to the 3F4 level, which decay radiatively to 3H6 with 1.9 μm emission. The cross-relaxation (CR) process 3H4+3H6→3F4+3F4 is extremely advantageous when the population of the 3F4 level is considered. It is referred to in the literature as a ”two-for-one pumping process”, in which the quantum efficiency can be reached as high as 2 for two Tm ions are raised to the upper laser level by one absorbed pump photon.
The emission cross sections are subsequently calculated by the Fuchtbauer-Ladenburg equation :Figure 3. It has a broad emission spectrum from 1600 nm to 2100 nm. The maximum emission cross section is 1.07 × 10−20 cm2 at 1762 nm, the second largest emission cross section is 0.81 × 10−20 cm2 at 1834 nm. Compared with the other crystals (Tm:YAG, Tm:YAP, Tm:YLF, and Tm:LLF) as shown in Table 1 [7–12], Tm/Mg: LT crystal exhibit larger emission cross section, indicating a potential for a wide tuning range and short pulse generation for this crystal.
The fluorescence decay curve of the 3F4 multiplet is shown in the inset in Figure 3. The measured lifetime of the crystal co-doped with MgO is about 4.5 ms, which is longer by about 80% in comparison with the lifetime of the crystal without MgO codoping (2.5 ms) . The lengthening of the measured fluorescence lifetime indicates that the codoping of MgO is an effective way to allow higher energy storage, making the Tm/Mg: LT crystal more suitable for laser generation.
Based on the above absorption and emission cross-section spectra, the gain cross-section spectrum G(λ) can be calculated by the following equation:Figure 4. Evidently, the gain cross-section becomes positive once the population inversion level reaches 30%, indicating that a low pumping threshold is achieved for the Tm3+: 3F4 → 3H6 laser operation. Moreover, it shows that a wide tunable wavelength range from 1800 to 2000 nm is expected when P is larger than 0.3, which suggests good potentiality for tunable and ultra-short pulse laser applications.
In our laser experiment, the pump source employed in the experiment was a fiber-coupled LD with a central wavelength at around 793 nm. The pigtail fiber had a diameter of 100 μm and a numerical aperture of 0.22. A couple of lenses with focal length of 50 mm were used to couple the pump laser into the Tm/Mg: LT crystal. A schematic diagram of the experimental setup of the Tm/Mg: LT laser is shown in Figure 5. The crystal was cut along the a-axis direction into small pieces of 3 mm × 3 mm × 6 mm. Both end surfaces of the crystal slab were parallel polished and antireflection (AR)-coated at lasing wavelength between 1800–1950 nm. The crystal was wrapped in an indium foil and mounted into a micro-channel copper heat-sink whose temperature was maintained at 20°C in the experiment to efficiently cool the crystal and avoid thermal fracture. The laser beam transmits parallel to the a-axis of the Tm/Mg: LT crystal (the length direction). The measured single-pass pump absorption under nonlasing was 50%. The input mirror M1 was a Plano mirror AR coated at 793 nm but highly reflecting (HR) at the laser wavelength between 1800–1950 nm. The output coupler M2 was also a Plano mirror with an output transmission of 1.5 % at the laser wavelength range and AR at 793 nm. The length between M1 and M2 was about 15 mm.
Figure 6(a) shows the output power as a function of the absorption pump power for the 1.5% transmission output coupling (OC). Laser emission was found to be centered at 1.92 μm for the 1.5% transmission OC. The maximum output power was 1.51 W with absorption pump power of 14 W, corresponding to the slope efficiency of near 38.5 %. The threshold value was 20.05 W (incident pump power). The laser spectrum of the 1.92 μm operation is indicated in the inset of figure 6(a). The beam quality of the laser light with the output coupler of T = 1.5% was measured using the 90.0/10.0 scanning-knife-edge method. Figure 6(b) shows the beam radius as a function of distance from the waist location. The M2 factor of the laser beam at output power of 1.51 W was best-fitted to be 1.80±0.1. For qualitatively investigating the photorefractive effect in Tm/Mg: LT crystal, the beam quality of the laser light with the output coupler of T = 1.5% under a lower output power (0.55 W) was also measured. The corresponding M2 factor was fitted to be 1.78±0.1. It is obvious to see that the M2 values have remained little changed under low laser power and high laser power, indicating that the photorefractive effect did not occur under laser operating.
Compared with the Tm/Mg: LN laser presented by R. Zhang et al (2.62 W output power at 1856 nm with a slope efficiency of 19.5% was obtained with a threshold value of 3.42W) , the Tm/Mg: LT laser has a higher threshold value (10 W), which can be ascribed to the fact that the Tm3+ ion concentration of Tm/Mg: LT crystal is lower than that of Tm/Mg: LN crystal. However, the Tm/Mg: LT laser has higher slop efficiency (38.5%). This may be caused by the following reasons: (1) the emission cross section and fluorescence lifetime at the lasing wavelength of Tm/Mg: LT (0.50×10−20 cm2, 4.5 ms) is larger than that of Tm/Mg: LN (0.28×10−20 cm2, 4.2 ms); (2) the diameter of pump source for Tm/Mg: LT laser (100 μm) is smaller than that for Tm/Mg: LN laser (200 μm). As can be seen in Table 1, the emission cross section at the lasing wavelength of Tm/Mg: LT is larger than that of Tm: YLF, however, X. M. Duan et al obtained a Tm: YLF laser oscillation of 9.8 W output power in 1.91 μm with a slope efficiency of as high as 51.4% , which is much higher than that of Tm/Mg: LT laser (38.5%). This may be mainly caused by the use of double-end-pumping method in the Tm: YLF laser, which could effectively improve the laser performance. In the future, using the double-end-pumping method may be an effective way to obtain a high efficiency diode pumped Tm/Mg: LT laser.
To study the photorefractive effect in Tm/Mg: LN crystal, the distortion of a transmitted TEM00 mode operating laser (λ =532 nm) beam through the crystal bar was investigated. Figure 7 shows the experiment setup. The laser was focused by means of a convex lens of focal length f (f=150 mm) into the Tm/Mg: LN crystal placed in the focal plane. The direction of the laser beam is parallel to the length direction (a-axis) of the Tm/Mg: LN crystal. The diameter D of the beam that irradiated the crystal can be calculated by the equation: D=2fλ/(πd), where λ is the wavelength of laser, d is the diaphragm diameter of the light shed. The power density in the Tm/Mg: LN crystal is given by the following equation :25, 26]. The ratio of radii along c-axis and b-axis, which represents the photorefractive effect, could be obtained based on the detected profile of the transmitted light spot by using beam diagnostics camera (LASERCAM HR, Coherent Co.). The measurement of the radii under three different power densities (2, 8, and 16 kW/cm2) for a period of 600 s was made. In order to improve the accuracy of the measurement of the radii, five sets of data were measured for every power density, and the average data are shown in Figure 8. It was obvious to see that the ratio of radii along c-axis and b-axis exhibited almost no change after a long time measurement, even under the power density of as much as 16 kW/cm2. It indicates that the long-term photorefractive effect of the Tm/Mg: LN crystal was not appeared. Moreover, Figure 9 show the original 532 nm laser beam profile and transmitted laser beam profile after 600 s irradiation of 16 kW/cm2 power density. From this figure we can see that two beam profiles were all most the same.
As Figure 6(a) shows, the optical-to-optical efficiency in Tm/Mg: LT crystal of nearly 5.0 % is low and only 50% of the pump power is absorbed by the crystal. This may be caused by the following reasons: (1) low doping concentration of Tm3+ ion in LT crystal result in the low optical-to-optical efficiency; (2) Scattering losses from Mg2+ ions in the gain medium could lead to increases in intracavity losses and the increase in the threshold value. In view of the cause mentioned above, it will also be interesting to optimize this laser results in our future research. The following ways may lead to an optimization of laser output: (1) We believe that a substantially increased output power could be achieved by optimization of the Tm dopant concentration and the optical quality of the crystal. Therefore, growing crystals with higher Tm-ion doping concentration is ongoing; (2) Changing the values of transmission OC will also be a way to improve the laser performance.
In conclusion, for the first time to the best of our knowledge, Non-photorefractive laser operation with a Tm/Mg: LT single crystal is demonstrated. The absorption spectra, fluorescence spectra, and fluorescence lifetime of the crystal were investigated. By applying the J-O approach, we obtained the intensity parameters Ω2,4,6, as well as other spectroscopic parameters that relate to laser performance. The results were compared with that of other Tm-doped crystals. The laser operation of a Tm/Mg: LT laser at 1.92 μm with 1.51 W output power has a slope efficiency of near 38.5%. As for as we know, it is the highest output power and slope efficiency reported for Tm/Mg: LT crystal. We believe that a substantially increased output power could be achieved by optimization of the Tm-dopant concentration and the optical quality of the crystal. We obtained that the co-doped of MgO not only can lead to the lengthening of the measured fluorescence lifetime of the 3F4 level, but also make Tm/Mg: LT crystal suitable for use as a laser host without restricted in photorefractive effect. Our results allow us to suppose that, Tm/Mg: LT single crystal looks very promising for high-peak power lasers owing to its high photorefractive damage threshold. Performing the tunable below 1.9 μm and Q-switched laser experiment in Tm/Mg: LT single crystal will be interesting endeavor in future studies.
We would like to thank the State Key Program for Basic Research of China (Grant No. 2010CB630703), the National Natural Science Foundation of China (Grant No. 51302283), the Shanghai Natural Science Fundation under Projects (No. 13ZR1463400), the Doctoral Foundation of Shandong Province (No. BS2012DX010), and the Science and Technology Project of Qingdao (No. 13-1-4-201-jch) for their financial support.
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