Tm:LuYO3 mixed ceramic was successfully fabricated by the solid-state reactive sintering method. The absorption cross section and emission cross section were studied at room temperature. The fluorescence lifetime of 3F4 energy level was fitted to be 2.6 ms. A continuous-wave (CW) laser operation of Tm:LuYO3 ceramic, pumped at 796 nm, was realized with the output power of 1.2 W and slope efficiency of 25.1%. A mode-locking (ML) laser operation of Tm:LuYO3 ceramic was demonstrated for the first time with pulse duration of 41 ps and pulse repetition frequency of 139.3 MHz.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
2 µm laser has attracted great interest owing to a wide variety of practical applications in eye-safe LIDAR, range-finding and medical surgery [1,2]. The Tm3+ ion is well known for its emission transition of 3F4 →3H6 around 2 µm . It could be pumped by commercial AlGaAs laser diode around 800 nm while the cross-relaxation process, 3H4+3H6→3F4+3F4, improves the quantum efficiency . Up to now, Tm3+ ions doped host materials have been researched widely such as YAG , GGG , CaYAlO4 [7,8], LiYF4 , CaF2  and KY3F10 .
Sesquioxides have attracted a lot of research interests for their excellent thermomechanical properties  and large energy-level splitting of the ground state which can give rise to a moderate spectral extending of the corresponding transitions. Due to the high melting points of sesquioxide single crystals (2450 ℃ for Lu2O3), it is extremely hard to grow sesquioxide crystals with general crystal growth methods. The sesquioxide transparent ceramic was thought as an alternative permitting for its easier and size-scalable fabrication. Furthermore, the transparent ceramic is less expensive because it is not required for the rhenium crucible, which is integrant for growing crystals. Tm3+-doped Lu2O3 [13,14], Y2O3 [15,16] and LuScO3 [17,18] ceramic lasers around 2 µm have been investigated. The continuous-wave and passively Q-switched regimes of Tm:LuYO3 ceramic  were also reported. Unfortunately, to the best of our knowledge, the ML laser operation of Tm:LuYO3 mixed ceramic has not been demonstrated. In this work, we discussed the spectral properties and CW laser operation of Tm:LuYO3 mixed ceramics. Furthermore, the ML Tm:LuYO3 laser operation was demonstrated for the first time.
2. Spectral characteristics
The mixed Tm:LuYO3, i.e., Tm:(Lu0.5Y0.5)2O3 ceramic was fabricated with the solid-state reactive sintering method using powders of Tm2O3, Lu2O3, and Y2O3 as raw materials . On the basis of laser experiments previously performed on 2 at.% Tm3+:Y2O3 and 3 at.% Tm3+:Y2O3 ceramics , thermally induced saturation of the output power was reported with the 3 at.% Tm3+:Y2O3 ceramic, while 2 at.% doped Tm3+ in the ceramic material showed better laser results without saturation, because of the large atomic mass difference between Tm and Y. According to Ref. , Lu2O3 features the insensitivity of thermal conductivity with the doping level because the mass of the substituted Lu ion is very similar to the mass of the dopant Tm3+. In the present work, the Tm3+ concentration of 3 at.% was selected in Tm:LuYO3 ceramic. The average grain size corresponding to Tm:LuYO3 ceramics was 1.65 µm. The room-temperature absorption spectrum of Tm:LuYO3 ceramics was measured by a spectrophotometer (Cary-5000, Varian, UV-VIS-NIR). The fluorescence spectrum and fluorescence lifetime at 1970 nm were obtained through Edinburgh spectrophotometer FSP920 excited at 796 nm.
Figure 1 shows the absorption cross section of Tm:LuYO3 ceramic from 300 nm to 2200 nm. There are six bands corresponding to Tm3+ transitions from 3H6 ground state to the 3F4, 3H5, 3H4, 3F2,3, 1G4, and 1D2 excited states. The absorption cross section of 3H6→3H4 transition at 796 nm, matching well with the emitting wavelength of AlGaAs laser diode, was calculated to be 3.8×10−21 cm2. The corresponding cross section of Tm:LuYO3 ceramic is similar with that of Tm:Lu2O3 ceramic (3.8×10−21 cm2 at 796 nm) , but higher than that of Tm:LuScO3 ceramic (3.5×10−21 cm2 at 793 nm ). The full width at half maximum (FWHM) at 796 nm was calculated to be 25 nm, which permits a large wavelength tolerance when pumped by AlGaAs laser diode.
The fluorescence spectrum and emission cross section, σem, of the 3F4→3H6 of Tm3+ in LuYO3 ceramic around 2 µm are shown in Fig. 2. Owing to the reabsorption effect of Tm3+ ions, which have a quasi-three energy level structure, the emission cross section at wavelength range from 1500 nm to 2000 nm was calculated from the measured absorption spectrum, see Fig. 1, using the modified reciprocity method :
The fluorescence decay curve of 3F4 manifold excited by 796 nm is plotted in Fig. 3. As can be seen, the fluorescence intensity shows a single exponential decay. The fluorescence lifetime τf of 3F4→3H6 transition was fitted to be 2.6 ms, which is lower than the calculated radiative lifetime τrad of 6.19 ms. The reduction of fluorescence lifetime may result from nonradiative quenching, such as the larger energy transfer to structural defects and unintentionally introduced impurities , energy transfer processes between the Tm3+ ions themselves and multiphonon decay, and so on. Similar results have been reported in Tm:Li2Gd4(MoO4)7  and Tm3+ doped silicate glass . The decay time is still larger than that of Tm:YVO4 (0.8 ms) , which reveals that Tm:LuYO3 is suitable for laser output around the 2.0 µm wavelength.
3. Laser experiments
3.1 CW laser operation
The CW laser experimental setup of Tm:LuYO3 ceramic is shown in Fig. 4. A fiber-coupled laser diode (core diameter of about 105 µm, numerical aperture of 0.15) emitting at about 796 nm was used as the pump source. The pump beam was collimated by a doublet lens (30 mm focal length) and then focused through another doublet lens (50 mm focal length). Thus, the pump beam size was expanded by a factor of 5/3, which, to a certain extent, increases the laser threshold but is more beneficial for the thermal alleviation. The Tm:LuYO3 ceramic was cut into 3×3×6 mm3 and both surfaces were polished without antireflection films. The ceramic was held in a copper block to strength the thermal contact. The copper block was sank with water cooled at 8 °C. The laser resonator was exposed in dried air. It can be seen in Fig. 4, the laser resonator was composed of a compact plano–concave configuration. The input mirror (IM) owns a high transmission of 94% at pump wavelength and a high reflection of 99.9% at lasing wavelength. The three output couplers with the transmissions of 3% (OC1), 5% (OC2) and 10% (OC3), were used to record the laser performance. During the experiments, the whole length of the cavity was about 48 mm.
Figure 5 shows the laser output powers of Tm:LuYO3 ceramic by using three OCs. Maximum output power of 1.20 W, slope efficiency of 25.1% and threshold of 0.53 W, were obtained with the OC1. Improved slope efficiency of 29.1% was achieved by using OC2 with a maximum output power of 1.10 W and threshold of 1.08 W. The highest slope efficiency of 33.1% was realized by OC3 with a maximum output power of 0.88 W and threshold of 1.5 W. The higher threshold of OC3 than that of OC1 and OC2 is because of the higher transmission of OC3 than that of OC1 and OC2. In addition, the relatively high threshold could be due to the high round-trip intracavity loss. Based on these data, for the quasi-three-level laser with reabsorption loss, the corresponding slope efficiency can be estimated by :32] corresponding to the quasi-three-level transition. T is the transmission of OC and L represents the round-trip loss. In this expression, the intracavity round-trip losses were calculated to be 1.51%, 1.48% and 1.41%, respectively by using the slope efficiencies of 25.1%, 29.1% and 33.1%. In fact, if the ηa for quasi-three level Tm-laser is in consideration of the non-unity, the thermal diffraction loss and deflection loss would be even larger. From Ref. , the 2 at.% Tm:Lu2O3 ceramic CW laser was reported with a maximum output power of 26 W and a 32.6% optical-to-optical conversion efficiency using a 11.6-mm long active rod and TOC = 6%. Moreover, H. Wang et al. , have achieved 2 at.% Tm:Y2O3 ceramic CW lasing operation, with a maximum output power of 7.25 W and slope efficiency of 40% using a 5% output coupler. Compared to laser performances of Tm:Lu2O3 and Tm:Y2O3 ceramics, power scaling of Tm:LuYO3 laser could be expected by improving the quality of the ceramic and power range of pumping source in the future.
The CW laser spectrum of Tm:LuYO3 ceramic is shown in Fig. 6. The spectrum profile has a multi-peak structure with the prominent peak wavelength at 2067 nm. The Tm:YAG laser at 2.02 µm with 180 µm absorption depth and the Ho:YAG laser at 2.09 µm with an absorption depth of 360 µm in water have been reported [33,34]. Therefore, the present Tm:LuYO3 laser at 2.067 µm might be favorable for practical medical applications.
3.2 ML laser operation
The scheme of the ML Tm:LuYO3 laser setup is shown in Fig. 7. The pump source was used with a commercial AlGaAs laser diode emitting at 796 nm. The pump beam was focused onto Tm:LuYO3 ceramic through two focus lenses with the same 100 mm focal length which led to a pump spot radii of about 40 µm. The Tm:LuYO3 ceramic with a size of 4×4×6 mm3 was optically polished and Brewster cut. Then the Tm:LuYO3 ceramic was wrapped by indium foil and firmly mounted in a water-cooled copper block with the circulating water temperature maintained at 13.0 °C. An X-type folded cavity was adopted in the ML laser experiment, in which the three folding cavity mirrors M1, M2 and M3 with ROC of -100 mm are all highly reflectively coated (reflectivity > 99.7% from 1850 to 2100 nm) and anti-reflectively coated (transmission > 95% at 796 nm). The SESAM was operated at 2µm with the modulation depth of 1.2% and the relaxation time of 10 ps (BATOP, SAM-2000-2-10ps-x).
The CW characteristics of the Tm:LuYO3 ceramic at 2 µm were discussed firstly. The laser output power decreased to be zero rapidly with the pump power increased. Excluding the possibility of variation of pump wavelength, the phenomenon could be resulted from the thermal lens effect in Tm:LuYO3 ceramic. When the SESAM was introduced, the maximum average output power of 121 mW was obtained. The laser spot had a round mode with a measured M2 factor of ∼1.2.
The laser output power arises first and decreases later in twice while the pump power increases which are corresponding to the two ML performances as shown in Fig. 8. The black data points are CW and unstable ML regime, and the red data points represent the stable ML regime. In the two ML, both the ML spectrum and the autocorrelations change little, and the drift of pump wavelength is relatively small. Besides, the stable ML could also be obtained with a small movement of M2 and regulation of SESAM, in which, however, the laser output power increases first and decreases later with the increase of pump power only once. Hence, that process is largely due to the thermal lens effect caused by high-power pump beam (focus spot size of about 40 µm). In the two ML, the status of laser resonator is on the edge of stable region, and the arising first and decreasing laser twice is because of the changes of stable region resulted from the thermal lens effect. And the higher ML threshold corresponds to the latter mode-locking region.
The ML pulse trains were detected by a high-speed mid-infrared detector (EOT, ET-5000) and recorded by a 500-MHz oscilloscope (Tektronix, DPO3054) as shown in Fig. 9. The mode-locked pulses had a period of ∼7 ns. Radio frequency spectra are shown in Fig. 10, measured by combining a RF spectrum analyzer (CETC-41, AV4037) with a mid-infrared photodetector, in which the fundamental RF spectrum shows a signal-to-noise ratio of 63 dB. Both clean fundamental and harmonic RF spectra indicate stable CW ML state without Q-switching. The mode-locked pulses had a repetition rate of 139.3 MHz, corresponding to the cavity length of 108 cm.
The autocorrelation trace and corresponding optical spectrum of the Tm:LuYO3 ceramic ML pulse are presented in Fig. 11. The pulse duration of autocorrelation trace was fitted to be 41 ps. The corresponding pulse spectrum is centered at 2061 nm and the FWHM bandwidth is 2.8 nm.
In conclusion, Tm:LuYO3 mixed ceramic has been successfully synthesized by a solid-state reactive sintering method. The absorption cross-section of Tm:LuYO3 ceramic was calculated to be 3.8×10−21 cm2 at 796 nm. The maximum emission cross-section corresponding to the transition of 3F4→3H6 was calculated to be 6.0×10−21 cm2 at 1937 nm and the fluorescence lifetime was fitted to be 2.6 ms. A continuous-wave laser operation of Tm:LuYO3 ceramic, pumped at 796 nm, was realized with the out power of 1.2 W and slope efficiency of 25.1%. A mode-locking Tm:LuYO3 ceramic laser pulse with the pulse duration of 41 ps and repetition rate of 139.3 MHz was reported for the first time. The results show that Tm:LuYO3 ceramic is a promising laser material operating in the region of 2 µm.
National Natural Science Foundation of China (61605069, 61621001, 61861136007); National Key Research and Development Program of China, National Basic Research Program of China (973 Program) (2016YFB1102202).
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