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Abstract
Tm3+-doped LuScO3 mixed sesquioxide ceramics were successfully fabricated by using a solid-state reactive sintering method. The absorption and emission spectra were measured at room temperature. Continuous-wave (CW) and passively Q-switched laser operation of Tm3+:LuScO3 ceramic were investigated for the first time to our knowledge. A CW output power of 211 mW with slope efficiency of about 8.2% was obtained. With single-walled carbon nanotube (SWCNT) as saturable absorber, a maximum average output power of 32 mW was achieved. The shortest pulse width was 0.59 μs at pulse repetition rate of 34.72 kHz.
© 2017 Optical Society of America
1. Introduction
Eye-safe laser sources operating at 2.0 μm wavelength region have attracted a great deal of attention due to their important applications, such as laser radar, remote sensing, spectroscopy, medical treatment, optical communications and metrology [1–4]. In general, 2.0 μm wavelength laser was realized in trivalent Tm3+- and Ho3+-doped laser materials, as well as their codoped materials. In terms of the potential eye-safe applications, Tm lasers are probably more appropriate than Ho lasers because most of Tm lasers can efficiently provide closer lasing wavelength to the absorption peak of water at about 1950 nm [5].
In fact, up to now, a great deal of Tm3+ lasers has been developed based on various host materials including oxides and fluorides, such as YAG [6], YVO4 [7], LuVO4 [8], GGG [9] and CaYAlO4 [10,11] for oxides, as well as LiYF4 [12], CaF2 crystal [13] and ceramic [14,15], and KY3F10 [16] for fluorides. Tm3+-doped sesquioxides Tm3+:RE2O3 (RE = Y, Sc and Lu) offer attractive gain media options for high power and ultrashort pulsed laser operations because of their good thermo-mechanical properties and large splitting of the ground-state that can result in a moderate spectral broadening of the optical transitions [17–20]. It is extremely difficult to grow sesquioxide single crystals with conventional crystal growth techniques because of their high melting points. But it is much easier to fabricate transparent sesquioxide ceramics because the sintering temperature is about 700 °C lower than its melting point [21]. Moreover, fabricating ceramics with large size and high doping concentration is not as difficult as growing crystals. In addition, fabricating ceramics is less expensive because it is not necessary to use expensive crucible. However, sesquioxide crystals have to be grown in an expensive Rhenium crucible [22].
The mixed rare earth ions doped sesquioxide LuScO3 provides a disordered crystal structure, which can induce broadened optical spectra, theoretically, and hence, can generally improve the laser performance in mode-locking regimes [23]. As reported by Koopmann et al. [24] in 2011, broadening of the peaks and a smoothing of the whole spectrum of Tm3+:LuScO3 are both evident, which, as just above mentioned, is of advantage to mode-locked laser operation. In [24], Ti:sapphire laser pumped continuous-wave Tm3+:LuScO3 crystal laser yielding a maximum output power of 705 mW was realized in a nearly concentric resonator. In 2013, Lagatsky et al. [25] further demonstrated a Ti:sapphire laser pumped mode locked Tm3+:LuScO3 crystal laser in a astigmatically-compensated Z-fold resonator, using SESAM as saturable absorber. However, 2.0 μm laser based on Tm3+:LuScO3 ceramic have never been reported, to the best of our knowledge. In this paper, Tm3+:LuScO3 ceramic is first-time fabricated for laser operation. Pumping with a more standard AlGaAs diode laser (therefore, making the system more compact and cost-efficient), Tm3+:LuScO3 ceramic lasers operating in continuous-wave and Q-switched regimes were then demonstrated preliminarily.
2. Spectral properties
The mixed Tm3+:LuScO3 ceramic was fabricated by using a solid-state reactive sintering method [26]. In Tm3+:LuScO3 ceramic, the cross-relaxation process converts one 3H4 excited state into two 3F4 upper laser states [27], a high Tm3+ concentration is required. However, too high Tm3+ concentration will increase the oscillation threshold. Rare earth ions doped sesquioxide ceramics have a larger volume concentration. On the basis of laser experiments previously performed on Tm:Y2O3 ceramics [28], the optimal Tm3+ concentration of 2 at.% has been selected in the mixed ceramics. The average grain size of Tm:LuScO3 ceramics was 1.65 µm. A sample was cut from the ceramics and two surfaces were polished for spectral measurements. Room temperature absorption spectrum was recorded with a UV–VIS–NIR spectrophotometer (Model Cary-5000, Varian, USA). The fluorescence spectra, as well as the decay curve at 1970 nm, were recorded using Edinburgh Instruments FSP920 spectrophotometer under 790 nm OPO excitation (pulse width 5 ns).
The room temperature absorption spectrum of Tm3+:LuScO3 ceramic from 300 to 2100 nm is presented in Fig. 1. Six bands are associated with Tm3+ transitions from the 3H6 ground state to 1D2, 1G4, 3F3-3F2, 3H4, 3H5, 3F4 excited states, respectively. The absorption cross section and the full widths at half-maximum (FWHM) for Tm3+ 3H6→3H4 absorption band at 793 nm were calculated to be 3.5 × 10−21 cm2 and 32 nm. The absorption cross section is comparable with that of Tm3+:Lu2O3 ceramic and crystal (3.8 × 10−21 cm2 at 796nm [18, 29]). The ceramic has a large FWHM, which permits high conversion efficiency when pumped by a 790 nm commercial AlGaAs laser diode.
Fig. 1 Absorption spectra of Tm3+:LuScO3 ceramic with (a) the optical density of Tm3+ ions and photo of Tm3+:LuScO3 ceramic; (b) the absorption cross section for the 3H6→3H4 transition of Tm3+ ions.
The emission spectrum was obtained at room temperature under 790 nm OPO excitation as shown in Fig. 2. The emission spectrum covers wavelength range from 1550 to 2200 nm corresponding to the transition from 3F4 to 3H6 of Tm3+. The emission half-width of the emission band centered at 1970 nm is approximately 75 nm for Tm3+:LuScO3 ceramic. However, according to [24], the emission spectra of Tm3+:Lu2O3 and Tm3+:Sc2O3 crystals peak at about 1940 and 1980 nm, respectively. Furthermore, Tm3+:Lu2O3 and Tm3+:Sc2O3 crystals both have obvious two-peak structures; the multi-peak structures of Tm:LuScO3 ceramic and crystal disappear because of the spectral broadening. Additionally, disordered Tm3+:LuScO3 ceramic (or crystal) exhibits smoothing of the whole spectrum, which could be helpful in obtaining ultrashort pulse laser by mode locking technology.
Figure 3 shows the fluorescence decay curve of the 3F4 multiplet. The measured decay curve shows singly exponential decaying behavior. The fluorescence lifetime of the 3F4→3H6 of Tm3+ was determined to be 3.2 ms, which is comparable with that of Tm3+:Lu2O3 ceramic (3.7 ms [18]) and Tm3+:Lu2O3 crystal (3.8 ms [29]). The large emission band and long lifetime indicate that Tm3+:LuScO3 ceramic could be very promising in ultrafast pulse generation when operated in mode-locking regime.
3. Laser experiments
3.1 Experimental setup
The arrangement of the diode-pumped continuous-wave and Q-switched Tm3+:LuScO3 ceramic lasers are schematically shown in Fig. 4. The pump source is a fiber-coupled diode laser with peak wavelength at about 790 nm, core diameter of 105 μm and a numerical aperture of 0.22. The coupling optics of the pump beam consisted of two doublet lenses with focal length of 30 mm and 50 mm, which leads to an expansion of the pump beam size by a factor of about 5/3, i.e. 175 μm of waist size of the pump beam. The laser resonator is a simple and compact two-mirror plane-concave configuration. The input mirror (IM) has a transmission of about 85% at the pumping wavelength and high reflection of more than 99.8% at lasing wavelengths. Two output couplers (OCs) having the same focal lengths of 50 mm, respectively with transmissions of about 0.8% (OC1) and 1.1% (OC2), was used to explore the laser performance. During the laser experiments, the length of the laser resonator maintained to be about 48 mm.
The laser gain medium was a 2 at% doped Tm3+:LuScO3 ceramic with cross section of 3 × 3 mm2 and thickness of 3 mm. A simple measurement of single pass absorption showed a maximum absorption ratio of about 52% of the pump power. For end pumping geometry, thermal effect of the laser gain medium, mainly induced by quantum defect, must be considered seriously, especially for the currently investigated Tm3+:LuScO3 ceramic with relatively low thermal conductivity. In this work, the solution for mitigating the thermal effect of Tm3+:LuScO3 ceramic was resorted to water cooling. The Tm3+:LuScO3 was wrapped with indium foil and then mounted inside a copper block and the copper block was connected to water-cooled chiller with temperature set at 14°C.
A SWCNT saturable absorber was used to modulate the intracavity loss for Q-switching. The fabrication detail of the SWCNT nanomaterial can be found in [30]. The SWCNT dispersion was transferred onto a 1-mm glass substrate using spin-coating method [31]. The linear transmission of the as-fabricated SWCNT saturable absorber was measured to be about 88.9% at the considered wavelengths using a Lambda 750 Spectrophotometer (Perkin Elmer Corp). Taking the transmission of blank glass to be about 92% into account, the SWCNT thin film itself was estimated to have a linear transmission of about 96.6%.
3.2 CW laser operation
Figure 5 shows output powers of the continuous-wave Tm3+:LuScO3 ceramic laser in free-running mode. The maximum output power reached 211 mW with slope efficiency of about 8.2% with respect to the absorbed power, achieved by using the 0.8% transmittive OC1. The output power shows rollover phenomenon when the absorbed power exceeded 3.45 W. Using the 1.1% transmittive OC2, the maximum output power decreased to 172 mW accompanying an increased threshold to 1.06 W (0.84 W of threshold for OC1). In addition, the slope efficiency was also reduced to 7.6%. Figure 6 shows the laser spectrum of the continuous-wave Tm3+:LuScO3 ceramic laser with peak wavelength of 1982 nm. In 2011, Koopmann et al. [24] reported a (1.0at%) Tm3+:LuScO3 crystal laser with maximum output power of 705 mW using Ti:sapphire laser as the pump source. Moreover, the reported laser oscillated between 2090 nm and 2115 nm simultaneously. Tuning the wavelength to 1982 nm led to a reduced output power of about 250 mW, which is comparable to our result. The present output power saturation could probably be interpreted as an indication of strong thermal lensing effect inside the laser gain medium since our Tm3+:LuScO3 ceramic has higher dopant concentration. However, in Koopmann’s report, the threshold was only as low as 38 mW, which has been far lower than that achieved in the present work. Good pump beam quality (Ti:sapphire laser) should be one of the reasons to explain their low threshold. The relatively high threshold indicated large round-trip intracavity loss, which can be estimated as follows.
Based on these data, for the quasi-three-level laser with reabsorption loss, the slope efficiency can be written by [32],
where is the slope efficiency; is the fraction of excited ions per absorbed pump photons, which we assumed here to be equal to 1; the efficiency with which absorbed pump photons are converted to laser photons, which contains all the geometrical factors associated with the conversion of the incident pump photons to laser photons; is the pumping wavelength; is the lasing wavelength; T is transmission of the OC and L is the round-trip loss of the laser cavity. Here was assigned a value of 0.98 according to [30] with respect to quasi-three-level transition. For OC1, the round-trip loss was calculated to be about 3.0%, while to be about 4.5% for OC2, which has probably indicated that the real loss should be larger than 3.0%. Although the laser gain medium has not been coated, most of the Fresnel reflection losses can be treated to stay within the resonator modes because of a good alignment of the laser material to the laser cavity. Thus, this large loss should be explained by the quality of the laser material, to great extent. Hence, power scaling could be expected by improving the quality of the Tm3+:LuScO3 ceramic in the future.3.3 Q-switched laser operation
For Q-switched laser operation, the maximum average output power was up to 32 mW achieved by using the OC1, as shown in Fig. 7. Moreover, comparing with the CW case, the threshold largely increased to 2.15 W. The laser spectrum of the Q-switched laser is also shown in Fig. 6, from which one can see that the peak wavelength shifted to 1976 nm.
Figure 8 shows that the shortest pulse width was about 0.59 μs and the corresponding pulse trains exhibited a repetition rate of 34.72 kHz. The whole evolutions of the pulse width, pulse repetition rate, pulse energy and pulse peak power are shown in Fig. 9. At threshold, the pulse width was about 1.31 μs with repetition rate of 18.78 kHz. With the increasing of the absorbed power, the pulse width decreased monotonously, while the pulse repetition rate increased monotonously and almost linearly. According to these parameter values, we estimated the maximum pulse energy and pulse peak power to be about 0.95 μJ and 1.58 W.
Fig. 8 Single pulse profile showing the shortest pulse width of 0.59 μs; inset: pulse trains at maximum output power.
Fig. 9 The dependences of (a) pulse width, (b) pulse repetition rate, (c) pulse energy and (d) pulse peak power on absorbed power.
4. Conclusion
In conclusion, Tm3+:LuScO3 ceramic has been successfully fabricated by using a solid-state reactive sintering method. The absorption and fluorescence spectra were measured to determine the pump source and emission lines. The first Tm3+:LuScO3 ceramic laser has been reported operating in CW and Q-switching regimes. For CW operation, the maximum output power reached 211 mW with slope efficiency of about 8.2%. The round-trip cavity loss of the Tm3+:LuScO3 ceramic laser was estimated to be more than 3.0%. For Q-switching, using SWCNT as saturable absorber, a maximum average output power of 32 mW was achieved and the narrowest pulse width was obtained to be about 0.59 μs at pulse repetition rate of 34.72 kHz. The corresponding maximum pulse energy and pulse peak power were about 0.95 μJ and 1.58 W, respectively.
After improving the quality, power and efficiency scaling of the Tm3+:LuScO3 ceramic laser could be expected. Further, ultrashort pulse laser based on the Tm3+-doped mixed sesquioxide ceramic could be realized by the technology of mode locking, which will be our next investigation in the near future.
Funding
National key Research and Development Program of China (2016YFB1102202); National Natural Science Foundation of China (61621001, 51672190)
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