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A diode-pumped continuous-wave and a Q-switched Tm:Y2O3 ceramic laser operating around 2050 nm were demonstrated. The maximum output power of 7.25 W, with a slope efficiency of 40% for continuous-wave operation and shortest pulse width of 115 ns at a repetition of 1 kHz for Q-switched operation, was achieved.
© 2017 Optical Society of America
1. Introduction
Lasers operating at 2 μm wavelength region have great potential applications in surgery, LIDAR systems, gas detection, plastics material processing, the pumping sources of Cr2+ lasers and mid-infrared optical parametric oscillators [1–3]. Tm3+ can be excited around 800 nm from the ground state to the 3H4 energy level (3H6→3H4). Then the upper laser level 3F4 is populated by a cross relaxation process (CR), leading to lasing transition from 3F4 to 3H6 at 2μm wavelength region. This excitation process yields two excited ions for each absorbed pump photon, and can support a high-slope-efficiency operation well beyond stokes limit. These advantages make Tm-doped solid-state lasers increasingly attractive for compact and efficient laser sources in the 2 μm wavelength region.
Sesquioxides, including Y2O3,Sc2O3 and Lu2O3, can be easily doped with rare earths. They are promising host materials for solid-state lasers which can ensure high power and ultrashort pulsed laser operations. Sesquioxides have strong Stark splitting and low phonon energy that can result in broad emission spectra and high quantum efficiency. Furthermore, the high thermal conductivity of sesquioxides is considerably attractive for high power laser operations. However, it is difficult to grow high quality large size single sesquioxide crystals with conventional crystal growth techniques because of the high melting-point (above 2400 °C) of sesquioxides [4, 5]. Transparent ceramics have attracted much attention because they can be fabricated with large size, high dopant concentration and composite structure in much lower temperature environments compared to single crystals. Furthermore, transparent ceramics not only have the optical and thermal properties that single crystals have, but also have better mechanical properties [6–9]. Efficient and high-power laser operations have been demonstrated in Tm-doped sesquioxide ceramic lasers. The distinguishing features of Tm3+-doped sesquioxides is that they have broader gain spectra at long-wavelength >2 μm. The emission wavelength of Tm3+ doped sesquioxide lasers can be expanded beyond 2.1 μm, so it can be an ideal alternative to Ho-doped lasers to directly pump mid-infrared ZnGeP optical parametric oscillators [10, 11]. In Ref [12], continuous-wave laser operation at 2066 nm with an output power of up to 26 W and a slope efficiency of 42% was obtained in a 811 nm laser diode (LD) pumped Tm:Lu2O3 ceramic laser. In Ref [13], an efficient 2064 nm Tm:Lu2O3 ceramics laser in-band pumped at 1670 nm was achieved with an output power of 23 W and an optical-to-optical efficiency of 51%. A proof-of-principle study of a 1.97 μm Tm:Lu2O3 ceramic disk laser intracavity pumped by a 1.2 μm semiconductor disk laser was also presented in Ref [14], obtaining a limited output power of 250 mW. In Ref [15], lasing in Tm:Y2O3 ceramics is obtained at wavelengths of 1.95 and 2.05 μm. The maximum output laser power at these wavelengths was 2.4 and 0.3 W, respectively.
In this paper, a diode-pumped continuous-wave (CW) and Q-switched Tm:Y2O3 ceramic laser was demonstrated. A comparison of the laser performance of Tm:Y2O3 ceramics with 1at.%, 2at.% and 3at.% Tm3+ doping concentration was made under the same experimental conditions. Better experimental results were achieved in a 2at.% Tm:Y2O3 ceramic. The maximum output power and slope efficiency in CW operation were 7.25 W and 40%, respectively. In the Q-switched laser system, the minimum pulse width of 115 ns under the absorbed pump power of 11.7 W and a repetition rate of 1 kHz was achieved. The absorption spectrum and fluorescence spectrum of Tm:Y2O3 ceramic were also investigated.
2. Experimental setup
Firstly, the absorption spectrum of the 2 mm-thick 2at.% Tm:Y2O3 ceramic was measured by a UV-VIS-NIR spectrophotometer (Lambda 950; Perkin-Elmer, Waltham, MA), as shown in Fig. 1. There are three strong absorption peaks located at 776 nm, 796 nm and 810 nm, which well match the emitting wavelength of high-power AlGaAs LDs. Figure 2 shows the fluorescence spectrum of the Tm:Y2O3 ceramic at room temperature measured with a spectrofluorometer (Edinburgh Instruments, FS980). A non-smooth multiple-spike structure can be observed. As can been seen in Fig. 2, there are two strong emission peaks located at 1932 nm and 2050 nm, suggesting that Tm:Y2O3 ceramic laser could simultaneously operate at multi-wavelengths.
The laser setup is shown schematically in Fig. 3. A linear cavity was employed for both CW and Q-switched laser experiments. A fiber-coupled 785 nm LD was used as the pump source. Under the maximum LD output power, the central emission wavelength of LD was located at 784.6 nm with 2.3 nm bandwidth. The output-coupling fiber of the LD had a core diameter of 400 μm and a numerical aperture of 0.22. The laser beam of the pump was reimaged into the ceramic with a spot radius of 200 μm by a lens assembly (F1 and F2) having 1:1 imaging ratio. The Tm:Y2O3 transparent ceramics with 1at.%, 2at.% and 3at.% Tm3+ doping concentration were optically polished, and both ends of the ceramics were antireflection (AR) coated at 760-810 nm and 1950-2150 nm. To begin with, the laser performance of several configurations of resonator has been experimentally studied, including plane-plane, plano-concave and concavo-concave resonator. It was found that both the highest output power and slope efficiency were obtained in the concavo-concave resonator. This was because concave-concave cavity has a better tolerance for the lasing beam deviated from the axis of cavity induced by scattering in polycrystalline ceramics. Changing the cavity length enabled higher output power, resulting in the cavity with mirrors placed close to the facets of the ceramic element for the CW laser operation. Finally, a curved mirror was used as the input mirror with a curvature of 300 mm which was high-transmission coated at the pump wavelength and high-reflection coated at the laser wavelength. Three curved mirrors were chosen as output mirrors with a curvature of 100 mm, and the transmittance of the output mirrors was 2%, 5% and 10% at 2 μm, respectively. The cavity length was about 12 mm in the CW laser experiment. In the Q-switched laser experiment, an AO Q-switcher (Gooch & Housego, QS027-2D-B5) was placed into the cavity for pulsed operations and the cavity length was increased to 76 mm. To reduce the generated heat during the experiments, the Tm:Y2O3 ceramic was wrapped with indium foil and inserted in a water-cooled copper block at a temperature of 15°C. The Q-switched laser pulse was detected by using a fast InGaAs photodiode (DET10D/M, Thorlabs) and recorded with an oscilloscope with 1GHz bandwidth, and 5Gs/s sampling rate (DPO7104C, Tektronix).
3. Experimental results and discussions
A comparison of the laser performance of Tm:Y2O3 ceramics with 1at.%, 2at.% and 3at.% Tm3+ doping concentration was made under the same experimental conditions with a 2% output coupler (Fig. 4). Better experimental results were achieved in a 2at.% Tm:Y2O3 ceramic. About 33%, 45.3%, and 60.2% of the pump power were absorbed during laser operation in the 8.5 mm long 1at.%, 2at.% and 3at.% Tm:Y2O3 ceramics, respectively. The Tm3+-doping concentration plays an important role in the laser performance of Tm lasers. On the one hand, a high Tm3+-doping concentration can enhance the cross relaxation process. The highly efficient cross relaxation process leads to a quantum efficiency of near two. On the other hand, the efficiency of the laser process can be lowered by upconversion processes and excited state absorption that start from the upper laser level 3F4. In the upconversion processes, one ion is lost (3F4→3H6) and anther ion is excited into 3H4 or 3H5. Especially, when the ion is excited to 3H5, a nonradiative process of 3H5→3F4 happens because the lifetime of 3H5 is very short, generating heat inside the gain medium. Consequently, further increasing the Tm3+-doping concentration can increase the thermal load in the gain medium, upconversion and excited state absorption process, lowering the efficiency of the laser process accordingly. As shown in Fig. 4, the laser efficiency of the ceramic with 2at.% Tm3+ ions is better than that with 1at.% Tm3+ ions which is due to the high level of population inversion in the gain medium with 2at.% Tm3+ ions. However, the laser efficiency decreases with the 3at.% Tm3+ concentration.
Figure 5 shows the CW laser output power versus absorbed power for the 2 at.% Tm:Y2O3 ceramic with different output couplers. The highest slope efficiency was 40% with the 5% output coupler. In this case, the maximum output power of 7.25 W under the maximum absorbed LD power of 20 W was achieved. The M2 factor of the beam was measured to be around 3.5. For the 2% and 10% output couplers, the maximum output power was 6.78 W and 5.84 W, corresponding to the slope efficiency of 37% and 34%, respectively. The low slope efficiency of the laser system can be attributed to the fact that the emitting wavelength of the LD pump source doesn't match well with the strong absorption peaks. As reported in Ref [16], a narrow-linewidth LD with the wavelength matching well with the peak of excitation spectrum is propitious to improve the conversion efficiency of Tm-doped lasers.
The laser spectrum was measured by a Thorlabs OSA205 optical spectrum analyzer, as shown in Fig. 6. There was no wavelength selector in our cavity. The cavity mirrors used in our experiment can sustain a broadband operation ranging from 1950 to 2150 nm, covering the fluorescence spectrum of Tm:Y2O3 ceramic. So the Tm:Y2O3 ceramic laser operated in the free-running mode. Due to the reabsorption losses at the lasing wavelengths below 2000 nm, the Tm:Y2O3 ceramic laser operated around the gain peak of 2050 nm. Under a low absorbed pump power of 5.4 W, the peak of lasing wavelength was located in 2051 nm (Fig. 6(a)). When the absorbed pump power was increased to 10 W, two emission peaks began to oscillate. Figure 6b shows the lasing spectrum obtained at maximum output power, with the two emission peaks locating at 2048 and 2054 nm. We attributed the wavelength variation to the change of inversion ratio that came with the change of pump power. The resultant change in the up-conversion rate further changed the emission wavelength. This multi-wavelength emission in Tm:Y2O3 ceramic laser ceramic may have potential applications in Doppler lidar and THz radiation generation.
A Q-switched laser system with an AO Q-switcher was also demonstrated in this paper. In the Q-switching experiment, the curved mirror with 10% transmittance was used as the output mirror to achieve the balance between the pulse width and average output power. Figure 7(a) shows the average output power and pulse width versus absorbed power under the repetition rate of 1 kHz, 3 kHz and 5 kHz respectively. As can be seen, average output power incresed linearly under the same repetition rate. When the repetion incresed, higher average output power can be obtained. The average power of the Q-switched radiation reached 2.5 W at the repetion rate of 5 kHz, corresponding to 92% of CW power in the same laser cavity. Figure 7(b) shows pulse width versus absorbed power under the repetition rate of 1 kHz, 3 kHz and 5 kHz respectively. Obviously, shorter pulse width could be obtained with lower repetition and higher pump power. Under the repetition of 1 kHz and absorbed power of 11.7 W, the shortest pulse width of 115 ns was achieved. Shorter pulse width could be obtained by increasing the pump power, but higher pump power might result the damage of the ceramic on the surface. Pulse train and single-pulse waveform with the pulse width of 115 ns at the repetition rate of 1 kHz is shown in Fig. 8.
Fig. 7 (a) Averge output power and (b) pulse width versus absorbed power under different repetition rates.
Fig. 8 Typical pulse train (top) and single pulse (bottom) with the pulse width of 115 ns at the repetition rate of 1kHz.
4. Conclusions
In summary, a LD pumped Tm:Y2O3 ceramic laser around 2050 nm was demonstrated both in CW and Q-switched operations. The maximum output power of 7.25 W with the slope efficiency of 40% was obtained in the CW operation. Dual-wavelength emissions can be observed with the increase of pump power. In the Q-switched operation, the shortest width was 115 ns at a repetition of 1 kHz.
Funding
National Natural Science Foundation of China (NSFC 61308047, 61605068 and NSAF U1430111); The Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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