The Er:YAG laser output properties at 1645 nm were studied first. The corresponding relations between the CW laser output power and the absorbed pump power are plotted in Fig .2a. A maximum output power was achieved to be 5.1 W with T = 10% output coupler, giving a slope efficiency of 41.3%. In order to investigate the polarization characteristic of the Er:YAG laser, a Glan-Taylor polarizer was applied to measure the laser power at different angles between the polarization direction of the laser output and the axis of the Glan-Taylor polarizer. Figure 2b presents the output laser powers versus variation of the angle. The extinction ratio of the Er:YAG laser was calculated to be 22.6 dB, which was conducted to enhance the absorption efficiency of the Tm:LuGGG crystal and reduce the thermal effect of the laser. Figure 2c displays the fitting line of the M-square factor measured by the knife-edge method at the output power of 4.8 W. The fitting results demonstrated that Er:YAG laser operated on TEM00 mode with \({\text{M}}_{\text{x}}^{\text{2}}\) of 1.44 and \({\text{M}}_{\text{y}}^{\text{2}}\) of 1.33. respectively. The two dimensions output beam spatial distribution was inserted in Fig. 2c, presenting a TEM00 mode Gaussian profile. Additionally, the output power versus time was recorded and shown in Fig. 2d. The RMS of the laser was calculated to be 0.88% during 1.5 h, exhibiting good stability. The laser spectrum centered at 1645.37 nm was inserted in Fig. 2d.
The performances of the Tm:LuGGG laser were first studied without the BF and using output couplers with transmittances of 1%, 1.5%, 3% and 5%, respectively. According to Tm-doped materials, in-band pumping scheme may bring many advantages of high absorption efficiency of the crystal, high laser slope efficiency, as well as reduced heat loading. Figure 3a describes the output power at 2 µm as a function of the in-band pump power. A maximum output power of 0.7 W was obtained with the T = 5% output coupler under a pump power of 1.75 W, corresponding to a slope efficiency of 46.3%. No thermal saturation phenomenon was observed during laser experiments at all four output couplers. Unfortunately, output couplers with higher transmittance were not available at the time of experiment. Hence, the output power may be further improved by using output coupler with higher transmittance and higher pump intensity. Based on the slope efficiency and reflectivity of the output coupler [12], the round-trip loss could be fitted to be 0.61% as presented in Fig. 3b. The relatively low round-trip loss indicated that the Tm:LuGGG crystal possessed reasonable quality. The laser output spectra at different output couplers were recorded and shown in Fig. 3c. The peak value of the wavelength appeared blue shift with increasing the transmittance of the output couplers.
The Tm:LuGGG laser can be wavelength tuned by using BF as a wavelength selector. As displayed in Fig. 4, the laser was tunable over 210 nm both with T = 1.5% and T = 3% output couplers. The tuning coverage ranged from 1854 to 2066 nm at T = 1.5%, while it was from 1849 to 2060 nm at T = 3%. Table 1 summaries tunable laser properties with the Tm-doped garnet structure crystal. Due to inhomogeneous broadening of emission spectrum, the tuning coverages of the Tm-doped disordered garnet structure crystals were relatively broad, especially in Tm:CLTGG and Tm:CLNGG crystals. Compared with previous studies of the Tm:LuGGG crystal laser employing typical 790 nm LD as pump source [11], the high brightness pump source at 1645 nm used in our experiment allowed shorter wavelength laser operation due to high population inversion. Considering tuning range and output power, Tm:LuGGG crystal is a relatively decent crystal for applications in tunable and ultrafast laser at 2 µm.
Table 1
Summary of tunable laser properties with the Tm-doped garnet structure crystal.
Materials | Tuning coverage (nm) | Tuning Wavelength (nm) | Maximum power (mW) | Slope efficiency (%) | Ref. |
Tm:CLTGG | 286 (Toc = 0.5%) | 1799–2085 | ~ 100 | 16.1% | [13] |
Tm:CNNGG | 168 (Toc = 0.5%) | 1885–2053 | 111 | - | [14] |
Tm:CLNGG | 224 (Toc = 0.5%) | 1848–2072 | 120 | - | [14] |
Tm:GAGG | 180 (Toc = 2.5%) | 1856–2036 | 600 | ~ 20.1 | [15] |
Tm:LuGGG | 149 (Toc = 1%) | 1907–2056 | ~ 700 | 16.5 | [11] |
Tm:LuGGG | 212 (Toc = 1.5%) | 1854–2066 | 560 | 33.9 | This work |