1 Introduction

The diode pumped solid state lasers with wavelength around 2 μm have important applications in laser medicine and laser remote sensing [1–3]. Among them 2 μm single-frequency lasers are important laser sources of Coherent Doppler Wind Lidars and Differential Absorption Lidars which are very important for the weather forecast and greenhouse gases measurement [4, 5]. Various techniques have been investigated to obtain 2 μm single-frequency laser operation, such as the microchip laser, the ring laser, the laser with intracavity etalons, the laser with a volume Bragg grating and the twisted-mode-cavity. Single-longitudinal-mode (SLM) operation can be achieved in a microchip laser in the 2 μm region, but it is not capable of high-power output because of the thinness of the active medium [6, 7]. Laser operation in a ring laser cavity is another technique to obtain 2 μm single-frequency laser output. Coluccelli et al. reported a diode-pumped single-frequency Tm:LiLuF4 ring laser with the output power of 120 mW and the slope efficiency of 12% [8]. Building a laser with intracavity etalons is also a method to obtain 2 μm single-frequency operation [9, 10]. In 2010, J. Li et al. reported a 2 μm single-frequency Tm:YAP laser with two etalons inside the cavity. The maximum output power of the 2 μm single-frequency Tm:YAP laser was 514 mW. The 2 μm single frequency laser can be also generated by using a volume Bragg grating as an output coupler. C.T. Wu et al. reported a diode pumped single frequency Tm:YAG laser with a volume Bragg grating. The 2 μm single frequency laser output was 457.3 mW, with a slope efficiency of 16.7% [11]. High power 2 μm single-frequency laser can be realized by using the non-planar ring oscillator (NPRO) [12–14]. C. Gao et al. reported a 2 μm single-frequency laser from a diode-pumped Tm:YAG non-planar ring oscillator with a sandwich structure [14]. Up to 867 mW 2 μm single-frequency lasers were obtained with the diffraction limited beam quality. Compared with the microchip laser, the ring laser, and the NPRO, the twisted-mode-cavity (TMC) is also a useful technique to obtain the 2 μm single-frequency laser operation [15]. The spatial hole-burning in the active medium is eliminated in the twisted-mode-cavity. In 2009, Y.S. Zhang et al. reported a 2 μm single-frequency Tm:YAG laser from the twisted-mode-cavity with a linear resonator. Up to 514 mW 2 μm single-frequency laser was generated from a diode-pumped Tm:YAG twisted-mode-cavity, and the slope efficiency was 9.7%. Because it was not easy to realize polarization control in higher pumping power, the 2 μm single-frequency output power could not be further increased in [15]. In this paper, we report a 2 μm single-frequency laser from an L-shaped twisted-mode-cavity Tm:YAG oscillator. To realize polarization control, a polarizer was used. Up to 1.46 W single-frequency laser was obtained, with a slope efficiency of 19.2%. To our knowledge, this is the highest 2 μm single-frequency laser from diode pumped Tm:YAG lasers with different resonators. The experimental setup and results of the 2 μm single-frequency twisted-mode-cavity Tm:YAG laser are reported below.

2 Experimental setup

The schematic of the experimental setup of a diode pumped single-frequency L-shaped twisted-mode-cavity Tm:YAG laser is shown in Fig. 1. The pump source was a fiber-coupled laser diode with the center wavelength of 785 nm and the core diameter of 100 μm. A coupling optics with a magnification factor of 1:2 was used behind the pumping fiber. The laser medium was a Tm:YAG crystal with a diameter of 3 mm and a length of 10 mm. The Tm doped concentration in the Tm:YAG crystal was 3.5 at%. The two surfaces of the Tm:YAG crystal were coated with high transmission coating at 2.02 μm (T>99.9%) and high transmission coating at 785 nm (T>95%). The Tm:YAG laser had a concave–concave resonator. The input concave mirror (R=200 mm) was coated with an antireflection coating at 785 nm and a high reflection coating at 2.02 μm. The output coupler (R=200 mm) had a transmissivity of 3.6% at 2.02 μm. For controlling the polarization state of the Tm:YAG laser, a polarizer was inserted inside the resonator. The polarizer was coated with high reflection coating for the s-polarized beam and high transmission coating for the p-polarized beam. Two 2 μm quarter-wave plates were placed beside the Tm:YAG crystal to build the twisted mode cavity. The principal axes of the quarter-wave plates were oriented with their fast axes perpendicular or parallel to each other, and at 45° to the direction of the polarizer. By optimizing the laser mode, an L-shaped resonator was designed. The mode size of the Tm:YAG laser inside the resonator was simulated by using the software LASCAD. In the middle of the Tm:YAG crystal, the radius of the oscillating mode was about 150 μm. The Tm:YAG crystal was mounted in a copper heatsink, and the temperature of the heatsink was controlled at 20°C by using a semiconductor TEC cooler. An infrared filter was used behind the output coupler for blocking the non-absorbed pumping beam.

Fig. 1
figure 1

Experimental setup of diode pumped 2 μm Tm:YAG laser with an L-shape twisted mode cavity

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3 Experimental results

The L-shaped twisted-mode-cavity Tm:YAG laser was experimentally investigated. When two quarter-wave plates were not inserted inside the resonator (the free-running mode), the Tm:YAG laser operated in the multi-longitudinal-mode. Figure 2 (blue square) shows the experimental result. The threshold of the free-running mode Tm:YAG laser was 0.9 W. When the laser diode output power was 10 W, the output power of the Tm:YAG laser was 2.06 W, with a slope efficiency of 23.9%.

Fig. 2
figure 2

Output power as a function of the pump power for the twisted-mode-cavity Tm:YAG laser free running mode ● single frequency mode

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When two quarter-wave plates were inserted inside the Tm:YAG cavity, single-frequency laser operation was obtained. For the twisted-mode-cavity Tm:YAG laser, the single-frequency operation was influenced by the orientations of the two quarter-wave plates and the orientation of the principal axes of the quarter-wave plate and the polarizer. Furthermore, the Tm:YAG crystal had heat-induced birefringence when the pump power was very high, and the birefringence of the crystal affected the polarization state of the oscillating mode. The orientation of the quarter-wave plate was adjusted when the laser diode output power was above 8 W, in order to balance the influence of the heat-induced birefringence of the Tm:YAG crystal. The experimental results of the single-frequency L-shaped twisted-mode-cavity Tm:YAG laser are shown in Fig. 2 (black dots). The threshold of the single-frequency Tm:YAG laser was 1.6 W. The maximum single-frequency output power was 1.46 W, with a slope efficiency of 19.2%. When the laser diode output power was above 9.25 W, the multi-longitudinal-mode operation began.

Figure 3 shows the spectrum of the L-shaped twisted-mode-cavity Tm:YAG laser. The spectrum was measured by a scanning Fabry–Perot (F–P) interferometer with a free spectral range of 3.75 GHz. Figure 3 (left) shows the spectrum of the Tm:YAG laser in the multi-mode operation when two quarter-wave plates were not inserted in the cavity. The upper trace was the F–P ramp voltage, and the lower trace was the signal of the Tm:YAG laser transmitted through the F–P interferometer. Figure 3 (right) shows the Tm:YAG laser in the single-longitudinal-mode. When we increased the current of the laser diode, the pumping power was increased and the mode jumping occurred in the twisted-mode-cavity Tm:YAG laser.

Fig. 3
figure 3

Spectra of the L-shaped twisted-mode-cavity Tm:YAG laser at different conditions: (left) spectrum of the L-shaped Tm:YAG laser in free running mode; (right) spectrum of the L-shaped Tm:YAG laser in single-frequency mode

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The beam quality of the Tm:YAG laser was measured by using a Pyro-III infrared CCD camera (Spricon Inc.) when the single-frequency output power was 1.46 W. The M 2 factors of the Tm:YAG laser were determined by measuring the beam radii of the Tm:YAG laser in the x- and y-directions along the beam propagation. The experimental results are shown in Fig. 4. The beam profile of the Tm:YAG laser is shown in Fig. 4(b). The M 2 factors were calculated to be 1.402 and 1.384 in the x- and y-directions, respectively.

Fig. 4
figure 4

Beam quality measurement of the twisted-mode-cavity Tm:YAG laser: (a) propagation of the second order beam radii; (b) beam profile

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4 Conclusion

In summary, a diode-pumped 2 μm single-frequency Tm:YAG laser with an L-shaped twisted-mode-cavity was demonstrated. The maximum single-frequency output power was 1.46 W, with a slope efficiency of 19.2%. The M 2 factors of the 2 μm laser at the maximum output power were 1.402 and 1.384 along the x- and y-directions, respectively. A Fabry–Perot interferometer illustrated that the twisted-mode technique in the 2 μm region can realize high power output.