We demonstrated a stable single-frequency laser operating at 2122 nm from a monolithic nonplanar Ho:YAG ring oscillator (NPRO). The Ho:YAG NPRO was resonantly pumped by a 1907 nm Tm:YLF laser built up by ourselves. The maximum multimode output power from the Ho:YAG NPRO was 9.66 W and the slope efficiency was 71.7%. With accurate adjustment of the pump position to make the laser oscillate in single frequency condition, an output power of 8.0 W was obtained with a slope efficiency of 61.4% and an optical-optical efficiency of 50.0%. The power stability of the Ho:YAG NPRO laser was 0.29% at maximum single frequency output power. The beam quality M2 factors were measured to be less than 1.1 in x- and y- directions.
©2013 Optical Society of America
High power single frequency lasers with eye-safe wavelengths around 2 μm have a number of important applications in the fields of medicine, coherent Doppler lidar, differential absorption lidar, and nonlinear frequency conversion to pump mid-infrared optical parametric oscillators. Due to good thermal and mechanical properties, Ho:YAG is an attractive laser material for high-power operation in the 2 μm wavelength range. However, Holmium-doped materials cannot be directly pumped by diode lasers in the 790 nm spectral region. A common approach is to use Tm and Ho co-doped crystals which can be pumped by 790 nm diode lasers. But Tm-Ho co-doped system has high quantum defect, and very strong up-conversion process which leads to reduce effective upper-state lifetimes and energy storage efficiencies, and increase thermal heat loading. Ho:YAG laser resonantly pumped by Tm doped laser can overcome most of the difficulties of Tm-Ho co-doped systems. The quantum defect of Ho:YAG is less 10%, then the heat loads in the crystal is relatively small. This system can produce high power and high energy output, and the efficiency is much higher than that of Tm doped system and Tm-Ho co-doped system.
Several techniques have been used to obtain single frequency lasers in 2 μm region, e.g. microchip cavity, coupled cavity, twisted-mode cavity, intra-cavity etalons and non-planar ring oscillators (NPRO) [1–5]. Due to compactness and without discrete optical components inside the cavity, the NPRO laser has low intra-cavity loss and stable high power single frequency output can be obtained. The first NPRO developed by Kane and Byer oscillated at 1064 nm single frequency by using Nd:YAG as the active material . In 1995, I. Freitag et al. reported that 1.8 W single frequency CW output was achieved by a diode-pumped monolithic Nd:YAG NPRO lasers with the pump power of 4 W and slope efficiency of 51% . In 2006, P. Burdack et al. demonstrated a longitudinally diode-pumped, monolithic ytterbium ion-doped YAG nonplanar ring laser and achieved a CW single frequency output power of 1 W with 45% slope efficiency . In 2008, B. Q. Yao et al. reported that single frequency output power of 7.3 W at 2.09 μm from a monolithic Ho:YAG NPRO pumped by a 50 W Tm-doped fiber laser, and the slope efficiency was 48.3% with respect to the pump power . In 2009, C. Q. Gao et al. demonstrated a monolithic double diffusion-bonded Tm:YAG NPRO pumped by a 785 nm laser diode. 867 mW single-frequency output at 2.01 μm was obtained with a slope efficiency and an optical–optical efficiency of 31.6% and 19.2%, respectively . In 2012, C. Q. Gao et al. reported a 1645 nm monolithic Er:YAG NPRO, with the single frequency laser output power of 6.1 W, and a slope efficiency of 55.2% .
In this paper, we demonstrate a monolithic Ho:YAG NPRO operating at 2122 nm with high-power and high-efficiency single frequency output, resonantly pumped by a Tm:YLF laser. 8.0 W single frequency laser output was obtained at 2122 nm, with a slope efficiency of 61.4% and an optical-optical efficiency of 50.0%. Figure 1(a) shows the emission cross section of the Ho:YAG crystal from 2000 nm to 2200 nm . Ho:YAG crystal has three main emission peaks in 2090 nm, 2097 nm and 2122 nm. The emission cross section of 2122 nm wavelength (0.5392 × 10−20 cm2) is much smaller than that of 2090 nm (1.1543 × 10−20 cm2) and 2097 nm (0.9633 × 10−20 cm2). So normally it is more difficult to obtain 2122 nm laser output. But in the same condition, the atmospheric transmission of 2122 nm wavelength is about 7% higher than that of 2090 nm wavelength, and 4% higher than that of 2097 nm wavelength in the optical transmission window, as shown in Fig. 1(b). In the Lidar detection system, it is a two-way process to emit the laser signal and detect the echo signal. So the intensity of echo signal is proportional to the square of the atmospheric transmission. Therefore, the 2122 nm laser has more advantages as the light source of Lidar than lasers with 2097 nm and 2090 nm wavelengths. We properly designed output transmission of the Ho:YAG NPRO in three wavelengths, and accurately adjusted the pump position to make the laser oscillate at 2122 nm. To our knowledge, this is the first time to obtain 2122 nm high power single frequency laser output with highest efficiency which is near the quantum limit efficiency.
2. Experimental setup
Figure 2 shows the experimental setup of the monolithic Ho:YAG NPRO resonantly pumped by a 1907 nm Tm:YLF laser. The Tm:YLF laser was built up by ourselves. The Tm-doping concentration was 3 at.% and the dimension of Tm:YLF crystal was 4 mm × 4 mm × 10 mm. The input mirror of cavity was a plano-concave mirror with the radius of 100 mm and the output mirror was a plane mirror with the transmittance of 8%. The maximum output of this laser was 16.0 W and the slope efficiency was 42.2% and the beam quality M2 factors were measured to be less than 1.1 in both directions. Since the Ho:YAG crystal has absorption spectrum around 1907 nm, we used an etalon to tune the wavelength of the Tm:YLF laser to the Ho:YAG absorption peak. The pump beam was collimated by using a 1:2 coupling optics and focused into the Ho:YAG NPRO by a spherical lens with the focus length of 100 mm. The beam radius of the pump laser was about 80 μm at the input surface of the NPRO. For the NPRO resonator, the effective gain region is in the path in front of the first total-reflection surface. The geometric length from the input surface to the first total reflection surface is 8.7 mm which is close to the Rayleigh length of the pump beam (7.6 mm). To achieve the single direction oscillation, a permanent magnetic field of 0.4 T was applied along the Ho:YAG NPRO. The Ho-doping concentration was 0.8 at.%. The dimension of Ho:YAG NPRO was 12 mm (width) × 12 mm (length) × 4 mm (height). The round-trip path length of Ho:YAG NPRO crystal is 30.6 mm. The input surface of the Ho:YAG NPRO was designed to have a high transmission coating at 1907 nm and 8.4% output coupling coating of the s-polarized beam at 2122 nm. For achieving the wavelength of 2122 nm, we made the output coupling coating of 2090 nm (11.4%) and 2097 nm (10.6%) bigger than that of 2122 nm. So 2122 nm laser suffers less loss and is easy to oscillate. The temperature of Ho:YAG NPRO was controlled at 17.6°C by using a thermal electric cooler (TEC).
3. Experimental results
The spectra of the output beams were monitored by using a scanning confocal Fabry-Perot interferometer with a free spectral range (FSR) of 3.75 GHz. At the same time, the wavelength of Ho:YAG NPRO laser was measured by EXFO WA-1000 wavemeter. The maximum output power of Ho:YAG NPRO laser was 9.66 W, but two modes were visible in one FSR as shown in Fig. 3(a), and Fig. 3(b) shows two wavelengths (2090 nm and 2122 nm). We carefully adjusted the position of pump beam, and stable single-longitudinal-mode oscillation was realized at 2122 nm as shown in Fig. 3(c). Figure 3(d) shows the wavelength at the highest single frequency laser output.
The laser output power as functions of the pump power is shown in Fig. 4. The maximum multimode output power was 9.66 W. In the multi-mode operation the pump absorption efficiency was 88.1%. The slope efficiency and optical-optical efficiency were 71.7% and 60.4%, respectively. After carefully adjusting the pump position and pump beam radius, up to 8.0 W single frequency output power at 2122 nm was obtained when the pump power was 16.0 W. In this case the slope efficiency was 61.4% with respect to the pump power. When the laser operated in single frequency condition, the pump absorption efficiency was measured to be 76.5%. With respect to absorbed pump power, the slope efficiency and the optical-optical efficiency are 80.3% and 65.4%, respectively.
The power stability of the single frequency Ho:YAG NPRO laser was measured by using a power meter (Molectron 3 Sigma, PM30 detector). At the highest single-frequency power level, the relative power stability is 0.29% in 30 minutes. We also measured the wavelength and wavelength stability of the Ho:YAG NPRO laser by using the EXFO WA-1000 wavemeter with a resolution of 10 pm. The wavelength of the single frequency laser is 2122.213 nm when the crystal temperature was controlled at 17.6°C. The wavelength stability of the Ho:YAG NPRO laser was measured to be less than 10 pm limited by the resolution of wavemeter in 250 seconds.
The frequency-tuning was realized by changing the temperature of the Ho:YAG crystal. According to the formula in , the frequency tuning of the laser with temperature is given by:Figure 5 shows the laser frequency of Ho:YAG NPRO laser as a function of the crystal temperature. The frequency tuning efficient of Ho:YAG NPRO laser is determined to be −1.70 GHz/°C, which agrees well with the theoretical calculation.
The beam quality of the output beam at the highest single frequency output power was measured. Figure 6 shows the measured beam radii at different positions along the beam propagation and the picture inserted is a two-dimensional beam profile of the laser beam. By fitting the measured data with a hyperbolic curve, the M2 -factors were calculated to be 1.016 and 1.053 in x- and y-directions, respectively.
We reported a single-frequency Ho:YAG NPRO laser resonantly pumped a Tm:YLF laser at room temperature. Up to 8.0 W single-frequency output power was obtained at wavelength of 2122 nm. The slope efficiency and the optical-optical efficiency are 61.4% and 50.0%, respectively. The tuning coefficient of the Ho:YAG NPRO laser is −1.70 GHz/°C. The relative power stability of Ho:YAG NPRO was 0.29% in 30 minutes. The measured M2 factors were less than 1.1 in both directions. This compact high power Ho:YAG NPRO laser operating at 2122 nm is a potential candidate for being a seed laser of an injection seeding system or pumping optical parametric oscillators.
The authors acknowledge much help from Prof. Yang Suhui and Li Jing from Beijing Institute of Technology. This work is partly supported by the National Science Foundation of China (61178027, 60908009) and Beijing Natural Science Foundation (4132036).
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