1. Introduction

The high-power lasers at 2 µm have attracted great research attention and it have been extensively investigated recently because of its attractive applications in the areas of medical treatment, optical communications and effective pump sources for 3 – 5 µm optical parametric oscillators (OPOs) [1–4]. One way to generate high power laser around 2 µm is by means of rare-earth ion doped crystals, including Tm³⁺, Ho³⁺, Tm³⁺-Ho³⁺, Cr³⁺-Tm³⁺-Ho³⁺ lasers [5, 6]. This paper focuses on the all-solid-state Tm³⁺ doped YAG lasers. The use of Tm³⁺ doped YAG has characteristics of long fluorescence lifetime, broader absorption and emission spectra, cross-relaxation introduced high quantum efficiency etc. and therefore it can become good candidate for high power lasing at 2 μm [7]. Besides, the absorption band of Tm³⁺ ions near 785 nm matches well with the commercially available laser diodes. Among various host matrices, YAG crystal posses the advantages of large heat conductivity and high mechanical strength, which allows high power operation with reduced risk of fracture [8, 9]. Thus, Tm:YAG material is a promising material for obtaining high power laser around 2 µm.

In the Tm³⁺ doped YAG crystal, the Tm³⁺ ions are excited into the ³H₄ state from ³H₆ state by absorbing pump radiation at ~785 nm, and the ions then relax down to the upper lasing level ³F₄. The laser radiation at 2 µm takes place between the lower Stark level of ³F₄ and the higher Stark level of ³H₆, which include 610 cm⁻¹, 730 cm⁻¹ [10, 11]. When the ions transition from lowest Stark sub-level in ³F₄ at 5556cm⁻¹ to ³H₆ level, resulting in the possible laser emissions at 2.02 μm, 2.07 μm, respectively, as shown in Fig. 1 .

 

Fig. 1 Energy level diagram of Tm: YAG lasers.

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High power Tm:YAG lasers at 2.02 μm have been reported by several groups. E. C. Honea et al. reported an 115 W continuous wave (cw) Tm: YAG laser with diode laser (LD) end-pumped structure under 3°C with the coolant mixture of 90% and water 10% alcohol [12]. K. S. Lai et al. achieved an 150 W cw LD side-pumped rod Tm:YAG laser under −10°C with the mixture of water and glycerol as the coolant [13]. Unfortunately, these lasers were all operated under low temperature, which may lead to freezing of the coolant or condensation at the rod surfaces. Our group has demonstrated the generation of cw 200 W output power in Tm:YAG laser at 2.02 µm under 8°C water-cool environment in 2011 [14], while a Q-switched Tm:YAG laser at 2.02 µm with output power of 171.4W under operated temperature of 8°C was also reported [15].

As for rod Tm:YAG laser at 2.07 μm, very little has been published. With Ti:sapphire laser pumping, R. C. Stoneman et al. reported a tunable Tm:YAG laser from 1.9 to 2.16 µm and the output-powers at 2.07 µm was only 280 mW [16].

For applications such as free-space optical communication, long-range communication requiring minimal atmospheric attenuation, the longer wavelength around 2 µm is superior [17]. Specially, the Tm:YAG laser with longer wavelength has less absorption loss in the ZnGeP₂ (ZGP) crystal [18].Therefore, compared with 2.02 µm, the Tm:YAGlaser at 2.07µm is more suitable for pumping ZGP-OPO.

In this paper, we report a high-power wavelength switchable LD side-pumped rod Tm:YAG laser system around 2 µm under 8°C water-cool environment. The system operates at either 2.07 or 2.02 µm depending on the transmission of the output coupler. The maximum cw output power at 2.02 µm is 77.1 W with a coupler of 10% transmission, while the maximum cw output power at 2.07 µm is up to 115 W by using a coupler of 5% transmission. To our knowledge, this is the first demonstration of wavelength switchable LD side-pumped 2 μm rod Tm:YAG laser with an output power over 100 W at 2.07 μm.

2. Experiment and result

The high-power rod Tm:YAG laser system around 2 µm is designed with two-rod scheme, in which two identical LD side pumping laser modules are employed. The experimental setup for the high-power rod Tm:YAG laser setup is shown in Fig. 2 . The system adopts a Plano-Plano symmetrical cavity.

 

Fig. 2 Schematic of the rod Tm:YAG laser around 2 µm

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There were five diode arrays in each laser module and each diode array consists of twelve diodes operating in cw-mode at wavelength of 785 nm and the total pump power is 1200 W. The rod Tm:YAG(doped with 3.5% Tm) used in our experiment is 4 mm in diameter and 69 mm in length with screw threads fabricated on the side surface with a 0.6 mm screw-pitch and a 0.1 mm depth to increase surface area for better cooling. Each end-face of the rod is bonded with an 18 mm un-doped YAG, which is especially necessary for reducing the reabsorption losses in the unpumped region for the side-pumping configuration and for reducing the chances of thermal fracture and lensing owing to bulging of the rod surfaces. The rod Tm:YAG- is cooled to 8°C by de-ionized water. M1 and M2 are rear mirror and output coupler (OC), respectively. M1 is HR-coated with reflectivity R > 99.5% at 2 μm, and two output coupler mirrors M2 are coated with transmission T_oc = 10% or 5% around 2 μm for wavelength switching. L1 and L3 are the distances between the M1 to laser module 1 and the M2 to laser module 2, respectively. L2 is the distance between two laser modules.

In order to design an optimum laser cavity, we measured the thermal focus length of each laser module as a function of LD pump power. The typical result is shown in the Fig. 3 . The thermal focus length f_T decreases with the increase of LD pump power, and f_T is ~109 mm at pump power of 594 W.

 

Fig. 3 The thermal focus length of a single laser module versus diode-pump power

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For obtaining high output power with high beam quality, the cavity parameters are optimized by computer simulation using ABCD propagation matrix formula for the fundamental mode. Figure 4 shows the simulation result with L1 = L3 = 25 cm and L2 = 50 cm for the calculated fundamental mode beam radius at the center of laser rod of laser module 2 as a function of thermal focal length. The laser system operates in the stable region under total LD pump power of 1,188 W, which corresponds to a thermal focus length around 109 mm according to the Fig. 3.

 

Fig. 4 Stability-zone calculations for the fundamental mode beam radius at the center of laser rod of laser module 2 as a function of thermal focal length.

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The output power of the rod Tm:YAG lasers at 2 μm is measured with a power meter (Ophir F300A-SH). The dependence of laser output power on the LD input power is shown in Fig. 5 . The laser threshold occurs at about 432 W of diode-pumping power for T_oc = 5%, and the maximum output power is up to 115 W under total LD pump power of 1,188 W at a cooling temperature of 8°C for the rod Tm:YAGs. While the laser threshold is around 619 W for T_oc = 10%, and the maximum output power is up to 77.1 W under total LD pump power of 1,188W. Compared with T_oc = 10%, the laser threshold for T_oc = 5% is lower, the conversion efficiency is higher and higher output power can be achieved. It is worth noting that the laser output power for both output couplers depends linearly on the pump power when the pump power is below 792 W. The slope efficiency is about 22.5% with T_oc = 5% and 21.1% with T_oc = 10%, respectively.

 

Fig. 5 Output power of rod Tm:YAG laser versus diode pumping power

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The emission wavelengths of the 2 μm rod Tm:YAG laser with different T_oc are recorded with an spectrometer (NIRQuest256-2.5, Ocean Optics). The laser spectra at maximum output power are shown in Fig. 6 . It can be seen from Fig. 6 that the central wavelength of the laser with T_oc = 5% is located at 2.07 μm at maximum output power of 115W. The wavelength is shifted to 2.02 μm when the output coupler is switched from T_oc = 5% to 10%. The dependence of the laser wavelength on the T_oc is attributed to reabsorption effect of quasi-three-level laser system [19, 20]. With the same pump power, a lower T_oc would result in a higher intracavity intensity. As a result, the lower laser sub-level within ³H₆ ground state manifold has higher reabsorption loss and therefore, the higher Stark sub-level within ³H₆ ground state manifold has a lower laser threshold and longer laser wavelength.

 

Fig. 6 Spectra of rod Tm:YAG laser with different output couplings. The central wavelength is 2.07 μm for the one with Toc = 5% and 2.02 μm for the one with Toc = 10%

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The value of beam quality M² can be derived from the measured beam profiles at various positions using a Pyro-III infrared CCD camera. Figure 7 shows M² value of a 2.07 μm rod Tm:YAG laser measured at an output power of 115 W. The M²_x and M²_y factors are found to be about 15.2 and 14.7, respectively, corresponding to an average value of M² = 15.

 

Fig. 7 Beam quality factor of a 2.07μm rod Tm:YAG laser measured at an output power of 115W

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The laser power stability is monitored at the full output power. Figure 8 shows the measurement result for the 2.07 μm rod Tm:YAG laser. It is found that, for a test interval of 30 min, the fluctuation of output power is less than ± 3%.

 

Fig. 8 Stability of a 2.07μm rod Tm:YAG laser measured at an output power of 115W

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3. Conclusions

In summary, we demonstrate here for the first time the high-power wavelength switchable LD-side-pumped Tm:YAG laser around 2 µm. The laser central wavelength is switchable between 2.07 and 2.02 µm by means of different OC transmission. The maximum output power at 2.07 µm is up to 115 W for T_oc = 5%. This is, to the best of our knowledge, the highest power reported so far for a rod Tm:YAG laser at 2.07 µm. The maximum output power at 2.02 µm is 77.1 W for T_oc = 10%. The novel spectral properties of the laser could be attributed to the reabsorption of light in the rod Tm:YAG laser medium.