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We first report a diode-pumped continuous wave orthogonally polarized dual-wavelength single-longitudinal-mode laser with a single c-cut Tm,Ho:LuLiF4 laser crystal. The simultaneous dual-wavelength single-longitudinal-mode laser near 2 μm is realized by using two uncoated intracavity Fabry-Perot etalons. The output wavelengths are 2064 nm in π-polarization and 2066 nm in σ-polarization respectively, which are orthogonal to each other. At the absorbed pump power of 1 W, the maximum single-longitudinal-mode output powers at 2064 and 2066 nm are 76 and 32 mW respectively. The orthogonally polarized dual-wavelength single-longitudinal-mode laser is possible to be applied to the 2 μm differential absorption lidar and the generation of THz radiation.
© 2013 Optical Society of America
1. Introduction
Diode pumped Tm-Ho-codoped solid state lasers operating in the 2 µm waveband are very interesting for various applications in coherent Doppler wind lidars, differential absorption lidars, range-finding, photo-medicine, and nolinear frequency conversion [1, 2]. Among those known Tm-Ho-codped materials, Tm,Ho:LuLiF4 (LLF) is a kind of excellent laser crystal because of high natural birefringence, low upconversion effect, and large energy spread of the manifolds [3–5]. Furthermore, it has good thermo-optical and thermomechanical properties. Its thermal conductivity (a-axis), linear expansion coefficient (c-axis), and thermal coefficient of refractive index dn/dT (c-axis) are 4.3 Wm−1K−1, 11 × 10−6 K−1, and −3.6 × 10−6 K−1 respectively [6]. For c-cut Tm,Ho:LLF crystal, the emission cross section of the Ho 5I7 manifold is about 0.887 × 10−20 cm2 at 2.06 μm, and the upper level lifetime of 5I7 manifold is as long as 14.8 ms [6]. At present, continuous wave (CW) and Q-switched Tm,Ho:LLF lasers have been widely researched [7, 8]. However, up to now, the orthogonally polarized dual-wavelength single-longitudinal-mode (SLM) Tm,Ho:LLF laser has not been reported.
Simultaneous dual-wavelength lasers have many applications in laser lidar, THz research, precision laser spectroscopy, medical application and so on [9, 10]. The SLM lasers around 2 µm with a small wavelength separation and orthogonal polarization are very attractive for differential absorption lidars measuring H2O and CO2 concentrations in the atmosphere. The lidars use two wavelengths that can be quickly switched between an on-line wavelength which is in resonance with the absorption band of measured molecule, and an off-line wavelength which is not absorbed by measured molecule. Furthermore, the 2 µm dual-wavelength lasers are also very interesting for the generation of coherent THz radiation by nonlinear difference frequency mixing, because 2 µm emission permits to use the nonlinear crystal such as ZnGeP2 and OP-GaAs which are not transparent at shorter wavelengths [11]. In the past years, for rare-earth doped solid state lasers, the research on the dual-wavelength emissions mainly focused on the Nd or Yb doped lasers at the wavelength range from 0.9 to 1.3 μm [12–20]. Recently, simultaneous output of two wavelengths at 2 µm spectral range has been widely interesting. Ju et al reported a dual-wavelength SLM Tm,Ho:GdVO4 microchip laser in 2008, which is cooled to the temperature of liquid nitrogen. The output wavelength emissions are at 2038.9 and 2050.1 nm from Ho ions, and the total output power was 98 mW [21]. Then, using Tm,Ho:YVO4 microchip crystal at cryogenic temperature, the SLM emissions at 2041.3 and 2054.6 nm were realized, and the total output power reached 285 mW [22]. In 2011, V. Jambunathan et al observed the dual-wavelength output in a diode- pumped Ho,Tm:KLu(WO4) system, one generated from Tm ions tuned in the 1854-1980 nm range and the other from Ho ions tuned in the 1971-2063 nm range [23]. More recently, M. Segura et al reported simultaneous CW laser emissions at 1922 and 1946 nm from the Tm:KLu(WO4) crystal, which is accompanied by polarization switching [11, 24]. In this paper, we report on a CW simultaneous dual-wavelength SLM Tm,Ho:LLF laser with orthogonal polarization, near room temperature. At the absorbed pump power of 1 W, the maximum SLM output powers at 2064 and 2066 nm are as high as 76 and 32 mW respectively.
2. Experimental setup
A simple plane-concave cavity configuration is shown in Fig. 1. The pump laser is a fiber-coupled laser diode temperature-tuned to 792 nm emission wavelength, with a maximum output power of 3 W. The diameter and numerical aperture of the fiber core are 100 μm and 0.22 respectively. A coupling optics system is used to focus the pump beam. The pump spot diameter in the crystal is about 100 µm. The total transmission efficiency of the beam-reshaping system is over 90% at 792nm. The c-cut Tm,Ho:LLF laser crystal has dopant concentrations of 5% Tm, 0.5% Ho with dimension of 5 mm × 5 mm × 2.5 mm. A dichromatic coating on the front face of the crystal is high transmitting at 792 nm, but is totally reflecting at 2 µm. The other face is antireflection coated at 792 nm and 2 µm. The curvature radius of the output coupler is 51.8 mm. The near hemispherical resonator is formed between the planar crystal front face and the output coupler, and the cavity length is about 50 mm. The crystal is wrapped with indium foils and held in a brass heat sink. Temperature of the heat sink is heldat 283K with a thermoelectric cooler. For the SLM operation, two uncoated fused silica Fabry-Perot (F-P) etalons, which are 0.1 and 1 mm thick respectively, are inserted inside the laser cavity. The output power is measured by a power meter (MolectronPM10). The output wavelength is measured with a monochrometer (WDG-30) and a InGaAs PIN photodiode. The spectrum of SLM is recorded by a scanning F-P interferometer with a free spectral range of 3.75 GHz and a fast InGaAs PIN photodiode connected with a Tektronix TDS3032B digital oscilloscope. The transverse output beam profile is measured with a beam analyzer (Electrophysics, MicronViewer 7290A).
3. Results and discussion
First of all, we investigate the Tm,Ho:LLF laser output performance in CW multimode operation without intracavity etalon. To obtain 2 µm orthogonally polarized laser, we make the 792 nm pump beam have a small angle with the cavity axis, which can thermally induce birefringence in the c-cut Tm,Ho:LLF. The output laser is separated into two orthogonally polarized laser beams with a polarizing beam splitter (PBS), and output power for individual polarization is measured simultaneously. The σ-polarization is defined as the polarization direction of output laser which has a smaller angle with the horizontal, and the π-polarization is orthogonal to the σ-polarization. Figure 2(a) shows the output power in each polarization as a function of the absorbed pump power. It can be noted that the π-polarization and σ-polarization laser simultaneously begins oscillating at the threshold pump power of 150 mW. The maximum output powers of the π-polarization and σ-polarization are 206 and 170 mW at the absorbed pump power of near 1 W, respectively. Furthermore, the total output power for the two polarization directions linearly increases with the absorbed pump power, and the corresponding slope efficiency is 46.2%. The measure optical spectrum at the maximum output power is shown in Fig. 2(b), and the central wavelength is 2066 nm.
Fig. 2 (a) Output powers versus the aborbed pump power, and (b) output spectrum of the laser with the central wavelength of 2066 nm for the multimode operation.
The SLM laser output at 2 µm can be realized by inserting two 0.1 and 1 mm thick uncoated etalons into the cavity [25]. The etalons can be used to not only realize the SLM oscillation of dual-wavelength laser, but also control the cavity losses of two orthogonal polarizations [13, 26]. When the thickness and the angle of etalons are changed, the SLM laser will achieve different wavelength oscillations [14]. By regulating the angle of the two solid etalons to make σ polarization have a higher loss [13], the orthogonally polarized dual-wavelength SLM laser at 2064 nm (π-polarization) and 2066 nm (σ-polarization) is realized. Figure 3(a) shows the output power as a function of absorbed pump power for the SLM laser. It can be noted that the 2064 nm and 2066 nm laser begins oscillating, when the absorbed pump power is increased to about 0.41 and 0.45 W respectively. The output powers of 2064 and 2066 nm lines first increase linearly with the absorbed pump power, with the nearly same slope efficiency. When the absorbed pump power is over 0.67 W, the output power of 2064 nm laser still increases linearly with the absorbed pump power, with a higher slope efficiency,however the output power of 2066 nm laser shows obvious saturation effect. The 2064 and 2066 nm laser lines result from the 5I7→5I8 transition and share the common upper level. The gain competition is not negligible between 2064 and 2066 nm, and becomes more evident at a high pump power. The saturation of output power at 2066 nm mainly results from the net gain increase of 2064 nm. The output powers at 2064 and 2066 nm reach their maximum values of 76 and 32 mW at the absorbed pump power of 1 W, respectively. The total output power of dual-wavelength SLM laser nearly linearly increases with the absorbed pump power, and the corresponding slope efficiency is 19%, as shown in Fig. 3(a). The slope efficiency in the SLM operation can be promoted by redesigning the etalon with lower optical loss. It is worthwhile to note that the dual-wavelength SLM laser shows good power stability, though there lies the obvious gain competition between 2064 and 2066 nm wavelengths over the absorbed pump power of 0.67 W. Figure 3(b) shows the measured optical spectra of the π-polarization and σ-polarization laser at the absorbed pump power of 1 W. It can be seen from Fig. 3(b) that the central wavelengths are 2064 nm (π-polarization) and 2066 nm (σ-polarization) respectively.
Fig. 3 (a) Output powers versus the aborbed pump power, and (b) output spectra of the laser with the wavelengths of 2064 nm and 2066 nm for the dual-wavelength SLM operation.
We note that the two polarization directions of the output laser take place rotation when the absorbed pump power changes. Figure 4(a) shows the transmitted output power as a function of the PBS rotation angle for the two absorbed pump powers of 0.83 W and 0.97 W. It can be noted that the rotation angle of the PBS, at which only π-polarization laser or σ-polarization laser is completely transmitted from the PBS, becomes larger when the absorbed pump power increases. Therefore the polarization directions of the output laser obviously change with the increase of absorbed pump power as shown in Fig. 4(b). The polarization angles of σ-polarization and π-polarization laser increase from near 0° and 90° to 70° and 160° respectively, when the absorbed pump power increases from 0.4 W to 1 W. One can see from Fig. 4(b) that the two polarization directions rotate the same amount and always keep orthogonal to each other. The corresponding rotation coefficient with the absorbed pump power is about 127°/W. The polarization rotation with the absorption pump power may be due to the thermally induced birefringence in the crystal and the orientation of the induced index ellipsoids not being aligned along the laser cavity axis [2].
Fig. 4 (a) The transmitted powers versus the rotation angle of PBS, and (b) the polarization angles of the orthoganal polarized laser versus the absorbed pump power, showing the rotation of the polarization direction.
The transverse output beam profile is measured under the absorbed pump power of 1 W, and shown in Fig. 5(a) and 5(b). It can be seen that the output beam is close to fundamental transverse electromagnetic mode (TEM00). Moreover, the beam radius at different positions along the beam propagation direction measured by the traveling knife-edge method is shown in Fig. 5(c), and the M2 factor is calculated to be 1.47.
Fig. 5 Far field beam profile, (a) two dimensional and (b) three dimensional. (c) The result of the beam quality measurement.
The mode spectra of the SLM laser are measured with a diagnostic air-gap scanning F-P interferometer as shown in Fig. 6. The input laser is detected by a InGaAs detector connected with a Tektronix TDS3032B digital oscilloscope. The mode spectrum of the dual-wavelength is shown in Fig. 6(a), and the mode spectra of 2064 and 2066 nm are shown in Fig. 6(b) and 6(c) respectively. As can be seen, the Tm,Ho:LLF laser operated on the SLM. Used to differential absorption lidars, the dual-wavelength SLM laser is very easy to realize the switching of output wavelengths due to its orthogonal polarization characteristics. Furthermore, to the generation of THz wave, the SLM operation of the dual-wavelength Tm,Ho:LLF laser can improve the purity of THz spectrum.
Fig. 6 F-P spectra of the dual wavelength SLM Tm,Ho:LLF laser. (a) Dual-polarization at 2064 and 2066 nm, (b) σ-polarization at 2066 nm, and (c) π-polarization at 2064 nm.
Finally, it is worthwhile to note that the Tm,Ho:LLF laser can also realized the single frequency operation at 2066 nm by careful tuning the angles of the two 0.1 and 1 mm thick etalons, when the 792 nm pump beam is along the cavity axis.
4. Conclusion
We have demonstrated, to the best of our knowledge, the first CW orthogonally polarized dual-wavelength SLM Tm,Ho:LLF laser, which emits at 2064 and 2066 nm. In the multimode operation, the central wavelength of the orthogonally polarized laser is 2066 nm, and the maximum total output power is 376 mW at 1 W absorbed pump power, corresponding to a slope efficiency of 46.2%. When the laser operates in the SLM by inserting two solid etalons into the cavity, the output wavelengths are at 2064 and 2066 nm, and the polarization directions are orthogonal to each other. At the absorbed pump power of 1 W, the output powers at 2064 and 2066 nm are 76 and 32 mW respectively. Furthermore, the rotation of the laser polarization direction as a function of absorbed pump power is investigated. The orthogonally polarized dual-wavelength SLM Tm,Ho:LLF laser is very suitable to 2 μm differential absorption lidars, coherent Doppler wind lidars, and THz wave generation.
Acknowledgments
This work is supported by the Specialized Research Fund for the Programe for New Century Excellent Talents in University (Grant No. NCET-11-269), the National Natural Science Foundation of China (Grant Nos. 11204048, 61275138, 10804022, 60878016), the Fundamental Research Funds for the Central Universities (Grant Nos. HEUCFZ1221, HEUCFZ11217), the Special Finemcial Grant from the China Postdoctoral Science Foundation (Grant No. 2013T60345) and the 111 project to the Harbin Engineering University (Grant No. B13015).
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