⇦Chinese Physics Letters, 2019, Vol. 36, No. 2, Article code 024201 A Stable Wavelength Operation Ho:YAG Laser with Orthogonally Polarized Pump * Jing-Liang Liu (刘景良), Xin-Yu Chen (陈薪羽), Rui-Ming Wang (王睿明), Chun-Ting Wu (吴春婷), Guang-Yong Jin (金光勇)** Affiliations Jilin Key Laboratory of Solid Laser Technology and Application, College of Science, Changchun University of Science and Technology, Changchun 130022 Received 11 November 2018, online 22 January 2019 *Supported by the Cooperation Foundation of Changchun Science and Technology Bureau under Grant No 17DY027, the Foundation of Education Department of Jilin Province under Grant No JJKH20181105KJ, and the Foundation of Jilin Province Science and Technology Department under Grant No 20180101033JC.
**Corresponding author. Email: jgycust@163.com Citation Text: Liu J L, Chen X Y, Wang R M, Wu C T and Jin G Y et al 2019 Chin. Phys. Lett. 36 024201    Abstract A stable wavelength operation Ho:YAG laser dual-pumped by two orthogonally polarized Tm:YLF lasers is reported. Under the cw operation mode, a laser output power of 24 W is measured. The corresponding optical-optical conversion efficiency is 44.75% and the slope efficiency is 50.12%. Under the Q-switched operation mode, the output maximum average power is 22.8 W at the re-frequency of 6 kHz. The corresponding optical-optical conversion efficiency and slope efficiency are 42.64% and 48.01%, respectively. The output central wavelength is 2090.73 nm, the linewidth is 0.40 nm, and the beam quality is $M^{2} < 1.6$. Moreover, the shift of the output central wavelength is less than 0.01 nm, and the linewidth shift is also less than 0.01 nm. DOI:10.1088/0256-307X/36/2/024201 PACS:42.55.Rz, 42.60.-v, 42.60.Gd © 2019 Chinese Physics Society Article Text LD-pumped 2 µm waveband solid-state lasers have been used in many fields, such as laser lidar,[1,2] medical surgery,[3,4] and material processing.[5,6] In addition, 2 µm waveband lasers are also excellent pumping sources commonly used in mid-infrared nonlinear frequency conversion technology.[7-9] Due to the small quantum loss of the Tm laser-pumped Ho laser and the long upper laser level lifetime of the Ho$^{3+}$ ion,[10,11] Ho-doped lasers are very suitable for high-power Q-switched laser output.[12-14] However, the stability of laser output wavelength is an important factor in applications of various fields, which is relatively less mentioned in literature for Ho:YAG laser systems. In this work, two orthogonally polarized 1.9 µm Tm:YLF lasers are used to a dual-pumped Ho:YAG laser. Under the cw operation mode, the Ho:YAG laser is demonstrated with an output power of 24 W. Under the acousto-optic Q-switched operation mode, the output average power is 22.8 W and the pulse width is 20.01 ns at the re-frequency of 6 kHz. The output central wavelength is 2090.73 nm, the linewidth is 0.40 nm, and the beam quality is $M^{2} < 1.6$. It is worth noting that during the whole operative process of the laser system, the shift of the output central wavelength is less than 0.01 nm, and the linewidth shift is also less than 0.01 nm. Figure 1 schematically shows the Ho:YAG laser system used in the experiment. In the experimental system, two orthogonally polarized Tm:YLF lasers were used as the pump source for the Ho:YAG laser system. Both of the Tm:YLF lasers were used fiber-coupled LD pumping structures which had a maximum output power of 25 W and the wavelength of 1908 nm. Since the two Tm:YLF laser sources were s-polarized and p-polarized, respectively, both the Tm lasers can be polarization-isolated from each other. This design effectively prevents the residual pump light from returning to the pump sources again and causing damage. Two TFPs were high transmission for s-polarized and p-polarized, respectively. The Ho:YAG crystal was $\Phi5\times20$ mm$^{3}$ in diameter and the Ho$^{3+}$ doping concentration was 1.0 at.%. The Ho:YAG crystal wrapped in indium foil was placed in a copper heat sink, which was cooled by water cooling to maintain its temperature at 18$^{\circ}\!$C. The resonant cavity of the laser system with a physical length of 150 mm consisted of a laser output coupling mirror (OC), a flat laser reflecting mirror (M1), and a 45$^{\circ}$ laser reflecting mirror (M2). The laser reflecting mirrors were coated high reflection at 2090 nm and high transmission at 1908 nm. The output coupling mirror was coated for a light transmittance of 30% for 2090 nm had a radius of curvature of 300 mm. The YAG F-P etalon with a thickness of 0.5 mm was placed at a certain angle in the cavity to achieve narrow linewidth laser output, and also to stabilize the laser output. The acousto-optic Q-switching device (AOM) was used for realizing the Q-switched pulse output of the laser system. The AOM had the rf frequency of 40.68 MHz and the maximum rf power of 50 W. To limit the output wavelength and the linewidth of the laser system, inserting the F-P etalon into the cavity is one of the simple and effective measures. Based on the principle of multi-beam interference, the transmittivity of the specific wavelength is adjusted, and the laser wavelength satisfying the oscillation condition is selected. The transmittivity function of the single etalon is given by $$\begin{align} T(\lambda)=\,&\frac{1}{1+({\frac{2F}{\pi }})^{2}\sin^{2}(\frac{\phi }{2})}\\ =\,&\frac{1}{1+({\frac{2F}{\pi }})^{2}\sin^{2}(\frac{2\pi nd\cdot \cos \alpha }{\lambda })}, \end{align} $$ where $F=\pi \sqrt R /(1-R)$ is the finesse of the etalon, $R$ is the reflectivity of the etalon for the selected wavelength, and $\phi =2\pi nd\cos \alpha /\lambda$ is the phase difference between the neighbor two beams in the etalon with $n$ the refractivity of the etalon, $d$ the thickness of the etalon and $\alpha$ the refraction angle of the beam incident etalon.
cpl-36-2-024201-fig1.png
Fig. 1. Schematic diagram of the Ho:YAG laser system.
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Fig. 2. The transmittivity curve of the etalon. (a) The transmittivity curve under different tilt angles of the etalon. (b) The transmittivity curve under different reflectivities of the etalon.
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Fig. 3. The output power of the Ho:YAG laser during two modes of operation.
Figure 2(a) shows the transmittivity curves under different tilt angles (i.e., incident angles) of the etalon. It can be seen that as the tilt angle changes, the transmittivity of the laser wavelength shifts significantly. Figure 2(b) shows the transmittivity curves under different reflectivities of the etalon. The conclusion that we can provide is that as the reflectivity increases, the transmittivity curve width is narrowed significantly, and the selectivity to a specific wavelength becomes better. Finally, the F-P etalon is chosen to have a reflectivity of 10% and a tilt angle of 3.56$^{\circ}$, which plays a role of stabilizing the wavelength and limiting the line width in the Ho:YAG laser system.
cpl-36-2-024201-fig4.png
Fig. 4. The single pulse energy and pulse width of the Ho:YAG laser at the re-frequency of 6 kHz.
cpl-36-2-024201-fig5.png
Fig. 5. The pulse sequence and pulse width diagram of Ho:YAG laser at the re-frequency of 6 kHz.
The output power of the Ho:YAG laser system under two operation modes was measured, which is shown in Fig. 3. From the figure it can be seen that both the curves show a linear growth trend. Under the cw operation mode, the output power of 24 W was obtained, the corresponding optical-optical conversion efficiency was 44.75% and the slope efficiency was 50.12%. Under the Q-switched operation mode, the output maximum average power is 22.8 W at the re-frequency of 6 kHz. The corresponding optical-optical conversion efficiency and slope efficiency were 42.64% and 48.01%, respectively. The Q-switched laser pulse profile was measured by a Tektronix DPO3050 oscilloscope with a HgCdTe detector. Figure 4 shows the dependence of the single pulse energy and pulse width of the Q-switched Ho:YAG laser on the pump power. It can be seen that as the pump power increases, the pulse width sharply narrows. Finally, the maximum single pulse energy is 3.8 mJ, and the corresponding peak power and pulse width are 180.78 kW and 21.02 ns. The pulse sequence and pulse width diagram at this time are shown in Fig. 5.
cpl-36-2-024201-fig6.png
Fig. 6. The laser output mode and beam quality of the Ho:YAG laser: (a) the laser output mode (2D), (b) the laser output mode (3D), and (c) the beam quality.
cpl-36-2-024201-fig7.png
Fig. 7. The shift of the output spectrum of the Ho:YAG laser.
cpl-36-2-024201-fig8.png
Fig. 8. The spectrum of the Ho:YAG laser at the maximum output power.
The experiment further measured the output spectrum and beam quality of the Ho:YAG laser system. Figure 6 shows the laser output mode and beam quality fitting curve of the Ho:YAG laser measured by the 90/10 knife-edge method. It can be seen that the output laser is the fundamental mode output, and the beam qualities in the $X$ and $Y$ directions are $M_{x}^{2}=1.53$ and $M_{y}^{2}=1.55$, respectively. The shift of the Ho:YAG laser output spectrum was also measured, as shown in Fig. 7. It can be seen that the output spectrum changes slightly with the laser output power, wherein the shift of the output central wavelength is only 0.01 nm, and the linewidth shift is also less than 0.01 nm. Figure 8 shows the spectrum of the Ho:YAG laser at the maximum output power with a central wavelength of 2090.73 nm and the linewidth of 0.40 nm. Finally, a Ho:YAG laser with a stable wavelength output was obtained. In summary, a stable wavelength operation Ho:YAG laser dual-pumped by two orthogonally polarized Tm:YLF lasers has been demonstrated. Under the cw operation mode, the laser output power of 24 W is measured. The corresponding optical-optical conversion efficiency is 44.75% and the slope efficiency is 50.12%. Under the Q-switched operation mode, the output maximum average power is 22.8 W at the re-frequency of 6 kHz. The corresponding optical-optical conversion efficiency and slope efficiency are 42.64% and 48.01%, respectively. The output central wavelength is 2090.73 nm, the linewidth is 0.40 nm, and the beam quality is $M^{2} < 1.6$. Moreover, the shift of the output central wavelength is less than 0.01 nm, and the linewidth shift is also less than 0.01 nm. This stable wavelength output Ho:YAG laser will play a role in many fields, and it will become an excellent pump source for mid-infrared nonlinear frequency conversion technology. References Intrapulse temporal and wavelength shifts of a high-power 21-μm Ho:YAG laser and their potential influence on atmospheric lidar measurementsThe healing effects on the biomechanical properties of joint capsular tissue treated with ho:YAG laserStone Retropulsion with Ho: YAG and Tm: YAG Lasers: A Clinical Practice-Oriented Experimental StudyA Mid-IR 14.1 W ZnGeP 2 Optical Parametric Oscillator Pumped by a Tm,Ho:GdVO 4 LaserWidely Tunable Middle Infrared Optical Parametric Oscillator Pumped by the Q-Switched Ho:GdVO 4 LaserHigh-Power Dual-End-Pumped Actively Q-Switched Ho:YAG Ceramic LaserEfficient mid-infrared laser using 19-µm-pumped Ho:YAG and ZnGeP_2 optical parametric oscillatorsHigh Power Q-Switched Dual-End-Pumped Ho:YAG LaserHigh power Ho:YAG laser pumped by two orthogonally polarized Tm:YLF lasersA 106W Q-switched Ho:YAG laser with single crystal146.4 W end-pumped Ho : YAG slab laser with two crystals
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