Solid-state lasers operating around a 2 μm waveband are in demand, because of their advantage of an eye-safe spectral range, for a variety of applications, such as atmospheric remote sensing[
Chinese Optics Letters, Volume. 18, Issue 9, 091401(2020)
Power and spectral analyses in diode-pumped c-cut Pbnm Tm:YAP laser
Detailed power and spectral analysis of a diode-pumped c-cut Pbnm 3 at.% Tm-doped yttrium aluminum perovskite (Tm:YAP) laser in a continuous wave (CW) operation is presented. The laser was experimentally examined in terms of the dependence on the transmittance and radius of curvature of the output coupling mirrors. At room temperature, for an output coupling transmission of 10.8%, the maximum output power of 6.35 W was obtained under a total absorbed pump power of 13.67 W with an optical-to-optical conversion efficiency of 46.5%. The highest slope efficiency of 60.4% was indicated. A detailed spectral analysis was presented. For its dependence on output coupler transmission, the Tm:YAP laser generates wavelengths at approximately 1940 nm or 1990 nm.
Solid-state lasers operating around a 2 μm waveband are in demand, because of their advantage of an eye-safe spectral range, for a variety of applications, such as atmospheric remote sensing[
Tm:YAP is another interesting gain medium. The thermal and mechanical properties of YAP are similar to those of the common host YAG. Additionally, in comparison with the Tm:YAG crystal, the emission cross section of Tm:YAP (
High power and efficiency Tm:YAP lasers in various configurations have been widely investigated in recent years[
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To achieve the expected laser output wavelength or maximize the laser gain to obtain a high-efficiency laser output, the axis cut of the laser crystal must be known. Owing to the quasi-three-level nature, most of the Tm lasers are blue-shifted as the output coupler (OC) transmission or the cavity losses increase.
The preferential free-running laser wavelength at room temperature for different axis cuts of Tm:YAP under various cavity output mirror transmissions was theoretically analyzed[
A Tm:YAP laser single-pass end-pumped by a fiber-coupled LD at 793 nm with a core diameter of 200 μm and a numerical aperture of 0.22 is shown in Fig.
Figure 1.Experimental setup of the diode-pumped Tm:YAP laser.
The
In the dependence of the OC radius of curvature, the Tm:YAP laser resonator physical cavity lengths changed from 47 to 55 mm. The input cavity mirror M1 was flat with high reflectivity in the wavelength range of 1.92 to 2.1 μm and high transmission at 793 nm. Considering the transmission losses on the coupling lenses and dichroic mirror M1, more than 90% of the available diode pump power was incident onto the crystal, which was 18.16 W, and the Tm:YAP crystal absorbed 75% of this pump power.
The plano-concave mirror OC of a different radius of curvature (
The diode-pumped Tm:YAP laser was studied during CW operation. The output power and spectral characteristics with different OCs were investigated in detail, with the results shown in Figs.
Figure 2.Output powers of the Tm:YAP laser under different output mirror transmissions as a function of the absorbed pump power for a radius of curvature of (a) 100 mm, (b) 200 mm, (c) 300 mm, and (d) 400 mm. The results of a linear fit and the calculated slope efficiencies are presented in the graphs.
Figure 3.Output spectra from the Tm:YAP laser in CW operation under different output mirror transmissions for a radius of curvature of (a) 100 mm, (b) 200 mm, (c) 300 mm, and (d) 400 mm.
The output power increased linearly with an increase in the absorbed pump power for each case. For the maximum CW absorbed pump power of 13.67 W, with an OC transmittance of
The output free-running spectra of the Tm:YAP laser were measured with an AQ6375 optical spectrum analyzer made by Yokogawa. No significant differences in the output laser wavelengths were observed in relation to the values of the OC
Because of the Tm:YAP laser levels feature, the operating wavelengths were shifted based on the output coupling transmittances. The change in the OC resulted in a change in the intracavity laser intensity. This behavior, in turn, changed the pump absorption of the Tm:YAP crystal because pump absorption is a function of laser intensity. With the same pump power, a lower
For the lowest OC, the developed laser characterized the highest output power. Unfortunately, this is an undesirable feature for the need to develop an efficient laser system that generates radiation with a wavelength of 1940 nm, which can be used as a pumping system for the Ho:YLF active medium.
For the higher OC transmittances, the generated laser wavelengths shifted to around 1940 nm, reaching 1935 nm for a
Figure 4.Output spectrum from the Tm:YAP laser for the output mirror transmission of 19.5% and the radius of curvature of 100 mm shown on a logarithmic scale.
Summarized experimental data of slope and optical-to-optical efficiencies with respect to the absorbed pump power are listed in Table
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The Tm:YAP laser generated beam spatial structure was measured with a pyroelectric NanoScan, a scanning-slit laser beam profiler that uses a lens with a focal length of
Figure 5.Measurement results of the Tm:YAP laser beam quality for the OC of 11.0% transmittance.
Figure 6.Measurement results of the Tm:YAP laser beam quality for the OC of 43.7% transmittance.
The beam profile in the 2D and 3D dimensions recorded by the pyroelectric NanoScan profiler for the maximum output power is shown in Fig.
Figure 7.Diode-pumped Tm:YAP laser generated beam spatial distribution recorded for the output coupler transmittance of 11.0%.
In conclusion, a high-efficiency Tm:YAP laser was demonstrated. By optimizing the LD emission to the absorption peak of Tm in YAP and the overlap efficiency of the pump-to-Tm-resonator mode, we achieved the highest slope efficiency of 60.4% with respect to the absorbed pump power. For the maximum absorbed pump power of 13.67 W (incident pump power of 18.16 W), the Tm:YAP laser yielded 6.35 W output power. In the spectral analysis, the increase in transmission of the OC resulted in the generation of a laser for lower wavelengths. To the best of our knowledge, simultaneous generation of two laser wavelengths in the Tm:YAP laser was presented for the first time. These experimental results offer guidelines for choosing the Tm:YAP laser parameters for achieving the expected spectral and energetic characteristics of the developed laser system.
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[17] W. Koechner. Solid-state Laser Engineering(2013).
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Jacek Kwiatkowski, "Power and spectral analyses in diode-pumped c-cut Pbnm Tm:YAP laser," Chin. Opt. Lett. 18, 091401 (2020)
Category: Lasers and Laser Optics
Received: Apr. 18, 2020
Accepted: May. 15, 2020
Published Online: Jul. 27, 2020
The Author Email: Jacek Kwiatkowski (jacek.kwiatkowski@wat.edu.pl)