Abstract
Solid-state lasers operating in the eye-safe 2 μm spectral region present significant interests for a number of advanced applications, such as gas sensing, coherent Doppler lidar, differential absorption lidar, medicine, and pumping sources for optical parametric oscillators[
The SLM laser can be obtained through some methods, such as microchip lasers, double-cavity lasers, nonplanar ring oscillators (NPROs), and lasers inserted with etalons and twisted-mode cavity (TMC) lasers. However, the output power is generally low by using the microchip laser or the double-cavity laser[
Tm-Ho co-doped crystals are attractive gain mediums used to generate a 2 μm laser; their significant performance involve an intensive energy storage linked to a long upper-level lifetime and a quantum efficiency of up to 2 because of cross relaxation.
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The
Figure 1.Energy-level diagrams for Tm, Ho:YAG[
In this Letter, we reported a diode-pumped tunable SLM Tm, Ho:YAG laser employing a TMC. The maximal SLM output power was 106 mW with the central wavelength of 2090.38 nm. The output laser is linear polarized, and the wavelength can be changed from 2089 to 2097 nm. To our knowledge, this was the first time for using TMC technology to obtain a SLM Tm, Ho:YAG laser.
The schematic layout of a Tm, Ho:YAG TMC laser is shown in Fig.
Figure 2.Schematic of the experimental setup.
Although the measurement of the thermal focal length is rough, it was helpful to make the Tm, Ho:YAG laser with a longer cavity to insert more components that the twisted-mode technology need. In view of the thermal focal length, the laser employed a plano-concave resonator with a cavity length of 67 mm. We selected a curvature radius of 100 mm and a transmission of 2% at 2 μm as the output coupler. The laser cavity is comprised of an input mirror, a pair of quarter-wave plates at 2.09 μm, two YAG etalons and an output coupler mirror. M1 is the input mirror coating with a high transmission at 785 nm and a high reflection at 2 μm. QWP1 and QWP2 are the pair of uncoated quarter-wave plates that are set on each side of the laser crystal. F-P1 and F-P2 are both uncoated YAG etalons with thicknesses of 0.05 and 0.1 mm, respectively. F-P1 is set at the Brewster angle to achieve a linear-polarization output laser. Different wavelengths of output laser could be achieved by changing the angle of F-P2. The dichroic filter is covered with a high-transmission coating at 2 μm and a high-reflection coating at 785 nm. The pump source is a 10 W fiber-coupled diode laser with a central wavelength of 785 nm. The wavelength could be tuned to match the absorption peak of Tm, Ho:YAG by changing the laser diode temperature. The fiber has an inner core diameter of 200 μm. The pump laser is focused into a Tm, Ho:YAG crystal with a beam diameter of 320 μm by a coupling system. The Tm, Ho:YAG crystal has dimensions of
The wavelength of the output laser was recorded by the Bristol 721 A IR spectrum analyzer. From Fig.
Figure 3.Spectrum of Tm, Ho:YAG laser under free running.
Figure 4.F-P spectrum of the free Tm, Ho:YAG laser.
Then we inserted the laser with F-P1. The etalon was set at the Brewster angle in order to achieve a linear polarization output. As seen from Figs.
Figure 5.Spectrum of the Tm, Ho:YAG laser with a 0.05 mm etalon.
Figure 6.F–P spectrum of the Tm, Ho:YAG laser with a 0.05 mm etalon.
The SLM Tm, Ho:YAG twisted-mode laser operating at 2090.38 nm was achieved when the pair of quarter-wave plates were inserted into the cavity and rotated to a suitable angle so that the fast axis of the quarter-wave plates were orthogonal and separately oriented at an angle of 45° to the polarization. The output spectrum of the SLM laser is shown in Fig.
Figure 7.Spectrum of the Tm, Ho:YAG laser with SLM operation.
Figure 8.F-P spectrum of the SLM Tm, Ho:YAG laser.
The wavelength stability of the SLM Tm, Ho:YAG laser is shown in Fig.
Figure 9.Wavelength stability of the SLM Tm, Ho:YAG laser.
Figure
Figure 10.Output power as a function of pump power.
To obtain the tunable wavelength, we inserted the F-P2 in the cavity. The wavelength could be tuned over a range from 2089.13 to 2097.32 nm by changing the angle of F-P2. From Table
Output Wavelength | Output Power of |
---|---|
2089.13 | 82.5 |
2090.38 | 106 |
2091.34 | 92.3 |
2093.17 | 79.7 |
2095.16 | 52.7 |
2096.12 | 61.5 |
2097.32 | 70.3 |
Table 1. Output Power with Wavelength
In conclusion, we report a tunable SLM Tm, Ho:YAG with a TMC laser. The maximum SLM output power of 106 mW at 2090.38 nm is achieved under the pump power of 5.9 W, corresponding to a slope efficiency of 4.86%. The output wavelength of the SLM laser can be tuned from 2089 to 2097 nm by adjusting the etalon. The SLM Tm, Ho:YAG laser can be used as a seed laser in coherent Doppler lidarand differential absorption lidar.
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