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935 nm-diode-pumped passively Q-switched Er:Yb:Sr3Gd2(BO3)4 pulse laser at 1.5–1.6 μm

https://doi.org/10.1016/j.optlastec.2021.107025Get rights and content

Highlights

  • The Er:Yb:SGB crystal has a broad and flat absorption band around 940 nm.

  • 1534 nm passively Q-switched laser was firstly realized in the Er:Yb:SGB crystal.

  • The Er:Yb:SGB crystal is a good 1.5–1.6 μm laser medium pumped by a 940 nm LD.

Abstract

Eye-safe 1.5–1.6 μm pulse laser passively Q-switched by a Co2+:MgAl2O4 crystal was realized in a Z-cut, 1.8 mm-thick Er:Yb:Sr3Gd2(BO3)4 crystal end-pumped by a 935 nm laser diode. A 1534 nm pulse laser with energy of 9.1 μJ, repetition frequency of 15.6 kHz, and duration of 29 ns was obtained at an absorbed pump power of 2.5 W. Polarization of the output laser beam was measured to be linear and parallel to the Y optical indicatrix axis. Benefited from a broad and flat absorption band between 920 and 950 nm, the Er:Yb:Sr3Gd2(BO3)4 crystal may be a good gain medium for 1.5–1.6 μm laser pumped by a common commercial diode laser around 940 nm.

Introduction

Eye-safe 1.5–1.6 μm solid-state lasers can be realized conveniently in some Er3+/Yb3+ co-doped materials, such as phosphate glass, as well as borate and silicate crystals, when a near infrared (900–1000 nm) laser diode (LD) is used to pump the Yb3+ sensitizing ions [[2], [3], [1]]. Generally, the common commercial LD has a broad emission bandwidth as well as a large wavelength-shift with operating temperature and output power. In order to realize a stable 1.5–1.6 μm laser operation in different operating environments, which is important for the applications in lidar, laser ranging, and remote sensing [[2], [3], [1]], a broad and flat absorption band of Yb3+ ions in the Er3+/Yb3+ co-doped material is required. A structural disordered crystal has multiple cation sites with different crystal field environments [4]. When the sites are randomly occupied by rare earth ions, the superimposition contribution of each type of site will cause an inhomogeneous broadening of the spectral band of the ions. Therefore, due to the advantages of broad absorption and emission bands like glass, as well as higher thermal conductivity than glass, the structural disordered laser crystals have been widely investigated as gain media, especially for ultrashort pulse and tunable lasers [5], [6].

Sr3Gd2(BO3)4 (SGB) crystal belongs to the orthorhombic system with space group Pnma [7]. The investigation of crystal structure shows that Sr2+ and Gd3+ ions occupy three different cation sites statistically in the SGB crystal [7]. Then, when rare earth ions are introduced into the crystal and replaced partially Gd3+ ions, the broad absorption and emission spectral bands have been observed in some rare earth ions doped SGB crystals due to the inhomogeneous broadening effect [7], [8], [9]. Up to now, a 1055 nm continuous-wave (cw) laser with a maximum output power of 2.82 W and a slope efficiency of 54%, as well as a 1049 nm passively mode-locked pulse laser with a duration of 189 fs, a repetition frequency of 52.2 MHz, and an energy of 3.4 nJ have been realized in an Yb:SGB crystal [7], [10]. Spectroscopic property and quasi-continuous-wave (quasi-cw) 1.5–1.6 μm laser have also been investigated for an Er:Yb:SGB crystal [9]. The stimulated emission cross section at the peak fluorescence wavelength of 1534 nm is about 0.8 × 10−20 cm2, which is similar to that of the Er:Yb:phosphate glass [3]. The fluorescence lifetime of the upper laser level 4I13/2 and the energy transfer efficiency from Yb3+ to Er3+ ions are 0.61 ms and 96%, respectively. Pumped by a 970 nm LD with a duty cycle of 2%, a 1564 nm quasi-cw laser with a maximum output power of 1.8 W and a slope efficiency of 18% has been obtained in the crystal [9].

In this work, absorption characteristic of Yb3+ ions in the near infrared band of 850–1075 nm was analyzed for the Er:Yb:SGB crystal. Then, the quasi-cw and passively Q-switched pulse 1.5–1.6 μm lasers were successfully demonstrated in the crystal end-pumped by a 935 nm LD.

Section snippets

Material property and experimental arrangement

A 1.8-mm-thick, Z-cut Er(0.8 at.%):Yb(19.5 at.%):SGB crystal with a cross section of 3 × 3 mm2 was cut from a single crystal grown by the Czochralski method, and then polished for spectral and laser experiments. Room-temperature polarized absorption coefficient spectra in 850–1075 nm and emission cross section spectra in 1430–1670 nm of the crystal are shown in Fig. 1 (a) and (b), respectively. It can be seen that the anisotropy of the spectra is weak. There is a broad and flat absorption band

Results and discussion

When the Co2+:MgAl2O4 saturable absorber was removed from the cavity, quasi-cw laser was realized in the Er:Yb:SGB crystal. Fig. 3(a) shows quasi-cw output power versus absorbed pump power for different OM transmissions. Because of 10% duty cycle of the used LD, the powers given in this work were the measured values multiplied by ten times. When the OM transmission was 1.4%, a maximum output power of 0.44 W was obtained at an absorbed pump power of 2.5 W. The slope efficiency η was 20.8%, which

Conclusion

The Er:Yb:SGB crystal has a broad and flat absorption band between 920 and 950 nm, and may be a good 1.5–1.6 μm laser gain medium pumped by a common commercial LD around 940 nm. End-pumped by a 935 nm LD, efficient quasi-cw and passively Q-switched pulse 1.5–1.6 μm lasers were successfully demonstrated in the crystal.

CRediT authorship contribution statement

Yujin Chen: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Visualization. Jianhua Huang: Resources. Yanfu Lin: Resources. Xinghong Gong: Resources. Zundu Luo: Formal analysis. Yidong Huang: Conceptualization, Methodology, Formal analysis, Writing - review & editing, Supervision, Project administration, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

We thank Dr. Qingguo Wang and Prof. Jun Xu from Tongji University for the use of their Co2+:MgAl2O4 crystal.

Declaration of Competing Interest

The authors declared that there is no conflict of interest.

Funding

This work was supported by National Key Research and Development Program of China (2016YFB0701002); Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000); Science and Technology Service Network Initiative of the Chinese Academy of Sciences (KFJ–STS–QYZX–069); Natural Science Foundation of Fujian Province (2019J01127)

References (19)

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