Novel Wavelengths Watt-Level Pr3+:LiYF4 Orange Lasers

We report the first demonstration on four new wavelengths of 609 nm, 613 nm, 618 nm in both $\sigma $ - and $\pi $ -polarized direction based on continuous-wave Pr $^{3+}$ :LiYF4(Pr $^{3+}$ :YLF) lasers. The transitions mechanism combined dual upper energy levels of $^{3}\text{P}_{1}$ and $^{1}\text{I}_{6}$ with Stark sub-levels of $^{3}\text{H}_{6}$ and $^{3}\text{F}_{2}$ was designated. The CW lasers achieve output power greater than watt levels at room temperature, with 2.36 W at 609 nm, 1.85 W at 618 nm, in the $\sigma $ -polarized direction and 1.93 W at 613 nm, 1.40 W at 618 nm, in the $\pi $ -polarized direction.

I. Introduction

High-power orange lasers are in great demand in many fields, such as biomedical applications [1], [2], laser displays [3], metal processing [4], optical communication [5], and deep UV generation [6]. The primary method of generating approximately 600 nm high-power lasers is currently nonlinear frequency conversion, which is mainly based on nonlinear materials, including KTP, LBO, BBO, and LiNbO₃ by sum-frequency generation [7], [8], [9], [10], [11], and other materials by the Raman effect [12], [13]. However, such systems based on nonlinear frequency conversion are inefficient, complex, and expensive. Optically pumped semiconductor laser (OPSL) technology is another promising method to generate yellow-orange lasers, but it is expensive, complicated, and challenging to generate over 600 nm lasers [14]. Lasers based on organic dyes are also proven technologies in this wavelength region, but the short lifetime of dyes is an inherent disadvantage. Therefore, laser diode (LD)-pumped yellow-orange lasers based on Pr-doped materials are recently developed solutions due to their advantages of high efficiency, simplicity, and low cost. Pr:YLF crystals have attracted much interest over the past decade for the generation of approximately 600 nm lasers since the low phonon energy contributes to excellent visible laser performance. However, up to now only 604 nm and 607 nm continuous-wave (CW) lasers are successful to operate in orange spectral region for Pr:YLF crystal [15], [16], they correspond to the maximum peaks of the -polarization and -polarization emission spectra, respectively, as shown in Fig 1. In 2014, P. W. Metz et al. demonstrated the performance of -OPSL pumped Pr:LiYF₄ laser with the CW output power of of 1.8 W at 607 nm and 1.5 W at 604 nm respectively [16]. High efficiency is obtained by using -OPSL as a pump source since the matched absorption peak and its outstanding beam quality. However, OPSLs operating at blue wavelengths are currently more expensive than InGaN-based diode lasers. In 2016, S. Y. Luo et al. reported a blue-InGaN pumped Pr:LiYF₄ laser at wavelengths of 604 nm and 607 nm, and their maximum output powers were 0.6 W and 1.1 W, respectively [17]. Although laser outputs of the watt-level in the orange region at 604 nm and 607 nm have been achieved, the research has focused only on laser emission at (~604 nm and ~607 nm) in the both - and -polarized direction [15], [16]. The 607 nm CW laser operation has also been reported in Pr:KYF, but the maximum output laser power is only 78 mW [17]. We can see from these spectra in Fig. 1 that there exist also several weaker peaks on the right side, and it is challenging to make these peaks laser operation to extend the orange laser wavelengths. Fig. 1.

Room temperature polarization-dependent emission cross-sections of Pr: YLF in orange spectral region.