Tuned lasers in the ∼3 μm mid-infrared spectral band are widely used in medical fields [1], detection, and spectral applications etc. [2],while wide-spectrum light sources are mainly used in biomedical imaging and industrial applications. By conducting spectral imaging on biological tissues, the components of different biomolecules can be distinguished and identified based on their "fingerprint" absorption characteristics in specific bands. In the online detection system, rapid and accurate spectral analysis can be achieved. Solid-state lasers doped with Er3+ and pumped by laser diodes can realize transition from the 4I11/2 to 4I13/2 energy levels [3], directly generating laser radiation. Due to its broad market and application prospects, it has received increasing attention [4]. Therefore, LD-pumped solid-state lasers doped with Er3+ have great application potential as tunable lasers in the ∼3 μm band (see Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10).

The absorption peaks of various laser crystals doped with Er3+ are around 970 nm, achieving high-performance 2.7-3 μm laser output [5]. At present, relatively matured Er3+ doped crystals include Er:YAG, Er:YSGG and Er:YVO4 etc. However, these materials have relatively high phonon energy. Therefore, the fluorescence decay time of the upper energy level (4I11/2) is shorter than that of the lower energy level (4I13/2), which leads to a self-termination phenomenon[6],[7],[8]. Er:YLF is a very promising material in the Er3+-doped gain media. Because of its phonon energy (for example, the energy of the Er:YAG phonon [9] is 865 cm−1, the energy of the Er:YSGG phonon is 728 cm−1 [10], and the energy of the Er:YLF phonon is 447 cm−1 [11]) and relatively low thermal conductivity (the thermal conductivity of Er:YAG is 11.72W·m−1·k−1 [12], the thermal conductivity of Er:YSGG is 6.83W·m−1·k−1, and the thermal conductivity of Er:YLF is 6.3W·m−1·k−1 [13]), it helps to reduce the non-radiative transitions and polyphonon relaxation probabilities between the upper and lower energy levels of the laser, enhancing the stability of the crystal, improving its optical performance, and without causing severe thermal effects. In the generation of mid-infrared 3 μm lasers, a higher Er3+doping concentration will achieve a greater cross-relaxation rate, and the quantum efficiency will also increase accordingly, however, the higher lifetime of the upper energy level of Er:YLF is conducive to population inversion, which leads to a lower laser threshold to achieve a higher laser output power and also brings benefits to wavelength tuning (Fig. 1). Meanwhile, Er:YLF has a wider emission spectrum compared to other gain media, thus it has a wide range of applications in the 3 μm band with significant application prospects in laser tuning. At present, the commonly used optical components for wavelength tuning include F-P etalons and birefringent filters, among which the F-P etalons change the loss of the longitudinal mode laser mode by altering the insertion angle of the etalons, thereby changing the wavelength of the output laser [14],[15].while the birefringence filter plate has the advantages of low insertion loss [16], high anti-damage threshold, wide tuning range and convenient operation making it one of the ideal wavelength tuning elements for continuously tunable lasers.

Since 1995, Jensen and several other researchers have conducted research experiments on LD side-pumped Er:YLF continuous lasers, achieving an average laser output of 1.1W at 2.8 μm [17]. As can be seen from the following figure, the Er:YLF crystal has a very wide emission spectrum, covering a range approximately at 2650 nm-2870nm [18]. It was not until Peter et al. achieved an LD side-pumped Er:YLF wavelength tunable laser with an output power of 4W and a tuning range of 2716 nm-2836nm by adding quartz birefringent filters to the resonant cavity [19].

The above data has successfully demonstrated that Er:YLF can perform laser emission in the mid-infrared spectrums under LD side pumping, and for laser generation in the mid-infrared emission region (2.7μm-3μm) [20], it is not necessary to use Er3+ with a high doping concentration [21]. It also proves that Er:YLF crystals can undergo wavelength tuning experiments. However, the experiment conducted by Peter et al. might be due to the fact that the material of the birefringent filter used was quartz, resulting in a relatively narrow tuning range. On this basis, a wider output wavelength range is still required. Therefore, this paper aims to introduce the research on LD end-pumped Er:YLF lasers with tunable output wavelengths based on two wavelength tuning methods: F-P etalons with thicknesses of 0.025 mm, 0.5 mm, and 1 mm, and MgF2 birefringent filters with thicknesses of 2 mm and 4 mm, in order to broaden the spectral output range.