Abstract
1. Introduction
Due to strong water absorption and high transmittance in the atmosphere, mid-infrared (MIR) lasers operating at the 2.7 µm wavelength band have attracted increasing attention and play a significant role in applications including medical, biological, remote sensing, free-space communication, etc.[
However, it is worth noting that -doped crystals face a serious “bottleneck” at the 2.7 µm laser transition, where the lifetime of the upper energy level () is much shorter than that of the lower energy level (), resulting in the self-terminating effect[
In this paper, an and co-doped YLF (Er,Pr:YLF) crystal has been successfully grown by the Bridgman method. The properties of absorption and emission spectra of the Er,Pr:YLF crystal were measured and analyzed. Based on the Judd–Ofelt (J-O) theory, the emission cross section and energy transfer efficiency between and were calculated. Furthermore, a diode-end-pumped continuous wave (CW) Er,Pr:YLF laser operating at 2659 nm was realized for the first time, to the best of our knowledge. All of these results indicate that the Er,Pr:YLF crystal has great potential for 2.7 µm laser generation.
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2. Experiments and Methods
The Er,Pr:YLF crystal was grown by the Bridgman method with the initial materials of 99.99% pure LiF, , , and with the molar ratio of 51.5:44.2:4:0.3[
Figure 1.Room temperature spectral properties of Er,Pr:YLF crystal: (a) absorption cross section (inset: absorption cross section within the range of 900–1060 nm); (b) fluorescence spectrum.
Figure 1(b) shows the fluorescence spectrum of Er,Pr:YLF with the spectral range of 2400–3000 nm, which was measured by Edinburgh Instruments (FLS920 and FSP920 spectrophotometers) excited by a 968 nm laser at room temperature. Two typical emission peaks located at 2685 and 2804 nm were observed. With the room temperature absorption spectra, based on the J-O theory, three typical J-O intensity parameters were calculated to be , , and , respectively[
The emission cross section () of the crystal can be calculated by the Fuchtbauer–Ladenburg (F-L) equation:
The energy transfer efficiency () of is an important factor to assess the effects of co-doped and can be calculated by the following equation:
Considering the beneficial spectral characteristics, a CW laser operation was realized. The experimental setup is shown in Fig. 2. A compact concave-plane cavity was designed with the cavity length of 14 mm. A fiber-coupled 976 nm LD with a core diameter of 200 µm and a numerical aperture of 0.22 was used as the pump source. The pump light was focused onto the crystal by a focus system with a focal length of 46.5 mm and a polarization ratio of 1:1. An uncoated -cut Er,Pr:YLF with dimensions of was wrapped with indium foil and mounted in copper block cooled by water at a temperature of 16°C. The input concave mirror with a radius of 200 mm was high-reflection (HR) coated at 2600–3050 nm and high-transmission (HT) coated at 900–1000 nm. The plane output couplers (OCs) with two different transmissions of 1% and 3% at 2600–3050 nm were used.
Figure 2.Schematic setup of LD end-pumped Er,Pr:YLF laser.
3. Results and Discussion
Figure 3(a) shows the laser output power as a function of the absorbed pump power with different transmissions of OCs. The maximum output power of 258 mW was obtained with a slope efficiency of 7.4%. The laser threshold was as low as 52 mW with an OC of 1%. However, the maximum laser output power 258 mW is lower than the reported value (1.1 W) for Er:YLF, which may be caused by the fact that the shortened lifetime of the upper level is detrimental to energy storage during diode CW pumping[
Figure 3.(a) Laser output power versus input power with different transmissions; (b) center emission wavelength of the Er,Pr:YLF laser.
The laser output spectrum was measured using an optical spectrum analyzer containing a grating spectrometer (Omni- 300, Zolix, China) and an InSb infrared detector (DInSb5-De01, Zolix, China), as shown in Fig. 3(b). The emission peak was located at 2659 nm, which is consistent with the measured fluorescence spectra. The output laser beam quality was analyzed by a laser beam profiler (NanoScan by Photoh, Inc). Figure 4 shows the measured laser beam quality factor and beam profile at the maximum output power. The output laser was operating in the single transverse electromagnetic mode with a beam quality factor measured to be and in the horizontal and vertical directions. The output laser was measured to be linearly polarized and parallel to the axis of the crystal with a polarization ratio of 9:1. The output laser stability was measured to be for 3 h of operation.
Figure 4.Laser beam quality of the Er,Pr:YLF laser. Inset: the far-field laser beam profile.
4. Conclusion
In conclusion, the spectroscopic and laser properties of Er,Pr:YLF crystals were studied. The absorption and fluorescence spectra were measured and analyzed by the J-O theory. The absorption cross section at 969 nm was calculated to be , while the emission cross section was determined to be at 2685 nm and at 2804.6 nm, respectively. Besides, the energy transfer efficiency from to was calculated to be 74.1%, indicating the effective deactivated function of the ion. Moreover, a diode-end-pumped CW Er,Pr:YLF laser operating at 2659 nm was realized for the first time, to the best of our knowledge. A maximum output power of 258 mW is obtained with a slope efficiency of 7.4%. The higher-efficiency, higher-power Er,Pr:YLF CW lasers are expected by optimizing the concentration of and ions. Our work demonstrates that the Er,Pr:YLF crystal should be a promising alternative for high-power and high-efficiency MIR laser generation.
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