Single- and dual-wavelength laser operation of a diode-pumped Nd:LaF3 single crystal around 1.05 μm and 1.32 μm
Introduction
In spite of the present supremacy and the growing interest of Yb-doped crystals like Yb:KGW, Yb:YAG, Yb:SFAP or Yb:CaF2 for the development of ultra-short pulse laser oscillators and/or ultra-high peak power laser chains around 1 μm, there is a renewed interest for a series of broad-band Nd-doped crystals which could offer nearly the same possibilities in terms of ultra-short pulse production and amplification along with a number of advantages such as lower laser thresholds and other possibilities such as laser operation at several laser wavelengths with the same or different polarization states for different applications. This is the case for instance of the recently re-investigated multisite (Nd,Y)- and (Nd,Lu)-doped CaF2 laser crystals with strongly overlapping emission bands peaking around 1.053 μm and 1.062 μm, emission bandwidths of about 30 nm at half maximum and emission lifetimes of the order of 325 μs [1], [2]. This is also the case of other well-known [3], [4], [5], [6], [7] but not completely explored crystals such as the structurally disordered oxide Nd:LMA (LaMgAl11O19), with overlapping emission bands peaking around 1.056 μm and 1.065 μm and an emission lifetime of about 300 μs [3], [4], and the single-site fluoride Nd:LaF3 with overlapping emission bands peaking around 1.047 and 1.062 μm and a radiative emission lifetime of the order of 735 μs [4], [5].
In the present communication we concentrate more particularly on the case of Nd:LaF3 for several reasons. We realized that the available spectroscopic data [3], [4], [7] were either incomplete or misleading because of reabsorption effects and that it would be worth to investigate the laser behavior and the laser characteristics of this crystal around 1.06 μm at multi-watt pump power levels and to examine the offered laser performance around 1.3 μm, what had never been reported before. The highest output power ever reported with Nd:LaF3 [8] did not exceed about 300 mW and it was measured for a combination of laser wavelengths around 1.04 and 1.06 μm, not for single- or true dual-wavelength laser operations.
Section snippets
Emission properties
Fig. 1 shows the emission spectra of Nd:LaF3 registered at room temperature in π (for E//c) and σ (E⊥c) polarization from 830 to 1380 nm. These spectra were obtained by exciting the crystal with a CW Ti:sapphire laser set at 808 nm and by using an OSA (Optical Spectrum Analyser). They were both calibrated between each other and corrected from the spectral response of the apparatuses. As usual, these spectra consist of three sets of bands located around 900 nm, 1050 nm and 1330 nm and assigned
Experimental conditions
As shown in Fig. 2, two types of linear and folded laser cavities were used for the laser experiments.
In both cases, however, the laser crystal was pumped with the same non polarized fiber-coupled 790 nm laser diode with a core diameter of 105 μm, a N.A. of 0.22 and a maximum output power of 25 W. Two doublet-lenses, with 30 mm and 60 mm focal lengths, were used to collimate and focus the pump beam inside the crystal. The pump spot was about two times magnified by these coupling optics in order
Conclusion
We have revisited first the emission properties of Nd:LaF3 by recording room temperature near-infrared polarized emission spectra from about 800 to 1400 nm. These spectra were used to derive either new or more reliable data such as emission cross sections and branching ratios than those reported in the past literature. Laser experiments were then fulfilled around 1060 nm and 1330 nm by using a 790 nm laser diode as pump source. In free-running mode, using a plano-concave linear laser cavity,
Acknowledgements
The authors wish to thank the financial support from the National Natural Science Foundation of China (61575164), the Specialized Research Fund for the Doctoral Program of Higher Education (20130121120043), the Fundamental Research Funds for the Central Universities (2013121022), the Natural Science Foundation of Fujian Province of China (2014J01251) and the High-End Foreign Expert Program (2014).
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