Sub-nanosecond pulse generation in a microchip Nd:La0.15Gd0.85VO4 laser
Introduction
Diode pumped passively Q-switched microchip lasers have highly compact structures, large pulse energy, and short pulse width, so they are well known as sub-nanosecond pulse sources for medical care, spectral analysis, and radar detection [1], [2], [3]. Sub-nanosecond microchip lasers contain saturable absorbers (SAs), such as Cr ion doped crystals (Cr4+:YAG and Cr2+:ZnS), gallium arsenide (GaAs), and semiconductor saturable absorber mirrors (SESAMs) [4], [5], [6], [7]. Cr4+:YAG is a typical partner with neodymium (Nd3+) or ytterbium (Yb3+) doped gain medium for generating ~1 μm high energy (μJ to mJ) and high peak power (kW to MW) pulses [4], [8], [9], [10], [11]. To date, many gain media such as garnet, tungstate, borate, and vanadate [8], [9], [10], [11], [12], [13], [14], [15], [16] have been reported for ~1 μm sub-nanosecond lasers.
Neodymium doped vanadate is commonly used as a gain medium in green microchip lasers. The large stimulated emission cross-sections enable vanadate microchip lasers to easily operate in continuous wave modes. But a barrier must be placed for sub-nanosecond laser oscillations. Researchers have explored several methods of constructing sub-nanosecond vanadate microchip lasers. Lightly doped and thick crystals combined with quasi-continuous wave pumping are one technique [9]. Highly doped thin crystals are another method [16], [17]. By changing the crystal orientation or host crystal field, smaller stimulated emission cross-sections can be obtained. This is the third method [18], [19], [20].
Compared to Nd:YAG crystals, Nd:GdVO4 possesses excellent thermal conductivity along the 〈0 0 1〉 and 〈1 0 0〉 directions [21]. But stimulated emission cross-section of Nd:GdVO4 at 1.06 μm is 2.7 times the value of Nd:YAG. In Nd:GdVO4 crystals, gadolinium (Gd) can be slightly substituted by lanthanum (La) ions, which have the largest ion radii in rare earth ions. Notably inhomogeneous broadened spectra of Nd:LaxGd1-xVO4 crystals were reported [22], [23], [24], [25], [26]. The maximum stimulated emission cross-section of Nd:La0.15Gd0.85VO4 is 1.3 times the value of Nd:YAG. This indicates that Nd:La0.15Gd0.85VO4 can be candidate materials for microchip laser gain media operating at short pulse range. To date, there are no reports on sub-nanosecond micro-type Nd:LaxGd1-xVO4 crystal lasers.
In this paper, we reported a passively Q-switched sub-nanosecond Nd:La0.15Gd0.85VO4/Cr4+:YAG microchip-type laser for the first time. The maximum stimulated emission cross-section of 3.62 × 10−19 cm2 was much smaller than that of Nd:GdVO4 crystals. Using lightly doped Nd:La0.15Gd0.85VO4 and adjusting the incident pump power, we obtained 880 ps pulses with a pulse energy of 3.1 μJ, corresponding to a peak power of 3.5 kW. When the incident pump power exceeded 5 W, the system transferred from a single picosecond pulse train into 0.9/1.1 ns two pulse trains.
Section snippets
Experimental design
The laser experimental design is illustrated in Fig. 1. A Nd:La0.15Gd0.85VO4 crystal (Nd ion concentration of 0.3 at.%) cut along the a axis was utilized in this experiment. A Cr4+:YAG crystal cut along the [1 0 0] direction with an initial transmission of 71% served as a saturable absorber. The optical apertures of the gain crystal and saturable absorber were 3 mm × 3 mm. The entrance surface of the Nd:La0.15Gd0.85VO4 crystal was coated for anti-reflection (AR) at 808 nm and high reflection (HR)
Experimental results and discussion
The stimulated emission spectra of the Nd:La0.15Gd0.85VO4 determined by the Füchtbauer-Ladenburg technique [27], [28] is shown in Fig. 2. The strongest emission was at 1062 nm, with a maximum stimulated emission cross-section of 3.62 × 10-19 cm2, which was much smaller than that of Nd:GdVO4 crystals. For the 4F3/2→4I13/2 transition, the largest stimulated emission cross-section was 1.88 × 10-19 cm2 at 1341 nm, corresponding to approximately half of the value at 1062 nm. For the 4F3/2→4I9/2
Conclusion
In summary, a sub-nanosecond Nd:La0.15Gd0.85VO4 microchip laser passively Q-switched with a Cr4+:YAG crystal saturable absorber was demonstrated. With a compact cavity, an average output power of 108 mW at 1063.71 nm was generated at an incident pump power of 4.91 W, corresponding to a single pulse width and repetition rate of 880 ps and 35 kHz, respectively. The calculated pulse energy and peak power were 3.1 μJ and 3.5 kW, respectively. Under a higher incident pump power of 5.19 W, an average
Declaration of Competing Interest
The author declared that there is no conflict of interest.
Acknowledgments
This study was supported by the Natural Science Foundation of Shandong Province, China (No. ZR2019QF010). The author thanks Prof. Honghao Xu, College of Physics, Qingdao University, for providing vanadate crystals.
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