A radio-frequency (RF) intensity-modulated light source at 532 nm was built for underwater ranging. The intensity of a narrow-linewidth laser at 1064 nm from a NPRO (Non Planar Ring Oscillator) was modulated via a Mach-Zehnder electrooptical modulator. The modulation frequency was tuned from 10 MHz to 2.1 GHz. The intensity-modulated light was amplified via a 2-stage laser diode-pumped Yb3+ doped large-mode-area fiber amplifier. A 15 mm long magnesium oxide doped periodically-poled lithium niobate (MgO: PPLN) nonlinear crystal was used to convert the 1064 nm light into 532 nm light via frequency doubling. The maximum output power at 532 nm was 2.56 W, the highest efficiency from the fundamental to second harmonic generation (SHG) was 22.6%. The watt level 532 nm light source was applied in underwater ranging experiments. Different modulation frequencies were applied to measure the distance of an object in the water. The turbidity of the water was changed by adding Mg(OH)2 powder, ranging accuracy of 6 cm was obtained at 2.5 m distance when the attenuation coefficient of the water was 1.72 m-1. In turbid water, higher modulation frequency was preferable for obtaining higher ranging accuracy.
KEYWORDS: Solar radiation, Absorption, Solid state lasers, Crystals, Solar concentrators, Manufacturing, Analytical research, Solid state electronics, Visible radiation, Remote sensing
Solar-pumped laser using solar radiation as pumping source converts sunlight to laser directly. It has the following advantages over other solid-state lasers: sufficient energy resource, clean and pollution-free, long service life, and low manufacturing costs. Up to now, researches on solar-pumped laser have been limited to the 1 μm band with Nd3+ doped gain medium. By analyzing the absorption spectrum of the existing solid-state materials, we have found that Tm3+ doped materials have strong absorption in visible light band where the solar radiation is strong. 2 μm eye-safe laser has a greater application prospect in the fields of remote sensing, LiDAR, medical treatment, and space optical communication. In this paper, the absorption spectrum and spectral overlap of two common laser crystals, Tm:YAG and Tm:YAP, with the solar radiation have been analyzed and calculated. The threshold pump power density of these two crystals were calculated as 1.14 Kw/cm3 , 1.434Kw/cm3 respectively. Tm:YAG crystal with lower threshold pump power density was selected as the gain medium and a two-stage pumping model was built. A Fresnel lens was used as the primary solar light concentrator. The secondary concentrator was a conic cavity with a defused high reflection surface. The laser rod with grooved side wall was installed in the conical cavity. Tracepro was used to optimize the whole setup. The work in this paper made a theoretical preparation for experimental research of 2 μm solar-pumped laser.
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