Optical parametric light generation is a unique way to obtain continuously tunable laser radiation in a wide spectral region. Relatively inexpensive and compact subnanosecond (100 ps - 1 ns) optical parametric generators (OPG) with broad wavelength tunability are needed for a variety of applications that do not require the high temporal resolution supplied by expensive and sophisticated ultrashort (<10 ps) laser systems, but nanosecond (>1 ns) time resolution is not sufficient. Such applications are spectroscopy, laser-induced fluorescence, detection of chemical materials, nonlinear microscopy, biochemical research and differential absorption lidar to name a few. Development and the reduced cost of the passively Q-switched micro-laser manufacturing technologies permits them to be used as a high energy pump source for subnanosecond OPGs. Such micro-laser pumped subnanosecond OPGs would allow the implementation of low-cost, compact, continuously tunable laser radiation sources suitable for the aforementioned applications, especially in near-infrared (NIR) and visible (VIS) spectral ranges where the demand is high. To date, no subnanosecond OPGs based on short (up to 2 cm in length) periodically poled (PP) crystals have been demonstrated that could also be continuously tunable over a wide spectral range. Long PP crystals are more expensive, therefore implementation and investigation of such OPGs would be commercially attractive. Here we investigate parametric light generator based on short (up to 2 cm in length) periodically poled lithium niobate (PPLN) crystal pumped by 473 ps pulses from a 1 mJ energy micro-laser. Experimentally measured spatial, temporal and energy characteristics of the generated light from different length PPLN crystals are shown and supplemented by numerical simulations of parametric generation in a nonlinear dispersive medium.
This work has received funding from European Regional Development Fund (project No. 01.2.2-LMT-K-718-03-0004) under grant agreement with the Research Council of Lithuania (LMTLT).
Laser radiation sources with wide wavelength tunability are of high demand in many applications including nonlinear microscopy, material science, spectroscopy, gas sensing, etc. Laser wavelength tunability is usually very limited, so optical parametric amplifiers (OPA) and optical parametric generators (OPGs) are used as sources of tunable laser radiation. Rapid progress in nonlinear optics enabled creation of a wide range of optical parametric devices with wavelength tunability from VIS to mid-IR spectrum range using pump from CW radiation to femtosecond pulses. However, there are very few optical parametric devices that operate in the subnanosecond duration range are (300 ps - 1 ns) and the few that have been demonstrated operate mainly in the IR spectral range. Since there is a demand of such sources in applications like laser-induced fluorescence, detection of chemical materials, etc., developing subnanosecond optical parametric devices remains an important task.
In this study we report, to the best of our knowledge, the first subnanosecond OPAs systems generating widely-tunable radiation in the visible spectrum range. The first demonstrated OPA is based on LBO crystal and the other one - on BBO crystal. To overcome difficulties related to low pump intensity and laser induced damage threshold (LIDT) of nonlinear crystals, we use continuum generated in a photonic crystal fiber as a seed source for the OPAs. Experimental data and numerical simulation reveal that good spectral, temporal and particularly energy characteristics of the seed radiation are crucial in order to achieve efficient parametric generation of signal radiation.
This work has received funding from European Regional Development Fund (project No. 01.2.2-LMT-K-718-03-0004) under grant agreement with the Research Council of Lithuania (LMTLT).
An R-on-1 laser-induced damage-threshold (LIDT) testing method was applied to test coated and uncoated magnesium oxide-doped periodically poled lithium niobate samples pumped by femtosecond Yb:KGW laser pulses at kilohertz and megahertz pulse repetition rates. LIDT values decreased by ∼1.5 and ∼38 times when increasing repetition rate from 100 to 571 kHz and to 76 MHz, respectively. We also investigated nonlinear absorption changes in lithium niobate crystals at the pulse repetition range from 60 to 600 kHz with trains consisting of 100 identical femtosecond pulses. Laser beam transmission in the crystal experienced a drop of ∼18 % from initial pulses of train to the next 40 pulses at intensities 40% to 15% lower than LIDT due to nonlinear absorption of 220 fs duration pulses at 1.03 μm.
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