LED-pumped actively Q-switched Nd:YLF laser has TEM00 beam quality

May 1st, 2019

While the “D” in diode-pumped solid-state (DPSS) lasers traditionally refers to coherent laser diodes, incoherent light-emitting diodes (LEDs) can, with some effort, be made to pump a variety of solid-state lasers, including fiber lasers. However, the resulting beams from these lasers, which in experiments have been either continuous-wave or free-running long-pulsed lasers, have been poor. Approaches to LED-pumping of lasers have concentrated on getting lasing to occur by making the LED pumping intensity as high as possible using high-power LEDs. Now, researchers at the Chinese Academy of Sciences, the University of Chinese Academy of Sciences, and the National Engineering Research Center for DPSSL (all in Beijing, China) are taking a different approach: keeping the LED power density relatively low while greatly improving the pump-light uniformity and matching the absorption characteristics of the working materials as best as possible to the LED light. The researchers also are relying on Q-switching to enable a much-more-stable pulsed output. Because active Q-switching has a lower input loss than passive Q-switching, they are using acousto-optic (active) Q-switching at a 27.12 MHz rate for the experimental neodymium-doped yttrium lithium fluoride (Nd:YLF) laser.

Four LED arrays from Osram (Munich, Germany), each with nine LEDs in a very tight line emitting at a peak wavelength of 819 nm and a full-width at half-maximum (FWHM) bandwidth of 32 nm, are placed against the square Nd:YLF crystal rod, illuminating it uniformly from all four sides. Operated in a quasi-continuous mode, the LEDs pump the laser such that, when the laser is free-running, the beam quality of the laser in two orthogonal directions is 16.9 and 20.7, respectively. With the acousto-optic Q-switch operating and an aperture placed in the laser cavity, a fundamental-mode Gaussian beam results. With LED pumping of only 14 W/cm2 peak power, the laser produces pulses with a duration of 452 ns and an energy of 10.6 μJ at a 1047 nm wavelength. The absorption spectrum of the LED pump was matched to the Nd:YLF absorption spectrum in a way that maximized the so-called effective absorption spectrum (EAS). Reference: T. Zhao et al., Opt. Lett. (2019); https://doi.org/10.1364/ol.44.001956.

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