12-ps megawatt peak power pulses out of a single laser oscillator with nonlinear pulse compression

We present a passively Q-switched Nd³⁺:YAG/Cr⁴⁺:YAG laser with subsequent nonlinear pulse compression. This miniature laser combines both the high pulse energy of several tens of micro Joules and the short pulse duration of <20 ps without any amplification. It is therefore readily usable for many ultrafast applications including micro machining and medical applications. With these parameters, the laser shows a new level of compactness in comparison to other sub-100 picosecond laser sources. We utilize a Nd³⁺:YAG and a Cr⁴⁺:YAG crystal in a flat-Brewster, Brewsterflat configuration, respectively, positioned closely and the Brewster faces adjacent to each other. Pumped with approx. 6W from a fiber-coupled, 808 nm laser diode, the miniature, passively Q-switched laser oscillator delivers a pulse energy of 54 μJ with a pulse width of 339 ps and a repetition rate of 8.5 kHz. These pulses are subsequently coupled into a 20 μm core dia. large mode area PM fiber. Following a fiber propagation of 1.65 m the pulses are spectrally broadened by about a factor 40 due to self-phase modulation. Thereafter the nearly linearly chirped laser pulses are compressed by a chirped volume Bragg grating (CVBG). The optimized laser output pulses have a pulse width of 11.8 ps and a pulse energy of 20 μJ. We measured the polarized (PER<20dB) beam quality to be close to the diffraction limit with an M²≈1.5. A 13 hours continuous, stable laser operation has indicated a good long term stability and reliability.