A Nd:YVO4 laser operating at 1064 nm generating a stable mode-locked train of 10 ps-long dark pulses with a 211 MHz repetition rate is presented. The mode-locking relies on a periodic loss modulation produced by intra-cavity sum-frequency mixing with a synchronous bright-pulse train from a mode-locked femtosecond Yb:KYW laser at 1040 nm. A modulation depth of 90% was achieved for the dark pulses, confirmed by cross-correlation measurements. The ultrafast loss modulation injects power into the Nd:YVO4 laser cavity modes beyond the laser gain bandwidth. At proper laser cavity length, the detuning interaction of these modes with the lasing modes leads to the generation of periodic ultrafast transients at frequencies above 1.5 THz.
Ultrashort-pulse solid-state laser systems are widely used today in a number of applications and often include stages of pulse shaping, spectral broadening, recompression, other processing steps. Experiments and applications in the femtosecond regime require reliable methods to characterize and measure the full electric field of the resulting pulses. Frequency-Resolved Optical Gating (FROG) is a common approach. The algorithms to retrieve pulses from measurement data typically employ gradient descent-based approaches, which tend to retrieve an average of the present measurement noise together with the pulse.Conventional FROG retrieval algorithms using a gradient descent approach use the experimental data to generate a new guess. Traditional algorithms might reach local minima instead of converging further to the global minimum for complicated signals and noisy datasets. Recent a generic optimization algorithm was published, which presents what is best described as a structured random search of the available input space [1]. Here we have adapted it for pulse retrieval from any FROG geometry. This new algorithm, which we call Line-search FROG, generates two complex values in the complex plane surrounding the current field value along a randomly generated directional vector.These three points can be considered to form a line in the complex plane, where the current field value intersects the line, and the two generated values make up the endpoints. These three points are evaluated by calculating the trace-area normalized FROG error G', after which the algorithm generates a new point on the line between the best two out of the three points and discards the worst point, effectively shrinking the line. This is repeated until a pre-set number of evaluations have been reached and the algorithm replaces the current guess with the best point of the current three, after which it starts over, generating a new directional vector and two new endpoints. If no better point was found during the iteration, the current guess is replaced with the second-best point from the previous iteration, thus "perturbing" the point a little bit, resulting in a reduced sensitivity to local minima. For a more precise description of the algorithm, we refer the reader to [2].
We demonstrate self-compression of 173 fs pulses centered at 1030 nm down to 19.5 fs through electro-optic phase modulation by the phonon-polariton waves generated in a phase-matched intra-pulse difference-frequency mixing.
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