Abstract
Semiconductor saturable absorber mirrors (SESAMs) have enabled a wide variety of modelocked laser systems, which makes measuring their nonlinear properties an important step in laser design. Here, we demonstrate complete characterization of SESAMs using an equivalent time sampling apparatus. The light source is a free-running dual-comb laser, which produces a pair of sub-150-fs modelocked laser outputs at 1051 nm from a single cavity. The average pulse repetition rate is 80.1 MHz, and the full time window is scanned at 240 Hz. Cross-correlation between the beams is used to calibrate the time axis of the measurements, and we use a non-collinear pump-probe geometry on the sample. The measurements enable fast and robust determination of all the nonlinear reflectivity and recovery time parameters of the devices from a single setup, and show good agreement with conventional nonlinear reflectivity measurements. We compare measurements to a rate equation model, showing good agreement up to high pulse fluence values and revealing that the samples tested exhibit a slightly slower recovery at higher fluence values. Lastly, we examine the polarization dependence of the reflectivity, revealing a reduced rollover if cross-polarized beams are used or if the sample is oriented optimally around the beam axis.
Highlights
Over the past several decades, ultrafast laser technology has progressed from laboratory demonstrations to widespread use in industry and scientific research
Since its mechanism is an intensity-dependent change in the size of the laser cavity mode, which leads to either reduced losses through an aperture or better overlap with the pumped region in the laser
The saturable absorption mechanism (1) is characterized by two parameters: a modulation depth ΔR which is the change in semiconductor saturable absorber mirror (SESAM) reflectivity after complete saturation of this mechanism; and a saturation fluence Fsat, which is the fluence at which a fraction 1/e of this loss has been saturated
Summary
Over the past several decades, ultrafast laser technology has progressed from laboratory demonstrations to widespread use in industry and scientific research. A key part of this progression has been the development of passively modelocked lasers producing ultrashort pulses. The core component in these lasers is a mechanism providing an intensity-dependent reduction in the net cavity losses, thereby favoring pulsed operation over continuous-wave lasing. Passive modelocking experiments used dye lasers, which led to pulse durations as short as 27 fs [2]. These were challenging systems to operate, limiting their use to research labs. In free-space laser oscillators (including solid-state lasers), the two dominant techniques used for ultrashort pulse generation are SESAM modelocking and Kerr-lens modelocking (KLM) [10]. Since its mechanism is an intensity-dependent change in the size of the laser cavity mode, which leads to either reduced losses through an aperture or better overlap with the pumped region in the laser
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