Simultaneous measurement of DC two-photon absorption signal offset and amplitude of the intensity autocorrelation in the focusing of femtosecond pulses

• Published in: Applied optics (2004) 2020-02, Vol.59 (6), p.1519
• Main Authors: Ramírez-Guerra, Catalina, Rosete-Aguilar, Martha, Garduño-Mejía, Jesús
• Format: Article
• Language: English
• Subjects: Amplitudes; Autocorrelation; Depth of field; Femtosecond pulses; Group delay; Nonlinear response; Photodiodes; Photon absorption; Photons; Pulse duration; Signal measurement
• ISSN: 1559-128X; 2155-3165
• DOI: 10.1364/AO.377678

In this work, the DC two-photon absorption signal offset (${{\rm DC}_{\rm TPA}}$DC ) and the amplitude of the autocorrelation (${{\rm A}_{\rm AC}}$A ) are measured simultaneously around the focal point of an apochromatic microscope objective using the z-scan autocorrelation technique. The ${{\rm A}_{\rm AC}}$A is obtained from the nonlinear sensor response given by the two-photon-absorption, generated in a GaAsP photodiode, for femtosecond laser pulses. We verify that the change in the ${{\rm DC}_{\rm TPA}}$DC signal along $z$z is coincident with the amplitude of the intensity autocorrelation, and that the highest amplitude of the AC is reached at the same position as the highest amplitude of the ${{\rm DC}_{\rm TPA}}$DC signal. The ${{\rm DC}_{\rm TPA}}$DC signal is typically used as a reference for the alignment in a collinear intensity autocorrelator, and we show that it can also be used as a practical procedure to estimate the depth of focus. The ${{\rm DC}_{\rm TPA}}$DC signal measurement allows us to locate the optimum spatial-temporal coupling given by the highest amplitude of the intensity autocorrelation. Additionally, we find a variation in the pulse duration within the same region due to the radially varying group delay dispersion.