Adaptive noise canceling for transient absorption microscopy.

Area of Science:

• Nonlinear optics
• Microscopy
• Laser physics

Background:

• Ultrafast fiber lasers offer compact and cost-effective alternatives to bulk lasers for nonlinear optical microscopy.
• High relative intensity noise (RIN) in fiber lasers challenges pump-probe measurements like transient absorption and stimulated Raman scattering.
• Label-free contrast from molecular vibrational and electronic structures is hindered by laser RIN.

Purpose of the Study:

• To assess the applicability of digital adaptive filtering for canceling laser RIN in a transient absorption microscope.
• To improve the signal quality and sensitivity of pump-probe measurements using ultrafast fiber lasers.

Main Methods:

• Digital adaptive filtering was implemented in MATLAB to cancel laser RIN.
• Digitized photodetector signals were processed by the adaptive filter and a software lock-in algorithm.
• Images were acquired using a resonant scanner with averaging, targeting Bi4Ge3O12 for nondegenerate two-photon absorption.

Main Results:

• Without adaptive noise cancellation, images predominantly reflected linear transmissivity and lacked pump-probe delay sensitivity.
• Adaptive noise cancellation successfully rejected RIN, restoring z-sectioning and sensitivity to pump-probe delay.
• Performance was ultimately limited by photodetector and analog-to-digital converter noise.

Conclusions:

• Digital adaptive noise cancellation effectively removes laser RIN in ultrafast fiber laser-based transient absorption microscopy.
• The technique recovers pump-probe signals even when averaging alone is insufficient.
• This method shows promise for label-free imaging, despite limitations from electronic noise.