Abstract
.Significance: Ultrafast fiber lasers are an attractive alternative to bulk lasers for nonlinear optical microscopy for their compactness and low cost. The high relative intensity noise (RIN) of these lasers poses a challenge for pump-probe measurements such as transient absorption and stimulated Raman scattering, along with modalities that provide label-free contrast from the vibrational and electronic structure of molecules.Aim: Digital adaptive filtering was applied to determine the applicability for canceling laser RIN in a transient absorption microscope with an ultrafast fiber laser source.Approach: Digitized signals from the transmitted probe and reference photodetectors were fed to an adaptive filter in MATLAB, running in a noise canceling configuration. This result was then fed to a software lock-in algorithm to demodulate the pump-probe signal. Images were built up one line scan at a time with a 3.5-kHz resonant scanner, with averaging. The imaging target was , which exhibits nondegenerate two-photon absorption at the pump/probe wavelengths used (530-nm pump and 490-nm probe).Results: Without adaptive noise cancellation, the lock-in output primarily passes the laser RIN within its detection bandwidth, resulting in images that closely follow the linear transmissivity and lack sensitivity to pump-probe time delay. With adaptive noise cancellation in front of the lock-in, the RIN rejection is enough to restore the z-sectioning and sensitivity to pump-probe delay, as expected for transient absorption. Results were limited primarily by noise from the photodetector and analog-to-digital converter.Conclusions: Digital adaptive noise cancellation, even when limited by electronics noise, can recover pump-probe signals by removal of laser RIN, under conditions where averaging alone fails.
Highlights
Pump-probe microscopy is an imaging modality that utilizes a femtosecond or picosecond pump laser pulse to prepare nonequilibrium electronic and vibrational states
Fiber-based ultrashort laser sources could have significant advantages over bulk oscillators for transient absorption microscopy and techniques based on a similar detection principle, such as stimulated Raman scattering (SRS) microscopy,[3] because of their permanent optical alignment, low cost, and compact size
Significant relative intensity noise (RIN) that is 27 ∼ 40 dB higher than shot noise limit is found even at high-frequency components,[5,6,7] making it difficult to reject with a lock-in
Summary
Pump-probe microscopy is an imaging modality that utilizes a femtosecond or picosecond pump laser pulse to prepare nonequilibrium electronic and vibrational states. The large laser relative intensity noise (RIN) of fiber-based laser sources increases the noise floor 20 to 30 dB above the shot-noise limit.[4] In SRS and transient absorption microscopy, RIN is usually avoided by modulating the pump at high frequency and employing lock-in detection. Significant RIN that is 27 ∼ 40 dB higher than shot noise limit is found even at high-frequency components,[5,6,7] making it difficult to reject with a lock-in Compensating for such noise by slow scanning and long averaging at each pixel introduces other problems. It is difficult to keep the signal and reference balanced in laser scanning microscopy because the signal amplitude is rapidly modulated by the spatially varying transmissivity of the sample To compensate these intensity fluctuations, several optical and electronic autobalanced detection methods have been developed
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