Abstract
Digital lock-in amplification (LIA) with synchronous digitization (SD) is shown to provide significant signal to noise (S/N) and linear dynamic range advantages in beam-scanning microscopy measurements using pulsed laser sources. Direct comparisons between SD-LIA and conventional LIA in homodyne second harmonic generation measurements resulted in S/N enhancements consistent with theoretical models. SD-LIA provided notably larger S/N enhancements in the limit of low light intensities, through the smooth transition between photon counting and signal averaging developed in previous work. Rapid beam scanning instrumentation with up to video rate acquisition speeds minimized photo-induced sample damage. The corresponding increased allowance for higher laser power without sample damage is advantageous for increasing the observed signal content.
Highlights
The signal to noise (S/N) of a nonlinear optical measurement is one of the critical defining properties dictating the available scope of use of emerging nonlinear optical microscopy techniques
High S/N is essential for high speed beam-scanning imaging, where each focal volume is often measured for only a few laser shots
Images from the lock-in amplification (LIA) and synchronous digitization (SD)-LIA techniques are shown in Figure 5, using a Nikon 10×/0.3NA objective
Summary
The S/N of a nonlinear optical measurement is one of the critical defining properties dictating the available scope of use of emerging nonlinear optical microscopy techniques. While in principle gated integration and LIA can be intuitively connected, in practice the instrumentation for each has become highly specialized Both techniques suffer performance losses relative to the shot-noise limit for detection at low light levels, where a S/N advantage arises from photon counting. SD is coupled with real-time statistical data analysis to extend the linear dynamic range of modulated signal detection seamlessly across the photon counting and signal averaging regimes.10 In this manner, the broader area indicated in the outermost box in Figure 1 can be spanned with a single measurement platform for modulated pulsed light detection with repetition rates exceeding 100 MHz. Preliminary efforts centered on homodyned SHG imaging, in which the signals were modulated through polarization modulation. Analysis where the distribution of pixel intensities may span a dynamic range of several decades
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