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Schematic of Mach-Zehnder interferometer for NLIR microscopy: OC, optical chopper; DM, dichroic mirror; PMT, photomultiplier tube; HPF, high-pass filter; LIA, lock-in amplifier. A pair of off-axis parabolic mirrors is used for the infrared beam expansion, and another short focal length parabolic mirror with an effective NA of 0.42 is used as an objective lens.
(a) Typical oscilloscope temporal traces of the interference signal measured by the PMT (yellow) and the high-pass-filtered signal (green). The time span is 60 ms. The inset shows the 2.5 kHz-modulated signal induced by the intensity-modulated infrared pump beam. (b) A typical NLIR image of the uniform balsam oil layer obtained from the randomly fluctuating signal (such signal as the one shown in green in (a)). The irregular black-and-white striped pattern is indicative of the nature of the noise.
Two coupled Mach-Zehnder interferometers for the compensation of phase fluctuation noise in NLIR microscopy: SMF, single-mode fiber. The channel phase of CH2 is adjusted to 90° to let the system work in dual-quadrature mode, which is analogous to the functioning of conventional dual-phase lock-in amplifiers.
(a) and (b) Oscilloscope traces of the PMT signals of the two channels (yellow for CH1, green for CH2) for two different channel phases, around 90° and 180°. (c) and (d) Corresponding high-pass filtered signals of (a) and (b). (e) and (f) NLIR images of a uniform balsam oil layer obtained from the corresponding signals in (c) and (d). See the text for details. The imaging time for the oil layer was 10 s. The scale bars are 5 μm.
NLIR images of 1-μm diameter polystyrene bead aggregates measured with a 3.5-μm infrared pump and 633-nm probe beams. (a) Two-channelresult. (b) Single-channel result. The squashed particle features in (b) are well recovered in (a). The intensity scale expressed by the modulated phase (radian) is indicated in the color bar. (c) and (d) Corresponding FFT results of the signal intensity profiles along the designated vertical lines on the beads images, (a) and (b). Notice the high frequency noise components in the shaded region of (d), which are absent in (c). The imaging time was 50 s. The scale bars are 5 μm.
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