On the role of interference in laser‐based mid‐infrared widefield microspectroscopy

Abstract

A laser's high degree of coherence leads to interferences, which-in the absence of precautions-can cause severe image distortions such as fringes and speckles and which thereby strongly hamper a meaningful interpretation of hyperspectral images in laser-based widefield microspectroscopy. While images and spectra of homogenous samples may already suffer from interferences, any structured object such as a tissue thin section will add to these distortions due to wavelength- and, in particular, sample-dependent phase shifts (structure sizes, absorption coefficients, refractive indices). This effect is devastating for the universal applicability of laser-based microspectroscopy especially in the mid-infrared (MIR), where cell sizes are of the same dimension as the wavelength of the illumination source. Here, we show that the impact of interferences is strongly mitigated by reducing the time-averaged spatiotemporal coherence properties of the illumination using a moving plus a stationary scatterer. In this case, the illumination path provides a pseudothermal radiation source and spatially resolved spectra can be obtained at the quality of the reference method, that is, Fourier-transform infrared microspectroscopy, without compromising spectral or spatial resolution.

Portable Spectroscopy

“…A further complication, especially for process, standoff, and imaging instruments used for solid phase studies, 343 is ''speckle.'' 344 This is the effect seen when a laser pointer illuminates a wall-you see highly non-uniform illumination, despite the beam itself having a nominal Gaussian profile.…”

“…A further complication, especially for process, standoff, and imaging instruments used for solid phase studies, 343 is “speckle.” 344 This is the effect seen when a laser pointer illuminates a wall—you see highly non-uniform illumination, despite the beam itself having a nominal Gaussian profile. Speckle reduction methods became increasingly important with the development of laser projectors for cinemas, 345 but as one paper observed, “there is no free lunch” in these processes.…”

Design Considerations for Discrete Frequency Infrared Microscopy Systems

“…(c) Laser illumination is homogenized by a rapidly rotating diffuser 133 to reduce the laser speckle for widefield recording. Reproduced with permission from the Schonhals et al 134 Copyright 2018 Wiley-VCH. (d) Commercial systems have enhanced accessibility of laser-based DFIR microscopy.…”

“…coherent sources are inherently subject to speckle arising from scattering in the optical train that compromises both morphological and spectral fidelity. Attempts to mitigate these issues and reduce the effect of laser speckle and coherent interference were developed, incorporating techniques such as spectral averaging, 132 rotating diffusers, 133,134,137 and time-delay integration, 139 but always at an expense of considerably increased acquisition time. We anticipate that increases in output power, wider tuning in single chips, greater wavelength ranges, and higher stability of turnkey QCL systems will emerge to drive innovations in IR imaging design in the near future.…”

Multicolor Discrete Frequency Infrared Spectroscopic Imaging

“…Hence, several additional techniques have been proposed to mitigate speckle effects. Rotating diffusers rapidly vary the speckle pattern while integrating the signal over time, ,, spectral averaging integrates the fluctuating speckle patterns as the laser tuner sweeps, and time delay integration averages speckle patterns by shifting the sample . Regardless of their individual efficacy for specific samples, none of these methods provide a general solution to scattering induced effects.…”