Characterization of double-deformable-mirror adaptive optics for IR beam shaping in hyperspectral imaging

Abstract

Vibrational microspectroscopy via Fourier transform infrared (FTIR) faces an experimental trade-off among the signal to noise ratio (SNR), acquisition time, spatial resolution, and sample coverage. This is mainly associated with broadband source type: e.g. low brightness thermal sources with high flux for large field of view imaging at low resolution, or lowétendue of synchrotron radiation infrared (SRIR) for diffraction-limited scanning microanalysis at high magnification. 1 Adaptive optics (AO), in this case deformable mirror (DM), is a potent tool in tackling the problem by modulating the intensity of high brightness structured SRIR beam toward a homogeneous field illumination for IR imaging at high magnification. The latter is required for an efficient coupling of SRIR source to a multi-pixel detector such as focal plane array (FPA). 2 Additionally, DM enables to achieve different shapes, optimized for different Cassegrain IR objective. Regardless, the quality of the generated beam relies upon the performance of the adaptive elements, i.e. actuators and their linear and reproducible response to the applied voltage. Moreover, the beam shaping capability of a single DM in controlling light beam position and angle is limited by its actuators influence function. In this work, we implemented two DMs for intensity shaping for the complex SRIR beam. A variation of multi-conjugate AO is implemented to characterize the performance of DMs and their actuators transfer function at multiple locations. An IR sensitive microbolometer array has been optically conjugated to the focal plane of individual actuators and the far-field of DM, in order to probe the corresponding actuating response. By analysing each actuator's response individually, a measure of linear independence, uniformity in response, and cross-coupling can be obtained in a spectral range, from visible to near and mid IR. Additionally, by assembling the vectorized version of each actuator response, the transfer matrix can be formed. This matrix describes the relationship between the actuation effect on the beam and the response of the IR microbolometer, at the given conjugate planes. Based on such discussion, we assess the stability of the deformable mirror for open-loop (i.e. without feedback) operation.

On the Limit of Detection in Infrared Spectroscopic Imaging

“…22,23 This progress also bodes well for IR imaging, in which ultrasensitive measurements are made even more challenging due to the signal being recorded from significantly smaller volumes of materials with low light throughput while requiring the use of additional optics and specialized detectors. IR imaging technology has expanded and diversified considerably in recent years with improvements in FT-IR imaging, [24][25][26] new designs to take advantage of synchrotron sources, [27][28][29][30][31] surfaceenhanced measurements utilizing plasmonic nanorod antennas, 32,33 QCL-based imaging instruments, [34][35][36][37][38][39][40][41][42][43] polarimetric imaging [44][45][46][47][48] and the emergence of hybrid techniques such as atomic force microscopy-IR imaging, 49,50 and photothermal microscopy. [51][52][53][54][55][56][57][58][59][60][61] Since many of these techniques are rapidly developing and their capabilities may not all be geared towards the chemical limit of detection, we focus this study largely on well-established FT-IR and QCL microscopes.…”