Inspection and imaging through scattering layers is a decades-old problem in optics. Unlike x-ray and ultrasonic imaging techniques, time-domain spectroscopy methods can provide detailed chemical and structural information of subsurfaces along with their depth. Although high-resolution time-of-flight measurement in time-domain spectroscopy provides 3D information, unfortunately it also induces an unwanted sensitivity to misalignments of the system and distortion of the layers themselves. Such high sensitivity to alignment and sample surface is a well known problem in time-domain and interferometric imaging, and is a major concern when the alignment error is comparable to the pulse wavelength. Here, we propose and implement an algorithmic framework based on low-rank matrix recovery and alternating minimization to remove such unwanted distortions from time-domain images. The method allows for recovery of the original sample texture in spite of the presence of temporal-spatial distortions. We address a blind-demodulation problem where, based on several observations of the sample texture modulated by undesired sweep distortions, the two classes of signals are separated with minimal damage to the main features. The performance of the method is examined in both synthetic and real data in the case of a terahertz time-domain system, and the successful reconstructions are demonstrated. The proposed general scheme can be implemented to advance inspection and imaging applications in THz and other time-resolved spectral imaging modalities.
['Alireza Aghasi', 'Barmak Heshmat', 'Albert Redo-Sanchez', 'Justin K. Romberg', 'Ramesh Raskar']
Sweep Distortion Removal from THz Images via Blind Demodulation