Supercontinuum sources for multimodal MIR-OCT imaging

: In this paper we present a multimodal OCT imaging system operating in the NIR and MIR spectral region. NIR/MIR Supercontinuum source based multimodal OCT provides new insights in scattering .materials. The system combines the modality of OCT and spectroscopy and enables a structural and specific characterization of different ceramics, polymers or composites.


Introduction
Recently the progress in Supercontiuum (SC) source developments has enables multiple new applications, for example in coherent and non-coherent imaging, in spectroscopy [1,2] or for data transmission. While at the beginning these SC sources have been covered the near infrared (NIR) range, meanwhile they are advanced also covering the mid infrared (MIR) range. For Optical Coherence Tomography (OCT) imaging these light sources are beneficial due to their low temporal coherence. The higher spatial coherence is rather detrimental for OCT, in particular for full field OCT versions. However, there are also challenges which have to be tackled by the integration into a raster scanning OCT system. In particular noise fluctuations and repetition rates must be adapted regarding the scanning rate of the OCT system which defines finally the usability of different versions of these SC sources. -We will show here a SC based multimodal MIR OCT system applicable in the field of non-destructive testing (NDT).

Measurements
Before the operational assemblage of the OCT system the NIR/MIR-SC source has been characterized with respect to its noise floor and spectral features. The relative pulse energy noise (REN) of the SC source is a key parameter and was characterized for different repetition rates (varied with an external trigger) and power levels. The variance of a sequence of 1000 time-integrated pulses, normalized by the mean value has yielded the relative pulse energy noise level (REN) of the SC source [3]. The REN provides a measure of the wavelength averaged noise. The pulse forms were measured using a multiple junction photovoltaic HgCdTe detector (PVM, VIGO) with a time constant of 0.7 ns and then post-processed in Python. The spectral characterization of SC has been performed by Fourier transform infrared spectrometer and monochromator measurements. The scheme of the (FD)-OCT system is depicted in Figure 1. As a multimodal system it consists of an OCT imaging modality and a spectroscopic modality (SM). For the first essential components are: the MIR SC source (based on ZBLAN fiber, 40 mW average power within OCT range), the OCT mirror based grating spectrometer (Czerny-Turner configuration), and the pyroelectric detector with synchronized chopper. For the spectroscopic part a microspectrometer (Fabry-Pérot tunable filter, FPTF) can be integrated, for instance enabled by a flippable mirror in the optical path. The optical setting has been simulated by ZEMAX, a comprehensive signal processing and aberration correction for the wide spectral range have been performed [4].
The FD-OCT system was also characterized and yields a sensitivity (SNR) of about 82 dB, a lateral resolution of 35 μm and an axial resolution around 40 μm. A spatial resolution, and effective probing depth for ceramic samples of up to 1.2 mm was achieved. The frame rate of the acquisition system is currently limited by 100 Hz.

Results
First the multimodal MIR OCT system has been exemplified by standard test pattern, like alumina and zirconia plane plates. Later our system was tested for more complicated and composed material systems, like paint and varnish samples for ceramic composites. The layered structures of the ceramic composites could be confirmed by MIR FD-OCT, while a comparable measurement by NIR OCT system failed for recognizing internal details in ceramics specimen, as shown in Figure 2.

Conclusions and Outlook
Summarizing, we have illustrated an elegant multimodal MIR FD-OCT system with pyroelectric line array as detector. We have demonstrated this system successfully for NDT applications of ceramics and polymer composite structures. The SC source enables to discover novel spectral ranges for OCT imaging and probing. We also highlight the spectroscopic imaging features of the multimodal system as well. The OCT system can still be improved by advances in the pyroelectric detector controlling system resulting in an even higher sensitivity for future. Prospectively, novel applications schemes for characterization of thin artificial teeth components or ceramic coated prosthetics, or for use in art (porcelain) conservation can be envisaged.
We acknowledge project funding by EU SUPUIR program and the Austrian national MICI and FTI research project.