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This paper comprehensively demonstrates the efficiency of balanced detection in a visible optical coherence tomography instrument employing a low-noise supercontinuum laser. By using an innovative technique for digitally aligning camera pixels, we achieved a noise floor reduction of up to 12.8 dB across the entire imaging depth range, particularly near the zero optical path difference between the interferometer arms. The instrument presented here operates at a central wavelength of 590 nm. It delivers high-resolution images with a sensitivity of up to 74 dB in a single spectrometer configuration and 92.8 dB in a balanced configuration. The enhancement in image contrast is exemplified through images of an optical phantom and in-vivo images of a human thumb and nail.

In this letter, a swept-source optical coherence tomography (SS-OCT) instrument employing an innovative scanning protocol for high-speed volumetric rate imaging is demonstrated. The optical source is a tuneable laser based on a supercontinuum source pumped with femtosecond pulses, followed by a time-stretched delay fibre. The instrument is equipped with an ultra-fast lateral scanner, based on a KTN crystal, driven at 100 kHz. The paper proves the utility of combining an ultra-fast lateral scanner with an ultra-fast swept laser to provide A-scans at a repetition rate of 40 MHz and an unprecedented 3D-OCT volume acquisition rate of 400 Hz.

In this paper, a fast dual resonance akinetic opticalswept source operating at 1550 nm is demonstrated. In-stead of modulating the optical amplifier gain reportedin our previous studies, here we employ a fiber intensitymodulator as a mode-locking element. A chirped fiberBragg grating is used to provide sufficient dispersion inthe laser cavity. A tuning range of 25 nm is obtained fora sweep frequency of ~900 kHz with a 6 dB drop-off insensitivity at 2.6 mm optical path difference.

Assessment of Ductile, Brittle, and Fatigue Fractures of Metals Using Optical Coherence Tomography Authors Gheorghe Hutiu, Virgil-Florin Duma, Dorin Demian, Adrian Bradu and Adrian Podoleanu Coordinates Assessment of Ductile, Brittle, and Fatigue Fractures of Metals Using Optical Coherence Tomography, Metals 8(2), 117 (2018). Abstract Some forensic in situ investigations, such as those needed in transportation (for aviation, maritime, road, or rail accidents) or for parts working under harsh conditions (e.g., pipes or turbines) would benefit from a method/technique that distinguishes ductile from brittle fractures of metals—as material defects are one of the potential causes of incidents. Nowadays, the gold standard in material studies is represented by scanning electron microscopy (SEM). However, SEM instruments are large, expensive, time-consuming, and lab-based; hence, in situ measurements are impossible. To tackle these issues, we propose as an alternative, lower-cost, sufficiently high-resolution technique, Optical Coherence Tomography (OCT) to perform fracture analysis by obtaining the topography of metallic surfaces. Several metals have been considered in this study: low soft carbon steels, lamellar graphite cast iron, an antifriction alloy, high-quality rolled steel, stainless steel, and ductile cast iron. An in-house developed Swept Source (SS) OCT system, Master-Slave (MS) enhanced is used, and height profiles of the samples’ surfaces were generated. Two configurations were used: one where the dimension of the voxel was 1000 μm^3 and a second one of 160 μm^3—with a 10 μm and a 4 μm transversal resolution, respectively. These height profiles allowed for concluding that the carbon steel samples were subject to ductile fracture, while the cast iron and antifriction alloy samples were subjected to brittle fracture. The validation of OCT images has been made with SEM images obtained with a 4 nm resolution. Although the OCT images are of a much lower resolution than the SEM ones, we demonstrate that they are sufficiently good to obtain clear images of the grains of the metallic materials and thus to distinguish between ductile and brittle fractures—especially with the higher resolution MS/SS-OCT system. The investigation is finally extended to the most useful case of fatigue fracture of metals, and we demonstrate that OCT is able to replace SEM for such investigations as well ...

Complex master-slave for long axial range swept-source optical coherence tomography Authors Manuel J. Marques, Sylvain Rivet, Adrian Bradu, and Adrian Podoleanu Coordinates Complex master-slave for long axial range swept-source optical coherence tomography," OSA Continuum 1, 1251-1259 (2018). Abstract Using complex master-slave interferometry, we demonstrate extended axial range optical coherence tomography for two commercially available swept sources, well beyond the limit imposed by their k-clocks. This is achieved without k-domain re-sampling and without engaging any additional Mach-Zehnder interferometer providing a k-clock signal to the digitizer. An axial imaging range exceeding 17 mm with an attenuation of less than 30 dB is reported using two commercially available swept sources operating at 1050 nm and a 100 kHz repetition rate. This procedure has more than trebled the range achievable using the k-clock signal provided by the manufacturers. An analysis is presented on the impact that the digitization has on the axial range and resolution of the system. ...

Down-conversion en-face optical coherence tomography Authors Adrian Podoleanu, Ramona Cernat and Adrian Bradu Coordinates Down-conversion en-face optical coherence tomography," Biomed. Opt. Express 10, 772-788 (2019). Abstract We present an optical coherence tomography (OCT) method that can deliver an en-face OCT image from a sample in real-time, irrespective of the tuning speed of the swept source. The method, based on the master slave interferometry technique, implements a coherence gate principle by requiring that the optical path difference (OPD) between the arms of an imaging interferometer is the same with the OPD in an interrogating interferometer. In this way, a real-time en-face OCT image can originate from a depth in the sample placed in the imaging interferometer, selected by actuating on the OPD in the interrogating interferometer, while laterally scanning the incident beam over the sample. The generation of the en-face image resembles time domain OCT, with the difference that here the signal is processed based on spectral domain OCT. The optoelectronic processor operates down-conversion of the chirped radio frequency signal delivered by the photo-detector. The down-conversion factor is equal to the ratio of the maximum frequency of the photo-detected signal due to an OPD value matching the coherence length of the swept source, to the sweeping rate. This factor can exceed 106 for long coherence swept sources. ...

Effect of an Anaerobic Fermentation Process on 3D-Printed PLA Materials of a Biogas-Generating Reactor 01 December 2022 {: .label .label-blue } New journal paper published in Materials! Authors Adrian Cioabla,Virgil-Florin Duma,Corina Mnerie,Ralph-Alexandru Erdelyi, George Mihai Dobre, Adrian Bradu, and Adrian Podoleanu Title and coordinates “[Enhanced resolution optoacoustic microscopy using a Effect of an Anaerobic Fermentation Process on 3D-Printed PLA Materials of a Biogas-Generating Reactor, Materials 15, 8571 (2022). Abstract 3D-printed materials are present in numerous applications, from medicine to engineering. The aim of this study is to assess their suitability for an application of interest today, that of testing of 3D-printed polylactic acid (PLA)-based reactors for biogas production using anaerobic digestion. The impact of temperature, pH, and aqueous phase on the tested bioreactor is investigated, together with the effect of the gaseous phase (i.e., produced biogas). Two batches of materials used separately, one after another inside the bioreactor were considered, in a realistic situation. Two essential parameters inside the reactor (i.e., pH and temperature) were continuously monitored during a time interval of 25 to 30 days for each of the two biogas-generating processes. To understand the impact of these processes on the walls of the bioreactor, samples of 3D-printed material were placed at three levels: at the top (i.e., outside the substrate), in the middle, and at the bottom of the bioreactor. The samples were analyzed using a non-destructive imaging method, Optical Coherence Tomography (OCT). An in-house developed swept-source (SS) OCT system, master–slave (MS) enhanced, operating at a central wavelength of 1310 nm was utilized. The 3D OCT images related to the degradation level of the material of the PLA samples were validated using Scanning Electron Microscopy (SEM). The differences between the impact of the substrate on samples situated at the three considered levels inside the reactor were determined and analyzed using their OCT B-scans (optical cross-section images). Thus, the impact of the biogas-generating process on the interior of the bioreactor was demonstrated and quantified, as well as the capability of OCT to perform such assessments. Therefore, future work may target OCT for in situ investigations of such bioreactors. ...

En-face OCT/fluorescence endomicroscopy for minimally invasive imaging using a robotic scanner Authors Manuel J. Marques, Michael R. Hughes, Khushi Vyas, Andrew Thrapp, Haojie Zhang, Adrian Bradu, Grigory Gelikonov, Petros Giataganas, Christopher J. Payne, Guang-Zhong Yang, and Adrian Podoleanu Title and coordinates En-face optical coherence tomography/fluorescence endomicroscopy for minimally invasive imaging using a robotic scanner, J. of Biomedical Optics 24(6), 066006 (2019). Abstract We report a compact rigid instrument capable of delivering en-face optical coherence tomography (OCT) images alongside (epi)-fluorescence endomicroscopy (FEM) images by means of a robotic scanning device. Two working imaging channels are included: one for a one-dimensional scanning, forward-viewing OCT probe and another for a fiber bundle used for the FEM system. The robotic scanning system provides the second axis of scanning for the OCT channel while allowing the field of view (FoV) of the FEM channel to be increased by mosaicking. The OCT channel has resolutions of 25 / 60 μm (axial/lateral) and can provide en-face images with an FoV of 1.6 × 2.7 mm2. The FEM channel has a lateral resolution of better than 8 μm and can generate an FoV of 0.53 × 3.25 mm2 through mosaicking. The reproducibility of the scanning was determined using phantoms to be better than the lateral resolution of the OCT channel. Combined OCT and FEM imaging were validated with ex-vivo ovine and porcine tissues, with the instrument mounted on an arm to ensure constant contact of the probe with the tissue. The OCT imaging system alone was validated for in-vivo human dermal imaging with the handheld instrument. In both cases, the instrument was capable of resolving fine features such as the sweat glands in human dermal tissue and the alveoli in porcine lung tissue. ...

Enhanced resolution optoacoustic microscopy using a picosecond high repetition rate Q-switched microchip laser 29 November 2022 {: .label .label-blue } New journal paper published in Journal of Biomedical Optics! Authors Gianni Nteroli, Giulia Messa, Manoj K. Dasa, Antti Penttinen, Antti Härkönen, Mircea Guina, Adrian Podoleanu, Stella Koutsikou, and Adrian Bradu Title and coordinates “Enhanced resolution optoacoustic microscopy using a picosecond high repetition rate Q-switched microchip laser,” Journal of Biomedical Optics 27(11), 110501 (2022). ...

Fast spectrally encoded Mueller optical scanning microscopy Authors Sylvain Rivet, Matthieu Dubreuil, Adrian Bradu, and Yann Le Grand Title and Coordinates Fast spectrally encoded Mueller optical scanning microscopy, Scientific Reports 9, 3972 (2019). Abstract Mueller microscopes enable imaging of the optical anisotropic properties of biological or non-biological samples, in phase and amplitude, at sub-micrometre scale. However, the development of Mueller microscopes poses an instrumental challenge: the production of polarimetric parameters must be sufficiently quick to ensure fast imaging, so that the evolution of these parameters can be visualised in real-time, allowing the operator to adjust the microscope while constantly monitoring them. In this report, a full Mueller scanning microscope based on spectral encoding of polarization is presented. The spectrum, collected every 10 $\mu$s for each position of the optical beam on the specimen, incorporates all the information needed to produce the full Mueller matrix, which allows simultaneous display of all the polarimetric parameters, at the unequalled rate of 1.5 Hz (for an image of 256 $\times$ 256 pixels). The design of the optical blocks allows for the real-time display of linear birefringent images which serve as guidance for the operator. In addition, the instrument has the capability to easily switch its functionality from a Mueller to a Second Harmonic Generation (SHG) microscope, providing a pixel-to-pixel matching of the images produced by the two modalities. The device performance is illustrated by imaging various unstained biological specimens. ...