Optical Coherence Tomography Sensor Research Articles

In-Mould OCT Sensors Combined with Piezo-Actuated Positioning Devices for Compensating for Displacement in Injection Overmoulding of Optoelectronic Parts

When overmoulding optoelectronic devices with optical elements, precise alignment of the overmoulded part and the mould is of great importance. However, mould-integrated positioning sensors and actuators are not yet available as standard components. As a solution, we present a mould-integrated optical coherence tomography (OCT) device that is combined with a piezo-driven mechatronic actuator, which is capable of performing the necessary displacement correction. Because of the complex geometric structure optoelectronic devices may have, a 3D imaging method was preferable, so OCT was chosen. It is shown that the overall concept leads to sufficient alignment accuracy and, apart from compensating for the in-plane position error, provides valuable additional information about the sample both before and after the injection process. The increased alignment accuracy leads to better energy efficiency, improved overall performance and less scrap parts, and thus even a zero-waste production process might be feasible.

Weld-penetration-depth estimation using deep learning models and multisensor signals in Al/Cu laser overlap welding

Al/Cu laser-welded overlap joints, in which weld-penetration depth significantly influences both joint strength and electrical conductivity, are widely applied in automotive battery cells. In this study, a unisensor convolutional neural network (CNN) model that predicts penetration depth using coaxial weld-pool images as input and multisensor CNN models that utilize additional photodiode signals are proposed. The penetration depth was estimated using an optical coherence tomography sensor. The coefficient of determination values for the unisensor and multisensor CNN models were between 0.982 and 0.985, and their mean absolute errors were between 0.0278 and 0.0302 mm. The short-term Fourier transform multisensor model presented the best performance in terms of prediction of penetration depth when applied to the photodiode signal. The proposed prediction models were validated using a gradually varying laser power experiment, which demonstrated the efficacy of this approach and its potential use in automotive applications. Keywords: Laser welding, Al/Cu overlap joint, Penetration-depth estimation, Image sensor, Photodiode, CNN, Deep learning.

A novel measurement approach based on optical coherence tomography for inline quality assessment of thermoplastic glass-fiber reinforced unidirectional tapes

Optical coherence tomography (OCT) has been found to be a powerful method for non-destructive testing of glass-fiber reinforced polymeric composites. Various authors have investigated the use of OCT in offline lab-scale detection of fiber orientation and defects in thermoset-based composites. This work evaluated the potential of OCT as a technique for detecting common defects in the continuous production of thermoplastic glass-fiber reinforced unidirectional (UD) tapes. Measurements were first conducted with a stationary OCT sensor using a spectral domain system to investigate: (1) insufficiently impregnated fiber regions, (2) unfilled gaps, (3) polymer accumulations causing fiber/matrix irregularities, and (4) rough tape surfaces. Optical microscopic analysis was used for validation. To overcome the limited maximum field of view of modern OCT setups, we then developed a novel inline test rig to accurately simulate process conditions and measure across the whole tape width. We show that OCT is a reliable method for acquiring cross-section information on tape quality both at the lab scale with a stationary sensor and inline with a sensor moving across the tape surface. Our OCT measurements were in excellent agreement with our offline microscopic investigations. OCT is a powerful, non-destructive and high-resolution method for quality assessment of glass-fiber reinforced UD tapes and has great potential for use in inline quality assurance systems.

PIN photodiode-based active pixel for a near-infrared imaging application in 0.35-μm CMOS

We present an active pixel for a spectral domain optical coherence tomography sensor on a chip, where optical components are realized in a photonic layer and monolithically integrated with the electronics, whereby light is brought to the pixels using waveguides. The core of the pixel is an amplifier with capacitive feedback (so-called capacitive transimpedance amplifier), apart from that, a correlated double sampling circuit is implemented within the pixel. The proposed active pixel is based on a PIN photodiode and fabricated in 0.35-μm high-voltage CMOS technology. We use three different epitaxial starting material thicknesses (20, 30, and 40 μm) in order to find the device with best performance. The pixel is optimized for high efficiency in a spectral range between 800 and 900 nm. We explain advantages in the spectral responsivity and crosstalk of this pixel over conventional p / n photodiode-based pixels in standard CMOS processes and over the pinned photodiode-based pixel. We also present measured pixel parameters and give comparison with prior work.

Measurement of the Intertablet Coating Uniformity of a Pharmaceutical Pan Coating Process With Combined Terahertz and Optical Coherence Tomography In-Line Sensing

We present in-line coating thickness measurements acquired simultaneously using 2 independent sensing modalities: terahertz pulsed imaging (TPI) and optical coherence tomography (OCT). Both techniques are sufficiently fast to resolve the coating thickness of individual pharmaceutical tablets in situ during the film coating operation, and both techniques are direct structural imaging techniques that do not require multivariate calibration. The TPI sensor is suitable to measure coatings greater than 50 μm and can penetrate through thick coatings even in the presence of pigments over a wide range of excipients. Due to the long wavelength, terahertz radiation is not affected by scattering from dust within the coater. In contrast, OCT can resolve coating layers as thin as 20 μm and is capable of measuring the intratablet coating uniformity and the intertablet coating thickness distribution within the coating pan. However, the OCT technique is less robust when it comes to the compatibility with excipients, dust, and potentially the maximum coating thickness that can be resolved. Using a custom-built laboratory scale coating unit, the coating thickness measurements were acquired independently by the TPI and OCT sensors throughout a film coating operation. Results of the in-line TPI and OCT measurements were compared against one another and validated with off-line TPI and weight gain measurements. Compared with other process analytical technology sensors, such as near-infrared and Raman spectroscopy, the TPI and OCT sensors can resolve the intertablet thickness distribution based on sampling a significant fraction of the tablet populations in the process. By combining 2 complementary sensing modalities, it was possible to seamlessly monitor the coating process over the range of film thickness from 20 μm to greater than 250 μm.

Calibration-free in-line monitoring of pellet coating processes via optical coherence tomography

The paper presents a new in-line measurement technique for determining the coating thickness and uniformity of pharmaceutical pellets during film-coating in a fluid-bed apparatus. Non-destructive and contact-free process monitoring was performed via an optical coherence tomography (OCT) sensor providing cross-section images. Through the OCT measurements, the coating thickness could be determined directly, without a chemometric calibration model required for the quantification. The direct integration of the OCT sensor head into the fluid-bed systems allowed continuous monitoring of the coating growth. Moreover, the in-line investigation of the intra- and inter-pellet coating uniformity was possible due to OCT׳s high acquisition rate. Results of the in-line OCT measurements were validated using both off-line OCT images and particle size analysis by performing an image analysis of samples that were periodically removed from the process during the production. Three batches were produced under the same process conditions demonstrating the reproducibility of the results. Rather than providing the temporal and spatial average of a large number of pellets, the OCT sensor characterized local pellet properties of up to 130 individual pellets per minute. Therefore, the OCT technology allows the operator to directly monitor the coating thickness and uniformity in sub-micron resolution, which makes it a promising in-line PAT method.