Low-coherence Interferometry Research Articles

Growth of homoepitaxial single crystal diamond by microwave plasma CVD in H2-CH4-O2 gas mixtures at high microwave power densities

Single crystal diamond growth by microwave plasma assisted CVD in oxygen-containing gas mixtures is attractive in view of possibility to realize a prolonged non-stoped synthesis process and to minimize defects and impurities in the produced material. We studied the homoepitaxial diamond growth on (100) oriented substrates in H2-CH4-O2 environment at high pressures (300 Torr) and high microwave power density (≈400 W/cm3) at variable O2 content (up to 3.5 %) in the plasma. A low-coherence optical interferometry and optical emission (OE) spectroscopy was used to measure in situ the growth rate and probe the plasma chemistry, respectively. Depending on CH4 content the growth rate dependence on the concentration [O2] either shows a maximum at certain [O2], or a monotonic decline, up to complete stop of the growth at some critical O2 percentage (specific for each methane content) in the mixture. We found CH, Hβ, C2 and C3 lines in OE spectra from the plasma to monotonically quenched with O2 adding, particularly, disappearance of the C2 line coincidences with growth rate going to zero. High-quality homoepitaxial diamond layers were produced at moderate growth rate at optimal gas composition and characterized with Raman spectroscopy. Moreover, a significant reduction in the stress on (111) oriented substrates owing to O2 addition was observed.

Two different visual stimuli that cause axial eye shortening have no additive effect

Previous studies identified two visual stimuli that can shorten the human eye by thickening the choroid after short-term visual stimulation, potentially inhibiting myopia: (1) watching digitally filtered movies where the red plane has full spatial resolution while green and blue are low-pass filtered according to the human longitudinal chromatic aberration (LCA) function (the “red in focus” filter), and (2) reading text with inverted contrast. This study aimed to determine whether combining these two stimuli would have an additive effect on axial length. Twenty-two emmetropic subjects were recruited to read text (standard and inverted contrast) for 30 min from a large screen, 2 m away, either unfiltered or filtered with the “red in focus” filter. Axial length was measured before and after each reading episode using low-coherence interferometry (Lenstar LS 900, Haag Streit). Reading text with conventional contrast polarity (dark letters on a bright background) resulted in no significant axial length change. Adding the “red in focus” filter did not alter the outcome. Consistent with previous findings, reading inverted contrast text made emmetropic eyes shorter. Surprisingly, when the text was combined with the “red in focus” filter, eyes became longer rather than shorter. A possible explanation for this contradictory result is that, for the text stimulus, the “red in focus” filter removes spatial information in the blue channel needed by the retina to use LCA analysis to thicken the choroid.

Harnessing forward multiple scattering for optical imaging deep inside an opaque medium

As light travels through a disordered medium such as biological tissues, it undergoes multiple scattering events. This phenomenon is detrimental to in-depth optical microscopy, as it causes a drastic degradation of contrast, resolution and brightness of the resulting image beyond a few scattering mean free paths. However, the information about the inner reflectivity of the sample is not lost; only scrambled. To recover this information, a matrix approach of optical imaging can be fruitful. Here, we report on a de-scanned measurement of a high-dimension reflection matrix R via low coherence interferometry. Then, we show how a set of independent focusing laws can be extracted for each medium voxel through an iterative multi-scale analysis of wave distortions contained in R. It enables an optimal and local compensation of forward multiple scattering paths and provides a three-dimensional confocal image of the sample as the latter one had become digitally transparent. The proof-of-concept experiment is performed on a human opaque cornea and an extension of the penetration depth by a factor five is demonstrated compared to the state-of-the-art.

Dispersion mismatch correction for evident chromatic anomaly in low coherence interferometry.

The applications of ultrafast optics to biomedical microscopy have expanded rapidly in recent years, including interferometric techniques like optical coherence tomography and microscopy (OCT/OCM). The advances of ultra-high resolution OCT and the inclusion of OCT/OCM in multimodal systems combined with multiphoton microscopy have marked a transition from using pseudo-continuous broadband sources, such as superluminescent diodes, to ultrafast supercontinuum optical sources. We report anomalies in the dispersion profiles of low-coherence ultrafast pulses through long and non-identical arms of a Michelson interferometer that are well beyond group delay or third-order dispersions. This chromatic anomaly worsens the observed axial resolution and causes fringe artifacts in the reconstructed tomograms in OCT/OCM using traditional algorithms. We present DISpersion COmpensation Techniques for Evident Chromatic Anomalies (DISCOTECA) as a universal solution to address the problem of chromatic dispersion mismatch in interferometry, especially with ultrafast sources. First, we demonstrate the origin of these artifacts through the self-phase modulation of ultrafast pulses due to focusing elements in the beam path. Next, we present three solution paradigms for DISCOTECA: optical, optoelectronic, and computational, along with quantitative comparisons to traditional methods to highlight the improvements to the dynamic range and axial profile. We explain the piecewise reconstruction of the phase mismatch between the arms of the spectral-domain interferometer using a modified short-term Fourier transform algorithm inspired by spectroscopic OCT. Finally, we present a decision-making guide for evaluating the utility of DISCOTECA in interferometry and for the artifact-free reconstruction of OCT images using an ultrafast supercontinuum source for biomedical applications.

Macular OCT's Proficiency in Identifying Retrochiasmal Visual Pathway Lesions in Multiple Sclerosis-A Pilot Study.

Optical coherence tomography (OCT) is a non-invasive imaging technique based on the principle of low-coherence interferometry that captures detailed images of ocular structures. Multiple sclerosis (MS) is a neurodegenerative disease that can lead to damage of the optic nerve and retina, which can be depicted by OCT. The purpose of this pilot study is to determine whether macular OCT can be used as a biomarker in the detection of retrochiasmal lesions of the visual pathway in MS patients. We conducted a prospective study in which we included 52 MS patients and 27 healthy controls. All participants underwent brain MRI, visual field testing, and OCT evaluation of the thicknesses of the peripapillary retinal nerve fiber layer (pRNFL), macular ganglion cell layer (GCL), and macular inner plexiform layer (IPL). OCT measurements were adjusted for optic neuritis (ON). VF demonstrated poor capability to depict a retrochiasmal lesion identified by brain MRI (PPV 0.50). In conclusion, the OCT analysis of the macula appears to excel in identifying retrochiasmal MS lesions compared to VF changes. The alterations in the GCL and IPL demonstrate the most accurate detection of retrochiasmal visual pathway changes in MS patients.

Effects of short-term exposure to red or near-infrared light on axial length in young human subjects.

To determine whether visible light is needed to elicit axial eye shortening by exposure to long wavelength light. Incoherent narrow-band red (620 ± 10 nm) or near-infrared (NIR, 875 ± 30 nm) light was generated by an array of light-emitting diodes (LEDs) and projected monocularly in 17 myopic and 13 non-myopic subjects for 10 min. The fellow eye was occluded. Light sources were positioned 50 cm from the eye in a dark room. Axial length (AL) was measured before and after the exposure using low-coherence interferometry. Non-myopic subjects responded to red light with significant eye shortening, while NIR light induced minor axial elongation (-13.3 ± 17.3 μm vs. +6.5 ± 11.6 μm, respectively, p = 0.005). Only 41% of the myopic subjects responded to red light exposure with a decrease in AL and changes were therefore, on average, not significantly different from those observed with NIR light (+0.2 ± 12.1 μm vs. +1.1 ± 11.2 μm, respectively, p = 0.83). Interestingly, there was a significant correlation between refractive error and induced changes in AL after exposure to NIR light in myopic eyes (r(15) = -0.52, p = 0.03) and induced changes in AL after exposure to red light in non-myopic eyes (r(11) = 0.62, p = 0.02), with more induced axial elongation with increasing refractive error. Incoherent narrow-band red light at 620 nm induced axial shortening in 77% of non-myopic and 41% of myopic eyes. NIR light did not induce any significant changes in AL in either refractive group, suggesting that the beneficial effect of red laser light therapy on myopia progression requires visible stimulation and not simply thermal energy.

3D nanoprinted catadioptric fiber sensor for dual-axis distance measurement during vitrectomy

Retinal damage is a common intraoperative complication during vitrectomy, caused by a complex interplay between the suction of the vitrectome, the cut- and aspiration rate, and the distance of the instrument to the retina. To control this last factor, we developed two miniaturized fiber-optic distance sensors based on low-coherence interferometry for direct integration into the vitrectome. Both sensors have a diameter of 250 µm, which makes them compatible with a 25G vitrectome. The first sensor measures distance in the lateral direction. The second sensor is capable of simultaneously measuring distance in both the lateral and the axial direction. Axial and lateral directions correspond to the direction of the cutter port of the vitrectome and the direction along the vitrectome’s shaft, respectively. In both sensors, a free-form mirror deflects and focuses the beam in the lateral direction. In the dual-axis distance sensor, an additional lens is integrated into the free-form mirror for distance measurement in the axial direction. The beam-shaping micro-optics at the tip of the sensor fibers were fabricated through two-photon polymerization and are selectively gold coated for increased reflectivity of the mirror. Distance measurements were successfully demonstrated in artificial samples and in ex vivo pig eyes with a back-end that uses a current-tuned VCSEL as a swept-source. We experimentally demonstrate that the complete sensor system can attain a SNRmax of up to 80 dB. The small dimensions of the developed sensors make them a potential solution for various other medical applications.

Next-generation endoscopic probe for detection of esophageal dysplasia using combined OCT and angle-resolved low-coherence interferometry.

Angle-resolved low-coherence interferometry (a/LCI) is an optical technique that enables depth-specific measurements of nuclear morphology, with applications to detecting epithelial cancers in various organs. Previous a/LCI setups have been limited by costly fiber-optic components and large footprints. Here, we present a novel a/LCI instrument incorporating a channel for optical coherence tomography (OCT) to provide real-time image guidance. We showcase the system's capabilities by acquiring imaging data from in vivo Barrett's esophagus patients. The main innovation in this geometry lies in implementing a pathlength-matched single-mode fiber array, offering substantial cost savings while preserving signal fidelity. A further innovation is the introduction of a specialized side-viewing probe tailored for esophageal imaging, featuring miniature optics housed in a custom 3D-printed enclosure attached to the tip of the endoscope. The integration of OCT guidance enhances the precision of tissue targeting by providing real-time morphology imaging. This novel device represents a significant advancement in clinical translation of an enhanced screening approach for esophageal precancer, paving the way for more effective early-stage detection and intervention strategies.

Low-coherence interferometry with homodyne demodulation capabilities for distributed fiber-optical sensors

Distributed fiber-optical sensors (DFOS) achieve the best results while interference registration schemes are used when controlled physical field lead to the fiber length and phase difference. DFOS based on fiber-optical phase-amplitude filters (FOPAF) have a simple and reliable design, consisting of distributed sensing elements (DSE) set. However, the main problem is the stabilization of its characteristics in different areas of DSE. On the other hand, the possibilities of low-coherence interferometry with homodyne light demodulation show the possibilities of stabilizing working points, but in the Michelson fiber-optical interferometric tandem scheme. The purpose is to substantiate the option of stabilizing the operation of DSE based on FOPAF, using low-coherence interferometry with homodyne demodulation in monitoring spatially distributed physical fields. The appropriate methods are considered. The analysis of the possible experimental setup based on the low-coherence interferometry with homodyne demodulation in the interferometric tandem scheme in comparison with the variants, based on the low-mode fiber-optical fibers is established. Promising scheme of the experimental installation is presented and the requirements for components are justified. The prospects of using low-coherence interferometry with homodyne demodulation for DFOS based on DSE with FOPAF are shown. The proposed low-coherence interferometry scheme with homodyne demodulation for DFPS on DSE with FOPAF has high sensitivity, is built on affordable equipment and, accordingly, has a low specific cost for the subsequent development of monitoring system.

Low-Coherence Homodyne Interferometer for Sub-Megahertz Fiber Optic Sensor Readout.

This study proposes a method for interferometric fiber optic sensor readouts. The method utilizes the advantages of the active homodyne demodulation technique and low-coherence interferometry. The usage of the tandem low-coherence interferometer enables modulating the reference interferometer without any changes to the sensor. This achieves high sensitivity, high stability, and a wide frequency band. A sensitivity of up to 0.1 nm (RMS) in the frequency range of 5 kHz is demonstrated by detecting acoustic signals with a fiber Michelson interferometer as a sensor.

Novel digital signal processing method for data acquired from Low Coherence Interferometry

Novel digital signal processing method for data acquired from Low Coherence Interferometry

Two different visual stimuli that cause axial eye shortening and choroidal thickening have no additive effect

Aims/Purpose: Previous studies identified two visual stimuli that can make the human eye shorter and potentially inhibit myopia: [1] watching digitally filtered movies that kept the red channel in focus but were low pass filtered in the blue and [2] reading text with inverted contrast. We have tested whether the two mechanisms may be additive, perhaps causing even more inhibition of eye growth.Methods: Fifteen emmetropic (average refraction: −0.6 ± 0.7 D) and fifteen myopic (average refraction: −3.7 ± 1.8 D, range −6 to −1.5D) young subjects (average age: 26 ± 4 years) read text from a large screen (65″) in a dark room at 2 m distance. Text with inverted contrast (bright letters on dark background) was digitally filtered in real time by custom‐developed software written in Visual C++ to present the red channel of the RGB colour format unfiltered while green and blue were blurred (red in focus) accordingly to the human longitudinal chromatic aberration function. Changes in axial length before and after 30 min of reading were measured using low coherence interferometry with auto positioning system (Lenstar LS‐900).Results: Unexpectedly, in emmetropes, significant eye elongation was found after 30 min of reading text with inverted contrast, combined with the red in focus filter (+5.7 ± 15.3 μm). As previously found, unfiltered text with inverted contrast made eyes shorter (−7.9 ± 14.7 μm, p < 0.05). Strikingly, no such changes were observed in myopic eyes, neither with the red in focus filter, nor when text with inverted contrast remained unfiltered.Conclusions: In emmetropes, the two stimuli had no additive effects but rather cancelled each other out. There may be a simple explanation: the red in focus filter made the text very blurry in the blue channel. Apparently, this removed the spatial information that is needed by the retina to detect defocus in the blue. Therefore, our finding supports the view that the emmetropic retina needs some spatial information in the blue to generate eye growth inhibiting signals—and they were lost in myopes.

Functional changes in the myopic retina compromise emmetropization

During emmetropization, the retina analyses the focus of the image and generates appropriate biochemical signals to control choroidal thickness and scleral growth. Using commercially available optical low coherence interferometry (the Lenstar LS 900), changes in eye length in the micrometre range can be picked up already after 30 min when young human subjects watch movies that were digitally filtered. We found that the emmetropic retina can distinguish real optical positive defocus from simulated/calculated defocus. It triggered choroidal thickening in the first case, and thinning in the second. Strikingly, the myopic retina was not responsive, and the choroid became thinner in both cases. Longitudinal chromatic aberration (LCA) would be a perfect signal to tell the retina where the focal plane is. We simulated chromatic defocus in our movies. Either the red or the blue plane of the RGB format was spatially filtered to simulate chromatic defocus. When blue was ‘in focus’, eyes became longer, but when red was ‘in focus’ eyes became shorter. Again, myopic eyes did not respond, suggesting that the myopic retina no longer responded to LCA. Taken together, we found that the myopic retina had undergone functional changes such that no growth‐inhibiting signal was generated. As a consequence, eye growth would become open loop, explaining why myopia does not inhibit itself.

Low Coherence Interferometry Measurement: An Algorithm for fast processing with low noise and phase linearisation

This work proposes a signal processing algorithm to analyse the optical signal from a Low Coherence Interferometric (LCI) system. The system uses a Mach-Zehnder (MZ) interferometer to interrogate a Fabry-Perot cavity, working as an optical sensor. This algorithm is based on the correlation and convolution operations, which allows the signal to be reconstructed based on itself, as well as, on the linearization of the signal phase, allowing the non-linearities of the actuator incorporated on the MZ interferometer to be compensated. The results show a noise reduction of 30 dB in the signal acquired. As a result, a reduction of 8.2 dB in the uncertainty of the measurement of the physical measurand is achieved. It is also demonstrated that the phase linearization made it possible to obtain a coefficient of determination (namely, R-squared) higher than 0.999.

Dual-Parameter Correction Method for Dispersion Compensation in Polarized Low-Coherence Interferometry with High Accuracy

Dual-Parameter Correction Method for Dispersion Compensation in Polarized Low-Coherence Interferometry with High Accuracy

Geometric phase for investigation of nanostructures in approaches of polarization-sensitive optical coherence tomography

Proposed paper presents the latest results in the framework of polarization-sensitive low-coherence interferometry related to new approaches for using the geometric phase to reproduce the polarization structure of a biological transparent anisotropic micro (nano) object. The polarization parameters of an anisotropic object are measured in real time on the basis of a modified Mach-Zehnder interferometer. The advantage of using the geometric phase is the diagnostic of polarization anisotropic surface (subsurface) nanosized layers in a non-contact, non-invasive manner.

Optical tomography in a single camera frame using fringe-encoded deep-learning full-field OCT.

Optical coherence tomography is a valuable tool for in vivo examination thanks to its superior combination of axial resolution, field-of-view and working distance. OCT images are reconstructed from several phases that are obtained by modulation/multiplexing of light wavelength or optical path. This paper shows that only one phase (and one camera frame) is sufficient for en face tomography. The idea is to encode a high-frequency fringe patterns into the selected layer of the sample using low-coherence interferometry. These patterns can then be efficiently extracted with a high-pass filter enhanced via deep learning networks to create the tomographic full-field OCT view. This brings 10-fold improvement in imaging speed, considerably reducing the phase errors and incoherent light artifacts related to in vivo movements. Moreover, this work opens a path for low-cost tomography with slow consumer cameras. Optically, the device resembles the conventional time-domain full-field OCT without incurring additional costs or a field-of-view/resolution reduction. The approach is validated by imaging in vivo cornea in human subjects. Open-source and easy-to-follow codes for data generation/training/inference with U-Net/Pix2Pix networks are provided to be used in a variety of image-to-image translation tasks.

Use of Optical Coherence Tomography in Dentistry.

Optical coherence tomography (OCT) is an optics-based imaging technique, which may be called an "optical biopsy." It can be used to acquire structural information about a tissue at a resolution comparable to histopathology. OCT is based on the principle of low-coherence interferometry where near-infrared (NIR) light is shown on a tissue sample and then cross-sectional images are obtained based on backscattered light and echo time delay. Two main types of OCT are characterized as time-domain OCT (TD-OCT) and Fourier-domain OCT (FD-OCT). The applications of OCT in dentistry can be broadly divided into two groups, i.e., assessment of pathologies and assessment of surfaces and interfaces. Lately, OCT has made its transition from experimental laboratories to mainstream clinical applications. Starting from the short-term training courses, clinicians working in specialities like oral pathology, oral medicine, and oral implantology may find it a useful tool for their practices. It is now clear that OCT will be considered a gold standard diagnostic tool for the detection and characterization of several conditions and lesions of the orofacial region. However, the next challenge will be to incorporate it into the undergraduate and postgraduate curriculum and train dental healthcare staff in the use of these devices.

Order Jump Correction Method Based on Dispersion Deviation Compensation for Polarization Low-Coherence Interferometry

Order Jump Correction Method Based on Dispersion Deviation Compensation for Polarization Low-Coherence Interferometry

Application of fiber optic sensors using Machine Learning algorithms for temperature measurement of lithium-ion batteries

Optical fiber sensors using low-coherence interferometry require processing of the output spectrum or interferogram to determine the instantaneous value of the measured quantity, such as temperature, quickly and accurately. Methods based on machine learning are a good candidate for this application. The application of four such methods in an optical fiber sensor of temperature is demonstrated. Using a ZnO-coated sensing interferometer and spectral detection, the sensor is intended for monitoring lithium-ion rechargeable batteries. While the performance of all methods was good, some of them seem to be better suited for this application. Full Text: PDF References B. Wrålsen et al., "Circular business models for lithium-ion batteries - Stakeholders, barriers, and drivers", J. Clean. Prod. 317, 128393 (2021), CrossRef T. Deng et al., "Measuring smartphone usage and task switching with log tracking and self-reports", Mobile Media & Communication 7, 3 (2019), CrossRef Smartphone Subscriptions Worldwide 2027. Statista, DirectLink T. Jerzyński, G. V. Stimson, H. Shapiro, G. Król., "Estimation of the global number of e-cigarette users in 2020", Harm Reduct. J. 18, 109 (2021). CrossRef Battery chemistries, DirectLink Y. Chen, et al., "A review of lithium-ion battery safety concerns: The issues, strategies, and testing standards", J. Energy Chem 59, 83 (2021), CrossRef T. Görgülü, M. Torun, A. Olgun, "A cause of severe thigh injury: Battery explosion", Ann. Med. Surg. 5, 49 (2016) CrossRef T. Maraqa et al., "Too Hot for Your Pocket! Burns From E-Cigarette Lithium Battery Explosions: A Case Series", J. Burn Care Res. 39, 1043 (2018). CrossRef Y. Chen et al., "A review of lithium-ion battery safety concerns: The issues, strategies, and testing standards", J. Energy Chem. 59, 83 (2021), CrossRef Q. Wang et al., "Thermal runaway caused fire and explosion of lithium ion battery", J. Power Sources 208, 210 (2012), CrossRef P. V. Chombo, L. Yossapong, "A review of safety strategies of a Li-ion battery", J. Power Sources 478, 228649 (2020), CrossRef P. Listewnik, M. Bechelany, J. B. Jasinski, M. Szczerska, "ZnO ALD-Coated Microsphere-Based Sensors for Temperature Measurements", Sensors 20, 4689 (2020), CrossRef M. Szczerska, "Temperature Sensors Based on Polymer Fiber Optic Interferometer", Chemosensors 10, 228 (2022), CrossRef M. Kruczkowski et al., "redictions of cervical cancer identification by photonic method combined with machine learning", Sci. Rep. 12, 3762 (2022), CrossRef