Back-reflected Light Research Articles

Polarimeters for the Detection of Anisotropy from Reflectance.

Polarimetry is used to determine the Stokes parameters of a laser beam. Once all four S0,1,2,3 parameters are determined, the state of polarisation is established. Upon reflection of a laser beam with the defined S polarisation state, the directly measured S parameters can be used to determine the optical properties of the surface, which modify the S-state upon reflection. Here, we use polarimetry for the determination of surface anisotropies related to the birefringence and dichroism of different materials, which have a common feature of linear patterns with different alignments and scales. It is shown that polarimetry in the back-reflected light is complementary to ellipsometry and four-polarisation camera imaging; experiments were carried out using a microscope.

Preserving polarization maintaining photons for enhanced contrast imaging of the retina.

The purpose of this study is to demonstrate the feasibility of using polarization maintaining photons for enhanced contrast imaging of the retina. Orthogonal-polarization control has been frequently used in conventional fundus imaging systems to minimize reflection artifacts. However, the orthogonal-polarization configuration also rejects the directly reflected photons, which preserve the polarization condition of incident light, from the superficial layer of the fundus, i.e., the retina, and thus reduce the contrast of retinal imaging. We report here a portable fundus camera which can simultaneously perform orthogonal-polarization control to reject back-reflected light from the ophthalmic lens and parallel-polarization control to preserve the backscattered light from the retina which partially maintains the polarization state of the incoming light. This portable device utilizes miniaturized indirect ophthalmoscopy illumination to achieve non-mydriatic imaging, with a snapshot field of view of 101° eye-angle (67° visual-angle). Comparative analysis of retinal images acquired with a traditional orthogonal-polarization fundus camera from both normal and diseased eyes was conducted to validate the usefulness of the proposed design. The parallel-polarization control for enhanced contrast in high dynamic range imaging has also been validated.

Application of diffuse optical back reflection spectroscopy for determining articular cartilage thickness in a clinical setting.

Develop a spectroscopic method to assess cartilage thickness during the arthroscopic examination. Currently, arthroscopy assesses cartilage damage visually; outcomes are based on the surgeon's subjective experience. Light reflection spectroscopy is a promising method for measuring cartilage thickness based on the absorption of light by the subchondral bone. In the presented study, in vivo diffuse optical back reflection spectroscopic measurements were acquired by gently placing an optical fibre probe on different locations of the articular cartilage of 50 patients during complete knee replacement surgery. The optical fibre probe consists of two optical fibers with a diameter of 1mm to deliver the light and detect back-reflected light from the cartilage. Centre to centre distance between the source and the detectorfibers was 2.4mm. Actual thicknesses of the articular cartilage samples were measured under microscopy using histopathological staining. Using half of the samples in the patient data, a linear regression model was formed to estimate cartilage thicknesses from the spectroscopic measurements. The regression model was then used to predict the cartilage thickness in the second half of the data. The cartilage thickness was predicted with a mean error of 8.7% if the actual thickness was less than 2.5mm (R2 = 0.97). The outer diameter of the optical fibre probe was 3mm, which can fit into the arthroscopy channel and can be used to measure the cartilage thickness in real-time during the arthroscopic examination of the articular cartilage.

Improved Finite-Key Security Analysis of Measurement-Device-Independent Quantum Key Distribution Against a Trojan-Horse Attack

Measurement-device-independent quantum key distribution (MDI QKD) is a promising method for remote key sharing that can eliminate all detector side-channel attacks. However, current security proofs often overlook the potential information leakages from the legitimate users' devices. In a quantum version of Trojan-horse attack (THA), a malicious eavesdropper can inject bright light into the sources of a MDI QKD system and then analyze the back-reflected light to obtain their setting choices, thereby compromising the final security. Here, we derive the finite-key security bounds of decoy-state MDI QKD in the presence of THA, which significantly outperform previous analyses in terms of the secret-key rate and transmission distance. Specifically, we analyze a symmetric three-intensity decoy-state MDI QKD protocol and an efficient four-intensity decoy-state MDI QKD protocol. Our results represent a fundamental step in guaranteeing the implementation security of quantum communication systems.

Diagnosis of Testicular Torsion and Differentiation From Other Pathologies Using Near-Infrared Spectroscopy

ObjectiveTo develop a near-infrared (NIR) spectroscopy device to diagnose testicular torsion with high sensitivity and specificity. Specifically, we aim to investigate the differentiation between testicular torsion from other pathologies such as orchidoepididymitis, varicocele, and hydrocele. MethodsTwo LEDs with wavelengths of 660 nm and 940 nm were used as light sources in the device. Each wavelength was sent to the testicle successively, and a photodiode detected back-reflected diffuse light. The ratio of the light intensities of 660 nm and 940 nm was used as a diagnostic parameter. A multi-center clinical trial was performed in 5 different hospitals. ResultsIn total, 62 patients in urology clinics with acute testicular pain have been recruited for the study. The developed NIR spectroscopy correctly defined all 8 testicular torsion cases. Besides, 3 orchidoepididymitis, 1 varicocele, and 3 hydrocele cases were correctly distinguished from testicular torsion. Only 1 hydrocele case was misdiagnosed as torsion. The range of the ratio was between 0.14 and 1.16 overall measurements. The ratio varied between 0.14 and 0.3 for the testicle with torsion. The ratio was between 0.49 and 1.16 for the normal testicle and testicle with other pathologies mentioned above. ConclusionWe have chosen the threshold ratio of 0.4 to differentiate between the normal and torsion testis and diagnosed all the torsion cases among all normal and other pathologies. The developed optical device to diagnose testicular torsion is inexpensive, user-friendly, and works based on objective criteria with high sensitivity and specificity in real time.

Back-Reflection in Resonant Fiber Optic Gyroscope and the Suppression Method

Backscattering and back-reflection are one of the main factors restricting the improvement of resonant fiber optic gyroscope accuracy. At present, most researches focus on backscattering. Due to the dependence on narrow linewidth laser, the interference of back-reflection light and the main light produces fluctuations in light intensity, which deteriorates the angle random walk of the resonant fiber optic gyroscope. This paper focuses on the back-reflection light outside the fiber ring resonator, taking the reflection on the end face of the photoelectric detector as an example, and simulates the influence on the detection noise of resonant fiber optic gyroscope. According to the frequency spectrum of intensity noise caused by back-reflection, a suppression method including length matching and frequency separation is proposed. The simulation and experimental results show that this method can effectively reduce the influence of back-reflection, and the angle random walk is improved by 6.06 dB with this method.

Photonic integrated circuit-based fiber-optic temperature and strain sensing system.

A low-cost, multi-function fiber-optic sensing system is highly desirable for physical security monitoring. Using the silicon photonic integrated circuit technology, we propose and demonstrate a compact fiber-optic sensing system which can simultaneously measure the temperature and strain information. A key enabler of the proposed system is an on-chip optical interrogator consisting of a two-dimensional grating coupler, four microring resonators, and four on-chip photodetectors. The interrogator conveys the temperature and strain information via measuring the center wavelength of a fiber Bragg grating and the polarization state of back-reflected light through a single-mode fiber.

The Role of Neuroglobin in Retinal Hemodynamics and Metabolism: A Real-Time Study.

PurposeIn this study, we used broadband near-infrared spectroscopy, a non-invasive optical technique, to investigate in real time the possible role of neuroglobin in retinal hemodynamics and metabolism.MethodsRetinae of 12 C57 mice (seven young and five old) and seven young neuroglobin knockouts (Ngb-KOs) were exposed to light from a low-power halogen source, and the back-reflected light was used to calculate changes in the concentration of oxygenated hemoglobin (HbO2), deoxygenated hemoglobin (HHb), and oxidized cytochrome c oxidase (oxCCO).ResultsThe degree of change in the near-infrared spectroscopy signals associated with HHb, HbO2, and oxCCO was significantly greater in young C57 mice compared to the old C57 mice (P < 0.05) and the Ngb-KO model (P < 0.005).ConclusionsOur results reveal a possible role of Ngb in regulating retinal function, as its absence in the retinae of a knockout mouse model led to suppressed signals that are associated with hemodynamics and oxidative metabolism.Translational RelevanceNear-infrared spectroscopy enabled the non-invasive detection of characteristic signals that differentiate between the retina of a neuroglobin knockout mouse model and that of a wild-type model. Further work is needed to evaluate the source of the signal differences and how these differences relate to the presence or absence of neuroglobin in the ganglion, bipolar, or amacrine cells of the retina.

Improved finite-key security analysis of quantum key distribution against Trojan-horse attacks

Most security proofs of quantum key distribution (QKD) disregard the effect of information leakage from the users’ devices, and, thus, do not protect against Trojan-horse attacks (THAs). In a THA, the eavesdropper injects strong light into the QKD apparatuses, and then analyzes the back-reflected light to learn information about their internal setting choices. Only a few recent works consider this security threat, but predict a rather poor performance of QKD unless the devices are strongly isolated from the channel. Here, we derive finite-key security bounds for decoy-state-based QKD schemes in the presence of THAs, which significantly outperform previous analyses. Our results constitute an important step forward to closing the existing gap between theory and practice in QKD.

A Review of Indirect Time-of-Flight Technologies

Indirect time-of-flight (iToF) cameras operate by illuminating a scene with modulated light and inferring depth at each pixel by combining the back-reflected light with different gating signals. This article focuses on amplitude-modulated continuous-wave (AMCW) time-of-flight (ToF), which, because of its robustness and stability properties, is the most common form of iToF. The figures of merit that drive iToF performance are explained and plotted, and system parameters that drive a camera’s final performance are summarized. Different iToF pixel and chip architectures are compared and the basic phasor methods for extracting depth from the pixel output values are explained. The evolution of pixel size is discussed, showing performance improvement over time. Depth pipelines, which play a key role in filtering and enhancing data, have also greatly improved over time with sophisticated denoising methods now available. Key remaining challenges, such as ambient light resilience and multipath invariance, are explained, and state-of-the-art mitigation techniques are referenced. Finally, applications, use cases, and benefits of iToF are listed.

Analysis of peripheral arterial disease (PAD) patients by laser speckle measurement techniques.

Diabetic foot is a well-known problem among patients suffering from peripheral arterial diseases (PAD). This article presents an optical sensor for contactless measurement of the anatomical site based on laser speckle techniques. The sensor illuminates the inspected tissue and analyzes the captured back-reflected light from the time-changing speckle patterns. An occlusion test was implemented to provide a statistical parameter to differentiate between a low perfused and a healthy foot. A clinical study of 15 subjects was conducted. The video was analyzed by two methods: dynamic laser speckle (DLS) and laser speckle contrast analysis (LASCA). Data analysis included several classification models, where the KNN model exhibited maximum performance. These findings suggest that a simple and inexpensive system for PAD monitoring can be designed for home use and/or in community clinics.

TE-mode magneto-optical isolator based on an asymmetric microring resonator under a unidirectional magnetic field.

Optical isolators are necessary components in photonic integrated circuits to ensure system stability by blocking the back-reflected light waves. In this study, a transverse electric mode magneto-optical isolator based on an asymmetric microring resonator without additional polarization rotators in series is demonstrated. Cerium-substituted yttrium iron garnet was integrated with silicon-on-insulator substrates by wafer bonding to break the Lorentz reciprocity on-chip after applying a unidirectional magnetic field. The isolator shows a 22 dB isolation ratio and a 4.3 dB insertion loss at an operation wavelength of 1572.62 nm.

Twice-Modulated Light in Fourier Transform Infrared (FT-IR) Spectrometers from Reflective Samples: Avoiding Distorted Intensity Values.

We document an artifact associated with the back reflection from samples or sampling accessories in Fourier transform spectrometers. Samples oriented normal to the incoming modulated beam can reflect light back to the interferometer and this light (the percentage dependent on the sample's refractive index) is modulated by the interferometer a second time resulting in light erroneously appearing at twice its true frequency. The phenomenon occurs across the spectrum but is most apparent when positive-going narrow absorption peaks at 1f display as negative-going peaks at 2f. We have redressed the artifact by implementing a rotation stage directly beneath the sample holder: As the stage is rotated through small angles relative to beam normal, the back-reflected light does not enter the interferometer and the artifact disappears. The observation is relevant to several IR sampling methods: gas/liquid cells, alkali halide pellets, reflectance accessories, etc.

Types of Optical Coherence Tomography for Cancer Diagnosis: A Systematic Review

Optical coherence tomography (OCT) is an emerging imaging technique that produces high contrast images that help distinguish different tissue layers by detecting the back-reflected near-infrared light. The technique is used to diagnose various diseases due to high contrast, three-dimensional imaging capability with high resolution, and a fast acquisition speed. The meta-analysis study was performed by the systematic review of literature in PubMed, Web of Science, Scopus, Google Scholar, Embase, and Cochrane Library using search terms relevant to “OCT”, "Carcinoma", and “cancers”. The various applications of different types of OCT are discussed in the detection and diagnosis of various cancers like colorectal cancer, breast cancer, skin cancer, brain cancer, prostate cancer, ovarian cancer and lung cancer.

A Fibre-Optic Platform for Sensing Nitrate Using Conducting Polymers.

Monitoring nitrate ions is essential in agriculture, food industry, health sector and aquatic ecosystem. We show that a conducting polymer, poly(3,4-ethylenedioxythiophene) (PEDOT), can be used for nitrate sensing through a process in which nitrate ion uptake leads to oxidation of PEDOT and change of its optical properties. In this study, a new platform is developed in which a single-mode fibre coated at the tip with PEDOT is used for nitrate sensing. A crucial step towards this goal is introduction of carbonate exposure to chemically reduced PEDOT to a baseline value. The proposed platform exhibits the change in optical behaviour of the PEDOT layer at the tip of the fibre as it undergoes chemical oxidation and reduction (redox). The change in optical properties due to redox switching varies with the intensity of light back reflected by the fibre coated with PEDOT. The proposed platform during oxidation demonstrates linear response for the uptake of nitrate ions in concentrations ranging between 0.2 and 40 parts per million (ppm), with a regression coefficient and a detection limit of 6.7 ppm. The procedure for redox switching is repeatable as the back reflection light intensity reaches of the initial value after reduction.

Fiber laser welding of AA 5754 in the double lap-joint configuration: process development, mechanical characterization, and monitoring

Aluminum alloys are widely spread in many industrial sectors due to their desirable characteristics as low density, good formability, high specific strength, and good resistance to corrosion. Autogenous laser welding is a technology that enables the use of these materials in the industrial process due to its high repeatability, reliability, and ease of automatization. In particular, in automotive applications, Al-alloys are welded in lap-joint configurations with more than 2 layers of material. The welding condition should be monitored in order to detect the complete penetration, hence guaranteeing the appropriate weld resistance. The use of non-invasive and coaxial monitoring solutions is highly desirable for the identification of weld defects during the process. This study investigates an autogenous laser welding process and monitoring in the double lap-joint configuration of sheets of AA 5754. First, the process parameters are investigated to identify the geometrical and mechanical characteristics of the resultant welding seams at different process conditions. The employed high-brilliance 3 kW fiber laser provided the possibility of reading the back-reflected light signal from an internal photodiode. The capability of this signal to be used as a non-invasive, coaxial, and remote monitoring system in order to predict the process outcome was tested. In the experiments the back-reflected light intensity could be correlated to the weld seam width at the second interface, as well as the strength of the joint to shear. Finally, the monitoring signal behavior was demonstrated under simulated weld defect conditions. The results show that weld anomalies such as lack of penetration, misalignment, and gap formation can be sensed through the monitoring approach.

Coherent Control of Light for Non-Line-of-Sight Imaging

Non-line-of-sight (NLOS) imaging relies on collecting light that is rendered incoherent from the multiple scattering events and is then post-processed to provide an estimate of the hidden scene. Here we employ coherent phase control of the outgoing laser beam phase front so as to refocus the beam behind the obscuring obstacle and then use the speckle memory effect to scan the focused spot across the scene. The back-reflected light intensity provides a direct measurement of the scene with a signal-to-noise ratio that is greatly improved when measured using a temporally gated detector. A spatial resolution of less than 1 mm is demonstrated, opening the way to high-resolution NLOS imaging.

Nonlinear Distortion Reduction Effect of Graded-Index Plastic Optical Fiber

We experimentally demonstrate that a graded-index plastic optical fiber (GI POF) can significantly reduce transmitted signal distortion caused by the nonlinear response of a multimode fiber (MMF) link based on a vertical-cavity surface-emitting laser (VCSEL). The low distortion of the signal transmission is attributed to the strong mode coupling in the GI POF, which suppresses the increment in signal distortion due to the external optical feedback, by reducing the self-coupling of problematic back-reflected light into the VCSEL cavity. The strong mode coupling is closely associated with polymer-specific microscopic heterogeneous structures in the fiber core materials, suggesting that the signal distortion can be further reduced by controlling the microscopic properties. The nonlinear distortion reduction effect of our developed GI POF will be invaluable for next-generation MMF links based on multilevel modulation in the ultrahigh-definition era.

Accuracy Improvement of Blood Glucose Measurement System Using Quantum Cascade Lasers

For non-invasive measurement of blood glucose levels, a measurement system based on mid-infrared, attenuated-total-reflection spectroscopy equipped with hollow optical fibers, a trapezoidal multi-reflection prism, and two fixed-wavelength quantum cascade lasers emitting different wavelengths is proposed. From the absorption spectra of lip mucosa measured by Fourier-transform infrared spectrometry, two wavelengths, 1152 cm-1 for absorption by glucose and 1186 cm-1 for the background, were chosen. To reduce measurement errors, the power distribution on the prism surface was investigated, and it was found that some high-intensity spots appear on the prism surface due to the coherency of the laser beam. This inhomogeneous power distribution causes measurement errors for slight movements of the lip mucosa. To homogenize the intensity distribution on the prism, a lens to excite higher modes in the fiber was introduced, and the incident angle was changed to suppress interference due to back-reflected light. These improvements increased the measurement stability, and in-vivo experiments demonstrated that the measured optical absorption correlates well with blood glucose levels.

Use of rare-earth elements to achieve wavelength-selective absorption in high-power fibre lasers

We have studied absorption spectra of optical fibres doped with rare-earth ions (Sm3+, Tm3+, Tb3+, Pr3+ and Ho3+) and demonstrated that they are potentially attractive for ensuring wavelength-selective absorption in fibre lasers. Such fibres can be used for pump diode protection against back-reflected light at the operating wavelength of neodymium, erbium, erbium – ytterbium or thulium lasers. A proper choice of a rare-earth element makes it possible to ensure a strong absorption (10 – 20 dB) at the operating laser wavelength in combination with high transmission (loss under a few tenths of a decibel) at pump wavelengths. To demonstrate the potential of the proposed approach, we have fabricated and investigated Tm3+-doped fibre compatible with the output fibre pigtails of standard semiconductor diodes used for pumping (core diameter, 105 μm; numerical aperture, 0.22). We have demonstrated the possibility of effectively suppressing light in the 1550-nm range by more than 20 dB even in the case of a high-power (up to 10 W) undesirable signal. Under such conditions, the total pump loss does not exceed 0.5 dB and can be further reduced by optimising the core composition and fibre design.