Laser Light Propagation Research Articles

Modeling pulsed dye laser treatment of psoriatic plaques by combining numerical methods and image-derived lesion morphologies.

Knowledge of the physical effects of pulsed dye laser (PDL) treatment of psoriatic lesions is essential in unraveling the remedial mechanisms of this treatment and hence also in maximizing in its disease-modifying potential. Therefore, the main objective of this study was to provide estimates of these physical effects (for laser wavelengths of 585and 595 nm), with the aim of identifying pathogenic processes that may be affected by these conditions. We modeled the laser light propagation and subsequent photothermal heating by numerically solving the transient diffusion and heat equations simultaneously. To this end, we used the finite element method in conjunction with an image-derived psoriatic lesion morphology (which was defined by segmenting blood vessels from a confocal microscopy image of a fluorescently labeled section of a 3 mm punch biopsy of a psoriatic lesion). The resulting predictions of the generated temperature field within the lesion were then used to assess the possibility of stalling or arresting some suspected pathogenic processes. According to our results, it is conceivable that perivascular nerves are thermally denatured, as almost all locations that reach 60°C were found to be within 18 µm (at 585 nm) and 11 µm (at 595 nm) of a blood vessel wall. Furthermore, activation of TRPV1 and TRPV2 channels in perivascular neuronal and immune cells is highly likely, since a critical temperature of 43°C is generated at locations within up to 350 µm of a vessel wall (at both wavelengths) and sustained for up to 700 ms (at 585 nm) and 40 ms (at 595 nm), while a critical temperature of 52°C is reached by locations within 80 µm (at 585 nm) and 30 µm (at 595 nm) of a vessel wall and sustained for up to 100 ms (at 585 nm) and 30 ms (at 595 nm). Finally, we found that the blood vessel coagulation-inducing temperature of 70°C is sustained in the vascular epithelium for up to 19and 5 ms at 585and 595 nm, respectively, rendering partial or total loss of vascular functionality a distinct possibility. The presented approach constitutes a useful tool to provide realistic estimates of the photothermal effects of PDLtreatment of psoriatic plaques (as well as other selective photothermolysis-based treatments), yielding information that is essential in guiding future experimental studies towardunraveling the remedial mechanisms of these treatments.

Correlation of signal feature importance and process parameters in water jet-guided laser cutting

Laser-based cutting can deliver high accuracy and speed during the process. However, there are still some technical challenges to be solved. First, the process itself requires precise focusing of the laser beam. When using continuous wave (CW)- or pulses in the μs- to ns-range for efficient materials removal, not only the targeted material volume is heated, but also the surrounding material due to heat diffusion. This decreases the quality of the cut. Coupling the light into a water-jet solves both issues: It ensures the delivery of focused light to the target surface, while cooling the affected material during processing. This work proposes a real-time monitoring system of the cutting process by exploiting unique physical phenomena related to the process. The water jet allows both the propagation of laser light towards the target while keeping it focused and the back-propagation of process light through the focusing system. This can be captured using suitable sensors, and saved using a data acquisition system. The acquired data can be used for process characterization using signal processing. In this work we demonstrate the direct correlation between different process parameters and statistical signal features. We show that reducing the signal to a few statistical features both in time and frequency by feature extraction does not reduce the information content, but instead makes it more robust to mis-classification while decreasing the classification time. It also opens up a wide range of future applications not only to process data, but also to control the process more precise.

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In order to deliver safe and effective transdermal deep tissue light therapy (tDTLT), the study simulates laser light propagation in an anatomical human knee model to assess the light transmittance and absorption‐driven thermal changes for various commonly used near‐infrared wavelengths (600‐1200nm) at multiple skin‐applied irradiances. A robust methodology and guidance for investigating high‐power laser therapy dosing prior to clinical research are provided, ensuring the safe and effective delivery of tDTLT.For further details please visit the article by Chironjeet Chaki, Luis De Taboada, and Kwong Ming Tse (e202200283).image

Three-dimensional irradiance and temperature distributions resulting from transdermal application of laser light to human knee-A numerical approach.

The use of light for therapeutic applications requires light-absorption by cellular chromophores at the target tissues and the subsequent photobiomodulation (PBM) of cellular biochemical processes. For transdermal deep tissue light therapy (tDTLT) to be clinically effective, a sufficiently large number of photons must reach and be absorbed at the targeted deep tissue sites. Thus, delivering safe and effective tDTLT requires understanding the physics of light propagation in tissue. This study simulates laser light propagation in an anatomically accurate human knee model to assess the light transmittance and light absorption-driven thermal changes for eight commonly used laser therapy wavelengths (600-1200 nm) at multiple skin-applied irradiances (W cm-2 ) with continuous wave (CW) exposures. It shows that of the simulated parameters, 2.38 W cm-2 (30 W, 20 mm beam radius) of 1064 nm light generated the least tissue heating -4°C at skin surface, after 30 s of CW irradiation, and the highest overall transmission-approximately 3%, to the innermost muscle tissue.

Laser light propagation in a turbid medium: solution including multiple scattering effects

We have shown that solutions to the radiative transfer equation for a homogeneous slab yield a zenith radiance reflectance that for collimated beam incidence in the nadir direction can be expressed in terms of the lidar ratio, defined as the extinction coefficient divided by the 180^circ backscattering coefficient. The recently developed QlblC method, which allows one to quantify layer-by-layer contributions to radiances emerging from a slab illuminated with a collimated beam of radiation, was used to show explicitly that in the single-scattering approximation the attenuated backscatter coefficient estimated by the new QlblC method gives the same result as the lidar equation. Originally developed for the continuous wave (CW) lidar problem, we have extended the new QlblC method to apply to the pulsed lidar problem. A specific example is provided to illustrate the challenge encountered for ocean property retrievals from space observations due to the fact that a very significant fraction of the signal is due to aerosol scattering/absorption; typically only about 10% (or less) comes from the ocean.Graphical abstract

Development of Novel Balloon-Integrated Optical Catheter for Endoscopic and Circumferential Laser Application.

The current study aims to demonstrate the feasibility of a novel balloon-integrated optical catheter (BIOC) to achieve endoscopic laser application for circumferential coagulation of a tubular tissue structure. Both optical and thermal numerical simulations were developed to predict the propagation of laser light and a spatio-temporal distribution of temperature in tissue. Ex vivo esophagus tissue was tested with 980nm laser light at 30 W for 90s for quantitative evaluations. In vivo porcine models were used to validate the performance of BIOC for circumferential and endoscopic laser coagulation of esophagus in terms of acute tissue responses post-irradiation. Optical simulations confirmed that a diffusing applicator was able to generate a circumferential light distribution in a tubular tissue structure. Both numerical and experimental results presented that the maximum temperature elevation occurred at 3-5mm (muscle layer) below the mucosa surface after 90 s irradiation. In vivo tests confirmed the circumferential delivery of laser light to a deep muscle layer as well as no evidence of thermal damage to the esophageal mucosa. The proposed BIOC can be a feasible optical device to provide circumferential laser irradiation as well as endoscopic coagulation of tubular esophagus tissue for clinical applications.

Upconversion phosphor thermometry for use in thermal barrier coatings

Measuring the temperature below the surface of a thermal barrier coating (TBC) using a thin phosphor layer is challenging primarily due to the absorption and scattering of laser excitation light and phosphor luminescence as they propagate through the coating. One way to increase phosphor luminescence could be to use upconversion phosphor thermometry, which is investigated in the current study. It is attractive because using longer excitation wavelengths reduces the absorption and scattering in TBCs as 8% wt. yttria-stabilize zirconia (8YSZ) generally has lower scattering and absorption coefficients around 1000 nm than at 532 and 355 nm. Therefore, the viability of upconversion to measure the temperature at the bottom of a TBC was evaluated for the first time and was compared with the more conventional downconversion phosphor thermometry. The current work involved an experimental study of several phosphors with lanthanides doped in the 8YSZ host, which were excited through downconversion by pulsed 355 nm and 532 nm laser light and through upconversion with 965 nm laser light. The YSZ:Er,Yb and YSZ:Ho,Yb phosphors show promise for upconversion phosphor thermometry. The experimentally acquired optical phosphor characteristics were used to simulate laser light and phosphor luminescence propagation in TBCs using Kubelka–Munk theory. This was to evaluate the signal strength with upconversion excitation compared to downconversion excitation. Upconversion excitation resulted greater signal strength from an embedded phosphor layer than 532 nm excitation and much higher than 355 nm excitation. Upconversion lifetime phosphor thermometry also resulted in improved phosphor lifetime temperature sensitivity. Coupled with reduced interference from background luminescence from impurities in TBCs with upconversion, it is a promising method for temperature measurements with the thermographic phosphor embedded in or underneath a TBC.

Estimation of laser light propagation in biological tissue during laser-tissue bonding using Monte Carlo simulation

Monte Carlo simulations are an elementary approach towards modeling light propagation in tissues. The detection of subsurface temperature in tissues during laser mediated therapies and microsurgeries is crucial for estimating associated thermal damage. MC simulation provides with the possibility to optimize the process. The approach has been used to model light propagation, associated energy deposition, and the temperature distribution inside the tissue. Understanding the extent of laser light transmittance and heat distribution within the tissue is crucial for minimizing damage to the adjacent biological tissues. The total photon weight absorbed estimates the total heat distribution within the volume. A three-layer heterogeneous tissue was specified consisting of only the epidermis, dermis, and the subcutaneous fat tissue. The hop, drop, and spin trail of photons depends on the optical properties of these layers. The energy deposition and temperature distribution estimation are obtained by the MC simulation method. A real-time measurement of the temperature profile was also performed. The experimental results were in close congruence with the simulation result. The simulation results show good reproducibility of the real temperature distribution. Monte Carlo method can, thereby, be used in estimation and optimization laser induced processes.

Sensing and coupling of optical channels in dynamic atmospheric turbulence using OAM beamlets for improved power and data transmission.

Propagation of laser light is distorted in the presence of atmospheric turbulence. This poses an issue for sensing, free-space optical communications, and transmission of power. The presented system offers a novel solution to mitigate the effects of turbulence. By rapidly probing a turbulent volume by varying a beam's spatial and phase characteristics, the best transmission mode can be determined and updated in real time. Unlike a traditional tip-tilt system, this scheme is fully electronic, and has a scalable architecture to leverage multiple optical transmission paths simultaneously. This optical control system greatly improves power efficiency and successful recovery of data through environments with strong turbulence.

Deriving Snow Depth From ICESat-2 Lidar Multiple Scattering Measurements

Snow is a crucial element in the Earth’s system, but snow depth and mass are very challenging to be measured globally. Here, we provide the theoretical foundation for deriving snow depth directly from space-borne lidar (ICESat-2) snow multiple scattering measurements for the first time. First, based on the Monte Carlo lidar radiative transfer simulations of ICESat-2 measurements of 532-nm laser light propagation in snow, we find that the lidar backscattering path length follows Gamma distribution. Next, we derive three simple analytical equations to compute snow depth from the average, second-, and third-order moments of the distribution. As a preliminary application, these relations are then used to retrieve snow depth over the Antarctic ice sheet and the Arctic sea ice using the ICESat-2 lidar multiple scattering measurements. The robustness of this snow depth technique is demonstrated by the agreement of snow depth computed from the three derived relations using both modeled data and ICESat-2 observations.

Light bullets in transparent dielectrics

This paper presents a retrospective analysis of the development of notions in nonlinear optics: from beam self-focusing and pulse filamentation to light bullets – wave packets extremely compressed in space and time during laser light propagation in the bulk of a transparent medium. We describe the state of the art in studies of mid-IR light bullets in condensed media and air.

Examining the Effects of Fluid Velocity Gradients on 4D Digital Holographic PIV/PTV Measurements

4D digital holographic PIV/PTV (4D-DHPIV/PTV) methods have demonstrated theoretical viability due to their relative ease of setup and high spatial resolution (Soria (2018)). This study investigates how velocity gradients related to different flow regimes and their magnitudes affect 3-component–3-dimensional (3C-3D) digital holographic PIV measurement uncertainty. The error introduced by velocity gradients within the interrogation volume is studied by simulating particles in a velocity field, with a given constant velocity gradient superimposed on a uniform flow from which a time-series of hologram pairs are generated and the 3C-3D velocity fields and their errors are determined using 4D-DHPIV/PTV Sun et al. (2020). Hologram pairs are simulated by modelling the propagation and particlediffraction of coherent laser light using the angular spectrum method (Goodman (1996)). The hologram reconstruction then involves direct reconstruction, followed by deconvolution, a particle position refinement and a hologram subtraction step (Sun et al. (2020)). The particle positions obtained from 4D-DHPIV/PTV are then used to resolve particle displacement measurements using 3D cross-correlation digital analysis with a 3D Gaussian fit to sub-pixel resolution (Soria (2006)). The effects of velocity gradients on the displacement uncertainty and bias error have been investigated by undertaking Monte Carlo simulations under a range of velocity gradient environments. Specifically, 5 common velocity gradients have been studied, which included pure strain, pure vorticity and x, y and z-directional shear. The results indicate that the novel 4D-DHPIV/PTV has poorer accuracy and precision in the z-propagation axis, resulting in larger minimum uncertainties and bias errors. The errors in the z axis are also significantly less affected by velocity gradients in the z direction when compared to the effects of x and y directional velocity gradients on x and y errors respectively. Furthermore, the rate of cross-correlation maximum and SNR decrease are approximately 1.36 times slower due to velocity gradients in the z axis than other axes.

Light propagation in a three-dimensional Rydberg gas with a nonlocal optical response.

We theoretically investigate the linear susceptibility and propagation of light in a three-dimensional (3-D) Rydberg gas under conditions of electromagnetically induced transparency. Rydberg atoms with two relevant S states are coupled via exchange interactions. When the gas is initially prepared in an entangled spin-wave state, this coupling induces a strong, nonlocal susceptibility whereby the photon field at one point of the medium acts as a source at a distant position. The nonlocal propagation occurs not only in the propagation direction but also in the paraxial direction. We discuss the absorption features and numerically simulate the 3-D propagation of probe laser light. Combined with the long-range exchange interaction, we show that the 3-D Rydberg gas is an ideal medium for studying nonlocal wave phenomena, in which the strength, range, and sign of the nonlocal interaction kernel can be widely tuned.

Monte Carlo Simulations of the photo-thermal cancer therapy of melanin

In this study, we performed Monte Carlo calculations to investigate photo-thermal effects on melanoma therapy. For this purpose, we simulated the light propagation in biological tissue. The propagation of laser light originates from a source point inside a single layer of light skin with a concentration of melanosomes $$(f_{\text{mel}} = 3\% )$$ . This single layer includes the epidermal with a thickness of 60 $$\upmu{\text{m}}.$$ Various important wavelengths are applied for photo-thermal therapy (PTT), such as Alexandrite laser, Ruby laser, and Nd:YAG, respectively. Accordingly, a single layer of skin is exposed to laser. It is demonstrated that the exposure to laser is a suitable option at a wavelength of $$\lambda =$$ 755 nm, which is useful to improve the therapeutic treatment compared to the lasers of wavelengths of $$\lambda =$$ 694 nm and $$\lambda =$$ 1064 nm, respectively. It is therefore possible to enhance the thermal treatment for light-skinned patients. The evaluation of temporal and spatial distributions is investigated versus the temperature, while the effects of relaxation time and thermal damage on the epidermal are involved.

Electromagnetic waves in an optical photonic lattice

Laser light propagation in a photonic lattice consisting of two parallel waveguide arrays is theoretically studied using the coupled mode method, with the interaction of each waveguide with the nearest neighbours and between the waveguides of the arrays being taken into account. Analytical expressions are obtained that make it possible to accurately predict the presence of localisation of light depending on the coupling constants. Particular solutions of a system of coupled waves are found, which describe strongly localised light propagating without transverse diffraction along the entire lattice at certain values of the coupling constants. The emergence of spatially limited transverse diffraction of light is predicted.

Numerical simulation of ophthalmic laser surgeries by a local thermal non-equilibrium two-temperature model

Purpose This paper aims to understand the laser–tissue interaction mechanism during ophthalmic laser surgeries through numerical analysis. The influence of laser parameters and the multipulse technique were investigated. Design/methodology/approach The ocular fundus was simplified as a multilayered homogenous medium model. Afterward, the multilayer Monte Carlo method was used to simulate the propagation and energy deposition of laser light, and a local thermal non-equilibrium two-temperature model was established to simulate the temperature variation of chromophores and surrounding tissue with different laser wavelength. Findings Through the model, the selective heating of chromophore (melanin and blood vessels) was clearly illustrated: 1) neglecting the laser energy absorbance by blood in the traditional model will cause significant errors in temperature calculation; 2) the non-thermal equilibrium heat transfer model was needed to obtain an accurate description of the thermal process when the dimensionless pulse width (tp*) is <105. For 532 nm Argon laser, the optimize tp* is around 105 and the appropriate energy density is 5 J/cm2; 3) multipulse technique makes the energy more concentrated within the melanin, thereby reducing the thermal damage in surrounding tissue, with most appropriate pulse number and duty cycle is 10 and 1/10. Originality/value Taking the blood absorption into account, the different temperature variations of melanin/vessels and surrounding tissue caused by the selective photo-thermolysis were simulated successfully. By understanding the mechanism of laser therapy, laser parameters and multipulse technique are suggested to improve the clinical results.

Magnetically and optically tunable terahertz radiation from Ta/NiFe/Pt spintronic nanolayers generated by femtosecond laser pulses

We generate terahertz (THz) transients by illuminating a few-nanometer-thick Ta/NiFe/Pt nanolayers with a train of linearly polarized 100-fs-wide laser pulses. The transients are ∼1-ps-wide free-space propagating bursts of electromagnetic radiations with amplitudes that are magnetically and optically tunable. Their spectral frequency content extends up to 5 THz, and the 3-dB cutoff is at 0.85 THz. The observed transient electromagnetic signals originate from the NiFe/Pt bilayer, and their amplitude dependence on the external magnetic field, applied in the sample plane, very closely follows the static magnetization versus magnetic field dependence of the NiFe film. For the same laser power, excitation with highly energetic, blue light generates THz transients with amplitudes approximately three times larger than the ones resulting from excitation by infrared light. In both cases, the transients exhibit the same spectral characteristics and are linearly polarized in the perpendicular direction to the sample magnetization. The polarization direction can be tuned by rotation of the magnetic field around the laser light propagation axis. The characteristics of our THz spintronic emitter signals confirm that THz transient generation is due to the inverse spin Hall effect in the Pt layer and demonstrate that ferromagnet/metal nanolayers excited by femtosecond laser pulses can serve as efficient sources of magnetically and optically tunable, polarized transient THz radiation.

Complex simplified Hirota’s forms and Lie symmetry analysis for multiple real and complex soliton solutions of the modified KdV–Sine-Gordon equation

The present work consists of detailed exploration of modified KdV–Sine-Gordon equation in integrable form, owning to two-component nonlinear channel for modeling laser light propagation. For validating the behavior of this equation in the sense of integrability, we use the Painleve test. The simplified Hirota’s technique with new complex forms is developed suitably to construct multiple-soliton solutions with complex structure for considered equation. Moreover, Lie symmetry analysis has been implemented for perceiving symmetries of MKdV–SG equation and then culminating the invariant solitary wave solutions. The new findings obviously reveal that simplified Hirota’s technique with complex structure would be highly proficient for fabricating new multiple complex soliton solutions to other nonlinear equations with integrable properties from mathematical physics and dynamical systems community.

Electromagnetically induced transparency in a spin-orbit-coupled finite-temperature cold atomic ensemble

We investigated the electromagnetically induced transparency in spin-orbit-coupled finite-temperature cold quantum gases. We showed that spin-orbit coupling can induce an additional frequency shift in the absorption line, which can be observed nondestructively from the electromagnetically induced transparency spectroscopy. As an alternative method this shift can be used to detect and measure the spin-orbit coupling strength. The spin-orbit coupling can also be used as a switch to control laser light propagation in this medium.

Light scintillation in soft biological tissues

ABSTRACT The applicability limits of the small complex phase perturbation (Rytov) theory for laser light propagation in soft biological tissues have been tackled. We have found that for typical coherent optical waves and for several types of optically soft biological tissues with known spatial power spectra, the weak regime of light-field fluctuations is limited to propagation distances of tens of microns. This result is in striking difference with oceanic or atmospheric propagation of light for which weak fluctuation regime may hold for tens of meters in the former case and kilometers in the latter. Furthermore, the scintillation indexes of plane, spherical, and Gaussian-beam waves are investigated in depth. Our results are envisioned to find applications in optical/infrared techniques for soft tissue diagnostics.