Supercontinuum Laser Research Articles

Active near‐infrared laser heterodyne system based on supercontinuum laser source for gas measurement

AbstractThe performance of an active near‐infrared laser heterodyne system using a supercontinuum light source as the light signal for trace species detection is experimentally demonstrated. The characteristics of the supercontinuum light source and the data processing methods used in heterodyne detection are described. The measurement of methane (CH₄) and carbon dioxide (CO₂) absorption spectra was carried out in the laboratory to evaluate the active laser heterodyne system, and high‐resolution absorption spectra of CO₂ and CH₄ were recorded simultaneously. The stability of the active laser heterodyne system is analyzed using Allan variance analysis of long‐term observation data. The active near‐infrared laser heterodyne detection system demonstrated in this paper has great potential for the development of industrial parks' gas monitoring.

750 nm laser based on an BaGa4Se7 optical parametric oscillator

The red edge effect of plants is extensively utilized in vegetation remote sensing, particularly by applying hyperspectral LiDAR (HSL) technology. This technology effectively captures spectral information from targets together with range measurements by processing recorded waveforms in the red-edge spectral bands. Despite its widespread use, there is still potential for enhancing the tuning accuracy and the energy output of each channel. What we believe to be a novel nonlinear crystal, BaGa4Se7 (BGSe), has been employed to achieve laser output in the red edge spectral band with a wide tuning range and high tuning precision for the first time. Successful generation of laser radiation at 1512 nm was achieved, with an angular tuning resolution of 35.9 nm/°. When the pump light energy was 17.81 mJ, the energy of the 1512 nm near-infrared laser was 3.210 mJ, with a slope efficiency of 31.2% and an optical-to-optical conversion efficiency (pump to signal) of 18.0%. Subsequent pumping of the second harmonic generation crystal KTiOPO4 (KTP) with the 1512 nm laser output from the BGSe optical parametric oscillator (OPO) facilitated the generation of 756 nm red light laser output. Angle tuning of the BGSe OPO eventually enabled the tunable output of the red edge spectral laser ranging from 701 nm to 780 nm with output energy of approximately 2 mJ, which is several orders of magnitude higher than traditional supercontinuum laser source solution. Such improvement becomes a solid cornerstone for long-range HSL applications.

Transportable hyperspectral imaging setup based on fast, high-density spectral scanning for in situ quantitative biochemical mapping of fresh tissue biopsies.

Histopathological examination of surgical biopsies, such as in glioma and glioblastoma resection, is hindered in current clinical practice by the long time required for the laboratory analysis and pathological screening, typically taking several days or even weeks to be completed. We propose here a transportable, high-density, spectral scanning-based hyperspectral imaging (HSI) setup, named HyperProbe1, that can provide in situ, fast biochemical analysis, and mapping of fresh surgical tissue samples, right after excision, and without the need for fixing, staining nor compromising the integrity of the tissue properties. HyperProbe1 is based on spectral scanning via supercontinuum laser illumination filtered with acousto-optic tunable filters. Such methodology allows the user to select any number and type of wavelength bands in the visible and near-infrared range between 510 and 900nm (up to a maximum of 79) and to reconstruct 3D hypercubes composed of high-resolution (4 to ), widefield images ( ) of the surgical samples, where each pixel is associated with a complete spectrum. The HyperProbe1 setup is here presented and characterized. The system is applied to 11 fresh surgical biopsies of glioma from routine patients, including different grades of tumor classification. Quantitative analysis of the composition of the tissue is performed via fast spectral unmixing to reconstruct the mapping of major biomarkers, such as oxy-( ) and deoxyhemoglobin (HHb), as well as cytochrome-c-oxidase (CCO). We also provided a preliminary attempt to infer tumor classification based on differences in composition in the samples, suggesting the possibility of using lipid content and differential CCO concentrations to distinguish between lower and higher-grade gliomas. A proof of concept of the performances of HyperProbe1 for quantitative, biochemical mapping of surgical biopsies is demonstrated, paving the way for improving current post-surgical, histopathological practice via non-destructive, in situ streamlined screening of fresh tissue samples in a matter of minutes after excision.

Dual-Wavelength LiDAR with a Single-Pixel Detector Based on the Time-Stretched Method.

In the fields of agriculture and forestry, the Normalized Difference Vegetation Index (NDVI) is a critical indicator for assessing the physiological state of plants. Traditional imaging sensors can only collect two-dimensional vegetation distribution data, while dual-wavelength LiDAR technology offers the capability to capture vertical distribution information, which is essential for forest structure recovery and precision agriculture management. However, existing LiDAR systems face challenges in detecting echoes at two wavelengths, typically relying on multiple detectors or array sensors, leading to high costs, bulky systems, and slow detection rates. This study introduces a time-stretched method to separate two laser wavelengths in the time dimension, enabling a more cost-effective and efficient dual-spectral (600 nm and 800 nm) LiDAR system. Utilizing a supercontinuum laser and a single-pixel detector, the system incorporates specifically designed time-stretched transmission optics, enhancing the efficiency of NDVI data collection. We validated the ranging performance of the system, achieving an accuracy of approximately 3 mm by collecting data with a high sampling rate oscilloscope. Furthermore, by detecting branches, soil, and leaves in various health conditions, we evaluated the system's performance. The dual-wavelength LiDAR can detect variations in NDVI due to differences in chlorophyll concentration and water content. Additionally, we used the radar equation to analyze the actual scene, clarifying the impact of the incidence angle on reflectance and NDVI. Scanning the Red Sumach, we obtained its NDVI distribution, demonstrating its physical characteristics. In conclusion, the proposed dual-wavelength LiDAR based on the time-stretched method has proven effective in agricultural and forestry applications, offering a new technological approach for future precision agriculture and forest management.

Exploring visible spectrum wavelengths in light transmission through wood material

Wood is a multiscale heterogeneous natural composite material with properties depending on its growing conditions and its genetic heritage. This variability is challenging for industries that work to perform homogeneous and reliable products. In industry, different non-destructive testing methods are in use to classify, grade, and select wood products to optimize their usage. Among them, the use of lasers to detect fiber orientation with different wavelengths. This orientation significantly influences the mechanical behavior of wood, including stress limits and stiffness. According to our knowledge, the use of laser diffusion still is limited to grain angle measurement. Our objective in this paper is to realize transmission light scattering maps for wood samples from several wood species (poplar, oak, Douglas fir, beech), and then identify the most suitable wavelength to study light diffusion in wood, depending on the property that will be measured. A supercontinuum laser is used over a wavelength range from 500 to 800 nm, allowing precise adjustment of the wavelengths. It was found that near-infrared light better scatters in the studied wood species than lower wavelength. However, the wavelength that gives the best contrast between earlywood and latewood depends on the sample studied and is not necessarily in the near infrared rays.

Supercontinuum laser-based gonio-scatterometer for in and out-of-plane spectral BRDF measurements.

The hyperspectral component of bidirectional reflectance measurements, namely from several hundred wavelengths upwards, is attracting growing interest for numerous applications in both optics and computer graphics. In this paper, we present a motorized hyperspectral bidirectional reflectance measurement bench that performs in-plane and out-of-plane measurements for isotropic materials using a supercontinuum laser covering the visible and near infrared range, with a sub-nanometer spectral accuracy. We describe the complete data processing chain, including a method for assessing the alignment error of the measurement bench. From these measurements, we verify the principles of non-negativity, energy conservation and Helmholtz reciprocity. We introduce criteria also to evaluate the validity of the Lambertian hypothesis for the bidirectional reflectance and its deviation from reciprocity, obtained from the measurements directly. We show the need for spectral bidirectional reflectance measurements for certain materials, rejecting the separable function approximation.

Mid-infrared tunable filter based on a femtosecond-written silica volume Bragg grating.

A silica volume Bragg grating (VBG) is used to filter the light of a mid-infrared (mid-IR) supercontinuum laser. The VBG with a 7 µm period was inscribed with 800 nm pulses with a 100 fs duration and the phase-mask technique over a glass thickness of 3 mm. Despite silica's absorption, the VBG allows obtaining a narrowband light source tunable from 2.9 to 4.2 µm with a full width at half maximum (FWHM) of 29 nm. This demonstrates the great potential of using femtosecond-written VBGs as highly tunable, yet selective, spectral filters in the mid-IR.

Advantages of internal reference in holographic shaping ps supercontinuum pulses through multimode optical fibers.

The use of wavefront shaping has found extensive application to develop ultra-thin endoscopic techniques based on multimode optical fibers (MMF), leveraging on the ability to control modal interference at the fiber's distal end. Although several techniques have been developed to achieve MMF-based laser-scanning imaging, the use of short laser pulses is still a challenging application. This is due to the intrinsic delay and temporal broadening introduced by the fiber itself, which requires additional compensation optics on the reference beam during the calibration procedure. Here we combine the use of a supercontinuum laser and an internal reference-based wavefront shaping system to produce focused spot scanning in multiple planes at the output of a step-index multimode fiber, without the requirement of a delay line or pulse pre-compensation. We benchmarked the performances of internal vs external reference during calibration, finding that the use of an internal reference grants better focusing efficiency. The system was characterized at different wavelengths, showcasing the wavelength resiliency of the different parameters. Lastly, the scanning of focal planes beyond the fiber facet was achieved by exploiting the chromato-axial memory effect.

Rapid non-destructive inspection of sub-surface defects in 3D printed alumina through 30 layers with 7 μm depth resolution

The use of additive manufacturing (AM) processes has grown rapidly over the last ten years like fused deposition modelling and stereolithography techniques. 3D printing offers advantages in ceramic component production due to its flexibility. To enhance quality and reduce resource consumption in ceramics industry, fast, integrated, sub-surface and non-destructive inspection (NDI) with high resolution is needed. This study demonstrates sub-surface monitoring of 3D printed alumina parts to a depth of ∼0.7 mm in images of 400 × 2048 pixels with a lateral resolution of 30 μm and axial resolution of 7 μm, using mid-infrared optical coherence tomography (MIR OCT) based on a 4 μm center wavelength MIR supercontinuum laser. We detected individual printed ceramic layers and tracked predefined defects through all four processing steps and demonstrated how a defect in the green phase could affect the final product. This research sets the stage for NDI integration in AM.

Fast and Wideband Optical Fiber Dispersion Measurement Using the Pulse Delay Method Based on Super-Continuum Laser

Fast and Wideband Optical Fiber Dispersion Measurement Using the Pulse Delay Method Based on Super-Continuum Laser

Compact and efficient high-power mid-infrared supercontinuum fiber laser source based on a noise-like pulse and germania fiber

Here, we demonstrate a compact and efficient high-power mid-infrared supercontinuum (MIR-SC) laser source based on a tunable noise-like pulse (NLP) fiber laser system and a short section of single-mode germania-core fiber (GCF). The NLP all-polarization-maintaining fiber laser system can deliver the maximum output power of ∼30.6 W and a broadband spectrum (∼1.8-2.7 µm) with a compact single-stage fiber amplifier. By directly pumping only ∼6.5 cm-long GCF with a core diameter of ∼3.5 µm, a MIR-SC (spectral coverage of ∼1.5-3.3 µm) with a maximum power of ∼25.2 W and a power conversion efficiency ∼81.2% is obtained, which represent the highest power and efficiency in any single-mode GCF-based MIR-SCs, to the best of our knowledge. Our study contributes to the high-power MIR-SC laser source with compact all-fiber configuration, and will prompt its practical applications.

Hyperspectral optical orbital angular momentum modulation from tunable structured waveplates.

Shaping the orbital angular momentum of optical pulses in the spectral domain is a means of managing light in space and time that offers many possible applications. However, these are limited by the small number of techniques available, whose flexibility does not yet rival that of the continuous regime. We propose here to implement a tunable hyperspectral management of the orbital angular momentum of a polychromatic light field. The main idea is to exploit the dispersive nature of geometric phase optical elements by intentionally choosing to work in a regime of high anisotropic optical retardance. An experimental proof of principle is demonstrated in the visible range using a supercontinuum laser and an optically thick, electrically controllable, liquid crystal structured wave plate.

Prototype development and evaluation of a hyperspectral lidar optical receiving system.

As a new type of active Earth observation technology, airborne hyperspectral lidar combines the advantages of traditional lidar 3D information acquisition and passive hyperspectral imaging technology, and it can achieve integrated imaging detection with a high spatial and hyperspectral resolution. Thus, it has become an important future direction of Earth surface remote sensing technology. This article introduces the design and development of an airborne hyperspectral imaging lidar system. The hyperspectral lidar adopts a focal plane splitting method, combined with an array of 168 optical fibers, to couple wide-spectral-range laser echo signals one by one to the corresponding single tube detector, achieving efficient splitting and precise coupling of supercontinuum laser pulse echo signals. This article proposes a fast synchronous calibration method that is suitable for hyperspectral imaging lidar systems. Results show that the spectral range of the hyperspectral lidar system is 400-900 nm, and the spectral resolution of single-fiber detection is greater than 3 nm. Notably, this article focuses on analyzing the abnormal detection channels based on the calibration results. With the test results of adjacent channels combined, the reason for the abnormal spectral bandwidth of channel 17 is analyzed as an example. This research points out the direction for verifying the design parameters of the hyperspectral lidar prototype and lays an important foundation for airborne flight test of the hyperspectral lidar.

CW mode-locked fiber laser by improved all-polarization-maintaining figure-9 cavity at 1064 nm

In order to realize broadband continuous wave (CW) mode locking by figure-9 fiber laser at 1064 nm, we propose an improved all-polarization-maintaining figure-9 cavity structure by adding gain fiber to both the loop and the linear arm in the laser. When the two sections of gain fiber are pumped simultaneously, the CW mode locking regime can be obtained. In this work, noise-like mode-locked pulses are generated with a central wavelength of 1064 nm and a chirped fiber grating (CFBG) is used for dispersion management. The net dispersion of the cavity is about 0.01 ps2 and the output power is 234 mW. The noise-like pulses are validated by autocorrelator, the peak duration on the autocorrelation trace pedestal is about 261 fs, and the pedestal duration is about 103 ps. Numerical simulations are performed on the noise-like pulses, which agree with the experimental results. This study has inspiration for mode-locked lasers with a central wavelength of 1064 nm, and has potential application value in fields such as supercontinuum generation, fiber sensing, laser microfabrication, and laser ablation.

Measuring spectral extinction with digital holography.

The optical extinction caused by a small particle, such as an aerosol particle, is an important measurable quantity. Understanding the influence of atmospheric aerosols on the climate, assessing visibility in urban environments, and remote sensing applications such as lidar all need accurate measurements of particle extinction. While multiple methods are known to measure extinction, digital in-line holography (DIH) features the unique ability to provide contact-free images of particles simultaneously with estimates for the extinction cross section. This is achieved through an integration of a measured hologram followed by an extrapolation. By means of a supercontinuum laser, we investigate the measurement of the cross section via DIH for stationary particles across a broad spectrum, from 440nm to 1040nm. The particles considered include a 50µm glass microsphere, a volcanic ash particle, and an iron(III) oxide particle. The results show the ability to estimate a particle's cross section to within 10% error across portions of the spectrum and approximately 20% error otherwise. An examination of the accompanying hologram-derived particle images reveals details in the images that evolve with wavelength. The behavior suggests a basic means to resolve whether absorption or scattering dominates a particle's extinction.

Microscopic transmittance spectroscopy using a supercontinuum laser source

A supercontinuum laser source emitted from an optical fiber is suitable for developing microscopic transmittance spectroscopy. The microscopic supercontinuum transmittance spectrum can be taken with a fiber probe or a reflective objective. The reflective objective is only suitable for measuring a very thin sample of a thickness less than 1 µm. Therefore, while measuring a relatively thick sample, the fiber probe is the better choice because it effectively simplifies the system without lowering the resolution. The spatial resolution of the microscopic transmittance spectrum as a function of the optical wavelength, measurement distance, and sample thickness is quantitatively evaluated for a fiber probe.

Poster Session II: The impact of eye's longitudinal chromatic aberration on visual acuity and accommodation response.

Chromatic composition of displays might affect vision and accommodation,possibly influencing myopia development.We investigated differences in visual acuity(VA) and accommodative lag(AccL) for steady accommodative demands (up to 5D,1D steps) with visual stimuli illuminated by monochromatic wavelengths (mono-λ 480,555&630nm,3nm bandwidth) and white light (WL).Data was obtained on 3 young emmetrope using an Adaptive Optics system with a supercontinuum laser,a DMD for stimuli,and a optunable lens to change vergence.Best focus for far was set at 555nm.VA was measured using QUEST(tumbling E).AccL was obtained from the peak shift of through-focus Visual Strehl,calculated from HS aberrometry.All subjects showed myopic shifts in blue consistent with longitudinal chromatic aberration.However,the response to wavelength differed across subjects.S#1 showed a sustained VA across distances(average VA standard deviation 0.054,-0.005logMAR),low AccL(slope:0.3 D/D) and systematic SA negative shift (slope:-0.04um/D),similar across mono-λ and WL.S#2 showed a more sustained VA,lower AccL and higher SA change in blue (0.09logMAR std,0.3D/D,-0.02um/D) than in WL(0.14logMAR std,0.52D/D,-0.013um/D).S#3 showed a steeper decrease in VA at near and higher AccL for mono-λ(0.218logMAR std,0.78D/D,on average)than in WL(0.05logMAR std,0.38D/D).Different subjects use chromatic cues in different ways to accommodate,likely affected by the interplay of chromatic blur,depth-of-focus and defocus sign perception.

Time‐stretch Chromatic Confocal Microscopy for Multi‐Depth Imaging

AbstractChromatic confocal technology has enabled multispectral information detection without any axial mechanical scanning. Especially, the chromatic confocal sensor is well applied in industry, such as device defect detection and ranging. However, for 3D imaging, chromatic confocal microscopy (CCM) still suffers from insufficient speed due to the slow refresh rate of a traditional spectrometer. To address this problem, a time‐stretch chromatic confocal microscopy (TSCCM) is presented for multi‐depth imaging. A supercontinuum laser is stretched and then focused on the sample using a home‐built chromatic lens, which disperses the laser to a different depth. The time‐of‐flight signal is collected by a high‐speed photodiode and is recorded and analyzed by a 3 GHz digitizer. A novel approach is achieved to significantly improve the speed of multi‐depth imaging with an up to 1 MHz A‐scan rate and 5 Hz volumetric imaging speed, which is an order faster compared with previous work of spectrometer‐based chromatic confocal microscopy. Multi‐depth volumetric imaging of nude‐mouse skin is performed to show the potential for biomedical applications. In this method, with its high A‐scan rate and multi‐depth imaging capability, a virtual tissue “light detection and ranging (LIDAR)” may be achieved.

Quantification of microvascular change of retinal degeneration in Royal College of Surgeons rats using high-resolution spectral domain optical coherence tomography angiography.

For research on retinitis pigmentosa in humans, the Royal College of Surgeons (RCS) rat is commonly used as the primary animal model since the disease process is similar. Therefore, it is necessary to understand how the disease develops and determine whether the treatment is effective. In this study, structural and microvascular change of retinal degeneration in RCS rats was assessed non-invasively on specific dates over 3.5months. Using a high-resolution spectral domain (SD) optical coherence tomography angiography (OCTA), the retinal degeneration in RCS rats, from day 14 until day 126, was qualitatively and quantitatively analyzed. Aside from the thinning of the retina thickness starting from 2 weeks of age, blood vessels in the deep layer of the retina also began to degenerate at about 4 weeks of age. Hole structures appeared at the inner nuclear layer and the inner plexiform layer by the age of 10 weeks. Observations of abnormal angiogenesis in the choroid began by 12 weeks of age. We conducted a longitudinal study of retina degeneration structure and vascular changes in an RCS rat model using a supercontinuum laser based high-resolution SD-OCTA. Combined with OCTA, OCT leads to a better understanding of photoreceptor pathology as retinal degeneration by identifying tissue and vessel loss.

Adaptive balanced detection spectral domain optical coherence tomography.

Balanced detection optical coherence tomography (BD-OCT) enables near-shot noise-limited imaging by suppressing wavelength-dependent relative intensity noise (RIN) originating from the light source. In spectral-domain BD-OCT (SD-BD-OCT), the level of RIN suppression relies on the co-registration accuracy of the spectra simultaneously captured by two independent spectrometers. However, existing matching methods require careful pre-calibration using a RIN-dominated dataset or subjective post-processing using a signal-dominated dataset. We developed an adaptive subpixel matching approach, referred to as adaptive balance, that can be applied to any SD-BD-OCT dataset regardless of RIN or signal level without the need for pre-calibration. We showed that adaptive balance performed comparable to or better than reported methods by imaging phantoms with varying spectrometer camera gain, exposure time, and supercontinuum laser repetition rate. We further demonstrated the benefits of adaptive balance in human retinal imaging.