Ultrafast electrooptic dual-comb interferometry

The femtosecond laser frequency comb has enabled the 21st century revolution in optical synthesis and metrology. A particularly compelling technique that relies on the broadband coherence of two laser frequency combs is dual-comb interferometry. This method is rapidly advancing the field of optical spectroscopy and empowering new applications, from nonlinear microscopy to laser ranging. Up to now, most dual-comb interferometers were based on modelocked lasers, whose repetition rates have restricted the measurement speed to ~ kHz. Here we demonstrate a novel dual-comb interferometer that is based on electrooptic frequency comb technology and measures consecutive complex spectra at a record-high refresh rate of 25 MHz. These results pave the way for novel scientific and metrology applications of frequency comb generators beyond the realm of molecular spectroscopy, where the measurement of ultrabroadband waveforms is of paramount relevance

A Broadly Tunable Surface Plasmon-Coupled Wavelength Filter for Visible and Near Infrared Hyperspectral Imaging

Hyperspectral imaging is a set of techniques that has contributed to the study of advanced materials, pharmaceuticals, semiconductors, ceramics, polymers, biological specimens, and geological samples. Its use for remote sensing has advanced our understanding of agriculture, forestry, the Earth, environmental science, and the universe. The development of ultra-compact handheld hyperspectral imagers has been impeded by the scarcity of small widefield tunable wavelength filters. The widefield modality is preferred for handheld imaging applications in which image registration can be performed to counter scene shift caused by irregular user motions that would thwart scanning approaches. In the work presented here an electronically tunable widefield wavelength filter has been developed for hyperspectral imaging applications in the visible and near-infrared region. Conventional electronically tunable widefield imaging filter technologies include liquid crystal-based filters, acousto-optic tunable filters, and electronically tuned etalons; each having its own set of advantages and disadvantages. The construction of tunable filters is often complex and requires elaborate optical assemblies and electronic control circuits. I introduce in the work presented here is a novel widefield tunable filter, the surface plasmon coupled tunable filter (SPCTF), for visible and near infrared imaging. The SPCTF is based on surface plasmon coupling and has simple optical design that can be miniaturized without sacrificing performance. The SPCTF provides diffraction limited spatial resolution with a moderately narrow nominal passband (\u3c10 \u3enm) and a large spurious free spectral range (450 nm-1000 nm). The SPCTF employs surface plasmon coupling of the π-polarized component of incident light in metal films separated by a tunable dielectric layer. Acting on the π-polarized component, the device is limited to transmitting 50 percent of unpolarized incident light. This is higher than the throughput of comparable Lyot-based liquid crystal tunable filters that employ a series of linear polarizers. In addition, the SPCTF is not susceptible to the unwanted harmonic bands that lead to spurious diffraction in Bragg-based devices. Hence its spurious free spectral range covers a broad region from the blue through near infrared wavelengths. The compact design and rugged optical assembly make it suitable for hand-held hyperspectral imagers. The underlying theory and SPCTF design are presented along with a comparison of its performance to calculated estimates of transmittance, spectral resolution, and spectral range. In addition, widefield hyperspectral imaging using the SPCTF is demonstrated on model sample

Hyperspectral LiDAR-Based Plant Spectral Profiles Acquisition : Performance Assessment and Results Analysis

In precision agriculture, efficient fertilization is one of the most important pursued goals. Vegetation spectral profiles and the corresponding spectral parameters are usually employed for vegetation growth status indication, i.e., vegetation classification, bio-chemical content mapping, and efficient fertilization guiding. In view of the fact that the spectrometer works by relying on ambient lighting condition, hyperspectral/multi-spectral LiDAR (HSL/MSL) was invented to collect the spectral profiles actively. However, most of the HSL/MSL works with the wavelength specially selected for specific applications. For precision agriculture applications, a more feasible HSL capable of collecting spectral profiles at wide-range spectral wavelength is necessary to extract various spectral parameters. Inspired by this, in this paper, we developed a hyperspectral LiDAR (HSL) with 10 nm spectral resolution covering 500~1000 nm. Different vegetation leaf samples were scanned by the HSL, and it was comprehensively assessed for wide-range wavelength spectral profiles acquirement, spectral parameters extraction, vegetation classification, and the laser incident angle effect. Specifically, three experiments were carried out: (1) spectral profiles results were compared with that from a SVC spectrometer (HR-1024, Spectra Vista Corporation); (2) the extracted spectral parameters from the HSL were assessed, and they were employed as the input features of a support vector machine (SVM) classifier with multiple labels to classify the vegetation; (3) in view of the influence of the laser incident angle on the HSL reflected laser intensities, we analyzed the laser incident angle effect on the spectral parameters values. The experimental results demonstrated the developed HSL was more feasible for acquiring spectral profiles with wide-range wavelength, and spectral parameters and vegetation classification results also indicated its great potentials in precision agriculture application

The Aerosol Limb Imager

Stratospheric aerosol has been measured globally from satellite platforms over the past three decades. The variability of the natural and anthropogenic sources and resulting effect on climate make continued and improved measurements a priority. Yet, few satellite instruments capable of measuring stratospheric aerosol currently exist, with a lack of planned missions to fill the gap left by the ultimate loss of current instruments. The Aerosol Limb Imager (ALI) is an optical remote sensing instrument designed to image scattered sunlight from the atmospheric limb. These measurements are used to retrieve spatially resolved information of the stratospheric aerosol distribution, including spectral extinction coefficient and particle size. Here we present the design, development and test results of an ALI prototype. The instrument design uses a large aperture Acousto-Optic Tunable Filter (AOTF) to image the sunlit stratospheric limb in a selectable narrow wavelength band ranging from the visible to the near infrared. Through the nature of the AOTF operation, ALI measures one orientation of the polarized limb radiance, rather than the historically observed total radiance. A modelling study on the impact of this approach on the retrievals shows that while there is no distinct advantage to the linearly polarized measurement, there are also no clear disadvantages assuming the somewhat lower overall signal levels can be handled in the instrument design or operation. The long term goal of this work is the eventual realization of ALI on a satellite platform in low earth orbit, where it can provide high spatial resolution observations, both in the vertical and cross-track dimensions. The ALI prototype was tested on a stratospheric balloon flight from the Canadian Space Agency (CSA) launch facility in Timmins, Canada, in September 2014. Preliminary analysis of the hyperspectral images indicate that the radiance measurements are of high quality, and these are used to successfully retrieve vertical profiles of stratospheric aerosol extinction coefficient from 650–950 nm, along with one moment of the particle size distribution

Analysis and Radiometric Calibration for Backscatter Intensity of Hyperspectral LiDAR Caused by Incident Angle Effect

Hyperspectral LiDAR (HSL) is a new remote sensing detection method with high spatial and spectral information detection ability. In the process of laser scanning, the laser echo intensity is affected by many factors. Therefore, it is necessary to calibrate the backscatter intensity data of HSL. Laser incidence angle is one of the important factors that affect the backscatter intensity of the target. This paper studied the radiometric calibration method of incidence angle effect for HSL. The reflectance of natural surfaces can be simulated as a combination of specular reflection and diffuse reflection. The linear combination of the Lambertian model and Beckmann model provides a comprehensive theory that can be applied to various surface conditions, from glossy to rough surfaces. Therefore, an adaptive threshold radiometric calibration method (Lambertian-Beckmann model) is proposed to solve the problem caused by the incident angle effect. The relationship between backscatter intensity and incident angle of HSL is studied by combining theory with experiments, and the model successfully quantifies the difference between diffuse and specular reflectance coefficients. Compared with the Lambertian model, the proposed model has higher calibration accuracy, and the average improvement rate to the samples in this study was 22.67%. Compared with the results before calibration with the incidence angle of less than 70 degrees, the average improvement rate of the Lambertian-Beckmann model was 62.26%. Moreover, we also found that the green leaves have an obvious specular reflection effect near 650-720 nm, which might be related to the inner microstructure of chlorophyll. The Lambertian-Beckmann model was more helpful to the calibration of leaves in the visible wavelength range. This is a meaningful and a breakthrough exploration for HSL.Peer reviewe

Spectral LADAR: Active Range-Resolved Imaging Spectroscopy

Imaging spectroscopy using ambient or thermally generated optical sources is a well developed technique for capturing two dimensional images with high per-pixel spectral resolution. The per-pixel spectral data is often a sufficient sampling of a material's backscatter spectrum to infer chemical properties of the constituent material to aid in substance identification. Separately, conventional LADAR sensors use quasi-monochromatic laser radiation to create three dimensional images of objects at high angular resolution, compared to RADAR. Advances in dispersion engineered photonic crystal fibers in recent years have made high spectral radiance optical supercontinuum sources practical, enabling this study of Spectral LADAR, a continuous polychromatic spectrum augmentation of conventional LADAR. This imaging concept, which combines multi-spectral and 3D sensing at a physical level, is demonstrated with 25 independent and parallel LADAR channels and generates point cloud images with three spatial dimensions and one spectral dimension. The independence of spectral bands is a key characteristic of Spectral LADAR. Each spectral band maintains a separate time waveform record, from which target parameters are estimated. Accordingly, the spectrum computed for each backscatter reflection is independently and unambiguously range unmixed from multiple target reflections that may arise from transmission of a single panchromatic pulse. This dissertation presents the theoretical background of Spectral LADAR, a shortwave infrared laboratory demonstrator system constructed as a proof-of-concept prototype, and the experimental results obtained by the prototype when imaging scenes at stand off ranges of 45 meters. The resultant point cloud voxels are spectrally classified into a number of material categories which enhances object and feature recognition. Experimental results demonstrate the physical level combination of active backscatter spectroscopy and range resolved sensing to produce images with a level of complexity, detail, and accuracy that is not obtainable with data-level registration and fusion of conventional imaging spectroscopy and LADAR. The capabilities of Spectral LADAR are expected to be useful in a range of applications, such as biomedical imaging and agriculture, but particularly when applied as a sensor in unmanned ground vehicle navigation. Applications to autonomous mobile robotics are the principal motivators of this study, and are specifically addressed

Supercontinuum radiation in fluorescence microscopy and biomedical imaging applications

Compact, high brightness supercontinuum sources have already made a big impact in the fields of fluorescence microscopy and other biomedical imaging techniques, such as optical coherence tomography and coherent anti-Stokes Raman scattering. In this review, we provide a brief overview on the generation and properties of supercontinuum radiation for imaging applications. We review specific uses of supercontinuum sources and their potential for imaging, but also their limitations and caveats. We conclude with a review of recent advances in UV supercontinuum generation, near-IR microscopy, exciting new potentials for the use of hollow-core PCFs, on-chip supercontinuum generation, and technologies to improve supercontinuum stability for certain applications.H2020 Marie Skłodowska-Curie Actions (MSCA) (722380); MedImmune; Infinitus (China) Ltd.; Engineering and Physical Sciences Research Council (EPSRC) (EP/H018301/1, EP/L015889/1); Wellcome Trust (089703/Z/09/Z, 3-3249/Z/16/Z); Medical Research Council (MRC) (MR/K015850/1, MR/K02292X/1)

Investigation of Narrowband Acousto-Optic Tunable Filters for Semiconductor Laser Tuning

Wavelength tunable sources find applications in areas of laser surgery, optical coherence tomography, material processing, and differential absorption light detection and ranging. Acousto-optic tunable filters (AOTFs) are wavelength selective devices which allow for better stability than mechanical tuning due to their lack of mechanical movement. Agile tunability is achieved by avoiding inertial effects. The aim of this thesis was to investigate the performance and limitations of AOTF wavelength controlled external semiconductor laser cavities. The three key components of a typical tunable source are outlined. Conventional tuning mechanisms are compared with AOTFs, and different gain media discussed, with semiconductors being identified as suitable for building AOTF tuned sources. Initially, two diffraction grating tuned sources were constructed. The first was a Thulium doped fibre laser which produced >1.3 W over 1920 – 2060 nm wavelength range which has since been delivered to OzGrav for characterising the absorption of low hydroxide OH bulk fused silica. The second source was constructed from a superluminescent diode with an Indium Phosphide (InP) active layer and produced >5 mW over 1650 – 1750 nm. This source was subsequently used for characterising a frequency shifting, quasi-collinear AOTF for its tuning relation, response to different driving conditions, and temporal response. Two tunable laser cavities were built using the quasi-collinear AOTF. The first was based on the InP superluminescent diode and demonstrated >10 mW over 1615 – 1775 nm. However, the laser displayed significant etalon effects in its spectral output. This motivated the construction of a ring-cavity based on an InP semiconductor optical amplifier. This laser produced >5 mW over 1600 – 1750 nm, with ~0.3 nm linewidths and no etalon effects observed in the output. Sweep rates of 10⁵ nm/s were demonstrated for both lasers without significant spectral broadening or power modulation. This is of similar magnitude to the fastest previously reported quasi-collinear AOTF tuned semiconductor lasers. Wavelength agility was demonstrated by switching between any two wavelengths in the gain band within the 29 μs interaction time of the AOTF. Finally, a frequency compensating AOTF device was utilised in the ring-cavity to investigate the effect of frequency shift on laser performance. Operating with no frequency shift allowed for a reduction in linewidth, but made the laser more sensitive to residual etalon effects which had not previously been observed with the frequency shifting AOTF ring-cavity. A +30 kHz frequency shift was significant enough to disturb these intracavity interference effects while being small enough to reduce the linewidth from the 0.3 nm observed with the frequency shifting AOTF to <0.1 nm. Future directions of research are discussed. These include operation at other wavelengths, and investigating dual wavelength operation.Thesis (MPhil) -- University of Adelaide, School of Physical Sciences, 201

A photon counting reconstructive spectrometer combining metasurfaces and superconducting nanowire single-photon detectors

Faint light spectroscopy has many important applications such as fluorescence spectroscopy, lidar and astronomical observations. However, long measurement time limit its application on real-time measurement. In this work, a photon counting reconstructive spectrometer combining metasurfaces and superconducting nanowire single photon detectors (SNSPDs) was proposed. A prototype device was fabricated on a silicon on isolator (SOI) substrate, and its performance was characterized. Experiment results show that this device support spectral reconstruction of mono-color lights with a resolution of 2 nm in the wavelength region of 1500 nm ~ 1600 nm. The detection efficiency of this device is 1.4% ~ 3.2% in this wavelength region. The measurement time required by this photon counting reconstructive spectrometer was also investigated experimentally, showing its potential to be applied in the scenarios requiring real-time measurement

Liquid crystal hyperspectral imager

Hyperspectral imaging is the collection, processing and analysis of spectral data in numerous contiguous wavelength bands while also providing spatial context. Some of the commonly used instruments for hyperspectral imaging are pushbroom scanning imaging systems, grating based imaging spectrometers and more recently electronically tunable filters. Electronically tunable filters offer the advantages of compactness and absence of mechanically movable parts. Electronically tunable filters have the ability to rapidly switch between wavelengths and provide spatial and spectral information over a large wavelength range. They involve the use of materials whose response to light can be altered in the presence of an external stimulus. While these filters offer some unique advantages, they also present some equally unique challenges. This research work involves the design and development of a multichannel imaging system using electronically tunable Liquid Crystal Fabry-Perot etalons. This instrument is called the Liquid Crystal Hyperspectral Imager (LiCHI). LiCHI images four spectral regions simultaneously and presents a trade-off between spatial and spectral domains. This simultaneity of measurements in multiple wavelengths can be exploited for dynamic and ephemeral events. LiCHI was initially designed for multispectral imaging of space plasmas but its versatility was demonstrated by testing in the field for multiple applications including landscape analysis and anomaly detection. The results obtained after testing of this instrument and analysis of the images are promising and demonstrate LiCHI as a good candidate for hyperspectral imaging. The challenges posed by LiCHI for each of these applications have also been explored