Light scattering in disordered media has been studied extensively due to its prevalence in natural and artificial systems. In photonics most of the research has focused on understanding and ...mitigating the effects of scattering, which are often detrimental. For certain applications, however, intentionally introducing disorder can actually improve device performance, as in photovoltaics. Here, we demonstrate a spectrometer based on multiple light scattering in a silicon-on-insulator chip featuring a random structure. The probe signal diffuses through the chip generating wavelength-dependent speckle patterns, which are detected and used to recover the input spectrum after calibration. A spectral resolution of 0.75 nm at a wavelength of 1,500 nm in a 25-μm-radius structure is achieved. Such a compact, high-resolution spectrometer is well suited for lab-on-a-chip spectroscopy applications.
Optical trapping of airborne particles is emerging as an essential tool in applications ranging from online characterization of living cells and aerosols to particle transport and delivery. However, ...existing optical trapping techniques using a single laser beam can trap only transparent particles (via the radiative pressure force) or absorbing particles (via the photophoretic force), but not particles of either type-limiting the utility of trapping-enabled aerosol characterization techniques. Here, we present the first optical trapping technique capable of trapping both transparent and absorbing particles with arbitrary morphology using a single shaped laser beam. Such a general-purpose optical trapping mechanism could enable new applications such as trapping-enabled aerosol characterization with high specificity.
The spatial coherence of laser sources has limited their application to parallel imaging and projection due to coherent artifacts, such as speckle. In contrast, traditional incoherent light sources, ...such as thermal sources or light emitting diodes (LEDs), provide relatively low power per independent spatial mode. Here, we present a chip-scale, electrically pumped semiconductor laser based on a novel design, demonstrating high power per mode with much lower spatial coherence than conventional laser sources. The laser resonator was fabricated with a chaotic, D-shaped cavity optimized to achieve highly multimode lasing. Lasing occurs simultaneously and independently in ∼1,000 modes, and hence the total emission exhibits very low spatial coherence. Speckle-free full-field imaging is demonstrated using the chaotic cavity laser as the illumination source. The power per mode of the sample illumination is several orders of magnitude higher than that of a LED or thermal light source. Such a compact, low-cost source, which combines the low spatial coherence of a LED with the high spectral radiance of a laser, could enable a wide range of high-speed, full-field imaging and projection applications.
Significance There has been an intense search for the ideal light sources for high-speed, full-field imaging applications ranging from next-generation microscopes and laser projectors to digital holography and photolithography. Traditional lasers, although providing the required brightness (i.e., power per mode), exhibit high spatial coherence, which introduces coherent artifacts such as speckle, corrupting image formation. At the other extreme, low spatial coherence sources such as thermal sources and light emitting diodes (LEDs) avoid speckle but lack sufficient power per mode for high-speed imaging. In this work, we demonstrate a new type of semiconductor laser based on a chaotic cavity, which combines low spatial coherence with high power per mode. Such a laser could enable a wide range of full-field imaging applications.
Optical frequency comb sources provide thousands of precise and accurate optical lines in a single device enabling the broadband and high-speed detection required in many applications. A main ...challenge is to parallelize the detection over the widest possible band while bringing the resolution to the single comb-line level. Here we propose a solution based on the combination of a frequency comb source and a fibre spectrometer, exploiting all-fibre technology. Our system allows for simultaneous measurement of 500 isolated comb lines over a span of 0.12 THz in a single acquisition; arbitrarily larger span are demonstrated (3,500 comb lines over 0.85 THz) by doing sequential acquisitions. The potential for precision measurements is proved by spectroscopy of acetylene at 1.53 μm. Being based on all-fibre technology, our system is inherently low-cost, lightweight and may lead to the development of a new class of broadband high-resolution spectrometers.
Modern lens designs are capable of resolving greater than 10 gigapixels, while advances in camera frame-rate and hyperspectral imaging have made data acquisition rates of Terapixel/second a real ...possibility. The main bottlenecks preventing such high data-rate systems are power consumption and data storage. In this work, we show that analog photonic encoders could address this challenge, enabling high-speed image compression using orders-of-magnitude lower power than digital electronics. Our approach relies on a silicon-photonics front-end to compress raw image data, foregoing energy-intensive image conditioning and reducing data storage requirements. The compression scheme uses a passive disordered photonic structure to perform kernel-type random projections of the raw image data with minimal power consumption and low latency. A back-end neural network can then reconstruct the original images with structural similarity exceeding 90%. This scheme has the potential to process data streams exceeding Terapixel/second using less than 100 fJ/pixel, providing a path to ultra-high-resolution data and image acquisition systems.
Frequency multiplexed coherent φ-OTDR Ogden, Hannah M; Murray, Matthew J; Murray, Joseph B ...
Scientific reports,
09/2021, Volume:
11, Issue:
1
Journal Article
Peer reviewed
Open access
Abstract
We present a comprehensive analysis of a frequency multiplexed phase-measuring φ-OTDR sensor platform. The system uses a train of frequency-shifted pulses to increase the average power ...injected into the fiber and provide a diversity of uncorrelated Rayleigh backscattering measurements. Through a combination of simulations, numerical analysis, and experimental measurements, we show that this approach not only enables lower noise and mitigates interference fading, but also improves the sensor linearity. We investigate the sensor dependence on the length of the pulse train and characterize the sensor performance as a function of range, demonstrating operation from 1 to 50 km. Despite its relative simplicity, this platform enables state-of-the-art performance, including low crosstalk, high linearity, and a minimum detectable strain of only 0.6
p
$$\varepsilon /\sqrt{\text{Hz}}$$
ε
/
Hz
in a 10 km fiber with 10 m spatial resolution and a bandwidth of 5 kHz.
We introduce special states for light in multimode waveguides featuring strongly enhanced or reduced spectral correlations in the presence of strong mode coupling. Based on the experimentally ...measured multispectral transmission matrix of a multimode fiber, we generate a set of states that outperform the established “principal modes” in terms of the spectral stability of their output spatial field profiles. Inverting this concept also allows us to create states with a minimal spectral correlation width, whose output profiles are considerably more sensitive to a frequency change than typical input wave fronts. The resulting “super-principal-modes” and “anti-principal-modes” are made orthogonal to each other even in the presence of mode-dependent loss. By decomposing them in the principal-mode basis, we show that the super-principal-modes are formed via interference of principal modes with close delay times, whereas the anti-principal-modes are a superposition of principal modes with the most-different delay times available in the fiber. Such novel states are expected to have broad applications in fiber communication, imaging, and spectroscopy.
Photophoretic trapping-Raman spectroscopy (PTRS) is a new technique for measuring Raman spectra of particles that are held in air using photophoretic forces. It was initially demonstrated with Raman ...spectra of strongly-absorbing carbon nanoparticles (Pan et al. 44 (Opt Express 2012)). In the present paper we report the first demonstration of the use of PTRS to measure Raman spectra of absorbing and weakly-absorbing bioaerosol particles (pollens and spores). Raman spectra of three pollens and one smut spore in a size range of 6.2–41.8µm illuminated at 488nm are shown. Quality spectra were obtained in the Raman shift range of 1600–3400cm−1 in this exploratory study. Distinguishable Raman scattering signals with one or a few clear Raman peaks for all four aerosol particles were observed within the wavenumber region 2940–3030cm−1. Peaks in this region are consistent with previous reports of Raman peaks in the 1600–3400cm−1 range for pollens and spores excited at 514nm measured by a conventional Raman spectrometer. Noise in the spectra, the fluorescence background, and the weak Raman signals in most of the 1600–3400cm−1 region make some of the spectral features barely discernable or not discernable for these bioaerosols except the strong signal within 2940–3030cm−1. Up to five bands are identified in the three pollens and only two bands appear in the fungal spore, but this may be because the fungal spore is so much smaller than any of the pollens. The fungal spore signal relative to the air-nitrogen Raman band is approximately 10 times smaller than that ratio for the pollens. The five bands are tentatively assigned to the CH2 symmetric stretch at 2948cm−1, CH2 Fermi resonance stretch at 2970cm−1, CH3 symmetric stretch at 2990cm−1, CH3 out-of-plane end asymmetric stretch at 3010cm−1, and unsaturated =CH stretch at 3028cm−1. The two dominant bands of the up-to-five Raman bands in the 2940–3030cm−1 region have a consistent band spacing of 25cm−1 in all four aerosols. Finally we discuss improvements to the PTRS that should provide a system which can trap a higher fraction of particle types and obtain Raman spectra over a larger range (e.g., 200–3600cm−1) than those achieved here.
•Photophoretic trapping-Raman spectroscopy (PTRS).•Raman spectra of a single pollen/spore trapped in air.•PTRS spectra of three pollens and one fungal spore.•Up to five Raman bands in the Raman shift region of 2940–3030cm−1.
The combination of optical trapping with Raman spectroscopy provides a powerful method for the study, characterization, and identification of biological micro-particles. In essence, optical trapping ...helps to overcome the limitation imposed by the relative inefficiency of the Raman scattering process. This allows Raman spectroscopy to be applied to individual biological particles in air and in liquid, providing the potential for particle identification with high specificity, longitudinal studies of changes in particle composition, and characterization of the heterogeneity of individual particles in a population. In this review, we introduce the techniques used to integrate Raman spectroscopy with optical trapping in order to study individual biological particles in liquid and air. We then provide an overview of some of the most promising applications of this technique, highlighting the unique types of measurements enabled by the combination of Raman spectroscopy with optical trapping. Finally, we present a brief discussion of future research directions in the field.
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