Limited Spectral Bandwidth Research Articles

Optical-digital joint design of multi-order diffractive lenses for lightweight high-resolution computational imaging

Multi-order diffraction lenses can focus different wavelengths at the same focal point by utilizing various diffraction orders. However, due to material dispersion, the focal position shifts when the incident wavelength moves away from the primary wavelength, resulting in limited spectral bandwidth. This paper presents a computationally optimized multi-order diffractive lens design and a related image reconstruction algorithm, enabling broadband high-resolution imaging with a single element. We introduce a new type of lens, the Spectral Multi-Order Diffractive Lens (SMODLs), which redistributes the energy across different diffractive orders to achieve relatively high focusing performance within specified wavelength bands. Subsequently. The relatively sharp spectral images generated by SMODL within these specified bands serve as priors for reconstructing images across the full spectrum. A prototype lens with a 40 mm aperture and a 320 mm focal length was designed and fabricated. The prior images' wavebands were designated as 525 nm to 575 nm and 675 nm to 725 nm, with peak Strehl ratios of 0.47 and 0.35, respectively. Finally, grayscale images within the 500∼800 nm wavelength range were reconstructed. Experimental results confirm the effectiveness of our approach, achieving high-resolution imaging across a 300 nm bandwidth with a single diffractive element.

Papr reduction through Gaussian pre-coding in DCO-OFDM systems

Visible light communication (VLC) utilizes commercially available light-emitting diodes (LEDs) for short-distance wireless data transmission, aligning with lighting standards for indoor wireless access. The inherent challenge of achieving high data rates in VLC due to the limited spectral bandwidth of white LEDs has led to the adoption of orthogonal frequency division multiplexing (OFDM). However, conventional OFDM presents issues with peak-to-average power ratio (PAPR), pushing LEDs into a nonlinear zone, resulting in energy inefficiencies and signal distortion that degrades the bit error rate (BER). The study focuses on pre-coding within DC-biased optical OFDM (DCO-OFDM) using a Gaussian matrix (GM) for PAPR reduction. The proposed Gaussian pre-coding method demonstrates superior PAPR performance compared to conventional discrete cosine transform (DCT) pre-coding method, especially when integrated with μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu$$\\end{document}-Law companding. Simulation results indicate that increasing the number of candidate GMs enhances PAPR performance, albeit with increased system complexity. The combination of DCT and Gauss pre-coding with μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu$$\\end{document}-Law companding exhibits a 2 dB improvement in PAPR, though at the expense of BER performance.

Epigenetic modification of gene expression in cancer cells by terahertz demethylation

Terahertz (THz) radiation can affect the degree of DNA methylation, the spectral characteristics of which exist in the terahertz region. DNA methylation is an epigenetic modification in which a methyl (CH3) group is attached to cytosine, a nucleobase in human DNA. Appropriately controlled DNA methylation leads to proper regulation of gene expression. However, abnormal gene expression that departs from controlled genetic transcription through aberrant DNA methylation may occur in cancer or other diseases. In this study, we demonstrate the modification of gene expression in cells by THz demethylation using resonant THz radiation. Using an enzyme-linked immunosorbent assay, we observed changes in the degree of global DNA methylation in the SK-MEL-3 melanoma cell line under irradiation with 1.6-THz radiation with limited spectral bandwidth. Resonant THz radiation demethylated living melanoma cells by 19%, with no significant occurrence of apurinic/apyrimidinic sites, and the demethylation ratio was linearly proportional to the power of THz radiation. THz demethylation downregulates FOS, JUN, and CXCL8 genes, which are involved in cancer and apoptosis pathways. Our results show that THz demethylation has the potential to be a gene expression modifier with promising applications in cancer treatment.

Organic Synthetic Photonic Systems with Reconfigurable Parity-Time Symmetry Breaking for Tunable Single-Mode Microlasers.

Synthetic photonic materials exploiting the quantum concept of parity-time (PT) symmetry lead to an emerging photonic paradigm-non-Hermitian photonics, which is revolutionizing the photonic sciences. The non-Hermitian photonics dealing with the interplay between gain and loss in PT synthetic photonic material systems offers a versatile platform for advancing microlaser technology. However, current PT-symmetric microcavity laser systems only manipulate imaginary parts of the refractive indices, suffering from limited laser spectral bandwidth. Here, an organic composite material system is proposed to synthesize reconfigurable PT-symmetric microcavities with controllable complex refractive indices for realizing tunable single-mode laser outputs. A grayscale electron-beam direct-writing technique is elaborately designed to process laser dye-doped polymer films in one single step into microdisk cavities with periodic gain and loss distribution, which enables thresholdless PT-symmetry breaking and single-mode laser operation. Furthermore, organic photoisomerizable compounds are introduced to reconfigure the PT-symmetric systems in real-time by tailoring the real refractive index of the polymer microresonators, allowing for a dynamically and continuously tunable single-mode laser output. This work fundamentally enhances the PT-symmetric photonic systems for innovative design of synthetic photonic materials and architectures.

Tailoring ultra-broadband vector beams via programming the electric field vector of light.

With spatially inhomogeneous polarization, vector beam (VB) has created substantial opportunities in both optics and photonics. However, the limited spectral bandwidth of VB generator hinders further advances for higher level of integration and functionality. Here, an innovative approach of programming the electric field vector of light is proposed to tailor arbitrary ultra-broadband VBs, in parallel among an unprecedented wavelength range over 1000 nm covering the visible and NIR band. We demonstrate the twisted nematic liquid crystals (TNLCs), specifically arranged in-situ by a dynamic programmable photopatterning, enable to directly manipulate the electric field vector of transmitted light into the VB as desired. Furthermore, the electrical responsiveness of TNLCs yields a dynamic multifunctionality between the VB and Gaussian beam. We anticipate this ultra-broadband VB generator would be promising for a variety of applications like optical manipulation, super-resolution imaging, and integrated optical communication system.

Broad-band spatial light modulation with dual epsilon-near-zero modes

Epsilon-near-zero (ENZ) modes have attracted extensive interests due to its ultrasmall mode volume resulting in extremely strong light-matter interaction (LMI) for active optoelectronic devices. The ENZ modes can be electrically toggled between on and off states with a classic metal-insulator-semiconductor (MIS) configuration and therefore allow access to electro-absorption (E-A) modulation. Relying on the quantum confinement of charge-carriers in the doped semiconductor, the fundamental limitation of achieving high modulation efficiency with MIS junction is that only a nanometer-thin ENZ confinement layer can contribute to the strength of E-A. Further, for the ENZ based spatial light modulation, the requirement of resonant coupling inevitably leads to small absolute modulation depth and limited spectral bandwidth as restricted by the properties of the plasmonic or high-Q resonance systems. In this paper, we proposed and demonstrated a dual-ENZ mode scheme for spatial light modulation with a TCOs/dielectric/silicon nanotrench configuration for the first time. Such a SIS junction can build up two distinct ENZ layers arising from the induced charge-carriers of opposite polarities adjacent to both faces of the dielectric layer. The non-resonant and low-loss deep nanotrench framework allows the free space light to be modulated efficiently <italic>via</italic> interaction of dual ENZ modes in an elongated manner. Our theoretical and experimental studies reveal that the dual ENZ mode scheme in the SIS configuration leverages the large modulation depth, extended spectral bandwidth together with high speed switching, thus holding great promise for achieving electrically addressed spatial light modulation in near- to mid-infrared regions.

Plasmonic interference modulation for broadband nanofocusing

Abstract Metallic plasmonic probes have been successfully applied in near-field imaging, nanolithography, and Raman enhanced spectroscopy because of their ability to squeeze light into nanoscale and provide significant electric field enhancement. Most of these probes rely on nanometric alignment of incident beam and resonant structures with limited spectral bandwidth. This paper proposes and experimentally demonstrates an asymmetric fiber tip for broadband interference nanofocusing within its full optical wavelengths (500–800 nm) at the nanotip with 10 nm apex. The asymmetric geometry consisting of two semicircular slits rotates plasmonic polarization and converts the linearly polarized plasmonic mode to the radially polarized plasmonic mode when the linearly polarized beam couples to the optical fiber. The three-dimensional plasmonic modulation induces circumference interference and nanofocus of surface plasmons, which is significantly different from the nanofocusing through plasmon propagation and plasmon evolution. The plasmonic interference modulation provides fundamental insights into the plasmon engineering and has important applications in plasmon nanophotonic technologies.

Characterization of the Frequency Response of Channel-Interleaved Photonic ADCs Based on the Optical Time-Division Demultiplexer

We characterize the frequency response of channel-interleaved photonic analog-to-digital converters (CI-PADCs) theoretically and experimentally. The CI-PADC is composed of a photonic frontend for photonic sampling and an electronic backend for quantization. The photonic frontend includes a photonic sampling pulse generator for directly high-speed sampling and an optical time-division demultiplexer (OTDM) for channel demultiplexing. It is found that the frequency response of the CI-PADC is influenced by both the photonic sampling pulses and the OTDM, of which the combined impact can be characterized through demultiplexed pulse trains. First, the frequency response can be divided into multiple frequency intervals and the range of the frequency interval equals the repetition rate of demultiplexed pulse trains. Second, the analog bandwidth of the CI-PADC is determined by the optical spectral bandwidth of demultiplexed pulse trains which is broadened in the OTDM. Further, the effect of the OTDM is essential for enlarging the analog bandwidth of the CI-PADC employing the photonic sampling pulses with limited optical spectral bandwidth.

Accessing depth-resolved high spatial frequency content from the optical coherence tomography signal

Optical coherence tomography (OCT) is a rapidly evolving technology with a broad range of applications, including biomedical imaging and diagnosis. Conventional intensity-based OCT provides depth-resolved imaging with a typical resolution and sensitivity to structural alterations of about 5–10 microns. It would be desirable for functional biological imaging to detect smaller features in tissues due to the nature of pathological processes. In this article, we perform the analysis of the spatial frequency content of the OCT signal based on scattering theory. We demonstrate that the OCT signal, even at limited spectral bandwidth, contains information about high spatial frequencies present in the object which relates to the small, sub-wavelength size structures. Experimental single frame imaging of phantoms with well-known sub-micron internal structures confirms the theory. Examples of visualization of the nanoscale structural changes within mesenchymal stem cells (MSC), which are invisible using conventional OCT, are also shown. Presented results provide a theoretical and experimental basis for the extraction of high spatial frequency information to substantially improve the sensitivity of OCT to structural alterations at clinically relevant depths.

Nonlinearity-Induced Nonreciprocity—Part I

Nonreciprocal electromagnetic devices allow signals to propagate asymmetrically along opposite directions, enabling essential functionalities for several microwave and optical technologies. Branching out from the conventional approach involving a dc magnetic bias, different avenues have been investigated over the past two decades to break reciprocity without the need for magneto-optical materials. In particular, nonlinearity-based approaches have recently attracted considerable attention, due to their simple implementation and bias-free operation, which make their fabrication and system integration particularly appealing. Nonetheless, these advantages are accompanied by limitations that need to be carefully assessed when using these devices for practical applications. In the first part of this work, we discuss the fundamental principles of nonlinearity-based nonreciprocal devices. We introduce a general model for two-port nonlinear resonators and show how to design a nonlinear system yielding highly nonreciprocal port-to-port transmission under continuous-wave excitation. We then analyze quantitatively the main limitations of these devices: the limited power and spectral bandwidth, the response under pulsed excitation, and the breakdown of nonreciprocity under simultaneous two-port excitation. In the second part, we will deepen our analysis by considering more complex scenarios involving coherent and incoherent two-port excitations and multiresonator systems offering more flexibility in the response.

Improvement and Enhancement of bandwidth of 5G Networks using Machine Learning

Radio-frequency-based systems are exhibiting severe bandwidth congestion as a result of the exponential development in the amount of data flow. Both cognitive radio technology and free-space-optical communication are examples of attempts to find solutions to the problems posed by high data rates and limited spectral bandwidth. Operating an optical wireless transmission system does not need the purchase of a license. Additionally, the accommodation of unlicensed users across the restricted frequency that is accessible to us is the foundation of the technology known as cognitive radio. Since Dynamic-Window Size systems do not need a license, they are very cost-effective, they can be readily deployed, and they provide a high bandwidth; hence, Dynamic-Window Size systems may be used to bridge with the existing Radio Frequency system. Within the framework of the proposed Dynamic-Window-Size system, the Radio Frequency link is modeled based on the Rayleigh distribution, whilst the Dynamic-Window-Size link experiences -IG composite fading. It is possible to determine both the moment-generating function (MGF) and its derivative. By making use of the formulas that were derived from them, various performance metrics, such as ergodic channel capacity, bit error rate (BER), and output power are calculated, along with the validations that are provided by asymptotic findings. In addition to this, a new closed-form identity is discovered that relates to a specific instance of Bessel's function. In addition to the convex optimization that was mentioned above for the purpose of optimizing the overlay and underlay power in the scheme that was presented, the performance of the Cognitive Radio network is evaluated by making use of a variety of pulse-shaping windows. Suppressing the side lobes of the primary users' (PUs') sub-carriers is a way to reduce the amount of interference that primary users cause for secondary users without harming the primary users' own transmissions. This study involves the creation of a variety of pulse-shaping windows across a variety of power allocation systems as well as an examination of how these windows compare to one another.

Amplitude and phase interaction in Hilbert demodulation of vibration signals: Natural gear wear modeling and time tracking for condition monitoring

In the context of automatic and preventive condition monitoring of rotating machines, this paper revisits the demodulation process essential for detecting and localizing cracks in gears and bearings. The objective of the paper is to evaluate the performance of the well-known Hilbert demodulation by providing a quantified assessment in terms of signal processing. For this purpose, vibration test signals are simulated guided by the analysis of real-world measurements. The database comes from a natural wear experimentation on a test bench at an industrial scale and without any fault initiation. In the proposed simulated model, the amplitude modulation is designed in a physical approach in order to be able to set up the number of faulty teeth and their location. The impact of a limited spectral bandwidth filtering is quantified not only for the amplitude but also for the phase modulation estimations. The interactions between the amplitude and phase estimations are discussed. A focus is made on the analytic signal ambiguity due to the non-uniqueness of the amplitude estimation. This property induces an original investigation when demodulating the residual generated after a time synchronous averaging. Finally, as the objective is a continuous surveillance of a machine, results are given for a sequence of real-world measurements in order to visualize the fault evolution through the demodulation process.

Hollow core fibers for optical amplification.

Hollow core optical fibers are normally passive light transport components. In contrast, within this Letter, we numerically investigate the possibility of using them as optical amplifiers, through the adoption of a novel fiber structure. We show that optical amplification can be achieved in hollow core fibers, where the cladding region is partially doped and composed of both resonant and anti-resonant elements. A balance between loss and glass/optical mode overlap is obtained, which allows efficient amplification over a limited spectral bandwidth. We discuss the case of a thulium-doped optical amplifier based on this novel technological approach.

7-cell hollow-core photonic bandgap fiber with broad spectral bandwidth and low loss.

The limited spectral bandwidth achieved in state-of-the-art hollow-core photonic bandgap fibers (HC-PBGF) has hindered its implementation in a wide range of applications. Here we demonstrate that broad spectral bandwidth and low loss can be simultaneously achieved in 7-cell HC-PBGF. Several 7-cell HC-PBGFs operating at 1550 nm telecom band and 1 μm laser band are present. One of the fibers exhibits a minimum loss of 6.5 dB/km at 1633 nm and a 3 dB bandwidth of 458 nm, approaching a bandwidth to central wavelength ratio of 26%. This is to our knowledge the broadest bandwidth achieved in triangular lattice HC-PBGF and the lowest transmission loss in 7-cell HC-PBGF.

Prediction of speech intelligibility using pseudo-binaural room impulse responses.

Head orientation (HO) affects better-ear-listening and spatial-release-from-masking, which are two key aspects in binaural speech intelligibility. To incorporate HO in speech intelligibility prediction, binaural room impulse responses (BRIRs) for every HO of interest could be used. Due to the limited spectral bandwidth of speech, however, approximate representations might be sufficient, which can be measured more quickly. A comparison was done between pseudo-BRIRs generated with a motion tracked binaural microphone array and a first order Ambisonics microphone using the spatial decomposition method (SDM). The accuracy of the Ambisonics/SDM approach was comparable to that of real BRIRs, indicating its suitability for speech intelligibility prediction.

Recurring obscuration in NGC 3783

Context. Obscuration of the continuum emission from active galactic nuclei by streams of gas with relatively high velocity (&gt;1000 km s−1) and column density (&gt;3 × 1025 m−2) has been seen in a few Seyfert galaxies. This obscuration has a transient nature. In December 2016 we witnessed such an event in NGC 3783. Aims. The frequency and duration of these obscuration events is poorly known. Here we study archival data of NGC 3783 in order to constrain this duty cycle. Methods. We use archival Chandra/NuSTAR spectra taken in August 2016. We also study the hardness ratio of all Swift XRT spectra taken between 2008 and 2017. Results. In August 2016, NGC 3783 also showed evidence of obscuration. While the column density of the obscuring material is ten times lower than in December 2016, the opacity is still sufficient to block a significant fraction of the ionising X-ray and extreme ultraviolet photons. From the Swift hardness ratio behaviour we find several other epochs with obscuration. Obscuration with columns &gt;1026 m−2 may take place about half of the time. Also, in archival X-ray data taken by the Advanced Satellite for Cosmology and Astrophysics (ASCA) in 1993 and 1996 we find evidence of obscuration. Conclusions. Obscuration of the ionising photons in NGC 3783 occurs more frequently than previously thought. This may not always have been recognised due to low-spectral-resolution observations, overly limited spectral bandwidth or confusion with underlying continuum variations.

Direct sampling of a light wave in air

Temporal characterization of a laser pulse is an essential task in many applications. Temporal characterization methods that are currently available support only a limited spectral bandwidth without information on the carrier-envelope phase (CEP) of the laser pulse or require complicated equipment in a vacuum environment. Here we demonstrate that an arbitrary time-dependent laser field can be directly sampled using subcycle tunneling ionization in a gaseous medium or in air. The subcycle ionization is used as a fast temporal gate for the direct sampling of the laser field. This unique approach enables the complete temporal characterization of the laser field, including its CEP, for a broad spectral range in ambient air, providing a universal tool for the precise measurement of the laser field.

Design and fabrication of resonator-quantum well infrared photodetector for SF6 gas sensor application

The infrared absorption of SF6 gas is narrowband and peaks at 10.6 μm. This narrowband absorption posts a stringent requirement on the corresponding sensors as they need to collect enough signal from this limited spectral bandwidth to maintain a high sensitivity. Resonator-quantum well infrared photodetectors (R-QWIPs) are the next generation of QWIP detectors that use resonances to increase the quantum efficiency for more efficient signal collection. Since the resonant approach is applicable to narrowband as well as broadband, it is particularly suitable for this application. We designed and fabricated R-QWIPs for SF6 gas detection. To achieve the expected performance, the detector geometry must be produced according to precise specifications. In particular, the height of the diffractive elements and the thickness of the active resonator must be uniform, and accurately realized to within 0.05 μm. Additionally, the substrates of the detectors must be completely removed to prevent the escape of unabsorbed light in the detectors. To achieve these specifications, two optimized inductively coupled plasma etching processes were developed. Due to submicron detector feature sizes and overlay tolerance, we used an advanced semiconductor material lithography stepper instead of a contact mask aligner to pattern wafers. Using these etching techniques and tool, we have fabricated focal plane arrays with 30-μm pixel pitch and 320×256 format. The initial test revealed promising results.

Terahertz imaging modalities of ancient Egyptian mummified objects and of a naturally mummified rat.

During the last few years, terahertz (THz) imaging has been used to investigate artwork and historic artifacts. The application of THz imaging to mummy investigations is very attractive since it provides spectroscopic information over a broad frequency range and its radiation has proven to be harmless to human cells. However, compared with the current standard imaging methods in mummy imaging-X-ray and computed tomography (CT)--it remains a novel, emerging technique whose potential still needs to be fully evaluated. Here, ancient Egyptian mummified objects as well as a naturally mummified rat have been investigated by two different THz imaging systems: a broadband THz time domain imaging system and an electronic THz scanner. The obtained THz images are compared with conventional CT, X-ray, and magnetic resonance images. While the broadband THz time domain setup permits analyses of smaller samples, the electronic THz scanner allows the recording of data of thicker and larger samples at the expense of a limited spectral bandwidth. Terahertz imaging shows clear potential for mummy investigations, although currently CT imaging offers much higher spatial resolution. Furthermore, as commercial mobile THz scanners become available, THz imaging could be applied directly in museums or at excavation sites.

Independent and continuous third-order dispersion compensation using a pair of prisms

Abstract The dispersion of a pair of prisms is analyzed by means of a ray-tracing method operating at other than tip-to-tip propagation of the prisms, taking into consideration the limited spectral bandwidth. The variations of the group delay dispersion and the third-order dispersion for a pair of prisms are calculated with respect to the incident position and the separation between the prisms. The pair of prisms can provide a wide range of independent and continuous third-order dispersion compensation. The effect of residual third-order dispersion on the pulse contrast ratio and pulse duration is also calculated. The residual third-order dispersion not only worsens the pulse contrast ratio, but also increases the pulse duration to the hundreds of femtosecond range for a tens of femtosecond pulse, even when the residual third-order dispersion is small. These phenomena are helpful in compensating for the residual high-order dispersion and in understanding its effect on pulse contrast ratios and pulse durations in ultrashort laser systems.