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A reconfigurable wearable textile antenna with wide frequency tuning range is demonstrated in this work. The frequency agility is realized by adding a coplanar reconfiguration module to a planar inverted-F antenna (PIFA) constructed in textile technology. The reconfiguration module consists of a small flexible printed circuit board (PCB) which integrates a tuning circuitry with commercial snap-on buttons as electronic-to-textile connectors. Using this module equipped with a varactor diode, the frequency reconfigurability of the PIFA is investigated and optimized parametrically for maximum tuning range, exploiting a smooth transition from a quarter-wave to a half-wave patch resonant mode. A fabricated prototype demonstrates a very wide tuning range of approximately 70% (or one octave) with stable directive radiation characteristics, as predicted by simulations. The robust performance and mechanical stability of the reconfigurable antenna in various on-body and bending configurations validates the viability of the antenna tuning concept and the practicality of the proposed coplanar reconfiguration module.

Optical isolators today are exclusively built on magneto-optic principles but are not readily implemented within photonic integrated circuits. So far, no magnetless alternative1–22 has managed to simultaneously combine linearity (that is, no frequency shift), linear response (that is, input–output scaling), ultralow insertion loss and large directional contrast on-chip. Here we demonstrate an electrically driven optical isolator design that leverages the unbeatable transparency of a short, high-quality dielectric waveguide, with the strong attenuation from a critically coupled absorber. Our concept is implemented using a lithium niobate racetrack resonator in which phonon-mediated13 photonic Autler–Townes splitting10,16,23,24 breaks the chiral symmetry of the resonant modes. We demonstrate isolators at wavelengths one octave apart near 1,550 nm and 780 nm, fabricated from the same lithium-niobate-on-insulator wafer. Linear isolation is demonstrated with simultaneously 39 dB contrast and 10 dB bandwidth up to ~200 MHz. Non-magnetic optical isolators are demonstrated using phonon-mediated photonic Autler–Townes splitting. The on-chip lithium niobate devices simultaneously achieve ultralow insertion loss and high contrast.

Microperforated panels (MPPs) are recognized as suitable absorbers for noise control applications demanding special clean and health requirements. While it is relatively easy to design single-layer MPPs for sound absorption in one-to-two octave bands at medium-high frequencies, the performance for low frequencies (below 600 Hz) leads to a rather narrowband absorption, similar to that of a Helmholtz resonator. However, multiple-layer MPPs can be designed as sound absorbers that yield low-frequency absorption in a wide frequency band. Recently, multiplecavity perforated panels have been proposed to improve the performance of MPPs in the low-frequency range. In this article, the capability of multiple-layer and multiple-cavity MPPs to provide sound absorption at low frequencies is analyzed. © 2021 Institute of Noise Control Engineering.

To satisfy the requirements for multi-standard frequency generation, mode-switching oscillators are developed to increase the frequency tuning range without incurring considerable phase noise penalty. However, this improvement in the tuning range is often limited by the parasitics of the circuitry for mode selection and tuning. To address this challenge, this work presents a dual-core octave-tuning VCO using a switch-less tertiary magnetic coupling loop. The design employs the benefits of transformer-switching without incurring switch loss and quality factor degradation and also the benefits of mode switching without the parasitics from the mode selection and tuning network. The VCO employs a non-constant $RC$ scaling for the capacitors’ bank, across the tuning bits, and positive feedback dc-coupled inductive-degenerated output buffer to maximize the tuning range. Implemented in 22 nm fully depleted silicon-on-insulator (FDSOI), the dual-core VCO achieves a 72% tuning at 8–17 GHz and a figure-of-merit-tuning ( $\text {FoM}_{T}$ ) of 208.8 dBc/Hz at 11 GHz. The chip operates from a 0.45 V supply with a power consumption of 17–33 mW and a core area of 0.39 mm2. To the best of our knowledge, the VCO achieves the highest $\text {FoM}_{T}$ and lowest supply for designs over 10 GHz.

This letter presents a new approach for reconfiguring substrate integrated waveguide (SIW) devices. It involves allowing or prohibiting the wave propagation by using a removable wall which is built from a series of drill holes. When the wall is required, the holes are filled with gallium-based liquid metal (LM), thus forming vias. When the wall is no longer required, the holes are emptied of LM. The method has been applied to a single-pole single-throw (SPST) SIW switch as well as a single-pole double-throw (SPDT) switch. Both switches perform well over a wide frequency bandwidth, approaching one octave. The SPST switch performs well from 2.4 to 4.3 GHz. In the ON-state (i.e., wall removed), the insertion loss (IL) is ≤0.5 dB. In the OFF-state (i.e., wall inserted), the isolation is 30 dB. The SPDT switch operates effectively from 4.7 to 7.2 GHz. The IL, between two connected ports, is 0.7 dB. The isolation between unconnected ports is 40 dB. The proposed approach will be applicable within a wide range of different reconfigurable microwave devices.

This paper presents a switched-power-combining RF power amplifier (PA) architecture, using evanescent-mode resonators as input splitters and output combiners. The input resonator also provides proper input matching to the transistor, eliminating the need for an input matching network, which reduces the overall size of the PA. The external couplings into the resonators are switchable to allow for activating one, two, or all three sub-PAs, depending on the power requirement. This results in a higher Output Backoff (OBO) efficiency, compared to a single PA. The presented concepts for the proposed architecture are supported by theoretical analysis and experimental validation. The implemented PA is based on Class-E sub-PAs, operates in the 1.2-2.4 GHz range, provides 36-40 dBm output power, and occupies an area of 41 × 82 mm2. Compared to the state-of-the-art amplifiers operating at this frequency range, this resembles an average of 80% reduction in size.

This paper reports on the design of frequency-dependent feed networks for linear antenna arrays with minimum half-power beamwidth (HPBW) variation in more than one octave of the frequency spectrum. The proposed approach is based on: i) frequency-dependent power dividers that gradually shift the RF power from the outer elements of the array to the inner ones with the increase of frequency and ii) uniformly or non-uniformly-spaced directive antenna elements. The operating principles and design trade-offs of the concept are presented through ideal simulations of four- and six-element antenna arrays shaped by isotropic or directive radiators. It is shown that by employing directive antenna elements and non-uniform spacing, minimum HPBW variation can be obtained in a broad bandwidth (BW, e.g., 3:1). For physical verification purposes, two frequency-dependent feed networks and Vivaldi antenna elements were designed, manufactured, and measured. The first network feeds a uniformly-spaced four-element array of Vivaldi antennas and demonstrates a 58% HPBW variation in a 3:1 of BW. The second network feeds a non-uniformly-spaced six-element Vivaldi array and results in a 67% HPBW variation in a 2.5:1 of BW.

We report the generation of an infrared (IR) supercontinuum spanning over an octave using a centimeter (6 cm) length of photonic crystal fiber (PCF) filled with MoS2, which has a considerably large nonlinear refractive index. The fiber is pumped with a femto-second laser pulse with an infrared wavelength of 2080 nm, pumping intensity of 336.76 GW/cm2, peak power of 0.75 GW, and pulse width of 150 fs. With the assistance of highly nonlinear MoS2, a supercontinuum bandwidth that remarkably extends from 1400 to 2700 nm below 20 dB is achieved. This supercontinuum bandwidth significantly exceeds the previously reported spectra for other nonlinear glass-based optical fibers, even with a fairly short fiber length. Furthermore, this short fiber length allows PCF to be more efficiently utilized, and is expected to be suitable for various applications, such as broadband light source through liquid-based fibers and waveguide-based optical communications, especially in the IR region.

We present the results of a theoretical calculation and an experimental study of double-sided antireflective processing of a dielectric plate by creating a specially profiled corrugated structure on its surface. Such a structure allows one to minimize the reflection coefficient of an electromagnetic wave to a level of about −30 dB within a range of at least one octave. This antireflective effect can be used in energy output windows of frequency-tunable generators operated in the millimeter- and submillimeter-wave bands. It can also be applied in windows of wideband spectroscopy devices used to diagnose a broad range of materials including both gases and condensed media, as well as in medical devices, in which the studied sample/object is placed in a vacuum-tight box. Theoretical calculations are based on solving numerically the complete equations of the electromagnetic field. Reflections from double-sided corrugated plates were studied experimentally using a Michelson interferometer. It is shown that the reflection coefficient is equal to no more than −23 dB in the 50–220 GHz frequency range. The antireflective profile was manufactured using a plate of radiation-processed fluorine F-4 (Raflon). This new material allowed us to manufacture an antireflective structure with a required accuracy.

Summary The gain bandwidth of a single-mode fiber is limited by the atomic transitions of one rare earth gain element. Here we overcome this long-standing challenge by designing a new single-mode fiber with multi-section core, where each section is doped with different gain element. We theoretically propose and experimentally demonstrate that this configuration provides a gain bandwidth well beyond the capability of conventional design, whereas the inclusion of multiple sections does not compromise single-mode operation or the quality of the transverse modal profile. This new fiber will be beneficial in realizing all fiber laser systems with few-cycle pulse duration or octave tunability., Graphical Abstract, Highlights • A theoretical study of the fiber with asymmetric gain profile is presented • A multi-section core fiber is demonstrated with unique manufacturing strategy • This fiber greatly extends gain bandwidth while maintaining a good beam quality, Optics; Fiber Optics; Optical Materials

SPM-enabled spectral selection (SESS) constitutes a powerful fiber-optic technique to generate wavelength broadly tunable femtosecond pulses. In the current demonstration, the maximum tuning range is 400 nm and the energy conversion efficiency from the pump source to the outmost spectral lobes is ∼25%. In this submission, we apply the particle swarm optimization method to the generalized nonlinear Schrödinger equation to identify the optimal parameters that maximize both the tuning range and the conversion efficiency. We show that SESS in an optical fiber with the optimized dispersion can deliver SESS pulses tunable in one octave wavelength range and the conversion efficiency can be as high as 80%. We further show the feasibility of experimental implementation based on specially designed fibers or on-chip waveguides.

Recent progress on supercontinuum generation of OAM modes in optical fiber is reviewed. Two-octave SC generation of up to OAM17,1 mode spanning from 1560 to 6250 nm in the high-index ring fiber will be discussed.

Microcombs have exhibited outstanding potential in the fields of optical frequency synthesizer, atomic clock, lidar, etc. To achieve a fully stabilized optical frequency comb, the microcombs should simultaneously have a nearly octave spectrum for self-referenced locking and a relatively small comb line spacing so that the beating frequency can be located in a comfortable photodetection bandwidth, e.g., 10 GHz. Unfortunately, it is still challenging for microcombs based on integrated microresonators, mainly due to the limitation on waveguide loss and dispersion control. Moreover, it is also highly desired that the microcombs can be stabilized in a simple way rather than complicated control.

Passive optical resonators are widely used for power enhancement, high harmonic generation and sensitive absorption spectroscopy [1] - [4] . The usual way of "filling" such a resonator with light is to couple in through a slightly transparent mirror, via constructive interference. We demonstrate an alternative method by using a thin wedged plate in the cavity, at an angle of incidence (AOI) close to Brewster’s angle. By adjusting the AOI on the plate, the cavity can be operated at different values of finesse without changing components. The main advantage, however, is the possibility of using low-dispersion mirrors, e.g., gold mirrors for the cavity, allowing for ultrabroadband resonant enhancement. We note that related ways of input coupling have been previously demonstrated. A parallel plate, one side of which was antireflection coated [5] and a plate with a slight wedge angle have been applied for input coupling [6] . However, both of these experiments were strongly limited in bandwidth and were conducted with continuous-wave lasers.

Recent policy changes to the United States radar S-band allocation requires that radar transmitters share the 3.45 to 3.7 GHz range with fifth-generation (5G) wireless communication systems. As more radar spectrum is designated for sharing, reconfigurable high-power radar transmitters are needed that can maintain performance while quickly adjusting operating frequency. At the heart of a frequency-agile radar transmitter is a reconfigurable matching network, placed between the power amplifier and the antenna, that can maintain optimal range performance in real time by adaptively matching the antenna to the amplifier. Our impedance tuner consists of laser diodes illuminating semiconductor-plasma switches to selectively expose six microstrip stubs and has an octave tuning range from 2–4 GHz. Using an advanced tuning algorithm, the tuner can optimize output power in approximately 260 µs under control of a software-defined radio platform, an improvement of three orders of magnitude over presently available high-power tuning technologies.

We developed a simple device to teach the concept of pH to visually impaired people through an experimental approach. An interface between a glass electrode and an open-source electronics platform based on easy-to-use hardware and software (Arduino Uno) was designed using an operational amplifier of ultralow input current. A headphone connected to digital output from Arduino Uno was employed to convert the electrical signals from pH to sound frequency. Using an audio file that emits sound frequencies, visually impaired students were trained to associate specific frequencies with pH values. There is a direct relationship between the logarithm of sound frequency from a given musical note and a pH value, which can be used to train students with low vision who would otherwise have difficulty observing pH values from visual indications. Sound frequency signals of 131–659 Hz that correspond to the piano musical scale of C3, which is one octave below middle C (small octave or the “Do” note in solfege), to E5, on...

In the safe driving of driverless vehicles, vehicle acoustic identification is one of the most important parts in driverless applications. During the process of driving the vehicle, there are many external interference factors, among them the vehicle acoustic signal is a complex multi-source signal, which is difficult to extract effective feature values. In this paper, an improved algorithm for extracting eigenvalues of vehicle acoustic signals is proposed to achieve a higher correct identification rate for different types of vehicles. Traditional frequency division method is improved by analyzing the characteristics of the 1/3 octave and the human ear Bark domain frequency sensing method. The algorithm extracts the effective energy feature values of different frequency ranges from the vehicle acoustic signals, and uses a probabilistic neural network for vehicle identification. Experimental results show that the recognition rate of trucks reaches 100%, the recognition rate of small vehicles, medium-sized vehicles and Buses reaches 90%, and the recognition rate of motorcycles is 85%. It indicates that the algorithm is feasible in vehicle type recognition and has a good recognition rate, and can be a reference for related vehicle identification researches.

Implementing optical-frequency combs with integrated photonics will enable wider use of precision timing signals.Here, we explore the generation of an octave-span, Kerr-microresonator frequency comb, using hybrid integration ofan InP distributed-feedback laser and a SiN photonic-integrated circuit. We demonstrate electrically pumped and fiber-packaged prototype systems, enabled by self-injection locking. This direct integration of a laser and a microresonatorcircuit without previously used intervening elements, like optical modulators and isolators, necessitates understand-ing self-injection-locking dynamics with octave-span Kerr solitons. In particular, system architectures must adjust tothe strong coupling of microresonator back-scattering and laser-microresonator frequency detuning that we uncoverhere. Our work illustrates critical considerations towards realizing a self-referenced frequency comb with integratedphotonics., 9 pages, 4 figures. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in APL Photonics 6, 026102 (2021) and may be found at https://doi.org/10.1063/5.0035452

We report the first femtosecond polycrystalline Cr:ZnS oscillator with an octave-spanning spectrum. Average power of the laser is 1.4 W at the mid-IR central wavelength 2.4 μm and the pulse repetition frequency 79 MHz.

We apply particle swarm optimization algorithm to optimize SPM-enabled spectral selection (SESS) and demonstrate highly efficient SESS which delivers MW peak-power femtosecond pulses with the wavelength tuning range exceeding one octave.

Low propagation loss in high confinement waveguides is critical for chip-based nonlinear photonics applications. Sophisticated fabrication processes which yield sub-nm roughness are generally needed to reduce scattering points at the waveguide interfaces in order to achieve ultralow propagation loss. Here, we show ultralow propagation loss by shaping the mode using a highly multimode structure to reduce its overlap with the waveguide interfaces, thus relaxing the fabrication processing requirements. Microresonators with intrinsic quality factors (Q) of 31.8 $\pm$ 4.4 million are experimentally demonstrated. Although the microresonators support 10 transverse modes only the fundamental mode is excited and no higher order modes are observed when using nonlinear adiabatic bends. A record-low threshold pump power of 73 $\mu$W for parametric oscillation is measured and a broadband, almost octave spanning single-soliton frequency comb without any signatures of higher order modes in the spectrum spanning from 1097 nm to 2040 nm (126 THz) is generated in the multimode microresonator. This work provides a design method that could be applied to different material platforms to achieve and use ultrahigh-Q multimode microresonators.

We developed a broadband terahertz wave circular polarizer which consists of a 2-dimensional array of 3-dimensional metallic helix. We evaluate the performance of polarizer using terahertz time domain spectroscopy and we confirmed that the array of helices transmits the circularly polarized terahertz wave with the opposite handedness as that of helices. The polarizer covers the frequency range from 150 GHz to 250 GHz, close to one octave. We obtained the ellipticity of circularly polarized THz wave close to unity in this frequency band.

A depressed cladding waveguide, written by the femtosecond laser beam in the dry tellurite 70TeO 2 – 22WO 3 – 8Bi 2 O 3 glass allows obtaining octave-spanning supercontinuum in the 1.5 – 3.4 µm range under pumping at 2.2 µm by 260 fs pulses. Theoretical simulations of nonlinear propagation of the ultrashot pulses are in good agreement with experimental results.

In this paper, a solid plasma antenna is proposed and analyzed. This solid plasma antenna is composed of silicon lateral PIN diodes, in which the i-region is exposed on the silicon wafer. The lateral PIN diodes in sufficient forward bias could be regarded and analyzed as plasmas, which could be a substitution of traditional metal in antenna structures, due to the carrier density in diodes maintaining at a high level. The solid plasma dipole antenna with bias feeding network has been designed and fabricated. The antenna resonant frequency could shift by step over an octave.

The noise generated at the interface between the wheels of vehicles and the road surface is well recognized in the literature worldwide. Many publications describe the phenomenon of reducing this kind of impact by silent road surfaces. A specific type of this noise is the sound generated by vehicles passing over the expansion joints of bridge structures. Due to the impulsive nature of this sound, it is very onerous for people living in the close vicinity of bridge structures. The passage of vehicles over expansion joints causes the formation of vibrations that are transmitted to the structural elements of bridge structures, which may cause the formation of the material sounds (especially arduous in the case of bridges with steel elements). An attempt to reduce this impact was made by making a prototype acoustic cover of the expansion joint on the selected bridge. The paper presents the results of research on the &ldquo, in situ&rdquo, acoustic effectiveness of this cover. Additionally, the noise was modelled in the object surroundings before and after the cover&rsquo, s application. The acoustic efficiency of the cover in the whole measured frequency range was 5.3 dBA. In the narrower frequency bands (1/3 octave bands), larger sound level reductions were observed. The maximum sound levels measured under the tested dilatation were less than 10.0 dBA lower than the maximum sound levels measured under the reference dilatation.

We show the characterisation of spectral broadening in the Tantalum Pentoxide waveguide system as a function of pump wavelength, showing spectra for central pump wavelengths of 0.9 to 1.5 um (150 fs, 80 MHz). We have achieved octave spanning spectra with approximately 5 mW of laser power coupled in the waveguide at 1 um pumping wavelength for a linear buried waveguide using a commercial source.

Presently, large cities have significant problems with noise pollution due to human activity. Transportation, economic activities, and leisure activities have an important impact on noise pollution. Acoustic noise monitoring must be done with equipment of high quality. Thus, long-term noise monitoring is a high-cost activity for administrations. For this reason, new alternative technological solutions are being used to reduce the costs of measurement instruments. This article presents a design for a versatile electronic device to measure outdoor noise. This device has been designed according to the technical standards for this type of instrument, which impose strict requirements on both the design and the quality of the device&rsquo, s measurements. This instrument has been designed under the original equipment manufacturer (OEM) concept, so the microphone&ndash, electronics set can be used as a sensor that can be connected to any microprocessor-based device, and therefore can be easily attached to a monitoring network. To validate the instrument&rsquo, s design, the device has been tested following the regulations of the calibration laboratories for sound level meters (SLM). These tests allowed us to evaluate the behavior of the electronics and the microphone, obtaining different results for these two elements. The results show that the electronics and algorithms implemented fully fit within the requirements of type 1 noise measurement instruments. However, the use of an electret microphone reduces the technical features of the designed instrument, which can only fully fit the requirements of type 2 noise measurement instruments. This situation shows that the microphone is a key element in this kind of instrument and an important element in the overall price. To test the instrument&rsquo, s quality and show how it can be used for monitoring noise in smart wireless acoustic sensor networks, the designed equipment was connected to a commercial microprocessor board and inserted into the infrastructure of an existing outdoor monitoring network. This allowed us to deploy a low-cost sub-network in the city of M&aacute, laga (Spain) to analyze the noise of conflict areas due to high levels of leisure noise. The results obtained with this equipment are also shown. It has been verified that this equipment meets the similar requirements to those obtained for type 2 instruments for measuring outdoor noise. The designed equipment is a two-channel instrument, that simultaneously measures, in real time, 86 sound noise parameters for each channel, such as the equivalent continuous sound level (Leq) (with Z, C, and A frequency weighting), the peak level (with Z, C, and A frequency weighting), the maximum and minimum levels (with Z, C, and A frequency weighting), and the impulse, fast, and slow time weighting, seven percentiles (1%, 5%, 10%, 50%, 90%, 95%, and 99%), as well as continuous equivalent sound pressure levels in the one-third octave and octave frequency bands.

The delayed optical nonlinearity of molecules is harnessed to generate a multi-octave supercontinuum and compress 280 fs pulses from a commercial Yb:KGW laser amplifier to sub-two cycle duration using an N2O filled hollow-core fiber.

Self-referenced dissipative Kerr solitons (DKSs) based on optical microresonators offer prominent characteristics including miniaturization, low power consumption, broad spectral range and inherent coherence for various applications such as precision measurement, communications, microwave photonics, and astronomical spectrometer calibration. To date, octave-spanning DKSs with a free spectral range (FSR) of ~1 THz have been achieved only in ultrahigh-Q silicon nitride microresonators, with elaborate wavelength control required. Here we demonstrate an octave-spanning DKS in an aluminium nitride (AlN) microresonator with moderate loaded Q (500,000) and FSR of 374 GHz. In the design, a TE00 mode and a TE10 mode are nearly degenerate and act as pump and auxiliary modes. The presence of the auxiliary resonance balances the thermal dragging effect in dissipative soliton comb formation, crucially simplifying the DKS generation with a single pump and leading to a wide single soliton access window. We experimentally demonstrate stable DKS operation with a record single soliton step (~80 pm) and octave-spanning bandwidth (1100-2300 nm) through adiabatic pump tuning and on-chip power of 340 mW. Our scheme also allows for direct creation of the DKS state with high probability and without elaborate wavelength or power schemes being required to stabilize the soliton behavior., Comment: 12 pages, 5 figures, 1 table, 5 pages supplementary with 4 figures

We demonstrate on-record spectral span microcombs (450–2000 nm) in an asymmetric optical microcavity. The two-octave-span intracavity emission is harnessed to a nanowaveguide through the chaos-assisted broadband coupling. © 2020 The Author(s)

We report octave spanning frequency combs generated by self-injection locking a distributed feedback laser to a Kerr microresonator. This method benefits from mitigated thermal transients and eliminates an optical isolator which simplifies photonic integration. ©2020 The Author(s)

A near octave comb spectrum ranging from 1100 to 2150 nm is generated at 406 mW pump power for AlN microresonators fabricated with standard photolithography. This work decreases the requirement for fabrication significantly.

Abnormal combustion phenomena are among the main hurdles for the introduction of hydrogen in the transportation sector through the use of internal combustion engines (ICEs). For that reason the challenge is to guarantee operation free from combustion anomalies at conditions close to the ones giving the best engine output (maximum brake torque and power). To this end, an early and accurate detection of abnormal combustion events is decisive in order to allow the electronic control unit deciding suitable correcting actions. In this work, an automotive size 4-cylinder 1.4 L naturally aspirated port-fuel injection spark ignition Volkswagen engine adapted to run on hydrogen has been investigated. Three distinct methods (in-cylinder pressure, block-engine vibration and acoustic measurements) have been employed to detect abnormal combustion phenomena provoked through the enrichment of the hydrogen-air mixture fed to the cylinders under a wide range of engine speeds (1000–5000 rpm). It has been found that the high-frequency components of the in-cylinder pressure and block engine acceleration signals obtained after a Fourier transform analysis can be used for very sensitive detection of knocking combustion cycles. In the case of the ambient noise measurements, a spectral analysis in terms of third octave bands of the signal recorded by a microphone allowed an accurate characterization. Combustion anomalies could be detected through more intense octave bands at frequencies between 250 Hz and 4 kHz in the case of backfire and between 8 kHz and 20 kHz for knock. Computational fluid dynamics simulations performed indicated that some characteristics of the engine used such as the cylinder valves dimensions and the hydrogen flow rate delivered by the injectors play important roles conditioning the likelihood of suffering backfire events.

We demonstrate spectral broadening and compression of amplified pulses from a titanium sapphire laser system using an argon-filled stretched, hollow-core fiber and an acousto-optic modulator based pulse-shaper. We characterize the pulses using pulse-shaper assisted collinear frequency resolved optical gating, pulse-shaper assisted D-scans, and D-scans using a variable path length water cell. The different compression and characterization approaches consistently compress the pulses down to < 6 fs, less than ∼1 fs from the transform limit. We discuss prospects for pulse shape spectroscopy with these broadband pulses, given our control over the spectral amplitude and phase.

We present a paradigm for integrated photonic devices based upon broadband slab-confined collimated beams that are launched with half-Maxwell fisheye lenses. Although it is challenging to match to the low-index focus of the lens whilst maintaining adequate field confinement for a well-defined point source, integrated dielectric slot waveguides prove highly suitable, yielding collimators of 90% efficiency and over one octave bandwidth. Terahertz technology will benefit from such broadband slab-confined beams to replace free-space optics, toward compact, mass-producible systems that do not require manual optical alignment. We present two prototype systems to demonstrate the versatility of this concept, namely a diagonally-set distributed Bragg reflector as frequency-division diplexer for terahertz communications, and an attenuated total reflection-based liquid sensor. Both are enabled by oblique in-slab reflections that are collected at a location other than the originating lens, which is not attainable using ordinary single-mode channel waveguides., Comment: 10 pages, 6 figures

We report on experimental generation of a 6.8 μJ laser pulse spanning from 1.8 to 4.2 μm from cascaded second-order nonlinear processes in a 0.4-mm BiB3O6 (BIBO) crystal. The nonlinear processes are initiated by intra-pulse difference frequency generation (DFG) using spectrally broadened Ti:Sapphire spectrum, followed by optical parametric amplification (OPA) of the DFG pulse. The highest energy, 12.6 μJ, is achieved in a 0.8-mm BIBO crystal with a spectrum spanning from 1.8 to 3.5 μm. Such cascaded nonlinear processes are enabled by the broadband pump and the coincident phase matching angle of DFG and OPA. The spectrum is initiated from the DFG process and is thus expected to have passive stable carrier-envelope phase, which can be used to seed either a chirped pulse amplifier (CPA) or an optical parametric chirped pulse amplifier (OPCPA) for achieving high-energy few-cycle mid-infrared pulses. Such cascaded second-order nonlinear processes can be found in many other crystals such as KTA, which can extend wavelengths further into mid-infrared. We achieved a 0.8 μJ laser pulse spanning from 2.2 to 5.0 μm in KTA.

Standardised measurements of sound attenuation of hearing protectors are performed in the frequency range from 125 Hz to 8 kHz. However, noise present at many workplaces contains significant components at higher audible frequency. Therefore, the knowledge about noise attenuation with earmuffs in the audible frequency range above 8 kHz is also necessary for proper hearing protection. The aim of this study was to obtain values of the noise attenuation with 27 commonly-used earmuffs models in the 1/3 octave bands of 10, 12.5 and 16 kHz. The measurements were conducted with a real ear at threshold (REAT) method with participation of subjects. The study showed that attenuation of earmuffs ranged from 24.7 to 42.8 dB, depending on model of earmuffs and frequency band. Furthermore, the measurements were performed with the use of acoustic test fixture which is designed especially for testing hearing protectors. Results obtained with the use of acoustic test fixture indicated that this measurement method can lead to values close to attenuation measured with participation of subjects. On the other hand, values obtained with the use of acoustic test fixture may differ average up to 14 dB from REAT method.

The real attenuation of hearing protection devices (HPD) can be assessed in the field using a method based on continuous field microphone-in-real-ear (F-MIRE) measurements. The two-microphone method provides an indicator called the measured noise reduction (NR∗), defined as the difference between the measured exterior (outside the protector) and interior (under the protector) sound pressure levels (SPL). The HPD’s attenuation expressed in terms of the more common insertion loss (IL) can then be obtained from NR∗ using compensation factors. For earmuffs, NR∗ has been shown to vary of up to 20 dB depending on the angle of incidence of the sound source. Therefore, there is a need to use sound incidence dependent compensation factors to relate NR∗ and IL. To evaluate these factors and more generally to improve the continuous F-MIRE method, a finite-element (FE) model of an earmuff on an ATF (acoustic test fixture) exposed to a directional sound field has been developed and its predictions compared with lab measurements for several incidence angles. Regarding the external microphone SPL and the NR∗, in one-third of octave bands, the model correlates very well with measurements for frequencies below 1250 Hz whatever the sound incidence. Above 1250 Hz, the FE model captures the trends, as a function of the incidence angle, but the agreement generally decreases with increasing frequency. A better correlation between the FE model and the experimental data is achieved for the variation of NR∗ (ΔNR∗) as a function of the sound incidence. Actions, such as (i) accounting for the headband in the model, (ii) refining the modeling of the sound source, (iii) improving the cushion modeling and (iv) better describing the backplate/cushion coupling conditions, are suggested to improve the model accuracy. To illustrate the potential of the modeling to improve the continuous F-MIRE measurement method, the FE model is used to determine an optimal position of the external microphone and to obtain estimates of exposure levels using the left and right ear exterior microphones.

We report the first demonstration of octave spanning frequency combs generated by self-injection locking a distributed feedback laser to a Kerr microresonator. This method benefits from mitigated thermal transients and eliminates an optical isolator which simplifies photonic integration.

A homogeneous THz Quantum Cascade Laser with emission spectra spanning over 1.65 THz in a bi-stable, voltage driven regime is presented. A nearly 1 THz emission spectrum in the stable regime showing a narrow electrical beatnote indicating frequency comb operation is observed. Such a narrow beatnote could then be injection locked to an external RF source with powers low as -55 dBm. By increasing the injection power and sweeping the injection frequency spectral emission state can be modified at nominally same bias. Together with the low threshold current density of 115 A/cm-2 this design could lead to octave spanning THz QCL frequency combs.

A wideband tunable active frequency selective surface absorber structure is studied in this paper. The band with reflection coefficient below -10dB extends from 0.9GHz to 5.4GHz, about six octave. The thickness of this absorptive radome is 20mm. This structure consists of three layers, the bottom is a metal reflector. The middle layer is a tunable active frequency selective surface absorber realized by varactor diode. The middle layer and the bottom layer is used to tune the absorption band from 0.9GHz to 2GHz, and it has the same reflection characteristics with metal plate from 2GHz to 5.4GHz. The upper layer is double square-loop with resistance, it is used to absorb waves from 2GHz to 5.4GHz. Therefore, the structure can achieve wideband wave absorption effect from 0.9GHz to 5.4GHz. The simulation results show that with the increase of incident angle, the absorbing property is deteriorating, but it still has certain wave absorbing property.

Industry 4.0 is focusing on modern maintenance techniques to minimize the downtime so that the production can be increased. In order to implement Industry 4.0, the maintenance department needs automatic condition monitoring and fault diagnostics techniques. In this study, octave analysis has been proposed for early detection of bearing faults. In order to show the effectiveness of the proposed methodology, detailed experimentations have been performed on a Machinery Fault Simulator (MFS). Octave analysis has been implemented on the vibrational data acquired through the accelerometers placed on the inboard and outboard bearing housing. The vibrational data of a healthy bearing has been compared with a faulty bearing at four different rotational speeds under two types of loading conditions: no load applied and a constant load applied near outboard bearing housing. The standard deviation has also been calculated for healthy and faulty bearings. The experimental results revealed that octave plots have high peaks for all rotational speeds and both loading conditions. The values of the standard deviation of faulty bearings were found high as compared to a healthy bearing. This research article proposes a simple approach that does not require signal processing. The results proved that octave analysis along with standard deviation can be used for early detection of rolling bearing faults.

We present experimental characterization of W-band serpentine waveguide TWT circuits fabricated by CNC micro-endmill machining. Cold test measurements of S-parameters demonstrate excellent agreement with simulation and precise fabrication repeatability between multiple circuits. The circuit wavelength dispersion was measured in both $W$ - and D-band waveguide bands, covering an octave in frequency, showing agreement with simulation from 85–170 GHz. We discuss details of the fabricated circuits and cold test results.

Accessing the molecular fingerprint region between 2 and 20 μm is a key aspect in modern metrology and spectroscopy. While the wavelength range from 2-5 μm can easily be addressed through nonlinear frequency conversion starting from well-matured 1 μm driving lasers, access to the deep mid-IR wavelength regime is difficult. This is, because of the limited transmission of non-oxide crystals (that offer high nonlinearity and good transmission for the aspired mid-IR idler) at the pump wavelength and/or multi-photon absorption. Shifting to a longer pump wavelength relieves these limitations. In this work we present an experiment based on intra pulse difference frequency generation (IPDFG) in GaSe driven by an ultrafast Tm-doped chirped pulse amplifier. This experiment led to an octave spanning mid-IR spectrum, covering the wavelength range between 7.2-16.5 μm (-10 dB width) with 450 mW of average power at 1.25 MHz repetition rate. This result outperforms comparable sources driven at 1 μm wavelength in average power and conversion efficiency, while providing much broader spectral coverage. To further facilitate the use of these promising sources in real-world spectroscopic applications, we have built a nonlinear amplifier, which, based on its compact and robust design is an ideal candidate in this respect. Optimizing the output ultimately led to high pulse quality 50 fs pulses with 250 nJ of pulse energy at 80 MHz of repetition rate and 20 W average output power, exceeding current designs in the anomalous dispersion regime by 1 order of magnitude. It is our ongoing effort to utilize this laser for parametric downconversion. Covering the wavelength regime beyond 5 μm wavelength would make it an enabling technology for next generation spectroscopy, fundamental and life sciences.

Dispersive mirrors operating in a broadband infrared spectral range are reported for the first time. The mirrors are based on Si/SiO2 thin-film materials. The coatings exhibit reflectance exceeding 99.6% in the spectral range from 2 to 3.2 µm and provide a group delay dispersion of -100 fs2 and -200 fs2 in this range. The fabricated mirrors are expected to be key elements of Cr:ZnS/Cr:ZnSe femtosecond lasers and amplifiers. The mirrors open a new avenue in the development of ultrafast dispersive optics operating in the infrared spectral range.

We overview Si/SiO2 mirrors operating in the spectral range 2-4 um. The coatings exhibit reflectance exceeding 99% and provide group delay dispersion of -200 fs2. The mirrors are key elements of Cr:ZnS/Cr:ZnSe femtosecond lasers and oscillators.

Parametric nonlinear optical processes allow for the generation of new wavelengths of coherent electromagnetic radiation. Their ability to create radiation that is widely tunable in wavelength is particularly appealing, with applications ranging from spectroscopy to quantum information processing. Unfortunately, existing tunable parametric sources are marred by deficiencies that obstruct their widespread adoption. Here we show that ultrahigh-Q crystalline microresonators made of magnesium fluoride can overcome these limitations, enabling compact and power-efficient devices capable of generating clean and widely-tunable sidebands. We consider several different resonators with carefully engineered dispersion profiles, achieving hundreds of nanometers of sideband tunability in each device when driven with a standard low-power laser at 1550 nm. In addition to direct observations of discrete tunability over an entire optical octave from 1083 nm to 2670 nm, we record signatures of mid-infrared sidebands at almost 4000 nm. The simplicity of the devices considered -- compounded by their remarkable tunability -- paves the way for low-cost, widely-tunable sources of electromagnetic radiation., Comment: 10 pages, 4 figures, submitted 1st of November 2018

A dual-core photonic crystal fiber (DC-PCF) of hexagonal lattice is designed with a tailored dispersion profile possessing a large air-filling fraction. Numerical investigation of the PCF reveals a great potential for supercontinuum (SC) generation extending 0.4 μm to 2.0 μm, when excited by a 50 fs input pulse of 800 nm pumping wavelength, 79 MHz repetition rate and a coupled average power of 135 mW corresponding to pulse energy of 1.71 nJ in a 10 cm DC-PCF. The PCF and the SC spectra thus showed desirable properties for applications in biological sensors, optical fiber communication, frequency metrology and optical coherence tomography.

We developed a broadband terahertz wave circular polarizer that consists of a two-dimensional (2D) array of three-dimensional metallic helices. Each helix operates in an axial mode of operation where the wavelength of resonance is comparable to the dimensions of the helix. We evaluated the performance of the polarizer using standard terahertz time domain spectroscopy, and we confirmed that the array of helices transmits a circularly polarized terahertz wave with opposite handedness as that of the helices. The polarizer covers the frequency range from 117 GHz to 208 GHz, close to one octave. We obtained the ellipticity of the circularly polarized terahertz wave close to unity in this frequency band.