Broadband Transmission Research Articles

Silicon‐Nitride‐Integrated Hybrid Optical Fibers: A New Platform for Functional Photonics

AbstractHybrid optical fibers that integrate exotic materials within more traditional silica glass architectures open a route for the development of highly functional all‐fiber photonic systems. Here, a compact hybrid optical fiber platform is reported formed by depositing a silicon nitride (SiNx ‐ nitride‐rich) nanolayer onto the surface of fused‐silica microfibers via plasma‐enhanced chemical vapor deposition. The SiNx thickness can be precisely tuned over a range of tens of nanometers, while maintaining an ultra‐smooth deposition surface, allowing for tunable coupling between the modes guided predominantly in the nanolayer and the fiber core. The effective indices of the hybrid modes display an anti‐crossing behavior under resonant conditions, resulting in a rich dispersion landscape that can be tailored via adjusting the SiNx thickness. By fabricating a SiNx‐silica hybrid microfiber with precise dispersion engineering and a low insertion loss, a flat supercontinuum spectrum spanning >1.5 octaves (−20 dB level) has been generated. The results demonstrate that SiNx‐silica hybrid microfibers can offer a unique combination of broadband transmission and wide tunablity of the mode properties, while still retaining the benefits of robust integration with conventional silica glass fiber networks, providing a rich playground for hybrid fiber‐based photonic systems.

Evaluating the effectiveness of Doppler frequency shift determination using pilots in broadband transmission

In underwater communications, reciprocal motion between transmitter and receiver has a significant impact on reception quality. In orthogonal broadband systems that provide high bit rates, this problem becomes more important, especially in the higher frequency range, where the absolute Doppler shift is the greatest. Due to the low propagation speed of acoustic wave underwater, a substantial difference exists between the Doppler shift for lower and upper frequencies of the utilized spectrum. Consequently, a frequency-independent Doppler shift factor is employed. One of the most popular methods for determining the Doppler shift is the use of pilots. The problem of selecting the number and determining the frequency of pilots in such a way as to obtain the lowest possible error rate was identified. Real-world testing was conducted in a multipath propagation environment with relative speeds of up to 1.5 m/s. The effectiveness of Doppler shift determination was evaluated by analyzing the bit error rate. The results of the conducted tests indicate that, based on the achieved error rate, it is sufficient to employ 7 pilots positioned at low frequencies.

Broadband high birefringence and single-polarization hollow-core anti-resonant fibers with an elliptical-like core

Due to the light-guiding mechanism of hollow-core anti-resonant fibers (HC-ARFs), the low overlap between core modes and anti-resonant layers poses a challenge in achieving high birefringence. To address this issue, we propose three high-birefringence HC-ARFs with an elliptical core and varying cladding compositions and conduct a detailed optimization and analysis for structure (c). Simulation results demonstrate that structure (a) and structure (b) provide high birefringence, broadband, and low-loss transmission properties. Specifically, structure (a) achieves a birefringence greater than 1.3×10−4 over a 680 nm wavelength range, and structure (b) achieves a birefringence greater than 1.6×10−4 over a 580 nm wavelength range, both maintaining fundamental mode (FM) loss below 1 dB/m. Structure (c) enhances birefringence by an order of magnitude and offers excellent single-polarization properties by introducing high-refractive-index material. At 1.55μm, structure (c) achieves a birefringence of 0.98×10−3, with a low FM loss for x-polarization of 0.01 dB/m and a polarization extinction ratio (PER) of 57450. Additionally, structure (c) exhibits low bend loss in the x-direction, with only 0.06 dB/m for a bend radius of 6 cm.

A switching norm based least mean Square/Fourth adaptive technique for sparse channel estimation and echo cancellation

To realize rapid data transmission, the broadband transmission technique is being extensively explored and applied in existing wireless communication systems. The multi-path channel in broadband wireless communication systems is sparse and this sparsity can be used as prior knowledge to estimate the channel. To make use of sparsity, this paper recommends a switching norm-based least mean square/fourth (SN-LMS/F) adaptive approach for sparse channel estimation and echo cancellation. The suggested SN-LMS/F is implemented by adding a soft parameter adjustment function (SPF) into the conventional LMS/F adaptive method's cost function and utilizes both the l0 and l1 norm to exploit system sparsity with reduced complexity. The simulated output indicates that the suggested SN-LMS/F adaptive technique provides a more desirable performance for sparse channel estimation and echo cancellation with reduced execution time.

RIS-Based Self-Interference Cancellation for Full-Duplex Broadband Transmission

RIS-Based Self-Interference Cancellation for Full-Duplex Broadband Transmission

Durable self-cleaning anti-fog and antireflective titanium-based micro-nano structures on glass by laser marker ablation

Hierarchical micro-nano structured glass surfaces were produced by a low-cost laser marker ablation of Titanium-coated glass. The samples were systematically analyzed in terms of composition, structure, morphology, transparency, and anti-fog performance. The influences of varying laser scan hatch distance and Titanium (Ti) coating thickness on the resulting surface properties were studied. The results showed that the surface exhibited a 30 μm-sized periodic hillock-hollow micro-structure, with widely dispersed nanoparticles of approximately 50 nm in diameter all over the micro-structure. The most favorable surface characteristics were achieved with a 250 nm Ti coating and a 0.03 mm hatch distance, resulting in a notable enhancement of the average transmission of the glass by 1.29 % over a wide spectral range spanning from 400 nm to 1100 nm. Additionally, the treated surface exhibited super-hydrophilicity, evidenced by a contact angle of 0°, thereby demonstrating excellent anti-fog performance in fog tests that remained effective for an impressive duration of 318 days. It also exhibits exceptional anti-fouling and self-cleaning attributes, which remain effective over a span of 459 days. This outstanding antifog self-cleaninng performance and broadband transmission enhancement was, for the first time, co-achieved for laser treated glass. These accomplishments hold substantial practical relevance in diverse applications.

Broadband metasurface bandpass filter with wide angular stability for the Ku-band

Broadband spatial spectral filters have consistently been in high demand due to their diverse and extensive applications across numerous fields in the past decades. In this paper, a broadband metasurface (MS) bandpass filter (BPF) with wide angular stability was proposed and investigated at Ku band. The unit-cell of the proposed MS BPF is consisted of the cross-shape structure (CSS) sandwiched with two layer identical four-square-patches (FSPs) and dielectric substrates. Finite element method (FEM) simulation indicates that the designed MS BPF has distinctive broadband transmission coefficient of over −3 dB from 12.75 GHz to 16.8 GHz with a relative bandwidth of 27.4 %, which is basically consistent with equivalent circuit model (ECM) calculation. The physical mechanism underlying the proposed MS BPF is elucidated through the introduction of impedance matching theory, electric field and surface current analysis. Further numerical simulation has confirmed that the designed broadband MS BPF exhibits valid performance across a wide spectrum of incident angles, encompassing both transverse electric (TE) and transverse magnetic (TM) polarizations. Finally, a prototype has been fabricated for measurements, and the proposed design has been validated through comprehensive analysis of simulated and measured results. Due to its excellent transmission properties, the proposed MS BPF shows potential application in such as radar, remote sensing, and satellite communication.

Frequency selective rasorber based on cross bend resonators for wideband transmission and absorption

The wideband absorption and transmission of frequency-selective rasorber (FSR) remain a persistent challenge in the application of radar devices. In this article, a novel high performance wideband FSR design based on cross bend resonators was proposed. The FSR consists of an upper absorption lossy layer, which offers broad absorption and transmission bands, and a lower bandpass frequency-selective surface that enables a highly selective transmission of incident electromagnetic wave. Full wave simulation results showed that this novel design achieves an absorption bandwidth of 83.7% with more than 90% absorptivity in the frequency range of 5.2–12.7 GHz. Furthermore, the passband’s fractional bandwidth for the insertion loss (IL) less than −3 dB is 33.9%, ranging from 14.9 to 21 GHz, with the minimum IL recorded at 0.69 dB at 17.7 GHz. To further verify the proposed method, a prototype FSR with 10 × 10 units of 120 mm × 120 mm was fabricated and the performance of the FSR was tested. The experiment results were in good agreement with the simulated results, and it showed a significant monostatic radar cross-section reduction in the frequency range of 5.3 GHz to 18.3 GHz compared with a metallic plane of the same size.

Three-dimensional acoustic propagation of noise from impact pile driving in a complex costal environment and its effects on large yellow croaker (Pseudosciaena crocea)

The effects of high-intensity impulsive noise generated by pile driving on fish are a major concern in environmental impact assessments. Numerical acoustic models are essential for predicting underwater-acoustic-related problems in complex coastal environments prior to offshore construction. However, underwater noise modeling for impact pile driving has often been performed using simplistic propagation models that are inadequate for three-dimensional (3D) environments. A 3D parabolic equation method (PE) was established in this study to better predict broadband transmission loss (TL) from impact pile driving in complex coastal environments and its influence on the large yellow croaker (Pseudosciaena crocea). The effects of 3D propagation were investigated using two realistic scenarios with different bathymetric complexities. The values and attenuation rate of the broadband TL for the steeply sloped bottom were significantly greater than those for the flat and weakly varying bottoms over 3 km. At a water depth of 5 m, a difference of approximately 10 dB was observed between the two TL scenarios at a distance of 4.5 to 5 km. The simulation results are in reasonable agreement with the field measurement data, with a difference of less than 3 dB. The zones of behavioral response and injury in the large yellow croaker were estimated using the For3D model. The results showed that the effects of the noise generated by the impact pile driving on the large yellow croaker were evident and three-dimensional. Therefore, 3D propagation effects should be considered when analyzing the influence of underwater noise on marine animals.

Design of Free-space Optical Communication Terminal Considering for Korean Domestic Weather Conditions

Modern military operations rely heavily on broadband communication and data transmission. Recently, the rising use of intelligent unmanned technology necessitates more frequencies. Free-space optical(FSO) communication can offer high-data-rate communications with high security and no need for licensing. Therefore, the FSO communication holds significant interest and potential in the defense industry. In this paper, we present design of a FSO communication terminal taking Korean domestic weather conditions into account. The domestic atmospheric attenuation is analyzed using several models and two-year meteorological information for a city in Korea, and this analysis is utilized to design the FSO communication terminal. The design results were verified using an FSO communication test bed, and we achieved an Ethernet bandwidth of approximately 1.86 Gbps at a distance of 1.3 km with the optical amplifier output power of the test bed set to about 20 dBm.

Broadband tunable transmission non-reciprocity in thermal atoms dominated by two-photon transitions

We propose a scheme for realizing broadband and tunable transmission non-reciprocity by utilizing two-photon near-resonant transitions in thermal atoms as single-photon far-detuned transitions can be eliminated. Our basic idea is to largely reduce the Doppler broadenings on a pair of two-photon, probe and coupling, transitions and meanwhile make the only four-photon transition Doppler-free (velocity-dependent) for a forward (backward) probe field. One main advantage of this scheme lies in that the transmission non-reciprocity can be realized and manipulated in a frequency range typically exceeding 200 MHz with isolation ratio above 20 dB and insertion loss below 1.0 dB by modulating an assistant field in frequency and amplitude. The intersecting angle between four applied fields also serves as an effective control knob to optimize the nonreciprocal transmission of a forward or backward probe field, e.g. in a much wider frequency range approaching 1.4 GHz.

Transmission Coefficient-Based Monitoring of Microwave Ablation: Development and Experimental Evaluation in Ex Vivo Tissue.

To assess the feasibility of monitoring transient evolution of thermal ablation zones with a microwave transmission coefficient-based technique. Microwave ablation was performed in ex vivo bovine liver with two 2.45 GHz directional antennas. A custom apparatus was developed to enable periodic switching between "heating mode" when power from the generator was coupled to the antennas, and "monitoring mode", when antennas were coupled to a network analyzer for broadband transmission coefficient ( s21) measurements. Experiments were performed with applied powers ranging between 30-50 W per antenna for 53-1219 s. Transient s21 spectra over the course of ablations were analyzed to determine feasibility of predicting extent of ablation zones and compared against ground truth assessment from images of sectioned tissue. A linear regression-based mapping between the two datasets was derived to predict ablation extent. Normalized average transmission coefficient initially rapidly decreased and then increased before asymptotically approaching steady state, with the transition time ranging between 53 s (45 W) and 109 s (30 W). Analysis of ground truth ablation zone images indicated time to complete ablation of 230-350 s. The relative prediction error for time to complete ablation derived from the s21 data was in the range of 1.6%-2.3% compared to ground truth. We have demonstrated the feasibility of monitoring transient evolution of thermal ablation zones using microwave transmission coefficient measurements in ex vivo tissue. The presented technique has potential to contribute towards addressing the clinical need for a method of monitoring evolution of thermal ablation zones.

Coherent Chaotic Communication Using Generalized Runge–Kutta Method

Computer simulation of continuous chaotic systems is usually performed using numerical methods. The discretization may introduce new properties into finite-difference models compared to their continuous prototypes and can therefore lead to new types of dynamical behavior exhibited by discrete chaotic systems. It is known that one can control the dynamics of a discrete system using a special class of integration methods. One of the applications of such a phenomenon is chaos-based communication systems, which have recently attracted attention due to their high covertness and broadband transmission capability. Proper modulation of chaotic carrier signals is one of the key problems in chaos-based communication system design. It is challenging to modulate and demodulate a chaotic signal in the same way as a conventional signal due to its noise-like shape and broadband characteristics. Therefore, the development of new modulation–demodulation techniques is of great interest in the field. One possible approach here is to use adaptive numerical integration, which allows control of the properties of the finite-difference chaotic model. In this study, we describe a novel modulation technique for chaos-based communication systems based on generalized explicit second-order Runge–Kutta methods. We use a specially designed test bench to evaluate the efficiency of the proposed modulation method and compare it with state-of-the-art solutions. Experimental results show that the proposed modulation technique outperforms the conventional parametric modulation method in both coverage and noise immunity. The obtained results can be efficiently applied to the design of advanced chaos-based communication systems as well as being used to improve existing architectures.

Durable self-cleaning anti-reflective and antifog micro-nanostructures fabricated by laser ablation of vanadium-coated glass surfaces

In this study, low-cost laser marker ablation of vanadium-coated glass was adopted to prepare self-cleaning, anti-reflective, and anti-fog micro-nano structures on its surface. The surface had a hierarchical micro-nano structure composed of a quasi-periodic microstructure and ∼100 nm-sized worm-like clusters interspersed with dispersed nanoparticles. The solar spectrum (AM 1.5) weighted average transmission of treated glass was enhanced by 3.87% over control glass in the wavelength range of 400–1100 nm. Importantly, the water contact angle of the laser-treated samples achieved and maintained 0° for a duration of 25 days, and the laser-treated samples achieved and maintained excellent anti-fog performance for 150 days. The antifog performance degraded substantially afterward, though was still noticeable compared to the reference even after 428 days of storage in the laboratory. Furthermore, co-achievement of exceptional self-cleaning anti-fog performance and broadband transmission enhancement through laser treatment of glass has rarely been previously reported.

Absorption-transmission-diffusion-type radar cross section reduction metasurfaces based on frequency selective absorber and polarization conversion chessboard structure

Absorption-transmission-diffusion-type radar cross section (RCS) reduction metasurfaces based on the frequency selective absorber and the polarization conversion chessboard structure are proposed in this paper. The RCS reduction metasurfaces consist of three parts, i.e. the top layer realized by a cross-shaped meander line loaded with lumped resistors for low-frequency absorption and high-frequency transmission, the mid-layer constructed by angular ring vias frequency selective surface (FSS) sandwiched with two square patches FSS for broadband transmission in mid-frequency zone, and the bottom layer employed chessboard metasurfaces composed of dual-arrow type polarization conversion units for scattering cancellation in high frequency zone. The physical mechanisms of the RCS reduction metasurfaces with multifunctional electromagnetic controlling including wave absorption, bandpass transmission, polarization reversal, and phase cancellation are also discussed and analyzed. Simulation and experimental results demonstrate a reflection of lower than −10 dB for normal incident linearly polarized waves over a frequency range of 2.5 GHz–18 GHz, with a relative bandwidth of 151.22%; transmission passband properties with insertion loss better to −3 dB in the frequency range of 5.62 GHz–11.0 GHz, with a relative bandwidth of 64.74%. Due to the resonance-coupling-resonance principle, the broadband transmission with low insertion loss was obtained. The RCS properties of the proposed multi-functional metasurfaces were further analyzed by numerical simulation and experimental measurement, and the results showed that RCS reduction of over 10 dB was achieved for normal incident different polarized waves at the frequency range of 2.5 GHz to 17 GHz. Even in the case of normal incidence, there was a deviation between the reflection characteristics and RCS reduction. The RCS reduction metasurfaces are expected to be employed in applications where multi mechanism collaborative control of electromagnetic waves is of extreme importance.

An ESPRESSO view of the HD 189733 system

Context. The development of state-of-the-art spectrographs has ushered in a new era in the detection and characterization of exoplanetary systems. The astrophysical community now has the ability to gain detailed insights into the composition of atmospheres of planets outside our Solar System. In light of these advancements, several new methods have been developed to probe exoplanetary atmospheres using both broadband and narrowband techniques. Aims. Our objective is to utilize the high-resolution and precision capabilities of the ESPRESSO instrument to detect and measure the broadband transmission spectrum of HD 189733b’s atmosphere. Additionally, we aim to employ an improved Rossiter–McLaughlin (RM) model to derive properties related to the velocity fields of the stellar surface and to constrain the orbital architecture. Methods. The RM effect, which strongly depends on a planet’s radius, offers a precise means of measurement. To this end, we divided the observation range of ESPRESSO into wavelength bins, enabling the computation of radial velocities as a function of wavelength. By employing a robust model of the RM effect, we first determined the system’s color-independent properties across the entire spectral range of observations. Subsequently, we measured the planet’s radius from the radial velocities obtained within each wavelength bin, allowing us to extract the exoplanet’s transmission spectrum. Additionally, we employed a retrieval algorithm to fit the transmission spectrum and study the atmospheric properties. Results. Our results demonstrate a high degree of precision in fitting the radial velocities observed during transit using the improved modeling of the RM effect. We tentatively detect the effect of differential rotation, with a confidence level of 93.4% when considering a rotation period within the photometric literature values, and 99.6% for a broader range of rotation periods. For the former, the amplitude of the differential rotation ratio suggests an equatorial rotation period of 11.45 ± 0.09 days and a polar period of 14.9 ± 2. The addition of differential rotation breaks the latitudinal symmetry, enabling us to measure the true spin-orbit angle, ψ ≈ 13.6 ± 6.9°, and the stellar inclination axis angle, i* ≈ 71.87−5.55°+6.91°. Moreover, we determine a sub-solar amplitude of the convective blueshift velocity, VCB ≈ −211−61+69 m s−1, which falls within the expected range for a K-dwarf host star and is compatible with both runs. Finally, we successfully retrieved the transmission spectrum of HD 189733b from the high-resolution ESPRESSO data. We observe a significant decrease in radius with increasing wavelength, consistent with the phenomenon of super-Rayleigh scattering.

Conductivity measurements in the 10–40 GHz band using Fabry–Pérot open resonator

In this paper we introduce a novel approach to measurement of electric conductivity of bulk, metallic samples. The method is based on a Fabry–Pérot Open Resonator operating in the plano-concave configuration. Because the resonator supports multiple TEM0,0,q modes, the proposed method provides properties of the sample-under-test at multiple frequencies in the band of 10 to 40 GHz with separation of ca. 1.5 GHz between consecutive frequency points. Contrary to other methods, it does not require that a sample is cut to any particular shape, as long as it is significantly larger than the TEM-mode beam waist and thicker that the penetration depth. It also provides high measurement accuracy that is comparable to alternative methods based on single-mode resonant setups and superior to the broadband transmission methods known in the literature. We demonstrate that the open resonator due to its relatively high Q-factor reaching 135000 can be successfully used in measurements of different samples of conductivity ranging from ca. 103 S/m (e.g. laminates based on carbon-fiber) up to the order of 107 S/m (e.g. aluminum alloys, copper). Using the measurement set-up we also demonstrate an interesting effect of strong frequency dependence of the effective conductivity of stainless steel. It is expected in all magnetic materials and results from large complex permeability of such samples. It is important to emphasize that no similar characterization campaigns are to be found in the literature.

Simultaneous multicolour transit photometry of hot Jupiters HAT-P-19b, HAT-P-51b, HAT-P-55b, and HAT-P-65b

ABSTRACT Accurate physical parameters of exoplanet systems are essential for further exploration of planetary internal structure, atmospheres, and formation history. We aim to use simultaneous multicolour transit photometry to improve the estimation of transit parameters, to search for transit timing variations (TTVs), and to establish which of our targets should be prioritized for follow-up transmission spectroscopy. We performed time series photometric observations of 12 transits for the hot Jupiters HAT-P-19b, HAT-P-51b, HAT-P-55b, and HAT-P-65b using the simultaneous four-colour camera MuSCAT2 on the Telescopio Carlos Sánchez. We collected 56 additional transit light curves from TESS photometry. To derive transit parameters, we modelled the MuSCAT2 light curves with Gaussian processes to account for correlated noise. To derive physical parameters, we performed EXOFASTv2 global fits to the available transit and radial velocity data sets, together with the Gaia DR3 parallax, isochrones, and spectral energy distributions. To assess the potential for atmospheric characterization, we compared the multicolour transit depths with a flat line and a clear atmosphere model. We consistently refined the transit and physical parameters. We improved the orbital period and ephemeris estimates, and found no evidence for TTVs or orbital decay. The MuSCAT2 broad-band transmission spectra of HAT-P-19b and HAT-P-65b are consistent with previously published low-resolution transmission spectra. We also found that, except for HAT-P-65b, the assumption of a planetary atmosphere can improve the fit to the MuSCAT2 data. In particular, we identified HAT-P-55b as a priority target among these four planets for further atmospheric studies using transmission spectroscopy.

Bi-layered coded metasurface for multi-functional hologram with broadband transmission and efficient reflection

Bi-layered coded metasurface for multi-functional hologram with broadband transmission and efficient reflection

Laser Glass deposition of single-mode glass fibers for the fabrication of chip-scale photonic circuits

The use of glass, particularly fused silica (FS), instead of polymers or semiconductors as optical waveguide material is advantageous due to broadband transmission, reduced propagation losses, and enhanced thermal and mechanical stability. In this contribution, laser glass deposition for the chip-scale fabrication of FS-based optical waveguides is investigated. We use a CO2 laser with emission at a wavelength of 10.6 µm to weld conventional single-mode glass fibers onto a FS substrate, with the aim of maintaining the waveguide properties of the fibers. Synchronized translational and rotational axis movements allow for positioning of the waveguides in arbitrary geometries. Furthermore, a CO2 laser-based cleaving method is introduced, which facilitates on-chip creation of waveguide end facets for optical coupling. An analysis of the cleave geometry in dependence on process parameters as well as optical coupling losses are presented. Coupling losses of 0.88 dB for a laser-cleave and propagation losses of 0.56 dB/cm for a 10 cm on-chip welded fiber were achieved. The results pave the way for on-chip integration of fiber-based systems like lasers, sensing devices or optical communication networks.