Entire C-band Research Articles

High anisotropy 2D large-size iron-based coral for C-band ultra-wide microwave absorption

Lightweight microwave-absorbing (MA) materials with broadband absorption in the C-band remain challenging to date. Recent studies have shown that promoting anisotropy in materials is expected to break the Snoek limit and enhance the absorption performance of magnetic materials in the C-band. In this paper, two-dimensional (2D) large-size coral-like iron-based composites (LCIC) were prepared using an NH4NO3-assisted blow molding separation strategy. The special structure with large shape anisotropy realizes a wide and strong natural magnetic resonance, leading to strong magnetic loss. The enhanced natural resonance effectively broadens the effective absorption bandwidth (EAB) of the LCIC in the C-band. The minimum reflection loss (RLmin) of LCIC is −41.64 dB, and the EAB is 5.42 GHz (3.83–9.25 GHz), covering the entire C-band. Strong natural resonance is necessary to improve the dissipation of magnetic losses in low-frequency electromagnetic waves. This work explores the important role of size effect in enhancing the C-band absorption properties of magnetic materials. It provides a feasible reference for the preparation of C-band broadband, strongly absorbing MA materials.

Ultra-short and highly efficient metamaterial Fresnel lens-assisted taper

This paper demonstrates the benefits of leveraging free-space optics concepts in the design of certain integrated photonic components, leading to a footprint reduction without compromising on performance. Specifically, we present ultra-short, highly efficient and fabrication-friendly mode-size converters based on metamaterial Fresnel lens-assisted tapers. This is achieved using a parameterized inverse-design approach, where the metamaterial phase shifters are realized using fabrication-friendly Manhattan geometries, by optimizing the width, length, and position of the phase shifters. This approach overcomes the limitations of the conventional method that uses local periodic approximation, which is not suitable for lenses with a short focal length and high numerical aperture. We also extend the free-space concept of compound lenses and demonstrate a doublet-based taper to further reduce the footprint. The devices are fabricated and experimentally characterized in terms of insertion loss and signal integrity at high data transmission rates, exhibiting high performance. For the singlet, it effectively achieves mode-size conversion from 15 μm to 0.5 μm within a 15 μm distance, leading to ×10 length reduction compared to a linear taper. The insertion loss is under 1 dB over the entire C-band. The doublet achieves the same mode-size reduction within a 10 μm distance, leading to ×15 length reduction compared to a linear taper. The insertion loss is near 1 dB over most of the C-band. In both cases, the signal integrity is maintained for up to 50 Gbit/s.

Enhancement of the microwave absorption performance of flaky FeSiCr powder through the compounding of cobalt ferrite

FeSiCr/CoFe2O4 composites were prepared by mechanical ball milling of cobalt ferrite prepared by hydrothermal method and flaky FeSiCr alloy powder. The phase and element of the composites was observed by XRD and XPS. The microstructure and surface elements of the composites were observed by SEM and EDS. The findings from the analyses confirmed the successful synthesis of the FeSiCr/CoFe2O4 composites. The results of EMP tests indicated that the electromagnetic parameters were balanced and the impedance matching was improved. When the mass percentage of cobalt ferrite was 20 %, the impedance matching was the best, and the RLmin reached −41.04 dB at 4.8 GHz. And the maximum EAB reached 5.4 GHz. By adjusting the thickness of the absorber, the EAB can be extended to 9.5 GHz (2.6–12.05 GHz), encompassing the entire C-band and X-band. The EMA performance of the composites has been greatly improved. FeSiCr/CoFe2O4 composites may serve as potential microwave absorbing materials in the future due to the wide range of economical raw materials, strong microwave absorption capacity and extensive absorption frequency range.

Optimizing Spatial Channel Networks (SCNs) in Hierarchical Optical Cross-Connect (HOXC) architectures: Impact of wavelength switching granularity on performance

The increasing demand for network bandwidth highlights the critical need to enhance optical transmission systems. Utilizing the entire C-band for a single optical channel(OCh) eliminates the requirement for wavelength switching, prompting the emergence of spatial channel networks (SCNs). SCNs transform the optical layer into hierarchical spatial division multiplexing (SDM) and wavelength division multiplexing (WDM) layers through the implementation of hierarchical optical cross-connects (HOXCs). A HOXC comprises a spatial channel cross-connect (SXC) for spatial bypass switching and multiple wavelength cross-connects (WXCs) for wavelength channel switching, ensuring efficient optical transmission. Therefore, the design of the HOXC plays a pivotal role in determining both the device cost and network performance in SCNs. This study investigates the impact of wavelength switching granularity on core-selective switch (CSS)-based HOXC architectures. We propose an adaptive dynamic Routing, Spatial Channel, and Spectrum Assignment (RSCSA) algorithm to solve routing and resource allocation problems in SCNs. By examining SCN designs with varying wavelength switching granularities, we assess the overall network deployment cost, considering both network throughput and spectrum resource utilization. Our findings indicate that adjusting the granularity of wavelength switching in SCNs can significantly affect device costs and performance, highlighting the importance of identifying an optimal SCN design that strikes a balance between these factors. These insights offer valuable guidance for the practical planning and management of future SCN deployments.

C-band wavelength tunable mode locked noise-like pulse source using an asymmetrically reflecting Fabry-Perot filter

We report a widely wavelength tunable mode locked noise-like pulse (NLP) source from an erbium doped fiber ring laser. By making use of the reflection of an asymmetric Fabry- Perot interferometer (FPI), continuous tunability of the central wavelength over the entire C-band (1530 nm to 1565 nm), with an independent control over bandwidth (3.5 nm to 17 nm) is demonstrated. Using a single control parameter, i.e. the length of the FPI, it is thus possible to select from a range of output spectral bandwidths and desired central wavelengths. Using the same control on the length of the FPI and intracavity polarization settings, generation of dual wavelength NLPs, temporally shifted noise-like pulse pairs, harmonic mode locked NLPs and loosely bound NLP doublets and NLP triplets are reported. Owing to the simplicity of the tuning mechanism and the adaptable output spectrum, we believe, such a laser will be useful for various applications like sensing, spectroscopy and imaging. The novel combination of a reflecting FPI-based filter, and the complex nonlinear polarization rotation (NPR) based mode locking mechanism, also offers the opportunity to further explore the rich physics of NLPs.

Circular photonic crystal grating design for charge-tunable quantum light sources in the telecom C-band

Efficient generation of entangled photon pairs at telecom wavelengths is a key ingredient for long-range quantum networks. While embedding semiconductor quantum dots into hybrid circular Bragg gratings has proven effective, it conflicts with p-i-n diode heterostructures which offer superior coherence. We propose and analyze hybrid circular photonic crystal gratings, incorporating air holes to facilitate charge carrier transport without compromising optical properties. Through numerical simulations, a broad cavity mode with a Purcell factor of 23 enhancing both exciton and biexciton transitions, and exceptional collection efficiency of 92.4% into an objective with numerical aperture of 0.7 are achieved. Furthermore, our design demonstrates direct coupling efficiency over 90.5% into a single-mode fiber over the entire telecom C-band. The hybrid circular photonic crystal grating thereby emerges as a promising solution for the efficient generation of highly coherent, polarization-entangled photon pairs.

Low-loss tantalum pentoxide photonics with a CMOS-compatible process.

We report a Ta2O5 photonic platform with a propagation loss of 0.49 dB/cm at 1550 nm, of 0.86 dB/cm at 780 nm, and of 3.76 dB/cm at 2000 nm. The thermal bistability measurement is conducted in the entire C-band for the first time to reveal the absorption loss of Ta2O5 waveguides, offering guidelines for further reduction of the waveguide loss. We also characterize the Ta2O5 waveguide temperature response, which shows favorable thermal stability. The fabrication process temperature is below 350°C, which is friendly to integration with active optoelectronic components.

A compact wideband self-isolated uniplanar quad MIMO antenna for C-band applications

The paper presents the development of a compact uniplanar MIMO antenna system for C-band applications. A quad-element MIMO system with uniplanar design arrangements for a compact 40 mm × 40 mm mainframe is analyzed. Four similar P-shaped radiators with a common ground form a self-isolating MIMO arrangement. After conducting a thorough parametric analysis, slots are added to the antennas to reduce their size and area. The overall area of the miniaturized radiator element is 16 mm2. The orthogonal mode MIMO covers an operating bandwidth of 5 GHz frequency range from 4 GHz to 9 GHz with the isolation of less than 15.5 dB and an overall gain of 4.6 dBi so it is fabricated and tested. The suggested antenna can cover the entire C-band frequencies (4 GHz-8 GHz) and a partial X-band frequency. The antenna provides a good radiation efficiency of 96%. The suggested antenna’s diversity characteristics, such as ECC (<0.13), TARC (< −10 dB), MEG, and CCL ensure good diversity performances. The SAR value of the MIMO is assessed, and the results are within bounds. The suggested MIMO system exhibits good efficiency, adequate isolation between individual antennas without the need for any decoupling mechanisms, increased antenna gain, and improved ECC while keeping a lower antenna size. The suggested compact quad MIMO system is appropriate for C-band applications.

Genetic algorithm-enhanced microcomb state generation

Microcavities enable the generation of highly efficient microcombs, which find applications in various domains, such as high-precision metrology, sensing, and telecommunications. Such applications generally require precise control over the spectral features of the microcombs, such as free spectral range, spectral envelope, and bandwidth. Most existing methods for customizing microcomb still rely on manual exploration of a large parameter space, often lacking practicality and versatility. In this work, we propose a smart approach that employs genetic algorithms to autonomously optimize the parameters for generating and tailoring stable microcombs. Our scheme controls optical parametric oscillation in a microring resonator to achieve broadband microcombs spanning the entire telecommunication C-band. The high flexibility of our approach allows us to obtain complex microcomb spectral envelopes corresponding to various operation regimes, with the potential to be directly adapted to different microcavity geometries and materials. Our work provides a robust and effective solution for targeted soliton crystal and multi-soliton state generation, with future potential for next-generation telecommunication applications and artificial intelligence-assisted data processing.

Low-loss and compact, dual-mode, 3-dB power splitter combining a directional coupler, a multimode interferometer, and a Y-junction.

Multimode power splitters are the fundamental building blocks in mode division multiplexing systems. In this paper, we propose a low-loss and compact, dual-mode, 3-dB power splitter for the two lowest TE modes combining three different structures, including a directional coupler, a multimode interferometer, and a Y-junction. The coupling length of the proposed device is only 7.2µm. For both T E 0 and T E 1 modes, the numerical simulation shows that the insertion loss is only less than 0.1dB and crosstalk is less than -20d B at the wavelength range of 1520-1580nm. The working bandwidth can cover the entire C-band. It offers a potential solution for a 3-dB power splitter of the two lowest TE modes.

High-resolution Si3N4 spectrometer: architecture & virtual channel synthesis and experimental demonstration.

Up-to-date network telemetry is the key enabler for resource optimization by capacity scaling, fault recovery, and network reconfiguration among other means. Reliable optical performance monitoring in general and, specifically, the monitoring of the spectral profile of WDM signals in fixed- and flex- grid architectures across the entire C-band, remains challenging. This article describes a two-stage spectrometer architecture amenable to integration on a single chip that can measure quantitatively the spectrum across the entire C-band with a resolution of ∼ 1.4 GHz. The first stage consists of a ring resonator with intra-ring phase shifter to provide a tuneable fine filter. The second stage makes use of an AWG subsystem and a novel processing algorithm to synthesize a tuneable coarse filter with a flat passband which isolates individual resonances of a multiplicity of ring resonances. The spectrometer is capable of scanning the entire C-band with high resolution using only one dynamic control. Due to its maturity and low loss, CMOS compatible Si3N4 is chosen for fabrication of the ring resonator and two cyclic AWGs. Complete spectrometer operation is demonstrated experimentally over a selected portion of the C-band. A novel virtual channel synthesis algorithm based on the weighted summation of the AWG output port powers relaxes the conventional AWG design requirement of a flat passband and sharp transition to stopband. The operation of the circuit is invariant to the optical path length between individual components and the algorithm corrects to some extent fabrication process variation impairments of the AWG channel spectra substantially improving robustness.

Multiplication of orbital angular momentum via multi-plane light conversion.

The multiplication of orbital angular momentum (OAM) modes using optical coordinate transformation is useful for OAM optical networks, but the scalability of this scheme is limited by the ray model. Here, we propose an alternative scheme for the scalable multiplication of OAM modes based on modified multi-plane light conversion (MPLC) that can extend azimuthal and radial indices of OAM modes supported by the multipliers and unlock a new degree of freedom for radial high-order OAM states that has been restricted in the zero order. The multiplication for 20 OAM modes with radial index p = 0 and 10 OAM modes with radial index p = 1 is performed in simulation and experiment. The 3-dB optical bandwidth corresponding to the purity of OAM modes covers the entire C-band experimentally. This novel, to the best of our knowledge, approach to manipulating OAM states provides valuable insights and flexible strategies for high-capacity OAM optical communication and high-dimensional optical quantum information processing.

Cavity-enhanced photoacoustic dual-comb spectroscopy

Photoacoustic dual-comb spectroscopy (DCS), converting spectral information in the optical frequency domain to the audio frequency domain via multi-heterodyne beating, enables background-free spectral measurements with high resolution and broad bandwidth. However, the detection sensitivity remains limited due to the low power of individual comb lines and the lack of broadband acoustic resonators. Here, we develop cavity-enhanced photoacoustic DCS, which overcomes these limitations by using a high-finesse optical cavity for the power amplification of dual-frequency combs and a broadband acoustic resonator with a flat-top frequency response. We demonstrate high-resolution spectroscopic measurements of trace amounts of C2H2, NH3 and CO in the entire telecommunications C-band. The method shows a minimum detection limit of 0.6 ppb C2H2 at the measurement time of 100 s, corresponding to the noise equivalent absorption coefficient of 7 × 10−10 cm−1. The proposed cavity-enhanced photoacoustic DCS may open new avenues for ultrasensitive, high-resolution, and multi-species gas detection with widespread applications.

Inverse design of an ultra-compact and large-bandwidth bent subwavelength grating wavelength demultiplexer.

Inverse design has attracted significant attention as a method to improve device performance and compactness. In this research, we employed a combination of forward design and the inverse algorithm using particle swarm optimization (PSO) to design a bent ultra-compact 1310/1550nm broadband wavelength demultiplexer assisted by a subwavelength grating (SWG). Through the phase matching at 1550nm and the phase mismatch at 1310nm, we rapidly designed the width parameters of SWG in the forward direction. Then the PSO algorithm was used to optimize the SWG parameters in a certain range to achieve the best performance. Additionally, we introduced a new bent dimension significantly reducing the device length while maintaining low insertion loss (IL) and high extinction ratios (ERs). It has been verified that the length of the device is only 7.8µm, and it provides a high ER of 24dB at 1310nm and 27dB at 1550nm. The transmitted spectrum shows that the IL values at both wavelengths are below 0.1dB. Meanwhile, the 1dB bandwidth exceeds 150nm, effectively covering the entire O-band and C-band. This approach has been proven successful in enhancing performance and significantly reducing the device footprint.

Variable Mode-Dependent-Loss equalizer based on Silica-PLC for Three-Mode transmission

We present a low-loss and compact silica-based planar lightwave circuit (PLC) mode-dependent loss (MDL) equalizer for three-mode transmission with a variable range of 2.5 dB. The proposed device utilizes a Mach-Zehnder interferometer (MZI) structure to selectively attenuate the LP01 mode while maintaining low insertion loss for the LP11a,b modes. We demonstrate that the device effectively reduces differential modal gain (DMG) in a three-mode transmission system. Additionally, we successfully equalize the pump-power dependence of the DMG in a two-LP mode erbium-doped fiber amplifier (EDFA) to less than 0.5 dB over the entire C-band. We also show that the variable range has a tradeoff relationship with the wavelength bandwidth, where too large a variable range results in non-negligible wavelength dependencies. Consequently, fixed equalizers are advantageous when large MDL compensation is required, while variable equalizers are effective for fine-tuning MDL of less than 1.2 dB.

Ni-Carbon Microtube/Polytetrafluoroethylene as Flexible Electrothermal Microwave Absorbers.

Flexible microwave absorbers with Joule heating performance are urgently desired to meet the demands of extreme service environments. Herein, a type of flexible composite film is constructed by homogeneously dispersing a hierarchical Ni-carbon microtube (Ni/CMT) into a processable polytetrafluoroethylene (PTFE) matrix. The Ni/CMT are interconnected into a 3D conductive network, in which the huge interior cavity of thecarbon microtube (CMT) improves impedance matching and provides additional hyper channels for electromagnetic (EM) waves dissipation, and the hierarchical magnetic Ni nanoparticles enhance the synergistic interactions between confined heterogeneous interfaces. Such an ingenious structure endows the composites with excellent electrothermal performance and improves their serviceability for application under extreme environments. Moreover, under a low fill loading of 3wt.%, the Ni/CMT/PTFE (NCP) can achieve excellent low-frequency microwave absorption (MA) property with a minimum reflection loss of -59.12dB at 5.92GHz, which covers almost the entire C-band. Relying on their brilliant MA property, an EM sensor is designed and achieved by the resonance coupling of the patterned NCP. This work opens up a new way for the design of next-generation microwave absorbers that meet the requirements of EM packaging, proofing water and removing ice, fire safety, and health monitoring.

Versatile wavelength tunable noise-like pulse generation from an erbium doped fiber laser using an intra-cavity Michelson interferometer

We report, versatile wavelength tunable mode locked noise-like pulses (NLP) from an erbium doped fiber ring laser, using a novel intra-cavity Michelson interferometer (MI) as a tunable filter. Our results demonstrate independent control of the central wavelength and the bandwidth using fine and coarse controls in one arm of the MI. The central wavelength is continuously tunable across the entire C-band (1530 nm to 1566 nm), while the bandwidth can be varied from 3 nm to 19.5 nm. The NLPs have a repetition rate of 1.307 MHz with pulse energies of ∼1 nJ. Additionally, harmonic mode-locked states and NLP pairs can also be produced using the same laser. On account of the adaptable output spectrum, we believe, such lasers will be useful for a variety of applications including sensing, spectroscopy and optical coherence tomography. The MI-based fiber laser can also serve as an ideal platform for gaining insight and exploring the rich dynamics of NLPs.

Brillouin laser spectrometer based on spectral compression

We introduce a spectrometer design that uses Brillouin lasing to perform broadband spectral compression. This approach enables the entire C-band (optical frequencies covering 4 THz, or 25 nm) to be compressed into a 230 MHz wide RF spectrum—allowing a 4 THz wide optical spectrum to be monitored continuously using a standard analog-to-digital converter. This technique is based on the linear dependence of the Brillouin frequency shift on optical wavelength. To use this dependence for spectral analysis, we couple the input optical spectrum into a fiber ring cavity where it acts as an optical pump, exciting a frequency-shifted Brillouin lasing spectrum. The Brillouin lasing spectrum is then referenced to a copy of the original optical spectrum using heterodyne detection, converting the Brillouin frequency shift associated with each lasing mode to the RF domain. The narrow linewidth of the lasing modes enables a precise measurement of individual lines with an uncertainty of 28 MHz (0.2 pm) across a 4 THz band at an update rate of 200 Hz. This simple approach, constructed using standard fiber-coupled telecom components, provides an efficient method for high-resolution, broadband spectral analysis.

An ultra-compact broadband polarizing beam splitter utilizing hybrid plasmonic waveguide

Polarizing beam splitter has rather significant applications in polarization diversity circuits and polarization multiplexing systems. In this paper, we present an asymmetric polarizing beam splitter utilizing hybrid plasmonic waveguide. The special hybrid structure with a hybrid waveguide and a dielectric waveguide can limit the energy of TE and TM modes to a different layer. Therefore, we can achieve beam splitting by adjusting the corresponding parameters of the two waveguides. First, we studied the influences of different structure parameters on the real part of the effective mode refractive index of the two waveguides, and obtained a set of parameters that satisfy the condition of strong coupling of TM mode and weak coupling of TE mode. Then, the performance of our proposed polarizing beam splitter is evaluated numerically. The length of the coupling section is only 4.1 µm, and the propagation loss of TM and TE modes is 0.0025 dB/µm and 0.0031 dB/µm respectively. Additionally, the extinction ratios of TM and TE modes are 10.62 dB and 12.55 dB, respectively. Particularly, the proposed device has excellent wavelength insensitivity. Over the entire C-band, the fluctuation of the whole normalized output power is less than 0.03. In short, our proposed asymmetric polarizing beam splitter features ultra-compactness, low propagation loss, and broad bandwidth, which would provide promising applications in polarization multiplexing system and polarization diversity circuits relevant to optical interconnection.

Joint WDM and OAM Mode Group Multiplexed Transmission Over Conventional Multimode Fiber

To exploit the advantages of multimode fiber (MMF)-based transmission and to improve the overall capacity, transmission of OAM modes over a single wavelength needs to be replaced by multichannel transmission with the proper experimental demonstration in terms of achievable bit error rates (BER), cross-talk(x-talk), etc. over the entire transmission band. In this paper, we have experimentally shown the successful transmission of two OAM mode groups multiplexed signals jointly with three channels using wavelength division multiplexing (WDM) over a 1 km conventional MMF, namely OM3. In this experimental study, each OAM mode over each WDM channel uses 10 Gbps of on-off keying (OOK) modulation format. At the receiver end, the achieved bit error rate (BER) is found well below the required pre-FEC BER of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$3.8\times 10^{-3}$</tex-math></inline-formula> without using any MIMO-DSP. To establish the efficacy of the signal transmission, the time evolutions of OAM mode x-talk and BER over wavelengths spanning the entire C-band have been experimentally measured. Under the influence of different x-talks, the BER performances of all three 50 GHz-spaced WDM channels for both OAM modes are measured. With proper adjustment of polarization at the receiver end, a BER value below the pre-FEC threshold is achieved over 900 seconds. The experimental results also revealed that the power penalty contribution of WDM x-talk is negligible compared to OAM mode x-talk.