Optical Stealth Research Articles

Pearl‐Inspired Colored Carbon Fibers with Electromagnetic Interference and Optical Camouflage Properties

Carbon fibers (CFs) are widely used in cutting‐edge and civilian fields due to their excellent comprehensive properties such as high strength and high modulus, superior corrosion and friction resistances, excellent thermal stability, light weight, and high electrical conductivity. However, their natural ultra‐black appearance is difficult to meet the aesthetic needs of today's civilian sector and the need for optical stealth in the military field. In addition, conventional coloring methods are difficult to effectively adhere to CF surfaces due to high crystallinity and highly inert surface caused by their graphite‐like structure. In this work, inspired by the nacre structural color of pearls, colored CFs with 1D photonic crystal structure are prepared by cyclically depositing amorphous (Al2O3 + TiO2) layers on the surface of carbon CFs through atomic layer deposition (ALD). The obtained CFs exhibit brilliant colors and excellent environmental durability in extreme environments. Moreover, the colored CFs also exhibit high EMI shielding effectiveness (45 dB) and optical stealth properties, which can be used in emerging optical devices and electromagnetic and optical stealth equipment.

Electrochromic and thermal-control broadband stealth device based on AZO/Ag/AZO configuration electrodes

Existing stealth technologies have limitations including single bands and functions, and static and passive applications, so that targets cannot remain stealthy if multi-source detection approaches are used. With the wide application of visible-infrared (Vis-IR) detection and guidance technologies, there is an urgent need to develop advanced techniques for visible smart stealth and Vis-IR compatible stealth. In view of this, an electrochromic and thermal-control broadband stealth device based on AZO/Ag/AZO configuration electrodes was developed. The influencing mechanisms of microstructural characteristics of the conducting layer in the AZO/Ag/AZO configuration on the visible transmission spectra and infrared (IR) reflection spectra were revealed. Furthermore, a conducting layer enabling IR stealth with high visible transmittance was optimized, on which a V2O5 multi-color electrochromic film layer was prepared via self-assembled deposition. In this way, the device integrates multiple optical mechanisms including the visible-induced transmission, IR radiation suppression, and electrochromism to modulate optical properties in multiple bands. A technical route that integrates visible smart stealth and active suppression of IR radiation was established, which is conducive to improving the adaptive fusion ability under a multi-domain background and multi-band compatible optical stealth.

Singular multi-wavelength and multi-waveband transparencies generated by P T-symmetric dumbbell optical waveguide networks.

In this paper, we investigate the singular multi-wavelength and multi-waveband transparencies generated by P T-symmetric dumbbell optical waveguide networks composed of two materials, and obtain the number regularity for the transparency wavelengths of one-unit-cell system and the general relationships for the transmission and reflection coefficients of multi-unit-cell systems. Consequently, three types of exact transparencies produced by multi-unit-cell systems are found based on the aforementioned formulas: (i)exact multi-wavelength unidirectional or bidirectional transparency as the same as those of one-unit-cell system; (ii)exact multi-wavelength bidirectional transparency at which one-unit-cell system cannot produce exact transparency, generated by adjusting the number of unit cells; (iii)exact multi-wavelength bidirectional transparency at which one-unit-cell system produces exact transparency, also generated by adjusting the number of unit cells. It provides theoretical foundations for developing highly sensitive and multi-wavelength optical filters. On the other hand, we also discover that multi-unit-cell systems can create approximate multi-waveband bidirectional transparencies by adjusting the number of unit cells, which provides scientific support for developing high-performance optical stealth devices.

Hierarchically structured stainless-steel surfaces with superior superhydrophobicity and anti-reflection

Superhydrophobicity and anti-reflection of stainless-steel surfaces have potential applications in the fields of solar energy absorption and conversion, stray light shielding, and optical stealth. Here, we propose an efficient method for fabricating hierarchically structured stainless-steel surface with superior superhydrophobicity and anti-reflection based on maskless electrochemical machining (maskless ECM) technology. The hierarchically structured surface not only had a contact angle of 154° without chemical modification, but also had an average reflectance of 1.44% in the wavelength range of 400–800 nm. The mechanism underlying optical properties was discussed. The hierarchical structure formed by maskless ECM is very suitable for mass production and large area fabrication.

Design of Reflective Tunable Structural Color Metasurface Based on Guided-Mode Resonance Filter and Sb2S3

In recent years, dynamically tunable structural color has attracted great interest. Here, we introduce the guided-mode resonance (GMR) filter and the phase-change material Sb2S3 to design a reflective optical metasurface to produce tunable structural color, in which the combination of the GMR filter, with narrow resonant wavelength, and the Sb2S3, with a much larger bandgap and higher refractive index, helps to produce high-quality tunable structural color. The simulation results indicate that through the phase transition between the amorphous and crystalline states of Sb2S3, the proposed metasurface can generate tunable structural color that can be perceived by the naked eye. Furthermore, the metasurface can sensitively sense environmental changes through changes in structural color. This work provides a new method for realizing dynamically tunable structural color, and paves the way for the application of controllable structural color in dynamic displays, optical stealth, colorimetric sensing, and other fields.

Cross-Scale Light Absorption Properties of Surface Bionic Microstructures for Spacecraft Stealth

To address the problem that the black coating for spacecraft optical stealth easily falls off, this study constructs a light-absorbing spacecraft surface based on a micro/nanostructure through imitating a natural light-trapping structure. In this paper, we first analyze the optical properties of a basic stealth structure with the finite difference time domain (FDTD) method and establish a mapping relationship between the light absorption rate of the basic stealth structure and its multiscale factors. Then, imitating the microstructural characteristics of the blackened parts of butterfly wings, we design a multilayered and multiscale complex stealth structure to achieve the optical stealth characteristics of low reflection and high absorption of sunlight on the surface of the spacecraft. Simulation analysis shows that the bionic microstructure can be used to change the optical properties of the metal surface to a certain wavelength band; the complex stealth structure designed based on the butterfly wing can absorb 80.18% of visible light and reduce the overall brightness of the high-orbiting spacecraft by four orders of magnitude.

Ultrasensitive and tunable multi-narrowband metamaterial absorber

In recent years, metamaterials have been widely used as a new type of optoelectronic device materials, such as optical stealth, holographic imaging, and metamaterial absorbers. In particular, the research of metamaterial absorbers with multi-narrowband, high absorption and tunability has attracted much attention. However, it is a great challenge to integrate multi-narrowband, high absorption, polarization independence, high sensitivity and tunability into a simple absorption structure. In this paper, we use FDTD to design a multi-narrow band, polarimetric independent, high sensitivity and tunability of metamaterial perfect absorber, which is a metal-dielectric-metal three-layer structure. From the bottom to the top of the structure, they are Au layer, SiO2 layer and Au pattern layer. The simulation results show that three resonance absorption peaks are obtained under the coaction of local surface plasmon resonance, magnetic resonance and cavity resonance. The absorption peaks are located at 878 nm, 1020 nm and 1276 nm, and the absorption rates are 98.7%, 99.3% and 99.5%, respectively. In addition, it is found that the number of absorption peaks can be changed by adjusting the top pattern layer structure. Further, the position of the absorption peak and the absorption rate can be altered by adjusting the geometric parameters of the structure. Besides its simple structure, the analysis also show that the structure has high sensitivity under different refractive index environment. Finally, it can also be used for optical conversion devices, such as optical absorption switches, splitters, optical modulators.

Structures, principles, and properties of metamaterial perfect absorbers.

Metamaterials are a kind of artificial material with special properties, showing huge potential for applications in fields such as infrared measurement, solar cells, optical sensors, and optical stealth. A metamaterial perfect absorber (MPA) is designed based on a metamaterial, featuring strong absorption, small volume, light weight, ultra-bandwidth, tunability and other characteristics. This paper introduces the absorption mechanism of MPAs from microwave to optical wave band, and four directions of absorber design are elaborated. Equivalent impedance matching, plasma resonance and interference effect are the main absorption mechanisms of MPA. Multiband perfect absorption, ultra-wideband and ultra-narrowband perfect absorption, polarization and angle insensitive absorption, and dynamically controllable tunable absorption are the main design aspects. Among them, the proposal of a dynamically tunable absorber realizes the dynamic absorption. Finally, the problems and challenges of metamaterial perfect absorber design are discussed.

Quantum noise ciphered optical stealth communication based on equivalent spectral encoding.

To provide secure and covert transmission for optical communication system at the same time, a quantum-noise stream ciphered optical stealth communication approach is proposed for the first time. In the proposed system, the optical pulses are time spread by a chromatic dispersion device and then phase modulated with optical codes at chip rate to realize an equivalent spectral encoding. Binary optical codes are converted to multiple level phase-shift keying signal according to Y-00 protocol. The encoded optical signals are attenuated to mesoscopic coherent states and sent to a public channel. The ciphered signals are stealthy transmitted under public channel noise. The availability and covertness of the optical stealth channel is verified by system simulations.

Analysis of beat noise in optical stealth transmission system

Optical stealth transmission system is more sensitive to noise than the conventional communication systems. In this paper, the effect of beat noise on optical stealth transmission system is analyzed for both stealth and public channel. According to the reported system, the optical stealth transmission system are classified to incoherent and coherent systems, the noise distributions and bit-error rates are derived for both of them. An experiment is setup to verify the theoretical derivations. The calculated bit-error rate results present the same changing law as the measured data in the experiment. The possible beat noise suppressing methods are also proposed and discussed. The impact of beat noise on the communication covertness is also addressed.

Effect of Laser Parameters on Optical Stealth Transmission System Performance

The performance of an optical stealth transmission system based on gain-switched laser depends largely on the laser parameters. Modulation frequency, bias current, and modulation current are considered to study the covertness and bit error rate performance of the optical stealth transmission system. According to optical stealth carrier generation with time spreading and all-optical encoding, the stealth signals are derived. A complementary encoding scheme is adopted in the system simulation. The simulation results show that the temporal and spectral characteristics of the generated stealth signal can be changed by adjusting the bias current, modulation current, and modulation frequency. However, there is a trade-off between bit error rate performance and covertness of the stealth channel. Under the premise of error-free transmission, the bias current and modulation frequency should be reduced and the modulation current should be improved to optimize the covertness of the stealth channel.

Tunable quad-band perfect metamaterial absorber on the basis of monolayer graphene pattern and its sensing application

In this paper, a classic three-layer metamaterial absorber (MA) is designed to achieve quad-band perfect absorption. The unit cell of the top graphene pattern layer is made up of a periodically arranged graphene split-ring nested with a cross-like graphene structure formed by four L-shaped strips. The middle dielectric spacer layer is SiO2 and the ground layer is gold. Simulation results show that four absorption bands with near unity peak absorption rate will occur in the interested frequency range of 3–11 THz. The absorption mechanism and the parameter dependency are discussed at first, which will provide guiding significances for the actual fabrication. Meanwhile, when the Fermi level of graphene is increased, the four absorption bands will have different degrees of blue-shift, and when the relaxation time of graphene is increased, the four absorption peaks will increase first and then decreases. As a result, dynamical tunability of the absorption properties can be achieved by changing the Fermi level or the relaxation time of the graphene without changing the geometry of the proposed MA. Further investigation indicates that the proposed absorber is independent of the polarization of the input electromagnetic (EM) wave and can maintain well absorption properties when the incident angle is changed within a broader range. Considering the potential sensing applications, it is shown that the proposed MA can be operated as a type of refractive index sensor with highest sensitivity of 2.97THz/RIU, and the maximum figure of merit (FOM) is 47.5. In terms of these unique performances, the proposed MA has broad application prospects in multispectral optical stealth, optical filtering, integrated photo detecting, sensing and so on.

Theoretical Modelling of Liquid Crystal Electro-Optical Material Display Elements to Achieve Optimal Low Power Consumption Ship Stealth

Advanced optical stealth techniques are of growing importance in the role of manned and unmanned aircraft or ship platforms. We consider here the concept of using passive reflective display elements rather than high power active light projection to achieve a low observable camouflaged ship platform. This paper presents practical consideration towards minimal radiated power user-driven requirements using adaptive liquid crystal electro-optical methods. Optical reflectivity for a multi-layer liquid crystal cell structure composed of a glass/ ITO/ aligning layer/ liquid crystal/ aligning layer/ ITO/ glass will support optical guided modes. Reflectivity as a function of angle and voltage are compared with theory generated from a Fresnel matrix formalism. Voltage device modulated reflectivities are then simulated across a ship platform to evaluate this method for low power consumption, with minimal radiated optical radiation.

Experimental demonstration of optical stealth transmission over wavelength-division multiplexing network.

We propose and experimentally demonstrate an optical stealth transmission system over a 200 GHz-grid wavelength-division multiplexing (WDM) network. The stealth signal is processed by spectral broadening, temporal spreading, and power equalizing. The public signal is suppressed by multiband notch filtering at the stealth channel receiver. The interaction between the public and stealth channels is investigated in terms of public-signal-to-stealth-signal ratio, data rate, notch-filter bandwidth, and public channel number. The stealth signal can transmit over 80 km single-mode fiber with no error. Our experimental results verify the feasibility of optical steganography used over the existing WDM-based optical network.

Optical Signal Processing and Stealth Transmission for Privacy

Optical encryption, key generation, and optical stealth transmission techniques for protecting the privacy of communication in optical networks are proposed and summarized. The signal processing methods based on fiber components provide ways to encrypt data and generate encryption keys at the speed of data transmission in optical fibers. Private and confidential communication is achieved without compromising the capacity and bandwidth of the optical network. Optical stealth transmission can hide the signal in plain sight. Because an eavesdropper can neither read the data nor detect the existence of the transmitted signal, optical stealth transmission provides a higher level of privacy. A hybrid system which includes both the public channels and stealth channels can effectively provide anonymous communication and defend against traffic analysis.

Optical steganography of code-shift-keying OCDMA signal based on incoherent light source

A method to enhance the security of the optical stealth channel is proposed. The proposed method uses the techniques of polarization modulator based code-shift-keying (CSK) data modulation and optical code-division multiple-access (OCDMA) en/decoding, where an incoherent light source is needed. Our analysis and experimental results show that the CSK-OCDMA signal, where a wavelength selective switch is used for optical en/decoding, has been stealthily transmitted over a 40-km wavelength-division multiplexing optical fiber link. In addition, the stealth channel only causes a power penalty less than 0.48 dB to the public channel, whereas both of these two channels can achieve error-free transmission (with bit error rate less than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> .

Optical stealth transmission based on super-continuum generation in highly nonlinear fiber over WDM network.

In this Letter, the optical stealth transmission carried by super-continuum spectrum optical pulses generated in highly nonlinear fiber is proposed and experimentally demonstrated. In the proposed transmission scheme, super-continuum signals are reshaped in the spectral domain through a wavelength-selective switch and are temporally spread by a chromatic dispersion device to achieve the same noise-like characteristic as the noise in optical networks, so that in both the time domain and the spectral domain, the stealth signals are hidden in public channel. Our experimental results show that compared with existing schemes where stealth channels are carried by amplified spontaneous emission noise, super-continuum signal can increase the transmission performance and robustness.

Complementary coding optical stealth transmission based on amplified spontaneous emission light source.

Complementary encoder of stealth signal is proposed and demonstrated for coding, modulating and enhancing the privacy of optical stealth transmission. With complementary encoding, the stealth signal carried by amplified spontaneous emission (ASE) light keeps the same characteristic to ASE noise and can be concealed well under public channel. The experiment results demonstrate the feasibility of the scheme and show the stealth signal has the same impact on public channel in transmission performance, compared to the ASE noise.

WDM optical steganography based on amplified spontaneous emission noise.

We propose and experimentally demonstrate a wavelength-division multiplexed (WDM) optical stealth transmission system carried by amplified spontaneous emission (ASE) noise. The stealth signal is hidden in both time and frequency domains by using ASE noise as the signal carrier. Each WDM channel uses part of the ASE spectrum, which provides more flexibility to apply stealth transmission in a public network and adds another layer of security to the stealth channel. Multi-channel transmission also increases the overall channel capacity, which is the major limitation of the single stealth channel transmission based on ASE noise. The relations between spectral bandwidth and coherence length of ASE carrier have been theoretically analyzed and experimentally investigated.

Stealth transmission of temporal phase en/decoded polarization-modulated-code-shift-keying optical-code-division multiple-access signal over synchronous digital hierarchy network with asynchronous detection

An innovative approach for security-enhanced optical stealth transmission in a synchronous digital hierarchy network is proposed and experimentally investigated. The security enhancement is achieved through a signal modulation format, so-called polarization-modulated-code-shift-keying, which is implemented with two superstructured fiber Bragg gratings-based optical-code-division multiple-access encoders and a polarization modulator. The proposed modulation format can provide a constant energy level for both bits 0’s and 1’s, which avoids secure vulnerability of single-stealth-user with on-off-keying modulation before coupling into the host channel and after the cascade of filters. Moreover, a self-made cost-effective gain-switched distributed feedback laser with relatively narrow spectrum is first employed as a stealth optical source, which greatly reduces the system cost and complexity. The stealth signal is recovered and detected asynchronously. The experimental results show high secure performance and robustness against eavesdropping, while keeping a bit error rate below forward error correction limit.