Abstract: Optical absorbers find uses in a wide array of applications across the electromagnetic spectrum, including photovoltaic and photochemical cells, photodetectors, optical filters, stealth technology, and thermal light sources. Recent efforts have sought to reduce the footprint of optical absorbers, conventionally based on graded structures or Fabry-Perot-type cavities, by using emerging concepts in plasmonics, metamaterials, and metasurfaces. Unfortunately, these new absorber designs require patterning on subwavelength length scales, and are therefore impractical for many large-scale optical and optoelectronic devices. In this article, we summarize recent progress in the development of optical absorbers based on lossy films with thicknesses significantly smaller than the incident optical wavelength. These structures have a small footprint and require no nanoscale patterning. We outline the theoretical foundation of these absorbers based on “ultra-thin-film interference”, including the concepts of loss-induced phase shifts and critical coupling, and then review several applications, including ultra-thin color coatings, decorative photovoltaics, high-efficiency photochemical cells, and infrared scene generators.

Abstract: Silicon photonics will provide low-cost, high-bandwidth and compact optical components for a wide range of applications in optical communications and interconnects. One of the cited key advantages is the capability of wavelength-division multiplexing (WDM). However, the nature of high-index contrast of silicon photonic devices leads to significant challenges when implementing on-chip WDM filters, which is one of the key components in WDM circuits. In this paper, we review several demonstrated silicon photonic WDM circuits based on monolithically integrated silicon nitride (SiN) arrayed-waveguide gratings (AWGs) and thermally tunable silicon microring filters. The use of SiN waveguides with lower index contrast than silicon waveguides enables the realization of high-performance AWGs. Meanwhile, they can evanescently couple to silicon waveguides with high efficiency. The thermally tunable silicon microrings can be used as modulators and wavelength (de)multiplexing filters to implement versatile WDM circuits. Reconfigurability of channel spacing and central wavelengths is achieved by individual tuning of the rings. In this paper, we review silicon photonic circuits for multiple-channel modulators, polarization-insensitive WDM receiver, and variable optical attenuators with multiplexer.

Abstract: Extraordinary optical transmission (EOT) is a term that refers to electromagnetic resonances through sets of subwavelength apertures in either a flat or a corrugated metal film, providing a larger transmission of electromagnetic fields than would be expected from the small aperture size. Since its discovery in 1998, EOT has been a very active research field, leading both to the discovery of new ways of enhancing optical transmission and to its application to sensing, color filters, metamaterials, lenses, optical trapping, enhancement of nonlinear effects, among others. This paper reviews the different mechanisms that lead to EOT, paying special attention to the new research areas and applications that have appeared in the last few years.

Abstract: Image-reject mixers (IRMs) play an important role in microwave and millimeter-wave receivers to eliminate the interference from the image frequency. The keys to realizing a wideband IRM are the precise 90° phase shifting and parallel frequency mixing within a large operational frequency range. In this paper, we propose and demonstrate a novel photonic scheme that can perform simultaneously wideband 90° phase shifting and parallel frequency mixing based on a dual-polarization dual-drive Mach–Zehnder modulator and an optical filter. An IRM is thus implemented. A proof-of-concept experiment is carried out. The experimental results show that the image rejection ratio of the proposed IRM reaches ∼60 dB, when the RF and LO frequencies are tuned in a frequency range of 10–40 GHz. In addition, since the optical filter is employed to select only the two +first-order RF and LO sidebands, the unwanted mixing spurs are all suppressed below the noise floor.

Abstract: The generation of a sub-diffraction longitudinally polarized spot is of great interest in various applications, such as optical tweezers, super-resolution microscopy, high-resolution Raman spectroscopy, and high-density optical data storage. Many theoretical investigations have been conducted into the tight focusing of a longitudinally polarized spot with high-numerical-aperture aplanatic lenses in combination with optical filters. Optical super-oscillation provides a new approach to focusing light beyond the diffraction limit. Here, we propose a planar binary phase lens and experimentally demonstrate the generation of a longitudinally polarized sub-diffraction focal spot by focusing radially polarized light. The lens has a numerical aperture of 0.93 and a long focal length of 200λ for wavelength λ = 632.8 nm, and the generated focal spot has a full-width-at-half-maximum of about 0.456λ, which is smaller than the diffraction limit, 0.54λ. A 5λ-long longitudinally polarized optical needle with sub-diffraction size is also observed near the designed focal point.

Abstract: We have demonstrated a rectangular microwave photonic filter (MPF) based on stimulated Brillouin scattering (SBS) effect in optical fiber and offering tunability on bandwidth, central frequency, and selectivity. A sweeping-pump multistage configuration with feedback control is implemented to achieve the rectangular MPF with high selectivity. The obtained 20-dB shape factor is as low as 1.056, which is, to our knowledge, the best reported result for MPF in gigahertz bandwidth. Furthermore, we solve the polarization-dependent SBS gain issue and realize a polarization-independent MPF. The SBS noise is reduced by adopting a multistage configuration to limit the gain at each stage. Finally, the filter selectivity for a four-stage configuration is as high as 57 dB for a 2.1-GHz bandwidth. In this case, the signal-to-noise ratio penalty is only 2.6 dB for a 4-Gbit/s orthogonal frequency division multiplexing signal in the quadrature-phase-shifted keying format.

Abstract: 2.5D photonic nanostructures with narrow-band diffraction characteristics have a vast range of potential applications in information storage, tunable lasers, optical filters, and biosensors. However, fabrication of 2.5D photonic devices over large areas remains expertise-dependent, inaccurate, and high-cost, limiting their widespread use in practical applications and consumer products. Here, large area printing of quasi 2.5D holograms is demonstrated in the visible spectrum. These holographic surface-relief gratings are hexagonally packed lateral microscale honeycomb pyramids consisting of vertical nanoscale steps. The consecutive steps act as Bragg gratings producing constructive interference of selective visible wavelengths. The 2.5D nanostepped pyramids exhibit coloration due to the narrow-band Bragg diffraction that is tuned in the visible spectrum and a wide angular range. Roll-to-roll processing allows for rapid nanoimprinting the 2.5D nanostepped pyramid arrays over large areas of acrylate polymer film on poly(ethylene terephthalate) substrate. The utilities of the 2.5D holograms are demonstrated by creating colorimetric refractive index and relative humidity sensors, quick response codes, fingerprints, signatures, and encrypted labels. It is envisioned that 2.5D holograms can be integrated with desktop dot-matrix printers for application in sensing, data storage, and security.

Abstract: We report on the development of a diode laser system - the “Faraday laser” - using an atomic Faraday filter as the frequency-selective element. In contrast to typical external-cavity diode laser systems which offer tunable output frequency but require additional control systems in order to achieve a stable output frequency, our system only lases at a single frequency, set by the peak transmission frequency of the internal atomic Faraday filter. Our system has both short-term and long-term stability of less than 1 MHz, which is less than the natural linewidth of alkali-atomic D-lines, making similar systems suitable for use as a “turn-key” solution for laser-cooling experiments.

Abstract: In this letter, we fabricated and characterized a linear variable filter based on a guided-mode resonance filter (GMRF) with gradient grating periods. The GMRF was first fabricated through nanoreplica molding on a plastic substrate, which was followed by the deposition of a thin TiO2 film. The grating periods of the GMRF vary from 250 to 550 nm with a 2-nm increment in each period consisting of 100 cycles. The results show that a 6-mm-long GMRF has a filtering range of 506–915 nm.

Abstract: The invention discloses a nanometer level three-dimension magnetic resonance molecule imaging device based on a diamond NV-color center, comprising a glass pedestal, a laser device, a diamond containing the NV-color center, a microwave pulser, a microscope object lens, a monochromatic optical filter, a nanometer convex lens, a distributed optical imaging lens and a packaging device. The laser is arranged inside the glass pedestal and is used for emitting laser to the outside; the diamond containing the NV-color center is arranged on the upper surface of the glass pedestal and the laser transmitted by the laser device is directly irradiated to the diamond; the microwave pulser is used for inputting the microwave pulse into the diamond; the microscope object lens enables the fluorescence transmitted by the NV-color center of the diamond to emit to the outside through the microscope object lens; the monochromatic optical filter is used for filtering the fluorescence transmitted by the NV-color center of the diamond; the nanometer convex lens is used for further collecting the filtered fluorescence transmitted by the NV-color center of the diamond; the distributed optical imaging lens is used for realizing the imaging function; and the packaging device is used for realizing temperature stabilization, shielding electromagnetism and isolation protection function.

Abstract: We propose and experimentally demonstrate a planar array of optical bandpass filters composed of low loss dielectric metasurface layers sandwiched between two distributed Bragg reflectors (DBRs). The two DBRs form a Fabry-Perot resonator whose center wavelength is controlled by the design of the transmissive metasurface layer which functions as a phase shifting element. We demonstrate an array of bandpass filters with spatially varying center wavelengths covering a wide range of operation wavelengths of 250nm around λ = 1550nm (Δλ/λ = 16%). The center wavelengths of each filter are independently controlled only by changing the in-plane geometry of the sandwiched metasurfaces, and the experimentally measured quality factors are larger than 700. The demonstrated filter array can be directly integrated on top of photodetector arrays to realize on-chip high-resolution spectrometers with free-space coupling.

Abstract: A tunable single bandpass microwave photonic filter (MPF) with an improved spurious free dynamic range (SFDR) using a dual-parallel Mach–Zehnder modulator (DP-MZM) and a phase-shifted fiber Bragg grating (PS-FBG) is proposed and experimentally demonstrated. The DP-MZM is employed to generate an equivalent phase-modulated (EPM) signal with an adjustable optical carrier to sideband ratio. The PS-FBG is used as an optical notch filter to remove one sideband of the EPM signal to convert the EPM signal to a single-sideband intensity-modulated signal. At the output of a photodetector, a microwave signal is detected. The entire operation is equivalent to a single passband MPF. The tunability is achieved by tuning the wavelength the optical carrier. The SFDR of the MPF is improved due to the gain enhancement by a partial suppression of the optical carrier. An experiment is performed. A single bandpass MPF with a passband width of 150 MHz and a frequency-tunable range of $\sim 5.5$ GHz with an improved SFDR by 11 dB is demonstrated.

Abstract: A miniaturized methane (CH(4)) sensor based on nondispersive infrared absorption is realized in MEMS technology. A high level of functional integration is achieved by using the resonance cavity of a linear variable optical filter (LVOF) also as a gas absorption cell. For effective detection of methane at λ = 3.39 µm, an absorption path length of at least 5 mm is required. Miniaturization therefore necessitates the use of highly reflective mirrors and operation at the 15th-order mode with a resonator cavity length of 25.4 µm. The conventional description of the LVOF in terms of the Fabry-Perot resonator is inadequate for analyzing the optical performance at such demanding boundary conditions. We demonstrate that an approach employing the Fizeau resonator is more appropriate. Furthermore, the design and fabrication in a CMOS-compatible microfabrication technology are described and operation as a methane sensor is demonstrated.

Abstract: We report the demonstration of an optical-frequency comb generator based on a monolithically integrated ring laser fabricated in a multiproject wafer run in an active/passive integration process in a generic foundry using standardized building blocks. The device is based on a passive mode-locked ring laser architecture, which includes a Mach-Zehnder interferometer to flatten the spectral shape of the comb output. This structure allows monolithic integration with other optical components, such as optical filters for wavelength selection, or dual wavelength lasers for their stabilization. The results show a -10 dB span of the optical comb of 8.7 nm (1.08 THz), with comb spacing of 10.16 GHz. We also obtain a flatness of 44 lines within a 1.8 dB power variation.

Abstract: Visible Light Communication (VLC) technology in indoor implementation is challenged by ambient light and other lighting noise, such as fluorescent lamp and bulb. The ambient light could create a DC offset or signal with specific frequency range. Thus, we propose analog filters design in the VLC Analog Front-End (AFE) receiver that can eliminate the ambient light noise. The proposed design uses DC offset removal, incorporated with automatic and manual adjustment mode. In automatic mode, we design the analog filter using High Pass Filter (HPF) which have fc = 10Hz; meanwhile, in manual mode we design a reference circuit using potentiometer and differential amplifier for direct current blocking. For reducing signal interference from lamp flickering, the proposed design uses Band Stop Filter (BSF) which has fc = 100Hz. The experimental results, both simulation and realtime, show that our proposed design can reduce signal interference and ambient light. We also test the design using PWM and BPSK modulation to evaluate Bit Error Rate (BER) performance.

Abstract: The efficiency of a thermal conversion system requires adequate radiative properties. In a thermophotovoltaic system, the optical filter plays a key role into the overall performance of the system. The purpose of this paper is to study a one-dimensional TiO2/SiO2 photonic crystal for application as thermophotovoltaic optical filter. The influence of the layers’ thicknesses, incidence angle, and number of periods on the spectral reflectance has been investigated through the transfer matrix method. It was found that, when one varies the number of layers from 6 to 12, an improvement of the optical properties of the spectral filter is observed. The surface wave through a film of photonic crystal generates a resonant transmission for a truncated structure. These results are in conformity with those found in the literature.

Abstract: A simple signal processing, low-cost technique for the fabrication of an optical oxygen sensor based on phase shift detection is described. The sensing film is based on the oxygen-sensing dye platinum tetrakis pentrafluoropheny porphine (PtTFPP) embedded in a polymer matrix. The feasibility of coating a photodiode with the oxygen-sensing film to fabricate an optical sensing device is investigated. The optical oxygen sensor is tested with regard to monitoring excitation voltage and oxygen concentration. The experiment results show that the maximum phase difference between 0% and 100% gaseous oxygen is 26°. A preparation procedure for coating photodiodes with the oxygen-sensing film that produces repetitive and reliable sensing devices is proposed. The developed optical oxygen sensor is low-cost, has simple signal processing, and lacks optical filter elements. The proposed sensor is a cost-effective alternative to traditional electrochemical-based oxygen sensors and provides a platform for other optically based sensors.

Abstract: An asymmetrical dual-core photonic crystal fiber (DC-PCF), which possesses all circular air holes, is proposed. By setting appropriate geometrical parameters, the wavelength-selective coupling property is realized, and a compact optical filter with a short length of 1.83 mm based on the DC-PCF is designed. The spectral transmission characteristics of the filter are investigated by the beam propagation method. The results demonstrate that the optical filter possesses a bandwidth of ~58 nm and small sidelobes. The proposed optical filter could be used in the integrated optical systems.

Abstract: Modern optical communications rely on high-resolution, high-bandwidth filtering to maximize the data-carrying capacity of fiber-optic networks. Such filtering typically requires high-speed, power-hungry digital processes in the electrical domain. Passive optical filters currently provide high bandwidths with low power consumption, but at the expense of resolution. Here, we present a passive filter chip that functions as an optical Nyquist-filtering interleaver featuring sub-GHz resolution and a near-rectangular passband with 8% roll-off. This performance is highly promising for high-spectral-efficiency Nyquist wavelength division multiplexed (N-WDM) optical super-channels. The chip provides a simple two-ring-resonator-assisted Mach-Zehnder interferometer, which has a sub-cm2 footprint owing to the high-index-contrast Si3N4/SiO2 waveguide, while manifests low wavelength-dependency enabling C-band (> 4 THz) coverage with more than 160 effective free spectral ranges of 25 GHz. This device is anticipated to be a critical building block for spectrally-efficient, chip-scale transceivers and ROADMs for N-WDM super-channels in next-generation optical communication networks.

Abstract: We present the first numerical investigation and comparison of 40-Gb/s lane rate electrical Duobinary, optical Duobinary and PAM-4 for NG-PONs incorporating low complex linear and nonlinear post-equalizations.

Abstract: This paper explores the potential of using multiple fibre Bragg grating-based sensors for acoustic emission (AE) detection, thus offering an effective alternative to conventional piezoelectric (PZT) sensors, especially where they have shown limitations in use, such as in the marine sector. A cascaded fibre optic acoustic sensor system, using optical filter signal demodulation has been developed and its performance extensively evaluated. To undertake this under standardized conditions, the optical sensor system was evaluated using a glass plate to detect the acoustic signal, followed by an evaluation using a metal plate to identify the location of acoustic sources, when subjected to sonotrode excitation, mimicking acoustic detection in cavitation detection. Under these circumstances, very good agreement has been reached between the outputs of the optical acoustic sensors and of the colocated PZT acoustic sensors. This work confirms the utility of these sensors—they can detect not only weak AE signals, but also enable multipoint simultaneous measurement, showing their potential for condition monitoring applications, especially in the marine sector.

Abstract: We present a novel methodology that is able to distinguish meaningful level shifts from typical signal fluctuations which, in a fiber monitoring context, is associated with the problem of identifying small losses within a noisy optical time-domain reflectometer (OTDR) profile. A two-stage regularization filtering can accurately identify the location of the significant level shifts with an efficient parameter-free algorithm. The developed methodology demands low computational effort and can easily be embedded in a dedicated processing unit. Our case studies compare the new methodology with the current available ones and show that it is the most adequate technique for fast detection of multiple unknown level shifts in a noisy OTDR profile.

Abstract: Integrated nanoscale photonic devices have wide applications ranging from optical interconnects and optical computing to optical communications. Wavelength demultiplexer is an essential on-chip optical component which can separate the incident wavelength into different channels; however, the experimental progress is very limited. Here, using a multi-component nano-cavity design, we realize an ultracompact, broadband and high-contrast wavelength demultiplexer, with 2.3 μm feature size, 200 nm operation bandwidth (from 780 nm to 980 nm) and a contrast ratio up to 13.7 dB. The physical mechanism is based on the strong modulation of the surface plasmon polaritons induced by the multi-component nano-cavities, and it can be generalized to other nanoscale photonic devices. This provides a strategy for constructing on-chip photon routers, and also has applications for chip-integrated optical filter and optical logic gates.

Abstract: We demonstrate a bandwidth and wavelength programmable silicon double microring optical filter that can be automatically, accurately, and repeatably set to an optimal Butterworth shape for maximum passband flatness. The algorithm uses a critical coupling condition to set the Butterworth shapes, followed by bandwidth tuning using a calibration dataset for the coupling coefficients. Silicon microrings with independently tunable phase shifts and coupling coefficients were designed for and fabricated at a foundry to demonstrate the method. Fabrication variations and thermal crosstalk are also compensated automatically.

Abstract: We present an extension of the adaptive spatial carrier frequency method which is proposed for fast measuring optical properties of fibrous materials. The method can be considered as a two complementary steps. In the first step, the support of the adaptive filter shall be defined. In the second step, the angle between the sample under test and the interference fringe system generated by the utilized interferometer has to be determined. Thus, the support of the optical filter associated with the implementation of the adaptive spatial carrier frequency method is accordingly rotated. This method is experimentally verified by measuring optical properties of polypropylene (PP) fibre with the help of a Mach–Zehnder interferometer. The results show that errors resulting from rotating the fibre with respect to the interference fringes of the interferometer are reduced compared with the traditional band pass filter method. This conclusion was driven by comparing results of the mean refractive index of drown PP fibre at parallel polarization direction obtained from the new and adaptive spatial carrier frequency method.