Ultra-narrow Bandpass Research Articles

Tunable Microwave Photonic Filter With Ultra-Narrow Passband and High Out-of-Band Rejection

Tunable Microwave Photonic Filter With Ultra-Narrow Passband and High Out-of-Band Rejection

Bandwidth modulation and pulse characterization of passively Q-switched erbium-doped fiber laser

We demonstrate the modulation of laser bandwidth by utilizing an ultranarrow tunable bandpass filter (UNTBF) in a passively Q-switched erbium-doped fiber laser. The passive Q-switch mechanism is enabled by using carbon nanotubes as saturable absorber at a Q-switched threshold of 35.5 mW. Based on spectral filtering effect introduced by the UNTBF, the 3 dB laser bandwidth can be tuned from 0.016 nm to 0.478 nm at a fixed pump power of 75.9 mW. The corresponding pulse behavior for each different bandwidth is characterized, and the results reveals that the pulse width can be as well tuned from 7.8 to 2.6 μs against the laser bandwidth, which agrees with the rule of time-bandwidth product. Correspondingly, the pulse repetition rate and the pulse energy vary from 16.23 kHz to 26.16 kHz and from 0.67 to 1.03 μJ respectively across the laser bandwidth. Further investigation of the pulse performance is performed against the pump power increment up to 107.2 mW. To the best of our knowledge, this is the first demonstration of spectrum bandwidth modulation in a passively Q-switched fiber laser, which can be useful for fully exploiting the possibilities of Q-switched pulse applications.

Quickly tunable ultra-narrow filter via a metal film waveguide.

The development of fast, efficient, and cost-effective tunable optical filters is a tireless pursuit of the goal in the field of optical signal processing and communications. However, the traditional filters have been limited by their complex structures, slow tuning speed, and high cost. To address this challenge, we present a tunable ultra-narrow bandpass filter, which is fabricated by a metal layer cladded in a high-parallelism and high-precision piezoelectric ceramic for an interlayer. Experimental results show a remarkable full width at half maximum of 51 pm and a fast response time of 800 ns. In addition, by cascading double filters, the wavelength of the output light has been fine-tuned from a Vernier effect. Moreover, we realize a tunable filter to select and output several ultra-narrow single peaks with 56% efficiency in the 2 nm range. Furthermore, it offers a wide tunable range, exceptional narrowband filtering performance, and fast piezoelectric response times. Hence, it is particularly well suited to applications requiring precise wavelength selection and control, opening new possibilities in the field of tunable optical filters.

Grating-based metasurfaces for ultra-narrow near-infrared bandpass filtering with wide out-of-band suppression

Here, we present a straightforward strategy for designing silicon grating-based metasurfaces tailored for narrow near-infrared bandpass filtering. By selecting appropriate structural parameters for the grating and including periodic groove perturbations within each grating slit, transverse guided mode resonances (GMRs) propagating perpendicular and parallel to the grating slit are created to provide wide out-of-band suppression and high-Q filter responses, respectively. The destructive and constructive interference between radiations from groove perturbations are then introduced to eliminate all GMRs except one, producing a single-band bandpass filter. Simply adjusting the period of the groove perturbations allows precise tuning of the passband’s central wavelength across the operational spectral range from 1350 nm to 1750nm, throughout which the passband exhibits a Q-factor exceeding 9,000 and the attenuation level outside the passband remains below 1%. Furthermore, our proposed narrow bandpass filters are found to be robust against the potential fabrication imperfections, such as variations in groove size and position.

Young’s double-slit experiment with undulator vortex radiation in the photon-counting regime

Young’s double-slit interference experiments with undulator vortex radiation were conducted, focusing on photon-counting regime. To isolate the second harmonic radiation in the ultraviolet range emitted from the helical undulator and achieve successful counting measurements, an ultranarrow bandpass filter was utilized under an extremely low-current mode of the electron storage ring. It was observed that the photon spots on the detector, after passing through the double slits, appeared to be randomly distributed. However, upon integrating these photon spots, it was confirmed that interference fringes with characteristic features of optical vortices, such as dark and broken/distorted stripes in the center, were formed. The reproducibility of these interference fringes was confirmed by calculating the optical path difference for the optical vortex reaching the double slits, as well as the optical path difference resulting from normal double-slit interference. Consequently, these findings indicate that even in the state of a single photon, the radiation emitted spontaneously by a high-energy electron in spiral motion possesses the nature of an optical vortex, characterized by a spiral wavefront.

All-dielectric metasurface based ultranarrow bandpass filter in optical C-band

Metasurface-based nanophotonics devices can potentially replace traditional bulky optical components such as optical filters, modulators, and switches. In this study, we report a dielectric metasurface-based ultranarrow bandpass filter operating at 1549.8 nm with the ability to tune the central wavelength within the C-band. The proposed tunable filter consists of a 2D periodic array of gallium arsenide blocks placed on top of a quartz substrate. The introduction of a symmetry-breaking defect results in an ultranarrow resonance with an FWHM of 0.31 nm at a 1549.8 nm wavelength when a TE-polarized wave is normally incident. The sharp, high Q factor resonance is understood to be governed by the symmetry-protected bound state in the continuum. The resonant response can be tuned by changing the periodicity, thickness, and asymmetry of the metasurface constituents. The multipolar analysis indicates the excitation of the electric quadrupole mode at the resonance wavelength. Our findings can be useful for the design of compact tunable ultranarrow bandpass filters for optical communications systems.

Single Longitudinal Mode Parity-Time Symmetric Brillouin Fiber Laser Based on Lithium Niobate Phase Modulator Sagnac Loop

A single longitudinal mode (SLM) parity-time (PT) symmetric Brillouin fiber laser (BFL) based on z-cut lithium niobate phase modulator (LN-PM) Sagnac loop with Hz level linewidth is proposed and experimentally verified. A 10 km single-mode fiber provide SBS gain, while an LN-PM Sagnac loop consisting of an LN-PM and two polarization controllers (PCs) is used to achieve PT symmetry. Two mutually linked feedback loops supporting orthogonally polarized light are created by utilizing inherent birefringence of LN waveguide from LN-PM without electrical signal modulation. One of the feedback loops experiences gain in a clockwise direction and the other loss in a counterclockwise direction. The polarization state of the stokes injected into the LN-PM is controlled by adjusting the PC, and SLM BFL can be achieved when the gain and loss are well matched and exceed the coupling coefficient to break the PT symmetry. Compared with existing BFL studies, this design does not call for frequency matching of several composite cavity structures or precise control of ultra-narrow bandwidth bandpass filters. In addition, a frequency-locking system based on Pound-Drever-Hall technology is employed to lessen the instability and external disturbances brought on by the ring cavity. In the experiment, the linewidth of PT symmetry BFL based on LN-PM Sagnac loop is 3.85 Hz, according to the measured linewidth of 77 Hz at the −20 dB power point. A 65 dB optical signal-to-noise ratio and a 43 dB maximum side mode suppression ratio are measured. Furthermore, the PT symmetry BFL's wavelength is tuned between 1550 and 1550.41 nm.

Estimation of electric field distribution of Hermstein’s glow corona discharge by spectral image processing

We present a spectral image processing method for estimating the distribution of the reduced electric field E/N for Hermstein’s glow corona discharge. This method analyzes spectral images of the discharge taken by an intensified charge-coupled device camera with ultra-narrow band pass filters at the pixel level. The method requires a shorter measurement time than optical emission spectroscopy and can reveal finer details of the spatial distribution. Experiments showed that the E/N of discharge at the tip of the electrode locally increases with an increase in the applied voltage. In particular, E/N at the tip of the electrode reaches a high value just before breakdown. Based on the maximum acquired values, the breakdown of discharge is estimated to occur when E/N ≥ 270 × 10−21–340 × 10−21 V m2. This study is expected to contribute to monitoring the discharge state of electrical equipment to prevent breakdown and ensure safety.

An ultra-narrow-band optical filter based on zero refractive index metamaterial

Owing to the photonic band gap effect and defect state effect, photonic metamaterials have received much attention in the design of narrow bandpass filters, which are the key devices of optical communication equipment such as wavelength division multiplexing devices. In this work, based on zero-index metamaterial (ZIM), a compact filter with both high peak transmission coefficient and ultra-narrow bandwidth is proposed. The photonic metamaterial with conical dispersion and Dirac-like point is achieved by optimizing the structure and material component parameters of dielectric rods with square lattice in air. It is demonstrated that a triply degenerate state can be realized at the Dirac-like point, which relates this metamaterial to a zero-index medium with effective permittivity and permeability equal to zero simultaneously. Electromagnetic (EM) wave can propagate without any phase delay at this frequency, and strong dispersion occurs in the adjacent frequency cone, leading to dramatic changes in optical properties. We introduce a ZIM into photonic metamaterial defect filter to compress the bandwidth to the realization of ultra-narrow bandpass filter. The ZIM is embedded into the resonant cavity of line defect filter, which is also composed of dielectric rods with square lattice in air. In order to increase the sensitivity of the phase change with frequency, the Dirac-like frequency is adjusted to match the resonant frequency of the filter. We study the transmission spectrum of the structure through the COMSOL Multiphysics simulation software, and find that the peak width at half-maximum of the filter decreases as the thickness of ZIM increases, and the peak transmittance is still high when bandwidth is greatly compressed. The zero phase delay inside the slab can be observed. Through field distribution analysis, the zero-phase delay and strong coupling characteristics of electromagnetic field are observed at peak frequency. The comparison with conventional photonic metamaterials filter is discussed. We believe that this work is helpful in investigating the realization of ultra-narrow bandpass filters.

Narrow linewidth parity-time symmetric Brillouin fiber laser based on a dual-polarization cavity with a single micro-ring resonator.

A narrow linewidth parity-time (PT) symmetric Brillouin fiber laser (BFL) based on dual-polarization cavity (DPC) with single micro-ring resonator (MRR) is proposed and experimentally investigated. A 10 km single-mode fiber provides SBS gain, while a DPC consisting of optical coupler, polarization beam combiner and a MRR, is used to achieve PT symmetry. Due to the reciprocity of light propagation in the MRR, the PT symmetry BFL based on DPC implements two identical feedback loops that are connected to one another, one with a Brillouin gain coefficient and the other with a loss coefficient of the same magnitude, to break a PT symmetric. Compared with existing BFL studies, this design does not call for frequency matching of compound cavities structures or without ultra-narrow bandwidth bandpass filters. In the experiment, the 3-dB linewidth of PT symmetry BFL based on DPC with single MRR is 11.95 Hz with the threshold input power of 2.5 mW, according to the measured linewidth of 239 Hz at the -20 dB power point. And a 40 dB maximum mode suppression ratio are measured. Furthermore, the PT symmetry BFL's wavelength is tuned between 1549.60 and 1550.73 nm. This design with single longitudinal mode output can be applied to high coherent communication systems.

Development of a filter-based near-surface remote sensing system to retrieve far-red sun-induced chlorophyll fluorescence

Observations of sun-induced chlorophyll fluorescence (SIF) by remote sensing have improved our understanding of the structural and physiological dynamics of vegetation. Substantial efforts have been made to measure SIF with ground-based sensing systems, but field observation data for various plant functional types are still sparse. This is partly due to the limited availability of commercial SIF measurement systems, the relatively high cost of hyperspectral spectroradiometers, and the difficulties of sensor calibration and maintenance in the field. We developed a filter-based smart near-surface remote sensing system for SIF (4S-SIF) to overcome the technical challenges of monitoring SIF in the field, which also decreased the sensor cost, thus enabling more comprehensive spatial sampling. To retrieve SIF, we combined ultra-narrow bandpass filters (full width half maximum <1.3 nm) and photodiode detectors to observe electromagnetic radiation at specific wavelengths (757, 761, and 770 nm). We confirmed that the spectral and radiometric performance of the bandpass filters was satisfactory to retrieve SIF by comparing them to a high-spectral-resolution spectroradiometer that served as a reference. In particular, we confirmed that the digital numbers (DNs) from 4S-SIF exhibited linear relationships with the DN from the reference spectroradiometer in each band (R2 > 0.99). In addition, we developed equations to correct for temperature-induced changes in filter transmittance, such that SIF can be reliably extracted in outdoor environments without the need to actively stabilize the temperature. Furthermore, we confirmed that the SIF signal from 4S-SIF had a strong linear relationship with the reference spectroradiometer-based SIF. Importantly, this relationship held even when the physiological mechanisms of the plant were altered by a herbicide treatment that induced substantial changes in the SIF signal (R2 = 0.85, relative RMSE = 0.22), which indicated that 4S-SIF could be used to retrieve SIF. We believe that 4S-SIF will be a useful tool for collecting in-situ SIF data across multiple spatial and temporal scales.

Parity-time-symmetric optoelectronic oscillator based on higher-order optical modulation.

An optoelectronic oscillator (OEO) for single-frequency microwave generation, enabled by broken parity time (PT) symmetry based on higher-order modulation using a Mach-Zehnder modulator, is proposed and demonstrated. Instead of using two physically separated mutually coupled loops with balanced gain and loss, the PT symmetry is realized using a single physical loop to implement two equivalent loops with the gain loop formed by the beating between the optical carrier and the ±1st-order sidebands and the loss loop formed by the beating between the ±1st-order sidebands and the ±2nd-order sidebands at a photodetector. The gain and loss coefficients are made identical in magnitude by controlling the incident light power to the modulator and the modulator bias voltage. Once the gain/loss coefficient is greater than the coupling coefficient, the PT symmetry is broken, and a single-frequency oscillation without using an ultra-narrow passband filter is achieved. The approach is evaluated experimentally. For an OEO with a loop length of 10.1 km, a single-frequency microwave signal at 9.997 GHz with a 55-dB sidemode suppression ratio and -142-dBc/Hz phase noise at a 10-kHz offset frequency is generated. No mode hopping is observed during a 5-hour measurement period.

Cover Image

The cover image is based on the Research Article Luminescence imaging of solar modules in full sunlight using ultranarrow bandpass filters by Germain Rey et al., https://doi.org/10.1002/pip.3563.

Luminescence imaging of solar modules in full sunlight using ultranarrow bandpass filters

AbstractPhotovoltaic (PV) electricity generation is the fastest growing energy source globally and also humanity's best opportunity to achieve the urgently required deep decarbonisation of the energy sector. As PV enters the terawatt scale, with millions of modules in a single PV power plant, quality testing of installed PV modules becomes indispensable to guarantee PV as a reliable long‐term source of electricity. We present a method that extends the use of photoluminescence (PL) imaging to field‐deployed solar modules in full sunlight. The method takes advantage of sunlight absorption in the Earth's atmosphere in a narrow spectral range around 1,135‐nm wavelength and recent developments in ultranarrow bandpass optical filter technology. The technical principles and experimental data are provided. This method lays the foundations for PL imaging, a powerful inspection method for the PV industry and research, to be applied to routine high‐volume inspection of fielded PV modules on large‐scale solar power plants.

Fabrication of an ultra-narrow band-pass filter with 60 pm bandwidth in green light band

The predecessor of China Optics was China Optics and Applied Optics Digest, which was founded in 1985. At that time, it was the only retrieval journal in the field of optics in China. At the end of 2008, China Optics and Applied Optics Digest was renamed

Ultrahigh spectral resolution single passband microwave photonic filter.

Microwave photonic filters (MPFs) with only one ultra-narrow passband are able to provide high frequency selectivity and wide spectral range, and they are of great importance in radio-frequency (RF) signal processing. However, currently all MPFs are limited by trade-offs between key parameters such as spectral resolution and range, tunability, and stability. Here, we report the first demonstration of a single passband MPF with unprecedented performance including ultrahigh spectral resolution of 650 kHz, 0-40 GHz spectral range, and high stability of center frequency drifting within ±50 kHz. This record performance is accomplished by breaking the amplitude equality of a phase-modulated signal via a Brillouin dynamic grating (BDG) which has an ultra-narrow reflection spectrum of sub-MHz. The results point to new ways of creating high performance microwave photonic systems, such as satellite and mobile communications, radars, and remote-sensing systems.

MicroPulse DIAL (MPD) – a diode-laser-based lidar architecture for quantitative atmospheric profiling

Abstract. Continuous water vapor and temperature profiles are critically needed for improved understanding of the lower atmosphere and potential advances in weather forecasting skill. Ground-based, national-scale profiling networks are part of a suite of instruments to provide such observations; however, the technological method must be cost-effective and quantitative. We have been developing an active remote sensing technology based on a diode-laser-based lidar technology to address this observational need. Narrowband, high-spectral-fidelity diode lasers enable accurate and calibration-free measurements requiring a minimal set of assumptions based on direct absorption (Beer–Lambert law) and a ratio of two signals. These well-proven quantitative methods are known as differential absorption lidar (DIAL) and high-spectral-resolution lidar (HSRL). This diode-laser-based architecture, characterized by less powerful laser transmitters than those historically used for atmospheric studies, can be made eye-safe and robust. Nevertheless, it also requires solar background suppression techniques such as narrow-field-of-view receivers with an ultra-narrow bandpass to observe individual photons backscattered from the atmosphere. We discuss this diode-laser-based lidar architecture's latest generation and analyze how it addresses a national-scale profiling network's need to provide continuous thermodynamic observations. The work presented focuses on general architecture changes that pertain to both the water vapor and the temperature profiling capabilities of the MicroPulse DIAL (MPD). However, the specific subcomponent testing and instrument validation presented are for the water vapor measurements only. A fiber-coupled seed laser transmitter optimization is performed and shown to meet all of the requirements for the DIAL technique. Further improvements – such as a fiber-coupled near-range receiver, the ability to perform quality control via automatic receiver scanning, advanced multi-channel scalar capabilities, and advanced processing techniques – are discussed. These new developments increase narrowband DIAL technology readiness and are shown to allow higher-quality water vapor measurements closer to the surface via preliminary intercomparisons within the MPD network itself and with radiosondes.

Ultrawide Photonic Bandgap and Ultrastrong Photonic Localization Produced by Series of Periodic Networks

AbstractA series of periodic networks is designed, in which all of the unit cells construct a series of fractal triangles, and their extraordinary optical characteristics for producing ultrawide photonic bandgap (PBG) and ultrastrong photonic localization are investigated. The results indicate that with increasing fractal generation, the number of equivalent triangular loops increases exponentially, and consequently the gap–midgap ratio of the PBG produced by the networks with the integer waveguide length ratio of 1:2 enlarges rapidly and tends to the limit at 200%. For the networks with the noninteger waveguide length ratio of (where ), an extremely narrow passband occurs in the middle of the wide PBG. Furthermore, the ultrastrong photonic localization is generated when the electromagnetic wave with the frequency of the all‐transmission peak in the ultranarrow passband propagates in the networks. This type of fractal triangular networks may have applications to design all‐optical devices, such as highly sensitive optical filters, highly sensitive all‐optical switches, and high‐performance photonic energy‐storage devices.

Development of a narrow stopband filter for spectroscopic fast ion deuterium-alpha measurements.

A new technique to attenuate the unshifted deuterium Balmer-alpha (D-alpha) emission is developed and tested for the fast ion D-alpha (FIDA) diagnostic. The unshifted D-alpha emission, at λ = 656.1nm, is around three orders of magnitude higher than the desired FIDA emission. Blocking the strong emission feature is essential to prevent blooming and light smearing on the CCD chip and scattered light contamination. The new method is a notch filter approach that utilizes the reflection from ultra-narrow bandpass filters to block the saturating signal before it enters the spectrometer. Collimated light from the fibers is reflected off the filter at a 15° angle of incidence. Measurements show that a center wavelength transmission of 0.006 and a blocking full width at half maximum of ∼1 nm are achieved by using a 200 μm fiber and a 20mm focal length collimator with two filters.

Polarimetric parity-time symmetry in a photonic system

Parity-time (PT) symmetry has attracted intensive research interest in recent years. PT symmetry is conventionally implemented between two spatially distributed subspaces with identical localized eigenfrequencies and complementary gain and loss coefficients. The implementation is complicated. In this paper, we propose and demonstrate that PT symmetry can be implemented between two subspaces in a single spatial unit based on optical polarimetric diversity. By controlling the polarization states of light in the single spatial unit, the localized eigenfrequencies, gain, loss, and coupling coefficients of two polarimetric loops can be tuned, leading to PT symmetry breaking. As a demonstration, a fiber ring laser based on this concept supporting stable and single-mode lasing without using an ultranarrow bandpass filter is implemented.