Transmission Bandpass Filters Research Articles

Polarization-independent long-wave infrared transmission bandpass filters based on guided mode resonance.

Subwavelength dielectric gratings based on guided mode resonances (GMRs) provide a compact, low-cost, and readily fabricable platform for narrowband spectral filters applied in numerous fields, such as sensors, spectrometers, and hyperspectral imaging. We numerically designed two long-wave infrared transmission bandpass filters based on Ge-ZnS-Ge thin film stacks and 2D zero-contrast gratings. The first filter is attributed to double GMR mode coupling, exhibiting a broadband high reflectance between 8 and 12 µm and a narrow transmission peak with a spectral linewidth of 53 nm and transmittance efficiency of 98% at the resonance. The other filter adopts a low-refractive-index material BaF2 as the substrate, which displays a lower-transmittance sideband from 8.6 to 9.4 µm, and a higher transmission efficiency up to 100% at the resonance. The physical mechanisms of two spectral responses are all relevant to guided mode coupling and Fabry-Perot resonance.

Electromechanically Tunable Frequency‐agile Metamaterial Bandpass Filters for Terahertz Waves

AbstractTunable terahertz (THz) transmission filters are essential for various key applications including miniaturized spectrometers, hyperspectral imagers, and channel selectors in high‐speed wireless communication systems. However, tunable THz transmission filters have remained elusive, so far. Here, an electromechanically reconfigurable microcantilevers integrated complementary metamaterial to function as a frequency‐agile THz bandpass transmission filter is reported. A large tunable range of 70% at central frequency of 0.455 THz is designed, and the experimental results show active switching of THz transmission frequency range of 54% at central frequency of 0.5 THz. The proposed electromechanically reconfigurable metadevice provides a large tunable range, uses electrical control, and is fabricated using complementary metal oxide semiconductor (CMOS) compatible materials and processes, thus making it highly attractive for the realization of plethora of miniaturized high‐performance THz components and devices.

Mid- to long-wave infrared computational spectroscopy using a subwavelength coaxial aperture array

Miniaturized spectrometers are advantageous for many applications and can be achieved by what we term the filter-array detector-array (FADA) approach. In this method, each element of an optical filter array filters the light that is transmitted to the matching element of a photodetector array. By providing the outputs of the photodetector array and the filter transmission functions to a reconstruction algorithm, the spectrum of the light illuminating the FADA device can be estimated. Here, we experimentally demonstrate an array of 101 band-pass transmission filters that span the mid- to long-wave infrared (6.2 to 14.2 μm). Each filter comprises a sub-wavelength array of coaxial apertures in a gold film. As a proof-of-principle demonstration of the FADA approach, we use a Fourier transform infrared (FTIR) microscope to record the optical power transmitted through each filter. We provide this information, along with the transmission spectra of the filters, to a recursive least squares (RLS) algorithm that estimates the incident spectrum. We reconstruct the spectrum of the infrared light source of our FTIR and the transmission spectra of three polymer-type materials: polyethylene, cellophane and polyvinyl chloride. Reconstructed spectra are in very good agreement with those obtained via direct measurement by our FTIR system.

Electromagnetically induced transparency in an integrated metasurface based on bright–dark–bright mode coupling

Based on a bright–dark–bright coupling model, a classical analog of an integrated electromagnetically induced transparency (EIT) metasurface at a microwave frequency band is numerically and experimentally demonstrated. The proposed EIT metasurface structure consists of three perpendicularly connected metallic elements: two split-ring resonators (SRRs) which couple directly with normally incident wave supporting the bright modes, and the I-shaped configuration treated as the dark mode. Due to the dual coupling excitation of the middle I-shaped configuration, two anti-parallel electric dipoles were formed in the upper and lower SRRs, resulting in the destructive interference. In addition, the wide-angle incidence and the different polarization-angle incidence of the proposed metasurface were calculated and studied. The group index is extracted and calculated to verify the slow light property within the transparent window. The research in this work reveals that the scheme based on multi-mode coupling is of referable value for constructing EIT-like metamaterials or high transmission bandpass filters.

Design of angularly tolerant zero-contrast grating filters for pixelated filtering in the mid-IR range.

Zero-contrast gratings (ZCG) can be used to implement narrow bandpass transmission filters. However, they suffer from poor angular tolerance, which hinders their use in pixelated applications. Combining ZCG with double-corrugation grating, we increase the resonance width and angular tolerance of the filter by more than 1 order of magnitude. Filters tunable around 4.6 μm with more than 90% transmission and compatibility with 140 μm pixel size are demonstrated.

Multilayered Aluminum Plasmonic Metasurfaces for Ultraviolet Bandpass Filtering

A multilayered aluminum-based metasurface is proposed toward realization of compact bandpass transmission filters for the ultraviolet spectral region. A systematic numerical study is presented that can assist in precisely tailoring the filter characteristics. The filter performance characteristics are benchmarked against previously reported filters. The proposed structures are CMOS process compatible and are suitable as replacements of bulky Wood’s filters; consequently, they are a promising step toward cost-effective UV photodetectors and multispectral imagers.

二维亚波长金属光栅多波长透射滤光片

二维亚波长金属光栅多波长透射滤光片

Bandpass filters based on phase-shifted photonic crystal waveguide gratings

In this paper, a bandpass transmission filter realized in phase-shifted waveguide gratings based on photonic crystals (PCs) is proposed. Phase-shift regions each composed of one period of photonic crystal (PC) waveguide are incorporated into PC waveguide gratings. The magnitudes of the phase-shifts are modified by involving small changes in the size of the border rods in the phase-shift regions. Using standard coupled-mode theory and finite-difference time-domain (FDTD) method, we show that by properly choosing the magnitudes of phase-shifts and the lengths of waveguide gratings, a flat-top and sharp roll-off response with a narrow bandwidth is theoretically and numerically achieved by the designed filter. A further analysis shows that the center frequency of the transmission band can be changed by altering the magnitude of the phase-shift and the response performance exhibits relaxed sensitivity to the phase-shift variation. As a specific application, we theoretically demonstrate a third-order Chebyshev bandpass filter based on compound phase-shifted PC waveguide gratings. The filter performance is suitable for dense wavelength-division-multiplexed (DWDM) optical communication systems with a channel spacing of 100-GHz.

Compact and integrated 2-D photonic crystal super-prism filter-device for wavelength demultiplexing applications

A two-dimensional photonic crystal (PhC) super-prism integrated with one-dimensional photonic crystal microcavity filters has been designed using the plane wave expansion (PWE) and 2-D Finite Difference Time Domain (FDTD) methods based on Silicon-on-Insulator (SOI) technology. The super-prism operates as a coarse spatial filter with an average response bandwidth of 60 nm, while the 1-D PhC microcavity filters operate as narrow band-pass transmission filters with an average filter response line-width of 10 nm. This work demonstrates the simultaneous operation of two photonic devices for de-multiplexing applications on a single platform that could be useful in future Photonic Crystal Integrated Circuits (PCICs).

Narrow Band-Pass Transmission Filter Consisting of Two Fiber Bragg Gratings with Different Periods

We developed a narrow band-pass transmission filter by connecting in series two fiber Bragg gratings (FBGs) with slightly different periods. The filter was fabricated by writing two FBGs directly onto the core of a single optical fiber. The center wavelengths of the element FBGs are 1549.71 nm and 1550.05 nm, their bandwidths are 0.36 nm and 0.42 nm, and their reflectances are 99.8% and 99.9%, respectively. The measured bandwidth (full width at half maximum, FWHM) of the fabricated band-pass filter is 0.052 nm, and the maximum transmittance is -12.5 dB. In order to reduce the transmission loss, we adjusted the period difference between the two element FBGs by temperature control, and achieved a maximum transmittance of -0.3 dB although the bandwidth slightly increased up to 0.092 nm. To obtain the narrower bandwidth of the transmission peak, we fabricated a series-type FBG filter using FBGs with higher reflectance, whose measured band-pass bandwidth was as narrow as 0.033 nm.

Analysis of the backreflected signal in an all-fiber bandpass Bragg transmission filter

We present an analysis and explanation of the presence of backreflected signal in an all-fiber bandpass transmission filter. The device is based on a 3-dB fused coupler and two Bragg gratings photoimprinted in the output arms of the coupler. Solutions to correct this problem are proposed.

Grating-based transmission bandpass filters using dispersion-matched mode conversion

A narrow-bandpass filter with a 15-dB signal-to-noise level is demonstrated using Bragg-grating assisted mode conversion within a tapered waveguide. The potential effects of such filters in wavelength-division multiplexing systems are discussed.

Diffractive narrow-band transmission filters based on guided-mode resonance effects in thin-film multilayers

New possibilities for realization of narrow-band transmission filters are introduced. Guided-mode resonance effects in thin-film multilayer structures incorporating a grating layer are shown to yield high-efficiency bandpass transmission filters. Calculated transmission characteristics for such filters are presented and contrasted with characteristics of corresponding thin-film interference filters.

Transmission bandpass guided-mode resonance filters

It is shown that the guided-mode resonance effects associated with waveguide gratings can be used to realize transmission bandpass filters. The key idea is the integration of the resonant waveguide gratings into a dielectric multilayer structure that efficiently reflects the off-resonance spectral components while passing the resonant part. This concept is applied to design multilayer transmission bandpass filters with high efficiency, narrow linewidth, symmetrical response, and low sidebands.

Specular reflectance of optical-black coatings in the far infrared

Far-infrared specular reflectance spectra of seven optically black coatings near normal incidence are presented. Seven photometric spectra were obtained using eleven bandpass transmission filters in the wavelength range between 12 and 500 μm, and three interferometric spectra were obtained for corroboration. Data on the construction, thickness, and rms surface roughness of the coatings are also presented. The chemical composition of three coatings can be distinguished from that of the others by a strong absorption feature between 20 and 40 μm, which can be largely attributed to amorphous silicate material. At 100 μm, the most and least reflective coatings differ by nearly 3 orders of magnitude. Inverse relationships observed between the spectra and the roughness and thickness of the coatings led to development of a reflecting-layer model for the measured reflectance. The model successfully describes the spectra at wavelengths outside the silicate absorption, and optical constants are deduced from a nonlinear least squares fit to the data. Parametric errors are estimated by chi-square analysis, and sensitivity tests are performed to determine which parameters control reflectance in different spectral regions.

Far Infrared Bandpass Filters and Measurements on a Reciprocal Grid

Transmittance and reflectance measurements on wirecloth and electroformed metallic meshes are presented in the region 0.01 <or= lambda/d <or= 5. The results were obtained under equal geometric optical conditions with a vacuum grating spectrometer. The measurements are used for the construction of a transmission bandpass filter in the wavenumber region nu > 18 cm(-1). In order to be able to construct a bandpass filter in the entire far ir, a reciprocal grid is investigated and found to be suitable as a transmission low pass filter characterized only by geometrical parameters.

Low-Q Microwave Filters

Waveguide bandpass transmission filters, each stage of which is composed of two similar irises spaced a distance s apart, are considered. The distance between stages for effectively quarter-wave coupling is shown to be s±1/4λ g/ g. The loaded Q of one stage is computed, and the frequency consciousness of the iris susceptance is shown to have no effect on the spacing condition, but to have a substantial effect upon the loaded Q, especially for low values of Q.

A Frustrated Total Reflection Filter for the Infra-Red

The Frustrated Total Reflection filter is a transmission band pass filter. It can be constructed for any part of the spectrum for which there exist transparent materials of different indices of refraction. Measurements are shown of a filter which was designed for the rocksalt infra-red region. This filter has two oppositely polarized pass bands which, at a particular angle of incidence, are at 412 and 5 microns, respectively. Both the pass bands are approximately 110 of a micron wide. They can be shifted over a range of approximately half a micron by tilting the filter.