Transmission Filters Research Articles

Ultra-broadband supercontinuum covering a spectrum from visible to mid-infrared generated by high-power and ultrashort noise-like pulses.

To facilitate a fiber-based supercontinuum generation system, single-mode fibers with different cutoff wavelengths are introduced to serve as shortpass filters to replace conventional reflective or transmissive filters. Meanwhile, an ytterbium-doped fiber amplifier is adopted to amplify the filtrated pulses, scaling their average power to the watt level up to 4.33 W. Through this approach, ultrashort high-power laser pulses of 1.56 µm and 1.06 µm wavelengths, which are commonly used in optical communications and industrial applications, can be generated by this single system. Furthermore, it is found that the noise-like pulses still maintain their temporal features, even after they undergo multiple optical processes including amplification, supercontinuum generation, and filtration. After that, the generated pulses at 1.06 µm were launched into a photonic crystal fiber to generate a supercontinuum of 1.85 W covering a spectral range from 560 nm in the visible region to 3.5 µm in the mid-infrared region. This is one of the widest records of spectrum in broadband supercontinuum generation.

Passband-Flattened Frequency-Tunable Optical Fiber Multiwavelength Filter with Composite Combination of Waveplates.

We propose a passband-flattened frequency-tunable optical multiwavelength filter with a composite combination of waveplates, which is realized by harnessing a polarization-diversified loop structure. The proposed filter comprises a polarization beam splitter (PBS), two polarization-maintaining fiber (PMF) segments of equal length, an ordered waveplate combination (OWC) of a half-wave plate (HWP) and a quarter-wave plate (QWP) before the first PMF segment, and an OWC of a QWP and an HWP before the second PMF segment. The second PMF segment is butt-coupled to one port of the PBS so that its slow axis is oriented at 22.5° for the horizontal axis of the PBS. Based on the filter transmittance derived through the Jones calculus, we found the orientation angle (OA) sets of the four waveplates, which could induce an extra phase shift Φ from 0° to 360° in the passband-flattened transmittance function. From the transmission spectra calculated at the eight selected OA sets, which caused Φ to increase from 0° to 315° by steps of 45°, it was confirmed that the passband-flattened multiwavelength spectrum can be continuously tuned by properly controlling the OAs. This indicates continuous wavelength tunability based on composite OWCs. Then, this theoretical prediction was verified by experimental demonstration.

Transfer matrix based reverse engineering approach to analyse annealing induced changes in optical transmission of TiO2/Ag/TiO2/Ag multilayer

Measured optical transmission spectrum of photonic structure comprising TiO2/Ag/TiO2/Ag multilayer is reverse engineered by employing transfer matrix method with three different multilayer models to analyse the factors responsible for the annealing induced changes in the transmission. The multilayer is made to have a single peak or a passband in the visible region, so that the spectral changes due to annealing can be observed more vividly. Prior to the multilayer deposition, single layer thin film of Ag and TiO2 are prepared using asymmetric bipolar pulsed dc sputtering and their optical constants such as refractive index and extinction co-efficient are determined. The multilayer is prepared by four-layer sequential deposition of Ag and TiO2 on BK7 glass substrate. The deposited TiO2/Ag/TiO2/Ag multilayer is annealed in the temperature range 100–300 °C. The optical transmission of the annealed multilayers is measured using spectrophotometer. The measured peak transmission and spectral bandwidth of the multilayer filter decreases with annealing, which is ascribed to inter-diffusion at the interfaces, and increasing absorption at the interfaces and in the silver layers of the multilayer. The ascription is resolved from the layers properties and interface parameters obtained by reverse engineering the measured transmission spectra. The rough interfaces in the multilayer are modeled as thin homogeneous layers following Bruggeman effective medium approximation, and the interface parameters are found to be varying with annealing.

Mid-wave infrared transmittance filters in suspended GaAs subwavelength gratings

The high-Q resonances observed in subwavelength gratings support many applications, including optical phase arrays, refractive index sensing, enhancement of nonlinear effects, and spectral filtering. Integrating these functions with active devices opens new possibilities for microscale optical systems. GaAs high-contrast gratings (HCGs) simplify the integration of resonant structures with active III–V semiconductor devices. These structures are particularly useful in the mid-wave infrared (5–7 μm), overcoming several limitations of the material systems presented in previous works. By selectively removing an underlying AlGaAs layer, a high index contrast is introduced between suspended GaAs gratings and air without using an exotic low-index substrate. Furthermore, epitaxial growth enables precise device thickness. In this work, we demonstrate GaAs/air HCG transmittance filters for operation in the mid-wave infrared. A numerical study investigates the dependence of filter performance on substrate proximity. We fabricate experimental filters using an HBr-based plasma etch and a selective wet etch. Optical characterization demonstrates quality factors >50 and transmittance peak efficiency >50%. The observed filter performance is comparable to other mid-wave infrared subwavelength grating filters. This work concludes with discussion of experimental nonidealities, emphasizing the considerable influence of grating size and optical measurement conditions. The suspended GaAs HCG filters show potential for direct integration with active devices such as photodetectors and lasers.

Radiative Transfer with Opacity Distribution Functions: Application to Narrowband Filters

Abstract Modeling of stellar radiative intensities in various spectral passbands plays an important role in stellar physics. At the same time, direct calculation of the high-resolution spectrum and then integration of it over the given spectral passband is computationally demanding due to the vast number of atomic and molecular lines. This is particularly so when employing three-dimensional (3D) models of stellar atmospheres. To accelerate the calculations, one can employ approximate methods, e.g., the use of opacity distribution functions (ODFs). Generally, ODFs provide a good approximation of traditional spectral synthesis, i.e., computation of intensities through filters with strictly rectangular transmission functions. However, their performance strongly deteriorates when the filter transmission noticeably changes within its passband, which is the case for almost all filters routinely used in stellar physics. In this context, the aims of this paper are (a) to generalize the ODFs method for calculating intensities through filters with arbitrary transmission functions, and (b) to study the performance of the standard and generalized ODFs methods for calculating intensities emergent from 3D models of stellar atmospheres. For this purpose we use the newly developed MPS-ATLAS radiative transfer code to compute intensities emergent from 3D cubes simulated with the radiative magnetohydrodynamics code MURaM. The calculations are performed in the 1.5D regime, i.e., along many parallel rays passing through the simulated cube. We demonstrate that the generalized ODFs method allows accurate and fast syntheses of spectral intensities and their center-to-limb variations.

Tunable properties of one-dimensional GaAs/AlAs-based photonic crystal containing two defect layers

The characteristics of an optical tunable filter are elicited theoretically in this study. Here, the characteristic matrix method is used to investigate the behavior of the defect modes in GaAs/AlAs-based one-dimensional defective multilayered photonic crystals consisting of two externally magnetized GaAs defect layers. The external magnetic field is applied to GaAs for the consideration of the defect mode in the middle of the structure. The effects on the defect modes for distinct values of the external magnetic field, electron density, and angle of incidence for both the TE and TM waves are analyzed. The numerical results convey that the frequency of the defect mode can be tuned effectively by changing the values of the physical parameters. Besides, the modes of propagation play a major role in the shifting trend of occurrence of the defect modes in the higher frequency region. The contour plots for electron density and incident angle show two clear defect peaks for both TE and TM waves. Moreover, the Brewster angle verifies the complete transmission through the defective symmetric optical media. The proposed structure can be used to design the narrowband and multichannel tunable transmission filters at the terahertz region.

Method of transmission filters to measure emission spectra in strongly scattering media.

We describe a method based on a pair of transmission filters placed in the emission path of a microscope to resolve the emission wavelength of every point in an image. The method can be applied to any type of imaging device that provides the light in the wavelength transmission range of the filters. Unique characteristics of the filter approach are that the light does not need to be collimated and the wavelength response does not depend on the scattering of the sample or tissue. The pair of filters are used to produce the spectral phasor of the transmitted light, which is sufficient to perform spectral deconvolution over a broad wavelength range. The method is sensitive enough to distinguish free and protein-bound NADH and can be used in metabolic studies.

Experimental investigation and simulation of phase-shifted fiber Bragg gratings

This work reports the inscription and investigation of phase-shifted fiber Bragg gratings. A method for the inscription of such structures that makes it possible to vary the full width at half-maximum of the transmission notch in the reflection band within the range of 8–76 pm is proposed. The influence of the amplitude of the induced refractive index modulation and length of the formed structure on the studied parameter is investigated and analyzed. The results are presented as curves for data collected experimentally and theoretically. The investigation conducted in this study makes it possible to manufacture structures with known widths of the transmission notches, opening up a wide spectrum of possibilities to design optical elements based on such specific Bragg grating structures (e.g., fiber-optic sensors, tunable transmission filters, and demultiplexers).

Design of linear variable optical filter for hyperspectral imaging

The design and simulation of a Linear Variable Optical Filter (LVOF) for the visible and near-infrared (VNIR) 450–900 nm region is described in this paper. The LVOF is designed and simulated using an all-dielectric and metal-dielectric-based induced transmission filter. We compared the design approaches based on all-dielectric and metal-dielectric filters in this paper. The LVOF filter based on the metal-dielectric method reduces design and fabrication complexities while also being cost-effective because no band rejection filters are deemed necessary. The metal-dielectric filter reported in the paper also has minimum spectral transmission of 70% as well as an FWHM of 2%–3% of central maxima.

편광 상이 고리 구조 내 복합 파장판 조합을 이용한 협대역 빗살 스펙트럼의 연속 파장 제어에 관한 연구

In this paper, we propose a novel birefringence filter based on polarization-diversity loop structure(PDLS) for continuous wavelength controlling of narrowband transmission spectrum. This filter consists of a polarization beam splitter(PBS), two high-birefringence fiber(HBF) of the same length, two half-wave plates(HWPs), and two quarter-wave plates(QWPs). We found a combination of the orientation angles of the four waveplates which could induce an additional phase difference φ from 0° to 360° to the narrowband transmittance function by considering the filter transmittance derived from Jones matrices. Then, we experimentally exhibited eight narrowband transmission spectra measured with a channel spacing of ∼0.1nm based on these theoretical calculations. As a result, we verified the effectiveness of the narrowband multiwavelength filter capable of continuously tuning its wavelength position when properly adjusting the combination of the orientation angles.

Comparison between flexible AC transmission systems (FACTs) and filters regarding renewable energy systems harmonics mitigation

Abstract Renewable energy growing is considered one of the main sources of the system harmonics. Thus, in this paper, Total Harmonic Distortion (THD) is reduced by using flexible AC transmission systems (FACTs). Static Compensator (STATCOM) and Static Synchronous Series Compensator (SSSC) are used and compared against basic harmonics filter. Cases studied are as follows: First, the system is studied without any harmonic limitation. Then each type of the chosen two types of (FACTs) is connected to the system. Finally, the harmonics filter is used as a reference to verify the (FACTs) effectiveness. MATLAB/SIMULINK is the tool to examine and simulate the system.

Fiber Bragg Grating Sensors for Dynamic Strain Measurements in Gasoline Direct Injectors

The strain in a gasoline direct injector was measured during a single injection process by using a fiber Bragg grating sensor installed along the axis of the injector stem in different operating pressure conditions. A fast FBG interrogator, based on linear transmission filters and direct detection, permits to measure dynamic strain up to 240 MHz frequency with a detection limit of 1.1 με and a dynamic strain resolution of 20.2 nε/√Hz. The FBG sensor was first calibrated for an internal pressure ranging from 50 to 250 bar and an energizing duration of 5ms; the FBG sensor allows to measure the static internal pressure of the injector inlet, which is linearly dependent on the FBG static strain, as well as the dynamic strain during the opening/spray/closing processes induced by pressure fluctuations, mechanical vibrations and the impact forces related to the injector needle bounces. The fast FBG interrogator, has been first validated by comparison with commercial interrogators, and then used to detect fast transients. The measured FBG strain variation provides important information on the dynamic behavior of the injector during the spray, confirmed by the analysis of the spray injection performed through a High Speed Video camera. The static and dynamic measurements demonstrate the effectiveness of the FBG sensors technology for monitoring the behavior of the injector and controlling the engine performance and emissions.

Graphene/porous silicon reconfigurable transmission filter operating at 1.55 and 1.53 µm

Graphene/porous silicon reconfigurable transmission filter operating at 1.55 and 1.53 µm

Dual-period guided-mode resonance filters for SWIR multi-spectral image sensors.

Typical guided-mode resonance (GMR) transmission filter design, which is based on a single ridge per period, necessitates multiple etching/fabrication steps for implementing an array of filters (having different transmission bands) on the same substrate. To address this problem, we demonstrate dual-period narrow bandpass GMR filters that offer more degrees of freedom, two periods and two fill-factors, for tuning the filter characteristics and achieving wider stop bands without changing the grating height. A set of six transmission filters with well-separated passbands in the short-wave infrared region was designed using COMSOL Multiphysics simulations and produced on the same silicon-on-quartz wafer in a single fabrication run. The $90\;{\unicode{x00B5}{\rm m}}\;{\times}\;90\;{\unicode{x00B5}{\rm m}}$ size filters exhibited passbands as narrow as 15 nm with peak-wavelength tunability over 200 nm, flat stop bands as wide as ${\sim}{400}\;{\rm nm}$, and peak transmittance reaching 87%. The experimental transmission spectra were in good agreement with the corresponding simulations. These findings pave the way for the realization of pixel size filter arrays for multispectral image sensors.

Absolute calibration of the spectral responsivity of thermal detectors in the near-infrared (NIR) and mid-infrared (MIR) regions by using blackbody radiation

Abstract. The Physikalisch-Technische Bundesanstalt (PTB) has set up an additional measurement approach for the absolute calibration of the spectral responsivity of detectors in the near-infrared (NIR) and mid-infrared (MIR) spectral range. This alternative method uses the radiation of a blackbody operating at about 1200 K with a precision aperture. The blackbody radiation can be calculated by Planck's law and is additionally spectrally selected by accurately characterized optical bandpass filters. Thus, a calibration of the spectral responsivity of a detector with respect to irradiance can be achieved at the bandpass wavelength of the applied transmission filters. If the aperture of the detector is known, the spectral responsivity can also be calculated with respect to radiant power. Thermopile detectors with known aperture size were calibrated in terms of their spectral responsivity with several bandpass filters in the spectral range between 1.5 µm up to 14 µm with relative standard measurement uncertainties between 5 % and 19 %. The obtained results are consistent with previous calibrations at PTB's national primary detector standard. Therefore, this additional measurement approach is a further validation of the existing primary method which is based on a cryogenic radiometer and extends the usable wavelength range.

Nonpolarizing narrow band metamaterial transmission filter based on electromagnetically induced transparency at visible wavelengths

A nonpolarizing narrow band metamaterial transmission filter based on EIT operating at visible wavelengths is demonstrated theoretically and experimentally. The two separated resonance components that are able to generate EIT effect are connected using a SiO2 film layer and the hardness of the metamaterial filter is further improved by adding a glass substrate. The theoretical results are calculated by using the finite-difference time-domain simulation method and a metamaterial filter sample is fabricated and measured to verify the simulation spectrum. The full width at half-maximums of simulation and measured spectra are 0.74 nm and 1.61 nm respectively. Electric field distributions are also calculated to illustrate the physical mechanism. The novel method might be helpful to improve the processability of the narrow band metamaterial transmission filter, and also to further promote the applications of this kind of metamaterial filter.

Enhancement of high transmittance and broad bandwidth terahertz metamaterial filter

Electromagnetic metamaterials can be used in optoelectronic devices because of their unique electromagnetic resonance properties. Many reported optoelectronic devices modulate transmission characteristics by changing geometric parameters. In this paper, a dual-band terahertz (THz) metamaterial filter based on vanadium dioxide (VO2) is measured and verified. Both transmission peaks are enhanced through changing the radius or thickness in experiments. It is found that this metamaterial filter is temperature sensitivity. Both transmission peaks and resonance frequencies can be modulated by heating or cooling. Finally, two transmission peaks are transformed into a broad transmission band with temperature increasing.

Adaptive Power Control with Selection Combiner over Nakagami-m Fading Channel

Acceptable Quality of Service (QoS) is of paramount importance in wireless communication system. However, QoS is affected by multipath propagation resulting in unreliable reception. Selection Combiner (SC) which is one of the techniques previously used to solve this problem is associated with poor performance due to fixed power level used in the technique. Hence, in this paper, an Adaptive Power Control (APC) technique using SC with a closed form expression over Nakagami-m Fading Channel is proposed. APC technique is developed using fixed power SC and Link Adaptation Algorithm (LAA). Ten thousand (10,000) bits are randomly generated and modulated using Quadrature Amplitude Modulation (QAM) scheme. The modulated signal is passed through the Square Root Raised Cosine (SRRC) filter for suitable transmission over the Nakagami-m fading channel. The faded signals at varying paths ‘L’ (2, 3, 4) are selected by Conventional Selection Combining (CSC) to instruct the transmitter for adjustment of power level based on the value of the received signal through LAA. Mathematical expression for the received signal using Probability Density Function (PDF) of Nakagami-m Fading Channel at varying paths is derived. The APC technique is simulated using MATLAB software and evaluated using Signal to Noise Ratio (SNR), Outage Probability (OP) and Bit Error Rate (BER) to determine the performance of the proposed technique. OP values of 0.0969 and 0.1069 are obtained for APC and CSC, respectively, at SNR of 6 dB with L of 4, while BER values of 0.0052 and 0.2599 are obtained for APC and CSC, respectively. The percentage reduction in OP and BER are 29.79% and 54.43%, respectively. The results obtained show that APC gives lower OP and BER values with increase in SNR when compared with fixed power SC due to self-adjustment of the allocated power. Therefore, the APC technique proposed can be used to enhance the performance of wireless communication systems. Keywords— Adaptive Power Control (APC), Selection Combiner (SC), Link Adaptation Algorithm (LAA), Bit Error Rate (BER), Outage Probability (OP), Probability Density Function (PDF) and Diversity Combiner (DC).

Suitability of blue filter ophthalmic lenses with respect to the use with mobile devices and computers

Blue light comprises the electromagnetic spectrum between approximately 380-500 nm. The sun is the main source of blue light, although we also receive emissions of this type of light by electronic devices and artificial lighting. Changes in lighting systems and lifestyle habits have led to the questioning of exposure to blue light emitted by artificial sources as a possible health problem. Faced with this potential problem, ophthalmic lens laboratories brought to market lenses with specific treatment against hazard blue light emitted by electronic devices. In this work we measure the spectral transmittance of 6 blue filter ophthalmic lenses from 4 different laboratories, Essilor, Hoya, Shamir and Kodak. We face the transmission spectra measured with the emission spectra of 3 different electronic devices, to be able to discuss the suitability of these blue filter lenses for use with electronic devices. We conclude that the transmittance of blue filters in the maximum blue light emission range is high above 90%, and that changing the correlated color temperature of electronic devices results in a greater reduction in exposure to hazard blue light. We also found that Shamir's Blue Zero and Essilor's Eye Protect System lenses block UVA light and have low transmittance in the range of 380- 420 nm, so they would be more effective for protection against blue light outdoors than for use with electronic devices.

A 1D Lyman-alpha profile camera for plasma edge neutral studies on the DIII-D tokamak.

A one dimensional, absolutely calibrated pinhole camera system was installed on the DIII-D tokamak to measure edge Lyman-alpha (Ly-α) emission from hydrogen isotopes, which can be used to infer neutral density and ionization rate profiles. The system is composed of two cameras, each providing a toroidal fan of 20 lines of sight, viewing the plasma edge on the inboard and outboard side of DIII-D. The cameras' views lie in a horizontal plane 77cm below the midplane. At its tangency radius, each channel provides a radial resolution of ∼2cm full width at half maximum (FWHM) with a total coverage of 22cm. Each camera consists of a rectangular pinhole, Ly-α reflective mirror, narrow-band Ly-α transmission filter, and a 20 channel AXUV photodetector. The combined mirror and transmission filter have a FWHM of 5nm, centered near the Ly-α wavelength of 121.6nm and is capable of rejecting significant, parasitic carbon-III (C-III) emission from intrinsic plasma impurities. To provide a high spatial resolution measurement in a compact footprint, the camera utilizes advanced engineering and manufacturing techniques including 3D printing, high stability mirror mounts, and a novel alignment procedure. Absolutely calibrated, spatially resolved Ly-α brightness measurements utilize a bright, isolated line with low parasitic surface reflections and enable quantitative comparison to modeling to study divertor neutral leakage, main chamber fueling, and radial particle transport.