Optical Bandpass Filter Research Articles

Tomographic imaging using multi-simultaneous measurements (TIMes) for flame emission reconstructions.

The method of tomographic imaging using multi-simultaneous measurements (TIMes) for flame emission reconstructions is presented. Measurements of the peak natural CH* chemiluminescence in the flame and luminescence from different vaporised alkali metal salts that were seeded in a multi-annulus burner were used. An array of 29 CCD cameras around the Cambridge-Sandia burner was deployed, with 3 sets of cameras each measuring a different colour channel using bandpass optical filters. The three-dimensional instantaneous and time-averaged fields of the individual measured channels were reconstructed and superimposed for two new operating conditions, with differing cold flow Reynolds numbers. The contour of the reconstructed flame front followed the interface between the burnt side of the flame, where the alkali salt luminescence appears, and the cold gas region. The increased mixing between different reconstructed channels in the downstream direction that is promoted by the higher levels of turbulence in the larger Reynolds number case was clearly demonstrated. The TIMes method enabled combustion zones originating from different streams and the flame front to be distinguished and their overlap regions to be identified, in the entire volume.

Temperature measurement of carbon dioxide using emission spectroscopy

This paper presents a relatively simple method for temperature measurement of hot carbon dioxide gas using an emission spectroscopy setup. The ν3 band emission at 4.3 µm is detected using an optical band-pass filter and a mercury cadmium telluride detector. A conversion methodology is introduced that calculates the detector voltage from the spectral radiance considering the spectral sensitivities of the optical components and the non-linear characteristics of the detector. Two radiation models, a line-by-line model and a random statistical narrow band model, are employed to calculate the spectral radiance at given flow conditions. Black body radiation is considered for radiance calculation for optically thick conditions. Temperatures under test conditions in a shock tube are determined using the proposed conversion methodology and are compared with the temperatures obtained from a computational fluid dynamics simulation. The accuracy and efficiency of the temperature measurement using the two radiation models and the black body curve are compared. It is confirmed that the proposed method is a reliable way to determine the temperature while using a relatively simple experimental setup. Detector voltages are presented for a wide range of temperature, pressure, and beam path length for applications under different test conditions.

Self-adaptive amplified spontaneous emission suppression with a photorefractive two-beam coupling filter.

Optical bandpass filters can be utilized to suppress parasitic broadband spectral power prior to laser amplification but are typically designed around specific frequencies or require manual adjustment, thus limiting their compatibility with highly tunable or integrated laser systems. In this Letter, we introduce a self-adaptive volume holographic filter using the dynamic two-beam coupling interaction in photorefractive BaTiO3, demonstrating -10dB suppression of amplified spontaneous emission noise surrounding a tunable 780 nm diode laser peak, with <2nm filter bandwidth and 50% power throughput. The spectral filtering is automatically centered on the lasing mode, with an estimated auto-tuning rate of 100 GHz/s under typical conditions. Furthermore, the filter suppression and bandwidth can be optimized via the two-beam coupling intensity ratio and angle, respectively, for versatile control over the self-adaptive filter characteristics.

Scalable SiPh-InP Hybrid Switch Based on Low-Loss Building Blocks for Lossless Operation

We design and experimentally demonstrate scalable 2 × 2, 4 × 4 and 8 × 8 silicon photonic (SiPh) thermo-optic switch exhibiting low loss, low crosstalk, low power penalty, and BER below 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10</sup> for payload data transmission. Less than 3.13 dB insertion loss (IL) and approximately 20.5 dB crosstalk is measured in the 8 × 8 SiPh banyan switch with thermal phase shifters. We also report on a semiconductor optical amplifier (SOA) in an indium phosphide (InP) technology platform with 25 dB gain and 7 dB noise figure enabling to transmit optical signals with large OSNR. Combining SiPh and InP technologies, we propose a lossless hybrid switch matrix with distributed SOA-based gain capable of transmitting data with near zero loss and low crosstalk over a large switching matrix. In hybrid SiPh/InP switches, the SOA gain compensates for the SiPh switch loss at the cost of amplified spontaneous emission (ASE) noise but mitigated by bandpass optical filters. Lower IL from the SiPh switch requires less gain from the SOAs leading to less OSNR degradation. Experimentally validated building blocks confirmed scalability up to 64 × 64 in SiPh-InP hybrid platform.

Modulation format-independent optical performance monitoring technique insensitive to chromatic dispersion and polarization mode dispersion using a multi-task artificial neural network.

We propose and experimentally demonstrate modulation format-independent optical performance monitoring (OPM) based on a multi-task artificial neural network (MT-ANN). Optical power measurements at a series of center wavelengths adjusted using a widely tunable optical bandpass filter (OBPF) are used as the input features for a MT-ANN to simultaneously realize high-precision optical signal-to-noise ratio (OSNR) and launch power monitoring and baud rate identification (BRI). This technique is insensitive to chromatic dispersion (CD) and polarization mode dispersion (PMD). The experimental verification in a 9-channel WDM system shows that for 10 Gbaud QPSK and 32 Gbaud PDM-16QAM signals with OSNR in the range of 1-30 dB, the OSNR mean absolute error (MAE) and root mean square error (RMSE) are 0.28 dB and 0.48 dB, respectively. For launch power in the range of 0-8 dBm, the MAE and RMSE of the launch power monitoring are 0.034 dB and 0.066 dB, respectively, and the identification accuracy for both baud rates is 100%. Furthermore, this technique utilizes a single MT-ANN instead of three ANNs to realize the simultaneous monitoring of three OPM parameters, which greatly reduces the cost and complexity.

Electrically Controlled Non-Polarizing Optical Bandpass Filtering in Liquid Crystal Planar Waveguide

This letter proposes an electrically controlled non-polarizing optical bandpass filter based on prism coupled planar optical waveguide (POW) structure that incorporates a liquid crystal layer, which works in the electrically controlled birefringent (ECB) mode and makes the central wavelength for TM passband tunable. The measured passband of TM wave, tuned from 526 nm to 599 nm, overlaps with that of the TE wave at 582 nm under driving voltage of about 3.70 V.

A novel setup for simultaneous two-photon functional imaging and precise spectral and spatial visual stimulation in Drosophila

Motion vision has been extensively characterised in Drosophila melanogaster, but substantially less is known about how flies process colour, or how spectral information affects other visual modalities. To accurately dissect the components of the early visual system responsible for processing colour, we developed a versatile visual stimulation setup to probe combined spatial, temporal and spectral response properties. Using flies expressing neural activity indicators, we tracked visual responses in the medulla, the second visual neuropil, to a projected colour stimulus. The introduction of custom bandpass optical filters enables simultaneous two-photon imaging and visual stimulation over a large range of wavelengths without compromising the temporal stimulation rate. With monochromator-produced light, any spectral bandwidth and centre wavelength from 390 to 730 nm can be selected to produce a narrow spectral hue. A specialised screen material scatters each band of light across the visible spectrum equally at all locations of the screen, thus enabling presentation of spatially structured stimuli. We show layer-specific shifts of spectral response properties in the medulla correlating with projection regions of photoreceptor terminals.

Tunable Optical Bandpass Filter via a Microtip-Touched Tapered Optical Fiber*Supported by the National Basic Research Program of China (Grant No. 2017YFA0304502), the National Natural Science Foundation of China (Grant Nos. 11974225, 11634008, and 11974223), the Fund for Shanxi “1331 Project” Key Subjects, the Program of State Key Laboratory of Quantum Optics and Quantum Optics Devices (Grant No. KF201809), and the

We demonstrate a tunable bandpass optical filter based on a tapered optical fiber (TOF) touched by a hemispherical microfiber tip (MFT). Other than the interference and selective material absorption effects, the filter relies on the controllable and wavelength-dependent mode–mode interactions in TOF. Experimentally, a large range of tunability is realized by controlling the position of the MFT in contact with the TOF for various TOF radii, and two distinct bandpass filter mechanisms are demonstrated. The center wavelength of the bandpass filter can be tuned from 890 nm to 1000 nm, while the FWHM bandwidth can be tuned from 110 nm to 240 nm when the MFT touches the TOF in the radius range from 160 nm to 390 nm. The distinction ratio can reach 28 ± 3 dB experimentally. The combined TOF-MFT is an in-line tunable bandpass optical filter that has great application potential in optical networks and spectroscopy, and the principle could also be generalized to other integrated photonic devices.

Evaluating spectral cytometry for immune profiling in viral disease.

In conventional fluorescence cytometry, each fluorophore present in a panel is measured in a target detector, through the use of wide band-pass optical filters. In contrast, spectral cytometry uses a large number of detectors with narrow band-pass filters to measure a fluorophore's signal across the spectrum, creating a more detailed fluorescent signature for each fluorophore. The spectral approach shows promise in adding flexibility to panel design and improving the measurement of fluorescent signal. However, few comparisons between conventional and spectral systems have been reported to date. We therefore sought to compare a modern conventional cytometry system with a modern spectral system, and to assess the quality of resulting datasets from the point of view of a flow cytometry user. Signal intensity, spread, and resolution were compared between the systems. Subsequently, the different methods of separating fluorophore signals were compared, where compensation mathematically separates multiple overlapping fluorophores and unmixing relies on creating a detailed fluorescent signature across the spectrum to separate the fluorophores. Within the spectral data set, signal spread and resolution were comparable between compensation and unmixing. However, for some highly overlapping fluorophores, unmixing resolved the two fluorescence signals where compensation did not. Finally, data from mid- to large-size panels were acquired and were found to have comparable resolution for many fluorophores on both instruments, but reduced levels of spreading error on our spectral system improved signal resolution for a number of fluorophores, compared with our conventional system. Furthermore, autofluorescence extraction on the spectral system allowed for greater population resolution in highly autofluorescent samples. Overall, the implementation of a spectral cytometry approach resulted in data that are comparable to that generated on conventional systems, with a number of potential advantages afforded by the larger number of detectors, and the integration of the spectral unmixing approach. © 2020 International Society for Advancement of Cytometry.

Magnesium fluoride as low-refractive index material for near-ultraviolet filters applied to optical sensors

This article describes the fabrication of MgF2 and MgO thin-film-based optical filters and compares the optical transmission of the filters over UV. The MgF2 thin-films were deposited by use of an e-beam technique and their optical properties were characterised by ellipsometry. The effect of substrate temperature on the optical properties was studied. The MgF2 optimum refractive indices were obtained with a substrate temperature between 200 °C and 300 °C. Optical simulations were performed to compare the performance of MgF2 and MgO in the fabrication of near-UV narrow bandpass optical filters. While MgO-based optical filters result in a higher transmittance peak intensity, especially at 350 nm, the MgF2 optical filters are narrower, present lower values of FWHM, a mean value of 20 nm. This feature could be especially relevant for specific applications on fluorescent optical sensors. Finally, a Fabry-Perot based on a MgF2/TiO2 optical filter was deposited, using an e-beam technique for the MgF2 thin-films and RF-sputtering technique for the TiO2 thin-films. The MgF2/TiO2 optical filter peak transmittance is approximately 70% close to 400 nm, as expected. The results are discussed with focus on applications in fluorescent optical sensors for peaks at 350, 370, 380 and 400 nm, respectively.

Dual-Channel Phase-Tunable Down Converter With LO Frequency Doubling

A dual-channel phase-tunable down converter with local oscillator (LO) frequency doubling is proposed by using a commercial dual-polarization dual-drive Mach-Zehnder modulator (DP-DMZM) and a specially designed dual-channel phase modulator (DP-PM). A radio frequency (RF) signal is injected to one of sub-DMZMs in the DP-DMZM, and a LO signal modulates another sub-DPMZM. An optical bandpass filter (OBPF) is used to filter out the unwanted sidebands, and only the +2nd-order LO and +1st-order RF sidebands are remained and output to the DP-PM. The DP-PM is composed by two parallel PMs with different modulation efficiency between orthogonal polarizations. Each bias voltage of the DP-PM can linearly vary the phase shift between the RF and LO modulated optical sidebands. After detected by a photodetector, the down converted signal with doubled LO frequency and dual-channel phase shift can be obtained. The principle of the proposed link is verified and the experiments are carried out. The experimental results show that the average spur suppression ratio of the down-converted signals is 29.15 dB. The spurious-free dynamic range (SFDR) of the down converter is 103.94 dB·Hz2/3. The continuous and dependent dual-channel phase shift with the full range of 360° can be experimentally achieved. The proposed down converter integrates multiple function in a link, and has the merit of high electrical purity and convenient operation.

Microwave Signal Detection Based on the Nonpolarimetric Frequency Down-Conversion Technique

We report a method for electro-optic (EO) microwave detection based on a nonpolarimetric frequency down-conversion technique. The detection principle is based on the coherent detection of the modulation sideband generated based on the interaction between the optical frequency comb and the microwave signal in the EO crystal. We theoretically analyzed the frequency characteristics of this technique. The theoretical calculation showed that the negative interference of the beat signals between the comb component and the modulation sidebands generated from the higher order comb components decreases the EO detection sensitivity, suggesting that the steeper roll-off characteristics of the optical bandpass filter will increase the sensitivity at the lower frequency side. We further demonstrate that the calculated frequency characteristic agrees well with the experimental measurements from 1 MHz to 10 GHz. Finally, we demonstrate the electric field visualization between the signal line and ground plane of the microstrip line. The proposed method is applicable to antenna measurements and measurements for electromagnetic compatibility.

Hybrid Fourier-domain mode-locked laser for ultra-wideband linearly chirped microwave waveform generation

We show the generation of a tunable linearly chirped microwave waveform (LCMW) with an ultra-large time-bandwidth product (TBWP) based on a hybrid Fourier-domain mode-locked (FDML) laser. The key device in the hybrid FDML laser is a silicon photonic integrated micro-disk resonator (MDR) which functions as an optical bandpass filter, to have strong wavelength selectivity and fast frequency tunability. By incorporating the integrated MDR in the fiber-based ring cavity to perform frequency-domain mode locking, an FDML laser is realized and a broadband frequency-chirped optical pulse is generated. By beating the frequency-chirped optical pulse with an optical carrier from a laser diode (LD) at a photodetector (PD), an LCMW is generated. The bandwidth of the LCMW is over 50 GHz and the temporal duration is over 30 µs, with an ultra-large TBWP of 1.5 × 106. Thanks to the strong tunability of the MDR in the FDML laser, the generated LCMW is fully tunable in terms of bandwidth, temporal duration, chirp rate, and center frequency.

Online determination of potassium and sodium release behaviour during single particle biomass combustion by FES and ICP-MS

This study focuses on the determination of alkali release from wood and straw pellets during combustion. The aim is to expand the knowledge on the K and Na release behaviour and to adopt chemiluminescence-based sensors for online monitoring of alkali detection which can be applied for the prevention of fouling formation in low quality biomass combustion plants. Flame emission spectrometry (FES) was used for optical detection of chemiluminescence spectra of K and Na using optical bandpass filters mounted on an ICCD (Intensified Charge Coupled Device) camera. FES data were verified by additional experiments with a single particle reactor (SPR) coupled with an inductively coupled plasma mass spectrometer (ICP-MS). Using both techniques, the release profiles of K and Na during a single pellet combustion at 1000 °C were determined and obtained K* and Na* emission intensities directly correlated with the results from the ICP-MS. It was determined that the emission intensity of alkali radicals depends on alkali concentrations in the samples and K and Na radical emission intensities increase with increasing alkali amounts in the samples. The ICP-MS data revealed that the release of K and Na mainly takes place during the stage of devolatilization. During devolatilization, almost all potassium and sodium are released from wood samples, while only 65–90% of K and 74–90% of Na are released from straw samples. Based on the results, the flame emission spectroscopy technique is capable to fully detect released alkali metals in the gas phase during combustion and proves a possibility to use flame emission sensors for monitoring the release of alkali species from biomass during combustion processes.

Single channel high transmission optical band-pass filter based on plasmonic Nano-cavities

This paper is concerned with the investigation of an optical band-pass filter based on subwavelength surface plasmon polaritons. The transmission characteristics are numerically analyzed by the finite-difference time-domain method, and simulation results reveal that the structure has a band-pass filtering characteristic. The metal–insulator–metal plasmonic nanostructure is implemented by several vertical rectangular cavities across an optical waveguide. The metal and dielectric materials utilized for the realization of the filter are silver and air, respectively. Furthermore, the performance can be efficiently modified by tuning the geometric parameters such as the cavities’ length and width and the coupling distance between them. The output transmission spectrum shows a single transmission peak over a wide wavelength range of 400 to 2000 nm, while the full width at half-maximum is about 200 nm. The high extinction ratio beyond 40 dB with the low insertion loss makes this filter comparable to those of other proposed band-pass filters. Besides, the operating wavelength of this filter can be expeditiously tuned by adjusting the geometric parameters, mainly the length of the center cavity. The small footprint of the structure contributes to the achievement of wavelength selection systems for optical communications in ultradense plasmonic integrated circuits.

Single-channel high-transmission optical band-pass filter based on plasmonic nanocavities

This paper is concerned with the investigation of an optical band-pass filter based on subwavelength surface plasmon polaritons. The transmission characteristics are numerically analyzed by the finite-difference time-domain method, and simulation results reveal that the structure has a band-pass filtering characteristic. The metal–insulator–metal plasmonic nanostructure is implemented by several vertical rectangular cavities across an optical waveguide. The metal and dielectric materials utilized for the realization of the filter are silver and air, respectively. Furthermore, the performance can be efficiently modified by tuning the geometric parameters such as the cavities’ length and width and the coupling distance between them. The output transmission spectrum shows a single transmission peak over a wide wavelength range of 400 to 2000 nm, while the full width at half-maximum is about 200 nm. The high extinction ratio beyond 40 dB with the low insertion loss makes this filter comparable to those of other proposed band-pass filters. Besides, the operating wavelength of this filter can be expeditiously tuned by adjusting the geometric parameters, mainly the length of the center cavity. The small footprint of the structure contributes to the achievement of wavelength selection systems for optical communications in ultradense plasmonic integrated circuits.

Miniaturised Infrared Spectrophotometer for Low Power Consumption Multi-Gas Sensing.

Concept, design and practical implementation of a miniaturized spectrophotometer, utilized as a mid-infrared-based multi gas sensor is described. The sensor covers an infrared absorption wavelength range of 2.9 to 4.8 um, providing detection capabilities for carbon dioxide, carbon monoxide, nitrous oxide, sulphur dioxide, ammonia and methane. A lead selenide photo-detector array and customized MEMS-based micro-hotplate are used as the detector and broadband infrared source, respectively. The spectrophotometer optics are based on an injection moulded Schwarzschild configuration incorporating optical pass band filters for the spectral discrimination. This work explores the effects of using both fixed-line pass band and linear variable optical filters. We report the effectiveness of this low-power-consumption miniaturized spectrophotometer as a stand-alone single and multi-gas sensor, usage of a distinct reference channel during gas measurements, development of ideal optical filters and spectral control of the source and detector. Results also demonstrate the use of short-time pulsed inputs as an effective and efficient way of operating the sensor in a low-power-consumption mode. We describe performance of the spectrometer as a multi-gas sensor, optimizing individual component performances, power consumption, temperature sensitivity and gas properties using modelling and customized experimental procedures.

Selecting optimal spectral bands for improved detection of autofluorescent biomarkers in multiphoton microscopy.

.Significance: In multiphoton microscopy, two-photon excited fluorescence (TPEF) spectra carry valuable information on morphological and functional biological features. For measuring these biomarkers, separation of different parts of the fluorescence spectrum into channels is typically achieved by the use of optical band pass filters. However, spectra from different biomarkers can be unknown or overlapping, creating a crosstalk in between the channels. Previously, establishing these channels relied on prior knowledge or heuristic testing.Aim: The presented method aims to provide spectral bands with optimal separation between groups of specimens expressing different biomarkers.Approach: We have developed a system capable of resolving TPEF with high spectral resolution for the characterization of biomarkers. In addition, an algorithm is created to simulate and optimize optical band pass filters for fluorescence detection channels. To demonstrate the potential improvements in cell and tissue classification using these optimized channels, we recorded spectrally resolved images of cancerous (HT29) and normal epithelial colon cells (FHC), cultivated in 2D layers and in 3D to form spheroids. To provide an example of an application, we relate the results with the widely used redox ratio.Results: We show that in the case of two detection channels, our system and algorithm enable the selection of optimized band pass filters without the need of knowing involved fluorophores. An improvement of 31,5% in separating different 2D cell cultures is achieved, compared to using established spectral bands that assume NAD(P)H and FAD as main contributors of autofluorescence. The compromise is a reduced SNR in the images.Conclusions: We show that the presented method has the ability to improve imaging contrast and can be used to tailor a given label-free optical imaging system using optical band pass filters targeting a specific biomarker or application.

Photonic generation of a parabolic-shaped microwave signal and dual-linear-chirp microwave waveform.

Based on an external modulation technique through a dual-polarized dual-parallel Mach-Zehnder modulator, a photonic technique is proposed for the generation of a microwave signal with a parabolic shape. An optically modulated waveform from the modulator is passed through an optical bandpass filter, which results in parabolic signals of 1 GHz and 9 GHz frequencies at the photo detector. A peak power spectral component of -30dBm power is obtained at 1 GHz and 9 GHz frequencies of the parabolic signal. Based on the present methodology, a parabolic signal of the desired frequency band can be obtained. The obtained signal is processed to generate a dual-linear-chirp signal by passing through the phase modulator. Here, a dual-linear-chirp microwave waveform with a chirp rate of 1.53Π×1019Hz/s is achieved at 6 GHz center frequency. The results are obtained through MATLAB simulation and verified by experimental results. A fair agreement is found between the result obtained through MATLAB simulation and the result obtained by experimental verification.

Using optically filtered high-speed imaging to characterise expansion tube operating conditions

Traditionally, Pitot rake test models have been used to take time- and spatially resolved impact pressure measurements in impulse facilities for flow characterisation. When expansion tubes are used for the study of hypervelocity planetary entry phenomena, generally the post-shock flow in the test section strongly radiates. This paper outlines a simple method which uses a high-speed camera in addition to a conventional Pitot rake to improve estimates of experimental test time and core flow size by imaging post-shock radiative emission over the probes in the Pitot rake. This method can also be used with specific narrow bandpass optical filters to examine when the emission from key species remains constant in the test flow in both time and space. The selection and suitability of various optical filters for a high-enthalpy Earth entry scenario are examined in this paper, as well as the effect of radiation from flow contamination. Experimentally, it was found that the radiative emission generally rises quite abruptly at the end of the test time and sometimes in situations where the pressure remains constant. It was also seen that different optical filters focusing on different spectral features can show their abrupt rise at different times, giving differing values for the end of the test time. Unfiltered measurements were found to be compromised by contaminant radiative emission which was seen to be randomly distributed in time and space towards the end of the test time.