Narrow Bandpass Filter Research Articles

Integrated photodetectors for compact Fourier-transform waveguide spectrometers

Extreme miniaturization of infrared spectrometers is critical for their integration into next-generation consumer electronics, wearables and ultrasmall satellites. In the infrared, there is a necessary compromise between high spectral bandwidth and high spectral resolution when miniaturizing dispersive elements, narrow band-pass filters and reconstructive spectrometers. Fourier-transform spectrometers are known for their large bandwidth and high spectral resolution in the infrared; however, they have not been fully miniaturized. Waveguide-based Fourier-transform spectrometers offer a low device footprint, but rely on an external imaging sensor such as bulky and expensive InGaAs cameras. Here we demonstrate a proof-of-concept miniaturized Fourier-transform waveguide spectrometer that incorporates a subwavelength and complementary-metal–oxide–semiconductor-compatible colloidal quantum dot photodetector as a light sensor. The resulting spectrometer exhibits a large spectral bandwidth and moderate spectral resolution of 50 cm−1 at a total active spectrometer volume below 100 μm × 100 μm × 100 μm. This ultracompact spectrometer design allows the integration of optical/analytical measurement instruments into consumer electronics and space devices.

Contactless thermal mapping of high-temperature solar receivers via narrow-band near-infrared thermography

We present the design, construction, and successful proof-of-principle demonstration of a thermal camera capable of seeing through glass and other radiatively participating media, and apply the device to map the temperature distribution of a steam-filled windowed solar cavity receiver operating at temperatures up to 1400 °C. This “glass piercing” thermal camera utilizes a narrow bandpass filter with a center wavelength of 772 nm to effectively see through glass and similar optical glazing materials, as well as infrared-active radiatively participating media, to allow direct contactless measurement of the inner surfaces of windowed cavity receivers. By utilizing a standard optical CMOS sensor, the device achieves much higher resolution at a lower cost than traditional thermal cameras operating in the mid-infrared. Using a calibrated photometry sphere, we experimentally determined the device sensitivity (grayscale value per spectral radiance value), which allows the spectral radiance and ultimately temperature to be determined from a simple grayscale image. The calibrated device was then applied to both a blackbody source operating at above 1000 °C, and a high-temperature solar cavity receiver operating above 1400 °C. The results serve as an experimental proof-of-concept for glass-piercing and radiatively-participating-gas-piercing thermography.

Simple method for broadband amplification in a polarizing-maintained Yb fiber amplifier with a narrow bandpass filter.

We have demonstrated a simple method for achieving broadband amplification and short-pulse generation in a polarizing maintained (PM) Yb fiber amplifier with the implementation of a narrow bandpass filter (NBF). By using an NBF with a spectral bandwidth of 3.3 nm at a wavelength of 1036 nm before the PM Yb amplifier, the spectral bandwidth was extended by self-phase modulation (SPM) in the PM YDF amplifier. The recompressed pulse duration of 125 fs obtained with the NBF was nearly three times shorter than that (350 fs) achieved without the NBF, and therefore the peak intensity of 0.35 MW with the NBF was over two times greater in comparison with that (0.16 MW) obtained without the NBF. We believe that this simple method is useful for developing an all-fiber laser system for high-power short pulse generation.

Turning point monitoring of narrow bandpass filters: the enormous strength of the error self-compensation effect.

It is generally accepted that turning point optical monitoring provides a very strong error self-compensation effect when used to control the deposition of narrow bandpass filters with quarter-wave or multiple-quarter-wave layer optical thicknesses. However, how strong this effect is has never been assessed before, to our knowledge. A recently developed general approach to estimating the strength of thickness error correlation and the strength of the associated error self-compensation effect allows, possibly for the first time, such an assessment. In this work, it is proved that, indeed, in the case of monitoring narrowband filters, the latter effect has enormous strength, many times greater than the strength of the error self-compensation effects when manufacturing all other types of optical coatings using various monochromatic and broadband monitoring techniques.

On the estimation of frequency residuals in a radio occultation experiment

ABSTRACT In the radio occultation (RO) experiments, the difference between observed and predicted Doppler, called frequency residuals, form basic input data to retrieve the vertical profiles of atmospheric parameters. In this paper, we compare different methods to estimate frequency residuals in a one-way downlink single frequency RO experiment, and propose a suitable method that not only improves the residual estimates but is easy to implement too. We show that the Doppler broadening in the received radio signals, which causes uncertainties in Doppler estimation, is predominantly due to changes in the line of sight velocity between the receiver and transmitter during the sampling period. Coupled with the random noises in the receiving system and fluctuations in plasma medium along the propagation path, the resultant uncertainty in Doppler estimates can become as large as 60 Hz. The expected phase changes due to the ionospheric dispersion/refraction, on the other hand, are of the order of 0.01–2 Hz only. To circumvent the issue, we re-sample the observed radio signals in smaller time packets. After removing DC constant shift, a narrow bandpass filter is applied to suppress small-scale fluctuations and Doppler and power of the received signals are estimated. Using Akatsuki radio signals, tracked at Indian Deep Space Network as test data sets, we compare our results with the residuals provided by the Akatsuki team and using the method used for analysing Cassini RO data. We show that our method provides a better estimate of frequency residuals with lesser standard deviations.

In situ monitoring of build height during powder-based laser metal deposition

A powder-based laser metal deposition (LMD) system can fabricate customised three-dimensional (3D) parts, layer by layer, based upon a computer-aided design (CAD) model. However, the deposition will not always feature the expected geometry due to excessive heat input and inconsistent powder flow. Due to the layer-by-layer nature of LMD, geometrical error in one layer is compounded in all following layers and may result in a build failure. Thus, it is critical to monitor online the track and layer height. This study developed an in situ monitoring system integrating a webcam and a narrow bandpass filter. The laser/powder defocus distance was extracted from the melt pool images, and the track/layer height was calculated from the laser/powder defocusing distance and preprogrammed layer spacing. The presented approach does not need additional illumination sources and is a nonintrusive online method. Therefore, it is a potential precursor to a feedback build height control system. It also can be used for measuring omnidirectional height, i.e. height in different build directions relative to the substrate, which has been tested by fabricating two thin-wall structures with customised shapes. These online-measured height data were successfully validated against dimensional measurements from an offline 3D scanner, thus demonstrating the online system’s potential utility in a feedback control system for ensuring acceptable part geometrical accuracy.

Analysis of Instantaneous Frequency Components of Speech Signals for Epoch Extraction

The major impulse-like excitation in the speech signal is due to abrupt closure of the vocal folds, which takes place at the glottal closure instant (GCI) or epoch in each cycle. GCIs are used in many areas of speech science and technology, such as in prosody modification, voice source analysis, formant extraction and speech synthesis. It is difficult to observe these discontinuities (corresponding to GCIs) in the speech signal because of the superimposed time-varying response of the vocal tract system. This paper examines the phase part of different frequency components of the speech signal to extract epochs. Three analysis methods to decompose the speech signal into different frequency components are considered. These methods are the short-time Fourier transform (STFT), narrow bandpass filtering (NBPF), and single frequency filtering (SFF). The locations of the discontinuities in the speech signal are obtained from the instantaneous frequency (IF) (i.e., the time derivative of the phase) of each of the frequency components. A method for automatic detection of epochs using the amplitude weighted IF is proposed. Performance of the proposed epoch detection method is compared with four state-of-the-art methods in clean and telephone quality speech. The performance of the proposed method is comparable with the performance of the existing epoch detection methods for clean speech but better for telephone quality speech.

Suppression Of Wall Reflection In PIV Images Over Rough Walls

Wall reflections in particle image velocimetry (PIV) measurements is one of the limiting factors in obtaining velocity information in the vicinity of rough walls. This work tests two different techniques to suppress wall reflections for time-resolved, two-dimensional particle tracking velocimetry (2D-PTV) and stereoscopic-PIV (stereo-PIV). The 2D-PTV was performed for a turbulent boundary layer developed on a rough wall in a water towing tank facility. To measure velocity in the vicinity of the rough wall, the near-wall region on the viewing window was covered with tinted papers to mitigate the wall reflection and maintain a visible reference plane of the towed wall. Using this technique allowed detection of particles at a wall-normal distance of approximately 300 μm (10 wall units) from the roughness valleys for an imaging system with 1 m standoff distance. To suppress the wall refection for the stereo-PIV over a rough wall performed in a wind tunnel, the roughness is coated with fluorescent paint to shift the reflected light to a higher wavelength. A narrow band-pass filter was then used on the imaging lenses, allowing only particle images to be recorded while extinguishing the majority of the reflected laser light from the surface. Reliable velocity measurements with this technique were obtained up to 500 μm (5.0 wall units) from the roughness valleys.

Reconfigurable high-Q terahertz filtering of VO2-based metamaterials using optical tunneling

Up to date, reconfigurable terahertz (THz) band-pass filters based on the design of metamaterials with vanadium dioxide (VO2) have been investigated extensively. However, VO2-based THz metamaterials with high-quality tunable narrowband transmission and good stop-band rejection have yet to be reported, which are essential for THz wave filtering components or systems. Here, we propose a reconfigurable filter in combination with a dielectric metastructure and a VO2 thin film. Utilizing the optical tunneling effects of a cavity layer, the filter has a high band-pass transmission up to 98% at 0.246 THz with a narrow bandwidth of 0.0009 THz, an ultra-high Q-factor of 273, and two broad nearly-suppressed sidebands. The transmission intensity can be modulated by the phase transition of VO2, and the modulation depth is as high as 96.4%. By efficiently tuning the cavity layer thickness, the filter can be designed for a series of transmission wavelengths and further accomplish the switch between the narrow band-pass filtering and wideband reflection. Moreover, the manipulation of the incident angle and polarization can be adopted as an additional degree to achieve a similar switching function. Our study implies a potential for developing various THz applications, where reconfigurable high-Q THz filtering is required.

Multipoint space-domain active fiber cavity ringdown technique for quasi-distributed magnetic field monitoring

A multipoint space-domain active fiber cavity ringdown (FCRD) sensing technique based on frequency-shifted interferometry (FSI) is proposed and investigated for quasi-distributed magnetic field measurements. Multiple FSI-based fiber ringdown cavities (RDCs) are cascaded together to constitute a sensor array, share one continuous-wave optical source and one slow balanced detector, and thus the system is economical and highly sensitive. In each RDC a bidirectional erbium-doped fiber amplifier (Bi-EDFA) is inserted to compensate the inherent cavity loss for achieving higher sensitivity. Compared to the time-domain active FCRD techniques, the amplifier has lower gain fluctuation because the continuous light (instead of pulse light) coupled into the cavity enables more stable optical power, and has lower amplified spontaneous emission noise due to the use of two narrow bandpass filters and differential detection. Magnetic fields in different locations are synchronously resolved in the space domain by measuring the ringdown distances of continuous light, rather than measuring the ringdown times of pulse light in the time domain as in conventional multipoint FCRD techniques. A dual-point active FSI-FCRD magnetic field sensor was experimentally built, and two side-polished fibers coated with magnetic fluid were incorporated inside the fiber cavities as the sensing probes. By applying the external magnetic field from 0 to 230 Gs, the sensitivities of 6.68 × 10−3 and 4.45 × 10−3/(km Gs) were respectively achieved, which are at least one order of magnitude higher than the conventional passive FCRD techniques. After repeated measurements, the obtained ringdown baseline stabilities of the two units were 0.91% and 2.00%, which were much higher than those of time-domain active FCRD sensing systems. Using power budget analysis, the maximum number of identical sensing units can be expected to be 48 over a 25.89-km distance under the nearly same experimental conditions. The proposed multipoint active FCRD sensing scheme has the merits of low cost, high sensitivity, good stability, and strong multiplexing ability, which would find good application prospects at electric grids, national defense, biology, and medicine for monitoring quasi-distributed magnetic field.

Integrated Tunable Sideband Suppressor for Optical Carrier Reuse in Millimeter-Wave Radio-Over-Fiber Transmission

An integrated Si photonic tunable sideband suppressor is presented for application of remote optical carrier reuse. This tunable sideband suppressor is constructed by a two-rings-coupled Mach Zehnder interferometer with a tuning function to synthesize a box-like, narrow band-pass filter through dual Fano resonance interference. By varying the resonant wavelengths of the ring resonators, we can tune the suppression band adapted to the working millimeter wave (mmWave) frequency. We demonstrate a suppression ratio more than 25 dB on both 28 and 39 GHz downlink mmWave radio-over-fiber (RoF) signals. By applying this component in a bi-directional mmWave RoF system, we observe the quality of the uplink signal modulated on the recycled optical carrier is almost the same as that modulated on a clean laser source without extra power penalty. Since the sideband suppressor is made by Si photonic circuits, it potentially can be integrated with other high-speed Si photonic components to implement an on-chip mmWave radio-over-fiber transceiver.

Extraction of Respiratory Activity from Pulse Oximeter Signals using Tunable Q-factor Wavelet Transform

Online monitoring of respiratory activity is essential in situations such as cardiopulmonary disorders, ambulatory monitoring, stress tests, sleep disorder investigations and post-operative hypoxemia. Extraction of respiratory activity from physiological signals having respiratory influence such as pulse oximeter’s photoplethysmographic (PPG) signals would be an alternative under clinical settings compared to that of all direct methods of recording respiratory signals such as spirometry, pneumography or capnography. The respiratory information can be extracted from PPG signal using a simple band pass filter, but the design of narrow band pass filter (NBPF) with classical filter design cannot be possible. In this paper, we present a simple method, based on tunable Q-factor Wavelet transform (TQWT), for extraction of respiratory activity from PPG signals. Advantage of TQWT stems from the fact that, the realization of practical narrow band pass filter with a specific Q-factor value can be designed, which motivated the authors to use for this application. The method is applied on, PPG data recorded from 15 healthy subjects; each consisting of simultaneously recorded PPG and respiratory signals. The extracted respiratory signals are compared with the original respiratory signals. Statistical parameters such as relative correlation co-efficient (RCC) in time domain as well as magnitude squared coherence (MSC) in frequency domain are used for performance evaluation along with error analysis using the accuracy rate (AcR) and normalized mean square error (NRMSE). Experimental results have shown a good acceptance for the extracted signal when compared with the originally recorded respiratory signal. The proposed technique could become an efficient approach for extraction of surrogate respiratory activity from PPG signals, avoiding usage of additional specialized sensor for respiratory monitoring.

Electrically tunable topological interface states in one-dimensional nanomechanical lattices

In optics, combining one-dimensional (1D) topological systems with lasers and nonlinear optics has promoted their rapid development. A 1D acoustic topological system can only achieve single-frequency acoustic waves localization, limiting its practicality. The implementation of tunable and integrated 1D topological systems will improve this situation. Here, we propose an electrically tunable and integrable 1D topological nanoelectromechanical phononic crystal. Different strong and weak couplings are formed by changing the overlapping degree of adjacent suspended circular membranes, accompanied by different topological properties for the phononic bandgaps. Topologically protected interface states exist at their interface, and these states are stable even after introducing various degrees of distortion. Narrow band-pass filters based on these interface states can be implemented, with adjusted operating frequency by different grid voltage. Our proposed electrically tunable and integrable structure holds potential for smart filtering and may facilitate the development of integrated electromechanical circuits.

Hyperspectral microscopy imaging based on Fourier ptychographic microscopy

Hyperspectral resolution, high spatial resolution, and a wide field of view (FOV) are the targets of optical spectral microscopy imaging. However, hyperspectral microscopy imaging technology cannot provide a wide FOV and a high spatial resolution at the same time. Fourier ptychographic microscopy (FPM) is a novel microscopy imaging technique that uses LEDs at varying angles to capture a series of low-spatial-resolution images that are used to recover images that have both high spatial resolution and a wide FOV. Since FPM cannot obtain the spectral resolution of the sample, in this paper, an efficient strategy based on the FPM system is proposed for the reconstruction of hyperspectral images. First, the traditional FPM setup is optimized, with a new experimental setup based on halogen lamp illumination and a narrow band-pass filter to capture a series of low-spatial-resolution images at different wavelengths. Second, a new algorithm, combining hyperspectral resolution imaging using interpolation compensation and a phase retrieval algorithm, is proposed to reconstruct high-spatial-resolution, wide FOV, and hyperspectral resolution images. Finally, we verified the feasibility and effectiveness of our experimental setup and algorithm by both simulation and experiment. The results show that our method can not only reconstruct high-spatial-resolution and wide FOV images, but also has a spectral resolution of 5 nm.

Investigation of measurement data of low-coherence interferometry at tilted surfaces in the 3D spatial frequency domain

Abstract The 3D transfer characteristics of interference microscopes and their effect on the interference signals occurring at surface slopes are studied. The interference image stacks acquired during a depth scan are 3D Fourier transformed. This allows a comprehensive frequency domain analysis of the interferograms. The double foil model introduced in a previous publication enables the interpretation of the signal spectra and the underlying transfer behavior of the interferometer using the concept of the Ewald sphere, which is limited by the numerical aperture (NA) of the imaging system. Analysis in the 3D spatial frequency domain directly discloses that the lateral dimensions of the transfer function depend on the axial spatial frequency. In this contribution we investigate measuring objects produced by Nanoimprint-Lithography. The corresponding signal spectra bear information that can be utilized to optimize the subsequent signal processing algorithms. These include envelope and phase evaluation procedures of the interference signals. A narrow bandpass filter is used to actively select certain frequency components in order to improve the robustness of the estimation of the envelope position. Although the shape and width of the envelope are affected, this procedure increases the reliability of the evaluation process and improves the accuracy of the measured topography especially at steeper surface slopes.

Multi-laser sensor for simultaneous multi-gas measurements using off-axis cavity-enhanced absorption spectroscopy with an opposite two-way configuration.

An opposite two-way off-axis cavity-enhanced absorption spectroscopy-based multi-gas sensor is reported. More than two lasers can be employed in the sensor for simultaneous detection of different gas species. An approximately two times improvement in magnitude of the 2f signal and the signal-to-noise ratio is achieved because the concave spherical mirrors outside each end of the cavity and the narrow bandpass filters before the detectors can act as re-injection mirrors to re-inject the light into the cavity in the scheme. The performance of the sensor is demonstrated by simultaneous measurement of CO2 and CH4 in the atmosphere. This Letter highlights a new, to the best of our knowledge, strategy for simultaneous multi-gas measurement in a single integrated cavity by employing as many as four lasers.

A filter-based Raman spectrometer for non-invasive imaging of atmospheric water vapor.

A new instrument was designed and developed to map the spatial distribution of water vapor concentration in the atmosphere. The high spatial resolution, sensitivity, and accuracy of the instrument enable new studies of the role of turbulence on clouds and aerosols in small-scale laboratory environments. The instrument exploits Raman scattering in a multi-pass laser configuration by using a set of narrow bandpass filters and a pair of charge coupled device imaging cameras in the 90° scattering geometry. The absolute concentration of water vapor was inferred from measured ratios of H2O and N2 vibrational Raman transitions. We have measured the number densities of water molecules in the atmosphere as low as 3.5 × 1017 cm-3, with an accuracy better than 20% and as high as 7.0 × 1017 cm-3 during minutes long observations. These measurements were taken within an imaging region 6cm in diameter, with a per-pixel resolution 2.60mm wide by 0.16mm tall and 1mm deep.

A Multi-Channel Ratio-of-Ratios Method for Noncontact Hand Video Based SpO$_2$ Monitoring Using Smartphone Cameras

Blood oxygen saturation (SpO<inline-formula><tex-math notation="LaTeX">$_2$</tex-math></inline-formula>) is an important indicator forpulmonary and respiratory functionalities. Clinical findings on COVID-19 show that many patients had dangerously low blood oxygen levels not long before conditions worsened. It is therefore recommended, especially for the vulnerable population, to regularly monitor the blood oxygen level for precaution. Recent works have investigated how ubiquitous smartphone cameras can be used to infer SpO<inline-formula><tex-math notation="LaTeX">$_2$</tex-math></inline-formula>. Most of these works are contact-based, requiring users to cover a phone&#x2019;s camera and its nearby light source with a finger to capture reemitted light from the illuminated tissue. Contact-based methods may lead to skin irritation and sanitary concerns, especially during a pandemic. In this paper, we propose a noncontact method for SpO<inline-formula><tex-math notation="LaTeX">$_2$</tex-math></inline-formula> monitoring using hand videos acquired by smartphones. Considering the optical broadband nature of the red (R), green (G), and blue (B) color channels of the smartphone cameras, we exploit all three channels of RGB sensing to distill the SpO<inline-formula><tex-math notation="LaTeX">$_2$</tex-math></inline-formula> information beyond the traditional ratio-of-ratios (RoR) method that uses only two wavelengths. To further facilitate an accurate SpO<inline-formula><tex-math notation="LaTeX">$_2$</tex-math></inline-formula> prediction, we design adaptive narrow bandpass filters based on accurately estimated heart rate to obtain the most cardiac-related AC component for each color channel. Experimental results show that our proposed blood oxygen estimation method can reach a mean absolute error of 1.26&#x0025; when a pulse oximeter is used as a reference, outperforming the traditional RoR method by 25&#x0025;.

Coupled Tamm plasmon polaritons induced narrow bandpass filter with ultra-wide stopband

Coupled Tamm plasmon polaritons induced narrow bandpass filter with ultra-wide stopband

Development of an Optical System Based on Spectral Imaging Used for a Slug Control Robot

The state-of-the-art technique to control slug pests in agriculture is the spreading of slug pellets. This method has some downsides, because slug pellets also harm beneficials and often fail because their efficiency depends on the prevailing weather conditions. This study is part of a research project which is developing a pest control robot to monitor the field, detect slugs, and eliminate them. Robots represent a promising alternative to slug pellets. They work independent of weather conditions and can distinguish between pests and beneficials. As a prerequisite, a robot must be able to reliably identify slugs irrespective of the characteristics of the surrounding conditions. In this context, the utilization of computer vision and image analysis methods are challenging, because slugs look very similar to the soil, particularly in color images. Therefore, the goal of this study was to develop an optical filter-based system that distinguishes between slugs and soil. In this context, the spectral characteristics of both slugs and soil in the visible and visible near-infrared (VNIR) wavebands were measured. Conspicuous maxima followed by conspicuous local minima were found for the reflection spectra of slugs in the near infrared range from 850 nm to 990 nm]. Thus, this enabled differentiation between slugs and soils; soils showed a monotonic increase in the intensity of the relative reflection for this wavelength. The extrema determined in the reflection spectra of slugs were used to develop and set up a slug detector device consisting of a monochromatic camera, a filter changer and two narrow bandpass filters with nominal wavelengths of 925 nm and 975 nm. The developed optical system takes two photographs of the target area at night. By subtracting the pixel values of the images, the slugs are highlighted, and the soil is removed in the image due to the properties of the reflection spectra of soils and slugs. In the resulting image, the pixels of slugs were, on average, 12.4 times brighter than pixels of soil. This enabled the detection of slugs by a threshold method.