Optical Notch Filter Research Articles

Polarization-independent topological corner states based on all-dielectric valley photonic crystals

Recently, topological edge states and corner states have provided new ways to manipulate light transmission and localization. Up to now, most works have focused on either transverse magnetic or transverse electric polarization. In contrast, dual-polarization photonic topological states have attracted extensive attention because of their potential applications in polarization-independent photonic devices. Previous study realizes the polarization-independent topological corner states by independently tuning the out-of-plane permittivity and the in-plane permittivity of the anisotropic elliptic metamaterial, which is difficult to realize in the optical regime. In this work, we achieve polarization-independent topological edge states and corner states based on all-dielectric fishnet photonic crystals made of isotropic material. Note that the frequencies of the topological edge states and corner states depend on the structure’s effective refractive index, which is related to the filling ratio of the dielectric material. By selecting a suitable filling ratio of the dielectric material, polarization-independent edge states and corner states are realized. In addition, we further construct a topological waveguide-cavity coupling system and demonstrate the function of a polarization-independent optical notch filter. Our work paves the way for the implementation of polarization-independent topological photonic devices.

Ultra-narrow linewidth micro-ring resonator for optical notch filter and methane sensing applications

Ultra-narrow linewidth micro-ring resonator for optical notch filter and methane sensing applications

Notch RGB-camera based SpO2 estimation: a clinical trial in a neonatal intensive care unit.

Regular and narrow-band RGB cameras are recently explored for contactless SpO2 monitoring. Regular RGB cameras with cross-band overlap provide a high signal-to-noise-ratio (SNR) in measuring the photoplethysmographic signals but possess high dependency on the spectra of incident light, whereas narrow-band RGB cameras have better spectral independence but lower SNR especially in dim lighting conditions, such as in the neonatal intensive care unit (NICU). This paper proposes a notch RGB camera based SpO2 measurement approach that uses an optical notch filter to attenuate the wavelengths of 580-605 nm of a regular RGB camera to improve the spectral independence while maintaining high SNR in signal measurement. The proposed setup was validated in the lab condition (e.g. dark chamber) against the existing solutions for visible-light based camera-SpO2 measurement and further verified in the NICU on preterm infants. The clinical trial conducted in the NICU with 22 preterm infants shows that the notch RGB camera can achieve a mean absolute error (MAE) less than 4% for SpO2 measurement. This is the first showcase of continuous monitoring of absolute camera-SpO2 values in the NICU.

Optical notch filters with tunable central wavelength and reconfigurable free spectral range on SOI platform

Conventional optical filter is typically easy to realize adjustable central wavelength and bandwidth, but it's quite challenging to obtain reconfigurable free spectral range (FSR), thus limiting its application in a variety of fields. In this paper, we propose novel schemes and implement two kinds of tunable and reconfigurable optical notch filters (TRNF) on Silicon-on-Insulator (SOI) platform for the first time. The experimental results indicate that the FSR reconfigurable range is 20 GHz–200 GHz for a five-stage TRNF based on microring resonators (MRR), and the central wavelength can be tuned over the whole free spectral range. Moreover, a five-stage TRNF based on Mach-Zehnder interferometers (MZI) offers a broader FSR reconfigurable range of 7.5 GHz–232.5 GHz. Our results will improve the spectrum usage efficiency of programmable photonic integrated circuits and pave the way for the development of next generation elastic optical network (EON).

Stochastic cooling notch filter developments for the high-precision spectrometer ring in the high intensity heavy-ion accelerator facility project.

Stochastic cooling of the high-precision spectrometer ring (SRing) at the High Intensity Heavy-Ion Accelerator Facility (HIAF) project in China, which is used mainly for experiments with radioactive fragment beams, is applied to speed up the cooling process of a stored ion beam. In this article, a new coaxial-type notch filter with an amplitude equalizer in the long branch and an optical-type notch filter with phase-stabilized optical fiber are discussed and evaluated for the SRing stochastic cooling system. Both prototypes of coaxial and optical notch filters are fabricated and tested. The minimum notch depth of coaxial and optical notch filters reaches to 26 and 40 dB, respectively. The performance of both coaxial notch filter and optical fiber notch filter is presented in this work. These developments will be used not only for the longitudinal stochastic cooling system but also have potential for the beam feedback system.

Customized designed notch filters and applied effects on glare and contrast sensitivity in patients with dry eye syndrome.

Glare visual acuity and contrast sensitivity are important indicators of visual quality. Studies have shown that the glare visual acuity and contrast sensitivity in dry eye patients tend to degenerate, further affecting their quality of life. The objective of this study was to investigate the effect of notch filters on glare VA and contrast sensitivity in patients with dry eye or with dry eye syndrome. 36 subjects in the 20‒65 age group were diagnosed as having dry eye disease or perceived dry eye syndromes themselves who were included after the initial screening with the OSDI questionnaire, and one was subsequently excluded as they had undergone retinal detachment surgery. Finally, 35 subjects (14 male and 21 female) with a mean age of 40.66 ± 15.62years participated in this study. All subjects wore their habitual prescriptions and four different filter lenses (namely 480, 620, dual 480 & 620 notch filter, and FL-41 tinted lens), and measured the parameters of glare visual acuity and contrast sensitivity using CSV-1000 and sine wave contrast test (SWCT), respectively. Student t-test and Repeated measurement analysis (R-ANOVA) were utilized by using SPSS 26.0 software. A dual-wavelength 480 & 620nm optical notch filter had a significant anti-glare effect decreasing glare disabilities or discomfort, and leading to better visual quality, the same effect was also shown on a 480nm notch filter lens. All participants showed a significant difference among the baseline, three notch filters (480nm, 620nm, dual-wavelength 480 & 620nm), and FL-41 tinted lens were used on SWCT_A (1.5cpd, F = 3.054, p = 0.019) and SWCT_E (18cpd, F = 2.840, p = 0.049); but did not show statistical different on SWCT_B (3cpd, F = 0.333, p = 0.771), SWCT_C (6cpd, F = 1.779, p = 0.159), and SWCT_D (12cpd, F = 1.447, p = 0.228). The baseline showed the best visual performance on CS at a low spatial frequency (SWCT_A, 1.5cpd), any filter might reduce the contrast sensitivity at low spatial frequencies in the clinical trial, whereas 480nm notch filter showed the best effectiveness on CS at a high spatial frequency (SWCT_E, 18cpd), the FL-41 lens that also filters out the 480nm band does not achieve the same effect. Moreover, patients with dry eye or those older than 40years old preferred optical multilayer notch filters to FL-41 tinted lenses. The 480- & 620-nm dual-wavelength and 480-nm single-wavelength notch filters have the best effect on the glare visual acuity and contrast sensitivity (CS) at high spatial frequencies in dry eye patients. The 620-nm notch filter performs better in CS at low and mid-low spatial frequencies; the FL-41 tinted lens performs poorly for glare VA and CS spatial frequencies examination. Patients with glare disabilities or CS disturbance at high spatial frequencies may choose a 480-nm notch filter lens, and patients who have CS disturbance at low spatial frequencies may consider a 620-nm notch filter for the prescription.

Design of tunable notch filter based on plasmonic and InGaAsP waveguide

A highly efficient compact tunable optical notch filter is proposed and analyzed using the 2D Finite Element Method (FEM). The proposed structure consists of a slanted stub plasmonic resonator, Metal–Insulator–Metal (MIM) waveguide, and InGaAsP as a third-order non-linear optical material. By altering the pumping state of the InGaAsP, the filtered wavelengths may be easily controlled continuously over 200 nm a range. The suggested notch filter can remove four narrow bands of wavelengths, each around 50 nm wide, and a transmission of about − 17 dB. The proposed filter’s key advantages are its high transmission coefficient and fabrication simplicity with compact size. For future integrated plasmonic devices such as outdoor visible light communications and optical imaging, the proposed filter can be manufactured using an oblique angle shadow evaporation technique.

Design and Fabrication of a Cost-Effective Optical Notch Filter for Improving Visual Quality

This study presents a multilayer design and fabrication of an optical notch filter for enhancing visual quality. A cost-effective multilayer design of notch filter with low surface roughness and low residual stress is proposed. A 9-layer notch filter composed of SiO2 and Nb2O5 with a central wavelength of 480 nm is prepared by electron beam evaporation combined with ion-assisted deposition. The optical transmittance, residual stress, and surface morphology are measured by a UV/VIS/NIR spectrophotometer, Twyman-Green interferometer and field emission scanning electron microscopy (FE-SEM). The transmittance of the notch filter at the central wavelength is above 15%, and the average transmittance of the transmission band is about 80%. The residual stress of the notch filter is −0.235 GPa, and the root mean square surface roughness is 1.85 nm. For improving the visual quality, a good image contrast can be obtained by observing the microscopic image using the proposed notch filter.

Multifunctional Optical Device with a Continuous Tunability over 500 nm Spectral Range Using Polymerized Cholesteric Liquid Crystals.

We report that polymerization makes a robust, practically applicable multifunctional optical device with a continuous wavelength tunable over 500 nm spectral range using UV-polymerizable cholesteric liquid crystals (CLCs). It can be used as a circular polarizer generating an extremely high degree of circularly polarized light with |g| = 1.85~2.00. It can also be used for optical notch filters, bandwidth-variable (from ~28 nm to ~93 nm) bandpass filters, mirrors, and intensity-variable beam splitters. Furthermore, this CLC device shows excellent stability owing to the polymerization of CLC cells. Its performance remains constant for a long time (~2 years) after a high-temperature exposure (170 °C for 1 h) and an extremely high laser beam intensity exposure (~143 W/cm2 of CW 532 nm diode laser and ~2.98 MW/cm2 of Nd: YAG pulse laser operation for two hours, respectively). The optical properties of polymerized CLC were theoretically analyzed by Berreman’s 4 × 4 matrix method. The characteristics of this device were significantly improved by introducing an anti-reflection layer on the device. This wavelength-tunable and multifunctional device could dramatically increase optical research efficiency in various spectroscopic works. It could be applied to many instruments using visible and near-infrared wavelengths.

A Compact TE-Notch Filter With Simultaneous Capability of TM-Suppression and Sensing Applications

An optical notch filter has been designed based on a silicon-slab Photonic Crystal (PhC) W1 waveguide. Birefringence originated usual performance degradation of notch filters has been eliminated by optimizing the slab-thickness and radius of the holes to embed a TE pass filter in the same structure. This resulted in TE and TM transmittances as $\approx$ $\mathrm{-1.5}$ dB and $\approx$ $\mathrm{-28}$ dB respectively at the wavelength $\mathrm{1550}$ nm. Structural parameters of the device are represented by normalizing them with respect to the lattice constant such that the structure can be scaled based on the application wavelength. Thereafter, generation of the mini stop band (MSB), i.e. notch-band, has been studied for various relative longitudinal-shifts of the lateral cladding for different filling fractions. Two different longitudinal-shifts are combined in a footprint of $\mathrm{9.8 \times 7.35}$ ${\mu \rm {m}^{2}}$ to realize a $\mathrm{10}$ nm notch-band having a notch suppression of $\approx$ $\mathrm{27}$ dB. Application of the notch-filter as a refractive index sensor has been investigated that exhibits its sensitivity as $\mathrm{500}$ nm/RIU. The potentiality of the device in applications like - bio-sensing (e.g., detection of morphological forms of certain bacteria, and some typical viral particles) and industrial measurements (typically, sensing of Ethyl Alcohol percentage in aqueous solution) has also been explored.

Photonics-Based CW/Pulsed Microwave Signal AOA Measurement System

A new microwave photonic system for determining the angle of arrival (AOA) of a continuous wave (CW) or pulsed microwave signal is presented. It is based on a dual-polarisation dual-drive Mach–Zehnder modulator followed by an optical notch filter for carrier suppression. Two orthogonal-polarised carrier- suppressed optical signals are detected by low-frequency photodetectors that generate two DC voltages. An incoming microwave signal AOA can be obtained from the ratio of the two DC voltages, without the need of a calibration procedure or measuring the incoming microwave signal power level, which are required in previously reported structures. The proposal system can be incorporated with a conventional radar receiver front end consists of mixers and filters, for measuring the AOA of a pulsed microwave signal. Experimental results demonstrate 0°–63° AOA measurement with less than 2° errors for an input microwave signal having different frequencies and different power levels.

Design and development of an optical reflective notch filter using the ion assisted deposition technique with stepwise modulated thickness for avionics applications.

This paper presents research work about the design and fabrication of a 44-layer optical reflective notch filter. The performance of the fabricated notch filter was studied at normal (0°) and oblique (45°) incidence angle. In addition, the paper also discusses a three-layer broadband antireflective coating on both sides of the multilayer stack to suppress the ripples in the passband region. The thickness-modulated reflective stack of the filter was designed by using the materials Al2O3 (1.63) and SiO2 (1.46). Optimization of the multilayer stack was carried out by using the damped least-squares algorithm. The theoretical and experimental results from the ion-assisted e-beam deposited samples for single notch reflective filters are presented. Good agreement in the design and experimental results was observed when the deposition process was controlled by time of evaporation. Further, the filter was characterized for the optical properties by using a UV-VIS-NIR spectrophotometer, surface morphology and protective properties using field emission scanning electron microscopy, a coherence correlation interferometer, and water contact angle.

Dual-chirp Fourier domain mode-locked optoelectronic oscillator.

Optoelectronic oscillators (OEOs) have been widely investigated to generate ultra-pure single-frequency microwave signals. Here, we propose and experimentally demonstrate a dual-chirp Fourier domain mode-locked (FDML) OEO to generate dual-chip microwave waveforms. In the proposed FDML OEO, a frequency-scanning dual-passband microwave photonics filter based on phase-modulation-to-intensity-modulation conversion using an optical notch filter and two laser diodes is incorporated into the OEO cavity. Fourier domain mode-locking operation is achieved by synchronizing the scanning period of the filter to the cavity round-trip time. Tunable dual-chirp microwave waveforms with a large time-bandwidth product are generated directly from the FDML OEO cavity in the experiment, which can be used in modern radar systems to improve its range-Doppler resolution.

Brillouin-Assisted Notch Filtering Based All-Optical Image Rejection Mixer

A novel approach for realizing a microwave photonic image rejection mixer is presented in this paper. It is based on using a widely tunable narrow-band optical notch filter implemented by stimulated Brillouin scattering (SBS) to remove the image signal. The image rejection mixer has a simple structure as it only requires a single integrated dual-polarization modulator for both frequency conversion and pump wave generation for the SBS process. The absence of electrical components enables the image rejection mixer to be operated over a wide frequency range with a high image rejection ratio. A technique is also proposed to suppress the noise generated by the SBS process in the image rejection mixer. Experimental results demonstrate a high and constant image rejection ratio of over 32 dB for the image rejection mixer operating over an RF signal frequency range of 11.8–19.6 GHz, and around −5 dB conversion efficiency.

Design of a High-Resolution Metal–Insulator–Metal Plasmonic Refractive Index Sensor Based on a Ring-Shaped Si Resonator

In this paper, a high-resolution refractive index sensor is proposed based on a novel metal–insulator–metal plasmonic topology. The structure is based on a Si nano-ring located inside a circular cavity. It acts as an optical notch filter with a quality factor equal to 269. The proposed filter topology is numerically simulated using the finite difference time domain method. It is shown that the proposed filter can also act as a refractive index sensor with a sensitivity of 636 nm/RIU and a fairly high figure of merit (FoM) equal to 211.3 RIU−1. It is shown that the sensor can easily detect a refractive index change of ± 0.001 for dielectrics whose refractive index is between 1 and 1.2. For the refractive index range of 1.33 to 1.52, the maximum FoM of the sensor is 191 RIU−1. The simplicity of the design and its high resolution are the two main features of the proposed sensor which make it a good candidate for biomedical applications.

Continuously Tunable and Bandwidth Variable Optical Notch/Band-Pass Filters Over 500 nm Spectral Range Using Cholesteric Liquid Crystals

We report continuously tunable and bandwidth variable optical notch and band-pass filters created by combining four left- and right-handed circular cholesteric liquid crystal cells without extra optical components. Filter performance was greatly improved by introducing an anti-reflection layer on the filter device. The filter comprised cholesteric liquid crystal wedge cells with continuous pitch gradient. The band wavelength position was spatially tuned from 470 nm to 1000 nm. The notch filters are polarization-independent in the spectral ranges. The band pass filters can be designed to be polarization-independent or polarization-dependent via cell alignment and the bandwidth can be reversibly controlled from the original bandwidth (60~18 nm). These tunable filter strategies could make a paved road to wavelength tunable spectroscopic instruments applications in the VIS and near infrared response spectral range.

Continuously Tunable Optical Notch Filter With Functions of a Mirror and a Beam Splitter

The objective of this study was to develop continuously tunable optical notch filters by combining four left- and right-handed circular cholesteric liquid crystal (CLC) cells without subsidiary optical components. By rotating each filter, the photonic band position could be continuously blue shifted over 100-nm spectral range. Using four notch filter sets with four different chiral dopant concentrations, spectral range from 440 to 870 nm could be covered. Since each CLC cell has only one chiral dopant concentration, it could be stable for a long time. Each filter is independent of polarization in the spectral range. Filter performance was highly enhanced by introducing an anti-reflection layer on the filter device. There was no light leakage inside the photonic band. Outside the band, transmittance was about 70%-100%. In addition, these filters had stable operation under extremely high laser intensity (~124 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of CW 532-nm diode laser and ~4.43 MW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of Nd: YAG pulse laser operation for 2 h) without showing damage. Such filters also have functions as a mirror and a beam splitter. Depending on CLC materials, this simple and easy strategy could be used to prepare filters for applications in VIS and NIR spectral range devices.

Silicon Photonics Enabled Hyper-Wideband RF Receiver With &gt;85% Instantaneous Bandwidth

We demonstrate the first-ever silicon photonics enabled hyper-wideband RF spread-spectrum link. A hybrid III/V-silicon photonic mode-locked laser encoded with a four-channel silicon photonic phase encoder is used to generate optical carriers that coherently demodulate a received RF signal spanning an instantaneous bandwidth greater than 85% of the center frequency (12/14 GHz). This allows low-speed (<3 GHz) electronics to be used in place of the traditional costly and high-power wide-bandwidth electronics used in an all-electronic hyper-wideband link. In addition, integrated highly tunable optical notch filters are successfully used to reject unwanted narrowband interference, with rejection ratio and tunability that surpass conventional wireless technology.

Tunable Single-Passband Microwave Photonic Filter Based on Integrated Optical Double Notch Filter

A novel-integration-based technique employing a cascaded pair of microring resonators on silicon-on-insulator platform that can achieve a tunable single-passband microwave photonic filter with improved shape factor and extinction ratio is presented. It is based on an integrated optical double notch filter using a cascaded pair of nonidentical microring resonators, in conjunction with optical phase modulation. This results in minimal net phase being introduced by the overall filter at radio frequencies falling outside the region of the notch stopband, thus enabling nearly full antiphase cancellation of the modulation sidebands, which results in the achievement of an improved filter shape factor and extinction ratio. Additionally, it features a bandwidth that is directly determined by the difference between the bandwidths of the two optical notch filters, rather than by their absolute individual bandwidths. Experimental results have verified the concept, and have demonstrated a single-passband filter having tuning range of 6–17 GHz, a shape factor of 1.78, shape-invariant tuning, and a good out-of-band suppression ratio of approximately 20 dB throughout the entire tuning range.

Differential photoplethysmogram sensor with an optical notch filter shows potential for reducing motion artifact signals

Objective: Photoplethysmography (PPG) has become the standard technology used in wearable health and fitness monitoring devices. Although, PPG works well at rest, trying to incorporate it for continuous monitoring is challenging, since the signal is inevitably corrupted by motion artifacts. This paper explores the feasibility of a reflective PPG sensor aiming at reducing the changes caused by motion in the non-pulsatile component of the signal, e.g., changes in venous filling. Approach: The tested sensor consisted of two back-to-back connected photodiodes and a light source with a peak wavelength around 575 nm. Additionally, the second photodiode was covered by a 575 ± 5 nm optical notch filter. Main results: When compared to standard PPG (single detector) on one test subject, the tested sensor provided 4.38 dB of attenuation for a change in venous blood during a gentle lift of upper extremity. Moreover, over 40 dB rejection was measured in the baseline resulting in a very small ‘direct current’ (DC) component. Furthermore, the baseline wander due to respiratory modulation tended to be smaller for the tested sensor. The sensor was not either susceptible for the changes in ambient light or the drift caused by the light source heating up. Significance: We believe that this is the first work that proposes the back-to-back connected photodiodes and a notch filter to reduce in-band noise in PPG. The positive results encourage to further optimize and integrate the sensor. The novelty of the method also brings new perspective to the PPG measurements.