Narrow-bandwidth Filter Research Articles

Integrated quantum communication network and vibration sensing in optical fibers

Communication and sensing technologies play crucial roles in various aspects of modern society. The seamless combination of communication and sensing systems has attracted significant interest in recent years. Without adding core devices, vibration-sensing functions can be integrated to build a quantum network with high efficiency and versatility. In this study, we propose and demonstrate a network architecture that integrates a downstream quantum access network (DQAN) and vibration sensing in optical fibers. By encoding the key information of eight users simultaneously on the sidemode quantum states of a single laser source and successively separating them using a specially designed narrow-bandwidth filter network, we achieved a secure and efficient DQAN with an average key rate of 1.94×104 bits per second over an 80 km single-mode fiber. Meanwhile, vibration locations with spatial resolutions of 131, 25, and 4 m at vibration frequencies of 100 Hz, 1 kHz, and 10 kHz, respectively, were implemented using the existing DQAN system infrastructure. The results indicate that the backward probe beam has a negligible effect on the DQAN system. Our integrated architecture provides a viable and cost-effective solution for building a quantum communication sensor network and paves the way for the functionality expansion of quantum communication networks.

A novel high-Q contour mode resonator

This work presents a novel silicon-based capacitive Edge-Radial Contour Mode (ERCM) resonator. Various loss mechanisms of the resonators are investigated via numerical calculations and simulations, the energy loss of the ERCM resonator is mainly dominated by thermoelastic and anchor loss. To suppress thermoelastic loss and anchor loss while maintaining a larger anchor radius, a supporting structure based on a four-ring release hole array was designed, which has significantly reduced anchor loss and motional impedance. By incorporating a four-ring spoke array within the circular disk, the resonator demonstrated more than 4 times enhancement in quality factor (Q) and an 81% reduction in motional impedance. The Q factor reaches 91,000 at frequency of 23.7MHz. Besides, the nonlinear behaviors of the resonator were studied. The nonlinear coefficients of the two resonators were modulated by optimizing the doping types to be 1.93×1010m-2 and -5.62×109m-2, respectively. These studies provide a deep understanding of the nonlinear behaviors of ERCM resonators and their potential application in tunable narrow-bandwidth filters and high-sensitive sensors.

Low noise signal processing for electrochemical biosensor using narrow bandwidth filter

Abstract Biosensor has an important role in several domains, including medical and environmental applications. The improvement of biosensor measurement system therefore offers a better measurement quality. The objective of this work is the development of the signal processing of electrochemical biosensor for advanced analysis and use. The noise of electrochemical sensor is considered rather higher than the other sensor due to many factors such as electrode area, solution velocity, and particle size in solution, which affect adsorption-desorption process. Moreover, the sensor signal processing also makes contribution to noise level. Therefore, it must be reduced and suppressed to obtain better measurement result. In this paper, a bandwidth optimisation method is proposed and used to improve measurement quality. The system is realized with a transimpedance circuit and low-pass and high-pass filter, so that it offers low noise characteristics. The developed proposed system has provided better improvement than the standard electrochemical biosensor system. The noise of the system can be suppressed to be lower up to 14.3% of the standard transimpedance circuit. This development is promising to give better biosensor measurement.

A novel narrowband MEMS filter with extensional mode resonators

This work proposes a novel electrostatic mechanically coupled narrowband filter based on width extensional mode (WEM) resonators. Rectangle plate WEM resonators have the potential for high quality factor (Q) and low motional resistance. The rectangle plate was slotted to reduce thermoelastic loss and optimize mode shape coefficient of WEMs. The Q values of coupled WEM resonators exceed 80,000, enabling a narrow bandwidth filter. A 3λ/4-length coupling beam filter (CBF) was demonstrated. The 3λ/4 coupling beam filter has a center frequency of 79.79 MHz and a bandwidth of 0.024 %. The CBF exhibits an impressive 26.39 dB stopband rejection and 6.97 dB insertion loss with 5.13 dB passband ripple. The MEMS filters presented in this work hold significant potential for applications in wireless communication systems.

A mechanically coupled MEMS filter with high-Q width extensional mode resonators

This work presents a novel radio frequency (RF) narrowband Si micro-electro-mechanical systems (MEMS) filter based on capacitively transduced slotted width extensional mode (WEM) resonators. The flexibility of the plate leads to multiple modes near the target frequency. The high Q-factor resonators of around 100 000 enable narrow bandwidth filters with small size and simplified design. The 1-wavelength and 2-wavelength WEMs were first developed as a pair of coupled modes to form a passband. To reduce bandwidth, two plates are coupled with a λ-length coupling beam. The 79.69 MHz coupled plate filter (CPF) achieved a narrow bandwidth of 8.8 kHz, corresponding to a tiny 0.011%. The CPF exhibits an impressive 34.84 dB stopband rejection and 7.82 dB insertion loss with near-zero passband ripple. In summary, the RF MEMS filter presented in this work shows promising potential for application in RF transceiver front-ends.

On-chip ultra-high rejection and narrow bandwidth filter based on coherency-broken cascaded cladding-modulated gratings

Bragg filters are of essential importance for chip-scale photonic systems. However, the implementation of filters with sub-nanometer bandwidth and rejection beyond 70 dB is hindered by the high index contrast of the silicon-on-insulator platform, which makes filters prone to fabrication imperfections. In this paper, we propose to combine coherency-broken cascading architecture and cladding modulation to circumvent the intrinsic limitation. The cascading architecture effectively prevents the accumulation of phase errors, while the cladding modulation offers additional design freedom to reduce the coupling coefficient. A bimodal Bragg filter with a testing-equipment-limited rejection level of 74 dB and a 40 dB bandwidth of 0.44 nm is experimentally demonstrated. The minimum feature size is 90 nm, which significantly relieves the fabrication constraints.

Design and analysis of tunable optical filter based on silicon-on-insulator photonic crystal for visible light communication

In this paper, tunable narrow bandwidth band-stop and band-pass optical filters for visible light communication (VLC) are designed and numerically investigated using a photonic crystal (PC) structure. Designing a tunable filter for visible light communication is crucial for enhancing spectral utilization and maximizing communication efficiency. A rectangular defect with optimized sides length utilized in the center of the PC structure based on the desired filtering characteristics. To obtain a favorable narrow bandwidth filter for visible light wavelengths, this defect is filled with a specific liquid crystal. In addition, an electric field is applied to the proposed structure in order to change the refractive index of the liquid crystal and achieve tunability in the desired range. The functional wavelength ranges of band-pass and band-stop filters are 550–570 nm and 565–585 nm, respectively. Applying an electric field causes a shift in the central wavelengths of the filters. It is shown that a 1.5 nm central wavelength shift for a band-stop filter and a 0.9 nm central wavelength shift for a band-pass filter can be achieved by changing the voltage source up to 1.5 V. In addition, the impact that the variation in the length of the rectangular defects’ sides has on the filtering characteristics of the proposed device is studied.

Design of all-sky airglow imaging spectrometers in the middle and upper atmosphere

In order to detect the distribution of gravity waves in the middle and upper atmosphere, an all-sky airglow imaging spectrometer is designed to measure the airglow. It has the detection advantages of ultrawide field of view (FOV), large relative aperture and high signal-to-noise ratio (SNR). It can obtain the spectral images of OH (715–850 nm / 880–930 nm) and OI (630 nm) in the height range of 87–250 km. The whole design method of telecentric objective splicing relay zoom group is used to effectively suppress the spectral drift less than 0.2 nm, which meets the requirements of narrow bandwidth filters. The field experiments in northwest China show that the all-sky airglow imaging spectrometer can continuously acquire airglow spectral images with high SNR in both OH channel and OI channel without focusing in the temperature range of 0–45 ℃, and the images obtained in OH channel have obvious gravity wave phenomenon after star point removal and geometric correction.

Narrow-bandwidth Bragg grating filter based on Ge-Sb-Se chalcogenide glasses.

Bragg grating (BG) filters play important roles in integrated photonics such as signal processing and optical sensing. In silicon-based counterpart photonic platforms, the application of narrow-bandwidth (Δλ) filters is often restrained by fabrication limitations. In this study, narrow-bandwidth BG filters based on Ge-Sb-Se chalcogenide materials are investigated. The structure of the filter is designed by optimizing the grating period, corrugation height, and grating number. The large corrugation of chalcogenide BG is more friendly and convenient for manufacturing process. The symmetric and asymmetric corrugation filters are then fabricated and characterized. Experimental results show a half-maximum bandwidth of 0.97 nm and 0.32 nm for symmetric and asymmetric filters, respectively, which demonstrates excellent narrow-bandwidth filtering performance of chalcogenide BG.

Low-noise Brillouin random fiber laser with Auto-tracking dynamic fiber grating based on a saturable absorption ring

A low-noise Brillouin random fiber laser (BRFL) with stabilized lasing output and narrow linewidth is proposed and demonstrated experimentally. Stimulated Brillouin scattering and Rayleigh scattering are exploited to work as the gain and random distributed feedback mechanisms, respectively. A simple unpumped Erbium-doped fiber (EDF) loop structure which consists of a section of EDF and an optical coupler is introduced into the BRFL cavity, forming a saturable absorption ring (SAR). When light with proper intensity is incident into the SAR, a dynamic fiber grating (DFG) can be built up to work as a narrow-bandwidth filter with the ability of auto-tracking the center frequency. The SAR can effectively filter out dense random longitudinal modes and suppress multi-mode resonances and frequency jitters by restricting the mode-competition-induced mode hopping within a limited frequency range of ∼ 1 MHz and auto-tracking the center frequency of light in the cavity, simultaneously. Thus, the output of BRFL with SAR is much more stabilized than existing free-running BRFLs. In the experiment, a stable lasing output with an ultra-narrow linewidth (less than2kHz) is obtained from the proposed BRFL. The measured relative intensity noise and frequency noise demonstrate that the noise level of the proposed laser is significantly lower than that of the free-running BRFLs. This proposed technique not only provides a solution to the dilemma of previously reported BRFLs with severe lasing instabilities, but also makes a step towards the development of practical and reliable BRFLs. This high-quality laser source with advantages of high stability, low cost, low threshold power, high coherence, narrow linewidth, and low noise can find useful applications in coherent optical detection, high-resolution spectrometers, microwave photonics, and optical sensing fields.

Multi-wavelength thulium-doped fiber laser at 2.05 μm incorporating a superimposed polarization-maintaining fiber Bragg grating

A switchable multi-wavelength thulium-doped fiber laser with a high optical signal-to-noise ratio (OSNR) and stability is proposed and experimentally demonstrated at 2.05 μm. For the first time, a superimposed polarization-maintaining fiber Bragg grating (SI-PM-FBG) was utilized as a multi-channel narrow bandwidth filter to select the lasing wavelengths in the laser cavity in the 2-μm band. It also introduced polarization hole burning (PHB) to inhibit the strong wavelength competition induced by homogeneous gain broadening in the thulium-doped fiber. By adjusting the two polarization controllers (PCs), four stable switchable single-wavelength operations were obtained. The OSNR reached 72 dB at a pump power of 2.89 W, and the maximum wavelength and power fluctuations were 0.01 nm and 0.162 dB, respectively. Four combinations of dual-wavelength operations were obtained by adjusting the PCs. The polarization states between each dual-wavelength lasing were orthogonal. The stability of the dual-wavelength operation was investigated in detail. The proposed laser also has the capability for three- or four-wavelength operation. The excellent performance enables this fiber laser to be used in atmospheric sensing and free-space optical communication.

Compressive laser scanning with full steady state wavefield for structural damage detection

This paper presents a compressive sensing-based high speed full-field laser scanning for structural damage detection. Conventional full-field laser scanning techniques with a high resolution spatial sampling rate are time and energy consuming to process the vast amount of data for large-scale inspection. To address this issue, a compressive full-field ultrasonic laser scanning is proposed in this study. In this technique, 2D random scanning patterns are used for compressive sensing in the inspection range of a laser scanning system. Random steady state ultrasonic responses are obtained by a Laser Doppler Vibrometer and a mirror tilting device with a single fixed frequency excitation by a mounted piezoelectric transducer. After the measurements are completed, a narrow bandwidth filter is applied to the compressed steady sate wavefield data to extract the steady state response at the excitation frequency using discrete Fourier transform (DFT). The full steady state response is reconstructed by using a Yall 1 algorithm which is a L1-minimization solver. Wavenumber based damage visualization techniques are then employed to estimate the damage sizes. For the performance evaluation, several simulation and experimental investigations are performed on aluminum and composite plates, which contain various types of damage, including corrosion and delamination. The results demonstrate that the proposed laser scanning technique based on compressive sensing could increase the efficiency of the damage detection and visualization process with high accuracy.

Lidar observations of the upper atmospheric nickel layer at Beijing ([formula omitted],[formula omitted

Ni atoms, produced in the Earth's upper atmosphere by meteoric ablation, are challenging to measure by lidar because the Ni atom density is low, the resonance scattering cross section is relatively small, and the transitions occur in the near-UV around 340 nm where absorption by the stratospheric ozone layer starts to become significant. A new Ni lidar has been designed and deployed at Yanqing station (40∘N,116∘E) Beijing, China over the period from 3 April 2019 to 31 December 2019. The combination of a frequency-doubled near-IR laser and narrow bandwidth filters enables stable and continuous operation. From 25 nights of measurements, the Ni peak density ranges from 98 to 460 cm−3 with the peak altitude between 80 and 88 km; the average peak density is 258±115cm−3. The average column abundances of Nickel varied between 1.52×108 and 6.0×108 cm−2 between mid-summer and mid-winter. An experiment was also carried out using the Ni lidar to determine the spectroscopic branching ratios from the excited Ni(3F4) state into 3 lower states; very close agreement with the literature values were obtained.

An Optical Pulse Shaping Scheme for Simultaneous Photonic Filtering and Digitizing Systems

The equivalent filter response in the simultaneous photonic filtering and digitizing system is determined by the temporal precision and duration time of optical pulses. To achieve narrow-bandwidth filter response at high central frequency, a pulse shaping scheme based on the time-frequency mapping and attenuation-delay coupling is presented. The effect introduced by attenuation-delay coupling is derived, and the method to search optimized parameters for target bandwidth, off-band suppression and central frequency is presented. Precise shaping of optical pulses with full width at half-maximum in sub-nanosecond order is realized in experiments, which can generate an equivalent filter with bandwidth of 0.93 GHz and central frequency of 39.9 GHz in the simultaneous photonic filtering and digitizing system.

Demonstration of a Sub-GHz Flat-Top Comb-Based RF-Photonic Filter Enabled by Fourth-Order Dispersion Compensation

We demonstrate a sub-GHz RF-photonic filter based on phase and amplitude control of a wideband all-fiber frequency parametric comb with 25-GHz tone spacing. The narrow bandwidth filter construction based on 221 comb tones was enabled by fourth-order dispersion compensation of the dispersive element. The filter integrity was further tested by retrieval of a set of digital modulated RF channels.

Low-Complexity Channelizer Based on FRM for Passive Radar Multi-channel Wideband Receiver

The low-complexity channelizer is essential in passive radar multi-channel wideband receiver using array antennas. The digital filter bank is usually used to realize the channelizer. In the paper, a low-complexity channelizer based on frequency response masking (FRM) is proposed to reduce the high computational complexity of the narrow transition bandwidth filter bank. A half-band filter is used to design the prototype filter of the proposed low-complexity channelizer based on FRM. Compared to the maximally decimated filter bank, the decimation coefficient of the non-maximally decimated filter bank can be different from the number of channels, which makes the low-complexity channelizer more flexible. The low-complexity channelizer based on FRM can be directly applied to a wideband receiver without the limit of the sampling rate. The simulation and complexity analysis results show that the proposed low-complexity channelizer is correct and significantly reduces computational and hardware complexities. The proposed low-complexity channelizer makes it possible to implement passive radar multi-channel wideband receiver on a single hardware processor; thus, it is suitable to applied in passive radar multi-channel wideband receiver using array antennas.

Interstellar communication: Short pulse duration limits of optical SETI

Previous and ongoing searches for extraterrestrial optical and infrared nanosecond laser pulses and narrow line-width continuous emissions have so far returned null results. At the commonly used observation cadence of $\sim 10^{-9}\,$s, sky-integrated starlight is a relevant noise source for large field-of-view surveys. This can be reduced with narrow bandwidth filters, multipixel detectors, or a shorter observation cadence. We examine the limits of short pulses set by the uncertainty principle, interstellar scattering, atmospheric scintillation, refraction, dispersion and receiver technology. We find that optimal laser pulses are time-bandwidth limited Gaussians with a duration of $\Delta t \approx\,10^{-12}\,$s at a wavelength $\lambda_{0}\approx1\,\mu$m, and a spectral width of $\Delta \lambda \approx 1.5\,$nm. Shorter pulses are too strongly affected through Earth's atmosphere. Given certain technological advances, survey speed can be increased by three orders of magnitude when moving from ns to ps pulses. Faster (and/or parallel) signal processing would allow for an all-sky-at-once survey of lasers targeted at Earth.

Dual-wavelength single-longitudinal-mode erbium-doped fiber laser with Mach–Zehnder interferometer and saturable absorber loop

A stable dual-wavelength erbium-doped fiber laser (DW-EDFL) in the single longitudinal mode (SLM) with a Mach–Zehnder interferometer (MZI) and a saturable absorber loop (SAL) is proposed and demonstrated experimentally. Dual wavelengths are obtained with different MZI arm lengths. After the first mode selection of the tunable fiber Bragg grating, SLM operation of the dual-wavelength is acquired by the SAL based on the Vernier effect and spatial hole burning, which induce an auto-tracking narrow bandwidth filter with an optical coupler and a 2 m unpumped erbium-doped fiber (EDF). The output of the DW-EDFL shows good performance, achieving 1.08 kHz at dual wavelengths of 1558.67 nm and 1558.77 nm. The interval of the dual wavelength is 0.1 nm, which has a good agreement with the theoretical value, and the peak powers are −5.508 dBm and −5.993 dBm, respectively under 170 mW pump power. Meanwhile, because of the automatic tracking of the wavelength of the SAL, the optical power instability and beating frequency fluctuation of the dual wavelength are less than 0.5 dB and 0.5 MHz, respectively, showing favorable stability.

Phase-shifted Solc-type filter based on thin periodically poled lithium niobate in a reflective geometry.

Configurable narrow bandwidth filters are indispensable components in optical communication networks. Here, we present an easily-integrated compact tunable filtering based on polarization-coupling process in a thin periodically poled lithium niobate (PPLN) in a reflective geometry via the transverse electro-optic (EO) effect. The structure, composed of an in-line polarizer and a thinned PPLN chip, forms a phase-shift Solc-type filter with similar mechanism to defected Bragg gratings. The filtering effect can be dynamically switched on and off by a transverse electric filed. Analogy of electromagnetically induced transparency (EIT) transmission spectrum and electrically controllable group delay is experimentally observed. The mechanism features tunable center wavelength in a wide range with respect to temperature and tunable optical delay to the applied voltage, which may offer another way for optical tunable filters or delay lines.

A high-speed, eight-wavelength visible light-infrared pyrometer for shock physics experiments

An eight-channel, high speed pyrometer for precise temperature measurement is designed and realized in this work. The addition of longer-wavelength channels sensitive at lower temperatures highly expands the measured temperature range, which covers the temperature of interest in shock physics from 1500K-10000K. The working wavelength range is 400-1700nm from visible light to near-infrared (NIR). Semiconductor detectors of Si and InGaAs are used as photoelectric devices, whose bandwidths are 50MHz and 150MHz respectively. Benefitting from the high responsivity and high speed of detectors, the time resolution of the pyrometer can be smaller than 10ns. By combining the high-transmittance beam-splitters and narrow-bandwidth filters, the peak spectrum transmissivity of each channel can be higher than 60%. The gray-body temperatures of NaI crystal under shock-loading are successfully measured by this pyrometer.