Variable Attenuator Research Articles

High-power 1.6 μm noise-like square pulse generation in an all-fiber mode-locked laser

We demonstrate generation of high-power and large-energy noise-like square pulses at 1612 nm in an all-fiber dumbbell-shaped mode-locked Er: Yb co-doped double-clad fiber (EYDF) laser. The custom couplers with high power handling keep the laser function well. Large-mode-area EYDF with high power handling and enough high pump power make it possible to obtain high output power in the oscillator. Compared with figure-eight structure, strictly all-fiber dumbbell-shaped structure without isolator and optimizing splicing loss could reduce intra-cavity loss and improve optical to optical efficiency, which could reduce heat accumulation and enhance the power carrying capacity of EYDF. In order to study the influence of in-band absorption on output wavelength, EYDF1 and EYDF2 with different in-band absorption coefficients are accessed to intracavity, respectively. It is directly demonstrated that regulation of in-band absorption is an effective way to control the output wavelength. Strong in-band absorption could restrain the emission of C-band and make the wavelength range above 1.6 μm obtain enough gain. Linear insertion loss is another important factor to affect the emission wavelength in EYDF fiber laser. At pump power of 8 W, maximum average output power with emission wavelength above 1.6 μm can reach 1.16 W, corresponding to a single pulse energy of 1.26 μJ. SNR of output pulse is 70 dB which indicates the high stability of mode-locking. In order to verify and evaluate influence of insertion loss on the output characteristics of mode-locked laser, a variable attenuator is inserted in experimental setup, allowing us adjust the linear loss of the cavity. By increasing pump power and adjusting PCs, mode-locked pulses could be obtained on the condition of large additional insertion loss. Even though ~ 10 dB additional insertion loss is introduced, the oscillation still could function at 1612 nm and keep stable mode-locked state. This result demonstrates our oscillation could bear strong additional loss and own strong robustness. If excessively large additional insertion loss is added, 1566 nm becomes the central emission wavelength and wavelength component at 1612 nm almost disappears. Our investigation supplies a direct guideline to design high-power fiber laser with emission wavelength above 1.6 μm.

Amplitude and Phase Tuning of Microwave Signals in Magnetically Biased Permalloy Structures

In this paper, a permalloy layer has been employed in the fabrication of a coupled line electromagnetic bandgap (EMBG) device to tune both amplitude and phase. A magnetically biased microwave coplanar configuration manufactured with evaporated permalloy has been measured, and a circuit modelling has been studied to evaluate the measured effects in terms of variable attenuation and phase shift. Starting from a permalloy made by the mixture 80% nickel and 20% iron content, we fabricated an electromagnetic bandgap (EMBG) structure based on a periodic arrangement of single sections of a transmission line with variable impedance, also including a central region with coupled lines. The bandpass characteristics of the EMBG device can be tuned by changing permalloy’s permeability through the application of a DC magnetic field $H_{0}$ (parallel to the plane of the structure). In particular, using a magnetic field up to 3000 Oe, it was possible to change the phase by ca. 45° and the amplitude by ca. 7 dB in the X band.

High frequency arbitrary waveform generator using a nonlinear transmission line

AbstractIn this article, a high‐frequency arbitrary waveform generator (AWG) based on Fourier synthesis theory is presented. A key element consists of a nonlinear transmission line (NLTL) that acts as a harmonic generator for large signals and as a phase shifter for small signals. The harmonics generated by the NLTL are separated into four channels using power dividers and each harmonic is bandpass filtered in each channel. The phase and the amplitude are then modulated in each channel by an NLTL phase shifter and a voltage variable attenuator. Finally, vector modulated signals from each channel are combined in Wilkinson power combiners to form any waveform shape. We used 0.5 GHz as a master signal and synthesized waveforms using all four channels.

240-GHz Four-Channel Power-Tuning Heterodyne Sensing Readout System With Reflection and Transmission Measurements in a 130-nm SiGe BiCMOS Technology

This article presents a fully differential power-tuning heterodyne on-chip sensing readout system at 240 GHz. The chip enables the measurement of not only the transmission parameter but also the reflection parameter to sense the permittivity of different materials by using four heterodyne mixer-based receiving channels connected to a dielectric sensing element. To facilitate the operation and characterization, three frequency multiplier chains are included to generate the required two identical radio frequency (RF) and one local oscillator (LO) subterahertz signals. RF frequency multiplier chain is configured to enable a tunable power level of the RF signal by using a variable attenuator. A chip prototype using 130-nm silicon–germanium (SiGe) BiCMOS is implemented with a size of 11 mm2 and dc power consumption of 2.7 W. The measured 10-dB bandwidth of 20.8% is achieved in a frequency range from 207 to 257 GHz with 14-dB measured power-tuning range. The transmission and reflection parameters’ measurements for copper and gummi show a differentiated value in terms of magnitude and phase, which demonstrates the sensing function of the proposed readout system.

Which attenuation curves for star-forming galaxies?

AbstractDust attenuation shapes the spectral energy distributions of galaxies and any modelling and fitting procedure of their spectral energy distributions must account for this process. We present results of two recent works dedicated at measuring the dust attenuation curves in star forming galaxies at redshift from 0.5 to 3, by fitting continuum (photometric) and line (spectroscopic) measurements simultaneously with CIGALE using variable attenuation laws based on flexible recipes. Both studies conclude to a large variety of effective attenuation laws with an attenuation law flattening when the obscuration increases. An extra attenuation is found for nebular lines. The comparison with radiative transfer models implies a flattening of the attenuation law up to near infrared wavelengths, which is well reproduced with a power-laws recipe inspired by the Charlot and Fall recipe. Here we propose a global modification of the Calzetti attenuation law to better reproduce the results of radiative transfer models.

Underwater image restoration through a combination of improved dark channel prior and gray world algorithms

Due to the absorption and scattering of light caused by particles suspended in water and the variable attenuation of light with different wavelengths, underwater images are typically characterized by low contrast, blurred features, and severe color bias. To improve the quality of underwater images, an underwater image restoration method is proposed. First, based on similarities between the underwater optical imaging model and the atmosphere model, an improved dark channel prior algorithm is used to enhance the original underwater image and an improved gray world algorithm is used to calibrate the image. Then, an improved wavelet fusion algorithm is adopted to combine the results produced by the two algorithms. The experimental results show that the proposed method can effectively improve feature clarity and color accuracy in underwater images with low quality and illumination, resulting in a better visual effect.

Monocular accommodation response to random defocus changes induced by a tuneable lens

Accommodation of the human eye relies on multiple factors and visual cues that include object size, monochromatic and chromatic aberrations, and vergence. Yet, even in monocular conditions, accommodation corrects for defocus. Studies of eye growth in chicks have addressed whether the retina can decode the sign of defocus as this may play a role for emmetropization and possibly also accommodation. However, findings have not been unambiguous and questions remain. Here, we report on monocular accommodation studies of emmetropic and myopic human subjects to clarify whether foveal vision drives accommodation in the correct direction by removing out-of-focus blur potentially before relying on other cues. Subjects viewed monocularly a green target at 1-meter distance while being presented with a random sequence of negative defocus step changes induced by a pupil-conjugated current-driven tuneable lens. The natural pupil was constricted by a pupil-conjugated motorized iris using three different diameters and target brightness was set with a liquid crystal variable attenuator. A Hartmann-Shack wavefront sensor with an infrared beacon captured real-time changes of defocus and Zernike polynomial coefficients up to 4th radial order. We find that the young adult eye accommodates reliably in the correct direction but with a latency of 300–700 ms. The findings are discussed in relation to an absorption model of light in outer segments that breaks the defocus symmetry and thus may serve as a plausible guide for accommodation and emmetropization.

4D spatiotemporal evolution of liquid spray using kilohertz-rate x-ray computed tomography.

Four-dimensional (x,y,z,t) x-ray computed tomography was demonstrated in an optically complex spray using an imaging system consisting of three x-ray sources and three high-speed detectors. The x-ray sources consisted of high-flux rotating anode x-ray tube sources that illuminated the spray from three lines of sight. The absorption, along each absorption path, was collected using a CsI phosphor plate and imaged by a high-speed intensified CMOS camera at 20kHz. The radiographs were converted to a quantitative equivalent path length (EPL) of liquid using a variable attenuation coefficient to account for beam hardening. The EPL data were then reconstructed using the algebraic reconstruction technique into high-speed time sequences of the three-dimensional liquid mass distribution.

Phase-Noise and Amplitude-Noise Measurement of DACs and DDSs.

This article proposes a method for the measurement of phase noise (PN, or PM noise) and amplitude noise (AN, or AM noise) of digital-to-analog converters (DACs) and direct digital synthesizers (DDSs) based on the modulation-index amplification. The carrier is first reduced by a controlled amount (30-40 dB) by adding a reference signal of nearly equal amplitude and opposite in phase. Then, residual carrier and noise sidebands are amplified and sent to a conventional PN analyzer. The main virtues of our method are: 1) the noise specs of the PN analyzer are relaxed by a factor equal to the carrier suppression ratio and 2) the capability to measure the AN using a PN analyzer with no need for the analyzer to feature AN measurement. An obvious variant enables AN and PN measurements using an AN analyzer with no PN measurement capability. Such an instrument is extremely simple and easy to implement with a power-detector diode followed by an FFT analyzer. Unlike the classical bridge (interferometric) method, there is no need for external line stretcher and variable attenuators because phase and amplitude controls are implemented in the device under test. In one case (AD9144), we could measure the noise over 10 decades of frequency. The flicker noise matches the exact 1/f law with a maximum discrepancy of ±1 dB over 7.5 decades. Due to the simplicity, reliability, and low background noise, this method has the potential to become the standard method for the AN and PN measurements of DACs and DDSs.

Robust countermeasure against detector control attack in a practical quantum key distribution system

In real-life implementations of quantum key distribution (QKD), the physical systems with unwanted imperfections would be exploited by an eavesdropper. Based on imperfections in the detectors, detector control attacks have been successfully launched on several QKD systems, and attracted widespread concerns. Here, we propose a robust countermeasure against these attacks just by introducing a variable attenuator in front of the detector. This countermeasure is not only effective against the attacks with blinding light, but also robust against the attacks without blinding light which are more concealed and threatening. Different from previous technical improvements, the single photon detector in our countermeasure model is treated as a blackbox, and the eavesdropper can be detected by statistics of the detection and error rates of the QKD system. Besides theoretical proof, the countermeasure is also supported by an experimental demonstration. Our countermeasure is general in sense that it is independent of the technical details of the detector, and can be easily applied to the existing QKD systems.

Multi-wavelength switchable erbium-doped fiber laser based on a hybrid filter incorporating a bi-tapered Mach–Zehnder interferometer and Sagnac loop

A multi-wavelength switchable C-band erbium-doped fiber laser based on a hybrid comb filter composed of a Sagnac loop and Mach–Zehnder interferometer (MZI) is designed and experimentally demonstrated. In the designed ring cavity fiber laser, the MZI is composed of bi-tapered single-mode fiber fabricated by etching with hydrofluoric acid. In the experiment, the wavelength intervals in the comb spectra of the Sagnac loop and MZI are 3.4 and 18.8 nm, respectively, and by combining the Sagnac loop and MZI, measurements of the hybrid comb spectrum are obtained. When the pump power is 91 mW, lasing occurs at 1529.2 nm. Adjustment of the variable attenuator (VA) can provide 1541.9 and 1558.6 nm single-wavelength switchable lasers, for which the maximum peak power difference is less than 3.59 dB, the signal-to-noise ratio (SNR) exceeds 19.25 dB, and the peak power fluctuation is less than 0.34 dB over a 30 min scan time at 26 °C. For dual-wavelength switchable laser emission, four different sets of wavelengths can be realized, and the SNR exceeds 15.57 dB with power fluctuations of less than 1.6 dB, under the same monitoring conditions. By further adjusting the VA, stable and tunable triple-wavelength lasing is also realizable; under these conditions, the SNR exceeds 15.02 dB and the power fluctuation is less than 0.91 dB. Furthermore, four- and five-wavelength lasing can also be achieved by adjusting the VA again, with SNRs exceeding 19.05 and 14.98 dB, respectively.

Simulations Find Our Accounting of Dust-obscured Star Formation May Be Incomplete

The bulk of the star-formation rate density peak at cosmic noon was obscured by dust. How accurately we can assess the role of dust obscured star-formation is affected by inherent biases in our empirical methods -- both those that rely on direct dust emission and those that rely on the inferred dust attenuation of starlight. We use a library of hydrodynamic simulations with radiative transfer to explore these biases. We find that for IR luminous galaxies that are in rapidly quenching systems (e.g. post-coalescence) standard luminosity-to-SFR relations can strongly overestimate the true SFRs. We propose using the $L_{IR}/L_{1.6}$ color to both help identify such systems and provide more accurate SFRs. Conversely, we find that the diagnostic UVJ plot misidentifies a subset of dusty star-forming galaxies. This is due to variability in the effective attenuation curves including being much grayer in the optical-to-near-IR regime than the Calzetti starburst law. This is in agreement with recent observations of IR-selected galaxies at cosmic noon. Our results support the view that we need a panchromatic approach from the rest-frame UV through the IR and SED modeling that includes realistic SFHs and allows for variable attenuation curves if we want to fully account for dust obscured star-formation across the epochs of greatest galaxy build-up.

Monolithically Integrated RF MEMS-Based Variable Attenuator for Millimeter-Wave Applications

This paper reports a millimeter-wave radio frequency (RF) microelectromechanical systems (MEMS)-based variable attenuator implemented by monolithically integrating CPW-based hybrid couplers with lateral MEMS varactors on a silicon-on-insulator (SOI) substrate. The MEMS varactor features a Chevron-type electrothermal actuator that controls the lateral movement of a thick plate allowing precise change of capacitive loading on a CPW line leading to a change in isolation between input and output. The proposed variable attenuator is successfully fabricated on an SOI substrate with a device footprint of 3.8 $\text {mm}\times 3.1$ mm. The fabrication process provides flexibility to extend this module and implement more complex RF signal conditioning functions, thus making it more appealing to realize a wide range of reconfigurable RF devices. The measured RF performance shows that the device exhibits attenuation levels ( $S_{{21}}$ ) ranging from 10 to 25 dB, at the center frequency of 60 GHz with a bandwidth of 4 GHz and a return loss of better than 20 dB. The device involves laterally moving 20- $\mu \text{m}$ -thick structures, hence offers reliable operation and eliminates MEMS problems like stiction, dielectric charging, and microwelding observed in surface micromachined thin membranes.

Basinga: A cell‐by‐cell GIS toolbox for computing basin average scaling factors, cosmogenic production rates and denudation rates

AbstractThe calculation of denudation rates from the measured cosmogenic nuclide concentrations in river sediments requires assumptions and approximations. Several different approaches and numerical tools are available in the literature. A widely used analytical approach represents the muogenic production with one or two exponentials, assumes the attenuation length of muons to be constant and also neglects temporal variations in the Earth's magnetic field. The denudation rates are then calculated directly and analytically from the measured concentrations. A second numerical and iterative approach was more recently proposed and considers a more rigorous muogenic production law based on pre‐calculated variable attenuation length of muons and accounts for temporal changes of the magnetic field. It also assumes a specific distribution of denudation rates throughout the basin and uses an iterative approach to calculate the basin average denudation rates.We tested the two approaches across several natural basins and found that both approaches provide similar denudation results. Hence, assuming exponential muogenic production and constant attenuation length of muons in the rock has little impact on the derived denudation rates. Therefore, unless a priori known distributions of denudation rates are to be tested, there does not appear to be any particular gain from using the second iterative method which is computationally less effective.Based on these findings, we developed and describe here Basinga, a new ArcGIS® and QGIS toolbox which computes the basin average scaling factors, cosmogenic production rates and denudation rates for several tens of drainage basins together. Basinga follows either the Lal/Stone or the Lifton/Sato/Dunai scaling schemes and includes several optional tools for correcting for topographic shielding, ice cover and lithology. We have also developed an original method for correcting the cosmogenic production rates for past variations in the Earth's magnetic field. © 2019 John Wiley & Sons, Ltd.

An automatic variable laser attenuator for IRMPD spectroscopy and analysis of power-dependence in fragmentation spectra

Infrared ion spectroscopy (IRIS) requires reproducibility and standard protocols in order to be fully appreciated as an analytical tool. An important parameter in IRIS measurements is the infrared laser power (energy per pulse), which is often difficult to control precisely on a day-to-day basis. We have developed an automatic variable attenuator to keep the laser power at a well-defined value, independent of IR frequency and time, while maintaining full overlap of the laser beam with the ion cloud in our quadrupole ion trap mass spectrometer. The device has been used to record IR spectra of the metabolite prostaglandin D2 at different laser pulse energies, allowing us to assess the effects on the IRMPD spectra of excessive ion depletion, a threshold pulse energy below which dissociation is not observed, and unobserved fragments due to electron detachment or fragmentation to below the low mass cut-off of the trap.

Variable aperture with graded attenuation combined with adjustable focal length lens.

In this paper, we demonstrate a variable aperture with graded attenuation combined with adjustable focal length lens actuated by hydraulic control. Two cylindrical chambers and a middle substrate are stacked to form the device body. An elastic film is fabricated in the middle substrate like a sandwich. In the initial state, the dyed liquid is fully covered on the elastic film. The variable aperture shows the state of the maximum optical attenuation. When the bottom chamber is injected with liquid, the elastic film can form a convex surface. The dyed liquid will be pushed to the side wall of the chamber by the raised elastic film and the optical attenuation can be varied by changing the volume of the injected liquid. The proposed device can achieve both the variable attenuator function and the variable-focus lens function. The experiments show that the variable aperture can obtain dynamic attenuation ranges from 33.01 dB to 0.71 dB, and the zoom liquid lens can reach 2.9☓magnifying power. The device can be applied in imaging systems and fiber-optic communications.

Performance analysis and small signal identification of time-resolved stand-off Raman spectroscopy system

Over the last decade, the time-resolved stand-off Raman spectroscopy has attracted considerable attention due to its daytime measurement model and fluorescent suppression. Suffering from the small Raman scattering efficiency, this system perusing large detection distance requires both large power consumption and optical system, which blocks its applications. In order to seek for high signal collection efficient, in this work, we designed a 2-inch refractive telescope system, and obtained 20 m detection distance using a frequency-doubled 532 nm Nd: YAG laser with pulse energy of 30 mJ. By adjusting the excitation energy through a variable attenuator, we investigate the interrelations of input power, the signal and the noise consisting of the internal and signal-induced noises. For both main peak and sub peak, the signal-induced noises are subject to a power function of the signal intensity, and can be filtered and smoothed by the norm-distribution-function weighted moving average method (WMAM) without the loss of the signal information. After transforming the correlation spectrum, defined by the multiplication of the noise and signal spectrum, iteratively by WMAM, we can identify the small signal of about 1 count beyond the internal noise limitation. With help of the iteration method, the requirement of integration time as well as the excitation energy can be greatly reduced.

A 3.5 GHz low insertion loss variation CMOS phase shifter

AbstractA 3.5 GHz low insertion loss variation phase shifter has been designed and fabricated on standard 0.18‐μm CMOS process. To achieve over 360° continuously phase tuning range, the phase shifter consists of a switch type phase shifter (STPS) and a reflection type phase shifter (RTPS). The phase‐invertible variable attenuator (PIVA) topology which can provide two states with 180° phase‐shift while maintaining low amplitude imbalance is adopted. The RTPS provides over 189° continuously phase tuning. The experimental results show that the 3.5 GHz phase shifter demonstrates over 360° continuously phase tuning range with low insertion loss variation of 1 dB at 3.5 GHz. The maximum insertion loss is 13.5 dB at 3.5 GHz and the dc consumption is zero. The chip size is 1.08 mm × 0.82 mm.

A 6-bit digital CMOS variable gain attenuator with large dynamic range and high linearity-in-dB for ultrasound imaging applications

A 6-bit digital CMOS variable gain attenuator (VGAT) with 42 dB dynamic range (DR) and high linearity-in-dB was presented. CMOS transistors were used as the shunt devices and attenuation gains with different distribution rules of the shunt transistors (W/L)s were analyzed and simulated in detail. A geometric series with a length of 42 and a ratio of 1.122 was finally adopted for the (W/L)s to achieve the 42 dB DR and 1 dB attenuation step size. Fabricated in a 0.18-μm CMOS technology, the proposed VGAT had been integrated in a 4-channel ultrasonic imaging receiver chip and occupied an area of about 0.5 mm × 0.37 mm. The gain errors were limited in the range from −1 dB to −1.3 dB at 27 °C and the DR of the VGAT at different temperatures was retained above 40 dB. The –3 dB bandwidth was about 1.9 GHz at the maximum attenuation and the S11 was better than −11 dB with a 30 MHz input.

Precise Wavelength Control of Yb-Doped Fiber Laser Using Fused Tapered Fiber Technology

We have proposed and demonstrated an all-fiber Yb-doped fiber laser (YDFL) that has adjustable peak positions and precisely controllable channel spacing. By exploiting a fused tapered fiber in one arm of a Mach-Zehnder interferometer, a multi-wavelength YDFL with finely controlled channel spacing has been achieved. By using another tapered fiber as a variable attenuator, both tunable single-wavelength and multi-wavelength lasing output have been obtained. We believe that the fused tapered fiber technology can be applied to the development of stable, wavelength-tunable all-fiber laser systems.