Optical Pulse Width Research Articles

Persistent photoconductivity and Emulating Ebbinghaus forgetting curve via characterization of excitatory synaptic transmission in a ZnO-based optoelectronic synapse with ultra-low power ([formula omitted] fJ) consumption

Optical synapses provide high bandwidth operation with low power consumption for neuromorphic computing. ZnO is established as a potential semiconductor for optoelectronic synapses with its high photosensitivity in the ultraviolet (UV) region. In this work, we focus on emulating Ebbinghaus forgetting curves via conductance decay and memory retention in a lateral synaptic device based on ZnO nanoparticles. We use optical stimulation using UV light of wavelength 375 nm. Retention of memory can be seen up to 2500 s showing long-term potentiation which is useful for memory and learning abilities. Long-duration memory retention using optical spiking is achieved and the decay characteristics of the memory are studied. A transition from short-term plasticity (STP) to long-term plasticity (LTP) can be induced by tuning the optical pulse width. Energy consumption is found to increase exponentially with the applied optical pulse width. The conductance decay follows the Ebbinghaus forgetting curve. We have demonstrated the effect of various measurement parameters such as pulse width, pulse frequency, and pulse amplitude on memory decay. The synaptic device can be operated with energy consumption as low as 1.2 fJ which paves the way for very low energy-consuming synaptic devices for neuromorphic applications.

Emulating Ebbinghaus forgetting behavior in a neuromorphic device based on low dimensional h-BN

Artificial synaptic devices that can mimic the biological synaptic functions of learning and forgetting are essential for the realization of neuromorphic computation, which could replace the von Neumann architecture. In this Letter, we have described a high-performing ultraviolet photodetector (wavelength 375 nm) using thin films of single-layer hexagonal boron nitride (h-BN) for potential use in fabricating a neuromorphic device. Furthermore, the classical Ebbinghaus forgetting curve can be optimized using various parameters such as the optical pulse width, number of pulses, and frequency of pulses. Our results show that the characteristic time constant (τ) has much more variability, indicating better performance control than the Ebbinghaus exponent (β). Furthermore, the performance of the optical synapse is very stable for low energy consumption, as low as 2–3 pJ.

Application of High-Speed Self-Aligned Focusing Schlieren System for Supersonic Flow Velocimetry

A self-aligned focusing schlieren (SAFS) system combines the field of view of a conventional schlieren system with the defocus blur of a focusing schlieren system away from the object plane. It can be assembled in a compact form, measuring 1.2 m (4 ft) in length in the described case. The depth of field is sufficiently shallow to distinguish specific spanwise features in a supersonic flow field within a 76.2 mm (3 in) wide test section. As a result, the boundary-layer perturbations on windows and window-material defects and surface imperfections are blurred. Analytical forms are derived for depth of field and vignetting of the SAFS system. A laser spark velocity measurement in Mach 2 flow is performed by tracking the blast wave of a laser spark using 500 kHz SAFS imaging with a 200 ns optical pulse width. The flow Mach number and stagnation temperature are measured by comparing the blast-wave dynamics to an analytical solution. Additionally, schlieren image velocimetry is performed by analyzing natural flow perturbations in 500 kHz SAFS images using a self-correlation method. Comparing the spectra of gas density perturbations from the core flow and a near-wall region reveals a significant difference, with high-frequency prevalence at the boundary-layer location.

Effect of Optical Pulsewidth on Transition From Linear to Nonlinear Mode of GaAs PCSS

Effect of Optical Pulsewidth on Transition From Linear to Nonlinear Mode of GaAs PCSS

Enhancement of Optical Gain in Colloidal CdSe/CdS/ZnS Quantum Dots through Nanosecond Optical Pumping.

Optical gain and lasing in colloidal nanocrystals are often hindered by sub-nanosecond rapid Auger non-radiative recombination, especially under continuous wave optical or electrical excitation. This study demonstrates amplified spontaneous emission (ASE) from CdSe/CdS/ZnS quantum dot (QD) solids through prolonged pulsed optical pumping over 10 ns. The incorporation of CdS and ZnS double shells on CdSe QDs effectively decelerates the Auger process in multiexcitonic states by extending the electron wave function and enhancing dielectric screening. Furthermore, we engineer smooth, densely packed QD solid films that efficiently guide the optical mode, achieving substantial net gain values under nanosecond pumping. The proposed approach helps observe ASE with gain thresholds of 0.84 and 1.5 mJ/cm2 under optical pumping pulse widths of 6 and 15 ns, respectively. This advancement can promote continuous pumping in colloidal QD gain systems, opening new avenues for optoelectronic applications.

On-chip all-optical second-order ordinary differential equation solver based on a single microdisk resonator.

This paper proposes an all-optical second-order ordinary differential equation (SODE) solver based on a single microdisk resonator. We validate the feasibility of our structure for constant and complex coefficient SODE solutions for Gaussian and super-Gaussian pulses. The results demonstrate a good agreement between the solutions obtained with the designed structure and those obtained through mathematical calculations for both constant and complex coefficient SODEs. We also discuss the influence of input optical signal pulse width on solution result deviations. Furthermore, we validate the capability of the designed structure to achieve tunable solutions for complex-coefficient SODEs with a tuning power of less than 10 mW. The device footprint is approximately 20×30 μm2, and it is 3-4 times smaller than the current smallest solving unit. The maximum Q-factor reaches 9.8×104. The proposed device avoids the traditional approach of cascading two resonators for SODE solving. Moreover, achieving mode alignment within the same resonator reduces the process challenges associated with aligning multiple devices in a cascade. Furthermore, it offers wider applicability for solving SODEs, namely, the ability to solve both constant and complex coefficient SODEs with complete derivative terms.

Monolithic Saturable Absorber with Gallium Arsenide Nanowires Integrated on the Flexible Substrate for Optical Pulse Generation.

In this work, we demonstrated a kind of flexibly monolithic saturable absorber (SA) with GaAs nanowires (NWs) on polyimide (PI) plastic substrate for broadband optical modulation at 1.0 and 1.5 µm, separately. The monolithic SA sample was prepared by the metalorganic vapor phase epitaxy (MOVPE) method. The crystal structure and element analysis were examined carefully by high-resolution scanning transmission electron microscopy (HRSTEM) and energy-dispersive X-ray spectroscopy (EDX). We observed a high-density distribution of NWs on the flexible substrate by scanning electron microscopy (SEM). In addition, linear and nonlinear optical properties of the sample were examined by testing the photoluminescence and absorption properties, which showed its potential application as an optical switch due to the pure semiconducting properties. After the characterizations, we experimentally demonstrated this monolithic SA for laser modulation at 1.0 and 1.5 µm, which yielded the minimum optical pulse widths of 1.531 and 6.232 µs, respectively. Our work demonstrated such a kind of monolithic flexible NW substrate-integrated device used for broadband optical modulation, which not only eased the integration process of NWs onto the fiber endface, but also proved the potential of easily integrating with more semiconducting nanomaterials (e.g., graphene, MoS2, …) to realize monolithic active flexible photonic systems, such as a microscale phase modulator, delay-line, and so on, paving an easy avenue for the development of both active and flexible photonic devices.

Fast, Energy‐Efficient InGaAs Synaptic Phototransistors on Flexible Substrate

AbstractPhotodetectors sensing the short‐wave infrared (SWIR) region have great potential due to their significant advantages in a variety of applications because SWIR light possesses both characteristics of visible light and infrared light. Among them, devices using photodetectors to mimic synaptic dynamics and functions have received a great deal of attention due to their capabilities to implement simplified neural systems. However, it is essential to develop synaptic devices that can operate fast with low energy consumption for more efficient implementation of neural systems. Here, a flexible InGaAs synaptic phototransistor with a fast operation speed of under 1 ms and low energy consumption in the atto joule level is developed to femto joule level which is superior to biological synapses (50 ms, 1–100 fJ). By using InGaAs which has high carrier mobility as a channel layer, weak light, and short optical pulse width, fast operation speed in the SWIR region with low energy consumption is obtained. Moreover, the devices demonstrate synaptic behaviors such as “excitatory post synaptic current”, “paired‐pulse facilitation”, “short term plasticity”, “long term plasticity”, and “learning‐experience behavior” as neuro‐synaptic applications. These results provide the possibilities for implementation of complex synaptic functions with fast speed and low power SWIR synaptic phototransistors.

Bubble structure evolution and electron injection controlled by optical cycles in wakefields

The evolution of bubble structure and electron injection in laser wakefield acceleration with different optical cycles is investigated through three-dimensional particle-in-cell simulations. Under fixed transverse and longitudinal ponderomotive force, the effect of optical cycles on the evolution of bubble structure and electron injection is studied by changing the laser wavelength. For a multi-cycle laser, electron acceleration is dominated by the ponderomotive force that produces symmetrical bubble and continuous injection. As the optical cycles decrease, the dominant effect of the electron acceleration can transition from the ponderomotive force to the carrier wave, and the carrier envelope phase shift can cause transverse oscillation of the bubble and periodic electron injection in the direction of laser polarization. The criterion for the dominant acceleration mechanism and the dependence of transition distance on the optical cycles and pulse width are obtained. The results are beneficial for manipulating electron acceleration and betatron radiation generation.

Denoising algorithm of [formula omitted] -OTDR signal based on curvelet transform with adaptive threshold

In this paper, a denoising scheme based on curvelet transform is proposed to improve the signal-to-noise ratio (SNR) for vibration sensing in phase-sensitive optical time-domain reflectometry (Φ-OTDR) systems. The noise level can be estimated based on non-vibration images, which can be used to set the threshold for curvelet coefficients. We further optimize the threshold according to the amplitude distribution of the curvelet coefficients. In the experimental demonstration, when the optical pulse width is 100 ns, the SNR of location information of the 100 Hz vibration events can be increased by 6.93 dB, which proves the effectiveness of the proposed method.

Theoretical analysis and experimental demonstration of gain switching for a PPM based UWOC system with picosecond pulses.

Shortening pulse width can improve the power efficiency and data rate of a pulse position modulation (PPM) based underwater wireless optical communication (UWOC) system at a fixed average optical power, which is more suitable for the energy-limited underwater environment. As a common method to generate short pulses, gain switching has the advantages of a tunable switching frequency and simple structure, facilitating the generation of high-order PPM signals. However, the output characteristics of electrical gain switching seriously affect the demodulation of PPM signals and limit the data rate. To study the performance of gain switching on a PPM communication system, simulation models of the semiconductor laser diode and the driving circuit are built to describe the generation of electrical and optical pulses. The pulse width, pulse peak value, and peak position of optical pulses are analyzed under different symbol durations and PPM orders. Furthermore, a 64-PPM/150-Mbps UWOC system with a 200-ps optical pulse width is demonstrated by using a gain-switched blue GaN-based laser diode in a water tank. The peak average power ratio (PAPR) is 19.5 dB. Via the statistical analysis of experiment results and the output characteristics of electrical gain switching, the main factor limiting the data rate attributes to the time delay fluctuation of gain switching. To the best of our knowledge, this is the first time that gain switching has been experimentally demonstrated and analyzed in a high-order PPM based UWOC system.

Lyapunov stability of singular planarsystems related to dispersion-managedsolitons in optical fiber

In this paper, we consider two singular planar differential systems which can describe the evolution of the optical pulse width and chirp for the so-called dispersion-managed solitons. Based on the method of third order approximation in combination with some quantitative information obtained by the upper–lower solutions method and the averaging method, some results on the existence and Lyapunov stability of the periodic solutions are obtained. Moreover, the formula of the first twist coefficient and a stability criterion of a nonlinear differential equation are also established.

Octave-Spanning Optical Frequency Comb Generation Using a Directly-Modulated Microlaser Source

A coherent optical frequency comb with a gigahertz line spacing is important in diverse fields, such as calibration of astronomical spectrographs and wavelength division multiplexing optical communications. In this paper, we realize octave-spanning 10-GHz optical combs using a directly-modulated 1.55 μm microlaser as a seeding source. A 6.4 ps transform-limited pulse is successfully generated from the directly-modulated laser output through optimal chirp compensation in a fiber. Moreover, after amplifying by an EDFA, the optical pulse width is further reduced to 140 fs by combing the self-phase modulation effect and the anomalous group velocity dispersion in commercial fibers. Finally, a coherent optical comb spanning from 900-2400 nm is produced in a home-made fluorotellurite fiber using the 140-fs pulse boosted to a few Watts as a seeding source. The experimental results demonstrate that optical combs generated from the directly-modulated microlaser have the potential for a fully stabilized frequency comb.

MHz Repetition Frequency, Hundreds Kilowatt, and Sub-Nanosecond Agile Pulse Generation Based on Linear 4H-SiC Photoconductive Semiconductor

In this article, an ultrahigh repetition frequency sub-nanosecond pulse generator based on vanadium-compensated semi-insulating (VCSI) 4H-SiC photoconductive semiconductor switch (PCSS) under high bias electric field is presented. In this work, the 0.8 mm-thick 4H-SiC PCSS was found to work with electric fields up to 200 kV/cm and power capacity of 176 kW. The VCSI 4H-SiC PCSS response test was performed with a 1 MHz, 1030 nm laser cluster driver with configurable optical pulsewidth and current viewing resistor (CVR) high frequency response circuit. In the experiment, the laser pulsewidth was varied between 300 and 500 ps, and 1.5 ns. The system generated a 1 MHz repetition frequency pulse with a minimum half-width of 365 ps and less than 100 ps of jitter. The results revealed higher response performance for single/continuous pulse triggered light of various pulsewidths. Due to the low absorption efficiency of 4H-SiC for non-intrinsic light, an axial optical reflection trap structure was utilized to boost the light absorption efficiency of PCSS. The proposed structure significantly improved the light absorption performance of PCSS devices. The photocurrent increase ratio of roughly 115% was obtained. This study presents the first results of MHz re-frequency high-power switching in bulk semi-insulating 4H-SiC.

Super Spatial Resolution Raman Distributed Temperature Sensing via Deep Learning

Raman distributed temperature sensing (RDTS) can obtain temperature information along with an optical fiber. Its spatial resolution is defined by the minimum spatial range that can be resolved by the system. The spatial resolution can be improved by reducing the optical pulse width. However, simultaneously, the signal-to-noise ratio is degraded, and higher speed equipment is required. Recently, deconvolution algorithms have been employed to improve spatial resolution without hardware modification. However, conventional deconvolution algorithms have limited effectiveness. Here, we propose and experimentally demonstrate an RDTS super spatial resolution deep convolutional neural network (SSRNet) to improve the spatial resolution with high fidelity. The convolution kernel of the RDTS is estimated from the falling signal at the fiber end rather than using the waveform of pulsed light. Then a modified RDTS model is built to generate a large amount of training data. Through optimizing the structure of SSRNet, the spatial resolution is increased from 4 m to 0.8 m which has reached the limit of the sampling rate of 250 MSa/s. The simulation and experimental results show that the restored results using SSRNet are more accurate with fewer artifacts than the results using a conventional deconvolution algorithm.

Pulse control of frequency and width for a real-time independently adjustable laser source

A set of semiconductor laser pulse seed sources based on an embedded chip is proposed. The greatest feature is that the optical pulse frequency and width can be independently adjusted in real time. The pulse seed sources can be switched independently and online from the gain-switched mode to the quasi-continuous wave mode to obtain optimal optical parameters for specific applications. To explore the physical mechanism of the semiconductor laser source, the rate equation that describes the carrier-photon transient change in a semiconductor laser cavity is numerically derived and solved. Subsequently, problems that need to be considered while designing the drive circuit are identified. The system evaluation indicates that the optical pulse frequency adjustment range is 250 Hz to 42 MHz, and the narrowest optical pulse output width is 80 ps. The pulse seed source can drive semiconductor lasers with different central wavelengths (1064, 1550, and 1970 nm), and can also simultaneously drive two semiconductor lasers and output dual-band optical pulses. It can be used as a seed source for general high-power optical systems, and exhibits good application value and extensive market prospects.

Multipath distributed acoustic sensing system based on phase-sensitive optical time-domain reflectometry with frequency division multiplexing technique

We propose a multipath distributed acoustic sensing (MP-DAS) system based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) and frequency division multiplexing (FDM) technique. With FDM introduced to Φ-OTDR, simultaneously monitoring of multipath vibration is realized with flexible functionality. In the proposed MP-DAS system, the parameters of each path can be adjusted individually according to the path's specific monitoring requirement. A dual path Φ-OTDR system with 1 km and 22 km measurement distance is demonstrated in the experiment. Through heterodyne detection and FDM technique, both amplitude and phase information are obtained for event locating and vibration sensing in each of the two paths, with a spatial resolution of 5 m and 10 m respectively. With changing optical power and pulse width of one path, its influence on the other path is discussed and analyzed. This proposed MP-DSA system based on FDM technique provides an efficient and cost-effective way for upgrading a conventional single DAS into an integrated DAS network, which shows great potential in many applications such as modern city civil structure monitoring with different demands.

Femtosecond pulse generation from external cavity diode laser based on self-mode-locking.

In this Letter, we report optical pulse generation from a single-section diode gain chip, employed in an external cavity geometry based on the self-mode-locking regime. The gain chip emits light at 1550 nm wavelength range. The external cavity is operated at various repetition rates, ranging from 1 to 2.5 GHz. An optical pulse width of approximately 650 fs is obtained by fitting a Lorentzian distribution. A low RF spectral width of 78.875 kHz is measured corresponding to a low pulse-to-pulse RMS timing jitter of 1.273 ps. This system paves the way towards ultra-compact, cost-effective, and chirp-compensated femtosecond laser pulse sources with adjustable repetition rates.

On the double-pole solutions of the complex short-pulse equation

In non-linear optics, it is well known that the non-linear Schrödinger (NLS) equation was always used to model the slowly varying wave trains. However, when the width of optical pulses is in the order of femtosecond ([Formula: see text] s), the NLS equation becomes less accurate. Schäfer and Wayne proposed the so-called short pulse (SP) equation which provided an increasingly better approximation to the corresponding solution of the Maxwell equations. Note that the one-soliton solution (loop soliton) to the SP equation has no physical interpretation as it is a real-valued function. Recently, an improvement for the SP equation, the so-called complex short pulse (CSP) equation, was proposed in Ref. 9. In contrast with the real-valued function in SP equation, [Formula: see text] is a complex-valued function. Since the complex-valued function can contain the information of both amplitude and phase, it is more appropriate for the description of the optical waves. In this paper, the new types of solutions — double-pole solutions — which correspond to double-pole of the reflection coefficient are obtained explicitly, for the CSP equation with the negative order Wadati–Konno–Ichikawa (WKI) type Lax pair by Riemann–Hilbert problem method. Furthermore, we find that the double-pole solutions can be viewed as some proper limits of the soliton solutions with two simple poles.

Airborne radiometric validation of the geostationary lightning mapper using the Fly’s Eye GLM Simulator

The Fly’s Eye GLM Simulator (FEGS) is a multi-spectral array of radiometers designed to provide a validation dataset for the geostationary lightning mapper (GLM). The main component of FEGS is a 5 × 5 grid of radiometers, each with a square 18 deg field of view, that are sensitive to a 10-nm wide spectral band centered on 780 nm to observe a neutral atomic oxygen emission triplet at 777.4 nm. FEGS was flown as a primary payload on the high-altitude NASA ER-2 aircraft as part of the GOES-R validation flight campaign in the spring of 2017. More than 14,000 lightning flashes were recorded over a variety of thunderstorm phenomenologies and geographical regions in the continental United States and Atlantic Ocean. Following the application of a quality control process, a subset of 9160 of these flashes occurring during daylight hours was suitable for analysis. This analysis has provided an independent, lower bound estimate of the average daytime optical flash detection efficiency (FDE) of GLM at 64%. The FDE varied significantly between storms and location ranging between 44% and 88%. An upper estimate of the time-integrated cloud-top radiance detection threshold of GLM during daytime observations is calculated to be 9 μJ / m2 sr nm. Dependencies of GLM performance on the temporal width, energy, and cloud-top area of optical pulses, and the background solar radiance conditions are presented and discussed.