Tunable Time Delay Research Articles

Experimental study on the ablation of stainless steel using multiple ultra-short laser pulses with tunable time delays

Results on the ablation of stainless steel using ultra-short single, double and quadruple pulses as a function of the fluence and the time delay between each laser pulse are presented. The investigations focused quantitatively on the ablated volume and the resulting ablation efficiency, and qualitatively on the topography of the structures produced, providing insights about physical mechanisms such as shielding, re-deposition of ablated particles, and accumulation of heat or energy. The results indicate a significant influence of fluence in combination with the time delay on the ablation process and the resulting ablation topography.

Tunable spin-wave delay line based on ferrite and vanadium dioxide

One of the key elements for modern microwave circuits is a delay line, which is widely utilized for the signal generation as well as processing. Spin-wave delay lines based on ferrite films provide a high delay time and small dimensions. Typically, the performance characteristics of such lines are tuned by the variation of an externally applied magnetic field characterized by some drawbacks. The phenomenon of a metal–insulator transition (MIT) in the phase change materials permits to improve the performance characteristics of the spin-wave delay lines. In particular, this concept allows to reduce the power consumption and improve the control speed of a delay time. Aim. Development of a tunable spin-wave delay line based on ferrite and vanadium dioxide films, as well as the study of its performance characteristics. Methods. Experimental investigations were carried out for the delay line composed of the yttrium iron garnet (YIG) and vanadium dioxide (VO2) films. The ferrite waveguide was fabricated from a single-crystal YIG film grown on a gallium gadolinium garnet substrate. A vanadium dioxide film was formed on a silicon dioxide substrate by DC reactive magnetron sputtering. The microwave measurements were carried out using the vector network analyzer R&S®R ZVA40. Results. It was shown that heating of the VO2 film induces a sufficient drop of its resistance that causes the transformation of the spin-wave dispersion characteristic. This leads to the decrease in the group velocity of the propagating waves providing a growth of a delay time. Namely, experimental structure of 5-mm length offers a tunable time delay range from 130 up to 150 ns at the operating frequency of 4.33 GHz. Conclusion. A proof-of-principle for the MIT control of the delay time composed on the YIG-VO2 structure has been presented. It was shown that a switch of VO2 film from the isolating into conducting state produces a 15% change in the delay time. The considered microwave delay lines look favorable for applications as a complimentary part to the traditional approach for general computing and microwave signal processing.

Investigation on Tunable Time Delays of a High-Frequency Microwave Photonic Signal Using a Distributed-Feedback Laser

Investigation on Tunable Time Delays of a High-Frequency Microwave Photonic Signal Using a Distributed-Feedback Laser

Photonics-Assisted Ultrawideband RF Self-Interference Cancellation With Signal of Interest Recovery and Fiber Transmission

Self-interference and chromatic dispersion are two main limiting factors that influence the recovery of the signal of interest (SOI) in in-band full-duplex (IBFD) radio-over-fiber (RoF) links. An ultrawideband photonic-assisted approach to radio-frequency (RF) self-interference cancellation (SIC) with high-speed SOI recovery and fiber transmission capability is proposed. The key to achieve the above optimum performances in a compact system is the combined use of fiber dispersion induced RF time delay and optical carrier phase-shifted double sideband (CPS-DSB) modulation. A high precision and tunable time delay matching between the reference signal and the SI signal can be achieved by dispersion induced RF time delay. Meanwhile, the equal amplitude, phase reversal, and dispersion-induced SOI power fading compensation conditions are satisfied simultaneously by manipulating the optical carrier phase of the CPS-DSB signal. For 25 km fiber transmission, 30 dB cancellation depth over 2.7 GHz bandwidth, 160 Mbit/s 16-quadrature amplitude modulation (16-QAM) SOI recovery, and good immunity to fiber dispersion are experimentally demonstrated. In addition, the proposed scheme facilitates multipath SIC by implementing an array of tunable lasers (TLs) with a shared electro-optic modulator at the reference branch.

True Time Delay Optical Beamforming Network Based on Hybrid Inp-Silicon Nitride Integration

We demonstrate a broadband and continuously tunable 1×4 optical beamforming network (OBFN), based on the hybrid integration of indium phosphide (InP) components in the silicon nitride (Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) platform. The photonic integrated circuit (PIC) comprises a hybrid InP-Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> external cavity laser, a pair of InP phase modulators, a Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> optical single-sideband full carrier (SSBFC) filter followed by four tunable optical true time delay lines (OTTDLs), and four InP photodetectors. Each OTTDL consists of eight cascaded thermo-optical micro-ring resonators (MRRs) that impose tunable true time delay on the propagating optical signals. The OBFN-PIC is designed to facilitate the steering of a microwave signal with carrier frequency up to 40 GHz over a continuous set of beam angles. We evaluate the performance of the OBFN-PIC to handle and process microwave signals, measuring the link gain, the noise figure (NF), and the spurious-free dynamic range (SFDR) parameters. Moreover, we assess its beamforming capabilities assuming that the OBFN-PIC is part of a wireless system operating in the downlink direction and feeds a multi-element antenna array. Using microwave signals at 5 and 10 GHz with quadrature amplitude modulation (QAM) formats at 500 Mbaud, we evaluate the performance of the OBFN-PIC under various configurations. We show that error-free performance can be achieved at both operating frequencies and for all the investigated beam angles ranging from 45° to 135°, thus validating its potential for high-quality beamforming performance.

Time-delay signature concealing electro-optic chaotic system with multiply feedback nonlinear loops

A novel time-delay signature (TDS) concealing electro-optic (EO) chaotic system with multiply feedback nonlinear loops is proposed and analyzed by numerical simulation. The proposed system employs mutual injection structure implemented by two asymmetric branches named as multiply feedback nonlinear loop which introduces an extra nonlinear factor to the system dynamic equation. The complexity of the chaos system is increased by introducing this multiply feedback nonlinear loop. The permutation entropy (PE) of the proposed system is improved to higher than 0.96 when feedback strength (β) equals 5. The proposed system can enter to chaos regime with a small β (β = 0.8). The TDS is concealed effectively due to the extra nonlinear factor introduced by multiply feedback nonlinear loop. Meanwhile, key-space of the proposed system is about 1012 times that of the classical EO system because more tunable time delay parameters are introduced. Furthermore, the performance of a secure communication system based on the proposed chaotic system is discussed, and the simulation results show that the system is sensitive to time delay parameters and robust to feedback strength, which proves the proposed system is suitable for secure communication.

Tunable Electromagnetically Induced Transparency-Like in Graphene metasurfaces and its Application as a Refractive Index Sensor

We present numerical and theoretical analysis of the tunable electromagnetically induced transparency-like (EIT-like) effect based on graphene metasurfaces. The unit cells of metasurfaces are composed of a pair of parallel graphene strips and a two-split rectangular graphene ring resonator, both of which are performed as bright modes. The physical mechanism behind the EIT-like effect results from the frequency detuning and hybridization coupling of two bright modes. The FDTD simulation results show an excellent agreement with the theoretical analysis based on the coupled Lorentz oscillators model. Moreover, the transparency window of EIT-like effect can be modulated not only by changing the geometry of the nanostructure, but also by adjusting the Fermi level of graphene, allowing for an actively tunable group time delay of the light. In addition, owing to the peak frequency of the transparency window is highly sensitive to the variation of refractive index of the surrounding media, we demonstrate a refractive index sensor with sensitivity of ∼ 6800 nm/RIU, and the calculated FOM can reach up to about 14.2. Therefore, our proposed nanostructure provides a feasible platform for slow light and sensing applications.

Mode hybridization analysis in thin film lithium niobate strip multimode waveguides

Mode hybridization phenomenon in air-cladded X-cut Y-propagating and Z-propagating thin film lithium niobate strip multimode waveguides is numerically studied and a mathematical relation between structural parameters leading to hybrid modes is formulated. Dependence of hybrid modes on waveguide dimensions, sidewall angles and wavelength is also analyzed. The results obtained are used to design lithium niobate on insulator (LNOI) taper for converting fundamental TM mode to higher order TE mode, and an optimum length for achieving a high conversion efficiency of 99.5% is evaluated. Birefringent Y-propagating LN and isotropic Z-propagating LN tapers are compared in terms of length, figures of merit, and fabrication tolerance. Tapers exhibit a broad bandwidth of 200 nm with an extinction ratio less than − 18 dB. The results of mode hybridization analysis are useful in design optimization of adiabatic tapers, tunable time delays, optical interconnects, mode converters and demultiplexers for mode division multiplexing (MDM) applications.

Tunable chiral-selective electromagnetically induced transparency-like effect in VO2-based metamaterial for slow light applications

A chiral metamaterial based on vanadium dioxide (VO2) is proposed to realize the ultrafast active control of an electromagnetically induced transparency-like (EIT-like) effect. The design of the tunable metamaterial is based on the heat-induced insulator-to-metal transition of VO2 and a tunable transmission peak with circular polarization selectivity is achieved. For the right circular polarization (RCP) laser incidence, the sharp EIT-like peak is generated and rises with the increasing signal powers, which has not happened in the left circular polarization (LCP) incident condition. The spin-dependent phenomenon in time domain is also observed and the slow light effect in the metamaterial yields a tunable time delay up to 4 ns. Therefore, the tunable EIT-like effect with chiral selectivity is presented and highlights its practical applications in programmable metamaterial regions and slow light devices, etc.

LNOI waveguide grating based true time delay line for tunable bandpass microwave photonic filter

A precisely tunable true time delay line (TTDL) based on lithium niobate on insulator waveguide grating is proposed. Utilizing the electro-optic effect of lithium niobate, active tuning of reflected wavelength and time delay can be realized. The numerical simulation results show that, with the optimized grating structure and flexible electrode distribution, wide tuning range and high tuning accuracy of time delay can be achieved by simply controlling the charging position of the electrodes. This TTDL is applied to a tunable bandpass microwave photonic filter (TBMPF), which using PM-IM conversion to suppressed dc signal to achieve bandpass. In the TBMPF, the TTDL plays the role of introducing the dispersion for PM-IM conversion, time delay between the taps and tuning of central frequency. This TTDL can reduce the system complexity and cost of microwave photonic filter.

Two-stage γ ray emission via ultrahigh intensity laser pulse interaction with a laser wakefield accelerated electron beam

In this study, we investigate the generation of twin γ ray beams in the collision of an ultrahigh intensity laser pulse with a laser wakefield accelerated electron beam using a particle-in-cell simulation. We consider the composed target of a homogeneous underdense preplasma in front of an ultrathin solid foil. The electrons in the preplasma are trapped and accelerated by the wakefield. When the laser pulse is reflected by the thin solid foil, the wakefield accelerated electrons continue to move forward and pass through the foil almost without influence from the reflected laser pulse or foil. Consequently, two groups of γ ray flashes, with tunable time delay and energy, are generated by the wakefield accelerated electron beam interacting with the reflected laser pulse from the foil as well as another counter-propagating petawatt laser pulse behind the foil. Additionally, we study the dependence of the γ photon emission on the preplasma densities, driving laser polarization, and solid foil.

Generation of two terahertz radiation pulses with continuously tunable frequency and time delay

We propose to generate two narrow band terahertz pulses radiated from two temporally modulated relativistic electron beams, which are generated in a photo-injector. The temporal profile of the drive laser is modulated by means of the paired chirped pulses beating technique, leading to the generation of two pre-bunched electron beams. Coherent transient radiation (CTR) is considered as the mechanism for terahertz radiation generation. The frequencies of the two terahertz pulses can be independently tuned by adjusting the paired beating frequencies, and the interval between the two terahertz pulses can be adjusted by the optical delay line.

Ring-core few-mode fiber for tunable true time delay line operation.

We propose, for the first time to our knowledge, tunable true time delay line operation for radiofrequency signals on a few-mode fiber link. In particular, the custom design of a 7-LP-mode ring-core few-mode fiber together with a set of 5 broadband long period gratings inscribed at the proper positions along the fiber allows 4-sample true time delay line tunability over a 20-nm optical wavelength range. We study the performance of the designed true time delay line in the context of reconfigurable microwave photonics signal processing by theoretically evaluating microwave signal filtering and optical beamforming networks for phased array antennas.

Photoacoustic thermorelaxation microscopy for thermal diffusivity measurement.

Thermal diffusivity is one of the main parameters to characterize the thermo-physical properties of materials, and advances in its measurement technique will have significant impact on materials science and related applications. Here a photoacoustic (PA) thermorelaxation microscopy is proposed as a new noncontact method to measure the thermal diffusivity. By delivering co-focused heating/probing laser pulse pairs with tunable time delays, the sample's in situ thermal relaxation behavior after the heating pulse excitation can be photoacoustically monitored based on the temperature-dependent property of the Grueneisen parameter. We theoretically deduced the dependence of the obtained PA thermorelaxation time on the thermal diffusivity, and the results coincided well with simulations. The feasibility of this method was validated by various industrial and biological samples. This method provides a new strategy for high-resolution thermal diffusivity measurement with flexible measurement conditions, prefiguring great potential for material and biological applications.

All optical control of comb-like coherent acoustic phonons in multiple quantum well structures through double-pump-pulse pump-probe experiments.

We present an advancement in applications of ultrafast optics in picosecond laser ultrasonics - laser-induced comb-like coherent acoustic phonons are optically controlled in a In0.27Ga0.73As/GaAs multiple quantum well (MQW) structure by a high-speed asynchronous optical sampling (ASOPS) system based on two GHz Yb:KYW lasers. Two successive pulses from the same pump laser are used to excite the MQW structure. The second pump light pulse has a tunable time delay with respect to the first one and can be also tuned in intensity, which enables the amplitude and phase modulation of acoustic phonons. This yields rich temporal acoustic patterns with suppressed or enhanced amplitudes, various wave-packet shapes, varied wave-packet widths, reduced wave-packet periods and varied phase shifts of single-period oscillations within a wave-packet. In the frequency domain, the amplitude and phase shift of the individual comb component present a second-pump-delay-dependent cosine-wave-like and sawtooth-wave-like variation, respectively, with a modulation frequency equal to the comb component frequency itself. The variations of the individual component amplitude and phase shift by tuning the second pump intensity exhibit an amplitude valley and an abrupt phase jump at the ratio around 1:1 of the two pump pulse intensities for certain time delays. A simplified model, where both generation and detection functions are assumed as a cosine stress wave enveloped by Gaussian or rectangular shapes in an infinite periodic MQW structure, is developed in order to interpret acoustic manipulation in the MQW sample. The modelling agrees well with the experiment in a wide range of time delays and intensity ratios. Moreover, by applying a heuristic-analytical approach and nonlinear corrections, the improved calculations reach an excellent agreement with experimental results and thus enable to predict and synthesize coherent acoustic wave patterns in MQW structures.

Wideband and low-error microwave frequency measurement using degenerate four-wave mixing-based nonlinear interferometer.

In this Letter, we report a wideband and low-error photonic instantaneous frequency measurement (IFM) system for microwave signals, which uses a nonlinear interferometer constructed with cascaded degenerate four-wave mixing (FWM) and in-between dispersion to introduce a tunable time delay. The microwave frequency is interpreted through interference of two idlers generated in cascaded FWM stages. This new approach allows parallel implementation of multiple IFMs without an overlap of idler wavelengths used for different IFMs, which is crucial to realize a wide frequency coverage and high accuracy without compromising the system practicality. A large, 40GHz measurement bandwidth and a low measurement error of approximately 0.4% have been successfully achieved.

Adaptive optical self-interference cancellation for in-band full-duplex systems using regular triangle algorithm.

In this paper, we propose an adaptive optical self-interference cancellation using regular triangle algorithm for in-band full-duplex systems. By using this algorithm, the manual adjustment of the tunable optical time delay line and attenuator is replaced with the adaptive program to change the delay and attenuation for achieving optimal cancellation point. The adjustment process is simplified as a convex function problem. We choose to attain the optimal cancellation point by directly and continuously sampling the power of the signal after cancellation and in turn adjust the time delay and attenuation according to the algorithm. In this way, the two paths in the self-interference cancellation system are precisely and automatically matched. By using our proposed algorithm, the interference signal over 300-MHz wideband is diminished to the noise floor, attaining 20-25 dB cancellation depth adaptively. Compared with other existing algorithms in both the experiment and simulation, our proposed regular triangle algorithm reaches the optimal point faster with 10-30% less number of samples from the near start region, and lowers 40-60% less number of samples from the farther start region.

Optical Beamforming Network for Multi-Beam Operation With Continuous Angle Selection

We propose an optical beamforming network (OBFN) for multi-beam operation and continuous selection of the beam angles. The OBFN is based on the use of wavelength-division multiplexing elements for the generation and manipulation of the signals that correspond to different beams and different antenna elements (AEs), and on the use of optical delay lines for the introduction of tunable time delays to these signals. We analyze the operating principle of the proposed OBFN in the downlink and uplink directions of a wireless system, and we experimentally demonstrate its proof of concept in the downlink direction up to the photodetection stage, using a $2\times 4$ setup based on commercial components. The implemented OBFN has the potential to support the steering of two beams and the excitation of four AEs. Using microwave signals with 15-GHz carrier frequency, high-order modulation formats, and symbol rates up to 1 Gbaud, we modulate two optical carriers and investigate the performance of the OBFN in four experimental cases, employing different combinations of signals, steering angles, and antenna excitation schemes. Through the analysis of the expected radiation pattern and off-line measurements of the BER of the decoded signals at the intended steering angles, we validate the feasibility of the proposed OBFN. The vision for actual implementations of the proposed OBFN would be enabled by means of photonic integration techniques.

Microwave Photonic Devices Based on Liquid Crystal on Silicon Technology

This paper reviews the recent developments in microwave photonic devices based on liquid crystal on silicon (LCOS) technology. The operation principle, functions and important specifications of an LCOS based optical processor are described. Three microwave photonic devices, which are microwave photonic notch filters, phase shifters and couplers, reported in the past five years are focused on in this paper. In addition, a new multi-function signal processing structure based on amplitude and phase control functions in conjunction with a power splitting function in a commercial LCOS based optical processor is presented. It has the ability to realize multiple time -shifting operations and multiple frequency-independent phase shifting operations at the same time and control multiple RF signal amplitudes, in a single unit. The results for the new multi-function microwave photonic signal processor demonstrate multiple tunable true time delay and phase shifting operations with less than 3 dB amplitude variation over a very wide frequency range of 10 to 40 GHz.

Generating attosecond x-ray pulses through an angular dispersion enhanced self-amplified spontaneous emission free electron laser

We propose a scheme that can generate attosecond x-ray pulses through an angular dispersion enhanced self-amplified spontaneous emission (ESASE) free electron laser (FEL). In this scheme, an angular dispersion section is implemented into a conventional ESASE set-up, together with a few cycle laser pulse to manipulate the electron beam effectively. During the procedure, the transverse-longitudinal phase space coupling is introduced into the electron beam in addition to the energy modulation and momentum compaction processes. As a result, the peak current of the electron beam will be strengthened significantly within a narrow width of the electron bunch, which can be used to generate ultrashort FEL pulses. Comparing to a conventional ESASE scheme, the proposed scheme can reduce the required modulation laser power by at least one order of magnitude. The signal-to-noise ratio of the final FEL radiation pulses can also be improved. In addition, we discuss the possibility of using this scheme to generate two-pulse two-color x-ray FEL with tunable central wavelengths and time delays for a conceivable x-ray pump/x-ray probe experiment.