Round-trip Frequency Research Articles

Engineering S, N-doped carbon nanosheets derived from thiazolothiazole-based conjugated polymer for efficient electrocatalytic oxygen evolution and Zn-air battery

Electrochemical oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) by using metal-free electrocatalysts is an emerging research interest for clean energy due to its cost affordability and eco-friendliness. However, only a few metal-free electrocatalysts are reported to exhibit robust OER and ORR performances, although OER and ORR are very important in various energy technology like metal-air batteries, fuel cells, etc. Herein, we have developed a rigid thiazolo [5,4-d] thiazole-based organic conjugated polymer, P-TzTz, and its polymer-derived N and S-doped carbon nanosheets for metal-free electrocatalytic OER and ORR. The polymer possesses a unique two-dimensional nanosheet structure with a moderate surface area of 72.6 m2/g and a wide range of pore sizes. The synthesized P-TzTz displayed a reasonable electrocatalytic OER performance with excellent long-term stability. The OER performance was further enhanced by pyrolyzing the P-TzTz at 800 °C to prepare the N and S-doped carbon nanosheet, P-TzTz-CNS800. The elevated surface area (228 m2/g) with uniform mesopores, enhanced electrical conductivity of P-TzTz-CNS800, and the presence of N, and S as the dopants for improving active sites made the P-TzTz-CNS800 a promising OER/ORR electrocatalyst as it showed the OER overpotential of 347 mV@10 mA/cm2 current density with more than 45 h long-range durability and 98% Faradic efficiency of oxygen evolution. Finally, to utilize the good OER efficacy of P-TzTz-CNS800-based metal-free electrocatalyst, a rechargeable Zn-air battery was fabricated as a proof of concept by taking P-TzTz-CNS800 as the air cathode. The Zn-air battery revealed a constant open circuit potential of 1.44 V and high charging/discharging stability over 40 h with a minimal loss in round trip frequency. Thus, the report corroborates the promising potential of P-TzTz derived N and S-doped carbon nanosheets as an efficient metal-free electrocatalyst as an air cathode for Zn-air battery for clean energy conversion.

Versatile Femtosecond Laser Synchronization for Multiple-Timescale Transient Infrared Spectroscopy.

Several ways to electronically synchronize different types of amplified femtosecond laser systems are presented based on a single freely programmable electronics hardware: arbitrary-detuning asynchronous optical sampling (ADASOPS), as well as actively locking two femtosecond laser oscillators, albeit not necessarily to the same round-trip frequency. They allow us to rapidly probe a very wide range of timescales, from picoseconds to potentially seconds, in a single transient absorption experiment without the need to move any delay stage. Experiments become possible that address a largely unexplored aspect of many photochemical reactions, in particular in the context of photo-catalysis as well as photoactive proteins, where an initial femtosecond trigger very often initiates a long-lasting cascade of follow-up processes. The approach is very versatile and allows us to synchronize very different lasers, such as a Ti:Sa amplifier and a 100 kHz Yb-laser system. The jitter of the synchronization, and therewith the time-resolution in the transient experiment, lies in the range from 1 to 3 ps, depending on the method. For illustration, transient IR measurements of the excited state solvation and decay of a metal carbonyl complex as well as the full reaction cycle of bacteriorhodopsin are shown. The pros and cons of the various methods are discussed, with regard to the scientific question one might want to address, and also with regard to the laser systems that might be already existent in a laser lab.

Ultra-narrow linewidth dual-cavity opto-mechanical microwave oscillator based on radial guided acoustic modes of single-mode fiber

A dual-cavity opto-mechanical microwave oscillator (OM-MO) for microwave photonic (MWP) generation with ultra-narrow linewidth based on radial (R) guided acoustic modes of a single-mode fiber (SMF) is proposed and investigated experimentally. The dual-cavity OM-MO consists of a 5 km SMF main ring, which provides forward stimulated Brillouin scattering (FSBS) gain, and a 300 m SMF subring that achieves single-frequency output of the R07 guided acoustic mode based MWP (R07-MWP) with Vernier effect. At 300 mW 980 nm pump threshold power, the 319.79 MHz R07-MWP is generated by adjusting polarization controllers based on nonlinear polarization rotation effect, corresponding to the 7437th harmonic of the 43 kHz cavity round trip frequency. The 3 Hz ultra-narrow linewidth of R07-MWP is achieved by decreasing the intrinsic linewidth of the passive ring resonator. The acoustic-mode and longitudinal-mode suppression ratios reach 22 and 36 dB, respectively. Within 20 min of the stability experiment, the power and frequency stability fluctuation of the R07-MWP are ±1 dB and ±0.05 MHz, respectively. This ultra-narrow linewidth MWP generation technology has great potential in the communication field, especially in long-distance wireless communication transmission.

The influence of built environment and socio-economic factors on commuting energy demand. A path analysis-based approach

Transport is the second energy consumer sector after housing in Algeria. In this article, we explore the ener- gy implication of commuting by considering a panel of socio-economic (SE) and built environment (BE) driving factors. The method is based on four steps: (i) The first step is to identify the main and potential drivers from the literature review and to propose a model that summarises the main assumptions that could explain the volume of commuting and the resulting energy consumption. (ii) In the second step, we designed and distributed 700 questionnaires in the municipality of Djelfa and retained 184 valid questionnaires in the final study sample. (iii) In the third step, we developed a method adapted to urban areas to quantify energy consumption as a function of the distance travelled, the type and density of occupation by means of transport and the type of fuel. (iv) The fourth step is to check the fit of the hypothetical model with a path analysis-based approach. The model developed identifies 15 factors, of which five have a direct impact and 10 have an indirect impact on the energy consumption of commuting. The model shows that building density and the age of the respondent can reduce the energy consumption of commuting by up to −15% and
 −12% respectively; whereas the number of cars by housing and the round-trip frequency could increase the energy consumption up to 38% and 27% respectively. Our results suggest a structuring role of the socio-economic characteristics of households in explaining the energy consumption of commuting.

InP integrated optical frequency comb generator using an amplified recirculating loop.

A novel realisation of photonically integrated optical frequency comb generation is demonstrated on indium phosphide (InP) using a generic foundry platform. The architecture, based on the amplified recirculating loop technique, consists of cascaded electro-optic phase modulators embedded within a short waveguide loop. While an injected continuous wave laser signal is recirculated by the loop, the modulators are driven with a modulation frequency corresponding to the round-trip loop length frequency. This results in many phase coherent, evenly spaced optical comb lines being generated. The choice of InP as an integration platform allows immediate optical amplification of the modulated signal by embedded semiconductor optical amplifiers, enabling loop losses to be compensated and expanding the comb across broad optical bandwidths. This approach reduces the requirement for external, high-power optical amplifiers, improving the compactness and power efficiency of the full system. The system was modelled to identify off-resonance behaviour, outlining limits in matching both the modulation frequency and seed laser frequency to the round-trip loop frequency for optimal comb line generation to be achieved. The experimental device occupied a fraction of the 6 x 2 mm2 InP chip and operated at round-trip loop frequencies of 6.71 GHz to produce 59 comb lines within a 20 dB power envelope. All comb lines exhibited strong phase coherence as characterised by low composite phase noise measurements of -105 dBc/Hz at 100 kHz. A second device is also presented with a shorter loop length operating at ∼10 GHz which generated 57 comb lines. Both loop configurations included short waveguide phase shifters providing a degree of tunability of the free spectral range with a tuning range of 150 MHz for small injection currents of < 2.5 mA.

Graphene saturable absorber mirror for passive mode-locking of mid-infrared QCLs

Passive mode-locking in quantum cascade lasers (QCLs) remains one of the huge challenges because of the fast relaxation time of the excited carriers which is typically in the range of sub-picoseconds. The use of conventional techniques such as the semiconductor saturable absorber mirror is inefficient because the spatial hole burning effect dominates the carrier dynamics. To overcome this effect, longitudinal transition structures with relaxation time around $$50~\mathrm {ps}$$ were proposed. However, mode-locking is assured with an external modulation at a cavity roundtrip frequency. In this paper, we demonstrate that a single-layer graphene used as a saturable absorber permits to generate stable pulses in such structures. The graphene is integrated with a highly reflective mirror to increase the internal electric field and achieve the saturation intensity. The dynamic of the QCL is modeled with Maxwell-Bloch equations while Maxwell-Ampere equation is used for the graphene layer by considering a nonlinear conductivity. This system of equations is solved using the one-dimensional Finite-Difference Time-Domain ( FDTD) method. To model the single-layer graphene of a $$0.33~\mathrm {nm}$$ thickness, a specific sub-cell is implemented based on Maloney method. Simulation results of a $${\mkern 1mu} 6.2~\mu {\text{m}}$$ QCL with a diagonal radiative transition show a generation of isolated pulses with a peak electric field of $$80~\mathrm {MV.m^{-1}}$$ and a duration of $$51~\mathrm {fs}$$ . The mode-locking remains stable for QCLs with a vertical transition having a relaxation time below $$5~\mathrm {ps}$$ .

Actively Mode Locked Raman Fiber Laser with Multimode LD Pumping

We present our recent experimental results on the pulsed regimes of Raman conversion of highly multimode laser diode (LD) pump radiation into the 1st and higher order Stokes radiation in multimode graded-index fibers. Three different linear cavities of Raman fiber laser with the modulation of losses (by acousto-optic modulator, AOM) or gain (by LD current) are explored and compared. An LD with wavelength of 976 nm is used for pumping enabling Raman lasing at wavelength of the 1st (1018 nm) and 2nd (1064 nm) Stokes orders. At ~27.2-kHz repetition rate corresponding to the laser cavity round-trip frequency (i.e., in the mode-locking regime), nanosecond pulses have been observed for both Stokes orders having the highest peak power of ~300 W in the scheme with bulk AOM and the shortest duration of 5–7 ns in the scheme with fiber-pigtailed AOM. At the same time, the beam quality of generated pulses is greatly improved as compared to that for pump diode (M2 &gt; 20) reaching the best value (M2 = 2.05) for the 2nd order Stokes beam in the scheme with the gain modulation and demonstrating also the most stable regime.

A Real-Time Matrix Iterative Optimization Algorithm of Booking Elevator Group and Numerical Simulation Formed by Multi-Sensor Combination

Elevators are an essential indoor transportation tool in high-rise buildings. The world is advocating the design concept of safety, energy-saving, and intelligence. We focus on improving operation speed and utilization efficiency of the elevator group. This paper proposed a real-time reservation elevator groups optimization algorithm, and a dynamic matrix iterative model has been established. The indoor navigation technology UWB is applied, which can help users to quickly find elevators. The manned equilibrium efficiency and running time equilibrium efficiency of elevator group are given. Moreover, the data filtering criterion formulas for user waiting time and elevator remaining space are defined. In this paper, three numerical examples are given. Example 1 is a single elevator in n-storey building. Example 2 is compared with different scheduling algorithms, such as FCFS, SSTF, LOOK, and SCAN algorithms, and the results show that our method has the advantages of short total running time and less round-trip frequency. At last, the matrix of numerical iteration results are visualized, and the data movement status of people on each floor can be observed. Example 3 introduced elevator group algorithms. For high-rise buildings, this paper adopts a high, medium, and low hierarchical management model; this model has high coordination, as well as fast response, batch process, and adaptive function. Finally, we also discussed and compared the complexity of single elevator and elevator group algorithms. Therefore, this method has great development potential and practical application value, which deserves further study.

Engineering the spectral bandwidth of quantum cascade laser frequency combs.

Quantum cascade lasers (QCLs) facilitate compact optical frequency comb sources that operate in the mid-infrared and terahertz spectral regions, where many molecules have their fundamental absorption lines. Enhancing the optical bandwidth of these chip-sized lasers is of paramount importance to address their application in broadband high-precision spectroscopy. In this work, we provide a numerical and experimental investigation of the comb spectral width and show how it can be optimized to obtain its maximum value defined by the laser gain bandwidth. The interplay of nonoptimal values of the resonant Kerr nonlinearity and cavity dispersion can lead to significant narrowing of the comb spectrum and reveals the best approach for dispersion compensation. The implementation of high mirror losses is shown to be favorable and results in proliferation of the comb sidemodes. Ultimately, injection locking of QCLs by modulating the laser bias around the round trip frequency provides a stable external knob to control the frequency-modulated comb state and recover the maximum spectral width of the unlocked laser state.

Sinusoidal Frequency-Modulated Waveforms Generated by a Phase-Modulated Frequency-Shifting Loop

We theoretically and experimentally investigate a phase-modulated frequency-shifting loop (PM-FSL) that includes an electro-optic phase modulator (EOPM) and a gain element, and is seeded by a single-frequency laser. By slightly detuning the modulation frequency of the EOPM off an integer multiple of the fundamental loop frequency, we generate an output waveform that exhibits a series of pulse doublets modulated at radio frequency (RF). We prove that the series consists of sinusoidal frequency-modulated (SFM) pulse doublets whose repetition rate and bandwidth are easily reconfigurable. We report the generation of the SFM waveforms with bandwidth above 7 GHz (limited by the detection bandwidth) by simply tuning the input RF tone over a span of a few kHz in the vicinity of 14.58 MHz (the round-trip frequency). The system is modeled using a time-delayed interference model that accounts for the modulation function of the EOPM, the loop delay time, and the detuning parameter. The model explains the formation of SFM pulse doublets and effectively reproduces all the experimental waveforms. This well-defined waveform may find applications in RF-optical signal processing and radar systems.

Stable downlink frequency transmission from arbitrary injection point with endless and quick phase error correction.

A stable frequency downlink transmission scheme, which delivers the frequency signal back to the central station from an arbitrary injection point along a radio-over-fiber (RoF) loop link, is proposed and demonstrated. The frequency signal at the arbitrary remote point is injected into the RoF loop link in both clockwise and counter-clockwise directions, simultaneously. The phase variation induced by the fiber loop link is obtained in real time with the help of a round-trip assistant frequency signal. The phase error can be exactly cancelled by a series of frequency mixing (i.e., up-conversion and down-conversion) among the signals. In the experiment, a 1.21-GHz frequency signal at an arbitrary remote point is downlink transferred to the central station in a 45-km fiber loop link. The result shows the overlapping Allan deviation (ADEV) of 1.04×10-12 at 0.1 s, 1.3×10-13 at 1 s and 1.1×10-15 at 104 s, respectively. The phase error correction operates entirely at the central station, leaving a simple and robust configuration of the remote site. No active adjusting part is integrated, and the all-passive compensation achieves an endless phase error correction range, as well as quick response to fiber delay changes.

Frequency comb investigation of monolithic mode‐locked GaSb‐based laser at 1.7 µm by heterodyne detection

In this work, monolithic mode-locked GaSb-based lasers at 1.7 µm emission wavelength with heterodyne measurement of the frequency comb are presented. Mode-locking operation was confirmed by measuring the optical beatnote at the round-trip frequency of 9.56 GHz and analysing the frequency comb. Mode-locking operation occurred from 0.1 to 3.3 V reverse bias voltages at the absorber section and driving currents from 160 to 400 mA at the laser section with a minimum beatnote width of 6.6 kHz. The frequency comb spans over 13 nm with a mode spacing of 0.10 nm, measured by heterodyne-detection with a distributed feedback laser.

Ultrafast response of harmonic modelocked THz lasers

The use of fundamental modelocking to generate short terahertz (THz) pulses and THz frequency combs from semiconductor lasers has become a routine affair, using quantum cascade lasers (QCLs) as a gain medium. However, unlike classic laser diodes, no demonstrations of harmonic modelocking, active or passive, have been shown in THz QCLs, where multiple pulses per round trip are generated when the laser is modulated at the harmonics of the cavity’s fundamental round-trip frequency. Here, using time-resolved THz techniques, we show for the first time harmonic injection and mode-locking in which THz QCLs are modulated at the harmonics of the round-trip frequency. We demonstrate the generation of the harmonic electrical beatnote within a QCL, its injection locking to an active modulation and its direct translation to harmonic pulse generation using the unique ultrafast nature of our approach. Finally, we show indications of self-starting harmonic emission, i.e., without external modulation, where the QCL operates exclusively on a harmonic (up to its 15th harmonic) of the round-trip frequency. This behaviour is supported by time-resolved simulations of induced gain and loss in the system and shows the importance of the electronic, as well as photonic, nature of QCLs. These results open up the prospect of passive harmonic modelocking and THz pulse generation, as well as the generation of low-noise microwave generation in the hundreds of GHz region.

Photon-Driven Broadband Emission and Frequency Comb RF Injection Locking in THz Quantum Cascade Lasers

We present homogeneous quantum cascade lasers (QCLs) emitting around 3 THz which display bandwidths up to 950 GHz with a single stable beatnote. Devices are spontaneously operating in a harmonic comb state, and when in a dense mode regime they can be injection locked at the cavity roundtrip frequency with very small RF powers down to -55 dBm. When operated in the electrically unstable region of negative differential resistance, the device displays ultra-broadband operation exceeding 1.83 THz (Δf/f=50%) with high phase noise, exhibiting self-sustained, periodic voltage oscillations. The low CW threshold (115 A·cm-2) and broadband comb operation (Δf/f=25%) make these sources extremely appealing for on-chip frequency comb applications.

On-Chip Dual-Comb Source Based on Terahertz Quantum Cascade Lasers Under Microwave Double Injection

High-power broadband terahertz dual-comb sources are of great importance for fast high-resolution spectroscopy, but are rare because of the lack of high-performance terahertz radiation sources. Here the authors demonstrate a broadband terahertz dual-comb source on a chip, based on electrically pumped quantum cascade lasers and microwave double injection. By injection locking the two lasers at slightly different round-trip frequencies, even weak microwave power can significantly broaden the dual-comb bandwidth. Furthermore, the double-injection technique allows direct evaluation of the carrier offset noise of the terahertz laser combs.

Resonances between fundamental frequencies for lasers with large delayed feedbacks.

High-order frequency locking phenomena were recently observed using semiconductor lasers subject to large delayed feedbacks. Specifically, the relaxation oscillation (RO) frequency and a harmonic of the feedback-loop round-trip frequency coincided with the ratios 1:5 to 1:11. By analyzing the rate equations for the dynamical degrees of freedom in a laser subject to a delayed optoelectronic feedback, we show that the onset of a two-frequency train of pulses occurs through two successive bifurcations. While the first bifurcation is a primary Hopf bifurcation to the ROs, a secondary Hopf bifurcation leads to a two-frequency regime where a low frequency, proportional to the inverse of the delay, is resonant with the RO frequency. We derive an amplitude equation, valid near the first Hopf bifurcation point, and numerically observe the frequency locking. We mathematically explain this phenomenon by formulating a closed system of ordinary differential equations from our amplitude equation. Our findings motivate experiments with particular attention to the first two bifurcations. We observe experimentally (1) the frequency locking phenomenon as we pass the secondary bifurcation point and (2) the nearly constant slow period as the two-frequency oscillations grow in amplitude. Our results analytically confirm previous observations of frequency locking phenomena for lasers subject to a delayed optical feedback.

Factors associated with round-trip carsharing frequency and driving-mileage impacts in London

This study draws on respondents from Greater London within the 2016/17 wave (n = 2640) of Britain’s Annual Survey of carsharing users, which we enrich with external data from the 2011 England and Wales Census and small-area income estimates. Focusing on round-trip carsharing users, we present multivariate analyses of frequency-of-usage of carsharing vehicles and impacts on annual vehicle miles traveled (VMT). Published attribute effects from other geographic and contextual circumstances are compiled and compared (where direct comparison is possible) with the specific attribute effects that we report in this paper. We demonstrate a statistically significant link between customer satisfaction with proximity of carsharing vehicles and VMT impacts. Car ownership (both current, and changes upon joining a carsharing service) is shown to have intuitive structural impacts. We find that frequent usage is associated ceteris paribus with increasing VMT after joining a carsharing service, and that subscribing to multiple types of carsharing is associated with frequent carsharing usage and a reduction in VMT. Interestingly, we did not find any significant effect of household income on either frequency-of-usage or VMT impacts.

High frequency modulation and injection locking of terahertz quantum cascade lasers

Due to intersubband transitions, the quantum cascade laser (QCL) is free of relaxations and able to work under fast modulations. In this work, the authors investigate the fast modulation properties of a continuous wave (cw) terahertz QCL emitting around 3 THz (∼100 μm). Both simulation and experimental results show that the 3 dB modulation bandwidth for the device can reach 11.5 GHz and the modulation response curve is relatively flat upto ∼16 GHz. The radio frequency (RF) injection measurements verify that around the laser threshold the inter-mode beat note interacts strongly with the RF signal and the laser can be modulated at the round trip frequency of 15.5 GHz.

5-ps-long terahertz pulses from an active-mode-locked quantum cascade laser

A significant research effort toward the generation of short terahertz (THz) pulses using quantum cascade lasers (QCLs) has been undertaken over the past few years. This is motivated by the desire to realize compact, electrically driven THz pulsed sources to study the dynamics of a variety of solid-state systems using ultrafast spectroscopy techniques, or to replace standard THz time domain spectroscopy systems for real-life applications. In this work we demonstrate the generation of 5-ps-long, transform-limited pulses by actively mode locking a 2.5 THz QCL. The pulse duration is a factor of 2 shorter than what has been reported to date with this technique, and has been achieved thanks to the use of a metal–metal waveguide, which favors lasing over a broader spectral bandwidth compared to previously employed single-plasmon waveguides. Active mode locking is obtained using a low-power radio-frequency (RF) signal to modulate the QCL current at the cavity round-trip frequency. The modulation does not affect the QCL emission spectrum. This is in striking contrast with previous reports of active mode locking where multimode emission was the direct consequence of the RF modulation.

Beat note analysis and spectral modulation of terahertz quantum cascade lasers with radio frequency injection

We demonstrate the electrical beat note analysis and radio frequency (RF) injection locking of a continuous wave (cw) terahertz quantum cascade laser (QCL) emitting around 3 THz (∼100 μm). In free running the beat note frequency of the QCL shows a shift of ∼180 MHz with increasing drive current. The beat note, modulation response, injection pulling, and terahertz emission spectral characteristics in the different current regimes I, II, and III are investigated. The results show that in the current regime I close to the laser threshold we obtain a narrower beat note and flat response to the RF modulation at the cavity round trip frequency. The pulling effect and spectral modulation measurements verify that in the current regime I the RF injection locking is more efficient and a robust tool to modulate the mode number and mode frequency of terahertz QCLs.