Rectangular Spectrum Research Articles

High SBS Threshold Fiber Laser With Spectral Management Using Binarized Multi-Tone Signal Modulation

In the high-power narrow-linewidth continuous fiber laser system based on the master oscillator power amplifier (MOPA) structure, the linewidth and spectral shape of the seed source are directly related to the SBS effect threshold which limits the maximum output power. It is currently under investigation which spectral shape has the highest laser power threshold. In this study, the spectral broadening and shape control of seed sources based on high-order phase modulation of binarized multi-tone signals is proposed. Combined with the heterodyne detected-based high-precision spectral measurement method, the measurement accuracy is improved from GHz to MHz level, which assists in optimizing the spectral shape. The broadened seed sources with rectangular, triangular, concave, and super-Gaussian spectra are experimentally demonstrated. And a maximum power of 2234 W is obtained with a rectangular spectrum with 10-GHz bandwidth. Theoretically, this scheme has multi-dimensional spectra tuning ability and can achieve arbitrary spectral types to explore the optimal seed source spectrum in diverse applications.

Generation of broadband flat millimeter-wave white noise using rectangular ASE slices mixing

Millimeter-wave white noise enables promising applications in MMW communication systems. However, current generation methods for this type of source suffer from small bandwidth and uneven spectrum. Herein, we propose and demonstrate an effective method for flat millimeter-wave white noise generation by slicing the broadband spectrum of amplified spontaneous emission (ASE) into two rectangular spectra and mixing on a photodetector. The bandwidths of the two rectangular ASE slices used for optical mixing are key factors affecting the bandwidth and power level of the flat-spectrum noise. As a feasible demonstration, the MMW white noise with a flatness of ±1.75 dB (@130–170 GHz) was experimentally achieved by mixing two rectangular ASE slices with bandwidths of 20 GHz and 60 GHz, respectively. Our proposed scheme takes full advantage of the bandwidth of high-speed photodetector and promises great potential for generating white noise with substantial flatness in the millimeter-wave and even terahertz bands.

Optimizing the spectrum of high power narrow linewidth fiber amplifier through the same complex degree of coherence

The spectra of narrow linewidth fiber amplifiers are closely related to both the stimulated Brillouin scattering (SBS) threshold in power scaling process and the combining efficiency in coherent beam combining (CBC) system. In this manuscript, the SBS thresholds of fiber amplifier with different spectral distributions (Gaussian, sinc2 and rectangular) have been compared under the same spectral complex degree of coherence (CDC), which could promise the same combining efficiency in CBC system. A SBS dynamic model is established to analyze the SBS process in fiber amplifier and a comparing experiment is also performed by measuring the SBS thresholds of different spectra that have the same CDC set to be 0.96. The FWHM linewidths of Gaussian, sinc2 and rectangular spectra are adjusted to be 1.1 GHz, 0.5 GHz and 1.06 GHz, respectively. The corresponding SBS thresholds are measured to be 108 W, 77 W, and 135 W. By contrast, the rectangular spectra could have most excellent capacity on improving SBS threshold in fiber amplifier under the same combining efficiency in CBC system. Overall, it could provide a feasible method on spectra designing in high power narrow linewidth fiber amplifiers used in CBC system.

Time advancement and distortionless of optical pulses via stimulated Brillouin scattering by broadband pump in optical fibers

The broadening Brillouin absorption spectrum of stimulated Brillouin scattering (SBS) in optical fibers by multiple monochromatic pumps is investigated. The results show that the bottom of the effective absorption spectrum becomes flatter with increasing the number of spectrum lines, and the spectral bandwidth is effectively expanded by multiple monochromatic pumps. but the spectral spacing should be tailored reasonably to ensure a broad linear range of the refractive index to reduce the pulse distortion. The time advancement and the pulse distortion induced by SBS pumped by broadband Gaussian and rectangular spectra are demonstrated respectively. The simulation results indicate that the pulse distortion induced by broadband rectangular spectral pump is smaller than that of broadband Gaussian spectral pump, and the time advancement can be improved effectively by increasing signal light power. We believe that these results are very promising for designing optical buffering or sensing components based on SBS fast light.

Physical-layer impairment estimation for arbitrary spectral-shaped signals in optical networks.

In long-haul fiber-optic networks, precise modeling of physical-layer impairments (PLIs) is crucial to optimizing network resource usage while ensuring adequate transmission quality. In order to accurately estimate PLIs, many mathematical models have been proposed. Among them, the so-called Gaussian noise (GN) model is one of the most accurate and simple enough to use on complex continental-size networks. However, the closed-form GN model assumes that the signals can be represented as having rectangular spectra, leading to a significant estimation error in typical cases when this assumption is violated. We propose the component-wise Gaussian noise (CWGN) PLI model that can account for arbitrary spectral-shaped demands. The CWGN model is computationally simple and suitable for most network management approaches. Results indicate that the CWGN model can prevent as much as a 136% overestimation of the PLIs resulting from the closed-form GN model applied to network lightpaths containing cascaded filters.

On the Optimization of Spread Spectrum Chirps Into Arbitrary Position and Width Pulse Signals. Application to Ultrasonic Sensors and Systems

This work presents in detail a new method for the optimization of rectangular spread spectrum excitation signals for its use in simple and low-cost ultrasonic pulsers, but that could be extended to other applications based on spread spectrum signals. Starting from rectangular linear frequency modulated (RLFM) chirps, it uses the transfer function of the transmitted signal and received echoes from reference specimens to iterate through a recursive algorithm to obtain arbitrary position and width pulse (APWP) signals with the desired bandwidth, maximizing the energy and the flatness of the spectrum, and enhancing the resolution and dynamic range of conventional chirp excitation signals. This optimization procedure can be repeated for any transducer and material, so that it achieves the best performance in each experimental environment. Such characteristics are ideal for Time-of-Flight estimation, imaging, and applications in which spectral regularity is needed, such as the Split Spectrum Processing (SSP) algorithm, which is used as example to test the performance of the proposed excitation signals. It also allows to specify and change the bandwidth reducing the need to change the transducers. The method is tested with different transducers (2 and 5 MHz focused transducers) and complex composite materials (aluminum-carbon aviation composite and high porosity GFRP composite) in immersion setup for imaging applications using SSP algorithm.

Generation of cylindrical vector dissipative soliton using an ultra-broadband LPFG mode converter with flat conversion efficiency

Dissipative solitons (DSs), which are pulses with high pulse energy and a broad rectangular output spectrum, have several applications. In a fiber system, the generation of higher-order DS requires a mode converter with a high and flat conversion efficiency that can cover the working waveband. The requirements on high and flat conversion efficiency as well as broadband working waveband is a great challenge for a traditional mode converter. This study presents an L-band mode-locked fiber laser with a 3-dB bandwidth of more than 47 nm and a first-order cylindrical vector (CV) mode DS output based on an ultra-broadband long-period fiber grating (LPFG). The fabricated mode converter is a novel cascading LPFG with an ultra-broadband working bandwidth and flat conversion efficiency which could meet the demand of DS mode conversion. The 15-dB bandwidth is 149.76 nm, from 1515.36 nm to 1665.12 nm. Furthermore, the conversion efficiency in the range of the output spectrum is 98.60 ± 0.23%. The single-, double-, and triple-soliton output characteristics and their real-time spectra are observed using time-stretched dispersion Fourier transformation (TS-DFT) technology. Experimental results demonstrate that the first-order CV mode DS laser can maintain its dynamics after mode conversion under the accuracy of detection system, implying that the LPFG works only as a mode converter. The broadband DS laser with CV mode output in the L-band has been presented for the first time in study, to the best of our knowledge. This study is also the first to detect real-time characteristics for a higher-order mode-locked fiber laser using the TS-DFT technology. The proposed type of LPFG is s good candidate for generating higher-order mode lasers with an all-fiber structure.

Agnostic sampling transceiver.

Increasing demands for data centers, backbone, access, and wireless networks require inventive concepts to transmit and distribute digital or analog signal waveforms. We present a new, extremely simple transceiver concept, fundamentally different from conventional approaches. It does not rely on high-speed electronics and enables transmission of various time multiplexed analog waveforms or digital data signals with the maximum possible symbol rate in the same rectangular optical spectral band B. The aggregate symbol rate of N signal channels corresponds to B or twice the used modulator's electro-optical bandwidth. By a modification of the system, it can be increased to three times the modulator bandwidth. The rectangular spectra can be further multiplexed into wavelength-superchannels without guardbands. To time demultiplex single signal channel, just another intensity modulator and a detector with an electrical bandwidth corresponding to the channel's baseband width (B/(2N)) is required. No optical filter, high-speed signal processing, or unconventional photonic devices are needed; thus, it has the potential to be easily integrated into any platform and provides an economical and energy-efficient solution for future communication networks and microwave photonic links.

Analysis of Non-Idealities in the Generation of Reconfigurable Sinc-Shaped Optical Nyquist Pulses

Optical sinc-shaped Nyquist pulses are widely used in microwave photonics, optical signal processing, and optical telecommunications due to their numerous advantages, like rectangular shape in the frequency domain, the orthogonality and the consequential possibility to use these pulses to transmit data with the maximum possible symbol rate. Ideal sinc pulses with the rectangular spectrum are just a mathematical construct. However, high-quality sinc pulse sequences offer the same advantages and can be generated by a phase-locked rectangular frequency comb with mode-locked lasers, intensity modulators, and integrated devices. Nevertheless, any non-idealities in the pulse and comb generation might lead to a degradation of the system performance, especially for metrology. Here, we investigate and analyze the effect of three major non-idealities, namely, the roll-off factor, the side band suppression ratio (SSR), and the ripple of sinc-shaped reconfigurable optical Nyquist pulse sequences based on 3, 5, and 9-line optical phase-locked frequency combs. We compare these results with the existing literature for the three-line comb followed by the experimental verification of the simulation results. We illustrate that by increasing the number of comb lines, the pulse sequences have superior performance and contribute to lesser root-mean-square (r.m.s.) error. We also discuss the trade-off between the r.m.s. error and the optical power loss for increasing the SSR.

Диффузионная динамика ридберговских состояний в поле излучения с непрерывным спектром

Redistribution of population and ionization of atomic Rydberg states due to multistep cascade transitions between states under the influence of radiation with continuous spectrum is considered. Consideration is carried up both by simulation of kinetic equations for state populations and with the use of a diffusion approximation based on the Fokker-Planck equation. On the example of sodium atom under the influence of radiation with rectangular spectrum a good agreement between the diffusion approximation and the kinetic equation simulation is shown. In the frame of the diffusion approximation some estimations are derived for the state populations, ionization rates and minimal spectral density of radiation necessary for significant ionization.

A modified 160 Gbit/s OTDM system based on QPSK Nyquist pulses and coherent detection

In this paper, we design and implement a 160 Gbit/s orthogonal time division multiplexing (OTDM) transmission system based on quadrature phase shift keying (QPSK) Nyquist pulses and coherent detection. We proposed an optimized Nyquist pulses generation scheme, which produce ultra-flat frequency combs with near-perfect rectangular spectrum and high suppression of out-of-band components by two cascaded Lithium Niobate Mach Zehnder modulators (LN-MZMs). Divided by variable length of optical delay line (Odl), 10GBaud QPSK Nyquist pulses are multiplexed to 80GBaud in a single wavelength. A set of training sequence (TS) which are inserted in front of data are employed to realize the inclusion of optical phase-locking and estimate channel transmission function. In this scheme, we make use of the TS overhead to achieve homodyne detection synchronization between local advanced receiver and Nyquist-pulses generator which is controlled by loop digital signal processing (DSP) module via an optical phase-locking loop. Meanwhile, for the advantage of the TS scheme and DSP, the Nyquist system compensates the residual physical dispersion and improves the transmission quality after 150 km transmission.

Reconfigurable optical generation of nine Nyquist WDM channels with sinc-shaped temporal pulse trains using a single microresonator-based Kerr frequency comb.

Sinc-shaped temporal pulse trains have a spectrally efficient, rectangular Nyquist spectrum. We demonstrate the simultaneous and reconfigurable optical generation of multiple Nyquist-shaped wavelength-division-multiplexed (WDM) channels having temporal sinc-shaped pulse trains as data carriers. The channels are generated through the insertion of coherent lines using cascaded continuous-wave amplitude modulation around the spectral lines of a microresonator-based Kerr optical frequency comb. For each of nine Kerr frequency comb lines, we insert sub-groups of uniform and coherent lines to generate nine WDM channels. The deviations from ideal Nyquist pulses for the nine channels at repetition rates of 6 and 2GHz are between 4.2%-6.1% and 2%-4.5%, respectively. Each WDM channel is modulated with on-off keying (OOK) at 6 Gbit/s. In addition, we show the reconfigurability of this method by varying the number of WDM channels, the generated sinc-shaped pulse train repetition rates, the duration, and the number of zero-crossings.

Synthesis of PEDOT:PSS using benzoyl peroxide as an alternative oxidizing agent for ESD coating and electro-active material in supercapacitor

Poly(3,4-ethylenedioxy thiophene) containing poly(styrenesulfonic acid) [PEDOT:PSS] is an important commercial polymer and most practical preparation method was developed at Bayer AG under the name ‘BAYTRON P synthesis’. In this work, we used benzoyl peroxide as an alternative oxidant for the preparation of PEDOT:PSS. Formation of PEDOT:PSS was confirmed from infrared, electronic absorption and XRD pattern. PEDOT:PSS was obtained in paracrystalline form with a very good pellet conductivity (6 S cm−1) and stability (up to 190 °C). Also, PEDOT:PSS was prepared in water as aqueous dispersion and this dispersion can be coated on various substrates. Electrochemical characterization of PEDOT:PSS electrode in aq.1 M H2SO4 solution was carried using cyclic voltammetry, charge-discharge and electrochemical impedance spectroscopy. Cyclic voltammetric pattern shows almost rectangular spectrum with specific capacitance value of 150 F g−1. One of the important outputs of this work, i.e. ESR value was very low (0.4 Ω) and found independent with cycle numbers.

Realization of both high-intensity and large-scale turbulence using a multi-fan wind tunnel

When experiments are conducted on the effects of freestream turbulence on the flow around bluff structures, generation of both high-intensity and large-scale turbulence in a wind tunnel is of major importance. We attempt this by developing a new type of wind tunnel named ‘multi-fan wind tunnel’ (MFWT), wherein airflow is driven by multiple fans. We focus on the use of shear layers generated between the outflows from adjacent ducts. Herein, we propose an efficient driving method in which an input signal composed of numerous sinusoidal waves is fed to each fan with quasi-random phases (‘random-phase mode’). It was confirmed that this mode generates approximately homogeneous turbulence within a short downstream distance. We found that pulsatile components were eliminated, which prevented the development of turbulence in our preceding experiments using the MFWT. The initial rectangular spectrum provided by forcing rapidly changes its shape as turbulence convects downstream, and asymptotes to a typical broad spectrum including a few decades of $$-5/3$$ power law. The intensity and the integral scale can be controlled by the amplitude of the input signal, and for an approximately isotropic region at large downstream stations, the intensity reaches 12–16% and the integral scale 0.6–0.7 m.

Потенциальная точность совместных оценок центральной частоты и ширины спектра сигнала с прямоугольным спектром

Потенциальная точность совместных оценок центральной частоты и ширины спектра сигнала с прямоугольным спектром

Application of Pulse Compression Technique in Fault Detection and Localization of Leaky Coaxial Cable

In this paper, an efficient technique in fault detection and localization of leaky coaxial cable (LCX) is presented. The approach utilizes a high-resolution reflectometry technique, which injects a reference signal into an LCX and handles the reflected signal to locate the fault. The reference signal adopts a linear frequency modulation signal by an approximately rectangular magnitude spectrum, which can ensure that the detection distance is large enough. And the pulse compression technique is used to improve the resolution of the reflected signal. Therefore, the approach can solve the contradiction between the detection distance and the high-resolution. The proposed method is verified by detecting various types of faults at different distances in MSLYFYVZ-50-9 LCX.

Rectangular optical filter based on high-order silicon microring resonators

The rectangular optical filter is one of the most important optical switching components in the dense wavelength division multiplexing (DWDM) fiber-optic communication system and the intelligent optical network. The integrated highorder silicon microring resonator (MRR) is one of the best candidates to achieve rectangular filtering spectrum response. In general, the spectrum response rectangular degree of the single MRR is very low, so it cannot be used in the DWDM system. Using the high-order MRRs, the bandwidth of flat-top pass band, the out-of-band rejection degree and the roll-off coefficient of the edge will be improved obviously. In this paper, a rectangular optical filter based on highorder MRRs with uniform couplers is presented and demonstrated. Using 15 coupled race-track MRRs with 10 μm in radius, the 3 dB flat-top pass band of 2 nm, the out-of-band rejection ratio of 30 dB and the rising and falling edges of 48 dB/nm can be realized successfully.

Nanotube mode locked, wavelength-tunable, conventional and dissipative solitons fiber laser

We report the generation of widely wavelength tunable conventional solitons (CSs) and dissipative solitons (DSs) in an erbium-doped fiber laser passively mode-locked by nanotube saturable absorber. The tuning ranges of CSs and DSs are ∼15 and ∼25 nm, respectively. In anomalous dispersion regime, the output CS exhibits symmetrical spectral sidebands with transform-limited pulse duration of ∼1.1 ps. In the contrastive case of normal dispersion regime, the DS has rectangular spectrum profile and large frequency chirp, which presents pulse duration of ∼13.5 ps, and can be compressed to ∼0.4 ps external to the cavity. This fiber laser can provide two distinct types of tunable soliton sources, which is attractive for practical applications in telecommunications.

Design of exposure system for the spectral sensitivity detection of the photosensitive material

To measure the spectral sensitivity of the photosensitive material, an exposure system for the spectral sensitivity detection of the photosensitive material is designed by use of commercial software of Code V and SolidWorks. The structural differences of the optical path are analyzed using line density of grating under diverse incident angle. The analysis result is that the incident angle of 25° and line density of 350 L/mm are optimum parameters of the exposure system. The experimental result in the system shows that the rectangular spectra of the size of 201.5 mm×80 mm can be achieved, the wavelengths cover the range of 360 nm~900 nm, the intensity of spectra can be adjusted in the range of 0.52 mW~10.65 mW when an 18 gray scale level of slices are inserted in the exposure system, and the resolution of spectra is 0.021 nm, which satisfies the requirement of the exposure system.

Effects of initial electron beam parameters of a linear accelerator on the properties of bremsstrahlung radiation in a radiotherapy setting

The dependence of the initial electron-beam parameters on absorbed dose distributions have been investigated using a CyberKnife radiotherapy accelerator (Accuray, United States). To describe the initial electron-beam characteristics, simulations of the linear electron accelerator are performed and the electron distributions in the beam of a linac output are analyzed. The radial distributions of electrons are assumed exponential, whereas the energy electron distributions are approximated by monoenergetic and rectangular spectra. There is no significant dependence of depth-dose curves in a phantom on the shape of the electron beam. Importantly, a clear dependence of the radiation field profile on the size of the electron beam is observed not just in the penumbra region, but also in the open part.