Bandwidth In Frequency Domain Research Articles

On the Connection Between Compactness of Pulse Shaping Function and ISI

This letter investigates the effect of compactness of a pulse shaping filter on the inter-symbol interference (ISI) by presenting a theoretical equation which directly expresses the bit error rate (BER) in terms of quantified compactness of the pulse. We numerically compute time-bandwidth product (TBP) of the considered pulses as the measure of compactness. To compute the TBP, we use four definitions of pulse width (i.e., time duration in time domain and bandwidth in frequency domain) including root mean square (RMS), full width at half maximum (FWHM), 99% energy concentration and - 40 dB bounded energy duration. We show that the standard pulses poses different compactness properties based on the defined TBP and the - 40 dB definition is the most effective definition to study ISI versus TBP. We also propose a new pulse which offers the least TBP based on - 40 dB definition among all standard pulses and show that this pulse outperforms these well-known pulses in terms of BER and bandwidth efficiency.

Two broadband chaotic signals generated simultaneously by semiconductor ring laser with parallel chaotic injection

Recently semiconductor ring laser (SRL) as a novel device has received much attention, for its special cavity allows the output light to propagate in two opposite directions, namely the clockwise mode and counterclockwise mode. SRL does not require gratings or cleaved facets for optical feedback and can be a candidate for small sized photonic integrated circuits which have been developed for secure data transmission, with chaotic carriers and high rate random bit generated. In this paper, we propose a method to obtain two broadband chaotic signals with high unpredictability degree by utilizing injected slave SRL and further explore the physical mechanism and injection conditions. Based on a conventional master-slave configuration, the proposed method obtains two modes of chaotic signals by master SRL with external cross feedback, which are injected in parallel to a slave SRL correspondingly. According to the well-known Lang-Kobayashi rate equations, we establish rate equations and numerically investigate the influences of frequency detuning and injection strength on bandwidth and unpredictability degree. We adapt the given definition of bandwidth and the normalized permutation entropy to respectively evaluate bandwidth and unpredictability degree of chaotic signals. Furthermore, we reveal the underlying physical mechanism of bandwidth enhancement and asymmetric bandwidth-enhancing region by analyzing the radiofrequency and optical spectra of intensity time series. The results show that two chaotic signals have similar routes to enhancing the bandwidth in frequency domain. In the unlocking injection area, two broadband and unpredictability-enhancing chaotic signals generated by slave SRL are simultaneously achieved by choosing appropriate control parameters. Analyses of optical spectra reveal that high-frequency periodic oscillation generated between injection chaotic signals and slave light via beating is the physical mechanism of bandwidth enhncment. The bandwidthenhancing domains of two chaotic signals are asymmetrical due to redshift of master SRL frequency, with external chaotic signals injected. Bandwidth-enhanced chaotic signals are easier to obtain in the domain of negative frequency detuning. The asymmetrical injections contribute to reducing the locking region and extending the bandwidthenhancing region under high injection strength. This conventional master-slave configuration composed of two SRLs can be easily implemented on chip and save other optical devices. The slave SRL subjected to parallel injection signals from master SRL can be used as a wideband unpredictability-enhancing chaotic source, which is extremely useful for the high capacity security-enhancing multiple chaotic communications, as well as for the potential applications of high speed random number generators.

Blind Deconvolution of Astronomical Images with a Constraint on Bandwidth Determined by the Parameters of the Optical System

Atmospheric turbulence severely restricts the spatial resolution of astronomical images obtained by a large ground-based telescope. In order to reduce effectively this effect, we propose a method of blind deconvolution, with a bandwidth constraint determined by the parameters of the telescope's optical system based on the principle of maximum likelihood estimation, in which the convolution error function is minimized by using the conjugate gradient algorithm. A relation between the parameters of the telescope optical system and the image's frequency-domain bandwidth is established, and the speed of convergence of the algorithm is improved by using the positivity constraint on the variables and the limited-bandwidth constraint on the point spread function. To avoid the effective Fourier frequencies exceed the cut-off frequency, it is required that each single image element (e.g., the pixel in the CCD imaging) in the sampling focal plane should be smaller than one fourth of the diameter of the diffraction spot. In the algorithm, no object-centered constraint was used, so the proposed method is suitable for the image restoration of a whole field of objects. By the computer simulation and by the restoration of an actually-observed image of α Piscium, the effectiveness of the proposed method is demonstrated.

Optical beat noise reduction in WDM-SCM access networks using modulated optical pulse train

We proposed a new method to suppress the optical beat interference (OBI) in wavelength-division-multiplexing/subcarrier-multiple-access network. The proposed scheme uses the modulated optical pulse train that has broad bandwidth in frequency domain. Since the OBI noise is inversely proportional to the optical bandwidth of beating optical signals, the generated OBI noise by beating of optical pulse train signals from several optical network units (ONUs) spreads over wide frequency range. In addition, the chance of optical beating is low because optical pulse trains from different ONU have different pulse timing. We have demonstrated the proposed idea experimentally. In beating experiment, the carrier-to-noise ratio was obtained over 30 dBc regardless of optical beating. In addition, 6-Ms/s 16 quadrature amplitude modulator data at 0.7 GHz was successfully transmitted up to 20 km with an average error-vector-magnitude of 4.5% in presence of optical beating.

B-ISDN waits for photonics to deliver

A variety of photonic switching techniques have been studied from the viewpoint of expanding switching-mode throughput. The basic issue discussed is how the high bandwidths of optical devices can be applied to switching nodes. It is shown that a high time-domain bandwidth allows high-speed signal transmission without waveform distortion, a high frequency-domain bandwidth allows a system to accommodate a large number of channels, which results in a high-capacity system, and a high space-domain bandwidth allows high-density interconnections in free space and resolves the pin-out bottleneck of LSIs and multichip modules (MCMs). Photonic space-division switching, photonic ATM switching, photonic frequency-division switching, and photonic free-space switching methods are described. >