Baseband Frequency Response Research Articles

Sequence Domain Harmonic Modeling of Type-IV Wind Turbines

A sequence domain (SD) harmonic model of a grid-connected voltage-source converter is developed for decoupling converter generated voltage harmonics from voltage harmonics in the external grid. The modeling procedure includes a derivation of the baseband frequency response for regular-sampled pulse width modulation and an analysis of converter generated voltage harmonics due to compensated dead-time. The decoupling capabilities of the proposed the SD harmonic model are verified through a power quality (PQ) assessment of a 3 MW Type-IV wind turbine. The assessment shows that the magnitude and phase of low-order odd converter generated voltage harmonics are dependent on the converter operating point and the phase of the fundamental component of converter current, respectively. The SD harmonic model can be used to make PQ assessments of Type-IV wind turbines or incorporated into harmonic load flows for computation of PQ in wind power plants.

Influence of tunneling rays to baseband frequency response of step‐index plastic optical fibers

AbstractThe transmission bandwidth of a stepped‐index plastic optical fiber (POF) is found to vary significantly with the location of incidence of the light at the POF edge plane. In order to determine the cause, the effect of a tunneling leaky ray on the baseband frequency characteristics of the POF is studied both numerically and experimentally. It is found that the transmission bandwidth becomes narrower due to excitation of tunneling leaky rays if the light is incident from a light source with a wide radiating angle, such as that in a light‐emitting diode (LED), on a position far from the center on the POF edge plane. © 2005 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 88(8): 12–17, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.20166

Optical Transmission using Super Luminosity LEDs as a Transmitter and a Receiver

Recently, technologies of super luminosity LEDs make rapid progress and visible blight LEDs are now commercially available. In this paper, we investigate to clarify the potential and limitation of super luminosity LEDs for optical transmission applications. Both sending and receiving properties are measured, such as baseband frequency response of direct modulation, wavelength dependences of responsivity. The peak wavelength of emission power and the peak wavelength of responsivity are measured. The interesting phenomenon is found that the two peak wavelengths are different and the peak detection wavelength is shifted to shorter wavelength when compared to the emission peak wavelength. This phenomenon occurs in measured all LEDs and also in laser diodes. Using a simple model, this short wavelength shift is explained. Basic optical transmission experiments using super luminosity LEDs as a transmitter and a receiver are constructed to confirm the photo-detection properties of super luminosity LEDs. We also discuss possible applications of super luminosity LEDs in optical transmission systems.

New topologies of fiber-optic delay line filters

Novel fiber-optic delay line topologies are theoretically investigated in this paper. Nonrecirculating and recirculating delay line loop (NDLL and RDLL) topologies are investigated. Calculated results are presented for KDLL and RDLL structures showing, respectively, the narrow baseband and notch frequency responses achievable with these structures.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Equalization and mode partition noise in all-plastic optical fiber data links

High-speed, short-distance data transmission over all-plastic step-index fiber (plastic optical fiber, or POF) is normally considered to be limited by intermodal dispersion. Theoretical calculations in this letter show that the baseband frequency response can be significantly improved using simple linear equalization. However, as the fiber length is increased, the sharp absorption attenuation peaks of PMMA fiber can potentially introduce mode partition noise. The bounds on practical laser characteristics and fiber lengths are explored; these results indicate that 530-Mb/s transmission over 100 m of 1-mm-diameter POF is feasible, thus potentially satisfying a significant segment of future computer interconnect applications. >

Bandwidth and rise time calculations for digital multimode fiber-optic data links

The use of multimode fiber in digital fiber-optic data links requires simple and accurate procedures for the calculation of data line component bandwidth and rise time responses. The author investigates source/fiber/detector interactions, to develop a simple model for multimode fiber system information capacity. Simplified expressions for the baseband impulse and frequency responses of each component are given along with relationships between the theoretical impulse response root mean square (RMS) width and the component rise time or bandwidth. The application of the central limit theorem to the system yields expressions for the composite system rise time and bandwidth which contain correction factors to those that would be obtained using the normal Gaussian assumptions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

PDM: A versatile digital transmission system for video

This report discusses a pulse density modulation (PDM) video transmission system. PDM is a one-bit digital coding scheme with advantages of transparency due to an inherently extended signal bandwidth and relatively simple decoding; these make it ideal for a variety of applications. The system outlined is designed to carry a single analogue video channel, together with several sound channels, over a 140 or 280 Mbit/s digital transmission-link. An alternative option to the sound channels will be to carry a 2.048 Mbit/s data channel. Since the analogue video input will be PDM coded, this will enable the system to carry video formats which have a flat base-band frequency response requirement of in excess of 20 MHz (such as HDTV), allowing a large degree of flexibility in use.

Some new aspects of filters for filter banks

The present paper deals with the design of filters for use in filter banks, i.e., with certain constraints on their transition bands. The goal is to achieve a baseband frequency response of a back-to-back analysis/synthesis filter bank system that is ideally flat in magnitude and linear in phase. Using the proposed method, this is possible for conventional polyphase filter banks, single-sideband (SSB) filter banks, and generalized quadrature mirror filter (GQMF) banks. The method provides several options, one of them even yielding subsystems with linear phase. In contrast to most previous approaches to this topic, the described design procedure is an analytical one and is not based on optimizations or iterative algorithms. Furthermore, two families of window sequences are proposed for use in the design procedure mentioned above, but are also suitable for any windowed Fourier filter design.

Theoretical analysis of optical fiber mode exciters constructed with alternate concatenation of step‐index and graded‐index fibers

AbstractMode exciters which consist of alternate concatenation of step‐index (S) and graded‐index (G) fibers are used widely for the measurement of transmission characteristics (optical loss and baseband frequency response) of multimode fibers. This paper presents a theoretical analysis for the mode exciters based on geometrical ray optics approximation. The mode power distribution controlling effect of the exciters, excited mode power distribution in the test fiber, and accuracy and reproducibility of optical loss measurements are clarified theoretically. It is shown that optical loss measurements under steady‐state distribution can be done by using either SGS or GSG mode exciters which consist of either small core or low NA fibers.

Signal-monitoring characteristics for laser-diode optical switches

Signal-monitoring characteristics of terminal voltage change across laser-diode optical switches (LDSW) have been studied. The terminal voltage changes are caused by the photovoltaic effect in the OFF state and by the quasi-Fermi level change in the ON state. The baseband frequency response characteristics are analyzed by small-signal analysis and coincide well with the measured characteristics. The 6-dB down frequency response bandwidths are verified to be very wide in both the ON and OFF states.

Launching condition dependence of bandwidth in graded-index multimode fibers fabricated by MCVD or VAD method

Baseband frequency response of a single or concatenated MCVD and VAD fibers measured by two representative launching conditions-equilibrium mode distribution (EMD) and overfill mode distribution (OMD)-is studied. From computations based on the WKB approximation method and from experiments, it is found that the OMD exciter on the average results in a greater 6-dB bandwidth than the EMD exciter for a single MCVD fiber, but is does a smaller bandwidth for a single VAD fiber. On the other hand, in concatenated fibers, the EMD exciter results in a greater bandwidth of about 10 percent in comparison with the OMD exciter. The concatenation factors for MCVD fiber and VAD fiber are different for EMD excitation, while they are nearly identical with each other for OMD excitation.

Frequency characteristics of self-sustained intensity oscillation of a laser diode using a delayed electrical feedback

Frequency characteristics of self-sustained intensity oscillation of a laser diode, introduced by a delayed electrical feedback using an optical fiber, is investigated. A step-index multimode fiber is utilized as an optical delay line, and it is found that the spectral profile of the self-sustained intensity oscillation follows the baseband frequency response of the fiber.

The resultant dispersion of fiberguide link

The mathematical separation of multimode dispersion and material dispersion on the overall dispersion of an optical fiber will be shown in this letter. The relations derived can be used to evaluate the resultant baseband frequency response of a fiberguide with the use of light sources of various spectral bandwidth. The approximate description of the baseband frequency response characteristics of individual fiber sections permits one to estimate the resultant transmission bandwidth of the fiberguide line.

Baseband frequency characteristics of the multimode fiber whose axis suffers from random bends

The effects of random bends on the baseband frequency response of the multimode optical fiber are very important in practical communication systems. But their theoretical investigation has not been fully carried out. Therefore in this paper we theoretically investigate the baseband frequency characteristics of the multimode fiber whose axis suffers from random bends with the help of the variational method. As a result, we obtain the phase characteristics as well as the amplitude characteristics and indicate that the phase distortion can be neglected within the bandwidth. Also, we confirm the existence of the quasi‐steady state, in which the transmission characteristics are fairly stabilized, although the bandwidth is not always proportional to the inverse square root of the fiber length.

Transmission characteristics of splices in graded-index multimode fibers

Losses and baseband bandwidth changes caused by a splice in a graded-index multimode fiber have been studied theoretically. Splices are classified into three categories, and their influence on mode conversion coefficients and baseband frequency response has been investigated. Transmission characteristics for splices in cascade are also investigated.

Profile dispersion characteristics in high-bandwidth graded-index optical fibers

To study profile dispersion effects on transmission bandwidth in high-bandwidth graded-index fibers, baseband frequency responses of various graded-index fibers with different profile parameters have been characterized in the 0.78-1.50-microm wavelength range. The test graded-index fibers were made by the MCVD technique. Intrinsic intermodal dispersions of these fibers were determined using a combination of a GaAlAs laser and a grating monochromator, a mode-locked Nd:YAG laser, an InGaAsP/InP laser, and stimulated Raman scattering in a single-mode fiber as the light sources. Transmission bandwidth functional dependences on wavelength are presented. The transmission bandwidth of the individual fibers changed by more than 400% within the experimental spectral range. The maximum transmission bandwidth so far observed was 3.74 GHz.km at 1.225-microm wavelength for a fiber with 0.20 N.A. Transmission bandwidth spectra were compared with computer calculated results based on Olshansky's theory.

Mode conversion at splices in multimode graded-index fibers

Mode conversions occurring at a splice in multimode graded-index fibers are investigated theoretically, and their effects on impulse responses are verified experimentally. The relation describing the mode behavior at a splice in existence of a geometrical offset and fiber parameter mismatches is derived by taking a skew ray as well as meridional ray into consideration. Then the mode transfer matrix is obtained to determine the variations in mode power distribution and impulse response due to the mode conversions occuring at a splice. The measured mean delay time difference between lower and higher mode pulse responses and baseband frequency response for spliced graded-index fibers are compared with the theory obtained from the transfer functions of individual fibers and the mode transfer matrix which describes the mode conversions at a splice due to a transverse displacement and fiber parameter mismatches.

Splice loss and mode conversion in a multimode fiber

Splice loss and mode conversion have been studied theoretically for a multimode fiber. Equations have been derived using electromagnetic theory for calculating splice loss and mode conversion in the general multimode fiber. Numerical examples are given for a square-law index fiber in both arc-fusion splice and V-groove splice cases. The influence of the refractive-index profile at the splice on splice loss and mode conversion coefficient is shown. The change in baseband frequency response due to mode conversion at splice is calculated.

Baseband frequency response of a graded-index fiber excited by a step-index fiber

Input modal power distribution and baseband frequency response of a graded-index fiber have been investigated theoretically and experimentally, when the fiber was excited by a step-index fiber. It is found that the bandwidth of the graded-index fiber is measured with good reproducibility and accuracy by using the step-index fiber as an exciter. An appropriate choice of step-index fiber parameters is made with respect to the test graded-index fiber.

Oscillation properties of AlGaAs DH lasers with an external grating

External control of the longitudinal oscillation mode of AlGaAs double heterostructure lasers using a diffraction grating is presented. Dependence of threshold current on selected mode wavelength is observed experimentally and is compared with a theory based on multilongitudinal mode rate equations. Baseband frequency responses for various bias currents above threshold are estimated by small-signal analysis and coincide well with experimental observations. Resonance frequency shifts to lower frequency and the resonance width is broadened by the optical feedback. Theoretical and experimental step pulse responses are also studied. Single longitudinal mode builds up rapidly. Relaxation oscillation observed after the oscillation builds up is less damped when mode selective feedback is provided. Output fluctuation frequency corresponds to the resonance frequency.