Solitons in dispersion-compensated optical fibers with loss

A dispersion-compensated optical fiber is proposed for optical soliton propagation. The scheme is studied by a variational approach. Numerical analyses show that the proposed scheme can retain the soliton property for a much longer distance than normal systems. It is shown that, after 48 km, the pulsewidth in the proposed system remains similar to the initial pulsewidth, whereas in the normal system, the pulsewidth has already reached 10 times the initial value. It is demonstrated that significant improvements in soliton propagation characteristics can be achieved by properly cascading fibers with different degrees of group velocity dispersion in finite steps     

Analysis of random PWM switching methods for three-level powerinverters

This paper presents a generalized theory which covers both two-level and three-level random pulsewidth modulation (RPWM) schemes. Various three-level RPWM schemes with low switching frequency are presented and compared with two-level schemes. Three-level RPWM schemes have less discrete harmonics and continuous noise than two-level RPWM schemes. They have desirable spectral characteristics and can be employed in high-voltage inverter-fed motor drives. Measurements have confirmed the theory and the attractive features of three-level RPWM schemes     

All-Optical Clock Recovery Using Erbium-Doped Fiber Laser Incorporating an Electroabsorption Modulator and a Linear Optical Amplifier

We demonstrated a 10-GHz all-optical clock recovery system using an erbium-doped fiber laser that incorporates an electroabsorption modulator and a linear optical amplifier. Stable pulses with peak power of 200 mW and pulsewidth of 6 ps are obtained. The output power and the pulsewidth of the recovered clock pulses are independent of the input data pattern. Stable optical clock can still be observed when the input data rate varies by more than 60% of the fundamental frequency without any optical tunable delay line inside the laser cavity. The scheme is essentially wavelength transparent for the whole C-band which recovers clock pulses from input data wavelength between 1525 and 1565 nm     

Limit of optical pulsewidth in the gain-switched DFB semiconductorlaser

We investigate the minimum optical pulsewidth that is obtainable in the gain-switched distributed-feedback semiconductor laser. The minimum obtainable optical pulsewidth is determined by either the period doubling or the saturation of modulation efficiency. The shift of the resonance frequency to the lower frequency side with increase of the modulation current brings about these phenomena. It is predicted theoretically that the minimum obtainable pulsewidth is about 0.2 times the period of the small-signal resonance. The minimum pulsewidth after compression with a dispersion compensating fiber is also discussed. Experimental measurements are in good agreement with the theory     

Effect of thermal noise and APD noise on the performance ofOPPM-CDMA receivers

The effect of thermal noise and avalanche-photodiode (APD) noise on the performance of optical overlapping pulse-position modulation/code-division multiple-access (OPPM-CDMA) systems with and without double optical hardlimiters is examined. Comparisons with the corresponding optical on-off keying/CDMA (OOK-CDMA) systems are presented as well. The maximum data rate that can be achieved under laser pulsewidth and bit error rate constraints is determined. It is shown that about 10 dB increase in the average power, with respect to the Poisson shot-noise-limited system, is required to compensate for the performance degradation due to thermal noise and APD noise. Moreover, it is indicated that for a bit error rate not exceeding 10^-9, a laser pulsewidth of 0.03 ns, and an average received optical power of -55 dBm, a data rate of more than 3 Gb/s can be achieved per channel when using OPPM-CDMA systems with double optical hardlimiters     

A novel three-phase utility interface minimizing line currentharmonics of high-power telecommunications rectifier modules

Based on the combination of a three-phase diode bridge and a DC/DC boost converter, a new three-phase three-switch three-level pulsewidth modulated (PWM) rectifier system is developed. It can be characterized by sinusoidal mains current consumption, controlled output voltage, and low-blocking voltage stress on the power transistors. The application could be, e.g., for feeding the DC link of a telecommunications power supply module. The stationary operational behavior, the control of the mains currents, and the control of the output voltage are analyzed. Finally, the stresses on the system components are determined by digital simulation and compared to the stresses in a conventional six-switch two-level PWM rectifier system     

Rapidly reconfigurable optical phase encoder-decoders based on fiber Bragg gratings

We demonstrate the capacity for fast dynamic reconfiguration of optical code-division multiple access (OCDMA) phase en/decoders based on fiber Bragg gratings and a thermal phase-tuning technique. The tuning time between two different phase codes is measured to be less than 2 s. An OCDMA system using tunable-phase decoders is compared with a system using fixed-phase decoders and, although the system using fixed-phase decoders exhibits a shorter output autocorrelation pulsewidth and lower sidelobes, the system using tunable-phase decoders has advantages of flexibility and a more relaxed requirement on the input pulsewidth.     

Two-photon optical bistability in polar molecules: vibrationalcoupling effects

Single-beam two-photon optical bistability in a two-level system where the levels have unequal electric dipole moments is analyzed theoretically. By means of unitary transformations the system is transformed into a two-level nonpolar system with a field-dependent effective transition dipole moment. Two-photon optical bistability is found to be favored as the difference between the dipole moments grows. When the excited electronic system is coupled to a vibrational normal mode of the nuclei, the susceptibility becomes three-valued and consequently a different type of optical bistability appears, which can coexists with the previous one     

Cavity optimization for minimum pulsewidth of gain-switchedsemiconductor lasers

The optimum photon lifetime for the minimum pulsewidth of a gain-switched laser is investigated both analytically and numerically. For a fixed pump power, the shortest pulse can be produced when the laser cavity is optimized to give an optical pulsewidth that is ~2.5 times the photon lifetime. This is in good agreement with experimental results     

Analysis of optical phase-conjugate characteristics of picosecondfour-wave mixing signals in semiconductor optical amplifiers

We have analyzed for the first time the optical phase-conjugate characteristics of picosecond four-wave mixing (FWM) signals in semiconductor optical amplifiers (SOAs) using the finite-difference beam propagation method (FD-BPM). We show that the optical phase-conjugate characteristics of the FWM signals are strongly dependent on input pump pulsewidths. As a typical example, we have demonstrated that SOAs act as an ideal phase-conjugator, within the confines of reversing the chirp of optical pulses, for a 10-ps input pump pulse and a ~2.2-ps linearly chirped input probe pulse. When the pulsewidth of pump pulse becomes short, the minimum compressed pulsewidth is obtained by using a fiber shorter in length than the input fiber, but having the same group velocity dispersion as the input fiber. For a much shorter pump pulse such as 1 ps, the short FWM signal can be obtained via the gating characteristics of the FWM. However, only a part of the phase information is copied to the FWM signal due to such gating characteristics. The phase information is also degraded due to the fast nonlinear effect in the SOA. Thus, the pulsewidth is not compressed by propagation through a dispersive medium     

Generation of 140 GHz optical pulses with suppressed amplitudemodulation by subharmonic synchronous modelocking of Fabry-Perotsemiconductor laser

140 GHz optical pulses with a pulsewidth of 2.7 ps are generated from a standard Fabry-Perot semiconductor laser using subharmonic optical injection. A new technique based on optical feed forward modulation is also presented to suppress the amplitude modulation associated with subharmonic injection     

Analysis and Verification of Two-Level Random Aperiodic PWM Schemes for DC–DC Converters

A typical two-level pulsewidth modulation scheme for a dc–dc converter generates a signal that has two values: 0 and 1. For a random aperiodic scheme, the switching period can be modulated in a random fashion with a known probability density function (pdf). In this paper, analytical expressions for both discrete and continuous spectra are provided for the case where different pdfs are used to modulate the switching signal of a dc–dc converter. Experimental results for a random aperiodic scheme that is used to optimize the output ripple of a buck dc–dc converter are provided to support the theoretical claims.      

Active picosecond optical pulse compression in semiconductoroptical amplifiers

A novel technique is proposed to compress and amplify a weak picosecond optical pulse by utilizing a copropagating intense pump pulse in semiconductor optical amplifiers. It is shown that, simply by adjusting the time delay between the two pulses, a controlled compression of the pulsewidth by up to a factor of ~4 with pulse energy amplification of up to ~16 dB can be achieved simultaneously. Furthermore, a compressed pulse shape with strongly damped wings is also observed. The shortest achievable pulsewidth is demonstrated to be dependent on the frequency detuning and pump pulse properties     

Space Vector Modulation Applied to Three-Phase Three-Switch Two-Level Unidirectional PWM Rectifier

This work presents a methodology to apply space vector modulation to a three-phase three-switch two-level Y-connected unidirectional pulsewidth modulation (PWM) rectifier. Converter switching stages are analyzed to determine switch control signals for space vector modulation. A switching sequence is proposed in order to minimize the number of switch commutations and to reduce the switching losses. Duty cycle functions are determined and the desired switching sequences are performed by a simple PWM modulator with no need of to determine the present sector of vector. For this propose is just necessary to impose the desired current sectors from input voltage references. The vector control structure used with the proposed modulation technique is also described. In order to validate the proposed modulation technique, experimental results are presented for a 20 kW prototype.     

Filter-Free Optically Switchable and Tunable Ultrawideband Monocycle Generation Based on Wavelength Conversion and Fiber Dispersion

In this letter, a novel filter-free scheme for all-optical ultrawideband monocycle generation based on wavelength conversion in semiconductor optical amplifier and group-velocity dispersion in single-mode fiber is proposed and experimentally demonstrated. The system is optically switchable in pulse polarity, and tunable in both pulsewidth and radio-frequency spectrum.     

Broad-band mid-span spectral inversion without wavelength shift of1.7-ps optical pulses using a highly nonlinear fiber Sagnacinterferometer

We demonstrate broad-band mid-span spectral inversion without wavelength shift of short optical pulses using a highly nonlinear fiber Sagnac interferometer. The 1.55-μm optical pulses with a 1.7-ps pulsewidth are transmitted over 40 km of standard fiber, showing the suitability of this technique for broad-bandwidth second-order dispersion compensation in high-bit-rate optical fiber transmission systems     

Picosecond dynamics of optical gain switching in vertical cavityemitting lasers

It is demonstrated that optically gain switched GaAs vertical lasers (VSELs) with short cavity lengths can generate pulses as short as 4 ps, with delays as short as 20 ps. A model of the large-signal response of VSELs to short optical input pulses is presented. The results demonstrate that the critical parameter of determining VSEL response time is the cavity length. A numerical solution to the rate equations is used to calculate the output pulsewidth and delay, given a specific cavity design and set of input conditions. Calculated results are compared to experimental data. Output pulse dependencies on input pulsewidth and power are examined for different cavity designs. The results of the calculations are used to generate design curves for future devices     

Zero-voltage and zero-current-switching PWM hybrid full-bridge three-level converter

This paper proposes a zero-voltage and zero-current-switching pulsewidth modulation hybrid full-bridge three-level (ZVZCS PWM H-FB TL) converter, which has a TL leg and a two-level leg. The voltage stress of the switches of the TL leg is half of the input voltage, and the switches can realize ZVS, so MOSFETs can be adopted; the voltage stress of the switches of the two-level leg is the input voltage, and the switches can realize ZCS, so IGBT can be adopted. The secondary rectified voltage is a TL waveform having lower high-frequency content compared with that of the traditional FB converters, which leads to the reduction of the output filter inductance. The input current of the converter has quite little ripple, so the input filter can also be significantly reduced. The operation principle of the proposed converter is analyzed and verified by the experimental results. Several ZVZCS PWM H-FB TL converters are also proposed in this paper.     

The Delta-Rectifier: Analysis, Control and Operation

The three-phase Delta-Rectifier is formed by a delta-connection of single-phase pulsewidth modulation (PWM) rectifier modules and has the advantage that it can provide full rated output power in the case of a mains phase loss. In this paper the Delta-Rectifier, implemented with a standard (two-level and/or three-level) boost converter, is analyzed based on an equivalent star connection. Analysis of the Delta-Rectifier shows a redundancy in the switching states concerning the input voltage formation. Furthermore, the Delta-Rectifier has reduced current ripple in the mains phase currents if the modulation is implemented with synchronized PWM. A disadvantage of two-level Delta-Rectifier is the higher voltage stress on the switching devices; however this is mitigated when the boost converter is implemented with a three-level topology as realized for a 10.5-kW laboratory prototype. The Delta-Rectifier concept is proposed based on theoretical considerations and is verified experimentally. The influence of non-idealities on the current ripple formation in the practical realization is analyzed and a high quality mains phase current is demonstrated     

A novel scheme for generating optical dark return-to-zero pulses and its application in a label switching optical network

We experimentally demonstrate a novel and simple method to generate a dark return-to-zero (RZ) pulse with tunable pulsewidth and extinction ratio by using a dual-arm LiNbO /sub 3/ intensity modulator. Our experimental results show that this dark RZ pulse signal can be used in a 10-Gb/s optical packet switching system as an optical label. In addition, we demonstrate that this dark RZ label can be easily erased using the gain saturation effect in a semiconductor optical amplifier.