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We simulate supercontinuum generation for various shapes of a dispersion varying As2S3 waveguides on MgF2 substrate with air cladding. The supercontinuum generation is simulated for pulses of 2000 W peak power and 50 fs pulse width centered at 1.55 μm wavelength. For a uniform waveguide the generated spectrum is considerably narrower than that for a non-uniform waveguide. This is because the non-uniform waveguide allows a continuous phase matching of the generated waves through nonlinear interaction. We have demonstrated that a high coefficient of nonlinear refractive index is necessary for generating supercontinuum with large bandwidths. Larger supercontinuum bandwidth is predicted for waveguides with increasing As2S3 thickness along the propagation direction.

A scheme to generate 50 GHz pulse train using rational harmonic mode locking technique is proposed and studied experimentally. The modulation frequency is adjusted to achieve a 50 GHz pulse train which has a pulse width of 5.25 ps. Numerical simulation results show good agreement with the experimental results.

We numerically demonstrate mid-infrared supercontinuum generation in a non-uniformly tapered chalcogenide planar waveguide. This planar rib waveguide of As2S3 glass on MgF2 is 2 cm long with increasing etch depth longitudinally to manage the total dispersion. This waveguide has zero dispersion at two wavelengths. The dispersion profile varies along the propagation distance, leading to continuous modification of the phase-matching condition for dispersive wave emission and enhancement of energy transfer efficiency between solitons and dispersive waves. Numerical simulations are conducted for secant input pulses at a wavelength of 1.55 μm with a width of 50 fs and peak power of 2 kW. Results show this proposed scheme significantly broadens the generated continuum, extending from ~1 to ~7 μm.

We have designed a tapered planar rib waveguide and numerically studied supercontinuum generation by the propagation of input pulses at 1.55 μm. The Air-SF57 glass-SiO 2 waveguide is 2 cm long, with a varying etch depth to manage the total dispersion. This proposed waveguide geometry significantly broadens the output spectrum caused by continuous modification of the phase matching condition for dispersive wave emission. The coherence property has also been investigated, demonstrating that fully coherent supercontinuum extending from ~1 μm to ~4.6 μm can be obtained with proper pumping conditions.

This article details the numerically studied coherence properties of the supercontinuum generated in a lead-silicate microstructured fiber taper, with an increasing core radius along the propagation distance that tailors the dispersion property. Simulations are conducted by adding quantum noise into the input pulse at 1.55 μm, and the complex degree of first-order coherence function and the overall spectral coherence degree are both calculated. Although the spectral broadening is comparable, the coherence degree is shown to vary with different pumping conditions. It decreases with higher peak power and longer duration due to the significant competition between the soliton-fission process and the noise-seeded modulation instability. By controlling the input pulse parameters, it is possible to generate a perfectly coherent supercontinuum with a flat broadened spectrum extending from ~1 μm to ~5μm in this fiber taper.

We propose and demonstrate a hybrid mode-locked erbium-doped fiber ring laser by combining the rational harmonic mode-locking technique and passive mode locking based on nonlinear polarization rotation in a highly nonlinear photonic crystal fiber. By carefully adjusting the modulation frequency and the polarization controllers in the cavity, a 30 GHz pulse train with improved stability and narrower pulse width is generated. The pulse width at 30 GHz using rational harmonic mode locking alone is 5.8 ps. This hybrid scheme narrows the pulse width to 1.9 ps at the repetition rate of 30 GHz. Numerical simulations are carried out that show good agreement with the experimental results.

A fiber ring laser which implements hybrid mode locking technique has been proposed and experimentally demonstrated to generate pulse train at 20 GHz repetition rate with ultrashort pulse width. Graphene and charcoal nano-particles acting as passive mode lockers are inserted into a rational harmonic mode-locked fiber laser to improve the performance. With graphene saturable absorbers, the pulse duration is shortened from 5.3 ps to 2.8 ps, and with charcoal nano-particles, it is shortened to 3.2 ps. The RF spectra show that supermode noise can be removed in the presence of the saturable absorbers. Numerical simulation of the pulse transmission has also been carried out, which shows good agreement with the experimental results.

An axially non-uniform tapered As 2 S 3 planar waveguide has been designed for mid-IR supercontinuum generation. The dispersion profile is varying along the propagation distance. Numerical results show this scheme significantly broadens the generated continuum, extending from ~1 μm to ~7 μm.

We numerically study the broadband mid-infrared supercontinuum generation in a non-uniform SF57 microstructured fiber. The dispersion of the fiber is tailored by linearly varying the air hole diameters along the propagation distance. The fiber has zero dispersion at two wavelengths. This allows a continuous shift of the higher zero dispersion wavelength to a longer wavelength. Results show this scheme can significantly broaden the generated spectrum.

A mode-locked erbium-doped fiber ring laser that is easy to set up is proposed and experimentally demonstrated to generate a high-repetition-rate optical pulse train with an ultrashort pulse width. The laser combines a rational harmonic mode-locking technique and charcoal nanoparticles as saturable absorbers. Compared to a solely active mode-locking scheme, the scheme with charcoal nanoparticles can remove the supermodes and narrow the pulse width by a factor of 0.57 at a repetition rate of 20 GHz. Numerical simulation of the laser performance is also provided, which shows good agreement with the experimental results.

The design and performance of two optical latches, the Set–Reset (SR) latch and D-Flip-Flop has been studied. These latches are the building blocks of large optical processors. The latches are built using two optical logic operations NAND and NOT. Both NAND and NOT operations are realized by using Mach–Zhender interferometer (MZI) utilizing semiconductor optical amplifier with quantum dot active region (QD-SOA). Nonlinear dynamics including carrier heating and spectral hole-burning in the QD-SOA are taken into account together with the rate equations in order to realize the all-optical logic operations. Results show that this scheme can realize the functions of Set–Reset latch and D-Flip-Flop at high speeds (∼250 Gb/s). The dependence of the output quality (Q factor) on QD-SOA parameters is also discussed.

A scheme to generate high speed optical pulse train with ultra short pulse width is proposed and experimentally studied. Two-step compression is used in the scheme: 20 GHz and 40 GHz pulse trains generated from a rational harmonic actively mode-locked fiber ring laser is compressed to a full width at half-maximum (FWHM) of ~ 1.5 ps using adiabatic soliton compression with dispersion shifted fibers (DSF). The pulse trains then undergo a pedestal removal process by transmission through a cascaded two photonic crystal fiber (PCF)-nonlinear optical loop mirrors (NOLM) realized using a double-ring structure. The shortest output pulse width obtained was ~ 610 fs for 20 GHz pulse train and ~ 570 fs for 40 GHz pulse train. The signal to noise ratio of the RF spectrum of the output pulse train is larger than 30 dB. Theoretical simulation of the NOLM transmission is conducted using split-step Fourier method. The results show that two cascaded NOLMs can improve the compression result compared to that for a single NOLM transmission.

The performances of all-optical logic gates XOR, AND, OR, NOR and NAND based on semiconductor optical amplifier (SOA) have been simulated including the effects of amplified spontaneous emission (ASE). For the parameters used, all-optical logic gates using SOA are capable of operating at speed of 80 Gb/s.

In this paper we theoretically study the broadband mid-IR supercontinuum generation (SCG) in a lead-silicate microstructured fiber (the glass for simulation is SF57). The total dispersion of the fiber can be tailored by changing the core diameter of the fiber so that dispersion profiles with two zero dispersion wavelengths (ZDWs) can be obtained. Numerical simulations of the SCG process in a 4 cm long SF57 fiber/fiber taper seeded by femto-second pulses at telecommunications wavelength of 1.55 µm are presented. The results show that a fiber taper features a continuous shift of the longer zero dispersion wavelength. This extends the generated continuum to a longer wavelength region compared to fibers with fixed ZDWs. The phase-matching condition (PMC) is continuously modified in the fiber taper and the bandwidth of the generated dispersive waves (DWs) is significantly broadened.

We report a dual-wavelength amplitude-equalized rational harmonic mode-locked fiber laser operating at up to 40 GHz based on the technique of nonlinear polarization rotation (NPR) in a highly nonlinear fiber. A theoretical simulation shows that NPR structure can introduce a self-feedback mechanism to stabilize the two simultaneously lasing wavelengths. By carefully adjusting the polarization controllers (PCs) in the cavity, amplitude-equalized pulses at two different wavelengths have been obtained.

When the two-photon absorption of a high intensity pump beam takes place in a semiconductor optical amplifier there is an associated fast phase change of a weak probe signal. A scheme to realize fast all-optical XOR logic function using two-photon absorption induced phase change has been analyzed. Rate equations for semiconductor optical amplifiers, for input data signals with high intensity, configured in the form of a Mach–Zehnder interferometer has been solved. The input intensities are high enough so that the two-photon induced phase change is larger than the regular gain induced phase change. The model shows that both XOR operation and pseudo-random binary sequence generation at 250 Gb/s with good signal to noise ratio is feasible.

We present experimental investigations of the dynamical properties of semiconductor optical amplifiers (SOAs) and their impacts in all-optical signal processing using SOAs. We introduce ultrafast optical signal characterization techniques to fully characterize the gain and phase dynamics of SOAs. We elucidate a consequence of the slow carrier recovery as pattern-dependent phase fluctuation in wavelength conversion of on-off -keyed signals. We also analyze the conversion of the phase fluctuation into amplitude fluctuation limiting the performance of all-optical xor operation. Finally, the performance of all-optical wavelength conversion of differential phase-shift-keyed signals is presented with emphasis on regeneration of the phase information.

We have studied high speed optical logic utilizing ultrafast two-photon absorption (TPA) induced phase change in semiconductor optical amplifiers (SOA). Results show that this scheme can realize all-optical logic and encryption at data speeds to 250 Gb/s.

An all-optical logic AND gate is demonstrated by using a semiconductor optical amplifier (SOA) based Mach-Zehnder interferometer (MZI). The AND results are numerically analyzed by solving the rate equation of SOA. Q-factor values have been calculated. The operation of the AND logic gate is experimentally demonstrated at 80 Gb/s. Operation at higher data rates is feasible using SOAs with shorter phase recovery time.

All-optical xor operation has been demonstrated using a semiconductor optical amplifier Mach-Zehnder interferometer (SOA-MZI) and delayed interferometer (DI) at 80 Gb/s. The DI is based on a polarization maintaining loop (PML) mirror. The results show using the PML-DI in series with the SOA-MZI improves the pulse quality of the xor result

All-optical OR operation has been demonstrated using a semiconductor optical amplifier (SOA) and delayed interferometer (DI) at 20 and 40 Gb/s. The DI is based on a polarization maintaining loop mirror. Q-factor of the operation is discussed through the numerical simulations. The results show the OR gate operation rate is limited by the carrier lifetime and the input pulse energy.

In this paper we review functional applications of semiconductor optical amplifiers. For example, wavelength conversion, optical demultiplexing, and optical logic elements are discussed, which are attractive from the point of view of potential applications in all-optical networks and optical time division multiplexed systems.

In this paper, we demonstrate a technique for suppressing the intensity noise of the erbium doped fiber lasers. We show that by introducing negative feedback into the system through the use of a self-biased intracavity loss modulator, the relaxation oscillation noise of the fiber laser could be suppressed by as much as 20 dB. To explain the observed noise reduction phenomenon, a simple theory based on teh linearized rate equation has been developed and also been supported by experimental measurements. We also derive a formula for estimating the maximum achievalbe intesity noise suppression ratio.

All-optical XOR functionality has been demonstrated experimentally using an integrated SOA-based Mach-Zehnder interferometer (SOA-MZI) at 20 and 40 Gb/s. The performance of the XOR results has been analyzed by solving the rate equation of the SOA numerically. The high-speed operation is limited by the carrier lifetime in the SOA. In order to solve the limitations imposed by carrier lifetime, a differential scheme for XOR operation has been experimentally investigated. This scheme is potentially capable of XOR operation to >100 Gb/s.

In this paper, we present an experimental demonstration of a high-quality optical pulse train generation at 80 Gb/s. The successful operation was achieved via two steps. In the first step, an ultrastable 2-ps (FWHM), 40-Gb/s pulse train was generated using the technique of fourth-order rational harmonic mode locking. The mode-locked laser was stabilized using a regenerative-type base-line extraction feedback technique. In the second step, an external fiber loop mirror consisting of several meters of polarization-maintaining fiber was used to double the repetition rate from 40 to 80 Gb/s. The output of the 80-Gb/s pulse train shows very small pulse-to-pulse amplitude fluctuation with good long-term stability.

In this article, we present a theory of the pulse train generated by a rational harmonic mode locked ring fiber laser. The pulse width is calculated as a function of the rational harmonic order and the optical transfer function of the modulator. The theoretical work is based on a time domain analysis, which predicts that the pulse width decreases when the rational harmonic order goes up. The pulse width as a function of the modulation amplitude and bias level of the modulator was measured, and the experimental results agree with the theory.

We demonstrate a stable dual-wavelength actively mode-locked erbium-doped fiber laser operating at 20 GHz. A highly nonlinear fiber is used to compensate the frequency chirp caused by regular single-mode fiber in the optical cavity. Dual-wavelength pulse trains with pulsewidths ~1.4 ps are obtained.

Erbium-doped fiber lasers at repetition rate of 40 and 80 GHz are demonstrated in an injection-locked and rational harmonic mode-locked ring cavity. The cavity has a polarization-maintaining loop mirror. The injection-locking pulse is generated at repetition rates of 20 and 40 GHz from another rational harmonic mode-locked fiber laser. When these pulses are injected into the second fiber laser, pulses at twice the repetition rates viz. 40 and 80 GHz are produced

A model is used to analyze the gain and phase dynamics of a semiconductor optical amplifier with a carrier reservoir (CR-SOA). Coupled rate equations are solved numerically. Due to the fast transition of carriers from the carrier reservoir layer to the active region, the CR-SOA is shown to have faster gain and phase response than a regular SOA. Increasing the injection current will also decrease the response times.

In this work, an optical short pulse generator is designed consisting of a pulse compressor and cascaded notch filter type repetition rate doublers. The performance characteristics such as pulsewidth and peak power as a function of design parameters are studied. The pulse compressor is optimized based on the simulation results. The 6-ps-wide pulses at 20-GHz repetition rate directly generated from mode-locked fiber laser are compressed to 1.25-ps-wide pulses. Using a set of polarization-maintaining fiber loop mirrors, the repetition rate is quadrupled and stable 1.45-ps-wide pulses at 80 GHz are achieved.

A multiwavelength fiber laser source is demonstrated with a semiconductor optical amplifier as the gain medium. A delayed interferometer is incorporated in the ring cavity serving as a comb-like multichannel filter. A stable 75-wavelength simultaneous operation spaced at 40 GHz with the extinction ratio of 40 dB is achieved. By tuning the cavity loss, the center wavelength of the generated lasing waveband could be varied by as much as 20 nm.

We have designed a tapered-rib waveguide and numerically studied the generation of supercontinuum using such waveguides. The Air-SF57 glass-SiO(2) waveguide is 3 cm long, with a varying etched depth to manage the total dispersion. Numerical simulations are conducted for input pulses at a wavelength of 1.55 μm with a width of 150 fs and peak power of 5 kW. The proposed waveguide geometry greatly broadens the output spectrum, extending from ∼1 to ∼6 μm, caused by the continuous modification of the phase-matching condition for the generated waves.

We have studied the broadband mid-IR supercontinuum generation in a lead-silicate tapered microstructured fiber for femto-second input pulses at 1550 nm. The supercontinuum generated extends from ~1000 to ~ 5000 nm for a suitably designed tapered fiber. The coherence properties of the supercontinuum depends on the input pulse parameters. It is possible to generate perfectly coherent supercontinuum with a flat broadened spectrum extending to ~5000 nm in this fiber taper.

In this paper we numerically study the coherence properties of the supercontinuum generated in a lead-silicate microstructured fiber taper, with an increasing core radius along the propagation distance which tailors the dispersion property. Simulations are conducted by adding quantum noise into the input pulse at 1.55 μm, and the complex degree of first-order coherence function and the overall spectral coherence degree are both calculated. Although the spectral broadening is comparable, the coherence degree is shown to vary with different pumping conditions. It decreases with higher peak power and longer duration due to the significant competition between the soliton-fission process and the noise-seeded modulation instability. By controlling the input pulse parameters, it is possible to generate perfectly coherent supercontinuum with a flat broadened spectrum extending to ~5μm in this fiber taper.

A fiber ring laser which generates ~ 570 ps wide pulse train at 40 GHz has been demonstrated using a photonic crystal fiber as a nonlinear optical loop mirror (NOLM). Theoretical simulation of the NOLM transmission has been carried out using the split-step Fourier method.

Semiconductor optical amplifiers are important for wide range of applications in optical networks, optical tomography and optical logic systems. For many of these applications particularly for optical networks and optical logic, high speed performance of the SOA is important. All optical Boolean operations such as XOR, OR, AND and NOR has been demonstrated using SOA based Mach-Zhender interferometers (SOA-MZI). A rate equation model for SOA-MZI has been developed. The model has been used to analyze the Set-Reset (S-R) latch, the gated S-R latch and the D-Flip-Flop devices. The modeling results suggest that the Flip-Flop circuits should work at high speeds. An optical pseudo-random bit stream (PRBS) generator is important for all-optical encryption circuits. A model of a PRBS generator using SOAMZI based devices has been developed. We show that a PRBS generator can work @ 80 Gb/s using regular SOAs and @ ~ 250 Gb/s or at higher speeds using two-photon absorption based processes in SOAs.

A 980-nm strained InGaAs quantum-well (QW) laser is the preferred pump source for an Er/sup 3+/-doped fiber amplifier for the next generation of lightwave communication systems because of lower noise, high power conversion efficiency, and low temperature sensitivity. Obtaining long lifetime, narrow far field, high power output in the fundamental transverse mode centered at 980/spl plusmn/5 nm, and planarity of the structure while maintaining low threshold current density (J/sub th/) and high differential quantum efficiency (/spl eta/) are the major challenges. Here, we report our work aimed at optimizing the design, growth, and fabrication of 980-nm lasers to address some of these issues. We demonstrate very low broad-area J/sub th/, of 47 A/spl middot/cm/sup -2/, operation up to 200/spl deg/C, and a very low linewidth enhancement factor of 0.54 of these lasers. We have also monolithically integrated 980-nm lasers with 850-nm GaAs QW lasers. To minimize coincorporation of nonradiative recombination impurities like oxygen and displacement of the p/n junction due to Be diffusion during MBE growth, we suggest that the Be doping should be dispensed with on the p-side of the GRIN region and the n-side GRIN region should be doped with Si. The optical properties of InGaAs QW's are insensitive to the type of the arsenic beam used, As/sub 2/ versus As/sub 4/. Although strained InGaAs QW lasers grown using As/sub 2/ at a constant substrate temperature as low as 570/spl deg/C have a lower J/sub th/, they also exhibit a 10-25% lower /spl eta/ as compared to the As/sub 4/. counterpart in which the AlGaAs cladding layers are grown at /spl sim/700/spl deg/C. To obtain a planar structure and to prevent the fabrication related defects, we have used a novel method in which the laser structure is first grown by MBE, and mesas are formed by in situ melt etching using SiO/sub 2/ stripes as a mask followed by regrowth of p/sup -/-p-n AlGaAs isolating layers by LPE. Compared to ridge waveguide (RWG) lasers, the buried heterostructure lasers so fabricated have significantly lower threshold current, higher power output; higher temperature operation, lower cavity losses, and kink-free light-current (L-I) characteristics, as expected. A CW power of 150 mW/facet at 986 nm was measured from a 400-/spl mu/m-long BH laser with 11-/spl mu/m active stripe width. A minimum threshold current of 2.5 mA was measured for lasers with 3.0-/spl mu/m active width and 300-400 /spl mu/m cavity length. The L-I characteristics of 500-, 800-, and 1300-/spl mu/m-long lasers with 3.0-/spl mu/m active width were linear up to the currents corresponding to a current density of 10 kA/spl middot/cm/sup -2/. At higher current densities, a sublinear increase of power with current was observed. Stable fundamental transverse mode operation was obtained up to 100-mW emitted power. >

We propose a novel method of all-optical OR operation using semiconductor optical amplifier Mach-Zehnder interferometers. We demonstrate all-optical OR operation at 42.8 Gb/s using a hybrid photonic integrated device.

When the two-photon absorption of a high intensity pump beam takes place in a semiconductor optical amplifier there is an associated fast phase change of a weak probe signal. A scheme to realize fast all-optical NAND logic function using two-photon absorption induced phase change has been analyzed. NAND gate is important because other Boolean logic elements and circuits can be demonstrated using NAND gates as a basic building block. Rate equations for semiconductor optical amplifiers (for input data signals with high intensity) configured in the form of a Mach-Zehnder interferometer has been solved. The input intensities are high enough so that the two-photon induced phase change is larger than the regular gain induced phase change. The model shows that both AND and NAND operation at 250 Gb/s with good signal to noise ratio is feasible.

Semiconductor optical amplifiers are important for wide range of applications in optical networks, optical tomography and optical logic systems. For many of these applications particularly for optical networks and optical logic, high speed performance of the SOA is important. All optical Boolean operations such as XOR, OR, AND and NOR has been demonstrated using SOA based Mach-Zhender interferometers (SOA-MZI). A rate equation model for SOA-MZI has been developed. The model has been used to analyze the Set-Reset (S-R) latch, the gated S-R latch and the D-Flip-Flop devices. The modeling results suggest that the Flip-Flop circuits should work at high speeds.

When the two-photon absorption of a high intensity pump beam takes place in a semi conductor optical amplifier there is an associated fast phase change of a weak probe signal. A scheme to realize fast all-optical XOR logic function using two-photon absorption induced phase change has been analyzed. Rate equations for semiconductor optical amplifiers, for input data signals with high intensity, configured in the form of a Mach-Zehnder interferometer has been solved. The input intensities are high enough so that the two-photon induced phase change is larger than the regular gain induced phase change. The model shows that both XOR operation and pseudorandom binary sequence generation at 250 Gb/s with good signal to noise ratio is feasible. Keywords: semiconductor optical amplifier, two-photon absorption, optical logic INTRODUCTION In future high-speed communication systems, optical logic gates are expected to play an important role in switching, signal regeneration, addressing, header recognition, data encoding and encryption, etc.[1]. In recent years, optical logic based on several different schemes have been demonstrated and reported, including that based on dual semiconductor optical amplifier (SOA) Mach-Zehnder interferometer (MZI)[2-3], semiconductor laser amplifier (SLA) loop mirror [4], ultrafast nonlinear interferometers [5], four-wave mixing process in SOA[6] or cross gain (XGM) or cross phase (XPM) modulation in nonlinear devices[7]. Among these schemes, the SOA based MZI has the advantage of being relatively stable, simple and compact. However, to the author’s knowledge, all these schemes have a lim itation of signal bit rate of up to 40 or 80 Gb/s. A scheme for higher data rate logic using quantum dot (QD) based optical amplifier[8,9] has been analyzed[10]. Another process that can result in a fast phase change of a probe signal is two-photon absorption of a pump beam[11-15]. Pump-probe experiments have shown that phase changes take place in a duration ~ 1 ps or less when the pump and probe signals are injected into a semiconductor optical amplifier[11-14]. In this work, we present a model of XOR and a pseudorandom binary sequence (PRBS) generator based on two-photon absorption (TPA) of pump beams. The proposed device is a Mach-Zehnder interferometer (MZI) each arm of which has a semiconductor optical amplifier (SOA). Th e pump beams are the data signals which are injected into each arm. The probe beam is a weaker signal injected into the middle arm. The pump and probe beams have single pulse durations ~ 0.5 ps. The concept of XOR using SOA based MZI is the same as has been described and demonstrated earlier for XOR[1-6]. The principal new element of the current an alysis is that the phase change ( GI ) due to the pump beam has a fast component, the magnitude of which is given by[11-13] GI = -(1/2) ED

A theory of small signal frequency response of four-wave mixing (FWM) in a traveling wave semiconductor optical amplifier has been developed. The bandwidth of FWM is >300 GHz. For large wavelength separation between the input signals, where the nonlinear gain effects dominate the generation of FWM, the bandwidth of the FWM signal exceeds 1 THz.

By using combined frequency and amplitude modulation of a cleaved-coupled-cavity semiconductor laser together with a phase-sensitive detection scheme, we have measured the chromatic dispersion in a single-mode silica fiber. The method presented can measure dispersion down to 0.03 psec/nm km and, with a slight modification to the measurement routine, also measure the total propagation time with a 10(-7) accuracy.

Semiconductor optical amplifiers are important for wide range of applications including optical networks, optical tomography and optical logic systems. For many of these applications particularly for optical networks and optical logic high speed performance of the SOA is important. The speed of operation of SOA is limited by the gain and phase recovery times in the SOA. We have demonstrated higher speed operation (i) for SOAs with a carrier reservoir layer, (ii) for SOAs with a multi-quantum well modulation doped active region, and, (iii) for SOAs with a quantum dot (QD) active region. The multi-quantum well SOA has been integrated with InGaAsP/InP based waveguides to build Mach- Zehnder interferometers (MZI). XOR optical logic has been demonstrated at 80 Gb/s using these SOA-MZI structures. XOR operation has been analyzed by solving the rate equation of the SOA, for SOAs with both regular and QD active region. Mach-Zehnder interferometers fabricated using SOA with quantum dot active region (QD-SOA) can be used for XOR operation at 250 Gb/s. Pseudo random bit stream (PRBS) generation using both regular and QD-SOA have been studied and their performance modeled. The model shows QD-SOA based devices can be used to produce PRBS generators that operate near 250 Gb/s.

The performance of all-optical XOR gate based on quantum-dot (QD) SOA MZI has been simulated. The saturation power, optical gain and phase response of a QD SOA has been analyzed numerically using a rate equation model of quantum dots embedded in a wetting layer. The calculated response is used to model the XOR performance. For the parameters used here, XOR operation at ~ 250 Gb/s is feasible using QD based Mach-Zehnder interferometers. The speed is limited by the relaxation time from wetting layer to the quantum dots.

We demonstrate simultaneous stabilized operation of a mode locked ring fiber laser at two wavelengths. At one of the wavelengths the mode locked operation is at 10 GHz and it is at 40 GHz at the second wavelength. The laser has an intracavity LiNbO3 modulator driven at 10 GHz. The 40 GHz pulses are obtained by rational harmonic mode locking. Pulses with widths in 5 to 8 ps range are obtained. Mode locking at 40 GHz for both wavelengths has also been demonstrated.

We simulated the evolution of self-similar parabolic pulses (similaritons) in a normally dispersive fiber amplifier. The rate of development of a Gaussian pulse into the asymptotic parabolic regime has been studied. The model has been applied to an optical transmission system with a fiber amplifier. By calculating the Q-factor, we numerically determined the signal to noise performance of the pulse train along fiber length. For the parameters we used, a 6m long fiber amplifier with 20 dB gain is capable of amplifying 200Gb/s initial chirp-free Gaussian pulses of duration 0.4ps with no distortion and additional noise. The trade-off between pulse width and amplifier length has been studied.

A scheme to realize fiber-based all-optical Boolean logic functions including XOR, AND, OR and NOT based on a semiconductor optical amplifier with a closely stacked Stranski-Krastanow InAs/GaAs quantum dot layers is proposed. Rate equations is given to describe the population dynamics of the carrier in the device, as well as the nonlinear dynamics including carrier heating and spectral hole-burning. The model is used to simulated the cross gain and cross phase modulation in the device that are related to the logic processes. Results show with QD excited state serving as a carrier reservoir, this type of QD device is suitable for high speed operations with ultra fast carrier and phase relaxation. All optical logic operation can be carried out at up to 250 Gb/s.

A model to study four-wave mixing (FWM) wavelength conversion in InAs-GaAs quantum-dot semiconductor optical amplifier is proposed. Rate equations involving two QD states are solved to simulate the carrier density modulation in the system, results show that the existence of QD excited state contributes to the ultra fast recover time for single pulse response by serving as a carrier reservoir for the QD ground state, its speed limitations are also studied. Nondegenerate four-wave mixing process with small intensity modulation probe signal injected is simulated using this model, a set of coupled wave equations describing the evolution of all frequency components in the active region of QD-SOA are derived and solved numerically. Results show that better FWM conversion efficiency can be obtained compared with the regular bulk SOA, and the four-wave mixing bandwidth can exceed 1.5 THz when the detuning between pump and probe lights is 0.5 nm.