Semiconductor Optical Amplifier Gain Research Articles

Numerical Simulation of Temporal and Spectral Variation of Gain and Phase Recovery in Quantum-Dot Semiconductor Optical Amplifiers

We numerically investigate the gain and phase recovery dynamics of quantum-dot (QD) semiconductor optical amplifiers (SOAs) by simultaneous consideration of the temporal and spectral variations. We solve 1088 coupled rate equations to simulate the carrier recovery dynamics of the inhomogeneously broadened QD ensemble, where QD electron and hole states are considered separately. The gain and phase recovery responses induced by an ultrafast pump pulse are calculated by considering different carrier relaxation time constants and spectral line shape functions involved in the successive carrier recovery process. The interband transition is assumed to have a Lorentzian line shape function and the intraband free-carrier absorption is described by the Drude model. We demonstrate that the actual behavior of the gain and phase recovery in QD SOAs can be clearly understood by simultaneously considering the temporal and spectral behavior of the gain and phase recovery responses, which are visualized by means of plots in the time and wavelength domains. We identify how the respective QD state-the ground state, the excited state, and an upper state-and quantum-well carrier reservoirs contribute to the gain and phase recovery of QD SOAs both temporally and spectrally.

SOA-Booster Integrated Mach–Zehnder Modulator: Investigation of SOA Position

Integration of a booster semiconductor optical amplifier (SOA) is an efficient way of overcoming losses in InP based Mach-Zehnder modulators. We analyze the impact of locating the SOA before and after the MZM, respectively, in terms of output power and signal integrity at 10 Gb/s, both experimentally and theoretically. For a device with 10 dB MZM loss it is found that, for a fixed power consumption, locating the SOA after the MZM provides 7-9 dB higher output power. This advantage is reduced for lower MZM losses but remains significant. With the SOA after the MZM, the gain is restricted by dynamic saturation effects (waveform distortion), which is not the case if the SOA is at the input of the MZM. The waveform distortion is accompanied by a spectral red-shift, which degrades the transmission performance. Simulations show that for MZM losses below ~4 dB, locating the SOA before the MZM can provide a higher power with no waveform distortion and negative chirp, at the cost of a higher SOA gain. For higher MZM losses it is unfeasible to locate the SOA before the MZM, due to a prohibitively large power consumption.

Electron–Hole Plasma-Induced Spectral Blueshift of Optical-Data Injection Mode-Locked Semiconductor Optical Amplifier Fiber Laser

Under an optical nonreturn-to-zero (NRZ) data injection at 10 Gbit/s, the 10-GHz mode-locking and pulsed return-to-zero (RZ) clock extraction from a semiconductor optical amplifier (SOA) based fiber ring is investigated in this paper. The diagnoses on gain and intracavity-power-controlled anomalous blueshifted spectrum and subpicosecond timing jitter are demonstrated. By increasing the injecting power of the optical NRZ data from -3 to 8 dBm into the SOA bias at different currents, the mode locking is completed with a dc level greatly decreasing from 480 to 50 ¿W (only 1.5% of the mode-locked pulse power at 3 mW), corresponding to a pulse/dc amplitude contrast ratio up to 18 dB. Increasing the SOA bias current up to 350 mA significantly suppresses the timing jitter from 1.8 ps to 345 fs, and the extracted RZ clock pulse is shortened from 55 to 27 ps. The pulsewidth of the amplified SOAFL is compressed from 11 ps to 836 fs after dispersion compensation. At constant data injection level, the increasing SOA bias or gain oppositely redshifts the mode-locked SOA fiber ring laser (SOAFL) spectrum by 5 nm. The amplifier spontaneous emission of SOA at short wavelength region ( ~ 1520 nm) is eliminated with increasing NRZ data power, whereas the mode-locking gain peak arises and blueshifts from 1558 to 1552 nm due to the band-filling effect. Such a blueshift in mode-locking spectrum becomes more significant in SOA at lower bias (or gain) condition. A theoretical model interprets the correlation between the nonlinear gain suppression-induced variation of electron-hole plasma in SOA and the blueshifted mode-locking SOAFL spectrum, which is occurred when the gain saturation condition for the SOA becomes more pronounced.

Semiconductor optical amplifier pattern effect suppression using Lyot filter

The feasibility of employing a Lyot filter to compensate for the pattern effect in a semiconductor optical amplifier (SOA) is experimentally demonstrated. The operation mechanism relies on the control of its comb-like transfer function by proper adjustment of the wavelength spacing and detuning to suppress the spectral components broadened owing to SOA gain saturation. The scheme is experimentally demonstrated on amplified 10 Gbit/s return to zero data.

Ultrafast gain and refractive index dynamics in GaInNAsSb semiconductor optical amplifiers

The gain and refractive index dynamics of dilute nitride antimonide semiconductor optical amplifiers are studied using heterodyne pump probe spectroscopy, both in forward and reverse bias regimes. In the forward biased absorption regime, both gain and refractive index relax on the same timescale indicating that both quantities are linked to the same relaxation process, interband recombination. Above transparency, in the forward biased gain regime, the gain and phase exhibit differing timescales resulting in a dynamical alpha factor that varies strongly with time. Reversed bias measurements suggest a recombination dominated absorption recovery where the recovery timescale increases with increasing reversed bias, possibly due to charge separation effects.

Low-Current N-Type Quantum Dash Semiconductor Optical Amplifiers

In this letter, we propose the use of light N-type doping to enhance the optical gain of the quantum dash semiconductor optical amplifier (SOA) at low applied current. We find that doping the dashes with light donor concentration increases the unsaturated optical gain and widens the optical gain spectrum range at low applied current. Also, our calculation reveals that the use of N-type doping is associated with a reduction in the slope efficiency and a decrease in the transparency current, which makes N-type quantum dash SOA attractive for low power dissipation applications.

Dual wavelength fibre laser with tunable channel spacing using an SOA and dual AWGs

In this paper, we propose and demonstrate a dual wavelength fibre laser (DWFL) based on the use of an inhomogeneously-broadened semiconductor optical amplifier (SOA) gain medium as well as two arrayed waveguide gratings (AWGs) together with two optical channel selectors (OCSs) and a broadband fibre Bragg grating (FBG) to generate dual wavelength output at variable channel spacings. The widest spacing obtained from the DWFL is 12.21 nm, while the narrowest spacing is 1.16 nm. The DWFL has good stability with only minor power fluctuations of less than 2 dB and a side mode suppression ratio (SMSR) of approximately 38.5 dB with fluctuations of less than 0.5 dB.

Semiconductor Optical Amplifier-Enabled Intensity Modulation of Adaptively Modulated Optical OFDM Signals in SMF-Based IMDD Systems

Detailed numerical investigations of the transmission performance of adaptively modulated optical orthogonal frequency division multiplexed (AMOOFDM) signals are undertaken, for the first time, in optical amplification- and chromatic dispersion compensation-free SMF IMDD systems using semiconductor optical amplifiers (SOAs) as intensity modulators. A theoretical model describing the characteristics of the SOA-based intensity modulators is developed, based on which optimum SOA operating conditions are identified. It is shown that the optimized SOA-based intensity modulators support a 30 Gb/s AMOOFDM signal transmission over a 80 km SMF, which doubles the transmission performance offered by directly modulated DFB lasers. The aforementioned performance enhancement is mainly due to a considerable reduction in the frequency chirp effect, resulting from the strong SOA gain saturation-induced decrease in SOA effective carrier lifetime. Relatively low extinction ratio and clipping of the SOA modulated signals are identified to be the key factors limiting the maximum achievable AMOOFDM transmission performance. In addition, results also indicate that both the optimum SOA operating conditions and the AMOOFDM transmission performance are insusceptible to variations in SOA parameters.

Poincare Sphere Method for Optimizing the Wavelength Converter Based on Nonlinear Polarization Rotation in Semiconductor Optical Amplifiers

We present a Poincare sphere (PS) model to optimize the wavelength converter (WC) based on nonlinear polarization rotation (NPR) in semiconductor optical amplifiers (SOA). The PS method is applied to describe the variations of ellipticipy angle and azimuth of SOA output state of polarization (SOP) induced by polarization-dependent gain (PDG) and polarization-dependent phase shift, respectively. The theoretical results reveal that the polarization dependence of the linewidth enhancement factor (LEF) and the variation of SOA gain are significant factors for the NPR. The evolutions of output SOP on the PS under quasi-continuum condition have been demonstrated experimentally. The experimental results agree well with our analysis.

Gain-controlled semiconductor optical preamplifier for the 100 Gbit/s 40 km Ethernet receiver

A numerical investigation of the performance of an automatic gain-controlled semiconductor optical preamplified receiver for a 4 x 25 Gbits/s wavelength division multiplexing transmission system with a 0-40 km reach is presented. We show that the control scheme acting on the semiconductor optical amplifier (SOA) gain increases the input power dynamic range of the optical receiver, thus allowing the transmission system to operate error free regardless of fiber length. In contrast, a fixed-gain optical receiver shows poor performance that is due to SOA nonlinearity and photodiode overload, which are well captured by the corresponding simulation models. The device represents a practical alternative to the next-generation high-speed Ethernet technology.

Novel monolithic integration scheme for high-speed electroabsorption modulators and semiconductor optical amplifiers using cascaded structure

A new monolithic integration scheme, namely cascaded-integration (CI), for improving high-speed optical modulation is proposed and demonstrated. High-speed electroabsorption modulators (EAMs) and semiconductor optical amplifiers (SOAs) are taken as the integrated elements of CI. This structure is based on an optical waveguide defined by cascading segmented EAMs with segmented SOAs, while high-impedance transmission lines (HITLs) are used for periodically interconnecting EAMs, forming a distributive optical re-amplification and re-modulation. Therefore, not only the optical modulation can be beneficial from SOA gain, but also high electrical reflection due to EAM low characteristic impedance can be greatly reduced. Two integration schemes, CI and conventional single-section (SS), with same total EAM- and SOA- lengths are fabricated and compared to examine the concept. Same modulation-depth against with EAM bias (up to 5V) as well as SOA injection current (up to 60mA) is found in both structures. In comparison with SS, a < 1dB extra optical-propagation loss in CI is measured due to multi-sections of electrical-isolation regions between EAMs and SOAs, suggesting no significant deterioration in CI on DC optical modulation efficiency. Lower than -12dB of electrical reflection from D.C. to 30GHz is observed in CI, better than -5dB reflection in SS for frequency of above 5GHz. Superior high-speed electrical properties in CI structure can thus lead to higher speed of electrical-to-optical (EO) response, where -3dB bandwidths are >30GHz and 13GHz for CI and SS respectively. Simulation results on electrical and EO response are quite consistent with measurement, confirming that CI can lower the driving power at high-speed regime, while the optical loss is still kept the same level. Taking such distributive advantage (CI) with optical gain, not only higher-speed modulation with high output optical power can be attained, but also the trade-off issue due to impedance mismatch can be released to reduce the driving power of modulator. Such kind of monolithic integration scheme also has potential for the applications of other high-speed optoelectronics devices.

Wavelength dependent ultrafast gain and phase dynamics in bulk SOAs over wide gain region

Based on the semiclassical density-matrix equations, ultrafast probe gain and phase dynamics in bulk semiconductor optical amplifiers excited by a 200 fs Gaussian pump pulse centered at 1,558 nm are numerically investigated and compared over a wide material gain region (1,460–1,650 nm). It is predicted that the recovery time of phase may be longer or shorter than that of gain depending on probe wavelengths. Besides, as functions of probe wavelength, the dynamic gain range and the dynamic phase range have different shapes. Moreover, the wavelength dependence of phase recovery overshoot is illustrated. At last, the probe chirp dynamics and the wavelength dependence of the chirp characteristics are described. These interesting features result from a restriction of the Kramers–Kronig relations and different ways of the competition or cooperation among the effects of spectral-hole burning, carrier heating and two-photon absorption on subpicosecond timescale over the whole concerned band.

Peak equalization of rational-harmonic-mode-locking fiberized semiconductor laser pulse via optical injection induced gain modulation

Optical injection induced gain modulation of a semiconductor optical amplifier (SOA) is demonstrated to equalize the peak intensity of pulses generating from the rational-harmonic-mode-locking (RHML) SOA based fiberized semiconductor laser. This is achieved by adjusting the temporal shape of the injected optical signal generated from a Mach-Zehnder intensity modulator, in which the DC biased level exceeding Vpi and the electrical pulse amplitude of 1.5Vpi are concurrently employed. Numerical simulation on the injected optical signal profile and the SOA gain during the inverse-optical-pulse injection induced gain modulation process are also demonstrated. After a peculiar inverse-optical-pulse injection, each pulse in the 5th-order RHML pulse-train experiences different gain from temporally varied SOA gain profile, leading the pulse peak to equalize one another with a minimum standard deviation of 2.5% on the peak intensity variation. The optimized 5th-order RHML pulse exhibits a signal-to-noise suppression ratio of 20 dB and a reduced variation on temporal spacing from 11 to 4 ps. The clock amplitude jitter is compress from 35.3% to 7.3%, which is less than the limitation up to 10% for 5th order RHML generation.

Polarization-insensitive quantum-dot coupled quantum-well semiconductor optical amplifier

The optical gain of a quantum-dot semiconductor optical amplifier is usually seriously dependent on polarization; we propose a quantum-dot coupled tensile-strained quantum-well structure to obtain polarization insensitivity. The tensile-strained quantum well not only serves as a carrier injection layer of quantum dots but also offers gain to the transverse-magnetic mode. Based on the polarization-dependent coupled carrier rate-equation model, we study carrier competition among quantum well and quantum dots, and study the polarization dependence of the quantum-dot coupled quantum-well semiconductor optical amplifier. We also analyze polarization-dependent photon-mediated carrier distribution among quantum well and quantum dots. It is shown that polarization-insensitive gain can be realized by optimal design.

Quantum-Dot Semiconductor Optical Amplifiers With Polarization-Independent Gains in 1.5-$\mu$m Wavelength Bands

We have demonstrated a polarization-independent gain in semiconductor optical amplifiers that have columnar quantum dots surrounded by strained side barriers in 1.5-mum wavelength bands. We obtained a polarization-dependent gain of 0.5 dB with a gain of 10 dB and a saturation output power of 18 dBm at a wavelength of 1.55 mum.

SOA gain recovery wavelength dependence: simulation and measurement using a single-color pump-probe technique

Measurements to date of the wavelength dependency of gain recovery time in semiconductor optical amplifiers (SOAs) have mostly used pump-probe techniques with a pump and probe operated on distinct wavelengths. Choice of pump wavelength, and its relative proximity to the probe wavelength, could influence measurements and impede unambiguous observation of wavelength dependence on recovery dynamics. We use a single-color pump-probe measurement technique to directly access the wavelength dependence of the gain recovery time in bulk InGaAsP SOAs. We used ultrashort pulses from a single mode locked laser to measure unambiguously the spectral dependency and temporal behavior of SOAs. Simulation results using a model that takes into account intra-band and inter-band contributions to SOA saturation, as well as experimental results for the SOA tested, show recovery rate dependency similar to gain spectrum.

SOA Fiber Ring Laser-Based Three-State Optical Memory

By exploiting three coupled semiconductor optical amplifier (SOA) fiber ring lasers, a compact, integratable, and polarization-independent three-state all-optical memory is demonstrated, with 40-dB extinction ratio for each state. Dynamic flip-flop operation between states is also demonstrated with switching pulse energy of 13.6 nJ. Due to the broad bandwidth of SOA gain spectrum, the wavelength range of set pulses is large. Finally, the transition behavior of state switching is investigated.

SOA based fiber ring laser with Fiber Bragg Grating

AbstractA semiconductor optical amplifier fiber ring laser (SOAFRL) utilizing a Fiber Bragg Grating (FBG) is demonstrated. The laser operates at a wavelength of 1547.64 nm, equal to the Bragg wavelength. By removing FBG, the SOAFRL shows a multi‐modes output due to the resonant characteristics of the semiconductor optical amplifiers (SOA's) gain. The output power ripple is observed around 5 dB. The experimental results show that when the saturation level is reached, the SOAFRL with the FBG shows a higher saturation current and peak power compared to that of the SOAFRL without the FBG. Mode competition in the cavity for the free‐running ring SOAFRL results in a lower peak power as well as a broader bandwidth as the current is increased. By incorporating the FBG in the SOAFRL, a lasing wavelength is generated and as such the side mode suppression ratio (SMSR) of the laser is significantly improved, resulting in a higher peak power at higher bias currents. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 3101–3103, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23895

Analysis of the Physical Impairments on Maximum Size and Throughput of SOA-Based Optical Burst Switching Nodes

In this paper, the physical limitations of semiconductor optical amplifier (SOA)-based switching nodes for optical burst switching (OBS) are investigated. The investigation covers two broadcast-and-select (BAS)-based architectures. Their maximum size and throughput are analyzed for nodes at different bit rates. As main impairments for signal degradation amplifier noise, crosstalk, SOA gain saturation and dynamics, and SOA chirp are evaluated by simulations. In conclusion, general rules for design burst switching nodes are established.

All-Optical Clock Recovery and Pulse Reshaping Using Semiconductor Optical Amplifier and Dispersion Compensating Fiber in a Ring Cavity

An all-optical clock recovery device capable of continuously recovering the clock frequency of optical return-to- zero signals from a 10- to 12.5-GHz repetition rate is reported. The recovered clock consists of chirp-free Gaussian pulses at a diffraction limit with low timing jitter of ~200 fs. We employ intracavity dispersion-compensating fiber (DCF) in order to eliminate chirp and asymmetry of semiconductor optical amplifier (SOA)-amplified pulses. This SOA+DCF ring configuration allows us to easily recover the clock over a 40-nm tuning range corresponding to the SOA gain bandwidth.