Quantum-well Semiconductor Optical Amplifiers Research Articles

Engineering Nonlinear Effects of Quantum-Well Semiconductor Optical Amplifiers for Applications in Optical Signal Processing

Quantum-well semiconductor optical amplifiers (QW-SOAs) have been widely used in all-optical signal processing functions because of their various nonlinear effects. For different optical signal processing functions, the requirements for performance of the QW-SOAs are different, and nonlinear effects of the QW-SOAs should be controlled selectively. Engineering nonlinear effects of QW-SOA is very important for different applications in optical signal processing. Here, a comprehensive QW-SOA model by combining the QW band structure calculation with the dynamic model is established to connect the structure parameters with the nonlinear effects of QW-SOA. Those characterized parameters of the QW-SOAs, such as gain coefficient, differential gain coefficient, refractive index change, differential refractive index change, linewidth enhancement factor, third-order susceptibility and polarization dependent gain, are used to characterize the nonlinear effects, and are calculated with different structure parameters based on this developed model. Take the wavelength conversion based on cross gain modulation effect and four wave mixing effect as examples, the structure parameters of the QW-SOAs are optimized for engineering the nonlinear effects and achieving the best output performance. This theoretical work presents a guide for choosing the optimized structure parameters while fabricating the QW-SOAs for different optical signal processing functions

Ultrafast gain recovery in a QW-SOA and its application for 40 Gb/s regenerative format conversion from NRZ-DPSK to RZ-OOK

We propose a scheme to realize gain recovery of the order of a few picoseconds in a typical quantum well semiconductor optical amplifier (QW-SOA) using a pump signal with amplitude dips. In the scheme, the gain recovery is determined by the fast carrier density depletion and carrier temperature heating processes. The ultrafast gain recovery is numerically verified by a completed model combining the QW band structure calculation with rate equations in the QW-SOA. In practice, a pump signal with amplitude dips can be partly characterized by a non-return-to-zero differential phase-shift keying (NRZ-DPSK) signal generated by a Mach–Zehnder modulator. Therefore, we experimentally demonstrate 40 Gb/s wavelength-converting and regenerative format conversion from NRZ-DPSK to return-to-zero on–off keying (RZ-OOK), which may be a key function to connect different kinds of modulation formats at gateway nodes between long-haul backbone networks and metro area networks.

Widely Continuous Tunable ps Pulse Train Generation Based on SOA Nonlinear Dynamics

We propose a simple setup based on nonlinear dynamics in a semiconductor optical amplifier (SOA) and solitonic compression for the generation of stable and tunable repetition rate picosecond (ps) optical pulses. A quantum-well SOA is used to amplify electrical-optic-modulated rectangular optical signal, during which the nonlinear amplification process induces front-edge distortion. Due to the non-zero linewidth enhancement factor, the front-edge distortion can be isolated by the optical filtration of the chirped frequency component, which eventually becomes Gaussian-like pulse train of tens of ps FWHM pulsewidth. Following a stage of solitonic compression and a stage of pedestal suppression, we are able to obtain a train of 1-ps high-quality pulses for which the root mean square timing jitter is $C$ -band.

A novel transmission line model for quantum well semiconductor optical amplifiers

This paper presents a new wideband model for quantum well semiconductor optical amplifiers (QW-SOAs) based on quantum well transmission line modelling method to facilitate the performance analysis of QW-SOAs. The dynamic process of electron-photon interactions during the photon propagation has been considered in this model. The gain and spontaneous emission spectra obtained by this model are in very agreement with those obtained by the analytical expression within a wide spectra band ranging from 1300 to 1700 nm. The temporal and spectral output waveforms of the pulse input signal having the peak power values of 0.1 and 1 mw are studied and it was found that the new model can accurately predict the temporal and spectral behaviours of the amplified output signal, spectral broadening, red shift and temporal distortion induced by the gain saturation.

Wavelength-dependent femtosecond pulse amplification in wideband tapered-waveguide quantum well semiconductor optical amplifiers.

In this paper, we study the wavelength-dependent amplification in three different wideband quantum well semiconductor optical amplifiers (QWAs) having conventional, exponentially tapered, and linearly tapered active region waveguide structures. A new theoretical model for tapered-waveguide QWAs considering the effect of lateral carrier density distribution and the strain effect in the quantum well is established based on a quantum well transmission line modeling method. The temporal and spectral characteristics of amplified femtosecond pulse are analyzed for each structure. It was found that, for the amplification of a single femtosecond pulse, the tapered-waveguide QWA provides higher saturation gain, and the output spectra of the amplified pulse in all three structures exhibit an apparent redshift and bandwidth narrowing due to the reduction of carrier density; however, the output spectrum in the tapered-waveguide amplifier is less distorted and exhibits smaller bandwidth narrowing. For the simultaneous amplification of two femtosecond pulses with different central frequencies, in all the three structures, two peaks appear in the output spectra while the peak at the frequency closer to the peak frequency of the QWA gain spectrum receives higher amplification due to the frequency (wavelength) dependence of the QWA gain. At a low peak power level of the input pulse, the bandwidth of each window in the tapered structure is larger than that of the conventional waveguide structure, which aggravates the spectrum alias in the amplification of femtosecond pulses with different central frequencies. As the peak powers of the two pulses increase, the spectrum alias in the conventional waveguide becomes more serious while there are small changes in the tapered structures. Also, we have found that in the amplification of a femtosecond pulse train, the linear-tapered QWAs exhibit the fastest gain recovery as compared with the conventional and exponentially tapered QWAs.

Analysis of strain effects on the dynamic spectra of a quantum well semiconductor optical amplifier using quantum well transmission line modelling method

This paper studies the strain (i.e. compressive (CS) and tensile (TS)) effects on the dynamic spectra of an amplified femtosecond pulse in a quantum well semiconductor optical amplifier (QW-SOA) using quantum well transmission line modelling (QW-TLM) method. Based on the analysis of band structure, the gain spectrum as well as the spontaneous spectrum of quantum well (QW) in the CS, unstrained (US) and TS are investigated using QW-TLM and it was found that in the CS QW, the magnitude ratio of the gain spectrum and the spontaneous emission spectrum is the largest. Furthermore, QW-TLM is adopted to investigate the dynamic spectral evolution of femtosecond pulse amplification in QW-SOAs and it was found that as the femtosecond pulse approaches the amplifier output, the centre frequency of the amplified femtosecond pulse spectra decreases and its bandwidth decreases. The output spectra of the amplified femtosecond pulse in QW amplifiers under the CS, US and TS cases are compared and the simulation results show that in a CS QW-SOA the spectral shape exhibits the largest magnitude and the smallest fluctuation due to the largest gain and the largest ratio between the gain and noise.

Experimental demonstration and devices optimization of NRZ-DPSK amplitude regeneration scheme based on SOAs.

We propose and experimentally demonstrate a novel scheme which can simultaneously realize wavelength-preserving and phase-preserving amplitude noise compression of a 40 Gb/s distorted non-return-to-zero differential-phase-shift keying (NRZ-DPSK) signal. In the scheme, two semiconductor optical amplifiers (SOAs) are exploited. The first one (SOA1) is used to generate the inverted signal based on SOA's transient cross-phase modulation (T-XPM) effect and the second one (SOA2) to regenerate the distorted NRZ-DPSK signal using SOA's cross-gain compression (XGC) effect. In the experiment, the bit error ratio (BER) measurements show that power penalties of constructive and destructive demodulation at BER of 10-9 are -1.75 and -1.01 dB, respectively. As the nonlinear effects and the requirements of the two SOAs are completely different, quantum-well (QW) structures has been separately optimized. A complicated theoretical model by combining QW band structure calculation with SOA's dynamic model is exploited to optimize the SOAs, in which both interband effect (carrier density variation) and intraband effect (carrier temperature variation) are taken into account. Regarding SOA1, we choose the tensile strained QW structure and large optical confinement factor to enhance the T-XPM effect. Regarding SOA2, the compressively strained QW structure is selected to reduce the impact of excess phase noise induced by amplitude fluctuations. Exploiting the optimized QW SOAs, better amplitude regeneration performance is demonstrated successfully through numerical simulation. The proposed scheme is intrinsically stable comparing with the interferometer structure and can be integrated on a chip, making it a practical candidate for all-optical amplitude regeneration of high-speed NRZ-DPSK signal.

Analysis of carrier heating effects in quantum well semiconductor optical amplifiers considering holes’ non-parabolic density of states

This paper studies effects of carrier heating on the amplified pico-second Gaussian pulse propagation in a quantum well semiconductor optical amplifier (QW-SOA) taking into account the holes’ non-parabolic density of states. In the analysis we have also considered the discontinuous distribution of energy band structure and the coupling between heavy hole, light hole and spin-orbit spilt-off bands. It has been found that at higher temperatures the wavelength at which the peak value of modal gain occurs decreases (small blue shift). The pulse amplification performances of QW-SOAs for two unchirped Gaussian input pulses having the peak power values of 10 and 100 mw have been investigated both in the presence and absence of the carrier heating effects. It has been found that carrier heating effects broaden the amplified output signal pulse width and lower the peak value of the amplified output pulse.

Single- and Dual-Channel DPSK Signal Amplitude Regeneration Based on a Single Semiconductor Optical Amplifier

We experimentally demonstrate that, based on operational condition optimization, a common quantum well (QW) semiconductor optical amplifier (SOA) has the ability of amplitude regeneration for return-to-zero (RZ) differential phase shift keying (DPSK) signals. For a single-channel RZ-DPSK regeneration scheme, a significant eye-opening enhancement and a negative power penalty of about 1.0 dB are obtained. For dual-channel RZ-DPSK regeneration, it can also be found that the eye-opening improvement and power penalty decrease in each channel. In addition, the DPSK regeneration scheme based on a single QW SOA is quite simple and stable.

RZ-DQPSK Signal Amplitude Regeneration Using a Semiconductor Optical Amplifier

An all-optical return-to-zero differential quadrature phase-shift keying (RZ-DQPSK) signal regeneration scheme is experimentally demonstrated. Thanks to the gain saturation effect, common quantum-well semiconductor optical amplifiers have the ability to regenerate the amplitude distorted RZ-DQPSK signal, so amplitude noise can be reduced while phase information will not be distorted. A significant eye-opening improvement and a negative power penalty of about 1 dB can be achieved for an 80 Gb/s RZ-DQPSK signal.

Theoretical investigation on gain recovery dynamics in step quantum well semiconductor optical amplifiers

In this paper, gain recovery dynamic characteristics in the step quantum well (QW) semiconductor optical amplifier (SOA) are theoretically investigated via a detailed model. We numerically solve the coupled rate equations including microscopically calculated carrier-phonon scattering rates between the carrier reservoir and the ground state on the basis of Fermi’s golden rule. The carrier-phonon scattering rates are given as functions of the width and height of the step in the QW of the SOA. It is demonstrated that the electron scattering rate in the step QW SOA depends on the potential parameters of the carrier reservoir region. Finally, it is shown that the SOA with a larger transition rate has a shorter carrier recovery time than the other SOAs via analyzing and comparing the gain and phase recovery dynamics in different types of SOA samples.

Gain Recovery Acceleration by Enhancing Differential Gain in Quantum Well Semiconductor Optical Amplifiers

It is demonstrated theoretically that the quantum well (QW) semiconductor optical amplifiers (SOAs) with high differential gain will show fast gain recovery dynamic characteristics. Based on the calculation of energy band structure, the effects of compressive strain and p-type modulation doping on the gain, the differential gain, and linewidth enhancement factor (a-factor) are investigated. The peak of the differential gain spectrum shifts to longer wavelength, and the peak value is significantly enhanced by increasing compressive strain. Meanwhile the gain is obviously increased by increasing the p-type modulation doping concentration. By comparing the gain and phase recovery dynamics in three different types of SOA samples, it is shown that the QW SOA with highest differential gain has the shortest gain recovery time among the three samples.

Optimized Quantum–Well Semiconductor Optical Amplifier for RZ-DPSK Signal Regeneration

In this paper, we present an optimized quantum-well (QW)-semiconductor optical amplifier (SOA) which is capable of regenerating phase-modulated signals, such as return-to-zero differential-phase-shift keying (RZ-DPSK) signal. Based on the optimized QW SOA, the amplitude fluctuations are suppressed while the phase information is preserved. The essential mechanism of the optimized QW SOA is the low linewidth enhancement factor (α-factor), which can be obtained by means of structural optimization. Based on the numerical model, the power fluctuations can be suppressed from 4.26 mW (33% of input peak power) to 1.59 mW (13% of output peak power) at 40 Gb/s. The average phase noise induced by SOA is as small as 50. Furthermore, the DPSK signal regeneration scheme based on a single QW SOA is quite simple and stable.

Packaged 1.5-$\mu$m Quantum-Well SOA With 0.8-W Output Power and 5.5-dB Noise Figure

We report the demonstration of a lensed-fiber-pigtailed InGaAsP-InP quantum-well semiconductor optical amplifier based on the slab-coupled optical waveguide (SCOW) concept. At a 5-A bias current and a wavelength of 1540 nm, the packaged SCOW amplifier (SCOWA) exhibits a record 0.8-W saturation output power, 13.8-dB small-signal gain, 5.5-dB noise figure, and a maximum electrical-to-optical conversion efficiency of 11%. The estimated coupling efficiency between the large (5.6 times 7.5 mum), fundamental SCOWA mode and the lensed fibers (6.5-mum spot size) is 90%.

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.

Analytical Model for Cross-Gain Modulation and Crosstalk in Quantum-Well Semiconductor Optical Amplifiers

An analytical model of the dynamic characteristics of a quantum-well (QW) semiconductor optical amplifier (SOA) is developed. Closed-form expressions for the optical gain and cross-gain modulation (XGM) for arbitrary input pulses are derived. The model takes into account the carrier capture and escape transitions between the QW and the continuum states. This model is also used to derive a closed-from expression for interchannel XGM crosstalk in multichannel SOA systems. The model/analysis provides insight into the effect of the SOA parameters on the performance of a wavelength-division multiplexed system. We found that crosstalk in a multichannel SOA system can be reduced by reducing the escape lifetime.

量子阱和量子点半导体光放大器中的慢光和快光

量子阱和量子点半导体光放大器中的慢光和快光

Carrier Dynamics of Quantum-Dot, Quantum-Dash, and Quantum-Well Semiconductor Optical Amplifiers Operating at 1.55 $\mu{\hbox {m}}$

We assess the influence of the degree of quantum confinement on the carrier recovery times in semiconductor optical amplifiers (SOAs) through an experimental comparative study of three amplifiers, one InAs-InGaAsP-InP quantum dot (0-D), one InAs-InAlGaAs-InP quantum dash (1-D), and one InGaAsP-In-GaAsP-InP quantum well (2-D), all of which operate near 1.55-mum wavelengths. The short-lived (around 1 ps) and long-lived (up to 2 ns) amplitude and phase dynamics of the three devices are characterized via heterodyne pump-probe measurements. The quantum-dot device is found to have the shortest long-lived gain recovery (~80 ps) as well as gain and phase changes indicative of a smaller linewidth enhancement factor, making it the most promising for high-bit-rate applications. The quantum-dot amplifier is also found to have reduced ultrafast transients, due to a lower carrier density in the dots. The quantum-dot gain saturation characteristics and temporal dynamics also provide insight into the nature of the dot energy-level occupancy and the interactions of the dot states with the wetting layer.

Slow-to-fast light using absorption to gain switching in quantum-well semiconductor optical amplifier

Room temperature quantum-well semiconductor optical amplifier with large input power is utilized in both the absorption and gain regime as an optical group delay and advance (slow and fast light), respectively. Material resonance created by coherent population oscillation and four wave mixing is tuned by electrical injection current, which in turn controls the speed of light. The four-wave mixing and population oscillation model explains the slow-to-fast light switching. Experimentally, the scheme achieves 200 degrees phase shift at 1 GHz, which corresponds to 0.56 delay-bandwidth product. The device presents a feasible building block of a multi-bit optical buffer system.

Reduced Recovery Time Semiconductor Optical Amplifier Using p-Type-Doped Multiple Quantum Wells

The effect of p-type doping of the active region of multiple quantum-well (MQW) semiconductor optical amplifiers (SOAs) has been studied. Spectrogram measurements of the dynamics of the SOAs reveal that using p-doped barriers for the MQWs has significantly reduced both gain and phase recovery times. 1/e phase recovery times as short as 11 ps were demonstrated using this approach