Quantum-Dot Semiconductor Mode-Locked Lasers and Amplifiers at 40 GHz

Mode-locked lasers (MLLs) and semiconductor optical amplifiers (SOAs) based on quantum-dot (QD) gain material will impact the development of next-generation networks, such as the 100 Gb/s Ethernet. MLLs presently consisting of a monolithic two-section device already generate picosecond pulse trains at 40 GHz. Temperature dependence of pulsewidth for p-doped devices, a detailed chirp analysis that is a prerequisite for optical time-division multiplexing applications, and data transmission experiments are presented in this paper. QD SOAs show superior performance for linear amplification as well as nonlinear signal processing. Using cross-gain modulation for wavelength conversion, QD SOAs are shown to have a small signal bandwidth beyond 40 GHz under high-bias current injection. This makes QD SOAs much superior to conventional SOAs.      

Theory of pulse-train amplification without patterning effects in quantum-dot semiconductor optical amplifiers

A theory for pulse amplification and saturation in quantum dot (QD) semiconductor optical amplifiers (SOAs) is developed. In particular, the maximum bit rate at which a data stream of pulses can be amplified without significant patterning effects is investigated. Simple expressions are derived that clearly show the dependence of the maximum bit rate on material and device parameters. A comparative analysis of QD, quantum well (QW), and bulk SOAs shows that QD SOAs may have superior properties; calculations predict patterning-free amplification up to bit rates of /spl sim/150-200 Gb/s with pulse output energies of /spl sim/0.2-0.4 pJ. The superiority of QD SOAs is based on: 1) the faster achievement of the regime of maximum gain in QD SOAs compared to QW and bulk SOAs and 2) the lower effective cross section of photon-carrier interaction in QDs.     

Progress in Epitaxial Growth and Performance of Quantum Dot and Quantum Wire Lasers

We report on interplay of epitaxial growth phenomena and device performance in quantum dot (QD) and quantum wire (QWW) lasers based on self-organized nanostructures. InAs QDs are the most explored model system for basic understanding of "near-ideal" QD devices. Vertically-coupled growth of QDs and activated phase separation allow ultimate QD wavefunction engineering enabling GaAs lasers beyond 1400 nm and polarization-insensitive optical amplification. A feasibility of QD semiconductor optical amplifiers at terabit frequencies using InAs QDs is manifested at 1300 and 1500 nm. 1250-1300 nm QD GaAs edge emitters and VCSELs operate beyond 10 Gb/s with ultimate temperature robustness. Furthermore, temperature-insensitive operation without current or modulation voltage adjustment at >20 Gb/s is demonstrated up to ~90 degC. Light-emitting devices based on InGaN-QDs cover ultraviolet (UV) and visible blue-green spectral ranges. In these applications, InN-rich nanodomains prevent diffusion of nonequilibrium carries towards crystal defects and result in advanced degradation robustness of the devices. All the features characteristic to QDs are unambiguously confirmed for InGaN structures. For the red spectral range InGaAlP lasers are used. Growth on misoriented surfaces, characteristic to these devices, leads to nano-periodi- cally-step-bunched epitaxial surfaces resulting in two principal effects: 1) step-bunch-assisted alloy phase separation, leading to a spontaneous formation of ordered natural super lattices; 2) formation of quantum wire-like structures in the active region of the device. A high degree of polarization is revealed in the luminescence recorded from the top surface of the structures, in agreement with the QWW nature of the gain medium. QD and QWW lasers are remaining at the frontier of the modern optoelectronics penetrating into the mainstream applications in key industries.     

WDM-POLSK Transmission Systems by Using Semiconductor Optical Amplifiers

The authors demonstrate the potential of using POLarization Shift Keying (POLSK) modulation format combined with semiconductor optical amplifiers (SOAs) in wavelength division multiplexing (WDM) transmission systems. The constant intensity of the POLSK modulation format allows for substantial removal of cross-gain-modulation impairments in SOAs so that practical amplifier spacing values (more than 100 km) are demonstrated in various different experimental configurations. Record WDM transmission experiments at 10 Gb/s by using SOA-based amplification are presented in short and medium reach system architectures using either single mode fiber or nonzero dispersion shifted fiber     

Time-Resolved Linewidth Enhancement Factors in Quantum Dot and Higher-Dimensional Semiconductor Amplifiers Operating at 1.55 $mu{hbox{m}}$

We report time-resolved measurements of the linewidth enhancement factors (-factors) , and , associated with the adiabatic carrier recovery, carrier heating, and two-photon absorption dynamical processes, respectively, in semiconductor optical amplifiers (SOAs) with different degrees of dimensionality-one InAs/InGaAsP/InP quantum dot (0-D), one InAs/InAlGaAs/InP quantum dash (1-D), and a matching InGaAsP/InGaAsP/InP quantum well (2-D)-all operating near 1.55- wavelengths. We find the lowest values in the QD SOA, 2-10, compared to 8-16 in the QW, and values of and that are also lower than in the QW. In the QD SOA, the -factors exhibit little wavelength dependence over the gain bandwidth, promising for wide-bandwidth all-optical applications. We also find significant differences in the -factors of lasers with the same structure, due to the differences between gain changes that are induced optically or through the electrical bias. For the lasers we find the QW structure instead has the lower -factor, having implications for directly modulated laser applications.     

Transmission performance analysis of 8/spl times/10 Gb/s WDM signals using cascaded SOAs due to signal wavelength displacement

We have analyzed the transmission performance of 8/spl times/10 Gb/s wavelength division multiplexing (WDM) signals due to crosstalk in cascaded conventional semiconductor optical amplifiers (SOAs). Using two different methods, the crosstalk over the whole gain bandwidth in SOAs is calculated to be 2-5 dB lower for the positive detuning. Then, transmission performance of 8/spl times/10 Gb/s WDM signals up to 6/spl times/40 km span in terms of receiver sensitivity is estimated over various transmission distances using cascaded SOAs for the positive signal wavelength displacement of 30, 40, and 50 nm. Especially for 50 nm detuning, transmission performances with and without using a reservoir channel are similar to each other. Our results suggest that SOAs can be used as an optical amplifier for displacement larger than 50 nm without using the reservoir channel.     

High-Speed Quantum-Dot Vertical-Cavity Surface-Emitting Lasers

We report on recent progress in high-speed quantum-dot (QD) vertical-cavity surface-emitting lasers (VCSELs). Advanced types of QD media allow an ultrahigh modal gain, avoid temperature depletion, and gain saturation effects. Temperature robustness up to 100degC for 0.96-1.25 mum range devices is realized in the continuous wave (cw) regime. An open eye 20 Gb/s operation with bit error rates better than 10^-12 has been achieved in a temperature range 25degC - 85degC without current adjustment. A different approach for ultrahigh-speed operation is based on a combination of the VCSEL section, operating in the CW mode with an additional section of the device, which is electrooptically modulated under a reverse bias. The tuning of a resonance wavelength of the second section, caused by the electrooptic effect, affects the transmission of the system. The approach enables ultrahigh-speed signal modulation. 60 GHz electrical and ~35 GHz optical (limited by the photodetector response) bandwidths are realized.     

Dynamic behavior of wavelength converters based on FWM in SOAs

As wavelength converters based on four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs) attract more attention, dynamic effects and wavelength dependent performance become key aspects to be investigated. Such issues are particularly important, as complex configurations are likely to be used to overcome challenges like tunability and polarization dependence. In this paper a numerical model is used to predict the dynamic performance of three FWM configurations and an analytical model is used to derive design rules. First, the wavelength dependent behavior of a wavelength converter is investigated and the requirement for a widely tunable converter is identified. Secondly, a configuration for extinction ratio (ER) improvement is studied and novel design rules are obtained analytically, tested experimentally and explained by the numerical model; experimental results with ER improvement at 10 Gb/s were achieved for the first time. The third configuration studied is a dual-pump arrangement enabling wide tunability. Fixed input/tunable output and tunable input/fixed output configurations are discussed in terms of optical signal-to-noise ratio and tunability. Design rules are extracted and verified for all three configurations that are likely to be deployed: simple wavelength converters, regenerating converters and tunable wavelength converters.     

InAs/InP Quantum-Dash Lasers and Amplifiers

InAs quantum-dash structures fabricated by self-assembly growth techniques and based on compound semiconductors lattice matched to InP substrates were used to realize long wavelength lasers and amplifiers for telecom applications. With this new type of laser material special properties of low-dimensional electronic systems can be utilized for device applications, which allow to realize new device features not possible by conventional device designs. In this paper a brief overview is given about application oriented material and device research on this wire/dot-like material system by highlighting laser and high-speed optical amplifiers. Broadband laser material with a gain bandwidth of more than 300 nm could be obtained to cover the extended telecommunication wavelength range between 1.4 and 1.65 . High-speed optical amplifiers could be realized by using this quantum-dash laser material with unique device performance, like multiwavelength amplification without any cross-talk at data rates of 10 Gbit/s and pattern-free and noise reduced signal amplification at saturation condition demonstrated up to 40 Gbit/s.     

A wavelength-tunable optical transmitter using semiconductor optical amplifiers and an optical tunable filter for metro/access DWDM applications

This paper proposes and demonstrates a widely tunable and precisely controllable optical transmitter that uses semiconductor optical amplifiers (SOAs) and an optical tunable filter (OTF) for metro/access dense-wavelength-division-multiplexing (DWDM) systems/networks. The transmitter consists of a fiber-ring-laser section based on SOA(s) and an OTF and a modulation section based on an SOA. First, the requirements imposed on the components intended for the fiber-ring laser are clarified through approximate analyses and basic experiments. To obtain low-noise characteristics, the lower limit of effective OTF bandwidth is investigated by considering the SOA carrier lifetime. For obtaining wavelength precision, the upper limit of the effective OTF bandwidth is derived by considering the SOA ripple effect. Next, the proposed transmitter is demonstrated, where all SOAs are integrated into one planar lightwave circuit (PLC) platform using spot-size converting technology. By designing the fiber-ring-laser section according to the clarified requirements, a wavelength control precision of /spl plusmn/1.75GHz is achieved while realizing the required relative intensity noise (RIN) from 1540 to 1560 nm. Finally, it is demonstrated that the transmitter can handle signals at up to 1.25 Gb/s.     

Distortion-free optical amplification of 20-80 GHz modelocked laser pulses at 1.3 /spl mu/m using quantum dots

Distortion-free amplification of modelocked laser pulses at 20, 40 and 80 GHz with a minimum pulse width of 710 fs is reported. The modelocked lasers and the semiconductor optical amplifiers are based on identical quantum dot material emitting at 1.3 /spl mu/m.     

Latest advances in high-performance light sources and optical amplifiers on silicon

Efficient light generation and amplification has long been missing on the silicon platform due to its well-known indirect bandgap nature.Driven by the size,weight,power and cost(SWaP-C)requirements,the desire to fully realize integrated silicon electronic and photonic integrated circuits has greatly pushed the effort of realizing high performance on-chip lasers and amplifiers moving forward.Several approaches have been proposed and demonstrated to address this issue.In this paper,a brief overview of recent progress of the high-performance lasers and amplifiers on Si based on different technology is presented.Representative device demonstrations,including ultra-narrow linewidthⅢ-Ⅴ/Si lasers,fully integratedⅢ-Ⅴ/Si/Si3N4 lasers,high-channel count mode locked quantum dot(QD)lasers,and high gain QD amplifiers will be covered.     

Polarization-insensitive SOA with a strained bulk active layer fornetwork device application

Current and wavelength characteristics for 1.55-μm semiconductor optical amplifiers (SOAs) containing bulk active layers with various tensile strains were investigated. The strain dependence of the gain-difference between the TE-mode and TM-mode (ΔG_TE-TM ) was almost linear having a waveguide-width over 0.8-1.5 μm. A -0.12% tensile-strained bulk SOA had very low ΔG_TE-TM of less than 0.8 dB for driving current ranging from 0 to 120 mA. The low-polarization-sensitive condition of SOAs with strained-bulk active layers was shown to yield very wide range in driving current and wavelength for network device applications     

Reduced crosstalk semiconductor optical amplifiers based on Type-IIquantum wells

It is shown theoretically that semiconductor optical amplifiers (SOAs) with type-II quantum-well active regions in SOAs will have up to 10 dB lower interchannel crosstalk at high (>2.5 Gb/s) data rates. That reduction is attained via the corresponding increase in saturation energy without any reduction in the efficiency     

Comparison of interferometric all-optical switches fordemultiplexing applications in high-speed OTDM systems

We have investigated three interferometric all-optical switches based on cross-phase modulation (XPM) in semiconductor optical amplifiers (SOAs), the semiconductor laser amplifier in a loop mirror (SLALOM) switch, the Mach-Zehnder interferometer (MZI) switch, and the ultrafast nonlinear interferometer (UNI) switch. Switching windows with different widths are measured under similar conditions for all three switching configurations. We introduce the integrated contrast ratio (ICR) as a measure to evaluate the performance of a switch from switching windows. Using the ICR, the switches are compared and their application is discussed as demultiplexer in optical time division multiplexing (OTDM) systems for data rates of 40, 80, and 160 Gb/s     

基于两只半导体光放大器级联结构的多波长光纤激光器

通过数值模拟对两只半导体光放大器(SOA)级联结构的静态增益饱和特性进行了理论研究.在不考虑自发辐射的情况下,分析了注入电流对两只SOA级联结构增益的影响.实验上构建了一种基于两只SOA级联结构的多波长光纤激光器,观测并分析了半导体光放大器的驱动电流和增益带宽对多波长输出结果的影响.在室温下,获得了基本符合ITU-T标准100 GHz的27个波长以上的稳定多波长输出,各信道输出功率不平坦度小于±3 dB,线宽小于0.102 nm,信噪比大于25 dB,总输出功率为1.94 mW,并且与由单只SOA构成的多波长光纤激光器进行了对比.     

注入锁定半导体激光器全光波长转换技术

波长转换器是光通信网络中的一个重要器件。而除半导体光放大器（SOA）外，半导体激光器也是进行波长变换的一种很好选择。基于半导体激光器的注入锁定波长变换技术具有转换带宽较大、啁啾小、消光比特性好、结构简单、成本低廉等诸多优点。将探测光与信号光同步注入法布里-珀罗（F-P）半导体激光器，可以通过信号光功率的变化控制激光器锁模与失锁，导致腔内纵模变化，探测光随之被共振放大或减弱，从而将信息由信号光转换到探测光频率上。从静态实验入手，对半导体激光器的注入锁定现象及光信号控制法布里-珀罗纵模移动等问题分别进行了研究。分析了动态转换激光器工作点的选取问题，在动态实验中实现了较宽范围的正相与反相波长转换，转换速率达到了10Gb／s。     

Characterization of the chirp and intensity modulation propertiesof an SOA-MZI wavelength converter

Wavelength converters based on cross-phase modulation in semiconductor optical amplifiers (SOAs) in the arms of a Mach-Zehnder interferometer (MZI) provide a key network element for reconfigurable optical networks that incorporate wavelength routing. For system design purposes, the properties of the intensity and phase (chirp) modulation of the wavelength-converted signal for an SOA-MZI wavelength converter are important. In this paper, the small-signal α parameter, which characterizes the converter chirp, and the conversion frequency response are characterized experimentally for an integrated all-active SOA-MZI wavelength converter. For both co- and counterpropagating signals, the variation of the a parameter along the interference and conversion curves is considered in detail. Three optical modulators with quite different chirp properties are used to generate the modulated input signal to the wavelength converter. The results demonstrate that the chirp of the wavelength-converted signal is primarily determined by the device properties and the intensity of the modulated input signal. The a parameter of the wavelength-converted signal is negative for noninverting operation and positive for inverting operation. An important contribution of the paper is the detailed assessment of this key device characteristic. The experimental characterization of the wavelength converter is incorporated into a device model that can be used to obtain the pulse response. Calculated and measured results for the time dependence of the intensity and chirp of the wavelength-converted signal are in good agreement. The conversion frequency response for the intensity modulation is also measured along the conversion curve. The 3-dB bandwidth is found to be generally about 8-10 GHz     

Improving the all-optical response of SOAs using a modulated holding signal

A method for increasing the all-optical modulation bandwidth of semiconductor optical amplifiers (SOAs) by use of a cross-gain-modulated (XGM) holding signal is suggested and analyzed. The bandwidth improvement is numerically demonstrated by studying wavelength conversion in an SOA-based Mach-Zehnder interferometer (MZI) at 160 and 40 Gb/s. The new scheme is predicted to improve the extinction ratio and the minimum mark output power, as well as to reduce the amplitude jitter of the wavelength converted signal.     

Wide-band polarization-free wavelength conversion based on four-wave-mixing in semiconductor optical amplifiers

Wide-band polarization free wavelength conversion based on four-wave mixing in semiconductor optical amplifiers (SOAs) subject to two pump beams has been studied in detail. With equalized forward and backward pump power, polarization independence of the converted signal was experimentally achieved when the wavelength detuning was larger than 1.24 nm. This is independent of the SOA bias current. When the wavelength detuning between the signal and one pump beam was fixed at 1.6 nm, the amplitude of the converted signal was nearly constant over a 58-nm wavelength range. A new theoretical analysis allows for signals of arbitrary polarization state to be considered. The experimental results are in good agreement with the theory developed here.