Holger Schmeckebier

Complete pulse characterization of quantum-dot mode-locked lasers suitable for optical communication up to 160 Gbit/s

A complete characterization of pulse shape and phase of a 1.3 microm, monolithic-two-section, quantum-dot mode-locked laser (QD-MLL) at a repetition rate of 40 GHz is presented, based on frequency resolved optical gating. We show that the pulse broadening of the QD-MLL is caused by linear chirp for all values of current and voltage investigated here. The chirp increases with the current at the gain section, whereas larger bias at the absorber section leads to less chirp and therefore to shorter pulses. Pulse broadening is observed at very high bias, likely due to the quantum confined stark effect. Passive- and hybrid-QD-MLL pulses are directly compared. Improved pulse intensity profiles are found for hybrid mode locking. Via linear chirp compensation pulse widths down to 700 fs can be achieved independent of current and bias, resulting in a significantly increased overall mode-locking range of 101 MHz. The suitability of QD-MLL chirp compensated pulse combs for optical communication up to 160 Gbit/s using optical-time-division multiplexing are demonstrated by eye diagrams and autocorrelation measurements.

80 Gb/s wavelength conversion using a quantum-dot semiconductor optical amplifier and optical filtering

Wavelength conversion of 40 Gb/s and 80 Gb/s return-to-zero on-off-keying signals using a quantum-dot semiconductor optical amplifier in combination with a delay interferometer as subsequent filter is demonstrated. The performance of the 80 Gb/s wavelength converter measured in terms of the bit-error ratio demonstrated here is the highest reported up to now for quantum-dot semiconductor optical amplifiers. The typical fast gain dynamics manifests itself in open eye diagrams of the converted signal. The slow phase dynamics of the carrier reservoir however induces severe patterning and requires compensation. Adaptation of the free-spectral range of the delay interferometer is necessary in order to mitigate these phase effects and to achieve error-free wavelength conversion.

Hybrid Mode Locking in Semiconductor Lasers: Simulations, Analysis, and Experiments

Hybrid mode locking in a two-section edge-emitting semiconductor laser is studied numerically and analytically using a set of three delay differential equations. In these equations, the external RF signal applied to the saturable-absorber section is modeled by the modulation of the carrier relaxation rate in this section. The estimation of the locking range where the pulse repetition frequency is synchronized with the frequency of the external modulation is performed numerically and the effect of the modulation shape and amplitude on this range is investigated. Asymptotic analysis of the dependence of the locking range width on the laser parameters is carried out in the limit of small-signal modulation. Our numerical simulations indicate that hybrid mode locking can be also achieved in the cases when the frequency of the external modulation is approximately twice and half of the pulse repetition frequency of the free-running passively mode-locked laser fP . Finally, we provide an experimental demonstration of hybrid mode locking in a 20-GHz quantum-dot laser with the modulation frequency of the reverse bias applied to the absorber section close to fP/2.

The Input Power Dynamic Range of a Semiconductor Optical Amplifier and Its Relevance for Access Network Applications

The input power dynamic range (IPDR) of a semiconductor optical amplifier (SOA) gives the input power range within which an SOA can be operated error free. It is among the most important parameters describing the usability range of an SOA in an access network. In this paper, we give design guidelines to maximize the IPDR at a given gain. Our IPDR description indicates that a large IPDR can be obtained if SOAs are designed properly. A particular large IPDR is predicted to be found for well-designed quantum-dot (QD)-SOAs. We apply both theory and experiment to a 1.3- μm QD-SOA and investigate the IPDR as a function of bitrate, wavelength, and bias current. Large IPDRs of 41 and 36 dB are found for single-channel experiments with a signal quality of Q2 = 12.6 dB at 2.5 and 40 Gbit/s, respectively.

Comparison of dynamic properties of ground- and excited-state emission in p-doped InAs/GaAs quantum-dot lasers

The dynamic properties of ground- and excited-state emission in InAs/GaAs quantum-dot lasers operating close to 1.31 μm are studied systematically. Under low bias conditions, such devices emit on the ground state, and switch to emission from the excited state under large drive currents. Modification of one facet reflectivity by deposition of a dichroic mirror yields emission at one of the two quantum-dot states under all bias conditions and enables to properly compare the dynamic properties of lasing from the two different initial states. The larger differential gain of the excited state, which follows from its larger degeneracy, as well as its somewhat smaller nonlinear gain compression results in largely improved modulation capabilities. We demonstrate maximum small-signal bandwidths of 10.51 GHz and 16.25 GHz for the ground and excited state, respectively, and correspondingly, large-signal digital modulation capabilities of 15 Gb/s and 22.5 Gb/s. For the excited state, the maximum error-free bit rate i...

Cross-Gain Modulation and Four-Wave Mixing for Wavelength Conversion in Undoped and p-Doped 1.3-

P-doped and undoped quantum dot (QD) semiconductor optical amplifiers (SOAs) having a similar chip gain of 22-24 dB are compared with regard to their static and dynamic characteristics. Amplified spontaneous emission (ASE) spectra reveal the influence of p-doping on the gain characteristics and the temperature stability. In contrast to QD lasers, p-doping does not significantly increase the thermal stability of QD SOAs. The static four-wave mixing efficiency is larger and more temperature stable in undoped devices, leading to a maximum chip conversion efficiency of -2 dB. Small-signal cross-gain modulation (XGM) experiments show an increase in the small-signal bandwidth from 25 GHz for the p-doped SOAs to 40 GHz for the undoped QD SOAs at the same current density. P-doped QD SOAs also achieve small-signal bandwidths beyond 40 GHz but at a larger bias. The XGM is found to be temperature stable in the range of 20°C to 40°C.

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The static and dynamic characteristics of degenerate four-wave mixing in a quantum dot semiconductor optical amplifier are investigated. A high chip conversion efficiency of 1.5 dB at 0.3 nm detuning, a low (< 5 dB) asymmetry of up and down conversion and a spectral conversion range of 15 nm with an optical signal-to-noise ratio above 20 dB is observed. The comparison of pumping near the gain peak and at the edge of the gain spectrum reveals the optical signal-to-noise ratio as the crucial parameter for error-free wavelength conversion. Small-signal bandwidths well beyond 40 GHz and 40 Gb/s error-free 5 nm wavelength down conversion with penalties below 1 dB are presented. Due to the optical signal-to-noise ratio limitation, wavelength up conversion is error-free at a pump wavelength of 1311 nm with a penalty of 2.5 dB, whereas an error floor is observed for pumping at 1291 nm. A dual pump configuration is demonstrated, to extend the wavelength conversion range enabling 15.4 nm error-free wavelength up conversion with 3.5 dB penalty caused by the additional saturation of the second pump. This is the first time that 40 Gb/s error-free wavelength conversion via four-wave mixing in quantum-dot semiconductor optical amplifiers is presented.

Quantum Dot Semiconductor Optical Amplifiers

We describe the formation of a strong pulse asymmetry in mode-locked quantum-dot edge-emitting two-section semiconductor lasers. A mode decomposition technique reveals the role of the superposition of different modal groups. The results of theoretical analysis are supported by the experimental data.

40 Gb/s wavelength conversion via four-wave mixing in a quantum-dot semiconductor optical amplifier

We consider a mode-locked (ML) quantum-dot (QD) edge-emitting semiconductor laser consisting of a reverse-biased saturable absorber and a forward-biased amplifying section. To describe the dynamics of this laser, we use the traveling wave model taking into account carrier exchange processes between a reservoir and the QDs. A comprehensive parameter study is presented and an analysis of mode-locking pulse broadening with an increase of injection current is performed. The results of our theoretical analysis are supported by experimental data demonstrating a strong pulse asymmetry in a monolithic two-section QD laser.

Strong pulse asymmetry in quantum-dot mode-locked semiconductor lasers

Carrier heating in quantum dot (QD) devices, which accompanies Auger capture of carriers from a carrier reservoir to discrete QD levels, is considered for the first time. Equations for carrier dynamics of QD structures are formulated and analyzed by taking into account the carrier heating. A numerical example shows that heating of carriers in a carrier reservoir of a QD structure can be much higher than that of bulk and quantum well devices. Auger capture carrier heating in QD devices can lead to a longer (more than a factor of 2 for the 90%-recovery time) relaxation time from a carrier reservoir to QDs.