D. Arsenijević

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.

Hybrid mode-locking in a 40 GHz monolithic quantum dot laser

Mode-locked semiconductor lasers are efficient sources of short optical pulses ideal for applications in high speed telecommunication systems. Especially promising for telecom applications is the new generation of quantum dot mode-locked lasers (QD-MLL) which demonstrate important advantages over the standard quantum well devices [1]. However, performance improvement is still an ongoing issue, in particular for the efficiency of hybrid mode-locking, — a commonly used technique to improve characteristics and synchronize the mode-locked pulses — namely the dependence of the locking regime on the frequency and power of the applied external signal. Based on our previous results on passive mode-locking in quantum dot lasers [2], we study experimentally and theoretically a monolithic two-section hybrid mode-locked quantum dot laser with periodically modulated reverse bias applied to the saturable absorber section.

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.

Large-Signal Response of Semiconductor Quantum-Dot Lasers

In this paper, the large-signal response of a quantum-dot laser is investigated. Based on experimental results, we show that including a dynamic device temperature as well as Auger recombination processes in the carrier reservoir is crucial to model the dynamic response of a quantum-dot laser for large variations of the pump current. A detailed analysis of the influence of temperature effects on the dynamics of the device is performed. Simulated eye diagrams are presented and compared with experimental results at the emission wavelength of 1.3 μm.

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...

1.3 µm range 40 GHz quantum-dot mode-locked laser under external continuous wave light injection or optical feedback

We have investigated the jitter, repetition frequency and spectra of a quantum-dot mode-locked laser operating at 40 GHz under continuous wave external light injection using an external cavity laser or under optical feedback. For the case of the continuous wave light injection we realized a shift of the optical emission of up to 15 nm. With optical feedback we have found a ninefold jitter reduction, down to a record low value of 1 ps over an integration range from 10 kHz to 1 GHz. Optical feedback fixes the repetition frequency while changing current and voltage applied to the diode. This is due to the fact that stable mode locking occurs at frequencies which are integers of the feedback loop frequency.

Phase noise and jitter reduction by optical feedback on passively mode-locked quantum-dot lasers

The noise properties of the pulse trains of passively mode-locked 40 GHz quantum-dot lasers subject to optical feedback are investigated in detail. Five different feedback regimes are discovered and the clearly identified regime of resonant optical feedback is further examined. The feedback parameters yielding minimum phase noise are determined. Here, the radio-frequency (RF)-line-width is reduced from its original value by 99% to 1.9 kHz. The corresponding pulse-to pulse jitter of 23 fs is a record low value for passively mode-locked 40 GHz quantum-dot lasers.

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.