S. Liebich

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.

Static gain saturation in quantum dot semiconductor optical amplifiers.

Measurements of saturated amplified spontaneous emission-spectra of quantum dot semiconductor optical amplifiers demonstrate efficient replenishment of the quantum-dot ground state population from excited states. This saturation behavior is perfectly modeled by a rate equation model. We examined experimentally the dependence of saturation on the drive current and the saturating optical pump power as well as on the pump wavelength. A coherent noise spectral hole is observed with which we assess dynamical properties and propose optimization of the SOA operating parameters for high speed applications.

High-Speed Small-Signal Cross-Gain Modulation in Quantum-Dot Semiconductor Optical Amplifiers at 1.3

Wavelength conversion using cross-gain modulation (XGM) in quantum-dot (QD) semiconductor optical amplifiers (SOAs) is investigated. Small-signal measurements reveal that the XGM bandwidth as well as the conversion efficiency strongly depends on the bias current. Thus, it is possible to tune the XGM by increasing the current from a low efficiency with a 10-GHz bandwidth to a very efficient one with bandwidths well exceeding 40 GHz. Two different saturation mechanisms are responsible for this pronounced influence of the bias current: 1) total carrier depletion that leads to a slow broadband cross-gain saturation and 2) spectral hole burning that causes spectrally narrow-band high-speed XGM. With increasing current, the saturation by depleting the carrier reservoir, which feeds the QDs, is minimized, and therefore, spectral hole burning becomes more dominant. Large-signal wavelength conversion experiments using 50 ps pulses indicate that efficient high-speed XGM is feasible for pump and probe signal detuning up to 10 nm. With increasing detuning, larger pulse broadening and a decreasing efficiency are observed, consistent with the small-signal results. The results on the QD SOAs are compared to conventional quantum-well devices.

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Small-signal cross-gain modulation of quantum dot based semiconductor optical amplifiers (QD SOAs), having a dot-in-a-well structure, is presented, demonstrating superiority for ultrahigh bit rate wavelength conversion. Optimization of the QD SOA high speed characteristics via bias current and optical pump power is presented and a small-signal 3 dB bandwidth exceeding 40 GHz is demonstrated. The p-doped samples investigated here enable small-signal wavelength conversion within a range of 30 nm, limited mainly by the gain bandwidth.

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The dynamics of nonlinear processes in quantum dot (QD) semiconductor optical amplifiers (SOAs) are investigated. Using small-signal measurements, the suitabilities of cross-gain and cross-phase modulation as well as four wave mixing (FWM) for wavelength conversion are examined. The cross-gain modulation is found to be suitable for wavelength conversion up to a frequency of 40 GHz.

40 GHz small-signal cross-gain modulation in 1.3 μm quantum dot semiconductor optical amplifiers

Mode-locked lasers (MLLs) and semiconductor optical amplifiers (SOAs) based on quantum dot (QD) gain material will impact the development of next generation networks like the 100Gb/s Ethernet. Hybrid mode-locked lasers consisting of a monolithic two section device presently already generate picosecond pulse trains at 40 GHz with an extremely low jitter in the range of 200 fs under optimum operating conditions. A detailed chirp analysis which is prerequisite for optical time division multiplexing applications is presented. QD SOAs are showing superior performance for linear amplification as well as nonlinear signal processing. Wavelength conversion via cross-gain modulation is shown to have a small signal bandwidth beyond 40 GHz under high bias current injection. This makes QD SOAs much superior to conventional SOAs.

Nonlinear properties of quantum dot semiconductor optical amplifiers at 1.3 \mm

We introduce and verify experimentally an interferometric scheme for performing cross-gain modulation. The scheme is used to demonstrate all-optical wavelength conversion with good extinction ratio in media with little or no cross-phase modulation nonlinearity.

Ultra high-speed nanophotonics

Wavelength conversion using cross-gain modulation in quantum dot semiconductor optical amplifiers is investigated. Small signal measurements reveal that with increasing bias the cross-gain modulation converts from low efficiency, limited to 10 GHz bandwidth to a very efficient one with bandwidths well exceeding 40 GHz. Two different saturation mechanisms are responsible for this pronounced influence of the bias current: a) total carrier depletion which leads to a slow broadband cross gain saturation and b) spectral hole burning which causes spectrally narrowband high speed XGM. With increasing current the saturation by depleting the carrier reservoir, which feeds the quantum dots, is minimized and therefore spectral hole burning becomes more dominant. Large signal wavelength conversion experiments using 50 ps pulses indicate that efficient high speed cross gain modulation is feasible for pump and probe signal detuning up to 10 nm. With increasing detuning larger pulse broadening and a decreasing efficiency is observed, consistent with the small signal results.

An Interferometric Configuration for Performing Cross-Gain Modulation with Improved Signal Quality

We numerically and experimentally demonstrate that high-speed small-signal cross- gain modulation of quantum-dot optical amplifiers can be improved by injecting carriers to excited states, which increases the amplifiers gain recovery time due to spectral hole burning.

Quantum dot semiconductor optical amplifiers for wavelength conversion using cross-gain modulation

We report on ultra-fast small signal sinusoidal cross-gain modulation using quantum dot semiconductor optical amplifiers exceeding a 40 GHz bandwidth in frequency and 40 nm in the wavelength domain.