M. Laemmlin

Slow and fast dynamics of gain and phase in a quantum dot semiconductor optical amplifier

Gain and phase dynamics in InAs/GaAs quantum dot semiconductor optical amplifiers are investigated. It is shown that gain recovery is dominated by fast processes, whereas phase recovery is dominated by slow processes. Relative strengths and time constants of the underlying processes are measured. We find that operation at high bias currents optimizes the performance for nonlinear cross-gain signal processing if a low chirp is required.

Theoretical and Experimental Study of High-Speed Small-Signal Cross-Gain Modulation of Quantum-Dot Semiconductor Optical Amplifiers

We numerically and experimentally investigate the high-speed small-signal cross-gain modulation (XGM) characteristics of a quantum-dot (QD) semiconductor optical amplifier (SOA). From a p-doped QD SOA operating at 1.3 mum, high-speed small-signal XGM responses up to 40 GHz are measured from low to high injection currents and improve at high injection currents. In the numerical model, we set up about six hundred coupled rate equations, where the carrier dynamics of QD electron and hole states are considered separately and the enhanced hole occupation due to p-type doping is included. The high-speed small-signal XGM spectra are calculated at various modulation frequencies and pump-probe detunings. We identify how the two separate XGM mechanisms of total carrier density depletion (TCDD) at low injection current and spectral hole burning (SHB) at high injection current affect the high-speed small-signal XGM behavior. From the measured and calculated results, we show that high-speed small-signal XGM responses of QD SOAs can be improved by injecting more carriers to the QD excited states, which enhances high-speed XGM induced by SHB rather than by TCDD.

Complete ground state gain recovery after ultrashort double pulses in quantum dot based semiconductor optical amplifier

Gain recovery dynamics are studied in electrically pumped quantum dot (QD) based semiconductor optical amplifiers (SOAs) after amplification of double femtosecond laser pulses using ultrafast pump-probe spectroscopy with heterodyne detection. The authors observe a distinct change in gain recovery in the ground state when a significant excited state population is achieved. A complete gain recovery is found when two 150fs pulses with 5ps time delay pass through the SOA in resonance to the ground state under high injection currents of 80–100mA. The obtained results open the way for ultrafast (>200GHz) operation in p-doped QD based SOAs at 1.3μm telecommunications wavelengths.

Static Gain Saturation Model of Quantum-Dot Semiconductor Optical Amplifiers

We theoretically investigate the gain saturation behavior of a quantum-dot (QD) semiconductor optical amplifier (SOA), focusing on spectral hole burning (SHB) and total carrier density depletion (TCDD). In the static gain model for a QD-SOA, SHB is modeled by the quantum-mechanical density matrix theory and TCDD is described by the shift of the global quasi-Fermi level. We calculate the gain saturation spectra of a QD-SOA at various injection current densities and qualitatively explain how high-speed cross-gain saturation responses can be affected by injection current density. From the quantum-mechanical description for SHB, we show that the optical power for 3-dB gain saturation due to SHB is proportional to the square of the homogeneous linewidth and the functionality of a QD-SOA can be changed by controlling device parameters such as doping density and barrier potential to adjust the homogeneous linewidth.

High-Speed Mode-Locked Quantum-Dot Lasers and Optical Amplifiers

Recent results on GaAs-based high-speed mode-locked quantum-dot (QD) lasers and optical amplifiers with an operation wavelength centered at 1290 nm are reviewed and their complex dependence on device and operating parameters is discussed on the basis of experimental data obtained with integrated fiber-based QD device modules. Hybrid and passive mode locking of QD lasers with repetition frequencies between 5 and 80 GHz, sub-ps pulse widths, ultralow timing jitter down to 190 fs, high output peak power beyond 1 W, and suppression of Q-switching are reported, showing the large potential of this class of devices for O-band optical fiber applications. Results on cw and dynamical characterization of QD semiconductor optical amplifiers (SOAs) are presented. QD amplifiers exhibit a close-to-ideal noise figure of 4 dB and demonstrate multiwavelength amplification of three coarse wavelength division multiplexing (CWDM) wavelengths simultaneously. Modelling of QD polarization dependence shows that it should be possible to achieve polarization insensitive SOAs using vertically coupled QD stacks. Amplification of ultrafast 80 GHz optical combs and bit-error-free data signal amplification at 40 Gb/s with QD SOAs show the potential for their application in future 100 Gb Ethernet networks.

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.

Direct correlation between a highly damped modulation response and ultra low relative intensity noise in an InAs/GaAs quantum dot laser

We describe modulation responses and relative intensity noise (RIN) spectra of an InAs/GaAs quantum dot laser operating near 1300 nm. A very large nonlinear gain compression coefficient yields a highly damped modulation response with a maximum 3 dB bandwidth of ~6.5 GHz and flat RIN spectra which reach as low a level as -158/-160 dB/Hz at frequencies up to 10 GHz.

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.