Lukas Drzewietzki

Timing jitter reduction of passively mode-locked semiconductor lasers by self- and external-injection: numerical description and experiments.

In this paper the influence of different feedback (FB) and synchronization schemes on the timing phase noise (TPN) power spectral density (PSD) of a quantum-dot based passively mode-locked laser (MLL) is studied numerically and by experiments. The range of investigated schemes cover hybrid mode-locking, an opto-electrical feedback configuration, an all-optical feedback configuration and optical pulse train injection configuration by means of a master MLL. The mechanism responsible for TPN PSD reduction in the case of FB is identified for the first time for monolithic passively MLL and relies on the effective interaction of the timing of the intra-cavity pulse and the time-delayed FB pulse or FB modulation together with an statistical averaging of the independent timing deviations of both. This mechanism is quantified by means of simulation results obtained by introducing an universal and versatile simple time-domain model.

High peak power and sub-picosecond Fourier-limited pulse generation from passively mode-locked monolithic two-section gain-guided tapered InGaAs quantum-dot lasers

We report on the development of a new generation of high-power ultrashort pulse quantum-dot lasers with tapered gain section. Two device designs are proposed and fabricated, with different total lengths and absorber-to-gain-section length-ratios. These designs have been informed by numerical simulations of the dynamic mode-locking regimes and their dependence on the structural parameters. One device design demonstrated a record-high peak power of 17.7 W with 1.26 ps pulse width and a second design enabled the generation of a Fourier-limited 672 fs pulse width with a peak power of 3.8 W. A maximum output average power of 288 mW with 28.7 pJ pulse energy was also attained. In addition, the integrated timing jitter of 2.6 ps and far-field patterns are demonstrated.

Experimental demonstration of change of dynamical properties of a passively mode-locked semiconductor laser subject to dual optical feedback by dual full delay-range tuning.

In this contribution we experimentally demonstrate the change and improvement of dynamical properties of a passively mode-locked semiconductor laser subject to optical feedback from two external cavities by coupling the feedback pulses back into the gain segment. Hereby, we tune the full delay-phase of the pulse-to-pulse period of both external cavities separately and demonstrate the change of the repetition rate, timing jitter, multi-pulse formation and side-band suppression for the first time for such a dual feedback configuration. In addition, we thereby confirm modeling predictions by achieving both a good qualitative and quantitative agreement of experimental and simulated results. Our findings suggest a path towards the realization of side-band free all-optical photonic oscillators based on mode-locked lasers.

Joint Experimental and Theoretical Investigations of Two-State Mode Locking in a Strongly Chirped Reverse-Biased Monolithic Quantum Dot Laser

The influence of different biasing conditions of a passively mode-locked strongly chirped two-section quantum-dot laser on its ground state (GS) and excited state (ES) emission is investigated both experimentally and theoretically. Numerical simulations performed with a modified time-domain travelling-wave model are in qualitative agreement with experimental results and allow to identify the origin of the experimentally observed mode-locking regimes: the complex time-dependent coupling between the ES and GS absorption bleaching in the saturable absorber section. This coupling, together with the difference in group velocities of ES and GS pulses, is found to be responsible for low-frequency inter-modulation signatures as observed experimentally and verified in the simulations.

Reverse-emission-state-transition mode locking of a two-section InAs/InGaAs quantum dot laser

Reverse-emission-state-transition mode locking in a two-section InAs/InGaAs quantum dot laser is experimentally investigated and confirmed by simulations. Stable mode locking starts on the first excited state (λ=1207 nm) and then, with increasing gain current, a transition to stable simultaneous two-state mode locking on excited state and ground state (λ=1270 nm) takes place. This particular state-transition occurs already at 0 V reverse-bias and at moderate gain-section currents. It is attributed to the strong active region chirping of the gain medium and in particular to a photon pumping process in the saturable absorber section.

Dual-State Absorber-Photocurrent Characteristics and Bistability of Two-Section Quantum-Dot Lasers

The peculiar behavior of the absorber photocurrent of two-section quantum-dot lasers with different gain-bandwidth chirping is studied numerically and by experiments. By biasing the absorber section of a strongly chirped laser with a self-generated positive voltage or with a reverse-bias, different emission-state lasing regimes also involving a smooth emission-state-transition from quantum-dot ground-state to excited state via simultaneous ground- and excited-state are observed. The gradual emission state and photocurrent transitions are simulated and are in excellent agreement with experiments. In contrast, in a medium-chirped laser a photocurrent and emission-state bistability with hysteresis is experimentally observed involving a transition from sole ground state to sole excited state. Thereby, no intermediate simultaneous dual-state emission regimes occur suggesting the exclusion of simultaneous dual-state emission and bistability. This peculiar bistable behavior is explained qualitatively and offers the potential to be exploited toward switching applications.

Two-state semiconductor laser self-mixing velocimetry exploiting coupled quantum-dot emission-states: experiment, simulation and theory

We exploit the coupled emission-states of a single-chip semiconductor InAs/GaAs quantum-dot laser emitting simultaneously on ground-state (λ(GS) = 1245 nm) and excited-state (λ(ES) = 1175 nm) to demonstrate coupled-two-state self-mixing velocimetry for a moving diffuse reflector. A 13 Hz-narrow Doppler beat frequency signal at 317 Hz is obtained for a reflector velocity of 3 mm/s, which exemplifies a 66-fold improvement in width as compared to single-wavelength self-mixing velocimetry. Simulation results reveal the physical origin of this signal, the coupling of excited-state and ground-state photons via the carriers, which is unique for quantum-dot lasers and reproduce the experimental results with excellent agreement.

Experimental Study of the Timing Jitter of a Passively Mode-Locked External-Cavity Semiconductor Laser Subject to Repetition Rate Transitions and Optical Feedback

We experimentally investigate the timing jitter (TJ) of a passively mode-locked external-cavity diode laser. Variation of the gain current and the absorber reverse bias voltage allows transitions from fundamental mode-locking up to seventh harmonic mode-locking. Hereby, a reduction of the TJ as a function of the harmonic mode-locking order is found. Furthermore, the application of optical feedback results in an additional reduction of TJ for almost the whole investigated operation range. In particular, the reduction increases with harmonic mode-locking order. The highest observed reduction of TJ amounts to a factor of 10 as compared with the free-running case which corresponds to a repetition-rate linewidth reduction by a factor of 100.

Picosecond pulse amplification up to a peak power of 42 W by a quantum-dot tapered optical amplifier and a mode-locked laser emitting at 126 µm

We experimentally study the generation and amplification of stable picosecond-short optical pulses by a master oscillator power-amplifier configuration consisting of a monolithic quantum-dot-based gain-guided tapered laser and amplifier emitting at 1.26 µm without pulse compression, external cavity, gain- or Q-switched operation. We report a peak power of 42 W and a figure-of-merit for second-order nonlinear imaging of 38.5  W2 at a repetition rate of 16 GHz and an associated pulse width of 1.37 ps.

Reverse ground-state excited-state transition dynamics in two-section quantum dot semiconductor lasers: mode-locking and state-switching

In this contribution reverse emission state transition of a two-section quantum dot laser at a saturable absorber bias of zero volt (short circuit) is presented where lasing and mode-locking starts first on the energetically higher first excited-state (ES) and then, with increasing gain current, additional lasing and mode-locking on the energetically lower ground-state (GS) takes place. A huge coexistence regime as well as temporal simultaneity of both GS and ES mode-locking is experimentally confirmed. At the onset of two-state mode-locking shorter pulse widths are found for the GS as compared to the ES at the same gain current. A considerable shortening of the ES pulse widths is observed when GS mode-locking starts. These state-resolved emission dynamics are confirmed by time-domain travelling-wave equation modeling. Finally, by electrically shortening the saturable absorber via an external variable resistor, a resistor Self-Electro-Optical Devices (SEED) configuration is exploited and tailored emission state control is achieved.