Progress in technology, theory and applications of high-frequency mode-locked diode lasers is reviewed. We present a systematic overview of a wealth of experimental and theoretical results and approaches reported recently in the field of physics, technology and applications of monolithic mode-locked laser diodes.

Monolithic and multi-gigahertz mode-locked semiconductor lasers: constructions, experiments, models and applications

We present a simple, yet powerful method for calculating nonlinear pulse propagation and pulse interaction in active semiconductor waveguides. The model is based on the density matrix equations, which under realistic operation conditions are shown to lead to an accurate description of the material gain, that includes the dynamics of carrier heating and spectral-hole burning (SHB). A very general and compact description of the amplifier dynamics in terms of an integral equation is derived. The model is used to analyze saturation effects in short pulse amplification and nondegenerate four-wave mixing. An analytical expression for the four-wave mixing response is derived, which extends previous results to the case of short and intense pulses. Saturation of the four-wave mixing signal is shown to be strongly pulsewidth dependent due to ultrafast gain dynamics, and self-phase modulation is shown to give excessive broadening of the conjugate pulse. Finally, the impacts on the noise characteristics are calculated and shown to explain recent experimental results.

Saturation effects in nondegenerate four-wave mixing between short optical pulses in semiconductor laser amplifiers

A detailed study of the design issues relevant to long-wavelength monolithic mode-locked lasers is presented. Following a detailed review of the field, we have devised a validated travelling wave model to explore the limits of mode-locking in monolithic laser diodes, not only in terms of pulse duration and repetition rate, but also in terms of stability. It is shown that fast absorber recovery is crucial for short pulse width, that the ratio of gain to absorption saturation is key in accessing ultrashort pulses and that low alpha factors give only modest benefit. Finally, optimized contact layouts are shown to greatly enhance pulse stability and the overall operational success. The design rules show high levels of consistency with published experimental data.

https://iopscience.iop.org/article/10.1088/1367-2630/6/1/179/pdf

Long-wavelength monolithic mode-locked diode lasers

The ultrafast gain and index dynamics in a set of InAs-InGaAs-GaAs quantum-dot (QD) amplifiers are measured at room temperature with femtosecond resolution. The role of spectral hole-burning (SHB) and carrier heating (CH) in the recovery of gain compression is investigated in detail. An ultrafast recovery of the spectral hole within /spl sim/100 fs is measured, comparable to bulk and quantum-well amplifiers, which is contradicting a carrier relaxation bottleneck in electrically pumped QD devices. The CH dynamics in the QD is quantitatively compared with results on an InGaAsP bulk amplifier. Reduced CH for both gain and refractive index dynamics of the QD devices is found, which is a promising prerequisite for high-speed applications. This reduction is attributed to reduced free-carrier absorption-induced heating caused by the small carrier density necessary to provide amplification in these low-dimensional systems.

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Spectral hole-burning and carrier-heating dynamics in InGaAs quantum-dot amplifiers

Analytical expressions for the optical response of active semiconductor waveguides are derived, taking into account the effects of propagation, gain, and index dispersion as well as pulse chirp. The dynamics of the gain and the index that are due to carrier density changes, carrier heating, and spectral holeburning are derived from semiclassical density-matrix equations, and instantaneous two-photon and Kerr effects are included phenomenologically. It is shown that dispersion of the gain strongly affects short-pulse pump-and-probe measurements, since it induces a coupling between the phase and the intensity of the probe pulses. Good agreement with published experimental data is obtained.

Theory of the ultrafast optical response of active semiconductor waveguides

Based on semiclassical density matrix equations, we derive a simple analytical expression for the pump‐probe response function of an active semiconductor optical waveguide. The results are in good agreement with published pump‐probe measurements of gain and refractive index dynamics in laser amplifiers. In particular, an experimentally observed delay in carrier heating is explained, and spectral effects not previously considered are shown to be significant under typical experimental conditions.

Response function for gain and refractive index dynamics in active semiconductor waveguides

The large signal dynamics of passively colliding pulse mode‐locked laser diodes is studied. We derive a model which explains modelocking via the interplay of gain and loss dynamics; no bandwidth limiting element is necessary for pulse formation. It is found necessary to have both fast and slow absorber dynamics to achieve mode‐locking. Significant chirp is predicted for pulses emitted from long lasers, in agreement with experiment. The pulse width shows a strong dependence on both cavity and saturable absorber length.

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Pulse‐shaping mechanism in colliding‐pulse mode‐locked laser diodes

A femtosecond four‐wave mixing technique is used to measure the ultrafast coherent dynamics of the optical polarization in semiconductor optical amplifiers. A heterodyne detection scheme enables us to measure a background‐free quasi‐degenerate four‐wave mixing signal even without spatial separation of the pump and probe beam. First results indicate that the polarization dephasing time close to the transparency point is on the order of 100 fs.

Time resolved four-wave mixing technique to measure the ultrafast coherent dynamics in semiconductor optical amplifiers

Spectral effects arising from linear gain dispersion are shown to strongly influence the ultrafast gain dynamics measured in active semiconductor waveguides using the pump‐and‐probe technique. Measurements on a AlGaAs amplifier under varying amounts of gain dispersion are in good agreement with a simple analytical theory derived from semiclassical density matrix equations.

Spectral effects in short pulse pump‐probe measurements

Spectrally resolved streak camera measurements of picosecond pulses emitted by hybridly colliding pulse mode-locked (CPM) laser diodes are presented in this letter Depending on the modulation frequency both blue-chirped (upchirped) and red-chirped (downchirped) pulses can be observed The two different regimes and the transition between them are characterized experimentally and the behavior is explained on the basis of our model for the CPM laser dynamics

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Papers

Response function for gain and refractive index dynamics in active semiconductor waveguides

Pulse‐shaping mechanism in colliding‐pulse mode‐locked laser diodes

Time resolved four-wave mixing technique to measure the ultrafast coherent dynamics in semiconductor optical amplifiers

Spectral effects in short pulse pump‐probe measurements

Chirp of monolithic colliding pulse mode-locked diode lasers