S. Azouigui

Low noise performance of passively mode locked quantum-dash-based lasers under external optical feedback

We report on a systematic investigation of the effect of external optical feedback on 17 GHz passively mode-locked two-section lasers based on InAs/InP quantum dashes emitting at 1.58 μm. Narrowing of mode-beating linewidth down to a record value of ∼500 Hz is demonstrated over a large operating range.

Effect of layer stacking and p-type doping on the performance of InAs∕InP quantum-dash-in-a-well lasers emitting at 1.55μm

The authors report the growth of 6-, 9-, and 12-layer InAs∕InP quantum-dash-in-a-well (DWELL) laser structures using gas source molecular beam epitaxy. Broad area laser performance has been investigated as a function of number of layers. The highest modal gain at 48cm−1 is achieved for an optimized nine-DWELL layer structure. The effect of layer stacking and p-type doping on the characteristic temperature is also reported. Nine-DWELL layer single mode ridge waveguide lasers showed high slope efficiency (0.2W∕A per facet) and output power (Pout=20mW), close to those of conventional quantum well devices.

Coherence collapse and low-frequency fluctuations in quantum-dash based lasers emitting at 1.57 μm

Optical feedback tolerance is experimentally investigated on a 600-mum-long quantum-dash based Fabry-Pérot laser emitting at 1.57mum. While quantum-dashes are structurally intermediate to quantum-wells and quantum-dots, the observed behaviour is distinctly like that of a quantum-well based laser but with greater stability. Coherence collapse and low-frequency fluctuation regimes are observed and are reported here. The onset of the coherence collapse regime is experimentally determined and is found to vary from -29 dB to -21 dB external feedback level when increasing the current from twice to nine times the threshold current.

Optical Feedback Tolerance of Quantum-Dot- and Quantum-Dash-Based Semiconductor Lasers Operating at 1.55

This paper reports on the tolerance of low-dimensional InAs/InP quantum-dash- and quantum-dot-based semiconductor lasers to optical feedback in the 1.55 mum window. For this purpose, the onset of coherence collapse (CC) is experimentally determined and systematically investigated as a function of different laser parameters, such as the injection current, differential gain, temperature, and photon lifetime. It is in particular found that for both material systems the onset of CC increases with the injection current in a similar way to bulk or quantum-well-based devices. Of most importance, we experimentally show that the differential gain plays a key role in the optical feedback tolerance. It is indeed shown to determine not only the range of the onset of CC but also the dependence of this threshold both on the temperature and laser cavity length. Increasing the operating temperature from 25degC to 85degC leads to a decrease of the onset of CC by a factor of only ~3 dB, well accounted for by the variation of the differential gain in this temperature range. We find no difference in the tolerance to external reflections of a truly 3-D confined quantum-dot-based laser and a quantum dash device of the same cavity length, which have similar differential gains. A tentative analysis of our data is finally carried out, based on existing models.

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The tolerance to external optical feedback of p-type doped InAs/InP quantum-dash-based distributed feedback (DFB) lasers is investigated for different values of the Bragg-grating coupling coefficient. We show that p-doping of the active layer not only enhances the differential gain but also results in small values of the linewidth enhancement factor, both parameters contributing to an increased tolerance to external optical feedback. A −18 dB onset of coherence collapse is reported for antireflection-coated devices, demonstrating the compatibility of quantum-dash-based DFB lasers with isolator-free operation.

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The tolerance to optical feedback of InAs∕InP quantum dash-based lasers is reported for several structures exhibiting differing values of the linewidth enhancement factor and damping factor. An analysis of the onsets of coherence collapse is carried out based on the experimental dynamic parameters extracted for each structure. It is shown that the relevant significant parameter to explain the optical feedback tolerance for this low dimensional material system is the differential gain.

Feedback-resistant p-type doped InAs/InP quantum-dash distributed feedback lasers for isolator-free 10 Gb/s transmission at 1.55 μm

Tolerance to optical feedback is investigated on quantum-dash based lasers emitting at 1.55 μm on both static and dynamic regimes. The onset of coherence collapse regime is experimentally determined using three criteria: optical spectrum broadening, relative intensity noise increase and bit error rate degradation. In spite of a relatively high linewidth enhancement factor (αH˜4.5), a -32 dB onset of coherence collapse corresponding to -24 dB maximum optical return loss tolerance from the system was achieved at 10 Gbps rate.

Systematic investigation of InAs∕InP quantum-dash based lasers under external optical feedback

This paper presents recent progress in the field of semiconductor lasers based on self-assembled quantum dots grown either on GaAs or InP substrates. Quantum dot (QD) based lasers are attracting a lot of interest owing to their remarkable optoelectronic properties that result from the three dimensional carrier confinement. They are indeed expected to exhibit much improved performance than that of quantum well devices. Extremely low threshold currents as well as high temperature stability have readily been demonstrated in the InAs/GaAs material system. The unique properties of quantum dot based active layers such as broad optical gain spectrum, high saturation output power, ultrafast gain dynamics and low loss are also very attractive for the realization of mode-locked lasers. Recent results in the field of directly modulated InAs/GaAs lasers emitting in the 1.3 μm window are discussed. We report in particular on temperature independent linewidth enhancement factor (or Henry factor αH) up to 85°C. This is a key parameter which determines many laser dynamic properties. Optical feedback insensitive operation of specifically band-gap engineered devices, compatible with high bit rate isolator-less transmission is also reported at 1.55 μm. Monolithic mode locked lasers based on InAs/InP quantum dashes have been investigated for 1.55 μm applications. Subpicosecond pulse generation at very high repetition rates (> 100 GHz) is reported for self-pulsating one-section Fabry Perot devices. Specific applications based on these compact pulse generators including high bit rate clock recovery are discussed.

Tolerance to Optical Feedback of 10 GBPs Quantum-Dash Based Lasers Emitting at 1.55 μm

We analyze the relaxation dynamics of quantum dot/dash lasers in terms of the energy exchange between the ground state and the wetting layer. We consider the case where both capture and escape times are of the same order of magnitude and determine the relaxation oscillation frequency and its damping rate. We show that the escape process may significantly affect the modulation characteristics and the tolerance to optical feedback.

Recent advances in long wavelength quantum dot based lasers

The effect of controlled optical feedback has been investigated for InAs/InP laser structures operating in the 1.55 μm fiber window. Mode locked lasers in particular show extremely small phase noise when subjected to optical feedback, implying a very low timing jitter which is of interest for many applications.