Ultrafast electroabsorption dynamics in an InAs quantum dot saturable absorber at 1.3μm

We report on experimental and theoretical studies of the stability regime of passive mode-locked quantum dot lasers, which is decisively larger than in quantum well lasers. A small range of Q-switched instability is observed at low gain currents. Transition to Q switching is inhibited due to fast damping of the relaxation oscillations. A double pulse mode-locking regime appears for longer cavities, and exhibits bistability and coupling to the fundamental mode-locking operation. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2822808͔Passive and hybrid mode-locked ͑MLd͒ quantum dot lasers ͑QDLs͒ are efficient sources of short pulses ideal for applications in high speed communication systems. 1,2 The mode-locking ͑ML͒ regime of quantum well lasers can exhibit peak power fluctuations induced by a Q-switching instability, which is detrimental for the laser performance. In most experimental situations, QDLs have only a small range of Q switching instability, a consequence of the strong damping of the relaxation oscillations induced by the fast carrier capture from the wetting layer to the dots.In this letter, we study the stability of the mode-locked regime in QDLs with pulse repetition rates of 20 and 40 GHz. We find that stable ML in QDLs occurs in a comparatively larger range of parameters than in quantum well lasers. We also demonstrate the bifurcation to a state of doubled pulse repetition frequency in the MLd regime in longer cavity devices and characterize its coupling to the fundamental ML regime.The devices used in this work are two-section QDLs with a total length of 1 mm integrated in a module and an unpackaged 2 mm device ͑saturable absorber length is always 1 / 10 of the total length͒. The active region was grown on a GaAs substrate, containing 15 self-organized InAs QD layers. The facet near the absorber was high refection coated, the front facet was as cleaved. Details of the growth and the processing are the same as described in Ref. 2.A number of different regimes have been observed experimentally: Q-switching modulated ML, pure ML and, finally, cw lasing. The stability domains of these regimes are displayed in Fig. 1 for the 1 mm QDL module. We measured the radio frequency ͑rf͒ spectra of the optical output in the range of 0 -50 GHz by means of a 55 GHz bandwidth photodetector and a 50 GHz electrical spectrum analyzer. The side band suppression ratio ͑SBSR͒ with respect to the fundamental ML frequency at 40 GHz was extracted for a large set of reverse bias voltages and gain currents and plotted in Fig. 1. The large area of proper ML with a SBSR better than 20 dB, i.e., with weak or absent bands besides the fundamental ML frequency, dominates the figure. The region of Q-switching modulated ML, i.e., the region with strong side bands in the range of a few gigahertz, is small compared to that observed with quantum well lasers. 3 Both regions are separated by a steplike change of the SBSR by more than 30 dB. In our experiments, we did not observe any dependence of the SBSR plot on the scanning dir...

Section: Applied Physics Letters 91 231116 ͑2007͒mentioning

confidence: 99%

Stability of the mode-locked regime in quantum dot lasers

Viktorov

Mandel

Kuntz

et al. 2007

“…The traces down to À6 V were fitted by double exponential functions with a ultrafast (bias insensitive) component originating from rapid QD 1ES to GS carrier relaxation 24 and fast (bias sensitive) component exhibiting faster recovery with increasing RB caused by tunnelling processes. 25 The inset shows the fitted timescales of $1.6 ps for the ultrafast component and exponential decrease of the fast component with timescale from $12.6 ps (0 V) down to $3.3 ps (À6 V) [ Fig. 3 inset].…”

mentioning

confidence: 99%

Electro-optical and lasing properties of hybrid quantum dot/quantum well material system for reconfigurable photonic devices

Thoma

Liang

Lewis

et al. 2013

We characterize the electro-optical and lasing properties of a hybrid material consisting of multiple InAs quantum dot (QD) layers together with an InGaAs quantum well (QW) grown on a GaAs substrate. Over 40 nm Stark shift of the InGaAs QW leading to 9 dB extinction ratio was demonstrated. Lasing operation at the QD first excited state transition of 1070 nm was achieved and together with < 10 ps absorption recovery the system shows promise for high-speed mode-locked lasers and electro-modulated lasers. (C) 2013 American Institute of Physics. (http://dx.doi.org/10.1063/1.4791565

“…The transmission is similar to the lower voltage case although as shown previously, the dynamics can now be fitted to a single exponential with $1-2 ps timescale 13 and commonly attributed to tunnelling into the wetting or barrier layers. 17 The tunnelling timescale primarily depends on the effective barrier thickness which greatly reduces at high reverse bias due to the large tilting of the band edge and so depends on the energy offset between the QD state and wetting layer. This offset is similar for both the high energy GS states and low energy ES states that are contained within the pulse bandwidth and so a similar tunnelling timescale should occur for both.…”

mentioning

confidence: 99%

Mixed state effects in waveguide electro-absorbers based on quantum dots

Piwoński

Pulka

Huyet

et al. 2011

Multi-pulse heterodyne pump-probe measurements are used to investigate the reverse bias dynamics of InAs/GaAs quantum dots in a waveguide structure. Using a femtosecond pulse, we simultaneously populate high energy ground states and low energy excited states and measure the resulting gain and phase dynamics over the bandwidth of the pulse. We identify a similar to 5 ps timescale in the phase dynamics which can be associated with low energy ground states outside the pulse bandwidth and may provide an explanation for the deterioration of monolithic mode locked laser performance at high reverse voltages. (C) 2011 American Institute of Physics. (doi:10.1063/1.3653287