Phase dynamics of InAs∕GaAs quantum dot semiconductor optical amplifiers

Abstract

The gain and phase dynamics of InAs/ GaAs quantum dot amplifiers are studied using single and two-color heterodyne pump probe spectroscopy. The relaxation of the wetting layer carrier density is shown to have a strong effect on the phase dynamics of both ground and excited state transients, while having a much weaker effect on the gain dynamics. In addition, the dynamical alpha factor may also display a constant value after an initial transient. Such behavior is strongly encouraging for reduced pattern effect operation in high speed optical networks. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2823589͔The physics of quantum dot ͑QD͒ photonic devices has been a subject of great interest since the development of self-assembly growth techniques. One of the principle motivating factors for the study of QD lasers was the possibility of substantially reducing the phase-amplitude coupling ͑␣ factor͒ of semiconductor lasers from values in the region of 3-5 for quantum well devices. 1 Among the expected benefits of such a reduction were low modulation chirp, reduced filamentation in high power devices, and reduced sensitivity to optical feedback. 1 To date, QD lasers have displayed a wide range of ␣ factors, the value displaying a strong dependence on the measurement conditions. 2,3 Recent analysis has shown that the low ground state ͑GS͒ gain saturation of QD devices coupled with a large amount of nonresonant carriers in the QD's excited states ͑ES͒ and barrier states can result in large values above threshold 4 and as a consequence, directly phase modulated devices have been demonstrated. 5 In addition, it has been proposed that the unique carrier dynamics of QD semiconductor optical amplifiers ͑SOAs͒ would lead to a reduction of patterning effects compared to conventional devices, for both linear and nonlinear applications. 6,7 As the understanding of relevant carrier relaxation processes in QD structures improves, additional device functionalities will be realized.Heterodyne pump probe spectroscopy has been established as an ideal tool to directly record the ultrafast gain and refractive index dynamics of SOA devices. Previous studies of the gain and phase dynamics of QD SOAs have been performed by Borri et al., who investigated the effects of carrier heating and spectral hole burning in InAs/ InGaAs QDs, 8 while related techniques have been used to measure the ␣ factor in various conditions. 9 Previously on InAs/ GaAs QDs, we demonstrated the importance of Augermediated carrier capture using a single color pump probe technique 10 and illustrated the effects of ultrafast hole relaxation on the gain recovery 11 using a two-color technique. In this letter, we report on two-color phase measurements of the same structure, and note the presence of a strong phase recovery component at the same time scale as the wetting layer carrier recovery. In addition, we combine these phase measurements with two-color gain measurements to calculate an ␣ factor for the device as a function of pump probe delay. The experimental...

Refractive index dynamics of quantum dot based waveguide electroabsorbers

“…This behavior is in contrast to the forward biased phase measurements reported previously, 13 in which the phase recovery dynamics exhibited much larger contributions at long timescales that implied the role of higher order dot and wetting layer carrier populations in the pump induced phase change and recovery. In our reversed bias case, the lack of large, long timescale components, and similarity of phase and gain dynamics in some spectral regions would suggest against such a strong role for nonresonant carriers in this case.…”

Ultrafast response of tunnel injected quantum dot based semiconductor optical amplifiers in the 1300 nm range

“…17 The phase dynamics of the ES is shown in panel (b) and exhibits a similar behaviour to that seen for the GS absorbing regime, albeit with a faster initial recovery linked to the faster ES gain recovery at this injection level. However, at increased injections, a sign change occurs in the phase even though the ES is still absorbing.…”

Interconnection between ground state and excited state gain in InAs/GaAs quantum dot semiconductor optical amplifiers

“…We assume an effective waveguide thickness (width) of 0.2 µm (2 µm), 6 dot layers, a constant waveguide loss of 5 cm -1 and a total pump (probe) pulse input energy of 1 pJ (7 fJ). The holes, due to their large effective mass and consequent closer level separation are assumed to thermalise on a much faster timescale than the pulse duration (~1 ps), while electron re-distribution is considered to be slower [6,10,11]. The propagation of the pulses is described by a standard propagation equation including TPA in which a TPA coefficient of 21 cm/GW is included [12].…”

“…The method incorporates the injection of a strong optical pump pulse into the waveguide, accompanied by a much weaker, variably time delayed probe pulse, whose purpose is to scan in time the amplitude and phase changes induced by the stronger pump pulse. Single and two colour pump-probe measurements have been reported recently of the gain and phase dynamics in InAs/GaAs QD SOAs [5,6]. In the case where the pump pulse is tuned to the maximum of the ES gain, an instantaneous reduction in probe amplitude was measured at both the ES and GS gain peak, and was attributed to fast scattering between the closely spaced hole levels [5].…”