Alvaro Gomez-Iglesias

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

The authors report a direct measurement of the absorption dynamics in an InAs p‐i‐n ridge waveguide quantum dot modulator. The carrier escape mechanisms are investigated via subpicosecond pump-probe measurements at room temperature, under reverse bias conditions. The optical pulses employed are degenerate in wavelength with the quantum dot ground state transition at 1.28μm. The absorption change recovers with characteristic times ranging from 62ps (0V)to∼700fs (−10V), showing a decrease of nearly two orders of magnitude. The authors show that at low applied fields, this recovery is attributed to thermionic emission while for higher applied fields, tunneling becomes the dominant mechanism.

Direct measurement of the group index of photonic crystal waveguides via Fourier transform spectral interferometry

The authors report a direct, single-shot measurement of the group index profile of photonic crystal waveguides, combining spectral interferometry with Fourier transform analysis. This technique’s versatility allows them to resolve subtle changes in dispersion and to quantify the “slow light” effect at the photonic crystal waveguide mode cutoff. For a waveguide 99μm long, they measure a group index up to 85, whereas for lengths of 397 and 695μm, they measure maximum values of 30 and 25, respectively. These results show the relationship between transmission characteristics and the maximum group delay observed in photonic crystals.

Temperature dependence of electroabsorption dynamics in an InAs quantum-dot saturable absorber at 1.3 μm and its impact on mode-locked quantum-dot lasers

This paper investigated absorption recovery in a 5-layer, p-i-n InAs quantum dot modulator that was 1.1 mm long and having a 6 μm ridge waveguide. The change in transmission of a weak probe optical pulse, induced by an intense pump pulse, has been measured as a function of time delay. The 250 fs pump and probe pulses are degenerate in wavelength with the transition at 1.3 μm of the QD ground state and so the temporal absorption dynamics could be mapped out. This measurement was performed with an increasing device temperature in order to assist in the removal of photogenerated carriers. For a test two-section QD mode-locked laser having a pulse repetition rate of 21 GHz, we have also observed that the pulse durations decreased significantly as the temperature was increased up to 70°C. At constant current and reverse bias values, the pulse durations were obtained over the temperature range of the pump-probe measurements. These data support the view that a reduction of the absorber recovery time plays the most important role in the shortening mechanism for the mode-locked pulses produced by this laser. Furthermore, these results also demonstrate the potential for higher repetition rate mode-locked QD lasers that can be resilient to temperature effects.

Electroabsorption and Electrorefraction in an InAs Quantum-Dot Waveguide Modulator

Optical transmission measurements are reported in the 1.3-mum region in an InAs quantum-dot (QD) waveguide modulator using a supercontinuum generator. A quantum-confined Stark shift of 15 nm was demonstrated in a five-layer QD stack with a field of 270 KV/cm. The electrically induced change in absorption has been measured and the change in refractive index (up to 0.001) has been estimated via the Kramers-Kronig relations. This highlights the potential of such a device as an electroabsorption or electrooptic phase modulator at telecommunication wavelengths.

Temperature dependence of electroabsorption dynamics in an InAs quantum dot saturable absorber at 1.3 μm

This paper investigated absorption recovery in a 5-layer, p-i-n InAs quantum dot modulator that was 1.1 mm long and having a 6 μm ridge waveguide. The change in transmission of a weak probe optical pulse, induced by an intense pump pulse, has been measured as a function of time delay. The 250 fs pump and probe pulses are degenerate in wavelength with the transition at 1.3 μm of the QD ground state and so the temporal absorption dynamics could be mapped out. This measurement was performed with an increasing device temperature in order to assist in the removal of photogenerated carriers. For a test two-section QD mode-locked laser having a pulse repetition rate of 21 GHz, we have also observed that the pulse durations decreased significantly as the temperature was increased up to 70°C. At constant current and reverse bias values, the pulse durations were obtained over the temperature range of the pump-probe measurements. These data support the view that a reduction of the absorber recovery time plays the most important role in the shortening mechanism for the mode-locked pulses produced by this laser. Furthermore, these results also demonstrate the potential for higher repetition rate mode-locked QD lasers that can be resilient to temperature effects.

Carrier heating in semiconductor optical amplifier-based Sagnac-type all-optical switches

We report the observation of ultrafast dynamics in interferometric switching using an InGaAs quantum-well laser amplifier. The switching windows, measured in a three-beam pump–probe interferometric set-up using sub-picojoule pulses of 800 fs duration at 1.57 µm wavelength, reveal an ultrafast (~2 ps) feature. This reshaping of the switching window edge was found to change substantially with the pump pulse energy, and is attributed to refractive index changes caused by the heating of the carrier distribution in the amplifier. A rate-equation model is used to analyse these results, showing very good agreement with the experimental evidence.

Ultrafast electroabsorption dynamics in a GaInNAs quantum well waveguide at 1.3μm

Time-resolved electro-absorption is reported for the first time in a GaInNAs quantum well p-i-n waveguide at 1.3 mum. A recovery of 55 ps demonstrates the potential for optical modulator devices.

Time-resolved carrier dynamics and the quantum confined Stark effect in a bilayer quantum dot waveguide at 1340nm

Quantum confined Stark effect and pump probe measurements are reported for the first time in a bilayer quantum dot waveguide at 1340 nm, showing a shift of 40 nm and ultrafast absorption recovery of 5ps.

Intraband Carrier Dynamics In Semiconductor Optical Amplifier‐Based Switch

We present interferometric switching experiments using femtosecond pulses with an InGaAs semiconductor optical amplifier (SOA). Our measurements reveal significant refractive index changes occurring within a few picoseconds. The timescale and power dependence of this ultrafast feature, together with the predictions of a phenomenological model, strongly suggest carrier heating as responsible.

Spatial dependence of gain nonlinearities in InGaAs semiconductor optical amplifier

Counter-propagating sub-picosecond pulses are used to monitor gain saturation along the waveguide of an InGaAs superlattice semiconductor optical amplifier at 1550nm wavelength. The functional form of the spatial dependence of gain saturation is found to depend on pulse energy. These observations are interpreted by combining the optical nonlinearities associated with interband carrier dynamics and carrier heating together and their respective time constants. We show that the results are consistent with the predictions of a propagation model. Implications for all-optical switching, particularly in the limit of full saturation across the whole amplifier, are discussed.