Jean-Louis Oudar

Ultrafast excitonic saturable absorption in ion-implanted InGaAs/InAlAs multiple quantum wells

We report on ultrafast excitonic nonlinearities in ion-implanted InGaAs/InAlAs multiple quantum wells. We find that irradiation with energetic O+ and Ni+ ions can reduce the carrier lifetime from 1.6 ns down to 1.7 ps without significantly altering the excitonic absorption properties, making efficient fast saturable absorbers in the 1.3–1.5 μm wavelength range.

Optical losses in plasma-etched AlGaAs microresonators using reflection spectroscopy

The optical losses in dry-etched monolithic microresonators have been studied as a function of their lateral dimensions. Cylindrical microresonators with various radii have been etched from a planar GaAlAs/GaAs microcavity with a very high quality factor (Q≅11 700). Measurements of the resonance linewidth, using Ti-sapphire laser spectroscopy allowed to study the degradation of the Q factor at small radii. The Q factor is four times smaller in 1.1 μm radius microresonators, compared to the unprocessed cavity. This degradation is attributed to optical scattering from sidewalls, whose efficiency is shown to scale with the guided mode intensity at the microresonator edge.

Quantum-well saturable absorber at 1.55μm on GaAs substrate with a fast recombination rate

We propose and realize a structure designed for fast saturable absorber devices grown on GaAs substrate. The active region consists of a 1.55μm absorbing GaInNAsSb quantum well (QW) surrounded by two narrow QWs of GaAsN with a N concentration up to 13%. Photoexcited carriers in the GaInNAsSb QW are expected to recombine by tunneling into the wide distribution of subband gap states created in the GaAsN QW. An absorption study shows that edge energy and excitonic peak intensity of the GaInNAsSb QW are not affected by the proximity of the GaAsN QWs. Pump-probe measurements provide information on the carrier relaxation dynamics which is dependent on spacer thickness, as expected for a tunneling process. We show that this process can be enhanced by increasing the N content in the GaAsN layers. Using this design, we have realized a monolithic GaAs-based saturable absorber microcavity with a 1∕e recovery time of 12ps.

Comparison of light- and heavy-ion-irradiated quantum-wells for use as ultrafast saturable absorbers

We have compared light- and heavy-ion irradiation of InGaAs/InAlAs multiple-quantum wells for ultrafast saturable absorption applications. Under heavy-ion impacts, defect clusters were produced, as observed via transmission electronic microscopy. By contrast, in proton-irradiated samples, only point defects were formed. Nonlinear absorption measurements were performed with excitonic resonance pumping. The relaxation time of absorption saturation (minimum value 2 ps) did not depend on the irradiating ion, and was practically independent of the pulse repetition rate (up to 10 GHz) and optical excitation fluence (0.1 mJ/cm2). We conclude that irradiating multiple-quantum wells with light ions is as effective as using heavy ions, when fabricating ultrafast saturable absorber devices operating at high bit rate and near bandedge wavelength.

All-optical reservoir computer based on saturation of absorption

Reservoir computing is a new bio-inspired computation paradigm. It exploits a dynamical system driven by a time-dependent input to carry out computation. For efficient information processing, only a few parameters of the reservoir needs to be tuned, which makes it a promising framework for hardware implementation. Recently, electronic, opto-electronic and all-optical experimental reservoir computers were reported. In those implementations, the nonlinear response of the reservoir is provided by active devices such as optoelectronic modulators or optical amplifiers. By contrast, we propose here the first reservoir computer based on a fully passive nonlinearity, namely the saturable absorption of a semiconductor mirror. Our experimental setup constitutes an important step towards the development of ultrafast low-consumption analog computers.

Ultrafast saturable absorption at 1.55 μm in heavy-ion-irradiated quantum-well vertical cavity

Measurements of absorption saturation in heavy-ion-irradiated InGaAs/InAlAs multiplequantum-well reflection-mode vertical-cavity devices have been performed with short pulses at 1.55 μm and repetition rates up to 10 GHz. The relaxation time was essentially independent of the pulse repetition rate and optical excitation fluence, with a lower value of 2.4 ps for an ion dose of 1012 cm−2. Efficient optical switching was obtained, with a saturation energy smaller than 12 pJ, a contrast ratio up to 3.5:1, and a switching amplitude up to 20% of the incident signal. A relaxation model accounting for capture and recombination on defect levels indicates an upper limit of 2 ps of the defect level recombination time.

Reduced threshold all‐optical bistability in etched quantum well microresonators

All‐optical bistability is demonstrated in GaAs/AlGaAs multiple quantum well microresonators fabricated by SiCl4 reactive ion etching. A fabrication process has been developed in order to obtain low threshold bistability. The studied samples are two‐dimensional 15×15 arrays of cylindrical microresonators of 4 μm diam and 6 μm height. Owing to lateral carrier and light confinement, bistability is observed with a strongly reduced threshold power, below 100 μW. This result was obtained without post‐etching surface treatment. The low bistability threshold suggests that the surface recombination rate is reasonably small, possibly due to some self‐passivation occurring during the etching process.

All-optical extinction-ratio enhancement of a 160 GHz pulse train by a saturable-absorber vertical microcavity

A vertical-access passive all-optical gate has been used to improve the extinction ratio of a 160 GHz picosecond pulse train at 1555 nm. An extinction ratio enhancement of 6 dB is observed within an 8 nm bandwidth. Such a device is a promising candidate for low-cost all optical reamplication and reshaping (2R) regeneration at 160 Gbits/s.

Analysis of thermal limitations in high-speed microcavity saturable absorber all-optical switching gates

The limitations owing to device heating and thermo-optic effects in high-speed quantum-well microcavity saturable absorber devices are investigated both theoretically and experimentally. A simplified theoretical description of the device electronic, thermal, and optical properties is developed and applied to the modeling of the device switching characteristics for reamplification + reshaping step (2R) all-optical regeneration. These predictions are compared to nonlinear optical measurements performed with switching pulses of fixed duration and variable repetition rate on two devices with significantly different thermal properties. It is shown that proper optimization of the device thermal properties is crucial to avoid the degradation of device performance at high bit rate. It is also shown that the negative effects of optically induced heating on the switching contrast may be compensated to some extent by operating the device on the long wavelength side of the microcavity resonance

Cascadability assessment of a 2R regenerator based on a saturable absorber and a semiconductor optical amplifier in a path switchable recirculating loop

We assess a new 2R regenerator based on a microcavity saturable absorber and a semiconductor optical amplifier. Cascadability is demonstrated and the impact of regeneration span is studied in a 10-Gb/s two-path recirculating loop. A wavelength study demonstrates the tunability of the device over 13 nm