T. Hessler

Direct observation of longitudinal spatial hole burning in semiconductor optical amplifiers with injection

Measurements of spontaneous emission from InGaAsP semiconductor optical amplifiers provide information on both the carrier density and temperature. By spatially resolving the light emitted along the active layer of the device, we find evidence of longitudinal spatial hole burning which results from amplified spontaneous emission in the structure and is modified by the injected optical signal. Under injection, we also observe pronounced asymmetry of the amplified spontaneous emission intensity from the two facets which we relate to the carrier density profile. The experimental results are in good agreement with numerical simulations. An analysis of the measured spectra reveals an unexpected very high temperature (400 K) and its decrease by at least 35 K in the middle of the device when light is injected

Observation of dark-pulse formation in gain-clamped semiconductor optical amplifiers by cross-gain modulation

We have measured the ultrafast simultaneous cross-gain and laser mode dynamics in a gain-clamped semiconductor amplifier perturbed by an intense detuned 150 fs pump pulse. Besides relaxation oscillations, we demonstrate the instantaneous formation of a dark pulse in the laser mode that repeats itself with a period given by the cavity round-trip time. The dark pulse sequence subsequently decays into two-mode beating and is shown to weakly cross modulate the amplifier gain. To describe dark-pulse formation a time- and spatially dependent model based on rate equations is necessary. The experimental results are in reasonable agreement with numerical simulations

Time-resolved relaxation oscillations in gain-clamped semiconductor optical amplifiers by pump and probe measurements

Pump and probe measurements with femtosecond resolution are performed on a gain-clamped semiconductor optical amplifier, a structure that contains a lasing mode. The corresponding relaxation oscillations are observed in the temporal gain recovery of the amplifier. The current dependence of the oscillations is well reproduced by a small-signal analysis of the laser rate equations. The gain recovery can be very fast, we measure damping rates of up to 40 GHz.

Optical speedup at transparency of the gain recovery in semiconductor optical amplifiers

Experimental demonstration of optical speedup at transparency (OSAT) has been performed on a 1 mm long semiconductor optical amplifiers (SOA). OSAT is a recently proposed scheme that decreases the recovery time of an SOA while maintaining the available gain. It is achieved by externally injecting into the SOA the beam of a separate high power laser at energies around the transparency point. Even though the experimental conditions were not optimal, a beam of 100 mW decreases the recovery time by a third when it is injected in the vicinity of the material transparency point of the device. This acceleration of the device response without detrimental reduction of the gain is found to be effective over a broad wavelength window of about 20 nm around transparency. The injection of the accelerating beam into the gain region is a less efficient solution not only because the gain is then strongly diminished but also because speeding is reduced. This originates from the reduction of the amplified spontaneous emission power in the device, which counterbalances the speeding capabilities of the external laser beam. Another advantage of the OSAT scheme is realized in relatively long SOAs, which suffer from gain overshoot under strong current injection. Simulations show that OSAT decreases the gain overshoot, which should enable us to use OSAT to further speedup the response of long SOAs.

Dynamical Density Matrix Theory of Multiphoton Transitions in Semiconductors

A dynamical theory of multiphoton transitions in semiconductors is developed using a density matrix approach. It is shown that, in dipole approximation, these transitions require a mixed parity of the basis stales and that band-diagonal couplings of the electromagnetic field to Bloch electrons have to be included. The general theory is outlined using the simple, but solvable model of a diatomic tight-binding chain. Possible extensions and applications of the approach are discussed.

150 fs Cross-Gain Modulation Experiments on Semiconductor Optical Amplifiers

get a bener understanding o f the fundamental physical processes involved in l h e interaction between a short optical pulse and a high density plasma. So far, the uluafast pain dynamics i n SOAs has been mainly studied by means of standard pump and probe (PBrP) measurements in which the wavelengths o f the pump and probe are identical. Here, we present a new experimental technique which is an ulwfast cross-gain modulation (XGM) behveen a I50 fa pump pulse and a weak cw probe signal. The advantages of the XGM o v a the P&P technique are two-fold. Firstly, the probe wavelength can be varied and therefore the gain recovery at different wavelengths around the pump wavelength can be obtained. Secondly, the probe sigoal i s a cw signal, thus having a very narrow linewidth and hence a high spectral resolution. Consequently, information is obtained on the spectral carrier population distributioo and ita

Auger recombination heating and hot phonon effect in semiconductor optical amplifiers

Summary form only. Few investigations on carrier density and temperature distribution of semiconductor optical amplifiers (SOA) have been performed despite their importance for gain and recovery times. We study the dependencies of these quantities on various parameters such as applied current, injected optical signal power, and active layer geometry. To get information on both density and temperature of the carriers, we performed spontaneous emission (SE) measurements.

Carrier heating in semiconductor optical amplifiers with injection

Summary form only given.Although semiconductor optical amplifiers (SOA) are required for all-optical devices and are to be used in future telecommunication networks, little is known about their internal carrier density and temperature distributions and their dependencies on various parameters such as applied current, injected optical signal power, and active layer geometry. Measurements of spontaneous emission (SE) provide information on both density and temperature of the carriers. Such an experiment has already revealed longitudinal spatial hole burning (LSHB) and carrier heating in an SOA working at 1.55 /spl mu/m. LSHB in this structure is caused by amplified spontaneous emission (ASE) and by injection of an optical signal. Here, we present for the first time to our knowledge, spatially resolved temperature measurements on SOAs.

Propagation effects on subpicosecond gain dynamics in semiconductor optical amplifiers

Summary form only given. Ultrafast gain dynamics in semiconductor optical amplifiers (SOAs) has recently received considerable interest because of the potential application of SOAs in high-speed optical networks. It also provides a better understanding of the fundamental physical processes involved in the interaction between a short optical pulse and a high density plasma. We have performed cross-gain modulation experiments with 120 fs time resolution on a 600 ym long SOA. A weak cw probe beam is injected into the SOA, together with a strong pump pulse. The probe output signal is time-resolved by upconversion.

Ultrafast Dynamics and Modelling of Semiconductor Optical Amplifiers for WDM Applications

We have developped a r e d program aimed at the sNdy of the duafast dynamical properties of Semiwnductor Optical Amplifiers (SOAs). This program is run in collsboration with the group of H. Melchior, and of H. Jaeckel in Zurich and the group of F. Devaux at Alcatel. Amongst the pmpsrties of these devices, we specifically m d y their possibilities in tern of switching and waveleog~k c.onvwsion. To this aim, we have developped a f m t a s e m d setup which allows to pmbe the cross gain dynamics of the amplifiers. A cw beam is fed into the amplifier and is modified by a stlong c m ~ s j m l ~ ~ pump beam. The changes ofthe nu beam arc then time =solved with 150 fs resolution in a non linw gate Such a set-up wmpponds to the configuration used for wavelength wnversiah and harr the peeat interest to provide at the same h e thc time resolution and the relative tunability of the twa laser b s over a wide range. We have shldied different configuration of amplifier systems [I], and compared their dynmical properties. The behavior of the amplifiers is modelled by a semiclassical model taking into m o u n t dyoamicaly the variations of the light intensity as well as of the carrier density along the amplifier [Z]. Tbe caracteristics that we measwe are also campared with a full quantum mechanical computation of the behavior of a high density plasma using Maxwell Bloeh semiwnductor equations. A number of results of importance me obtaioed, both at the level of the basic undemandiag (fo example the higher scamring rate for hole-hole scattering which is directly measured as well as wmputed, or the importance of two photon absnrplion) as well as at the level of device behavior in an actual configuration (we wmpme for example the superior Maviorofgain clamped SOAs for lower cmss-tauc)