J.L. Pleumeekers

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.

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

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 Gain Dynamics in Semiconductor Optical Amplifiers by Cross-Gain Modulation

Ultrafast gain dynamics in semiconductor opticel amplifiers (SOAs) is of great interest because of the potential application of SOAs in high-speed optical networks. It also enables to obtain a better understanding of the fundamental physical praccsses involved in the interaction between a short optical pulse and a high density plasma. Here, we present experimental results obtained by a 150 fs CmSgain modulation technique. This novel technique enables lis to probe the gain recovery at different wavelengths arouud tile pump wavelength with a high spectral resolution. The pump wavelength itself can be varied over B large region. We will present results obtained with the pump io the gain region as well M in the absorption region. The results are obtained on a 500 pm long SOA operated a t 250 m 4 , providing a maximum gain of 29 dB a t 1530 urn. The experimental results show complex sub-picosecond gain dynamics, attributed to different physical pmcesses, such BS two-photon absorption, spectral hole burning and carrier heating. From these results the different time constants for these processes are extracted. Their dependence on both pump arid probe wavelength is inveatigated. By changing the probe wavelength whiie keeping the pump wavelength fixed. we are able to obtain the dependence of C lan aud agj8T on wavelength. The experimental results arc compared with a detailed theoretical analysis based on the numerical solution of the semiconductor Bloch equatioos. Apart from coherent manybody effects the inodel contains ali carrier-carrier scattering trim8 up to second order (Born approximation) that contribute to the thermalization of the particle distribution functions as well as to polarization transfer and decay. The use of microscopic collision integrals iiiscead af phenomenological relaration and dephasing times is essentiai to obtain agreement between theo re t id and expcrimmtal results.

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)

Crosstalk Due to Longitudinal Spatial Hole Burning in Gain-Clampled Semiconductor Optical Amplifiers

Gain-clamped semiconductor optical amplifiors (GCSOAs) me promising candidates for wavelength division multiplexing (WDM) applications due to their low inter-channel crosstalk and high saturation output power, obtained by the gain-clamping [I]. However, in this presentation we will show that, although thegain is clamped, some inter-channel orossldk still occurs due to longitudinal spatial hole burning. A strong longitudinal spatial hole burning (LSHB) oecum in GCSOAs due to the low mirror reflections (% 1%) wich me required to provide the high internal chip gain (m 20 dB) 111. By means of a detailed numerical model we will analyse the dependence of the LSHB on injection current snd input powers. We will show that when an optical signal is injPctPd iiito the GCSOA, the carrier density distribution inside the active layer changes largely, resulting in a small variation of the gain spectrum. Therefore, a kind of cross-gain rnodiilat,ion OCCUTS, leading t o inter-channel crosstalk when the GCSOA is used as a multi-channel amplifier in WDM-applications. However, our modeling rPsult8 show that the introduced crosstalk is rather small (less than 0.6 dB gain variation) and occurs mainly at high signal input powers, when the lasing mode is nearly switched off. Rrtl iermore, r e will siiow thal, by employing asymnetric B r a g mirror reflectivities in the GCSOA-cavity, the carrier density profile can be adapted, which can lead to a significmt redirtinn of the LSHB-induced crosstalk.

Direct Observation in the Temporal Domain of Relaxation Oscillations in a Semiconductor Laser

Pump and Probe measurements with femtosecond pulses are performed on a Gain-Clamped Semiconductor Optical Amplifier (a special laser structure). The relaxation oscillations of the carriers density are observed directly in the time domain, Reasonable values of the differential gain and the nonlinear gain factors are extracted from the current dependence of the oscillations.