Measurement Of Ultrafast Oscillations In Vertical Cavity Lasers After Pulse Perturbation

Abstract

The temporally resolved emission of an electrically biased vertical caviw surface emitting laser (VCSEL) after perturbation with a short optical pulse has been measured with a high resolution up-conversion setup. Transverse multimode devices show very fast oscillations of the laser emission at frequencies corresponding to the lateral mode spacing. Introduction Vertical-cavity lasers are predestined devices for high speed optical communications. The small cavity volume of VCSELs promotes high photon densities and therefore high resonance frequencies of the devices. There are some reports on ultrafast oscillations in VCSELs with frequencies exceeding 50 GHz [1,2]. Normally they are attributed to ultrafast relaxation oscillations based on the oscillatory energy transfer between the electronic and the photonic system. But they are far beyond measurements of relative intensity noise (RIN) and small signal modulation [3]. In this paper we present results on investigations of the transient response of a running VCSEL to a perturbation of the intrinsic photon density. Our investigations are restricted to perturbations with pulses having the same photon energy as the VCSEL in order to get no transient carrier heating caused by carrier-carrier scattering. Laser Structure We investigate VCSELs with 3 I~.2Gao.8As/GaAs quantum wells emitting in the 980 nm wavelength regime [4]. The devices are grown on n-doped GaAs substrate, which is transparent for the emission wavelength of the device. Therefore an optical pulse with a center wavelength equal to the emission wavelength of the VCSEL can be coupled through the substrate into the active region. Current is injected through a ring contact on top of the device and through the substrate. Current confinement is achieved either through proton-implantation in the top mirror or through selective oxidation of a thin AlAs layer after mesa etching. The optical fields of the proton-implanted devices are gain and thermally induced index guided, whereas the oxidized devices are mainly index guided by the oxide aperture. Measurement Setup Fig1 shows the measurement setup. The sample is mounted such that a 100 fs pulse from a titanium sapphire (Ti:Sp) laser is coupled fiom the substrate side into the VCSEL cavity, and the device luminescence is monitored at the epitaxial side. In this setup we have the advantage that the intense backscattered light fiom the mirror of the VCSEL does not cause problems in the detection system. The laser luminescence can be measured either by an optical sampling scope with a time resolution of 25 ps or by up-conversion [5,6]. For that purpose the VCSEL output and a fraction of the Ti;Sp pulse are collimated on a crystal and mixed using type I phasematching. The polarization plane of the linearly polarized Ti:Sp pulse can be rotated