INSTABILITIES IN THE POWER SPECTRUM OF MODE-LOCKED SEMICONDUCTOR LASERS

Abstract

Sideband instabilities in the intensity spectrum of synchronously mode-locked external cavity semiconductor lasers are investigated as a function of the cavity length and frequency detuning. A sideband spectrum with distinct narmw lines is observed in the optical power spectrum for modulation frequencies smaller than the inverse photon round trip time and is attributed to fr&quency and phase modulation of the modelocked pulse train. 1.INTRODUCTION: Mode-locking of semiconductor lasers has been widely used for the generation of coherent short light pulses on the picosecond and subpiposecond timescale applying either active, passive or hybrid mode-locking techniques. With purely active mode-locking coherent pulses with 5ps duration have been obtained /I/. Apart from its use as a source for very short light pulses the mode-locked laser may serve as a model system for the investigations on nonlinear dynamics and instabilities. We have performed systematic studies of the power spectrum of an actively modelocked semiconductor laser as a function of the detuning between the modulation frequency f and the inverse photon round trip time f . mod res 2.EXPERIMENTAL: We use an external cavity configuration (cavity length 1.87~1) with an one facet antireflection coated GaAs / (GaA1)As CSP laser (Hitachi HLP1400) in the experiments. The residual reflectivity of the coated facet 4 is smaller than 10 . An intracavity etalon (bandwidth 10nm) is employed for spectral bandwidth con(''present address : Research Institute for Technical Physics of the Hungarian Academy of Sciences. H-1325 Budapest. Hungary Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988296 C2-406 JOURNAL DE PHYSIQUE trol and tuning. The cw threshold in the external cavity configuration is about the same as for the solitary laser (1~~=54.5mA) without antireflection coating. Mode-locking is achieved by modulating the prebiased diode ( I ~ ~ * I ~ ~ ) with the current pulses from a comb generator (HP33002A), which have a duration of 130ps and an amplitude of typically 15V at an impedance of 50 Ohms. The optical output was analyzed by a fast photodiode (risetime loops) and an electrical spectrum analyzer (DC-2GHz). HR Etalon MO ARcoated Laser APD ca.8OMHz 0.5 -1 W HP33002A HP33150A HLPUM) TEK4514 MSGllA Fig.l:Experimental setup. 3.RESULTS: Under optimum mode-locking conditions, fmod=fres=80MHz, the power spectrum consists of a series of equally spaced narrow lines of width 150kHz and signal to noise ratio 50dB at multiples of the modulation frequency fmodt indicating proper mode-locking. The power spectrum shows an unstructured noise band of width 13MHz for positive detuning (fmod-f >O). This broad noise band res is symmetric with respect to the inverse resonator round trip time fres9 independent on detuning. Its height and width are a measure for the amplitude and phase fluctuations in qualitative agreement with the results of mode-locked dye lasers /2/. Opposite, the power spectrum shows a characteristic structure of narrow sidebands (Fig.21, if the laser is detuned a few 10kHz to the negative side. This sideband structure is symmetric with respect to the modulation frequency fmod