Experimentall Verification of Trailing Pulse Suppression in Actively Mode-L,ocked Laser Diodes by Gain Quenching

Abstract

Actively mode-locked laser diodes (LDs) with an exlemal mirror configuration suffer from trailing pulses which is caused by the residual reflection at an LD facet even with an antireflection coaling. When the cavity length is relatively long (- few GHz) and the bias and RF cunents are large. we also observe trailing pulses caused by an excess gain. We proposed recently a new method for obtaining high-power shon optical pulses without such trailing pulscs[l]. A pan of the output is reflected back into the LD with an orthogonal polarization and an appropriate time delay. Trailing pulses caused both by the excess gain and the residual reflection at an LD facet are suppressed. In this presentation. we demonstrate the effectiveness of the proposed idea using 1.55 pm buried heterastructure InGaAsP LDs and have achieved the reduction of the trailing pulses caused by the excess gain. The experimental setup is shown in Fig. 1. The cavity side facet of an LD was antireflection coated. A laser cavity was constructed with the opposite facet of the LD and a grating, and was formed only for the light with the horizontal polarization (called TE polarization) by inserting a polarizer into the laser cavity. A part of the intra-cavity laser power was spilt off using a beam splitter and reflected back into the cavity using a suitably positioned LD amplifier which was tilted ahout 45“ to obtain the maximum amplification. The light was reflected hack hy the outside facet of the LD amplifier. The beam passes through a polarization rotator (45” rotation) and a polarizer, so that the beam reflecred back to the LD has an orthogonal polarization (called TM) The inlensity of the 1’M polarized reflected light can be changed by changing the LD amplifier current. If the distance from the LD to the LD amplifier is longer than that from the LD to the grating, the TM polarized light arrives slightly after the TE main pulse and effectively quenches the “accumulated” carriers in the LD which causes unwanted trailing pulses.