Investigation of directional coupler near-end cross talk by laser-induced refractive-index shifts

The near-end cross talk of InGaAsP-InP waveguide directional couplers is investigated. It is shown that the cross-talk level is strongly dependent on the optical path difference between the two link-end arms of the coupler. By locally heating one of these arms with an argon laser beam this path difference, and hence the couplers' near-end cross talk, can be tuned. The ultimate limit for the near-end cross talk is determined by stray light and has therefore approximately the same level as the far-end cross talk.