Optical components improve reliability
Optical communications systems have progressed rapidly from research laboratories into commercial applications. Due to their enormous information-handling capacity, they have already been established as point-to-point transmission links and interconnecting electrical nodes. Capable of transmitting larger amounts of information over longer distances than conventional electronic communications networks, optoelectronics offer support for the growing bandwidth demands from the internet and mobile telephone services. In optical systems, developed to a level of great sophistication, electronic data signals are converted into light signals which are emitted through optical isolators into an optical fibre. These optical components increase both performance and reliability of the laser transmitters by eliminating one of the main noise sources, interferences from reflected light signals. To further reduce the manufacturing cost of laser diode packages and avoid expensive beam alignment techniques, a monolithically integrated laser-isolator system has been proposed within the ISOLASER project. Researchers at the Alcatel Thales III-V Lab deposited an electrical contact consisting of a transversely magnetised ferromagnetic metal close to the guiding region of an InP-based optical amplifier. Magnetisation perpendicular to the light propagation direction induced different non-reciprocal modal absorption in each propagation direction. In order to provide the highly selective gain needed to compensate the modal loss in the forward propagation direction, a novel multiple-quantum well material was developed. Specifically, tensile-strained multiple-quantum wells were grown instead of compressive MQWs, usually used in optoelectronic devices. The extension of the optical isolator's wavelength to the full range of the telecommunications window (1.3-1.55μm) was achieved by using AlGaInAs/InP material system. Due to the larger conduction band offset compared to the most widely used InGaAs/InGaAsP/InP, the current needed for transparency in the forward direction was lower. The advantage of this approach is that the waveguide optical isolator has essentially the same structure as the laser source with which it is to be integrated. Furthermore, its experimental demonstration opens the way to the fabrication of an integrated optical waveguide isolator.