Photonic crystal planar waveguide comprising a mode converter and optical system comprising the waveguide coupled to an optical fibre

The circuit has a guide (12) formed in a photonic crystal membrane (11) at a surface of a substrate (10a), and a mode adapter (13) coupled to the guide. The membrane comprises a central point forming the adapter having a section gradient in termination of the guide. The central point is suspended, so as to permit the propagation of modes in a symmetrical manner. The membrane is made of silicon or III-V type material such as gallium arsenide, gallium aluminum arsenide, gallium-indium phosphide, indium phosphide or gallium indium arsenide phosphide. Independent claims are also included for the following: (1) a method for fabricating a photonic crystal circuit (2) an optical system comprising a photonic circuit and an optical fiber.