Provided is a stable key sheet in which, even if a key top is operated so as to be shifted sidewise, no excessive change, such as great tottering of the key top or overlapping of key tops each other, is caused. It concerns compounds comprising a specifier linked to two or more of the same or different leaving groups via a self-eliminating multiple release spacer or spacer system, which compounds upon a single activation step, in particular removal or transformation of the specifier, release at least two leaving groups. If the waveguide structure has positive PDL, for example, the output fibre may be aligned with the TE mode axis to reduce TE coupling loss relative to TM coupling loss, thus inducing a negative coupling PDL. Pressure-dependent elements that change geometries andor the pressure-based openingclosing of one or more bypass channels may be used for this purpose.