In an input-output optical system of spatially optical coupled type, a spacer is provided in at least one of input and output optical systems and interposed between a lens array block and a fiber block without blocking an optical path for ensuring a distance corresponding to a focal distance of collimating lenses. The homopolymers are important for applications where a high density is needed to ensure certain mechanical properties like abrasion resistance, indentation resistance, pressure resistance, topload resistance, modulus of elasticity, or morphology and additional advantages such as melt processability. The enclosure presents an inlet, outlet, and air flow plenum, with the fan directing air flow along a path between the inlet and outlet. The high K charge blocking and charge storing medium is disposed on the tunnel medium. A drop velocity modulation device varies a relative velocity of first and second drops of selected drop pairs causing first and second drops of selected drop pairs to form combined drops having a third charge state.