A system and method of optically routing wavelength channels from within a plurality of optical inputs to any of a plurality of optical outputs. The screens are aperture rings, arranged concentric to a symmetry axis of the vaporising crucible in a vacuum coating unit, by way of a screen holder. As a result, the ring oscillator frequency is stabilized with respect to those parameters. An input signal is received from the amplifier at a gate terminal of the secondary transistor.