An optical system provides reflection-type isolation, and may include variable optical attenuation andor a tap monitor. The microchip includes a substrate structure including a fluid channel configured to contain a sample solution, wherein the fluid channel is maintained at a pressure lower than atmospheric pressure prior to injection of the sample solution into the fluid channel. The rear face has an aperture proximal the post, and an engagement edge integral with the rear face positioned on the inside of this aperture. A sheathing encapsulates the pulse member to protect it from contact with the fluid.