Low-noise optical probes

An optical probe for non-invasive measurement of characteristics of a medium, said optical probe comprising:

• an emitter which transmits optical radiation;
• a detector configured to detect said optical radiation transmitted by said emitter and attenuated by said medium;
• a flexible circuit assembly extending between said emitter and said detector, said flexible circuit assembly having electrical circuit paths connecting to said emitter and said detector;
• and
• a substrate forming a surface of said flexible circuit assembly said substrate having an optical obstruction in the path along said surface between said emitter and said detector to reduce light piping between said emitter and said detector, said optical obstruction comprising an aperture through said flexible circuit and said substrate, said aperture configured to receive a fingertip when said optical probe is affixed to a finger, said aperture stabilizing said finger within said optical probe so as to reduce optical decoupling between said emitter and said detector.