Gas detection based on evanescent coupling from waveguides in bulk substrates to downhole fluids

An apparatus for estimating a parameter of interest of a downhole fluid in a borehole intersecting an earth formation, the apparatus comprising:

• an evanescent wave measurement instrument configured for conveyance in a borehole intersecting an earth formation, the borehole filled with a downhole fluid, the evanescent wave measurement instrument comprising:
• a substrate configured for contact with the downhole fluid such that at least a portion of the substrate is immersed in the downhole fluid;
• a waveguide formed in an interior of the substrate, the waveguide comprising a local modification of a refractive index of the substrate, and having a configuration geometrically configured to generate an interaction between the downhole fluid and an evanescent wave arising from electromagnetic energy propagating in a segment of the waveguide responsive to application of electromagnetic energy to the waveguide;
• a detector configured to generate measurement information indicative of the downhole fluid in response to electromagnetic signals received from the waveguide, the electromagnetic signals produced responsive to the interaction.