The EVAS evanescent wave absorption sensor consists of a sapphire fiber wound around a vertical PTFE shaft.

The ends of the fiber are tapered up to facilitate the coupling of light into and out of the probe.  The design permits the adjustment of the interaction length by more than an order of magnitude to accommodate the optimal analysis of spectral features with widely different absorption coefficients.

The EVAS is especially well suited for measurements in turbid fluids.

At a Glance

• Useful range: 300 nm - 4000 nm
• Active length: 30 cm for EW-6
• Fiber end diameter: 170 um
• Effective fractional power in evanescent wave: 1% for EW-6
• For l = 1,000 nm and input/output NA=0.39

This illustrates that sapphire fiber evanescent wave absorption by the 1.9 µm band in water is essentially unchanged when graphite particles sufficient to produce a bulk loss coefficient of 300 cm^-1 is introduced.